Under the Partial Test Ban Treaty (PTBT), all nuclear tests except for those underground are banned. Under the Non-Proliferation Treaty (NPT), only the permanent members of the UN Security Council are legally allowed to possess nuclear weapons. Given the public outcry over fallout that led to the PTBT and the worries over widespread nuclear proliferation that led to the NPT, it’s clear that we require something beyond pinky promises to verify that countries are meeting the terms of these treaties.
But how do we do so? How can you tell when a country tests an atomic bomb? How can you tell who did it? And how can one differentiate a bomb on the surface from a bomb in the atmosphere from a bomb in space from a bomb underwater from a bomb underground?
I’m going to focus on two efforts to monitor nuclear weapons: the national security apparatus of the United States and the Comprehensive Test Ban Treaty Organization (CTBTO) Preparatory Commission’s International Monitoring System (IMS). Monitoring falls into five categories: Atmospheric Radionuclide Monitoring, Seismic Monitoring, Space-based Monitoring, Hydroacoustic Monitoring, and Infrasound Monitoring.
Atmospheric Radionuclide Monitoring
Nuclear explosions generate radionuclides, either by dispersing unreacted fuel, as direct products of fission, or by interactions between neutrons and particles in the air or ground. These radionuclides are widely dispersed from any surface testing, while only a few fission products (mainly various radionuclides of the noble gas xenon) can escape from properly conducted underground tests.
For the purposes of minimizing fallout, underground tests are obviously preferred. But because they only emit small amounts of one particular radionuclide, they are much harder for radionuclide monitoring to detect.
Detecting physical particles is relatively easy. There are 80 IMS stations scattered around the world. Each is equipped with an air intake and a filter. Every day, the filter is changed and then prepared for analysis. Analysis involves waiting a day (for irrelevant radionuclides to decay), then reading decay events from the filter for a further day. This gives scientists an idea of what radioactive elements are present.
Any deviations from the baseline at a certain station can be indicative of a nuclear weapon test, a nuclear accident, or changing wind patterns bringing known radionuclides (e.g. from a commercial reactor) to a station where they normally aren’t present. Wind analysis and cross validation with other methods are used to corroborate any suspicious events.
Half of the IMS stations are set up to do the more difficult xenon monitoring. Here air is pumped through a material with a reasonably high affinity for xenon. Apparently activated charcoal will work, but more sophisticated alternatives are being developed. The material is then induced to release the xenon (with activated charcoal, this is accomplished via heating). This process is repeated several times, with the output of each step pumped to a fresh piece of activated charcoal. Multiple cycles ensure that only relatively pure xenon get through to analysis.
Once xenon is collected, isotope analysis must be done to determine which (if any) radionuclides of xenon are present. This is accomplished either by comparing the beta decay of the captured xenon with its gamma decay, or looking directly at gamma decay with very precise gamma ray measuring devices. Each isotope of xenon has a unique half-life (which affects the frequency with which it omits beta- and gamma-rays) and a unique method of decay (which determines if the decay products are primarily alpha-, beta-, or gamma-rays). Comparing the observed decay events to these “fingerprints” allows for the relative abundance of xenon nuclides to be estimated.
There are some background xenon radionuclides from nuclear reactors and even more from medical isotope production (where we create unstable nuclides in nuclear reactors for use in medical procedures). Looking at global background data you can see the medical isotope production in Ontario, Europe, Argentina, Australia and South Africa. I wonder if this background effect makes world powers cautious about new medical isotope production facilities in countries that are at risk of pursuing nuclear weapons. Could Iran’s planned medical isotope complex have been used to mask nuclear tests?
Not content merely to host several monitoring stations and be party to the data of the whole global network of IMS stations, the United States also has the WC-135 “Constant Phoenix” plane, a Boeing C-135 equipped with mobile versions of particulate and xenon detectors. The two WC-135s can be scrambled anywhere a nuclear explosion is suspected to look for evidence. A WC-135 gave us the first confirmation that the blast from the 2006 North Korean nuclear test was indeed nuclear, several days before the IMS station in Yellowknife, Canada confirmed a spike in radioactive xenon and wind modelling pinpointed the probable location as inside North Korea.
Given that fewer monitoring stations are equipped with xenon radionuclide detectors and that the background “noise” from isotope production can make radioactive xenon from nuclear tests hard to positively identify, it might seem like nuclear tests are easy to hide underground.
That isn’t the case.
A global network of seismometers ensures that any underground nuclear explosion is promptly detected. These are the same seismometers that organizations like the USGS (United States Geological Survey) use to detect and pinpoint earthquakes. In fact, the USGS provides some of the 120 auxiliary stations that the CTBTO can call on to supplement its fifty seismic monitoring stations.
Seismometers are always on, looking for seismic disturbances. Substantial underground nuclear tests produce shockwaves that are well within the detection limit of modern seismometers. The sub-kiloton North Korean nuclear test in 2006 appears to have been registered as equivalent to a magnitude 4.1 earthquake. A quick survey of ongoing earthquakes should probably show you dozens that have been detected that are less powerful than even that small North Korean test.
This probably leads you to the same question I found myself asking, namely: “if earthquakes are so common and these detectors are so sensitive, how can they ever tell nuclear detonations from earthquakes?”
It turns out that underground nuclear explosions might rattle seismometers like earthquakes do, but they do so with characteristics very different from most earthquakes.
First, the waveform is different. Imagine you’re holding a slinky and a friend is holding the other end. There are two mains ways you can create waves. The first is by shaking it from side to side or up and down. Either way, there’s a perspective from which these waves will look like the letter “s”.
The second type of wave can be made by moving your arm forward and backwards, like you’re throwing and catching a ball. These waves will cause moving regions where the slinky is bunched more tightly together and other regions where it is more loosely packed.
These are analogous to the two main types of body waves in seismology. The first (the s-shaped one) is called an S-wave (although the “S” here stands for “shear” or “secondary” and only indicates the shape by coincidence), while the second is called a P-wave (for “pressure” or “primary”).
Earthquakes normally have a mix of P-waves and S-waves, as well as surface waves created by interference between the two. This is because earthquakes are caused by slipping tectonic plates. This slipping gives some lateral motion to the resulting waves. Nuclear explosions lack this side to side motion. The single, sharp impact from them on the surrounding rocks is equivalent to the wave you’d get if you thrust your arm forward while holding a slinky. It’s almost all P-wave and almost no S-wave. This is very distinctive against a background of earthquakes. The CTBTO is kind enough to show what this difference looks like; in this image, the top event is a nuclear test and the bottom event is an earthquake of a similar magnitude in a similar location (I apologize for making you click through to see the image, but I don’t host copyrighted images here).
There’s one further way that the waves from nuclear explosions stand out. They’re caused by a single point source, rather than kilometers of rock. This means that when many seismic stations work together to find the cause of a particular wave, they’re actually able to pinpoint the source of any explosion, rather than finding a broad front like they would for an earthquake.
The fifty IMS stations automatically provide a continuous stream of data to the CTBTO, which sifts through this data for any events that are overwhelmingly P-Waves and have a point source. Further confirmation then comes from the 120 auxiliary stations, which provide data on request. Various national and university seismometer programs get in on this too (probably because it’s good for public relations and therefore helps to justify their budgets), which is why it’s not uncommon to see several estimates of yield soon after seismographs pick up on nuclear tests.
Space Based Monitoring
This is the only type of monitoring that isn’t done by the CTBTO Preparatory Commission, which means that it is handled by state actors – whose interests necessarily veer more towards intelligence gathering than monitoring treaty obligations per se.
The United States began its space based monitoring program in response to the Limited Test Ban Treaty, which left verification explicitly to the major parties involved. The CTBTO Preparatory Commission was actually formed in response to a different treaty, the Comprehensive Test Ban Treaty, which is not fully in force yet (hence why the organization ensuring compliance with it is called the “Preparatory Commission”).
The United States first fulfilled its verification obligations with the Vela satellites, which were equipped with gamma-ray detectors, x-ray detectors, electromagnetic pulse detectors (which can detect the electro-magnetic pulse from high-altitude nuclear detonations) and an optical sensor called a bhangmeter.
Bhangmeters (the name is a reference to a strain of marijuana, with the implied subtext that you’d have to be high to believe they would work) are composed of a photodiode (a device that produces current when illuminated), a timer, and some filtering components. Bhangmeters are set up to look for the distinctive nuclear “double flash“, caused when the air compressed in a nuclear blast briefly obscuring the central fireball.
The bigger a nuclear explosion, the larger the compression and the longer the central fireball is obscured. The timer picks up on this, estimating nuclear yield from the delay between the initial light and its return.
The bhangmeter works because very few natural (or human) phenomena produce flashes that are as bright or distinctive as nuclear detonations. A properly calibrated bhangmeter will filter out continuous phenomena like lightning (or will find them too faint to detect). Other very bright events, like comets breaking up in the upper atmosphere, only provide a single flash.
There’s only been one possible false positive since the bhangmeters went live in 1967; a double flash was detected in the Southern Indian Ocean, but repeated sorties by the WC-135s detected no radionuclides. The event has never been conclusively proved to be nuclear or non-nuclear in origin and remains one of the great unsolved mysteries of age of widespread atomic testing.
By the time of this (possible) false positive, the bhangmeters had also detected 41 genuine nuclear tests.
The Vela satellites are no longer in service, but the key technology they carried (bhangmeters, x-ray detectors, and EMP detectors) lives on in the US GPS satellite constellation, which does double duty as its space-based nuclear sentinels.
One last note of historical errata: when looking into unexplained gamma-ray readings produced by the Vela satellites, US scientists discovered gamma-ray bursts, an energetic astronomical phenomenon associated with supernovas and merging binary stars.
Undersea explosions don’t have a double flash, because steam and turbulence quickly obscure the central fireball and don’t clear until well after the fireball has subsided. It’s true that radionuclide detection should eventually turn up evidence of any undersea nuclear tests, but it’s still useful to have a more immediate detection mechanism. That’s where hydroacoustic monitoring comes in.
There are actually two types of hydroacoustic monitoring. There’s six stations that use true underwater monitoring with triplets of hydrophones (so that signal direction can be determined via triangulation) which are very sensitive, but also very expensive (as hydrophones must be installed at a depth of approximately one kilometer, where sound transmission is best). There’s also five land based stations, which use seismographs on steeply sloped islands to detect the seismic waves underwater sounds make when they hit land. Land based monitoring is less accurate, but requires little in the way of specialized hardware, making it much cheaper.
In either case, data is streamed directly to CTBTO headquarters in Vienna, where it is analyzed and forwarded to states that are party to the CTB. At the CTBTO, the signal is split into different channels based on a known library of undersea sounds and explosions are separated from natural phenomena (like volcanos, tsunamis, and whales) and man-made noises (like gas exploration, commercial shipping, and military drills). Signal processing and analysis – especially of hydrophone data – is a very mature field, so the CTBTO doesn’t lacks for techniques to refine its estimates of events.
Infrasound monitoring stations are the last part of the global monitoring system and represent the best way for the CTBTO (rather than national governments with the resources to launch satellites) to detect atmospheric nuclear tests. Infrasound stations try to pick up the very low frequency sound waves created by nuclear explosions – and a host of other things, like volcanos, planes, and mining.
A key consideration with infrasound stations is reducing background noise. For this, being far away from human habitation and blocked from the wind is ideal. Whenever this cannot be accomplished (e.g. there’s very little cover from the wind in Antarctica, where several of the sixty stations are), more infrasound arrays are needed.
The components of the infrasound arrays look very weird.
What you see here are a bunch of pipes that all feed through to a central microbarometer, which is what actually measures the infrasound by detecting slight changes in air pressure. This setup filters out a lot of the wind noise and mostly just lets infrasound through.
Like the hydroacoustic monitoring system, data is sent to the CTBTO in real time and analyzed there, presumably drawing on a similar library of recorded nuclear test detonations and employing many of the same signal processing techniques.
Ongoing research into wind noise reduction might eventually make the whole set of stations much more sensitive than it is now. Still, even the current iteration of infrasound monitoring should be enough to detect any nuclear tests in the lower atmosphere.
The CTBTO has a truly great website that really helped me put together this blog post. They provide a basic overview of the four international monitoring systems I described here (they don’t cover space-based monitoring because it’s outside of their remit), as well as pictures, a glossary, and a primer on the analysis they do. If you’d like to read more about how the international monitoring system works and how it came into being, I recommend visiting their website.
This post, like many of the posts in my nuclear weapon series came about because someone asked me a question about nuclear weapons and I found I couldn’t answer quite as authoritatively as I would have liked. Consequently, I’d like to thank Cody Wild and Tessa Alexanian for giving me the impetus to write this.
The following is the annotated speakers notes for a talk I gave on nuclear weapons today. I’d like to claim that it was a transcript, but after practicing from these notes for almost a week, I ended up giving the talk mostly ex tempore. Like I always do.
Note:The uncredited photos were created by the US government and therefore have no copyright attached. All other images are either original (and therefore covered by the same license as the rest of the blog) or are credited and subject to the original license (normally CC-BY of some sort).
Hi I’m Zach.
This will be a backwards explanation of nuclear weapons; I don’t have time to cover it all so instead of covering the boring stuff like how fission works, I’m going to talk about the strategic realities surrounding the use of nuclear weapons.
Let’s actually do this thing like you’re a bunch of kids; I’m going to assume you’re always asking me “why?”. So at the highest level: this is a presentation about nuclear weapons.
Why am I doing this?
Like maybe a lot of you, I’ve been worried about nuclear weapons of late. My worrying actually started in September 2016. I don’t know if you remember, but that was the first time it seemed like Trump might really win. And then I think a lot of us had to grapple with what that meant.
And the biggest question there was “could this mean the end of the world?”
I was worried about the end of the world because I knew Trump might end up with the nuclear launch codes and all I really knew about nuclear weapons was that they were really dangerous. At this point I was very much in the pop culture mode of “these are the things that end the world in blockbuster movies”.
Of course before I could really take this fear seriously, I had to think about why Trump might actually use nuclear weapons. Like I was pretty sure he wasn’t going to nuke Tuvalu just for fun.
Here’s what the payoff matrix looks like for nuclear war between major powers. Everyone is pretty happy doing nothing, although they’d be happier if they could wipe out their pesky rivals . Unfortunately, their rivals want to avenge themselves if they’re going to die.
The decision-making algorithm that tells us we’re going to stick to doing nothing is General Metarationality. We know how our opponents will act in response to our actions and we avoid actions that will cause strong sanctioning. And I don’t know of any sanctions stronger than getting nuked.
This whole thing works because everyone understands it. The logic is so inescapable and the probable actions of your enemies are so obvious that the whole edifice survives, even though doing nothing isn’t technically even the Nash equilibrium.
But this is just theory. How do you ensure mutually assured destruction in practice?
That’s a question people have been asking since the 1950s. By now everyone’s agreed that there’s a right way to do it and a wrong way to do it. The wrong way is to stick a bunch of missiles in a desert and call it a day. The right way is to come up with three separate ways of delivering your warheads, spend billions and billions of dollars on them, and call it a day.
Here we have the three methods that everyone’s chosen – and I want to make it clear that this is arbitrary; three others would work just as well . As it stands though, the conventional nuclear triad is Nuclear armed bombers, like the US B-52 or B-2, Intercontinental Ballistic Missiles (ICBMs) like the US Minuteman III, and submarine launched ballistic missiles (SLBMs), like the US Trident II. The idea with this triad is that it’s impossible for an enemy to launch a first strike so devastating that they take out your whole ability to respond.
Planners are always vaguely trying to build up their capacity for a “first strike” (remember the payoff matrix before; all nuclear powers would like it best if they could win a nuclear war). The first strike idea is this pernicious thought that maybe if you nuke someone else hard enough, you’ll take out all their nukes and just win. No one has ever felt confident in their ability to pull off a first strike, which is good because if someone ever was, nuclear war would become inevitable.
But why do we care about first strikes and MAD?
Because MAD has made civilization destroying nuclear war the default form of nuclear war, at least as far as all of the non-regional nuclear powers are concerned. With respect to Trump, it means that any nuclear war he starts with China or Russia, America’s traditional nuclear adversaries, would be really bad.
Now we all know that Trump is basically in Putin’s pocket. Because of this, I wasn’t very worried about a nuclear war with Russia; I always figured that if things got heated with Russia, Trump would fold.
At the time I first did this research – remember, this was September 2016, before we found out that Xi Jinping was more than a match for Trump – I thought nuclear war with China would become a lot more likely if Trump was president. So I looked into the Chinese nuclear arsenal and there I found the question that unlocked my understanding of nuclear weapons.
China’s premier missile is the silo-based Dongfeng 5. It has a range of about 12,000km and is tipped with a 5 Mt warhead.
