Page 48 - Radiochemistry and nuclear chemistry

Nuclei, Isotopes and Isotope Separation 37

where r is the centrifuge radius and v r the speed of rotation. The separation factor a ~- 1
+~.
Estimated values for present centrifuges operating on UF 6 are: length 1 - 2 m; radius 0.2
- 0.4 m; rotational speeds of 50 - 80 x 103 rpm. The enrichment obtainable in one stage
is limited by the material strength of the centrifuge bowl; the present materials limit v r r to
< 800 m s -1. Typical separation factors are 1.4 - 3.9 per stage, thus about 10 stages are
required to enrich 235U from 0.72% to 3 % with a 0.2% tail. As compared, a diffusion
plant would require ~ 1300 stages. The corresponding figures for production of 80 % 235U
are ~ 45 and ~ 3600, respectively. Though rather few stages are required to upgrade
natural uranium to reactor quality, a very large number of centrifuges are needed to
produce large quantities of enriched material.
Current centrifuge technology requires ~ 3 % of the power consumed by a diffusion plant,
or 50 - 100 kWh SWU -1. This makes their environmental impact minimal, as compared
to gas diffusion plants, which require substantial electric power installation and cooling
towers with large water vapor effluent. Smaller plants of a few MSWU/y are economical,
and their output can be readily multiplied by installation of parallel processing lines. The
very large number of centrifuges required due to their small size (each having a capacity
of g 15 SWU/y) and limited life-time does not lead to excessive construction costs due to
continuous mass production on-site. On the whole, centrifuge separation now seems to have
a lower enrichment cost than large scale diffusion plants. Centrifuge performance has been
increased by a factor of ~ 25 since 1980 and is predicted to improve further.
Many plants are now operating, the largest known being near Ekaterinburg (Russia, 9
MSWU/y; Russian centrifuge plants have total capacity of ~ 19 MSWU/y), Almelo (the
Netherlands, 1.7 MSWU/y), Capenhurst (United Kingdom, 1.1 MSWU/y) and Gronau
(Germany, 1.1 MSWU/y).

2.8.7. Other methods of isotope separation

In theory all physicochemical procedures are capable of isotope separation. Some other
methods which have been studied include distillation, solvent extraction, ion exchange,
photoionization and photoexitation.
Tons of D20 are purified annually in India by cryogenic distillation of hydrogen. Tenths
of kilograms of pure 13C and 15N have been produced at the Los Alamos Scientific
Laboratory through distillation of NO and CO at liquid air temperature. At the same time
a fractionation between 160 and 180 occurs.
A continuous ion-exchange isotope separation process for uranium enrichment has been
developed in Japan. Few details of this process have been disclosed. However, it is known
that "reflux" is obtained by oxidation and reduction of U 4+ and UO22 +. A demonstration
plant with a capacity of 2 kSWU/y is in operation at Hyuga.
A method of separation, involving passage of a mixture of UF 6 and helium or hydrogen
at very high velocities through a nozzle, as seen in Fig. 2.11, has been developed by E. W.
Becker in Germany and in South Africa. The technique is sometimes referred to as "static"
or "stationary-walled ~ gas centrifugation. The separation factor is typically 1.01 - 1.03 per
stage, i.e. about three times better than in the gaseous diffusion process, and offers great
possibilities for further improvements. Thus while the diffusion process requires about 1200