Page 46 - Radiochemistry and nuclear chemistry

Nuclei, Isotopes and Isotope Separation 35

This reaction is quite undesirable as HF is highly corrosive and solid UO2F 2 can plug the
pores of the membranes. The tubing and cells of a plant are made principally of nickel and
teflon to overcome the corrosion problems.
(d) In order to transport the large gas volumes and to keep the proper pressure drop
across the membranes, a gaseous diffusion plant requires a large number of pumps. A large
cooling capacity is needed to overcome the temperature rise caused by compression of the
gas.
The work required to enrich uranium in 235U increases rapidly with the 235U content of
the product. Because of varying domestic prices on natural uranium, as well as varying
content of 235U in uranium obtained from used reactor fuel elements, so-called toll
enrichment has been introduced. In this case, the purchaser himself provides the uranium
feed into the separation plant and pays for the separative work required to make his desired
product out of the uranium feexl provided. Separative work is defined as

Separative work = W V(xw) + P V(xp) -F V(XF) (2.56)

where the separation potential V(xi) is defined by

V(xi) = (2x i - 1) ln{xi/(1 - xi) } (2.57)

As seen from (2.56), separative work has the dimension of mass, and can be thought of as
the mass flow rate multiplied by the time required to yield a given quantity of product. The
cost of isotope separation is obtained by assigning a value to one separative work mass unit
(kgSW or SWU). A 1 GWe nuclear light water reactor station requires about 180 x 103
SWU in initial fueling and then 70 - 90 x 103 SWU for an annual reload.
In w the number of stages and the interstage flow relative to the product flow was
given for enrichment of 235U from its natural isotopic abundance of 0.71% to a value of
80%. With a waste flow in which the isotopic abundance of 235U is 0.2%, (2.48) shows
that for each mole of product obtained 156 moles of feed are necessary. In more recent
designs the concentration of 235U in the waste is increased to ~ 0.3 % to minimize cost.
Isotope separation through gaseous diffusion is a very energy-consuming process due to the
compression work and cooling required. An annual production of 10 MSWU requires an
installed capacity of - 2900 MW in present plants, or ~ 2500 kWh SWU-1. Improved
technology may reduce this somewhat. Gaseous diffusion plants are known to exist in
Argentina, China, France, Russia and the United States. The combined capacity of these
plants was about 40 MSWU/y at the end of 2000.

2.8.5. Electromagnetic isotope separation

During the Manhattan Project of the United States, electromagnetic separation was used
to obtain pure 235U. This process is identical in theory to that described for the mass
spectrometer. The giant electromagnetic separators were called calutrons (California
University Cyclotrons) and were after World-War II used at Oak Ridge National Laboratory
to produce gram amounts of stable isotopes of most elements up to a purity of 99.9 % or
more. Large capacity electromagnetic separators have also been developed and operated in