A brief digression: when we talk about nuclear weapons and say “ton”, “kiloton”, or “megaton”, we’re referring to the explosion that would be created by a given mass of TNT. So, the warhead on the DF5 explodes with the same force as you’d expect from 5 million tonnes of TNT. Everyone always compares yields to Hiroshima and Nagasaki. I hate this and think it’s stupid – for reasons I’ll get into in just one minute – but I’ll do it anyway. The DF5 releases 250 times as much energy as the 20 kt bomb that destroyed Nagasaki.
Anyway, the DF5 is 5Mt. The premier missile used by the US is the Trident II. It also has a range of 12,000km, it’s launched from a ballistic missile submarine, and it is armed with eight W88 warheads, each of which has a yield of 475kt.
And this was confusing, because we generally think of the US as more advanced than China when it comes to military technology – and here it definitely is! So why does China have bigger nukes?
That’s our key question right there.
So I just told you I hate the Hiroshima comparison. Here’s why: it assumes that nuclear weapons scale linearly. Get twice the yield and you should get twice the destruction, right? But very few things in the real world are linear. Nuclear weapons certainly aren’t.
There are actually 5 or 6 ways a nuclear weapon can kill you. There’s the shockwave, which knocks over buildings. There’s the gamma ray burst, which make death inevitable even as you appear to recover. There’s the thermal radiation, which can give you third degree burns, even if you’re kilometers distant. There’s the central fireball, which rips apart everything it touches. And then there’s the neutron burst and the X-rays and all the other ionizing radiation sources.
Each of these scales differently, but all of them are sublinear. This means that as a nuclear weapon gets bigger, it gets less efficient. The number of people you kill per additional ton of yield is much higher when your yield is 20kt than when it is 5Mt.
Some of these scaling effects are really complicated because of interactions with the ground or the air, but two are simple enough that I can give you a quick explanation of how to calculate them.
When it comes to shockwave, I want you to imagine a sphere. The amount of stuff in that sphere is proportional to the radius of that sphere, r, to the third power. Energy is just the capacity to do work, in this case, move stuff. If you want to figure out the amount of stuff energy can move – say move in a city destroying shockwave – you move this equation around a bit and you end up with the cube root of energy. To double the range of the shockwave, you need eight times as much energy.
For thermal radiation, I want you to think about the surface of a sphere. The size of the surface is proportional to r to the second power. Now we have a set amount of thermal radiation at the start that gets spread evenly around the whole surface of the sphere as flux, even as the sphere grows. So, you get ten meters out and the energy is spread out one hundred times as much as it was at one metre. You multiplied the radius by ten and saw the energy go down by a factor of one hundred. This also means that if you add in 100 times as much energy, the radius with a given flux will only grow ten-fold. The destructive radius (for any given destructive radiation effect) is proportional to the square root of the initial energy.
In both these cases, this means you’re facing severe diminishing returns. The 5 Mt Chinese warhead isn’t 10x as powerful as the 475kt American bomb. It’s between 2 and 3 times more powerful.
It gets worse for the Chinese warhead. It’s error radius is 800m, about 10x the 70m error radius of the Trident II. When aimed at a specific hardened target, like say a silo or a fortification, a target that needs to be hit with a certain amount of energy, the Chinese weapon is actually between 3x to 5x less likely to damage it than the American one, even though it’s much bigger. That’s not even to mention that there are eight American warheads on each missile.
They’re on these things called Multiple Independent Re-entry Vehicles, or MIRVs for short. Each one can pick its own target. Add all this up and the Trident II missile is something like 24x to 40x more dangerous than the DF5, despite looking less powerful at first glance.
The Chinese warhead is big because they haven’t mastered accuracy or MIRVs. With those, size matters much less.
24x or 40x or whatever is nice and all, but why doesn’t America go for broke and pack their thing full of 5 Mt warheads too? Wouldn’t that be best?
Well that’s because space and especially weight is at a premium on a rocket. The heavier it is, the shorter its range. There’s this whole laborious process called “miniaturization” that all nuclear weapons programs have to master. You detonate your test bomb in a big fixed installation, but then you need to make it small enough that you can fit it on a missile. That’s hard.
If you look at the real experts – not the pundits on CNN, but the brilliant folks at 38North or Ploughshares – you’ll see that there’s a lot of anxiety about North Korea “miniaturizing” their nuclear weapons. Jong-un say they have. We don’t know if he’s telling the truth or not. Miniaturization is the difference between some scary seismic readings and a crater where Tokyo used to be. If North Korea can get their physics package (the nuke part of the warhead) down to 400, 500kg, then they’ll have room to put on a heat shield. Then they’ll have an ICBM.
Not a triad. So, there’s still time for a first strike. But they’re working on SLBMs. Soon, maybe in a decade, they’ll be a “real” nuclear power. That’s bad for the US. But it’s really bad for China. Right now, China is actually more at risk from North Korea than the US, according to many analysts. It’s actually gotten so bad that China has set up missile defenses between North Korea and Beijing.
These probably won’t work if push comes to shove, but that’s a story for another day.
So to summarize:
The major nuclear powers are China, Russia, and the USA
Mutually Assured Destruction is guaranteed by a nuclear triad and has kept these powers from nuking each other.
As long as the triad lasts, first strikes will bring massive retaliation
Retaliation means that you have to do a certain amount of damage to certain targets. You can achieve this with really big nukes, or really precise nukes.
Scaling means that 10x the yield does not bring 10x the destructive power. Conversely, accuracy gives a lot of bang for your buck. 10x accuracy means 100x or 1000x damage to a specific target.
Don’t use Hiroshima as a unit of measure, because people will assume that destruction is linear and overestimate how bad things will be
North Korea can’t do anything until they miniaturize a nuke. It’s unclear if they have yet.
In response to a question about the risk of Pakistani nuclear weapons falling into the wrong hands, I explained that this would be locally really bad, but drew the distinction between events that are bad for a localized group of people (like the Taliban nuking Karachi) and events that are bad for the human race (a MAD-level nuclear exchange between China and the US). If you’re worrying about the existential risk posed by nuclear weapons, the first is really just noise, except insofar as it can make the second more likely by increasing tensions all around.
In response to a question about disarmament, I talked about the New START treaty and the need to distinguish between warheads that are stored (most of them, at least for Russia and America) and warheads ready to go (1,550 for both the US and Russia, if they’re sticking to their treaty obligations). I stressed the need for further treaties like New START to slowly reduce the number of active (and therefore existentially dangerous) nuclear weapons in the arsenals of major powers.
 I was questioned pretty heavily on this pay-off matrix. Several people thought that Do Nothing should be preferred to Attack. I have two things to say to this:
In an iterated game with this sort of matrix, the highest payoff comes when people cooperate the most. So while at any given point in time attacking might be preferred, once you take into account that real life is iterated, doing nothing is a better long term strategy.
All of us born after the Cold War, or even born after the 60s, cannot adequately understand what it was like to live in a world where it really did seem like the Soviets might “bury us”. Faced with that kind of existential threat, a first strike seemed like an appealing option. In this globalist age, it does seem much worse to launch a first strike, especially because major powers do major mutual trade.
 If questioned here, I was going to mention carrier based bombers (France tried this for a while) and nuclear tipped cruise missiles (the US may move in this direction). ^
Using this presentation
The slides are available here. This content, like everything else original on my blog, is covered by the CC-BY-NC-SA v4 International license. If you present this, please reference this blog and include a link. If you are a student and using this presentation is against the academic policies of your institution, I’d ask that you please refrain from plagiarizing it.
“We knew the world would not be the same. A few people laughed, a few people cried, most people were silent. I remembered the line from the Hindu scripture, the Bhagavad-Gita… ‘Now, I am become Death, the destroyer of worlds.'” – J Robert Oppenheimer, on the reaction to the successful test of the first atomic bomb.
Because I keep talking about it piecemeal with various people and wanted to collect everything I’ve said in one place. Because some people are more scared then they need to be and some people are more blasé than they really should be. Because I care about elevating the level of the discourse (which is often really poor). Because I’m scared that people might actually endorse some of the really terrible proposed solutions to this crisis and I want them to understand why they won’t work.
The real experts are currently busy briefing politicians and making clipped statements to the media. Therefore, it falls to verbose hobbyists like myself to try and make sense of every cryptic utterance and disseminate some of what the experts are saying more widely.
1.3 Why is does North Korea have a nuclear program anyway?
There are a lot of theories here. I’m going to walk you through my favourite. See these men?
Pictured: Muammar Gaddafi and Saddam Hussein. Images courtesy of Wikipedia Commons.
Both of those men once ran countries. Now they’re deposed and dead. The common factor? America. Call it imperialism. Call it empire building. Call it promoting democracy or protecting freedom. Call it exacting justice on two terrible butchers. From one perspective or another, all of those are the truth. What matters to North Korea is that these men tangled with America, they didn’t have nuclear weapons, and now they’re dead.
As far as I know (and the bloody purges at the start of his reign probably attest to this), Kim Jong-un doesn’t want to die. If he has a nuclear deterrent, he might fancy himself safe from any American led attempts at regime change and/or ending his horrific prison camp system.
The Saddam Hussein regime in Iraq and the Gaddafi regime in Libya could not escape the fate of destruction after being deprived of their foundations for nuclear development and giving up nuclear programmes of their own accord
2.1 What should I know about nuclear weapons to understand this crisis?
It can be helpful to understand a bit about how nuclear weapons work before reading about using them. Here’s a very quick and slightly simplified rundown.
Nuclear weapons liberate energy from the nuclei of atoms. These can’t just be any atoms. You need the right version of the right atom to get a nuclear reaction. The ones relevant here are deuterium and tritium (forms of hydrogen with additional neutrons), plutonium-239 (commonly called “weapon grade plutonium”) and uranium-235 (“highly enriched uranium”).
There are two types of atomic reactions used in nuclear bombs. In fission weapons, plutonium or uranium atoms are split apart by the energy of a free neutron. This releases more neutrons setting in motion an unstoppable chain reaction (until the energy of it blows the fuel apart). The reaction is started by creating a critical mass. Weapon grade plutonium and highly enriched uranium are inherently unstable; at any given moment, a small number of atoms of either will be breaking apart, releasing neutrons. Get a large amount of either in one place (or compress an existing sample with explosives) and you’ll have enough neutrons to start the reaction.
Fusion is the opposite. In fusion, you slam two atoms together so hard that they merge. In fusion weapons, the fuel is a mix of deuterium and tritium (or a molecule called lithium deuteride, that turns into deuterium and tritium when exposed to neutrons). When you push these together hard enough, you get helium, energy, and a very, very energetic neutron. This neutron can then start fission reactions. In many thermonuclear weapons the true destructive power comes after these neutrons hit a very large outer shell of uranium, which then fissions very violently.
Fusion weapons are often called hydrogen bombs, because isotopes of hydrogen are used in them, or thermonuclear weapons, because high temperatures (among other things) are used to initiate the process of fusion. Not all bombs that use fusion are as destructive as “true” thermonuclear weapons (i.e. the things experts normally mean when they say “thermonuclear weapons”). It is possible to put a bit of deuterium and tritium into an “ordinary” fission bomb in order to generate some extra neutrons from fusion and speed up the chain reaction. This allows for more of the fuel to be used before it scatters itself around the landscape and increases the yield of the bomb.
Yields are commonly measured in kilotons (kt; equivalent to 1000 tons of TNT) or megatons (Mt; equivalent to 1,000,000 tons of TNT). A kiloton bomb is enough to do serious damage to a large city. A megaton bomb will utterly devastate it. Yields vary widely with design, but in general you’d expect a simple fission weapon to yield somewhere between 5 and 50 kilotons; a boosted weapon would normally yield between 25 and 150 kt; a fusion weapon can yield anywhere from 50 kilotons to 50 megatons. These ranges are just guidelines and have to do more with what is an efficient use of nuclear materials than anything else; you could make a one megaton boosted fission bomb (although that actually is the upper limit on what you can do without multi-stage fusion), but this would be very wasteful compared to creating a similarly destructive thermonuclear weapon.
Having a high yield in a small package is very important for miniaturization, the process of making a functioning atomic bomb small enough for delivery on a missile. When it comes to missiles, the smaller (and lighter) the warhead, the better. A lighter warhead allows a missile to travel further, a key requirement for countries like North Korea or America, with very distant adversaries.
North Korea successfully tested a missile in July with a range of 10,000 km (6,210 miles). This range is enough to reach the continental US and classify the missile as an Intercontinental Ballistic Missile (ICBM). In addition, a missile tested in 2016 had a range of 12,000km (7,450 miles).
The United States has successfully shot down mock intermediate and medium range ballistic missile (IRBM/MRBM) in tests of its Terminal High Altitude Area Defense (THAAD) anti-missile system and mock ICBMs with its Ground-Based Midcourse Defense (GBM) anti-missile system.
North Korea claims that their nuclear weapons (including this latest one) are small enough to be mounted on their missiles (i.e. successfully miniaturized). Leaked intelligence suggests some of their earlier bombs are, but it’s unclear if that applies to this latest one as well.
It is unknown if US ground missile defense systems could successfully intercept an ICBM aimed at the continental United States or IRBM/MRBM aimed at US possessions or allies closer to North Korea (e.g. Hawaii, Guam, Japan, South Korea).
2.3 What are your best guesses for what we don’t know?
Oh my. Please remember that these are guesses.
2.3.1 Is this weapon fusion or boosted fission?
We won’t know for sure if the weapon the North Koreans detonated was “merely” a boosted fission bomb or a multistage fusion bomb until isotope analysis is completed (and even then, the results could be inconclusive or unreleased). I’m unwilling to hazard a guess here because I can make a plausible case either way. On one hand, boosted fission seems likely because it’s much easier than staged thermonuclear weapons. On the other, the North Koreans previously claimed to have detonated a thermonuclear bomb that clearly fizzled (if it indeed had a fusion stage). It doesn’t seem impossible that this failed test gave them the information necessary to make a successful multi-stage thermonuclear weapon.
I previously mentioned that testing would be necessary before any country could hope to reliably deploy multi-stage thermonuclear weapons. This is because there are a lot of unknowns in these weapons and it is hard to get them right. It’s much less surprising to see a country get their staged thermonuclear bomb right on the second try than it would be had they done it on their first.
There’s one final possibility, although it seems less likely. North Korea could have resurrected the old Sloika (layer-cake) nuclear weapon design. This is technically a thermonuclear weapon, but it requires a disproportionally (compared to its power) large mass of high explosives to work and lacks many of the desirable properties of the more conventional (staged) Teller-Ulam design (like the ability to chain as many additional stages as you’d like). The Sloika is currently regarded as a dead end in weapon development, but if the North wanted an impressive explosion to scare off the Americans and didn’t have any intent to ever put it on a rocket, it might be a good choice for them.
I don’t know. I want to believe that they haven’t successfully miniaturized this device (and that Kim Jong-un is posing with a fake in this picture). The first successful detonation of a multi-stage thermonuclear weapon required an 82-ton facility (Soviets mocked it as a “thermonuclear installation”). I find it hard to believe that in less than a year, North Korea could go from miniaturizing fission weapons to miniaturizing thermonuclear weapons, but it is possible that they have.
The recently released picture of Kim Jong-un with a “nuclear weapon” is certainly supposed to evoke a miniaturized multi-stage weapon. The distinct double humped shape (compare it to the single sphere of last year’s “disco ball of death“) suggests that there are two separate stages.
But this is a propaganda shot. Literally anything could be inside the enclosure in the pictures North Korea released (I actually think fissile material is the least likely thing to be in there, just based on how close Jong-un is to the thing; which isn’t to say that it couldn’t be identical in appearance to their actual weapons). It could be a true representation of their latest nuclear weapon designs, or it could be filled with lead. No one but Jong-un and his propagandists and senior subordinates know for sure.
Last year, North Korea claimed that a 10kt detonation was the successful test of a thermonuclear weapon capable of destroying the entire United States. We can’t trust official pronouncements about their nuclear weapons program. We can only trust the scarce scraps of hard evidence they leave.
So, in this case, I think we’re going to have to wait for more US intelligence leaks before we know either way.
2.3.3 Does the heat shield work?
It might depend on the payload. Doctor John Schilling, writing for 38 North (a North Korea focused blog run by Johns Hopkins) believes that the heat shield failed for one of the two ICBM tests this summer. He thinks that North Korea has a successfully tested a heat shield that will work with very light payloads, but has been unsuccessful building one suitable for heavier payloads (such a heatshield would need to be rather light itself).
Depending on the mass of the miniaturized North Korean bombs, they might have a heatshield suitable for striking targets on America’s east coast, or they might not be able to reach even there. It does seem likely that they can reach Hawaii or Alaska with their current proven heat shield design.
