For this to all make sense, we should start with a brief review of atomic theory.
All matter is made up of atoms. Atoms have an outer shell of negatively charged electrons (more accurate descriptions exist, but I’m not going to delve into them; throughout this section I’m going to use simplified models wherever they’ll do the topic justice) and an inner core containing uncharged neutrons and positively charged protons.
The number of protons in an atom determines which element the atom is. All atoms with two protons are helium, all atoms with six protons are carbon, and so on. Much of the time, elements will have the same number of electrons as they have protons, so that the charges cancel each other out. Forms of elements with differing numbers of electrons are called ions. Ionization is a very common phenomenon. You observe it whenever you see lightning or dissolve salt in water.
Neutrons aren’t as simple; there’s no 1:1 correspondence or linear ratio between the number of neutrons in an atom and the number of protons. Even more confusingly, any sufficiently large sample of most elements will contain atoms that have a different number of neutrons than is most commonly observed.
More concretely, while hydrogen atoms normally have no neutrons, they sometimes have one or two neutrons. Hydrogen with one neutron is commonly called deuterium; hydrogen with two neutrons is commonly called tritium. These unique names bely the fact that regardless of the number of neutrons, the elemental classification of the atom is hydrogen; in almost all cases, the atoms behave identically, regardless of number of neutrons. A deuterium atom, hydrogen atom, and oxygen atom could come together to form what would essentially be water. In fact, you drink water in this configuration every single day!
Different forms of an element separated by the number of neutrons are called isotopes. Normally, chemists assume that isotopes will be in their naturally occurring proportions, which are heavily biased towards stable isotopes. If a specific isotope is being referred to, then it will be referenced with the name of the element and the total number of protons and neutrons; for example, uranium-235.
Isotopes are broken up into two categories: stable isotopes and unstable isotopes. Stable isotopes are at a fundamental resting state. If not acted upon by external forces, they will never change form (to a reasonable first approximation). Unstable isotopes are not at this fundamental ground state and will eventually return to it. The process of returning to the ground state radiates energy. For this reason, unstable isotopes are also sometimes called radioisotopes.
Unstable isotopes also have a characteristic half-life – the amount of time necessary for half of the element to break down into other elements through decay. Elements with a shorter half-life are more unstable, emit more radiation each second, and break down more quickly. Elements with a longer half-life are more stable, emit less radiation each second, but also persist much longer.
The energy released when an unstable isotope reverts to the ground state is commonly termed radiation – the same radiation produced by nuclear weapons, nuclear reactors, and nuclear waste. It mainly comes in four flavours: α-particles, β-particles, γ-rays, and free neutrons.
α-particles are high energy helium nuclei. Because they have a relatively high mass, they tend not to travel far and are unable to penetrate obstacles. A single sheet of paper can stop anα-particle – but their danger shouldn’t be underestimated. An unstable isotope of polonium that decayed and producedα-particles was used in the murder of Alexander Litvinenko.
β-particles are high energy electrons. Since they have less mass and interact less with matter, they can travel much further than α-particles. It takes a few millimetres of aluminum to stop a β-particle.
γ-rays are what many people think of when they think of radiation.γ-rays are photons, the same as radio waves, microwaves, and light. γ-rays have much more energy than more innocuous photons, which causes them to have much smaller wavelengths. Here’s a good rule of thumb about photons: they interact with and can be intercepted by things about the size of their wavelength. TV antennas give you a clue as to the size of radio waves, the mesh or your microwave the size of microwaves, and the tiny rods and cones in your eyes are sized just right for visible light.
γ-rays are much smaller than visible light – they’re sized just right for electrons. This means that they can travel very far, as electrons are very small and any individualγ-ray is unlikely to hit one. Once a γ-ray does hit an electron, it will transfer most of its energy to it, ionizing the atom. This can be very dangerous to humans, killing cells if enough ions are created in them and damaging DNA even when the cells survive. To block out γ-rays, you need to put a lot of electrons between you and them. Four metres of water, two metres of concrete, or 30 centimetres of lead will do the trick.
Breakdown by ejecting free neutrons is comparatively rare. It generally only occurs in significant amounts in the isotopes uranium-235 and plutonium-239.
Any ejected neutron will eventually hit another atom. When it collides, it can cause the atom to immediately fission, releasing more neutrons or be captured by the atom. Whether capture or fission occurs depends on the energy (read: speed) of the neutron. If the neutron is moving at the correct speed, fission will occur. Otherwise the neutron will be captured or bounce off. If a stable isotope captures a neutron, the result is almost always an unstable isotope. Therefore, neutron radiation is the only kind of radiation that can make other substances radioactive.
The observable characteristics of a fission reaction depend on how many neutrons are released (on average) in each collision and subsequent breakup or absorption; none will be immediately released in an absorption, but multiple can be released in a breakup, giving a wide range of possible average values. In nuclear power plants, on average only one neutron is created by each emitted neutron. This causes a slow and steady “burn” of the uranium or plutonium fuel, producing heat that can be harnessed for power. As long as each neutron produces at least one more neutron, the result is a nuclear chain reaction.
In nuclear fission weapons, each emitted neutron generates multiple new neutrons. This quickly leads to a large proportion of the fuel being consumed and turned into absolutely colossal amounts of energy. The smallest tactical nuclear weapons are equivalent to the detonation of dozens of tonnes of TNT, the largest equivalent to the detonation of millions of tonnes. This is where phrases like kilotons (kt – equivalent to 1,000 tonnes of TNT) or megatons (Mt – equivalent to 1,000,000 tonnes of TNT) come from.
The mass at which this out of control reaction takes place is called critical mass. Beyond the critical mass, the reaction is supercritical. Critical mass varies with purity (how great a percentage of the isotopes are the fissionable ones) and shape. A sphere is the shape with the lowest critical mass. This is because in a sphere, the greatest percentage of emitted neutrons are emitted back into the bulk of material – a sphere minimizes the surface area for any given volume. The critical mass of plutonium-239 is 11kg (equivalent to a 10cm sphere), while uranium-235 has a critical mass of 56kg (equivalent to a 17cm sphere).
Criticality can occur with smaller amounts of isotopes than the critical mass in certain cases. Critical mass is tied to density. If you increase the density, you decrease the critical mass by making collisions more likely. You can also lower the critical mass by employing a tamper, a layer around the core that reflects escaping neutrons back towards it, or a moderator (like water) that reduces the speed of the fastest neutrons to one more optimal for sparking further fission.
There is one other type of nuclear weapon. Fusion weapons push atomic nuclei together to form new, heavier nuclei. Fission/fusion weapons aren’t an either/or proposition though. Many nuclear weapon designs incorporate multiple stages. For example, some designs will use the energy from fission to start a fusion reaction and get most of their power from this. Other designs make use of a small amount of fusion to release extra neutrons, which allows more of their fuel to be consumed.
Additional Reading: Proton, Neutron, Radioactive Decay, Nuclear Chain Reaction, Electromagnetic Radiation, and Isotope