A STUDY OF MINOR ISOTOPES IN ENRICHED REPROCESSED URANIUM*

The question has been posed as to whether the concentrations of minor isotopes in a withdrawal stream of a uranium enrichment plant might be a signature of the type of enrichment process used or whether transient or off-design operations of the enrichment process might be inferred from this information. To address this question, research has been conducted to analyze the enrichment of minor isotopes in reprocessed uranium in a gas centrifuge enrichment plant (GCEP). The enrichment of the minor isotopes for normal design conditions has been calculated. The GCEP is studied for start-up conditions of the cascade and for off-design operation of the cascade. The focus of the study is enrichment cascades producing low enriched uranium (LEU) for nuclear power reactor fuel. For the gas centrifuge enrichment plant, the separation factor, a, for U235/U238 varies with feed rate. The separation factors for each of the minor isotopes are functions of a, and hence, they are functions of feed rate. A model of separation factor versus feed rate is used for a hypothetical centrifuge to illustrate the problem. Cascade calculations are performed for a feed stream with isotopic concentrations typical for reprocessed uranium, and the concentrations of minor isotopes in the withdrawal streams are analyzed for a range of product rates. Multi- component calculations are performed using a fixed plant analysis with a modified version of the code PCFX4.