North Korea has every incentive to play down the mass of their weapons and play up the strength of their heat shield, which is what makes determining the likelihood they can successfully strike America so challenging.
2.3.4 Can THAAD and GCM defend America (and its allies)?
THAAD and GCM have both succeeded in their last few tests, but it’s unclear how closely these tests mimic reality. Unfortunately, success is relatively new for the GCM system. Previously it’s failed about as often as it has succeeded. Real missiles will probably be even harder to successfully target than the dummies it’s been tested on.
THAAD has been fairly reliable, at least in its last few tests. But it is currently only deployed to protect a few US bases in Korea. Seoul is not within its range and even if it was, THAAD wouldn’t be able to protect the South Korean capital (and its millions of inhabitants) from the conventional artillery aimed at it by North Korea. There are also THAAD launchers in Guam, Hawaii, and Alaska, giving those territories some modicum of protection.
I honestly don’t know what probability to assign to these systems making a successful interception of a North Korean missile. I think the THAAD is more likely to succeed than the GCM, but I have no hard numbers to put on either.
North Korea’s nuclear program has existed for more than three decades. But for many people, the latest tests are the first time they’ve really sat up and taken notice. To a certain extent, this makes sense. Before Kim Jong-un took over from his father, there had only been two nuclear tests and both of them were of fairly small bombs (the first was under 2kt, the second under 5kt).
If this is the first you’re seriously hearing about the crisis, it can help to get some of the historical context.
3.1 How expensive has the program been?
That’s a hard question to answer. The total cost direct cost is possibly between $1.1 billion and $3.2 billion, but it’s really hard to put hard numbers on anything that goes on in North Korea.
In addition to whatever North Kore has actually paid for its program, there’s the indirect costs. The program has led to international sanctions, the latest round of which will cost North Korea something like a billion dollars in exports. That doesn’t necessarily mean that their economy will shrink by a billion dollars though. The economic capacity that was consumed by the exports will still exist, but it will have to be used less efficiently (and may suffer from shortages of raw materials purchased with those exports). It will become harder for North Korea to acquire anything that it itself cannot produce and it will become less able to import food in the event of a famine or poor harvest. Those are both costly.
There’s also the opportunity cost. North Korea is incredibly impoverished, such that $1-3 billion dollars represents 3.5% to 10.5% of its entire yearly economic output. Had this been invested in a more economically useful fashion (e.g. in manufacturing or mining) North Korea would probably have a higher GDP. The opportunity cost of using this money in such a wasteful way cannot help but compound – that is to say the gap between what is and what could have been will only grow larger.
Here, I think a qualitative answer is best. The nuclear program has been incredibly expensive, but also – given that it is an excellent shield against regime change – worth it, at least from the perspective of Kim John-un.
3.2 Okay, but it’s cheap compared to the $61.3 billion the US spent on nuclear weapons in 2011. How can they get so much with so little?
I can think of two reasons for the discrepancy. First, the Manhattan Project created nuclear weapons from scratch. When the Manhattan Project started, nuclear weapons really were just a theoretical pipe dream. By demonstrating that nuclear weapons were possible, the Manhattan Program removed the theoretical question entirely.
But the Manhattan Project helped in ways beyond just demonstrating the technology was possible. Many other nuclear programs got help directly or indirectly from Manhattan Project scientists. Even the Soviet Union relied on the Manhattan Project to jump start their own nuclear weapons program (via the spy Klaus Fuchs, among others).
Of the nuclear powers, only America and India completed their nuclear programs without outside assistance, spies in other nuclear programs, or researcher exchanges. South Africa received assistance from Israel (and possibly France). Israel got assistance from France. France and the UK had scientists participate in the Manhattan Project. China got assistance from the USSR. The USSR conducted the aforementioned spying on the Manhattan Project. Pakistan received assistance from China (and possibly the United States) and in turn provided assistance to North Korea.
The other reason for the cheap price tag is domestic. In America, the government cannot force scientists or labourers to work on atomic weapons and must pay a wage commensurate with each employee’s skills. The American government cannot force someone who finds atomic weapons distasteful to work on them against their will. For example, Joseph Roblatt was able to leave the Manhattan Project, even in the middle of all the paranoia stirred up by World War II.
North Koreans have none of that luxury. They work for whatever pittance the government chooses to give them and are executed or sent to prison camps if they refuse. There is no room for conscientious objectors or for negotiating on salary. Put plainly, the North Korean nuclear program is much cheaper than other nuclear programs because it is underlain with slavery and coercion.
3.3 How did things get so bad?
To rip off one of my favourite authors, “slowly, then all at once”.
There was an agreement to denuclearize North Korea signed by Clinton and Kim Jong-Il in 1994, when the North first began to make progress on its nuclear program. This agreement would have provided the North with proliferation-resistant nuclear power plants and free oil as those new power plants were constructed, as well as eventual sanctions relief and normalization of relations with the United States and South Korea. In return for this, North Korea agreed to remain bound by the Non-Proliferation Treaty (NPT) and submit to monitoring of its nuclear sites.
But this wasn’t a fully binding treaty and congress never secured the funds (it was signed right before the first midterm election of Clinton’s presidency, where Republicans took back the house). Delays repeatedly occurred on the American side and I’m not sure that the North Koreans ever fully suspended their nuclear program. No normalization of relations occurred, no sanctions were lifted, and George W. Bush eventually cancelled the agreement. North Korea soon announced that they were again developing nuclear weapons.
The nuclear program rapidly accelerated after Kim Jong-Il’s death in 2011. I’m of two minds about this. I’ve seen people claim that Jong-un has poured resources into the program to help prop up his standing internally, which certainly seems in keeping with his self-preservation instinct. But I also wonder if this could just be the natural result of North Korean scientists becoming more experienced and proficient in nuclear weapons production.
Either way, there have been four nuclear tests since Jong-un took power, three of them since 2016. The rapidity of these recent tests, their pairing with tests of missiles, and Trump’s bellicose response have combined to make the stand-off feel much direr than it has been at any other point in my life.
3.4 How many nuclear weapons does North Korea have? How does this compare to the US?
North Korea’s nuclear warhead count is unclear, but estimates range from 12 to 60.
There’s a big difference between prepared warheads, unassembled potential warheads in storage, and fissile material that can be used in warheads. When people estimate the number of warheads, they’re normally estimating the fissile material that the North Koreans could possess, probably assuming it’s all eventually going to active warheads. This assumption could be wrong if something other than fissile material – maybe highly technical bomb components? – is actually the limiting factor in North Korean warhead production.
The US has 1,550 active warheads . These are the warheads that could be quickly deployed. The rest of its stockpile is in various states of readiness. I think some of them could be used relatively quickly (i.e. in a day or two), while others could be used only after a significant amount of refurbishment or preparation.
If North Korea has many active warheads (e.g. 60), an American first strike becomes impractical. It would be very hard to guarantee that all of them were destroyed (thereby preventing retaliatory strikes against the US or US troops in South Korea). Inactive nuclear weapons would still present a threat in the aftermath of a successful first strike, but it’s a threat that can be mitigated by sufficient damage to the chain of command or the logistic structure of the North Korean army.
Likewise, raw fissile material can be mostly neutralized as a threat by eliminating the state infrastructure necessary to turn it into finished warheads (it could still be used to create dirty bombs, but these are far less of a threat than nuclear warheads). It takes labour and speciality components to turn enriched fissile material into a reliable and functional weapon, prerequisites that are difficult to fulfill if the state that normally supplies them has collapsed.
I should also mention that very few (if any) of North Korea’s active warheads will be similar to the most recent test detonation. Many of their weapons will be relatively weak pure fission devices (similar in strength to their previous nuclear tests). Now that they have a warhead capable of ~150kt yields, they’ll certainly try and ramp up production of it (assuming that it’s at all practically useful and doesn’t weigh several tonnes), but that will take time.
Some experts seem to think that North Korea has much more access to enriched uranium than plutonium. This will further slow down their ability to build new weapons in the ~150kt range, at least if they want those weapons to be miniaturized .
3.5 How bad would it be if North Korea used nuclear weapons?
The latest North Korean weapon would (if it actually had a yield of 150kt and these casualty estimates are accurate) kill almost 300,000 people in LA, 270,000 people in SF, about 550,000 people in Tokyo, or 490,000 people in Seoul. If you want to get a sense of the destruction, you can play around with it on NukeMap. For cities on the US West Coast or in Asia and Europe, use a ~150kt bomb. For the East Coast, a 5-20kt bomb is probably more realistic (if one can be delivered at all) .
The danger is greatest for South Korea and Japan. Their cities are much denser (so nuclear weapons are more devastating) and much closer to North Korea (making it easy for the North Koreans to deliver larger warheads on missiles). There is also less in the way of missile defenses protecting major Asian cities, making bombs aimed at them much more likely to succeed.
That said, if North Korea ever used nuclear weapons, the greatest loss of life would be inside North Korea.
Each Ohio class submarine can carry several times as many warheads as North Korea possesses. One Ohio-class submarine with a full complement of warheads has almost the same nuclear arsenal as France.
If an Ohio class submarine were to unleash its payload on North Korea, the country would cease to exist in any meaningful way. Every single major popular centre would be irrevocably devastated. It would be destruction unlike the world has ever seen. It would make Hiroshima and Nagasaki look like child’s play. It would be the scourging of an entire country with nuclear hellfire.
Trump’s speech, where he promised “fire and fury unlike the world has ever seen” wasn’t hyperbole. It was a statement of fact. A single US nuclear ballistic missile submarine could easily make good on his threats. A single US nuclear tipped missile could make good on his threats.
(There are 14 Ohio class submarines, by the way.)
3.5.1 I’ve heard that nuclear weapons cause an electromagnetic pulse (EMP). How much damage could North Korea do with this?
Like most questions about nuclear weapon damage, this depends on several factors.
First, there’s a common misconception that a normal anti-material nuclear detonation (e.g. one within a few kilometers of the ground) creates an EMP effect that can do widespread damage. This is technically true – there is a large EMP effect – but practically irrelevant because the electromagnetic pulse will only really affect areas already ravaged by the bomb. Absent the other effects, it certainly would do significant damage, but it’s hard to think of a case where the most damage to a city attacked by a nuclear weapon will come from the EMP.
The strength of this electromagnetic pulse depends on the type of bomb, its altitude, and the local strength of the magnetic field (the stronger the field, the stronger the EMP). The ideal nuclear weapon for producing EMP effects is a single stage weapon that produces a greater-than-average portion of its energy output in the form of gamma radiation and does this as quickly as possible .
I don’t think North Korea has resources to invest in optimising for EMP effects. Development would probably require tests, which themselves require an expenditure of the government’s limited stockpile of fissile material. Since cost-effective and material-effective EMP weapons are normally single stage, North Korea would risk weakening their deterrent posture if they conducted these tests (to the US listening in with seismographs, it would look like they had regressed in their program and were failing to achieve fusion).
It also appears that most electronics, especially unplugged electronics would survive an EMP almost entirely unscathed. Computers, phones, and cars would largely be undamaged, but power lines would be heavily affected. This would be bad, but also probably not irrecoverable. A bunch of things would have to go horribly wrong for an EMP attack on America to cause more casualties than a thermonuclear attack on a large city. For this reason, I suspect North Korea’s would favour attacking population centres in any retaliatory second strike over high altitude EMP-producing bursts.
3.6 How do we get North Korea to give up its nuclear weapons program?
That is the most important question. President Trump likes to assert that China could get North Korea to stop. I once thought this was true, but I’ve abandoned that position as I’ve become better informed on the topic. If we give up on the idea that China can magically get North Korea to stop, it’s difficult to conceptualize North Korea giving up its weapons program. We don’t have a lot of examples of this occurring; the only singular history has to give us come from South Africa, which was briefly a nuclear power but later gave up its weapons. The parallels – both were international pariahs who felt weapons were necessary against an encroaching threat – offer perhaps the only blueprint for the denuclearization of the Korean peninsula.
3.6.1 How come China can’t make North Korea stop?
China once saw North Korea as a buffer against American influence or aggression. North Korea was the fifth Chinese buffer zone  – one of the client kingdoms that surround the Han heartlands of the state. To some extent, that’s still true. North Korea does provide a buffer between American allied South Korea and China. But at this point, North Korea is also a significant threat to China’s security.
The relationship between China and North Korea has significantly deteriorated since Kim Jong-un became leader. Jang Song Thaek – the uncle that Jong-un had executed – was one of the primary conduits for diplomacy between Pyongyang and Beijing. With his death, bilateral relations are largely stalled. Apparently, China hasn’t even been able to send an envoy to North Korea in more than a year.
Even before that though, mistrust characterized the relationship between Beijing and Pyongyang (on both sides). Kim Il-sung was almost executed by the Chinese communist party early in his life. Additionall disputes arose between the two countries during the Korean war and many of them haven’t been resolved since. There were even border skirmishes between the two nations in the late 1960s (I fact I didn’t know until I began researching for this section).
I don’t know why I didn’t realize this until I had it pointed out to me by 38north.org, but throughout history, client kingdom relationships have rarely been characterized by meek submission on the part of the client . If you want an example of a heavily dependent ally that America cannot effectively control, look no further than Benjamin Netanyahu of Israel. In addition to ignoring American requests to stop settlements, he resolutely opposed Obama, even crossing the normal red line of meddling in American domestic politics. Why should we expect China’s client states to behave any differently than America’s?
At this point, China seems to believe they’ve lost any ability to control North Korea. They responded to the latest North Korean missile test with the test of an anti-ballistic missile system of their own. The location of this system? Between North Korea and Beijing. This is not something allies do. This isn’t even something that disinterested parties do. Pakistan and the UK both have nuclear weapons, but the US has put no effort into building missile defenses against either of them. China fears and mistrusts North Korea more than the United States fears and mistrusts Pakistan (which incidentally is also another excellent example of a rocky relationship between client and suzerain).
All of this means that a solution for the present crisis will not come only from Beijing. The engagement of Beijing is key to bringing North Korea to the table – we can’t accomplish anything without them – but we can no longer foist responsibility for North Korea onto China.
3.6.2 Why did South Africa end its nuclear weapons program?
In the 1970s South Africa was internationally isolated. It was banned from major sporting events and faced coordinated economic and military sanctions. It was fighting two separate guerilla wars and one conventional war. Thanks to intervention by Cuba and the USSR, (white) South Africans legitimately felt like they might soon be overrun by communists.
In this climate, they saw nuclear weapons as a salvation and a guarantee of independence. They could not use nuclear weapons to pacify their own people, but they thought that nuclear weapons might buy them breathing room and permanent protection from communism. For this, a token nuclear deterrent was enough – it’s unclear if their weapons were even usable, or if they intended to use the threat of them to prompt international aid if their borders were ever threatened .
There was good reason for the world to sanction South Africa. Its apartheid system was despicable. It conducted one of the largest forced removals of people in history. It had a government without any principled claim to legitimacy. It was at war with its neighbours and had banned all dissent from its black citizens.
Many in South Africa wanted to prop up the system indefinitely. Many knew they were complicit in a great evil, but they feared death if apartheid were ever to unravel.
Does any of this sound familiar? South Africa had the same foundational paranoia that North Korea’s Kim dynasty currently possesses.
Here’s what happened. The sanctions – especially the sports bans – took their toll, demoralizing white South Africans. The Soviet Union fell, ending communism as an existential threat. Demographics forced the government to realized that they could only fight the tide of history for so long. F.W. de Klerk negotiated peace with the Angolans, the Namibians, the Cubans, and the ANC. He secured immunity for the state actors that had propped up apartheid. Then he dismantled his country’s nuclear weapons, followed shortly by his government.
This, I think, is the blueprint we must follow for North Korea. We should follow it not because it’s particularly attractive, but because it is the only blueprint we have.
3.6.3 How could we convince North Korea to give up its nuclear weapons?
First, the Americans and North Koreans would have to accept the current Chinese proposal, which would see North Korea pause its nuclear program and the US cancel joint military exercises with South Korea. This is actually similar in principle to the trilateral treaty that ended the conflict in Namibia and Angola. As a result of that treaty South Africa withdrew its forces, Cuba did the same, and Namibia ran democratic elections.
If there’s any backsliding or reluctance at all on the part of North Korea, then we can use some of the sticks that were particularly effective against South Africa, especially the sports ban (which seriously demoralized white South Africans). North Korea is currently allowed to compete in both the Olympics and FIFA. That should change. For as long as nuclear tests continue, all North Korean athletes should be banned from international competition. The North Korean government cares a lot about its successes in athletics (seeing them as proof of the power of juche), so taking that away from them would be a potent psychological blow.
