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.