Fissile Material

Fissile Material	The major difficulty in producing an atomic weapon is not building the bomb itself, but producing the fissile material -- uranium or plutonium. ²³⁵U can produce a chain reaction; ²³⁸U can not. But ²³⁵U is quite a small fraction of the natural mineral (0.7%). Therefore one of the main problems in producing a uranium weapon is separating the two isotopes found in natural uranium. They cannot be separated by chemical means.
Isotope Separation Gas Diffusion	One method of separating these isotopes is called a gas diffusion process. It is based on the fact that the mass of ²³⁵U is smaller than that of ²³⁸U. This means that in a gas containing a mixture of the two isotopes the ²³⁵U's move a little faster. The gas is forced through a porous barrier, and the ²³⁵U's get through a little more quickly than the ²³⁸U's. One ends up with a gas which is slightly "enriched" in ²³⁵U. This gas is forced through the barrier again, making it still more enriched. Repeating this process many times, we can get uranium that is about 90% ²³⁵U, which is sufficient for weapons. For use in the most common type of nuclear reactor, uranium enriched to 3% ²³⁵U is necessary. It is not actually uranium metal that is used, but compounds of uranium that are easier to vaporize. These are uranium hexaflouride (UF₆) or uranium tetrachloride (UCl₄). The principle is the same: the molecule has a different mass depending on which uranium isotope it is made of. The gas diffusion process was used by the U.S. to make enriched uranium for the first atomic bomb dropped in World War II and later to make enriched uranium for military and civilian purposes. It is, however, extremely expensive, a single plant requiring a capital investment of billions of dollars. Today cheaper methods are available.
Electromagnetic Separation	Electromagnetic separation involves creating a beam of uranium ions -- that is, atoms from which one or two electrons have been removed. These electrically charged particles move in the region of a strong magnetic field, which makes their path curved. The ions with larger mass (²³⁸U) move in a slightly different, less curved, path than the ²³⁵U's. This is an application of the equation, "acceleration = force/mass", where the force on the two isotopes is the same, provided they have the same electrical charge, so the one with larger mass has a smaller acceleration.
Centrifuge	The centrifuge method involves placing gaseous UCl₄ or UF₆ in a rapidly rotating container. The particles with larger mass drift to the outside and are collected; the gas that remains is slightly enriched in ²³⁵U. The same principle is used in biological and chemical laboratories, to separate heavy particles that may be suspended in a liquid. As in the gas diffusion method, each use of the centrifuge enriches the uranium by a very small factor. There must be a sequence of many operations, involving thousands of centrifuges, to get up to 90% ²³⁵U.
Laser Separation	Analysts concerned with the spread of nuclear weapons have been concerned about the potential of using lasers to separate the uranium isotopes. Because of their different masses, there is a very small difference in the frequency of laser light which the two isotopes absorb. This difference can be used to electrically charge the ²³⁵U's in a uranium vapor, and not the ²³⁸U's . Laser separation would be very inexpensive, and require much less energy input compared to the centrifuge method. Hence it would be more easily available to small nations and sub-national groups, and more easily hidden. A group in Australia developed a workable method for this technique, and General Electric recently (August, 2011) requested federal approval for building a major production facility. Although the details are classified and secret, the risk of opening a new and relatively inexpensive path to nuclear weapons is serious. The issue promises to be a major controversy during the next two years.
Plutonium Reprocessiong	Plutonium is made by placing uranium(²³⁸U) in an intense flux of neutrons, produced by a nuclear reactor. A series of nuclear reactions induced by these neutrons produces plutonium. The reactor operates for several months or years, and is allowed to cool down. The fuel rods are then subjected to a variety of chemical processes to separate plutonium from other elements. This chemical procedure is called reprocessing.
The Nuclear Weapons Complex	The U.S. has not made enriched uranium for weapons for many years, because there has been a stockpile adequate for what it considered its military needs. It does operate a number of reactors specifically for military purposes. These produce plutonium for the ignition of hydrogen bombs and warheads as well as tritium (the isotope ³H) for the fusion reaction. These reactors, together with reprocessing plants and factories for fabricating nuclear weapons, make up what is called the "nuclear weapons complex".
Production Cutoff	Many people believe that the most effective way to reduce the risk of nuclear war is to formulate a verifiable treaty cutting off the production of fissile material. Such a treaty would include strict rules for accounting for all fissile material, including that removed from retired missiles. A fissile cutoff treaty would support efforts to stop nuclear proliferation, inhibit the development of a new arms race in the case of increased tensions anywhere in the world, and create a safer environment for peaceful use of nuclear power. The U.S. has in principle supported such a treaty for more than 10 years, although little progress has been made in negotiations. The Bush administration, in 2004, withdrew support for verification measures (on the grounds that they would compromise military secrets). Lacking verification, a treaty would be ineffective.

KEY CONCEPTS

Fissile material: ²³⁵U and Pu
Isotope separation: Chemical means cannot be used.
Gas diffusion
Laser separation
Electromagnetic
Centrifuge

Reprocessing
The nuclear weapons complex
A fissile cutoff treaty