If an American suspension of military drills fails to bring North Korea to the table, America will have strengthened its position with China at the same time as North Korea presents yet another embarrassment to Beijing. This will make it easier to coordinate even more damaging sanctions on Pyongyang. If Jong-un continues on this path, he risks well and truly alienating China, which would deeply cripple North Korea’s economy. I think at some point (e.g. if China gets pissed off enough that it threatens to stop guaranteeing North Korea against an attack), Kim Jong-un would have to blink and start bargaining with the powers arrayed against him
There are two paths that can be followed once the North freezes its nuclear program and America abandons its military drills. In the first, we can go back to where we were in the 1990s, but this time do it right. I’m personally pessimistic that this can lead to long term security, because totalitarian regimes and democracies can almost never co-exist, especially side by side. If North Korea remains under juche, some conflict with America will eventually escalate, ruin any existing deal, and lead to renewal of weapon’s research. I’m not opposed to buying time (every day where North Korea and America aren’t on a hair trigger is a day where far fewer people are at risk of dying!), but I’d also like to see this conflict settled for good.
Hence, the second path. It starts off like the first, with the world steadily upping the pressure on Kim Jong-un. But here, instead of just making this about nuclear weapons, we make it personal and we offer him a personal escape from his current situation . A guaranteed life of ease may not be owning a country, but it competes favourably with being dead. The goal here would be to remove Jong-un and replace him with someone able to undertake the Korean equivalent of the Khrushchev Thaw or Deng Xiaoping’s reforms.
This would go hand in hand with the negotiations following the suspension of military drills and might involve the following:
America removes all of its troops from South Korea
Kim steps down as Supreme Leader. He and all of his cronies are guaranteed a state pension for as long as they live.
North Korea agrees to abandon its nuclear program and accedes to the NPT and (after verification of the programs dismantling) the NSG.
A transitional government is put in place in North Korea. Realistically, this government will have to be heavily influenced by Beijing, but that shouldn’t rule out eventual re-unification.
I hate this plan. The only end that feels fitting for Kim Jong-un involves a firing squad.
A nuclear war between North Korea and America will (at a minimum) kill millions. Every day that tensions remain this high on the peninsula risks that eventuality. The current state of uneasy paranoia is unacceptably dangerous . Even a more stable stand-off, punctuated by brief periods of tensions this bad is too much of a risk.
North Koreans are not served by Kim Jong-un walking free and never facing justice. But they’re served even less by dying in a country turned into a conflagration.
I don’t know if this plan could work. I don’t know if there’s the political will. I don’t know if Trump or Jong-un can thread the needle, or walk the knife’s edge, or whatever metaphor you want to use for what would be an intensely difficult process. But I’m convinced that this plan, or something similar is the only way we can permanently de-escalate tensions on the Korean peninsula and remove North Korean weapons of mass destruction.
That’s the other reason I wrote this FAQ. Because I want people to have all of the context for this crisis. I want you to understand the true scope of devastation that any military response to North Korea would entail. I want you to understand that China cannot control North Korea. I want you to understand that missile defense is cold comfort. I want you to understand that we have done this before and we can do it again but that it will be hard and unsatisfying.
If you’ve made it this far, I have a favour to ask of you. Check my work. Make sure what I’ve written is correct. If I’m wrong, help me to understand this crisis even better. And if it checks out, tell other people what we know. Spread it as far as you can. Tell your friends, your coworkers. Tell your parents, your children. Help people understand what we have to do.
 For illustrative purposes, note that this means 175,000 to 380,000 fatalities if detonated above downtown LA or 270,000 to 760,000 fatalities if detonated above downtown Tokyo. For more on yield, see my post on nuclear weapon effects. ^
 If it is following the limits outlined in the New START treaty with Russia. ^
 It requires much more in the way of conventional explosives to compress a uranium primary than a plutonium primary. Uranium has a higher critical mass than plutonium, which has the consequence of requiring a greater initial mass or greater compression before fission can be obtained. Either way, this requires more explosives to start the thing. My understanding is that multi-stage fusion bombs are never started with gun-type primaries, making implosion a necessity and eliminating one option for making uranium weapons more explosive-efficient. If you want to efficiently miniaturize a bomb, you need to bring along as little conventional explosives as possible. It’s this need that has driven technologies like boosted fission. ^
 For maximum casualties, use an airburst. To see fallout, use a surface burst. Airbursts are favoured against soft targets, like cities, ports, and military bases. Ground bursts are used against hardened targets, like nuclear silos or government bunker complexes.
In large nuclear weapons (and 150kt is large by any reasonable standard), most of the fatalities come from the shockwave and thermal radiation (as opposed to the central fireball or prompt radiation exposure). When a bomb is detonated closer to the ground, there’s much less of a shockwave and fewer people are exposed to dangerous thermal radiation, but some of the soil becomes radioactive and is dispersed as dangerous fallout. ^
 I don’t know this for sure, because undisclosed. But I would bet several thousand dollars that one is there. ^
 Missiles with multiple warheads mount them on multiple independent re-entry vehicles, or MIRVs. I’ve seen this verbed, as in “those missiles were MIRVed with eight warheads each”. Each re-entry vehicle can pick an independent target (within some radius of the initial target) as it re-enters the atmosphere. Hence the name.
Technically, the Trident II missiles can carry 14 MIRVed warheads, but treaties limit them to 8. Both the US and Russia are allowed (by bilateral treaty) to have up to 288 nuclear tipped sub-launched ballistic missiles (SLBMs), with up to 1152 warheads carried on those missiles (this is in addition to the maximum number of warheads allowed per missile). ^
 Gamma rays cause electromagnetic pulses by ionizing electrons in the upper atmosphere. These electrons circle magnetic field lines, producing a large oscillating electric and magnetic field, as well as acting as a giant coordinated synchrotron array. The gamma rays emitted from these synchrotrons cause a second, longer lasting and less intense pulse that can nonetheless damage systems weakened by the first pulse. ^
Single stage weapons more efficiently produce EMPS (compared to multi-stage weapons) because the first stage of multi-stage weapons can pre-ionize the air before gamma rays from the second stage reach it. Once air is ionized, the EMP will likely induce an opposite direction current in it, which will cancel out some of the EMP effect.
When gamma rays are produced extremely quickly (here, “quickly” really means “with little gap between production of the first and production of the last”), there is little chance for this opposite current to reduce the strength of the pulse. ^
 The reason for this is almost always domestic. While it might be better for a country as a whole to reap the benefits of a close relationship with their protector, this is often hard for the leader of a country to pull off without appearing to be a foreign puppet (which is the sort of thing that leads to losing elections or dying in a coup, depending on how political systems are set up to transfer power). Seen this way, Kim Jong-un’s domestic paranoia is one of the driving forces of his estrangement from Beijing. See also The Iron Law of Institutions. ^
 This isn’t without precedent. During the Yom Kippur war, Israel assembled several nuclear weapons in plain view of US intelligence gathering assets. This is thought to have contributed (although it is unclear how much) to the subsequent American decision to re-supply Israel, replenishing its material losses from the early stages of the war. ^
 Offering an attractive escape is key. Ratcheting up the pressure without one just makes nuclear war more likely. We’re competing here with “90% chance I get to keep running my country, 10% chance I die horribly”, or the like. If we can’t make an offer that can attractively compete with this, we should avoid squeezing Kim too tightly, just in case he reacts (apocalyptically) poorly. ^
 When tensions are this high, accidents can easily start nuclear wars. Accidents happen. Let’s say (and I do not particularly believe these numbers are correct, but they are illustrative) you expect one accident a year and 30% of accidents cause a nuclear war. After five years, there is an 83% chance that a nuclear war will have broken out. It’s this small but consistent chance for a horrendous death toll that I so desperately want us to avoid. ^
I recently read The Singularity is Near as part of a book club and figured a few other people might benefit from hearing what I got out of it.
First – it was a useful book. I shed a lot of my skepticism of the singularity as I read it. My mindset has shifted from “a lot of this seems impossible” to “some of this seems impossible, but a lot of it is just incredibly hard engineering”. But that’s because I stuck with it – something that probably wouldn’t have happened without the structure of a book club.
I’m not sure Kurzweil is actually the right author for this message. Accelerando (by Charles Stross) covered much of the same material as Singularity, while being incredibly engaging. Kurzweil’s writing is technically fine – he can string a sentence together and he’s clear – but incredibly repetitious. If you read the introduction, the introduction of each chapter, all of Chapter 4 (in my opinion, the only consistently good part of the book proper), and his included responses to critics (the only other interesting part of the whole tome) you’ll get all the worthwhile content, while saving yourself a good ten hours of hearing the same thing over and over and over again. Control-C/Control-V may have been a cheap way for Kurzweil to pad his word count, but it’s expensive to the reader.
I have three other worries about Kurzweil as a futurist. One deals with his understanding of some of the more technical aspects of what he’s talking about, especially physics. Here’s a verbatim quote from Singularity about nuclear weapons:
Alfred Nobel discovered dynamite by probing chemical interactions of molecules. The atomic bomb, which is tens of thousands of times more powerful than dynamite, is based on nuclear interactions involving large atoms, which are much smaller scales of matter than large molecules. The hydrogen bomb, which is thousands of times more powerful than an atomic bomb, is based on interactions involving an even smaller scale: small atoms. Although this insight does not necessarily imply the existence of yet more powerful destructive chain reactions by manipulating subatomic particles, it does make the conjecture [that we can make more powerful weapons using sub-atomics physics] plausible.
This is false on several levels. First, uranium and plutonium (the fissile isotopes used in atomic bombs) are both more massive (in the sense that they contain more matter) than the nitroglycerine in dynamite. Even if fissile isotopes are smaller in one dimension, they are on the same scale as the molecules that make up high explosives. Second, the larger energy output from hydrogen bombs has nothing to do with the relative size of hydrogen vs. uranium. Long time readers will know that the majority of the destructive output of a hydrogen bomb actually comes from fission of the uranium outer shell. Hydrogen bombs (more accurately thermonuclear weapons) derive their immense power from a complicated multi-step process that liberates a lot of energy from the nuclei of atoms.
Kurzweil falling for this plausible (but entirely incorrect) explanation doesn’t speak well of his ability to correctly pick apart the plausible and true from the plausible and false in fields he is unfamiliar with. But it’s this very picking apart that is so critical for someone who wants to undertake such a general survey of science.
My second qualm emerges when Kurzweil talks about AI safety. Or rather, it arises from the lack of any substantive discussion of AI safety in a book about the singularity. As near as I can tell, Kurzweil believes that AI will emerge naturally from attempts to functionally reverse engineer the human brain. Kurzweil believes that because this AI will be essentially human, there will be no problems with value alignment.
This seems very different from the Bostromian paradigm of dangerously misaligned AI: AI with ostensibly benign goals that turn out to be inimical to human life when taken to their logical conclusion. The most common example I’ve heard for this paradigm is an industrial AI tasked with maximizing paper clip production that tiles the entire solar system with paper clips.
Kurzweil is so convinced that the first AI will be based on reverse engineering the brain that he doesn’t adequately grapple with the orthogonality thesis: the observation that intelligence and comprehensible (to humans) goals don’t need to be correlated. I see no reason to believe Kurzweil that the first super-intelligence will be based off a human. I think to believe that it would be based on a human, you’d have to assume that various university research projects will beat Google and Facebook (who aren’t trying to recreate functional human brains in silica) in the race to develop a general AI. I think that is somewhat unrealistic, especially if there are paths to general intelligence that look quite different from our brains.
Finally, I’m unhappy with how Kurzweil’s predictions are sprinkled throughout the book, vague, and don’t include confidence intervals. The only clear prediction I was able to find was Kurzweil’s infamously false assertion that by ~2010, our computers would be split up and worn with our clothing.
It would be much easier to assess Kurzweil’s accuracy as a predictor if he listed all of his predictions together in a single section, applied to them clear target dates (e.g. less vague than: “in the late 2020s”), and gave his credence (as it stands, it is hard to distinguish between things Kurzweil believes are very likely and things he views as only somewhat likely). Currently any attempts to assess Kurzweil’s accuracy are very sensitive to what you choose to view as “a prediction” and how you interpret his timing. More clarity would make this unambiguous.
Furthermore, we’ve already began to bump up against the limit on clock speed in silicon; we can’t really run silicon chips at higher clock rates without melting them. This is unfortunate, because speed ups in clock time are much nicer than increased parallelism. Almost all programs benefit from quicker processing, while only certain programs benefit from increased parallelism. This isn’t an insurmountable obstacle when it comes to things like artificial intelligence (the human brain has a very slow clock speed and massive parallelism and it’s obviously good enough to get lots done), but it does mean that some things that Kurzweil were counting on to get quicker and quicker have stalled out (the book was written just as the Dennard Scaling began to break down).
All this means that the exponential growth that is supposed to drive the singularity is about to fizzle out… maybe. Kurzweil is convinced that the slowdown in silicon will necessarily lead to a paradigm shift to something else. But I’m not sure what it will be. He talks a bit about graphene, but when I was doing my degree in nanotechnology engineering, the joke among the professors was that graphene could do anything… except make it out of the lab.
Kurzweil has an almost religious faith that there will be another paradigm shift, keeping his exponential trend going strong. And I want to be really clear that I’m not saying there won’t be. I’m just saying there might not be. There is no law of the universe that says that we have to have convenient paradigm shifts. We could get stuck with linear (or even logarithmic) incremental improvements for years, decades, or even centuries before we resume exponential growth in computing power.
It does seem like ardent belief in the singularity might attract more religiously minded atheists. Kurzweil himself believes that it is our natural destiny to turn the whole universe into computational substrate. Identifying god with the most holy and perfect (in fine medieval tradition; there’s something reminiscent of Anselm in Kurzweil’s arguments), Kurzweil believes that once every atom in the universe sings with computation, we will have created god.
I don’t believe that humanity has any grand destiny, or that the arc of history bends towards anything at all in particular. And I by no means believe that the singularity is assured, technologically or socially. But it is a beautiful vision. Human flourishing, out to the very edges of the cosmos…
Yeah, I want that too. I’m a religiously minded atheist, after all.
In both disposition and beliefs, I’m far closer to Kurzweil than his many detractors. I think “degrowth” is an insane policy that if followed, would create scores of populist demagogues. I think that the Chinese room argument is good only for identifying people who don’t think systemically. I’m also more or less in agreement that government regulations won’t be able to stop a singularity (if one is going to occur because of continuing smooth acceleration in the price performance of information technology; regulation could catch up if a slowdown between paradigm shifts gives it enough time).
I think the singularity very well might happen. And at the end of the day, the only real difference between me and Kurzweil is that “might”.
In an effort to make my nuclear weapons post series a one stop resource for anyone interested in getting up to speed on nuclear weapons, I’ve decided to add supplementary materials filling any gaps that are pointed out to me. This supplementary post is on laser enrichment.
Enrichment is one of the more difficult steps in the building of certain nuclear weapons. Currently, enrichment is accomplished through banks of hundreds or thousands of centrifuges, feeding their products forward towards higher and higher enrichment percentages.
Significant centrifuge plants are relatively big (the Natanz plant in Iran covers 100,000m2, for example) and require a large and consistent supply of energy, which often makes it possible spot them in satellite imagery. The centrifuges themselves require a recognizable combination of components, which are carefully monitored. If a nation were to suddenly buy up components implicated in centrifuge design, it would clearly signal its intention to increase its enrichment capacity.
Recently, laser enrichment has emerged as an additional vector for proliferation. Properly called SILEX (separation of isotopes by laser excitation), this new technology has the potential to make enrichment (and therefore proliferation easier). This post discusses how laser enrichment works and puts the threat it represents in context. It’s both a summarization (and simplification) of the recent paper on laser enrichment in published in Science & Global Security by Ryan Snyder and the product of my extensive background reading on nuclear weapons.
How It Works
Like gas centrifugation, laser enrichment requires gaseous uranium hexafluoride (Hex). While the preparation of uranium hexafluoride doesn’t represent a significant technical challenge (compared to all of the rest of the work of building a nuclear weapon), it’s still the sort of work that most reasonable chemists try to avoid. “Requires work with a poisonous, corrosive, radioactive gas” will never be a selling feature of enrichment work.
Laser enrichment also requires a large laser capable of outputting 10.2µm light (which must be converted to 16µm using Raman scattering off of H2 gas), capable of pulsing 30,000 times per second. This appears to be just barely possible with current technology and impossible with off the shelf technology. It’s the sort of thing that would have to be custom assembled.
Also requiring custom assembly is the enrichment cell, which must have a nozzle capable of injecting a supersonic stream of uranium hexafluoride in such a way as to minimize post-injection expansion. The cell also must have an optically transparent window for your laser to shine through and must have several egress lines – peripheral ones for enriched product and a central one for the jet to flow out of.
