1 INTRODUCTION

Because of the importance of M5U at compositions above natural abundance, originally for military purposes and more recently for nuclear electricity generation, great effort has gone into investigation and development of many processes for enriching 23SU. This chapter deals only with those processes that have been used on an industrial scale, those that seem likely to become of future industrial importance, and those that illustrate the shortcomings of the processes used industrially for separating the isotopes of light elements when applied to heavy elements such as uranium.

Discussion of processes for industrial separation of uranium isotopes cannot be as complete as the discussion of deuterium separation in Chap. 13. The detailed technology of the most economical and most promising processes is subject to security classification and to proprietary restrictions. Nevertheless, processes for enriching uranium can be described in sufficient detail to make their principles clear and to illustrate the similarities and differences between them and processes for separating isotopes of light elements.

For a more detailed discussion of uranium isotope separation than is possible in this chapter, reference may be made to papers on this subject presented at the four International Conferences on the Peaceful Uses of Atomic Energy sponsored by the United Nations at Geneva, to the Proceedings of the International Conference on Uranium Isotope Separation sponsored by the British Nuclear Energy Society in London in April 1975 [B20], to the Chemical Engineering Progress Symposium Series volume on uranium enrichment [B14], the articles on diffusion separation methods [Hll, S3] in the Encyclopedia of Chemical Technology, informative reports ORO-684, 685, 690, and 694 on uranium enrichment activities of the U. S. Atomic Energy Commission (AEC), and the authoritative monograph on uranium enrichment edited by Villani [Via].

The processes used most extensively for separating isotopes of light elements, distillation and chemical exchange, become progressively less selective as the atomic weight increases and are ineffective for uranium.

The processes used most extensively for separating uranium isotopes, gaseous diffusion and the gas centrifuge, are much less efficient than distillation for light elements, but are impaired less by an increase in molecular weight, so that they are the preferred methods for uranium. Table 14.1 compares the separation factors for these four processes when applied to mixtures

Table 14.1 Representative separation factors for isotope separation processes

Separation factor for isotopic mixture: Process Property* H2-HD 14NO-1sNO “’UFg-^F*

+a*, relative volatility. K, exchange equilibrium constant: * for HD-H2 О exchange; § for 14NO — H1SN03 exchange;* for 235UF6-23&UF5 NOF exchange. m2,m!, molecular weight of heavy, light component. va, peripheral speed, 500 m/s. R, 8314 J/(kg-mol’K). T, 300 K.

of H2 and HD, 14NO and 15NO, and J35UF6 and 238UF6. Although many features besides separation factor enter into choice of the preferred process, it is clear that the higher values for gaseous diffusion and the gas centrifuge give these processes a substantial advantage over distillation and chemical exchange for uranium isotope separation.

Section 2 of this chapter deals with the isotopic content of uranium. Section 3 lists the principal processes for separating uranium isotopes on an industrial scale and describes briefly projects using these processes. Section 4 gives a detailed description of the gaseous diffusion process, which until now has produced almost all of the world’s enriched uranium. Section 5 is a parallel treatment of the gas centrifuge process, which is emerging as an effective competitor of gaseous diffusion. Section 6 describes aerodynamic processes that separate uranium isotopes through composition differences developed when mixtures of 23SUF6 and 238UFe are subjected to high linear or centrifugal accelerations in flowing gas streams. Such processes are in an advanced stage of development and are to be used industrially in Brazil and possibly South Africa. The remainder of this chapter discusses in less detail other processes not yet ready for industrial use. Mass diffusion (Sec. 7) and thermal diffusion (Sec. 8) are clearly not economical for uranium isotope separation but are described briefly because they illustrate isotope separation principles in the comparison with gaseous diffusion, and have been used to advantage for other elements. Laser-based processes (Sec. 9) appear very promising and may, with sufficient development, become the most economical means of separating uranium isotopes.