GAS CENTRIFUGE CASCADE STUDY FOR MAXIMUM ASSAYS DURING START-UP

Research has been conducted to investigate the maximum possible enrichment that might be temporarily achieved in a facility that is producing enriched uranium for fuel for nuclear power reactors. The purpose is to provide information to evaluate if uranium enrichment facilities are producing 235U enriched within declared limits appropriate for power reactors or if the facilities are actually producing more highly enriched uranium. The correlation between feed rate and separation factor in a gas centrifuge cascade shows that as flow decreases, the separation factor increases; thereby, creating small amounts of higher enriched uranium than would be found under optimum design operating conditions. The research uses a number of cascade enrichment programs to model the phenomenon and determine the maximum enrichment possible during the time transient of a gas centrifuge cascade. During cascade start-up, the flow through the centrifuges begins at lower than design stage flow rates. Steady-state cascade models have been used to study the maximum 235U concentrations that would be predicted in the cascade. These calculations should produce an upper bound of product concentrations expected during the transient phase of start-up. Due to the fact that there are different ways in which to start a cascade, several methods are used to determine the maximum enrichment during the time transient. Model cascades were created for gas centrifuges with several product to feed assay separation factors. With this information, the models were defined and the equilibrium programs were used to determine the maximum enrichment level during the time transient. The calculations predict in a cascade with separation factor 1.88 designed to produce enriched uranium for the purpose of supplying reactor fuel, it would not be unreasonable to see some 235U in the range of 8-20%. Higher assays produced during the start-up period might lead inspectors to believe the cascade is being used in an undeclared manner. Therefore, results of this research may prove to be valuable to inspectors of declared enrichment cascades.