Finally, if you want to make this maximally efficient, you’re going to need a mirror set up so as to have your laser pass through the gas twice. This corrects for the circular shape of the laser. Without this mirror, you won’t have enough coverage at that edges of the gas and you’re only going to operate at 78.5% of the maximum efficiency.
The whole setup looks like this:
Once you’ve assembled all of this, you’re good to start enriching.
Remember, natural Hex is largely made up of 238UF6 and is only about 0.7% 235UF6. The purpose of enrichment is to increase the percentage of 235UF6 in the gas until it is almost entirely made up of this isotope of uranium.
The process SILEX uses to achieve this is relatively simple. You run the Hex and a carrier gas (the paper says SF6) through this system at supersonic speeds and low temperatures while pulsing the laser so as to hit the jet just as it leaves the nozzle. If you’ve tuned your wavelength as directed, then photons from the laser will kick any 235UF6 molecules they hit into a heightened vibrational state (called the v3 vibrational mode), while doing nothing to the 238UF6 molecules that make up most of the Hex.
235UF6 in the v3vibrational mode will eventually revert to a lower energy (or “ground”) state, but it is unlikely to spontaneously revert to a ground state during the few milliseconds it takes to traverse the cell. For the purposes of SILEX, 235UF6 in the v3vibrational mode will remain in that mode unless something acts on it to change it. To improperly anthropomorphize a particle for a second, this is “bad” for the excited 235UF6, because it “wants” to be at a lower ground state.
The excited 235UF6 could get external “help” from a collision with 238UF6 (this collision would allow it to release a photon and revert to its ground state), but this is unlikely if you keep the overall concentration of UF6 in the carrier gas low (the paper recommends 5%). This is in fact exactly what is done, because efficiency is maximized when 238UF6 doesn’t get a chance to collide with 235UF6.
When you put Hex in a carrier gas like SF6, you’re going to see the formation of transitory dimers. These are temporary, weak bonds between one Hex molecule and one SF6 molecule. These bonds are fairly stable, unless the Hex is in the v3 (or similar) vibrational mode. If dimer formation occurs between v3235UF6 and SF6, the dimer is very short-lived. The excited 235UF6 dumps all of its extra energy into the dimer bond, resulting in a lot of recoil; both the 235UF6 and the SF6 go flying apart in opposite directions. It’s the dimer formation that causes a very different outcome from a simple collision with 238UF6.
This recoil tends to push 235UF6 to the edges of the stream. A skimmer positioned around the outlet collects this enriched product. Note that it won’t be entirely enriched; the outside edges of the jet will have plenty of 238UF6 because the jet is going to be mostly 238UF6 – or at least, it will be when natural or lightly enriched uranium is the input.
If you were doing this on an industrial scale, you’d set a bunch of these cells up in series, with the enriched product of each cell being the feed for the next. In this way, you’d get the same sort of cascade towards higher enrichment as you would with centrifuges.
Laser enrichment might be more space and energy efficient than centrifuge arrays.
I have to say might because there’s some uncertainty here. A few key parameters that determine ease of proliferation using SILEX are missing. This isn’t because of censors removing them for security reasons. It’s because this technology is so new that there are serious question marks hanging over it. SILEX has shown promise in lab scale experiments, but there doesn’t yet exist any proof that SILEX will be superior to centrifuge enrichment when it comes to enriching uranium on an industrial scale. Given that the pilot project has been stalled since GE pulled out, it may be quite a while before we know if SELIX will fulfill its promise or not.
It looks like a SILEX would allow a country with technology on the level of Iran to enrich the same amount of uranium with only 59% of the floor area. This would make enrichment a bit easier to hide, but would do nothing to stop leaks. It was human intelligence, not satellite photos that allowed the west to discover the work at Natanz.
The error bound on SILEX energy consumption is large enough that it’s unclear if there would be a power consumption benefit or cost for rogue states switching to SILEX from indigenous centrifuge technology. State of the art American centrifuges still beat SILEX on floor space and they may beat it in energy use.
Estimates for SILEX efficiency span an order of magnitude and in the upper two-thirds of that range it seems to be a clear winner (in terms of amount of energy required per percent enrichment). I couldn’t see any consensus on the relatively likelihood of high vs. low actual efficiency, but I would personally bet that a lot of the probability distribution exists near the bottom of the allowed efficiencies. I haven’t worked in nuclear science, but I have done chemistry, and my experience is that few processes perform as well on an industrial scale as you might expect from efficiency calculations done at laboratory scale.
Enrichment with SILEX is quite possibly easier than enrichment with centrifuges. That is to say, even if SILEX doesn’t allow rogue nations to enrich more efficiently, it might allow them to enrich at all. SILEX requires some advanced optics knowledge and the lasers needed aren’t exactly available off the shelf, but they are easier to design and build than specialized enrichment centrifuges.
Before centrifugation became the preferred method of isotope separation for nuclear weapons (and nuclear energy), gaseous diffusion was used. Gaseous diffusion plants use truly prodigious amounts of space and energy. There is absolutely no way that these things can be hidden or disguised as something else.
With the advent of centrifuges, proliferation became significantly easier. Countries used to be faced with no good path to a functioning bomb. Plutonium is relatively easy to acquire and separate, but it is very difficult to build a successful implosion weapon (and impossible to do so without alerting anyone with test detonations). Uranium was relatively difficult to enrich, which closed off the option of a simpler gun assembly weapon (it is impossible to build a gun assembly weapon using plutonium).
If you want a nuclear arsenal and don’t care that gun assembly weapons are wasteful and less useful for staging, then the advent of uranium enrichment via centrifugation was a boon to you. Gun assembly weapons don’t even necessarily require test detonations, which allows for the (slim) possibility of entirely clandestine nuclear arsenals – assuming enough uranium can be secretly enriched.
SILEX may eventually exacerbate this problem, to the point where any country with access to uranium could conceivably build a relatively low yield bomb (say a dozen or two kilotons).
At present, the technology is too new for this to be true. SILEX almost certainly has a few kinks left to be worked out. Trying to work them out at the same time as your country builds a new nuclear program isn’t ideal. Best to wait for India or Pakistan to figure them out and then leak them to you in exchange for favours or missile technology (this has been North Korea’s approach to nuclear weapons and it has worked quite well).
In a decade, SILEX may make proliferation even easier. I don’t think it will make it easy to the point where Al Qaeda or Daesh can attempt to build nuclear weapons (can you imagine Daesh setting up a high-energy laser laboratory in Raqqa?). But I do worry that countries like Saudi Arabia or the Philippines might see the calculus around proliferation change enough to justify their building of a small arsenal of uranium weapons.
That would be a disaster for world peace and stability.
Governments are already reacting to threat posed by SILEX by adding necessary components to export ban and international watch lists. If any nation buys up a bunch of laser components over a short time without a good explanation, the international community will now suspect enrichment. I’m sure there are many men and women in the basement of the Pentagon and CIA headquarters now watching all laser equipment sales for more subtle signals of gradual stockpiling. Don’t think for a second that SILEX somehow represents a cheat code for proliferation. It’s still untested and unproven and governments and international organizations are already taking steps to reduce the proliferation risk.
Most nuclear technology is dual use. Uranium enrichment by centrifugation has made proliferation easier. It also increased the energy return on investment from burning uranium in power plants from ~40x to over 1500x (see here if you want to double check my calculations). Because of centrifugation, nuclear power plants could permanently end our dependence on oil if coupled with new battery technologies (and upfront capital and political will to build them).
SILEX could further increase the energy return on investment, making nuclear power plants even more economical. But SILEX also has the potential to make proliferation easier. It’s still a new, experimental technology and it might not even pan out. Until we know for sure, it is certainly best for the world to proceed with caution.
Nuclear weapons represent an existential risk. I’ll let 80,000 Hours speak for me for a minute:
A survey of academics at the Global Catastrophic Risk Conference by Oxford University estimated a 1% chance of human extinction from nuclear wars over the 21st Century. …
Luke Oman estimates the probability “for the global human population of zero resulting from the 150 Tg of black carbon scenario in our 2007 paper [delving into the effects of a single nuclear exchange] would be in the range of 1 in 10,000 to 1 in 100,000.” This being said, we think this estimate is too low, as it doesn’t account for the potential for weaknesses in their model or the risk of a societal collapse causing a permanent reduction in humanity’s ability to reach its potential (which is nonetheless an existential risk even if people remain).
If you’re interested in reducing the existential risk that nuclear weapons pose, I’ve identified a few areas where you may be able to make a difference.
8.1 Tactical Weapons
Countries have begun to reduce stockpiles of tactical weapons and put those that remain under better centralized control. No one ever wanted a fresh lieutenant in charge of the nuclear weapon that could eventually set off World War III – it just took everyone a while to realize this.
Still, it seems like this has primarily been possible because of the collapse of the Soviet Union. When the USSR seemed poised to overrun Europe, killing the commies was given priority over keeping humanity alive. Increasing regional tensions between Russia and NATO may result in a resurgence of tactical weapons.
Treaties that ban weapons under a certain yield, or require all nuclear warheads to have locks that can only be released by the civilian leadership of a country would be an excellent way to reduce the risk of conventional warfare leading to a nuclear exchange.
8.2 Arms Reduction Treaties
Not all arms reduction treaties are created equal. The Strategic Offensive Reductions Treaty (SORT) expired on the same day it came into full effect and set non-specific limits; while it may have reduced the total number of nuclear weapons deployed, it probably did this by causing the early retirement of already obsolescent systems. In addition, SORT had no verification provisions. We literally have no way of knowing if it actually had an effect.
On the other hand, the New Strategic Arms Reduction Treaty (New START) has a robust verification mechanism, including demonstrations that technology has been fully decommissioned and eighteen inspector visits each year. New START sets specific limits on ICBMs, SLBMs, nuclear armed bombers, and total deployed warheads. It will be in full force for at least three years, but might be extended longer. It comes up for review in 2019, so convincing the US and Russia to renew it will be very important.
8.3 Anti-ballistic missiles
The US ABM system represents a real threat to global peace. If it is demonstrated to be effective, we could see China rapidly increase its nuclear arsenal. If it’s expanded to the East Coast of the US, or Europe, we could see Russia do the same.
If you live in America, pressuring your congressional representative or senator to vote against any measures increasing funding for the ABM system could be very important. You can call it a waste of taxpayer money, demand it not be built in your backyard, etc.
If you live near one of the current ABM sites, or are near one of the sites for potential expansion, you can engage in direct action.
In addition to organizing protests (it should be easy to get people uneasy about nuclear weapons near them), you can attempt to bog down any expansion or new construction in and endless morass of red tape. If a system is being built near you, you should attend any community meeting you can, be as obstinate as possible, and jump on any zoning violation, rushed environmental assessment, or other bureaucratic mistake like a rabid pit bull. This won’t be very effective if new ABM sites are built entirely within existing military bases, but if even a single support strut has to go up for municipal approval, there’s potential to make an impact.
Current ABM sites are Fort Greely in Alaska and Vandenberg Air Force Base, California. Proposed eastern sites are SERE Remote Training Site in Maine, Fort Drum in New York, Camp Ravenna Joint Military Training Center in Ohio, and Fort Custer Training Center in Michigan.
Neither piece offers firm suggestions for the most effective charity and I lack the expertise to do my own evaluation. Both OpenPhil and 80,000 Hours suggest that there may not be much room for more funding, although OpenPhil suggests that effective anti-nuclear advocacy may be underfunded.
Having covered the practicalities of nuclear physics, nuclear weapon design, and nuclear weapon effects, we may now turn our attention to the strategies that have grown out of these physical realities.
7.1 Tactical and Strategic Weapons
Broadly speaking, there are two kinds of nuclear weapons – tactical and strategic. This post has been focused primarily on strategic nuclear weapons, high yield weapons capable of destroying cities and hardened targets. Tactical nuclear weapons have smaller yields, allowing them to be hypothetically used on a battlefield that contains friendlies.
The line between the two gets somewhat blurred with the highest yield tactical weapons. Is a 5kt bomb tactical or strategic? No one really has a clear answer. These already crystal clear waters get muddied further when you add in “dial-a-yield” weapons, which can yield anywhere from <1kt to ~100kt. On the low end, they’re definitely tactical. But at the high end, they’re clearly strategic.
Most of the treaties that deal with nuclear weapons don’t cover tactical weapons. This is a bit of a problem, because tactical weapons are perhaps the easiest way that a conventional conflict could escalate into a nuclear conflict. It goes like this: my army is losing, so I have a fire-team use a tactical nuke launched from a recoilless rifle on your densest concertation of tanks. The 100t weapon totally destroys the formation, swinging the battle in my favour.
The nuke I used on you, the opposing general, is not a strategic weapon, so I don’t need codes from higher up or another person to agree with my decision.
You are now losing because those tanks occupied a key position. You reply with a 1kt tactical nuke of your own, fired from an 8″ howitzer 20km behind your own lines. It takes out 3,000 of my infantry. Satisfied, you go back to conventional war.
But I’m pissed off, so I dial up one of my short ranged tactical missiles to a 10kt yield. There’s a plume of smoke from the rocket launch, a bright flash, and a crater where your army used to be. I’m satisfied with a job well done, but your side is pretty enraged. So they call in a 50kt strategic nuke on an intermediate range ballistic missile that wipes out my forces and camp.
A few more levels of escalation and the ICBMs and SLBMs begin to fly. Once you believe nuclear war is inevitable, the only thing to do is try for a successful first strike and pray for the best.
7.2 First Strike, Second Strike, Counterforce, Countervalue
A first strike is an attempt to destroy an opponent’s nuclear arsenal before they can launch it at you. SLBMs or on station stealth bombers are the only real way to pull this off. The flight time for ICBMs between probable belligerents is much too long for the missiles to reach targets before those targets have a chance to respond.
Targets include airbases where nuclear bombers are known to be based, the known location of any mobile ICBMs, missile silos, docks where nuclear submarines may be resupplying, command and control systems, and key nuclear weapons decision makers. Separately, any nuclear submarines that have be detected will be attacked by the hunter-killer submarines shadowing them.
In all likely nuclear exchanges between larger nuclear powers (NATO/Russia, NATO/China, China/Russia, China/Pakistan, China/India, India/Pakistan, Pakistan/Israel and neglecting North Korea due to the primitive nature of its nuclear program), this won’t be enough. Some of the nuclear weapons will survive.
Baring a truly unlikely display of self-sacrifice and forgiveness, these remaining weapons will immediately be fired at the aggressor in a second strike – a retaliatory attack.
First strikes are predicted to be mostly counterforce, that is to say, aimed at an enemy’s military forces in general and their nuclear forces in particular. There will be civilian casualties, because there always are, especially with weapons as indiscriminate as nukes, but civilian casualties aren’t the goal of a first strike.
A second strike, on the other hand, would be primarily countervalue, that is to say, aimed at the most valuable targets an enemy possesses. Major cities, knowledge centres, and industrial centres are the primary targets in a countervalue strike. Civilian casualties are kind of the point and consequently will be rather high.
7.3 Mutually Assured Destruction
Nuclear policy has for decades been based on the idea of mutually assured destruction (MAD). Mutually assured destruction requires all nuclear armed powers to be committed to a massive countervalue retaliatory strike against any country which deploys nuclear weapons against them. Furthermore, this threat must be credible – enemy decision makers must believe it is real and can be carried out if necessary.
For MAD to ensure stability, two things must be true:
No actor can destroy the entire nuclear arsenal of another in a first strike.
No actor can defend against a second strike well enough that they will escape unscathed.
While the current MAD equilibrium is relatively stable, it has come under threat from both the US and the USSR in the past. The closest we’ve come to nuclear self-annihilation has been those times when MAD had begun to break down.
The Cuban Missile Crisis was ultimately about first strike capability. The US had based ballistic missiles in Italy and Turkey that gave it the easy ability to target the USSR – and Soviet missiles (of which there were only 20 that could hit the continental United States from Soviet territory). With these forward deployed missiles and timely reconnaissance from the U2 spy plane, it had become perhaps possible for the US to wipe out the Soviet Union and face minimal retaliation.
When the Soviets began to build missile sites in Cuba, the problem became mutual. Suddenly each side had first strike capability. If you want to look for the hand of God in human affairs, I would suggest centring your search here. Conditions were riper for a first shot than a Mos Eisley cantina. It’s a genuine miracle that no one launched missiles.
The negotiated resolution to the Cuban Missile Crisis was publically all about Cuba. Kennedy promised to never invade again and Khrushchev promised to remove the missiles. But in secret, Kennedy promised to remove all of the US missiles based in Italy and Turkey. First strike capability was removed and equilibrium restored.
