A common misunderstanding about thermonuclear weapons (those that employ tritium-deuterium fusion as well as the fission of uranium or plutonium) is that most of the extra energy produced comes from fusion. In fact, the great majority comes from additional fission encouraged by neutrons produced by the fusion reaction. Each atom that undergoes fission generates 180 million electron volts (MeV) of energy, equivalent to 74 terajoules per kilogram. Tritium-deuterium fusion produces only 17.6 MeV per incident, though the materials that undergo fusion are far less massive than those that undergo fission.
- A neutron generator bombards the plutonium pit of the primary (fission device).
- Exploding-bridgewire or slapper detonators initiate the high explosive shell around the pit.
- The pit is compressed to a supercritical density.
- The pit undergoes nuclear fission, aided by the neutron reflecting properties of a shell made of beryllium, or a material with similar neutron-reflection properties.
- The fission process in the primary is ‘boosted’ by the fusion of tritium-deuterium gas contained in a hollow chamber within the plutonium.
- The x-rays produced by the primary are directed toward the secondary through an interphase material.
- Within the secondary, heat and compression from the primary induce the production of tritium from lithium deuteride.
- Tritium and deuterium fuse, producing energy and high-energy neutrons.
- Those neutrons help induce fusion within a uranium-235 pit within the secondary (called the spark plug). Layers of uranium-235 may alternate with layers of lithium deuteride, and the whole secondary may be encased in a sphere of uranium-235 or 238. This tamper holds the secondary together during fission and fusion. Uranium-235 or 238 will also undergo fission in the presence of neutrons from fusion.
Throughout this process, the whole device is held together by a uranium-238 (depleted uranium) case. This is to ensure that the reactions proceed as far as possible before the whole physics package is blasted apart.
One important security feature can be built into the detonators that set off the explosive shell around the primary. By giving each detonator a fuse with a precisely set random delay, it is possible to ensure that only those who know the timing of each detonator can cause the bomb to explode as designed. If the detonators do not fire in a very precisely coordinated way, the result is likely to be the liquefaction of the plutonium core, followed by it being forced out of the casing as a fountain of liquid metal. Nasty as that would be, it is better than the unauthorized detonation of the weapon.
The detonators are also an important safety feature since their ability to cause very stable explosives to detonate means that the high explosive shell can be made of something that doesn’t detonate easily when exposed to shock or heat. That is an especially valuable feature in a world where bombs are sometimes held inside crashing planes, and where fires on submarines can prove impossible to control.