Liquid Metal Fast Breeder Reactor Program. Volume III. Environmental Statement Page: 64 of 622

                                
6A.l.l.2.4 Potential Effect of Advanced Enrichment Processes on Resource
Conservation
Some Characteristics of Enrichment by Gaseous Diffusion
Natural uranium introduced as feed to a gaseous diffusion enrichment plant is
separated into a product enriched in U-235 content and a waste stream, or tails,
depleted in U-235 content. The tails assay (i.e., residual U-235 content of the
tails expressed in weight percent) can be selected more or less independently of
feed and product assays, but the actual choice of a tails assay involves a number
of practical considerations.
The tails assay of the AEC gaseous diffusion plants is currently set at 0.3% U-235.
As shown in the simple cost curve in Figure 6A.1-10, this tails assay is consonant
with essentially minimum unit cost for a 3.2% enriched product,* assuming $8 per
pound of U308 feed concentrate price and $36 per separative work unit. These assump-
tions are close to prices that have prevailed in the past, but must be expected to
change. Figure 6A.1-ll shows that the tails assay for optimum product cost would
have to be shifted downward as feed price increases, all other factors remaining
the same.7
For 3.2% enriched product, a 0.3% tails assay places about 63.9% of the incoming
U-235 into the enriched product; the balance, 36.1%, remains in the tails. More
efficient utilization of U-235 could be achieved by setting the tails assay at a
lower value. At 0.1%, for example, the resultant recovery would be 88.7% of the
U-235, corresponding to a 28.1% reduction in the quantity of feed required per
unit of enriched product. The consequence, however, would be the spending of a
larger fraction of separative work capability on stripping the tails to the lower
assay, with a resultant increase in cost of more than $50 per kilogram of enriched
product and a substantial decrease in annual product output (Table 6A.1-3).
Thus, the use of low tails assays in the currently available separations plants, for
more complete U-235 recovery, would be unfavorable to product costs and would hasten
the time when additional separations capacity would be needed to satisfy the
presently growing product requirements. As feed prices increase, there will be an
economic incentive to set tails assays at lower values, but this would also be at
*Also true for a product of any other enrichment derived from $8 concentrate and
$36 per separative work unit, although product cost increases with increased
product assay.

6A.1-19