The second threat to MAD came from anti-ballistic missiles (ABMs). By the 1960s, both the US and USSR were working on ABM systems. The Soviets were building a network of ABMs around their capital Moscow, while the Americans were building a similar system around their missile silos in North Dakota.
These systems were imperfect and could be overwhelmed by MIRVs. But both sides were worried about the future; what if their enemy figured out perfect missile defense before they did? Both the US and USSR knew that If one side could gain a critical advantage in missile defense, they would be able to launch a first strike with impunity. The bitter irony was that systems designed to protect against nuclear attacks were making global nuclear war more likely.
This crisis was also defused through diplomacy. Both sides understood the risks and decided they weren’t worth it. The first Strategic Arms Limitation Treaty (SALT I) limited both the USSR and US to two ABM sites. Hot on the heels of this was the Anti-Ballistic Missile Treaty which revised the limit downwards to one site and limited the number of ABMs at each site to 100.
Unfortunately, the US unilaterally withdrew from this treaty in 2002 to begin work on a new missile defense system. If this system ever becomes operational to the point that the US can expect to survive a second strike from Russia or China, nuclear war will become imminent.
7.4 The Nuclear Triad
The delicate balance of MAD is maintained by the nuclear triad: ICBMs, SLMBs, and conventional bombers. Once a nation possesses all three legs of the triad it becomes almost impossible to remove their nuclear capability in a first strike.
Each leg of the triad has a purpose. ICBMs are static, but are relatively cheap and can often be launched quickly. SLBMs are more expensive and slightly harder to launch (they require accurate positioning for targeting) but are very hard to destroy. And conventional bombers have the greatest flexibility in avoiding attack, plus a very long range courtesy of in-air refueling.
These aren’t the only three methods that can round out a triad though. Really, the important thing is having three credible and disparate systems, such that it is impossible to remove your ability to make a countervalue second strike. Space based weapons (forbidden by treaties), air launched missiles, carrier based nuclear bombers, or nuclear torpedoes could all be used in place of one of the “standard” legs of the triad.
Only India, China, the United States, and Russia have confirmed nuclear triad capability. Israel is suspected of having a nuclear triad, but refuses to confirm or deny this assertion (Israel does wink suggestively when asked, which has led basically all experts to assume that they do in fact possess a nuclear triad).
7.5 Current Nuclear Strategy
Every country charts its own course on nuclear weapons. From public statements and acknowledged procurements, it’s possible to get some idea of each country’s strategy, but you have to understand that they really don’t broadcast these things. I mean, they broadcast them, but we shouldn’t take those statements at face value. There are a host of reasons – diplomatic, strategic, domestic, that prevent leaders from being entirely candid with their nuclear plans.
When reading into strategies, I focus on questions like: what are the known capabilities of deployed weapons? How many nuclear weapons can a country deploy? What delivery methods does a country possess? Where are its weapons based? What advances in technology are politicians highlighting in public speeches? Where have they faced technical difficulties? What countries are they friendly with? Unfriendly with?
I would also recommend avoiding the common pitfall of obsessing over the total number of warheads a country possesses. This number is much less important than the count of operational or deployed warheads. In any significant nuclear exchange, it is unlikely that any warheads but those immediately at hand for deployment will be used.
This section represents my best guesses at the current nuclear strategies of various countries. Please treat these as speculation, not as fact.
Russia wants to keep its status as a major nuclear power, but it needs to do it on a tighter budget than the USSR had. This means a focus on land based ICBMs, no truly stealthy fighters, and limited resources for its SLBM program.
Current sanctions on Russia have disrupted Russian supply chains and applied pressure on Putin to slash military spending – right as he becomes more confrontational with the rest of the world. Russia can’t afford to fall behind in the nuclear arms race. Its second strike capability is the only thing stopping the US from giving it much harsher ultimatums. Significant budget cuts would put this second strike capability under threat. But on the other hand, Russia can only afford its current military budget for so long.
This statement falsified if: real stealth bombers enter service with a sticker price of >$500,000,000 per unit, Russia manages a string of successful SLBM launches in 2017, the international sanction regime against Russia collapses with Trump as US president.
China’s nuclear arsenal is less advanced than that of Russia or the USA. MIRV-ready missiles have been rolled out only in the last decade and many of their missiles still lack MIRV capability (but they do incorporate some decoys and countermeasures). In addition, Chinese missiles are kept unfueled and without warheads in place, which drastically increases their second strike response time. They make up for this with a massive network of decoy silos, real silos, and tunnels built into the mountains of Central China. China only has a handful of nuclear missile submarines and its conventional bomber force is fairly obsolete.
China’s nuclear policy is explicitly second strike only. Based on all the facts above and what I know about China, I’m inclined to believe this. Historically, China has never cared much about what happens outside of China (broadly defined, of course). Since China already enjoys massive conventional supremacy over its neighbours, it has no need of nuclear weapons to intimidate them.
This statement falsified if: China renounces no first use, China threatens a non-nuclear state with nuclear attack, China has >10 active ballistic missile submarines by 2020, China develops a new dedicated heavy bomber.
7.5.3 India and Pakistan
Neither of these countries have sprung for forces really capable of mutually assured destruction and only India maintains a full nuclear triad. Instead of adopting MAD, they instead both aim to have forces just big enough to make a nuclear attack by the other not worth the risk.
Since these countries really only need to be able to deter each other (and possibly China), they’re freed from the need to spend to keep up with the US or Russia. Both India and Pakistan lack the ability to launch a truly significant countervalue strike in response to a first strike from the US or Russia. Given how unlikely the US or Russia launching a first strike on India or Pakistan is, this is a sensible approach.
This statement falsified if: Either of these countries test Mt range thermonuclear weapons, either of these countries develops an ICBM capable of targeting the continental United States (range >11,000km), either of these countries increases nuclear funding by >200%.
7.5.4 UK and France
These countries keep nuclear weapons because they’re members of the UN Security Council and it comes with the territory. Neither has a particularly robust nuclear force (the UK only has American made Trident SLBMs, France has indigenous SLBMs and nuclear capable bombers). It’s largely a prestige thing though. Neither country has been particularly enthusiastic about the nuclear rigmarole (and its price tag) since the end of the Cold War.
Nuclear policy in the UK and France is closely tied to the nuclear policy of NATO, although both countries do maintain some ability to conduct nuclear war on their own terms. Neither country has ruled out using nuclear against non-nuclear states in response to attacks with conventional forces and France has specifically mentioned that they would be willing to use nuclear weapons against countries that sponsor terrorism against them.
All this being said, it is unlikely that France or the UK would be the ones to start a global nuclear war.
This statement falsified if: Either the UK or France increases their supply of operational warheads, either the UK or France develops a full nuclear triad.
Now, people are less sure. Has North Korea gained full ICBM capability? Or is this more bluster and staged photographs? There’s probably a dozen men and women in the Pentagon (and in many other places) who would love to know the answer for sure. My personal guess, based on the evidence (and the bluster) is that North Korea has a missile design that in theory could target the US, but they’ll need a year or three to get it working reliably. I don’t think they’ll pull off a successful test this year, but I won’t be surprised if they pull one off in 2018 or 2019. That also seems to be the view of the Ploughshares Fund, an anti-proliferation non-profit.
Under Kim Jong-il, the prevailing belief was that the nuclear program was a bargaining chip in order to get free food or other concessions from other countries. Its purpose under Kim Jong-un is less clear. Despite punishing sanctions, Jong-un has maintained and expanded the nuclear weapon and missile programs started by his father. Whether he is willing to trade them in for concessions or wishes to use them in an attempt at unification is unknown.
This statement falsified if: North Korea successfully tests a true ICBM (range >5,500km) with successfully atmospheric re-entry by the end of 2017 and analysts believe it has enough additional payload for a miniaturized bomb.
Israel’s nuclear weapons remain unacknowledged, because Israel has pledged to not be the country that “introduces” nuclear weapons to the Middle East. Israel’s statement should be given all of the skepticism one would give to Bill Clinton using the word “is”. Israel may intend “introduce” to mean “acknowledge” or “deploy”, but we’re all pretty sure they don’t intend “introduce” to mean what it literally does.
Israel wants to have nuclear weapons as the ultimate hedge against military aggression by its neighbours. In addition, it wants to ensure that none of its neighbours possess them. Given the clear support for genocide that some of its neighbours have expressed, this position is understandable. If it appears likely that one of its regional foes will develop nuclear weapons, Israel is likely to launch a conventional attack to stop their development. If a conventional attack fails, a nuclear one is not out of the question.
Many sources talk about how Israel holds nuclear weapons as a “Samson option” and is prepared to use them to utterly annihilate an enemy if it looks like they are in a position to destroy Israel. This is actually how I’d expect most countries to behave, so I think the obsession with the Samson Option in Western reports on Israel’s nuclear program has more to do with the story it makes than a real difference between, say, France and Israel.
Israel probably possesses a full nuclear triad. Since it does not confirm or deny its nuclear program, there are no publically available official details that would allow us to be sure of this. It does make sense though. Israel has the technical know-how to pull off the tricky parts of a triad, like SLBMs.
In the future, we can expect Israel to continue to hold onto its arsenal and continue to neither confirm nor deny its existence. I don’t think Israel is a particularly likely candidate to touch off a nuclear war, as it is unlikely to use nuclear weapons against another nuclear armed state. That said, Israeli use of nuclear weapons would almost assuredly result in many civilian casualties and is still an eventuality that basically everyone would like to avoid.
This statement falsified if: Israel joins the NPT and allows inspectors into its nuclear facilities, Israel publically acknowledges its nuclear arsenal, Israel does not launch an airstrike against any nuclear program started by another Middle Eastern country.
Iran does not possess nuclear weapons, but as recently as 14 years ago was probably working on them. Most analysts believe that this work mostly stopped with the US invasion of Iraq. Iran had no particular desire to become (more of) an international pariah for developing nuclear weapons, but couldn’t accept the risk of Iraq developing them without an Iranian counter. Iran remembered the nerve agent attacks that Iraq unleashed (with the assistance of the US) during the Iran-Iraq war and felt that any developments in Iraqi weapons of mass destruction had to matched.
Iran has been prevented from coming clean about its past development of nuclear weapons by the belief (rightly or wrongly) than any admission will result in sanctions or attempts at regime change from Western actors.
Iran does have a well-developed civilian nuclear program. Despite the wailing and gnashing of teeth among hawks, the current nuclear deal should prevent any breakout towards nuclear weapons. The deal includes a robust enforcement and inspection regime that has most global powers convinced that Iran won’t be able to restart nuclear weapons work secretly.
Donald Trump’s talk of reversing this deal is just that: talk. It isn’t a bilateral deal between Iran and the USA, it’s a multilateral deal between Iran, the Permanent members of the UN Security Council, and Germany. The US could unilaterally re-impose sanctions, but there would be a significant diplomatic cost for basically no gain; it took a network of international sanctions to bring Iran to the negotiating table the first time around, so it is doubtful that US sanctions alone would change anything. The Iranian economy isn’t very integrated with the US economy, further diminishing the sting of any unilateral sanctions. Honestly, the US would probably suffer just as much from a fresh round of sanctions as Iran would.
This statement falsified if: Iran renounces the nuclear deal, Iran leaves the NPT, Iran refuses to allow inspectors access to a site they wish to visit, inspections turn up clear evidence of nuclear weapons development done since 2005.
7.5.8 The United States of America
When Vladimir Putin goes to his magic mirror and asks “mirror, mirror on the wall, who’s the greatest nuclear power of all?”, the answer is invariably “the United States”. In every nuclear metric that matters (so, discounting the total number of warheads), the United States reigns supreme. It has the best stealth bombers, the most accurate missiles, and the biggest fleet of nuclear submarines. As the world’s one remaining superpower, the United States is the only country that is able to mount even a half-way decent first strike – although it probably can’t launch a successful first strike against any triad state.
I don’t know what US nuclear policy will look like going forward. If Donald Trump maintains good relations with Putin, then a nuclear exchange with Russia will be (even more) unlikely. I do think a nuclear exchange with China has become slightly more likely as a result of Donald Trump’s election, but I hope the risk is relatively low.
No matter how you cut it, the risk of a nuclear exchange is – and always has been – low. But no one truly knows how low “low” is. Is it 10% over the Trump presidency? 1%? Whatever it is, I wish it was lower.
I also wish Trump was less of a loose cannon. I can’t really make predictions about America’s nuclear policy over the next few years because the information I have is too heavily weighted towards hyperbole and bluster.
All the nukes in the world are useless unless you have a way to get them to their targets. Aside from outlandish and potentially suicidal methods like suitcase nukes or nuclear artillery, there are three main ways of doing this: bombers, intercontinental ballistic missiles (ICBMs), and submarine launched ballistic missiles (SLBMs).
The only nuclear weapons ever used in anger were delivered by B-29 bombers, the Enola Gay and the Bockscar. Because the Allies had attained near total air-superiority over Japan at the time of the bombings, it was possible for these bombers to go in without any real escort. They were accompanied only by weather reconnaissance and observation planes.
In a modern nuclear exchange, total air superiority would probably be required for a country to be able to openly deliver a bomb. If a nuclear bombing is attempted with anything less than total superiority, the attacker can expect to have all remaining anti-air defenses thrown at them.
Absent total air superiority, the only reasonable way to deliver a nuclear weapon via bomber is stealth. Conventional bombers can make a play at this by flying below radar, but this is very risky and doesn’t guarantee success. In states of heightened alert, the bomber would have to avoid both radar and settlements where residents might alert the target to the coming attack.
Stealth aircraft can make a better go at delivering bombs, especially when the target is on high alert. All new combat aircraft are billed as having some stealth capabilities, but the only true stealth bomber is the US B-2 Spirit. Its virtually invisible to most means of detection (it shows up about as well on radar as a large bird or a sphere a foot in diameter would) and can carry 18 tonnes of bombs 11,000km (this gives it a range of about 5,000km, neglecting in-air refueling).
The main disadvantage of bombers then, is the difficulty that they all (except the B-2) face in arriving at their target both intact and at the correct altitude to drop a bomb. There are a couple of significant advantages to bombers though.
They’re controlled by people. Really well-trained, smart people who know what they’re doing. Tell a pilot that it’s up to her to prevent a retaliatory strike on her home country and she’s going to be very motivated to carry out her mission. Maybe she’ll come up with a new trick and make it to her target. If she doesn’t, maybe one of her squadron mates will. Send in a dozen bombers and you force your enemy into a problem of resource allocation. They may end up spread too thin to stop them all. And if just one gets through…
Having pilots also makes bombers fairly accurate. The steep fall-off in weapon intensity with distance from the target (as discussed earlier in §5.1) demands a fair amount of accuracy and pilots can be significantly more accurate than some ICBMs.
Bombers are also the delivery method that’s hardest to stop at the source. During periods of heightened alert, countries will always keep nuclear armed bombers in the air. Any sign of a nuclear attack and they’ll be released for bombing runs on their prearranged targets. Unlike ICBs, there is no silo you can take out to stop them.
Intercontinental ballistic missiles are missiles capable of delivering nuclear warheads at least 5,500km from their launch site. Intercontinental ballistic missiles are distinguished from cruise missiles by their use of rocket engines and their suborbital trajectories. They leave the atmosphere during their boost phase and come back down on top of their target, using a guidance system and fins for limited maneuvering and final targeting once back inside the Earth’s atmosphere.
US and Russian ICBMs use multiple independent targetable re-entry vehicles (MIRVs). A single ICBM can carry many MIRVs, which allow it to simultaneously strike several targets. MIRVs were invented in response to anti-ballistic missiles, nuclear tipped missiles that detonate near incoming enemy missiles, causing them to fizzle or frying their electronics. The advent of MIRVs made missile defense mostly pointless, because it is much easier to build bigger ICBMs with more MIRV capability than it is to expand missile defense systems to deal with more incoming MIRVs.
MIRVs were one of the main factors pushing down bomb yields. The scaling factors we went over earlier mean that smaller weapons do damage much more efficiently than larger ones. It was MIRVs that made the delivery of these smaller weapons economical. Without MIRVs, launching 8 smaller warheads would require launching 8 smaller missiles, which would end up much more expensive than launching one really huge missile. With a MIRV, you get the best of both worlds – one missile, many warheads.
Improvements in US and Russian missile accuracy over the last 50 years have also driven the decrease in the warhead yields commonly fielded by those countries. Early ICBMs were accurate to within about half a kilometer, which necessitated big warheads if you wanted to be sure of annihilating your target (this is especially important when the target is hardened, like your enemy’s nuclear arsenal is liable to be). Modern US and Russian missiles are now accurate to something like 200m, which (using cubic scaling to approximate destruction) allows for the same change of destroying the target with about 1/15 the yield.
