When a power company or other customer wishes to obtain Ep kg of uranium enriched to yp weight fraction, the usual arrangement is for the customer to purchase [(yp ~xw)/(zF ~xw)]Fp kg of natural uranium with zp = 0.00711, deliver it to a uranium enrichment plant providing toll enrichment services, and pay for an amount of separative work S calculated from

The U. S. DOE sets the tails assay хц> in transactions with its customers; in 1977 this tails assay was хцг = 0.002. In the future, it is likely that customers will be given some latitude in the choice of xw so as to minimize the sum of the costs of separative work and natural uranium feed.

Substitution of these values into (12.153) and using (12.144) for ф yields

c yp

rr — (ERDA, 1977) = (2xp -1) In t-^— +258.0964 yp-6.7039 (12.154)

£p 1 —yp

This equation has been used in the U. S. “Standard Table of Enriching Services” [U2]. This text has used хц/ = 0.003 as a more probable value of the tails assay in enrichment transactions after 1977. With zp = 0.00711 and xw = 0.003, Eq. (12.153) becomes

щ; (this text) = (2xP — 1) In + 219.5666 yP — 6.4300 (12.155)

The second column of Table 12.9 gives, for different values of the product weight fraction 235U, yp, the kilograms of natural uranium feed required to produce 1 kg of product, Ep/EP,

from the material-balance relation

Ep yp—0.003

£> (this teXt> = 0.00711 — 0.003 (12.156)

The third column gives the number of separative work units required to produce 1 kg of product, S/Ер, from Eq. (12.155). The units of S are kilograms of uranium, but are conventionally referred to as SWUs (for separative work units).