1. Fission: splitting heavy atoms
Every nuclear weapon starts with fission. A few very heavy atoms, uranium-235 and plutonium-239, are unstable enough that when a stray neutron hits their nucleus, the nucleus splits in two. The split releases energy, and it also throws out two or three new neutrons. Those neutrons hit neighbouring nuclei, which split and release more neutrons. This is a chain reaction.
In a nuclear power station the chain reaction is slowed and controlled. In a bomb it is allowed to run away. Each generation of the reaction takes about ten nanoseconds, so in under a microsecond around eighty generations have passed and a few kilograms of metal have released as much energy as thousands of tonnes of TNT. The bomb has already blown itself apart by then, which is why only a small fraction of the fuel ever reacts. Little Boy, the Hiroshima bomb, fissioned less than a kilogram of its 64 kilograms of uranium.
Critical mass
The reaction only runs away if enough fuel is packed closely enough that neutrons hit other nuclei before escaping. That amount is the critical mass: about 50 kg for a bare sphere of uranium-235, about 10 kg for plutonium, and much less when the fuel is surrounded by a reflector that bounces neutrons back in. The whole engineering problem of a fission bomb is to keep the fuel safely below critical until the moment of use, then make it supercritical as fast as possible.
2. Two ways to build a fission bomb
Gun-type
The simplest design. One piece of uranium is fired down a barrel, like an artillery shell, into a second piece. Together they exceed critical mass and the reaction starts. It is so reliable that the Hiroshima bomb was used without ever being tested. Its weakness is that it only works with uranium, it is slow, and it wastes most of the fuel.
Implosion
A hollow or solid sphere of plutonium is surrounded by carefully shaped blocks of conventional explosive. When they detonate together, the shock wave crushes the sphere to a fraction of its size. Compressed plutonium goes supercritical, and a tiny neutron source at the centre starts the reaction at exactly the right moment. This was the Trinity test device and the Nagasaki bomb. It is far more efficient and it is the design behind every modern weapon.
3. Fusion: the hydrogen bomb
Fission bombs top out at a few hundred kilotons. To go further, designers use fusion, the process that powers the Sun. When hydrogen isotopes, deuterium and tritium, are heated to tens of millions of degrees and squeezed hard, their nuclei merge into helium and release even more energy per kilogram than fission does.
A thermonuclear weapon uses a fission bomb as the match. In the Teller-Ulam design the fission "primary" explodes first; its X-rays, travelling at the speed of light, flood a radiation case and compress a separate fusion "secondary" before the blast wave arrives. The secondary fuses and, in most weapons, also drives a final round of fission in a uranium casing. This staging is what makes yields of megatons possible, and in principle there is no upper limit.
Almost every warhead deployed today is a thermonuclear weapon of a few hundred kilotons: the US W87 is about 300 kt, Russia's Topol-M warhead about 800 kt. Bombs in the megaton range are rarer now because several smaller warheads spread across an area destroy more than one huge one.
4. What "yield" means
Yield is the energy a weapon releases, expressed as the mass of TNT that would release the same energy. A kiloton (kt) is 1,000 tonnes of TNT; a megaton (Mt) is a million tonnes. Hiroshima was about 15 kt. The largest bomb ever detonated, the Soviet Tsar Bomba of 1961, was about 50 Mt.
Destruction does not scale with yield in a straight line. Blast pressure and heat spread out in three dimensions, so the radius of destruction grows with the cube root of the yield. A bomb eight times more powerful only doubles the destroyed radius, and a thousand times more powerful only multiplies it by ten. That is why the numbers in the simulator do not jump as much as people expect when they switch from 15 kt to 800 kt.
5. What the explosion does
The energy comes out in three forms, in this order.
- Heat and light, about 35 percent. The fireball shines brighter than the Sun for a few seconds. Skin burns out to kilometres away, fires start across the whole area, and people looking at the flash are blinded.
- Blast, about 50 percent. A shock wave of overpressure followed by hurricane winds. At 5 psi of overpressure most houses collapse; at 20 psi reinforced concrete fails. Most immediate deaths come from collapsing buildings and flying debris.
- Radiation, about 15 percent, split between the prompt burst of gamma rays and neutrons in the first minute and the fallout that settles over the following hours. For weapons above about 50 kt the blast and heat reach further than lethal prompt radiation does, so radiation mostly matters for the survivors.