China lags behind on ICBM technology, with Chinese missiles only accurate to within 800m of their target. This results in China using much larger warheads (4-5 Mt) on its ICBMs than the 475kt US MIRV capable W88 or the 200-300kt warheads the Russians favour. It is counter-intuitive yet true that routinely using larger warheads is the sign of a less capable nuclear power.
ICBMs are stored on mobile launching units, or in missile silos away from large cities and built into mountains or underground. Both strategies are designed to protect missiles from enemy attack. Keeping ICBMs mobile makes them very hard to target with other missiles – where they are when the missile is launched may not be where they are when it arrives. Add this to the problem of finding the damn things in the first place and you have relatively good protection.
Missile silos were most useful when missile accuracy was much worse than it currently is. Missile silos are supposed to be impervious to everything but a direct hit from another nuclear weapon. Once a difficult task, this is now fairly possible. If nothing else, missile silos are useful because they will divert some enemy attacks away from populated areas. Still, the expense and relative uselessness of nuclear silos mean that no one is clamouring to build more of them. The Russians are pivoting towards mobile ground based launchers, while the USA is focusing its efforts on SLBMs.
ICBMs have several advantages over bombers. Missiles can endure g-forces, altitudes, and heat that would kill humans many times over. Taken together, this lets missiles launch with very rapid acceleration, travel at very high speeds, and use very efficient trajectories; basically they arrive on target much more quickly than a bomber can. They’re also much harder to intercept. Modern ICBMs come with a variety of countermeasures, from dummy MIRVs, to aluminum balloons, all of which make countering them extremely difficult.
The Iron Dome suggests that >90% missile interception might be technically possible, but intercepting ICBMs remains much more difficult than intercepting short range rockets, so don’t expect much progress anytime soon.
One disadvantage of ICBMS are their highly visible launches and re-entries. A successful stealth bombing provides no warning at all and a successful SLBM attack might provide only 2-10 minutes of warning. ICBMs take something like 30 minutes to cover intercontinental distances (say, Russia to the USA, or China to the USA). During this time targets can be evacuated and retaliatory strikes prepared.
At one point, it made sense to have a firm distinction between SLBMs (which had fairly small ranges) and ICBMs. For modern weapon systems, the ranges are effectively the same. Gone are the days where submarines had to sneak right up to the coast to attack their targets. If you were to look at a globe centred on a modern US or Russian ballistic missile submarine, you’d see all the targets it can hit.
Modern nuclear armed submarines are also very stealthy. As long as they remain submerged they cannot be found by satellites. SONAR and thermal imaging remain about the only things that can find them. Active sonar will smoke a sub out pretty quick – at the cost of letting the submarine know it’s been found and giving away the position of the intercepting vessel. Passive SONAR can occasionally find them, but the ocean is big and nuclear submarine captains know a whole bunch of tricks for masking their SONAR signature.
Thermal imaging can find nuclear powered submarines by the wake of warm water they leave behind (warmed after cooling their nuclear reactors), but savvy captains (and all of them are) can minimize their changes of detection by cruising along boundaries between zones of different temperatures. These zones are so irresistible to subs that a French and British submarine managed to collide as they both (probably) followed one. This was obviously pretty embarrassing to both countries, but should stand as a testament to how damn stealthy these things are. Had either captain realized what was going on, he would have changed course.
Submarines powered by nuclear reactors can run for decades without any refueling. The only practical limit on how long a nuclear sub can stay submerged is the food supply. Water is collected from the ocean and air is recycled much as it would be on the ISS.
SLBMs have all the advantages of ICBMs, with the added advantage of stealth. The US has embraced them wholeheartedly. Its SLBMs are more accurate and more heavily MIRVed than its land based missiles; the Ohio class nuclear submarine carries 24 Trident II SLBMs with up to 8 W88 475kt warheads, each with an effective range of 11,000km and an average error less than 90m. The Russians, Chinese, and others have been less enthusiastic with their nuclear subs. Russia in particular has been having a lot of trouble getting new SLBM systems into active use.
This highlights the one real disadvantage of SLMBs. They’re expensive and technologically complicated. Just getting a missile to successfully launch from beneath the ocean is challenging enough. Add in stealth requirements and a nuclear reactor and it’s no wonder that the sea floor is littered with sunken nuclear submarines, primarily of Soviet origin (one unlucky Soviet nuclear sub actually sunk twice).
You could reduce the technical complexity by using a more traditional diesel-electric submarine, but you’d sacrifice much of the submarine’s endurance and ability to stay submerged. Batteries can only power a submarine for a few days (or hours, if it’s moving as fast as it possibly can) and diesel engines will quickly suffocate the crew if run underwater. Add to this the limited amount of diesel any sub can carry and nuclear submarines become the only real way to do long duration submerged deterrence patrols.
To understand the effects of nuclear weapons, you first need to understand how those effects scale with weapon yield.
Modern bombs are much smaller than the Tsar Bomba. The standard US nuclear warhead, the W88, is a “mere” 475kt, a yield that is 100x less than that of the Tsar Bomba. On the other hand, the W88 weighs in at 360kg, 75x lighter.
This may seem like a poor trade, but it’s actually a very good one, due to the fundamental properties of explosive scaling. Scaling factors are very important to weapons. They determine the stable equilibriums that designs fall into. For example: we have tanks instead of mechs because strength scaling and mass scaling together make tall vehicles very vulnerable to weapons.
Scaling factors for all nuclear weapon effects (the fireball, the shock wave, and electromagnetic radiation) are different, but we can use the scaling factor of electromagnetic radiation as an example.
A nuclear weapon emits a set amount of energy as photons – gamma rays and X-rays, as well as IR, UV, and visible light. These photons radiate out approximately equally in all directions. The energy that the photons carry is fixed, but the area that the energy covers isn’t. We call the amount of energy per square meter “intensity”.
Intensity is incredibly important. It’s what drives the differences in climate between Mercury (surface temperature: 427ºC) and Pluto (surface temperature: -218ºC). In nuclear weapons, the intensity of radiated photons determines if buildings are set afire from the heat or people die of radiation sickness.
Since energy is constant, intensity will depend only on the area the energy is spread out over. Luckily, it’s pretty easy for us to calculate this area as a function of distance from the bomb. Dividing the total energy by the surface area of a sphere of a certain radius, we get the following equation for intensity:
Where E0 is the initial energy of the bomb and d is the distance from the epicentre of the explosion.
Ten meters out from a 10kt (in the SI unit, joules, this is 41.8 TJ) bomb, the energy per square metre 33,300 MJ. Ten times further away, the energy per square metre is 333 MJ, a 100-fold decrease.
To get the desired destructive power, a certain intensity is necessary. An intensity of 372kJ per square meter is necessary to give third degree burns, for example. Ignoring loss from the air, you get this effect from a 10kt bomb out to about 3km. Increase the bomb size 100-fold to 1Mt, and the radius expands only ten-fold, to 29.9km.
These examples illustrate one of the fundamental scaling factors of nuclear weapons. The photon effect radius of a nuclear weapon is proportional to – at best – the square root of the power of the weapon. This means that there are intense diminishing returns to increasing the power of a weapon. Increase its power 25 times and the radius of destruction becomes five times bigger. Increase the power 50 times and the radius only increases sevenfold.
Blast radius and fireball size scale differently than radiation, but in all cases the scaling is square root or worse. In fact, you’ll soon see that radiation scales better than nearly every other destructive effect.
Taking into account this scaling factor, the W88 is (by one measure) 10x less destructive than the Tsar Bomba, but 75x lighter – a trade-off that makes much more sense. Keep this relationship in mind whenever nuclear weapons are described in terms of “the power of the atomic bomb that destroyed Hiroshima”. The policy implications of this relationship will also become clear later. For now, it’s important that you simply understand that relative destructive power and yield don’t correspond 1:1.
Thus far, we’ve restricted our discussion of nuclear weapon detonation to the technical. There’s a cascade of neutrons and a bunch of energy is released as a by-product – that much is clear. But what does all that energy do? How do we get from neutrons to levelled cities and mushroom clouds?
There isn’t as much clear, well-curated information on this topic as I’d like, but perhaps that’s only to be expected. As near as I can tell, here’s what happens:
A lot of heat (and some photons and neutrons) get generated by fission or fusion
The fission of an atom releases a huge amount of energy. Only 7% of this in the form of fission neutrons and gamma rays; most is in the kinetic energy of the atom fragments, which will be travelling at about 4% of the speed of light.
Fusion reactions output mainly high energy neutrons. These are captured by the dense tamper around the fusion core, allowing them to cause fission, or to impart their kinetic energy to another atom. Most fusion neutrons that don’t collide with the tamper will escape.
About one microsecond after detonation, the core and tamper of a bomb will be a cloud of plasma a couple metres in diameter, with temperatures exceeding 10,000,000ºC.
Via blackbody radiation, the plasma emits x-rays. These x-rays heat the surrounding air to a similar temperature.
The very air becomes incandescent, releasing a bright pulse of light.
Depending on yield, distance, and atmospheric conditions the pulse can be permanently or temporarily blinding for anyone who looks at it.
As well as visible light, IR and UV light is emitted. The combined radiance of all of this light can set buildings on fire or give people severe burns.
The superheated air in the very centre of the explosion pushes against the surrounding air, acting like a giant piston and producing a massive shock front.
Compression in the shock front causes it to briefly become a dense plasma, temporarily blocking the light from the incandescent air closer to the core.
The compressed air of the shock front begins to expand rapidly, dropping in temperature and becoming clear. The fireball in the centre can once again be seen.
This shock wave travels at approximately the speed of sound. Whenever it hits something (a house, a vehicle, a tree), it causes damage via two mechanisms.
Static overpressure: the direct hammer-blow of all that dense air striking at once.
Dynamic overpressure: the drag associated with the wind of the shock-front’s passage, which can tear, tumble and drag objects.
Back in the centre of the blast, there’s still a bunch of hot air. It will quickly cool to the point where it no longer glows, but it will still be much brighter than the surroundings.
This hot air begins to rise, pulling up debris from the ground (at this point, there will be plenty) into the vacuum its rising creates. The hot air also expands and the outside edges cool, which causes rotating internal air currents that help to draw in more air from below. This forms the characteristic mushroom cloud of a nuclear explosion.
Between the vacuum caused by the rising mushroom cloud and the vacuum caused by the initial shockwave, a steady wind will blow back towards ground zero until equilibrium is reached. This wind will further toss around debris and can help to fan the flames of any fires that have started.
The nearby detonation of an atomic bomb is utterly devastating to infrastructure. Everything within the central fireball will be annihilated. The size of the fireball varies: 150m for a 10kt blast (slightly smaller than Hiroshima), 720m for the modern 475kt W-88 favoured by America, and 4.62km for the monstrous 50Mt Tsar Bomba. Ideally, fireball effects would scale as the square root of the yield. In actuality, they scale a bit worse than this, as the fifth root of the square of the radius (i.e. the 2.5th root). I’m not sure exactly what causes this, possibly some weird property of plasma, or just general fluid dynamics oddness.
As deadly as the fireball is, most of the damage from a nuclear explosion comes from the shockwave. Weapon designers carefully model the shockwave and use these insights to come up with an optimal detonation height for various effects – like levelling reinforced buildings or destroying residential areas. Models take into account the “Mach stem”, the angle at which some of the wave will be reflected off the ground and constructively interfere with the rest of it, leading to increased destruction.
To get an idea of the damage a shockwave causes, we can look at the radius within which the maximum pressure increase is 20 psi and the radius within which the maximum pressure increase is 5 psi. 5 psi is the blast pressure required to level residential buildings, while 20psi will demolish even heavy concrete buildings.
Unlike radiation effects, blast pressure increases with cube root of the yield. This scaling – like the shockwave itself – is driven by heat. The pressure air exerts is directly proportional to its temperature (this relationship is linear and given by the ideal gas law: PV = nRT). To get a 5 psi overpressure at a certain radius, the bomb must ultimately heat all the air within the radius to a temperature that will cause a 5 psi increase in pressure. The energy taken to heat a material is proportional to the mass of material to be heated. The mass of air is proportional to its volume. And volume of a sphere is really easy to calculate: . Chaining all these observations together, we’re left with radius increasing as the cube root of explosive power.
This means that for every 1000-fold increase in bomb power, you’ll see a 10-fold increase in destructive radius. In terms of overpressure effects, the W88 is about 4.7 times less destructive than the Tsar Bomba, while being 75 times lighter. This trade-off comes up even better than for radiation.
[Data Source: Nukemap 2.42] 5 psi trend line has equation , 20 psi trend line has equation .
5.2.3 Direct Radiation
Nuclear weapons produce two types of damaging radiation (they’re both photons, just at different energy levels): thermal and ionizing. Ionization radiation (mainly gamma rays) is responsible for causing acute radiation poisoning and later cancers. Thermal radiation (light, from IR to UV) is responsible for the horrendous burns nuclear weapons survivors often bear.
In a vacuum, both of these would exhibit scaling with the square root of the yield, as discussed earlier. In real world conditions though, there’s another key factor in play: the atmosphere. Air is actually fairly good at absorbing gamma radiation. For every 150m that a group of gamma rays travels through the air at sea level, about half of them are absorbed. This is cold comfort if you ever find yourself at ground zero of a nuclear explosion, but it means anyone more than a few kilometres away has fairly little to fear from direct radiation. Once atmospheric absorption is taken into account, the radius where gamma radiation is lethal increases with approximately the 6th root of the yield. This means that even very large bombs scarcely irradiate more people than their smaller counterparts.
As blasts become larger, fewer and fewer people are affected by direct radiation. The radius of complete destruction (20 psi overpressure) is smaller on 10kt bombs than the radius at which radiation fatalities are common. But the differences in scaling means that by the time a bomb is 575kt the radii are equal. Beyond this size, the radius of complete destruction will be larger than the radius of radiation effects and most casualties will come from the shockwave, not radiation.
IR, visible, and UV light are much less affected by the atmosphere. If the atmosphere absorbed light at the same rate it absorbs gamma rays, the sun would appear 717 billion billion times brighter on an airplane at cruising altitude as it does at sea level. This obviously isn’t the case – the sun is approximately as bright in both cases. This difference in absorption means that the radius at which nuclear weapons cause burns scales with close to the square root of the yield (the small amount of energy absorbed by the air does mean that it scales slightly slower in atmosphere than in vacuum).
[Data Source: Nukemap 2.42] 50% Deaths trend line has equation , 3rd Degree burns trend line has equation .
Part of the horror of nuclear weapons comes from fallout, the lingering radiation left behind after a nuclear detonation. Fallout doesn’t last forever (and remember, the more unstable and radioactive an element is, the quicker it breaks down into something stable), but while it is around it can sicken and kill. Fallout can also cause illness and death far from the site of the initial blast, increasing the human toll of any nuclear weapon detonation.
Fallout is composed of unstable, radioactive isotopes that are scattered after a nuclear bomb is detonated. Fallout is mainly comprised of fission by-products (some of these are stable, but many are themselves radioactive and must conduct additional decay before they become stable) and left over fuel (remember, fission efficiency is almost never higher than 40%, leaving 60% of the fuel weight in radioactive waste). Depending on bomb detonation height, there may also be neutron activated fallout. Neutron activated fallout arises when stable elements are bombarded with neutrons and either capture the neutron, or fission from the energy of the collision. Since nuclear weapons tend to be detonated high in the air (with the hope of maximizing the shock wave), neutron activated fallout is a small portion of the total fallout. Either way, all fallout behaves similarly.
Some of the fallout will be blown up into the stratosphere by the force of the blast, or carried up in the wake of the rapidly rising fireball. These fallout products end up distributed fairly evenly across the whole globe. Nuclear testing before the partial and complete test bans resulted in a dramatic rise in strontium-90 and caesium-137 levels in humans all over the globe. These isotopes can cause cancer and other problems if ingested in significant amounts and the threat posed by steadily increasing global levels was enough to prompt the test ban treaties.
Stratospheric fallout is the minority; most of the fallout remains in the area of the blast. It’s still dispersed in the atmosphere, but its subject to local winds, not the global jet stream. The largest particles begin to fall immediately. Not all of them will be radioactive – some of the falling matter will be simple vaporized dirt or water – but it will never be safe to assume that any particles falling in the wake of a nuclear explosion are benign.
A high altitude burst doesn’t guarantee that the particles that rain down will be fallout free. Even if no neutron activation occurs, some of the material that rains down will be contaminated with radioisotopes produced by the blast. After the Castle Bravo nuclear test, coral contaminated with radioisotopes fell like snow over a large area, causing severe radiation burns whenever it touched human skin.
