Although the isotopes of an element have very similar chemical properties, they behave as completely different substances in nuclear reactions. Consequently, the separation of isotopes of certain elements, notably 235 U from 238 U and deuterium from hydrogen, is of great importance in nuclear technology. The fact that isotopes of an element have such similar gross physical and chemical properties, however, makes their separation unusually difficult and has necessitated the development of processes and concepts especially adapted to this purpose. Despite the novelty of some of these isotope separation techniques, they have features in common with distillation and other familiar separation methods, and study of isotope separation is helpful in under­standing more conventional separation methods.

1 USES OF STABLE ISOTOPES

Table 12.1 lists separated isotopes that are being produced on a significant industrial scale. In addition to these, separated isotopes of practically all natural elements are being produced in research quantities by the U. S. Department of Energy (DOE) and by the atomic energy agencies of England, France, the Soviet Union, and other nations.

1.1 235 U

23SU is the separated isotope of by far the greatest industrial importance, with the value of annual production throughout the world of the order of a billion dollars. Uranium enriched from the natural level of 0.7 percent to from around 1.5 to 4 percent is used as fuel in power reactors moderated by natural water or graphite.

235 U enriched to 90 percent or higher, mixed with thorium, is proposed as fuel for the high-temperature gas-cooled reactor, the light-water breeder reactor, and the thorium-fueled CANDU type of heavy-water reactor, and as an alternative fuel for light-water reactors. In these reactor systems fission of 235 U is supplemented by five times or more as many fissions from 233U produced by neutron absorption in thorium, as outlined in Chap. 3. Highly enriched 233U is used as fuel for research or testing reactors, where the highest attainable neutron flux is wanted, and in compact power reactors, where high power density is needed.