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 233 U from 238 U and deuterium from hydrogen, is of great importance in nuclear technology. Table 1.5 lists isotopes important in nuclear power applications, together with their natural abundance and processes that have been used or proposed for their separation. In addition to applications mentioned earlier in this chapter, Table 1.5 includes the use of 2D and 6 Li as fuel for fusion power, a topic treated briefly in Sec. 9, following.

The fact that isotopes of an element have very similar chemical and gross physical properties makes their separation particularly difficult and has necessitated the development of concepts and processes especially adapted for this purpose. In almost all isotope separation processes the degree of separation obtainable in a single stage is very small, so that many identical stages must be used for practical, useful separation. An example of this is the use of more than 4000 stages in the Oak Ridge gaseous diffusion plant. Chapter 12 describes principles that have been developed for dealing with separation processes that consist of a large number of similar stages, and hence are applicable to all methods of isotope separation.

Table 1.5 indicates that for isotopes of the light elements hydrogen, lithium, and boron, separation methods used or proposed include distillation, electrolysis, and chemical exchange. These methods for separating isotopes of light elements are described at length in Chap. 13, with principal application to deuterium. Mention is also made of methods for concentrating 13C, ISN, 170, and 180. These are isotopes of elements important in living systems that are used extensively as stable tracers in biological and medical research.

None of the conventional separation processes, such as distillation, ion exchange, or solvent

extraction, has been used for large-scale separation for isotopes of uranium or other heavy elements. To separate isotopes of uranium or other heavy elements that exist in gaseous form at convenient temperatures, it has been necessary to use gaseous diffusion, gas centrifugation, or one of the other novel processes described in Chap. 14. Gases to which these processes are applicable include xenon, MoF6, WF6, and UF6.

Another process that can be used to separate isotopes of all elements on a small scale, but that is too costly for large-scale production, is the electromagnetic method, which is based on the principle of the mass spectrometer. The electromagnetic method separated the microgram amounts of 235 U used to show [Nl] that this was the fissile isotope of uranium and was later employed by the Manhattan District to produce the first kilogram quantities of 235 U. The cost was so high, however, that the electromagnetic method was replaced by gaseous diffusion. The electromagnetic method is now used [Kl] to produce research quantities of separated isotopes of nearly all naturally occurring mixed elements. As the electromagnetic method is a physical rather than a chemical engineering process, it is not described further in this text.