In the first day after a bomb is detonated, half of the local fallout will be deposited – unless it rains, in which case even more will fall. Rain is actually quite likely after a nuclear weapon detonation because fine particles dispersed in the atmosphere (like the dirt drawn up in a mushroom cloud) can help start the process of raindrop formation. Rain makes decontamination even harder, as the radioactive ions that travel down with raindrops tend to bond to external surfaces, requiring sandblasting to remove them.
The danger of the fallout varies with local atmospheric conditions, the size of the bomb, the efficiency of the bomb, and where the bomb detonates. Winds can disperse fallout, affecting more people, but giving each person a lesser dose. Bombs with a high efficiency create less fallout than bombs with lower efficiency. Bombs that get more of their power from fusion will generally have less fallout than a bomb of the same yield that gets more energy from fission.
Because of the potential variance in conditions, there are few general rules about fallout. I’ve seen something called the “Seven Ten” rule bandied about. The rule claims that every sevenfold increase in time after the detonation leads to a tenfold decrease in radiation from fallout. So after seven hours there will be ten times less radiation compared to the first hour and after forty-nine hours (approximately 2 days) there will be one hundred times less radiation.
I spent a lot of time confused by this. Remember half-lives from earlier? Instead of a fixed ratio of times (i.e. seven times) producing a fractional drop in radioactivity (i.e. ten times), I thought there should be fixed period of time that produces a fractional drop in radioactivity. If there really is a tenfold drop in radiation after seven hours, then it means that the half-life of the isotopes in the fallout would average out to 2.1 hours. Which should predict there would be a one-hundred hold decrease in fallout after fourteen hours (The equation is which gives approximately 1/100). This definitely isn’t the case, which was the cause of my confusion.
I think the 7-10 rule is correct because of the mixture of isotopes we see in fallout. The most common isotopes in fallout (and their half-lives are): neptunium-240 (61.9 minutes), neptunium-239 (2.1 days), uranium-237 (6.75 days), iodine-131 (8 days), tritium (12 years), caesium-134 (20 years), strontium-90 (28.8 years), caesium-137 (30 years), technetium-99 (211,000 years), and zirconium-93 (over 1,000,000 years). The widely disparate half-lives lead to a variety of zones, each of which is dominated by a different half-life.
The 7-10 rule is an approximation that more or less fits the wonky data that results from this mixture. But it isn’t a very good approximation – it’s accurate to within 25% for the first two weeks and accurate within 50% for the first six months. After that, fallout is dominated by isotopes with similar half-lives and fits an exponential decay model, rendering the 7-10 rule basically useless.
Iodine-131 is one of the most famous fallout products and is primarily dangerous if consumed. Once in the body, it will be concentrated in the thyroid, where it can cause immediate damage or eventual cancer. This effect can be minimized with iodine tablets, which essentially “fill-up” the thyroid with safe iodine, leaving no room for iodine-131. Recommended dosage of iodine varies with the severity of the radioactive contamination and isn’t recommended for anyone over 40 except in cases where they may face acute thyroid damage. It’s also recommended to avoid drinking milk if you suspect any contamination with iodine-131.
Neptunium-239 and uranium-237 are responsible for most of the gamma radiation in fallout in the early days after a nuclear blast (neptunium-240 dominates for a few hours, but promptly fades). This is actually fairly good. After 20 days, there will be 1000-fold less radiation from neptunium-239 as there was immediately preceding the blast. Uranium-237 is a bit longer lived, requiring about 67 days for a 1000-fold decrease in radioactivity. Whether this represents a safe level is dependent on the initial amount and the relative abundance of neptunium-239/uranium-237 and other elements.
If you know or suspect that your area has been exposed to fallout, minimize your time outdoors. Your dwelling will offer excellent protection against alpha and beta particles and some protection against gamma radiation. If you must venture outside, wear many layers of clothes to protect yourself from beta particles. Don’t drink water without filtering it first. Try to avoid food produced after the fallout started, especially milk. Food produced before the nuclear exchange should be safe, as long as the outside is washed with clean water. Peeling fruit will also suffice. You can relax these precautions as the weeks and months wear on. In the years after fallout occurs, be aware of your surroundings. Hot spots (with fatal levels of radiation) may exist for several years. If you see dead trees or animals with no signs of rot, head in the other direction quickly.
This is just the barest of outlines. There are many guides out there that will go into specifics. Unfortunately, many guides have an implicit agenda (either: “scare people into becoming anti-nuclear activists”, or “keep our population from panicking about nukes”). If you know of a good, scientifically accurate guide, please link it in the comments. As additional reading, I’ve provided the official US Government planning guide, which seems to be accurate. In addition, I’ve linked to a more technical overview of fallout production and decay (Residual Nuclear Radiation and Fallout).
The last section required that you take it on faith that nuclear weapons are hard to design. Now it’s time to get into the nitty-gritty details of weapon design and understand why that is.
Nuclear explosions require a critical mass of the right unstable isotope. But there’s no safe way to store an assembled critical mass. As soon as you get to the critical mass, the chain reaction starts and an explosion will occur without drastic countermeasures.
All nuclear weapon design ultimately starts with this problem of assembling a critical mass in situ (and only ever in situ).
The first atomic bombs used one of two methods: gun assembly or implosion. These methods are still used to this day in fission weapons or in the fission first stage of multiple stage weapons.
The gun assembly method was used in Little Boy bomb dropped on Hiroshima. In this method, sub-critical hemispheres (or a ring and a cylinder) are combined by conventional explosives into a critical mass (inside a neutron reflective tamper, which further decreases the necessary fissile mass).
This method is highly inefficient and suffers from “fizzling”. Basically, the pieces tend to become supercritical before they fully touch. The massive energy released by this reaction will then blow the pieces far apart before the reaction can complete. Only 1% of the uranium in the Hiroshima bomb was used up in the explosion. The rest was scattered and eventually rained down as fallout.
The only possible fuel in gun-assembly bombs is U-235. Plutonium fissions much more quickly, leading to the sub-critical pieces being blown apart before any significant mass of the plutonium can fission.
Due to these problems, this method has mostly fallen out of use, except in applications where only one dimension can be large (such as artillery shells).
Implosion type weapons start with a sub-critical mass and use explosives to compress it until it becomes critical. As the density increases, fewer neutrons escape the mass without triggering further fission events, leading to criticality despite the lower than normally necessary mass. An outer shell directly around the core that reflects neutrons can further decrease the mass necessary for a detonation.
The fizzling problem that plagues the gun-type device is also present here – eventually, the nuclear chain reaction will overwhelm the force imparted by the conventional explosives and blow the fuel apart, stopping the fission progress. Tamper design plays a factor in this; due to its very strong and heavy uranium tamper, the Fat Man plutonium bomb held together for a few hundred additional nanoseconds, allowing it to attain 20% efficiency.
In addition to tampers, there are various strategies to mitigate the breakup, all adding cost or technical complexity. For example, some nuclear weapons add a layer of aluminum-beryllium alloy between the explosives and the tamper to slow down the explosive shock wave and cause the core to be held compressed for slightly longer. This tweak can marginally increase the efficiency of a weapon.
The core of a nuclear weapon is very small and relatively light (compared to conventional munitions). Most of the size and weight of a nuclear weapon comes from the explosives, tamper, and superstructure necessary to hold it all in the right place. The Fat Man bomb was egg shaped, 3.3 metres long and 1.5m in diameter at its thickest. The whole thing weighed 4.67t. the plutonium core was only 6.4kg, 0.14% of the total weight.
There’s no point having a weapon you can’t deliver. Methods that worked to deliver bombs across the 2500km that separated US island airbases and Japan weren’t feasible when dealing with the vast expanse of the Pacific, or the 8500km between the missile silos in the central United States and Moscow. Mass is the principle limit – the lighter its payload, the further a missile or bomber might fly.
Making weapons more powerful and miniaturizing them are two sides of the same coin. Every innovation that makes a bomb blast bigger can also potentially allow a smaller weapon to have the same effect as a larger weapon.
There are many unique challenges to miniaturizing weapons, such as correctly focusing explosions to create the desired pressure pattern. Regardless of the specific features necessary for miniaturization, it’s important to understand why miniaturization is such a big deal. North Korea, for example, has only possibly succeeded in miniaturizing its weapons to the point that its missiles can carry them any significant range. Until it thoroughly conquers this technical hurdle, only its closest neighbours have anything to fear.
4.4 Boosted Weapons
Boosted weapons were the first atomic bombs to make use of both fusion and fission, although they still get almost all of their energy from fission. Boosted weapons are all of the implosion type. Instead of a solid core, a boosted weapon will use a core with a hollow centre. In this hollow will be an equal mixture of tritium and deuterium.
Detonation proceeds as normal in the implosion assembly method, but with one extra wrinkle. The intense heat and pressure of the plutonium explosion compresses the deuterium/tritium mix to the point where these atoms begin to fuse into helium. Deuterium has two neutrons and one proton; tritium has three neutrons and one proton. Helium most commonly has two of each. Which means that every single fusion event also releases a neutron and imparts into that neutron a great amount of kinetic energy – it will be moving. These neutrons are the goal of the fusion stage.
Recall that fission chain reactions occur because neutrons from each fission event create further fission events. Adding a bunch of bonus neutrons to the mix puts the whole process on steroids. It helps that fusion neutrons move even faster than fission neutrons, so when they cause fission to happen, even more neutrons are released.
Using a boosted weapon, efficiency can be doubled. Which means that the same mass can give twice the yield, or the same yield can be attained with half the mass (because of this, boosting is one of the principle mechanisms of miniaturization).
There are a host of other benefits to boosting. Because the efficiency is increased, a smaller, lighter tamper can be used (these days they’re made of beryllium, which reflects neutrons but is not nearly as dense as uranium). Because the tamper is lighter, less explosives are required to compress the whole package. Boosting also gives a higher efficiency, so there’s no need for an additional focusing layer of aluminum/beryllium, leading to further savings in direct mass and mass of explosives necessary to drive the implosion.
All of these, combined with the reduction in plutonium mass necessary to get the same yield means that bombs have shrunk dramatically since the earliest days of nuclear weapons. The Fat Man bomb was 4.67t and 3.3m by 1.5m. It had a yield of 20kt. The first boosted weapon, the US “Swan” had a weight of just 47.6kg and measured 58cm long by 29.5cm in diameter, with a yield of 15kt, there was only a small loss of power for a 100x decrease in mass.
While it’s possible to build bombs in the >100kt range using boosted fission, this is rarely done. Fission remains fairly inefficient, so the amount of fissile material and tritium that must be used to get these yields quickly becomes wasteful. Very high yields remain the sole providence of fusion bombs, which can achieve these yields much more cheaply (recall how difficult and expensive enrichment is – to put a dollar value on it, the US pays $5,000 per gram of weapons grade plutonium-239).
There’s only one real drawback to boosted weapons: tritium. Tritium has a half-life of just 12 years. Even worse, it breaks down into helium-3, a form of helium with a single neutron that really “wants” to have two neutrons instead. Because of this, helium-3 “poisons” nuclear weapons by capturing neutrons that would otherwise cause fission. This means that boosted weapons must be periodically serviced and the helium-3 removed and replaced with fresh tritium. This is expensive (tritium cost $30,000 per gram in 2003 but is probably much more expensive now; we can’t calculate the cumulative cost because no government is willing to share the amount of tritium that is in each of their nukes, but it’s probably large) and almost certainly tricky (again, I can’t be sure, because no one releases their refueling procedures).
The Alarm Clock (named because Teller thought it would “wake up” the United States to the possibilities present by fusion weapons) or Sloika (Russian for a type of layered cake dessert) is a single stage boosted fission weapon that gets up to 20% of its yield from fusion (compare the ~1-2% that boosted weapons get from fusion).
The Sloika is made up of three layers. In the centre is a classic fission core, like in the normal implosion detonation design. Surrounding this is lithium-6 deuteride. The outer layer is unenriched uranium-238.
When this whole contraption is detonated (and detonating it takes a lot of explosives due to the three layers), three reactions occur in sequence. First, the core goes critical and fissions, ejecting a bunch of neutrons. Second, these neutrons hit the lithium-6 deuteride, which first cause lithium-6 to fission into tritium, which promptly fuses with the deuterium in the intense heat. The fusion reaction releases a lot of heat energy, a lot of radiation, and a lot of very fast neutrons. Finally, these fast fusion neutrons hit the natural uranium and cause it to fission. Since fusion neutrons are about 14 times more energetic than fission neutrons, they can cause fission even in unenriched uranium-238. The neutrons the uranium-238 releases during fission actually lack the energy to cause a self-sustaining chain reaction within itself, but they are capable of converting lithium-6 into tritium and helping the tritium fuse with deuterium. For a brief moment, this bomb has a stable multi-step chain reaction (fusion à high energy neutrons à fission à lower energy neutrons à lithium converted to tritium à fusion). Then it tears itself apart.
The Sloika derives most of its explosive power from relatively cheap natural uranium, which makes it a more cost effect way of getting yields in the 100s of kilotons compared to pure or boosted fission. It’s also safer, because most of its yield comes from a fuel incapable of spontaneous fission. Despite this it can’t truly compete with fusion weapons for cost effectiveness and lacks the ability to scale to the Mt range (as the amount of explosives necessary to detonate it grows infeasibly large at higher yields).
Because of these inadequacies, the Sloika proved a dead end in weapon design and is not used in any current nuclear weapons.
The Teller-Ulam design is necessary to economically achieve multi-megaton yields with devices small enough to be delivered intercontinentally. Teller-Ulam weapons (also known as thermonuclear bombs or H-bombs) use multiple stages of fission and fusion and incorporate concepts previously seen in boosted, implosion, and Sloika weapons. Here’s what the design looks like:
The primary is a standard boosted implosion bomb, normally with a yield in the range of 10-20kt. The secondary is made up of a “sparkplug” of highly enriched plutonium or uranium, a supply of lithium-6 deuteride (the fusion fuel), and a natural or depleted uranium tamper (made primarily of uranium-238). In many designs, the bomb is also packed tightly with a plastic foam.
Detonation begins with the primary, which works exactly like the boosted implosion design described above (while boosting isn’t strictly necessary, boosting is important for reducing the size of the weapon and so in practice will be used in almost all modern thermonuclear weapons). As the primary detonates, three separate forces begin to exert immense pressure on the secondary.
First is X-ray radiation. The primary produces x-rays (in addition to neutrons and gamma rays), which can be mirrored by the outer casing to focus on the secondary. These x-rays have momentum and therefore exert pressure when they collide with the secondary. The large amount of high energy x-rays exerts a pressure equivalent to tens of millions of atmospheres, beginning the process of compression.
Plasma from the foam also plays a role in compression. X-rays aren’t just hitting the secondary. They’re also being absorbed by the foam. No plastic can withstand such an intense barrage of x-rays – so it almost immediately converts into plasma. As this plasma tries to expand in the confines of the bomb, it exerts even more pressure on the secondary, probably an order of magnitude more than the x-ray radiation alone.
Finally, the x-rays cause the tamper to begin to vaporize. Gaseous uranium boils off the surface under the intense x-ray bombardment. For every action, there must be an equal and opposite reaction and such is the case here. The uranium boiling off the surface pushes the rest of the uranium into the fusion fuel and it pushes it hard. The ablation pressure in a thermonuclear weapon is an appreciable fraction of the pressure at the centre of the sun, equivalent to many billions of Earth atmospheres.
Which one of these ultimately causes the secondary to detonate is intensely classified. What we do know is that one (or more) of these eventually causes the fissile “sparkplug” to go supercritical, which pushes on the fusion fuel from another direction and provides neutrons to activate it. Shortly after the sparkplug ignites, fusion begins.
Fusion provides a bit less than half of the total power of the average thermonuclear bomb. The rest comes from the fission of the uranium tamper with the very fast neutrons produced in the fission process.
Total yield varies from a hundred kilotons to a dozen megatons. It’s actually possible to make one bomb design that can have multiple yields (called “dial-a-yield”), by varying the amount of tritium in the primary (and therefore the pressure under which the secondary is compressed).
The Teller-Ulam design has the same disadvantage as boosted weapons: constant maintenance is required to replace the unstable tritium necessary to boost the first phase. This disadvantage pales beside the massive power of this design. It’s the only economical (in terms of cost, weight and size) way to achieve yields in the megaton range.
While a two-stage design is by far the most common, there is no theoretical cap on the number of stages this weapon can have. Each stage provides the compressive force to ignite the next. In this manner, yields up to a gigaton of TNT can be achieved, although a bomb this large would be very difficult to deliver, even with the general efficiencies of this design. The largest bomb ever detonated, the Soviet Tsar Bomba used three stages, massed 27t, and had a total yield of 50 Mt (although it used lead tampers to minimize fallout; had the tampers been uranium the yield would have been 100Mt – and fallout from the fission would have irradiated large swathes of the USSR).