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 Kistiakowsky's teams, making him, a physical chemist, as critical as any of the nuclear physicists.

Increasing fission efficiency: mechanical compression

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.

Solid core issues

The original designs were solid cores of plutonium, in direct contact with the innermost sphere of a nickel- and gold-plated polonium ball, the "urchin" neutron generator. Solid cores have several disadvantages:

1. If the chemical explosive is in contact with the core, the maximum pressure is limited to the internal detonation wave, not more than 400 kilobars.
2. Especially with plutonium, the metallurgical characteristics of the metal limit the increase of density possible through pure mechanical compression.

The most basic steps to increasing pressure is to reflect the shock wave back at the core, and by convergence of multiple explosive waves. In a solid core design, implosion starts at the urchin initiator in the center, so the outer layers of the pit will provide some reflection. Reflection occurs whenever there is a change in density. This can be increased, by some extent by inserting one or more layers, of increasing density, between the explosive, the very dense tamper, and the pit. The multilayer technique can be used only 2 or 3 times.

Shock convergence is one of the reasons that the explosive system resembles a soccer ball, made up of many curved segments, each with a detonator in the center, the detonators triggered near-simultaneously. The additional compression is limited by the ratio between the radius of the fissile coure and the outer radius of the implosion system. Unfortunately for the designer, getting a large ratio means the entire system grows in size and weight.

Yet another problem in solid pit systems is the Taylor wave. This wave is a sharp pressure drop immediately behind the shock wave propagating from the chemical explosion. If this wave is not suppressed or reduced, by the time the compression reaches the innermost part of the pit, the outer regions of the pit may have returned to normal pressure. To deal with the Taylor wave problem, more concentric layers come into play: intermediate density (e.g., aluminum) pushers' between the explosive and the tamper. Remember, the explosive detonation propagates in all directions, toward the pit and away from the pit. Pusher layers reflect the away-from-pit wave back at the pit, and tend to cancel the Taylor wave.

Levitated pit issues

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.

There are additional roles for the tamper. When the pit begins to disassemble as the fission chain reaction begins, the tamper does not immediately vaporize. The denser and tougher the tamper, the more containment (still measured in microseconds) it will give to the pit, increasing the neutron flux density and thus the efficiency of reaction.

Other mechanical approaches

Even more exotic compression schemes, such as flying plate and linear implosion, exist, but are beyond the technical level of this discussion.

Neutron reflection

Neutrons are reflected by light atomic weight materials, including the explosives themselves. For greater reflection of neutrons back into the pit during the early part of fission, the roughly spherical explosives are surrounded by a neutron reflector, usually of beryllium. Neutrons generated by the pit, again in a process of microseconds, increases core neutron density.

Improving fission efficiency: external neutron sources

Having the initiator "urchin" at the very center adds to the problems of a solid core. Two alternatives exist, both using deuterium-tritium reactions to generate neutrons, over a very short, but not instantaneous, period:

1. Linear accelerators, which look vaguely like hair dryers attached to the outside of the sphere
2. Internal deuterium-tritium mixes in cavities small enough not to interfere with explosive compression

Improving fission efficiency: boosted fission

The latter technique can be turned into a method of boosting the efficiency of the entired device, by adding a metered quantity of tritium into a Primary. Under the conditions of implosion, tritium, hit by neutrons, will generate neutrons more efficiently than fissionable materials, although the conditions for a fusion chain reaction do not exist in a boosted fission device. These neutrons have much higher energy than fission neutrons.

Again, there is a concern with having too much internal gas and turning the pit into a hollow sphere with undesirable explosive wave reflectinon characteristics. To avoid this, the boosting tritium has to be somewhere inside the outermost shell, but still inside the compression system. Sublette suggests that, in U.S. weapons, the gas is "between the outer shell and the levitated pit. Here the collapsing thin shell would create multiple reflected shocks that would efficiently compress the gas to a thin very high density layer. There is evidence that US boosted primaries actually contain the boosting gas within the external shell rather than an inner levitated shell. The W-47 primary used a neutron absorbing safing wire that was withdrawn from the core during weapon arming, but still kept its end flush with the shell to form a gas-tight seal."

In fission and fusion weapons with variable yield, the amount of yield is probably controlled by the amount of tritium injected. While this would appear to be a simple process, it apparently is not; there have been various reports on information that suggest that at least one of the Indian tests had a partial fizzle yield in what appears to have been a boosted fission Primary.