Fission device

From a nontechnical standpoint, nuclear fission is the mechanism that causes the intense energy release of a fission weapon. In this context, the nucleus of a radioactive element, such as plutonium, is struck by a subatomic particle, a neutron. When the unstable nucleus captures the neutron, it splits into two new nuclei, releases energy, and emits two new neutrons.

If the fission were only of one nucleus, the energy release would be infinitesimal. When the system is constructed such that the emitted neutrons hit other nuclei and cause additional fissions, the process of a chain reaction exists. The size and density of the material needed to sustain a chain reaction defines the critical mass. In a nuclear power reactor, the rate of the chain reaction is carefully controlled, with strict limits on the rate of neutron generation.

In a bomb, however, the more neutrons that can be captured in a short time, the higher the yield. Obviously, the bomb cannot be transported while in a chain reaction. The challenge of fission bomb design is to change the physical state of the fissionable material, such that the rate of generation and capture of neutrons are maximized -- and before the energy released physically disrupts the material.

Compression systems

To change that state, the material needs to be compressed, in an extremely precise manner, by pressure waves created by the explosion of conventional explosives. There are two basic ways to do this:

• gun-type compression, where, conceptually, a "bullet" of fissionable material is fired down a barrel into a "target" of fissionable material. Neither the bullet nor the target form a critical mass by themselves, but they do when combined
• implosion systems, where, conceptually, pressure is applied symmetrically around a spherical and subcritical mass. When the shock waves converge on the subcritical mass, they compress it, increasing its density until it reaches critical mass.

All modern fission weapons, or fission Primaries to trigger fusion reactions, use the implosion process. The design of the explosive system used for implosion is extremely complex; the reason that there was only one bomb test before the attacks on Japan was that it was not certain implosion would work. The weapon used on Hiroshima was gun-type.

A concern today is that non-national terrorists, if they could obtain enough fissionable material, would use the gun-type because it is simpler, although less efficient. That lack of efficiency means that much more fissionable material is needed than in an implosion system, so that implosion still might be attempted. If, however, the implosion system fails to compress symmetrically, it may only scatter radioactive material, or create a fizzle yield of minimal force.

Neutron generators

For an efficient bomb, there must be a controlled source of neutrons applied to the critical mass. Other refinements maximize the number of neutron captures and fissions before the material flies apart.

Implosion system design

To improve the performance of a fission device, the most important consideration is maximizing the amount of explosive force directed into the core by the implosion subsystem. There are only microseconds to do this, as the Primary will, soon after an uncontrolled chain reaction starts, break apart with the energy generated by fission.

Many of the methods of increasing the force into the core, and, indeed, in a fusion Secondary, perhaps counterintuitively depend on carefully placed empty spaces, or spaces filled with plastic foam that will quickly turn into gas. As the implosion explosives detonate, ignoring the special csses of such things as linear implosion, the first goal is to make the compression wave as symmetrical as possible. To achieve such a wave, the first requisite is that the multiple explosive "lenses" surrounding the core detonate simultaneously. This requires precise switching of intense current bursts, which most commonly involves switching devices called krytons. Krytons still remain on critical technology export controls, but they are now dual-use. Another application is forming the shock wave in lithotrypters, which are medical devices that use shaped shock waves to pulverize kidney stones, gallstones, and the like.

The explosive lenses themselves are usually constructed of layers of fast and slow explosive, to help form the appropriate waveform. In the Manhattan Project, the chemical explosive systems were the responsibility of George Kistiaknowsky's teams, making him, a physical chemist, as critical as any of the nuclear physicists.

Returning to the issue of voids, bomb designer Theodore Taylor alluded generally to the then-classified techniques: "When you drive a nail, do you put the head of the hammer on top of the nail and push?" While Taylor would not elaborate, he probably was referring, at the least, to levitated pits and mass drivers.

Remember that the implosion device is made of multiple concentric spheres or sphere-like structures. One of these is called the tamper, and is physically between the explosives and the fissionable material in the central pit. The levitated pit design puts an air gap between the inside of the tamper and the outside of the pit, the pit held in place only by thin wires or foam. This air gap allows the compression wave to become smoother, and, with the slight delay before it hits the pit, have more time for more explosions to contribute to the shock wave.