Isotope separation is the process of concentrating specific isotopes of a chemical element by removing other isotopes. The use of the nuclides produced is varied. The largest variety is used in research (e.g. in chemistry where atoms of "marker" nuclide are used to figure out reaction mechanisms). By tonnage, separating natural uranium into enriched uranium and depleted uranium is the largest application. This process is crucial in the manufacture of uranium fuel for nuclear power plants and is also required for the creation of uranium-based nuclear weapons (unless uranium-233 is used). Plutonium-based weapons use plutonium produced in a nuclear reactor, which must be operated in such a way as to produce plutonium already of suitable isotopic mix or grade. While chemical elements can be purified through chemical processes, isotopes of the same element have nearly identical chemical properties which makes this type of separation impractical, except for separation of deuterium.
Techniques There are three types of isotope separation techniques:
Those based directly on the atomic weight of the isotope. Those based on the small differences in chemical reaction rates produced by different atomic weights. Those based on properties not directly connected to atomic weight, such as nuclear resonances. The third type of separation is still experimental; practical separation techniques all depend in some way on the atomic mass. It is therefore generally easier to separate isotopes with a larger relative mass difference. For example, deuterium has twice the mass of ordinary (light) hydrogen and it is generally easier to purify it than to separate uranium-235 from the more common uranium-238. On the other extreme, separation of fissile plutonium-239 from the common impurity plutonium-240, while desirable in that it would allow the creation of gun-type fission weapons from plutonium, is generally agreed to be impractical.
Enrichment cascades All large-scale isotope separation schemes employ a number of similar stages which produce successively higher concentrations of the desired isotope. Each stage enriches the product of the previous step further before being sent to the next stage. Similarly, the tailings from each stage are returned to the previous stage for further processing. This creates a sequential enriching system called a cascade. There are two important factors that characterize the performance of a cascade. The first is the separation factor, which is a number greater than 1. The second is the number of required stages to get the desired purity.
Commercial materials To date, large-scale commercial isotope separation of only three elements has occurred. In each case, the rarer of the two most common isotopes of an element has been concentrated for use in nuclear technology:
Uranium isotopes have been separated to prepare enriched uranium for use as nuclear reactor fuel and in nuclear weapons. Hydrogen isotopes have been separated to prepare heavy water for use as a moderator in nuclear reactors. Tritium is both a nuisance in the coolant / moderator of water moderated reactors and a valuable product; it is thus sometimes separated from the coolant. Lithium-6 has been concentrated for use in thermonuclear weapons. Tritium is commonly produced from lithium-6 which is often enriched for this purpose. Some isotopically purified elements are used in smaller quantities for specialist applications, especially in the semiconductor industry, where purified silicon is used to improve crystal structure and thermal conductivity, and carbon with greater isotopic purity to make diamonds with greater thermal conductivity. Isotope separation is an important process for both peaceful and military nuclear technology, and therefore the capability that a nation has for isotope separation is of extreme interest to the intelligence community.
Alternatives The only alternative to isotope separation is to manufacture the required isotope in its pure form. This may be done by irradiation of a suitable target, but care is needed in target selection and other factors to ensure that only the required isotope of the element of interest is produced. Isotopes of other elements are not so great a problem as they can be removed by chemical means. This is particularly relevant in the preparation of high-grade plutonium-239 for use in weapons. It is not practical to separate Pu-239 from Pu-240 or Pu-241. Fissile Pu-239 is produced following neutron capture by uranium-238, but further neutron capture will produce Pu-240 which is less fissile and worse, is a fairly strong neutron emitter, and Pu-241 which decays to Am-241, a strong alpha emitter that poses self-heating and radiotoxicity problems. Therefore, the uranium targets used to produce military plutonium must be irradiated for only a short time, to minimise the production of these unwanted isotopes. Conversely, blending plutonium with Pu-240 renders it less suitable for nuclear weapons. If the desired goal is not an atom bomb but running a nuclear power plant, the alternative to enrichment of uranium for use in a light-water reactor is the use of a neutron moderator with a lower neutron absorption cross section than protium. Options include heavy water as used in CANDU type reactors or graphite as used in magnox or RBMK reactors. Obtaining heavy water however also requires isotope separation, in this case of hydrogen isotopes, which is easier due to the bigger variation in atomic weight. Both magnox and RBMK reactors had undesirable properties when run with natural uranium, which ultimately led to the replacement of this fuel with low enriched uranium, negating the advantage of foregoing enrichment. Pressurized heavy-water reactors such as the CANDU are still in active use and India which has limited domestic uranium resources and been under a partial nuclear embargo ever since it became an atom bomb state in particular relies on heavy water moderated reactors for its nuclear power. A big downside of heavy water reactors is the enormous upfront cost of the heavy water.
Methodology
Diffusion
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