Evaluation of sup 236 U as a fertile isotope in Fast Flux Test Facility fuel

Future plans for fueling the Fast Flux Test Facility (FFTF) core are based on the application of enriched uranium fuel instead of the mixed plutonium-uranium oxide currently in use. Recycled enriched uranium, which contains {sup 236}U, {sup 235}U, and {sup 238}U, is a potential uranium source for this fuel. In addition, a uranium metal alloy and uranium oxide are both candidate fuel forms. To evaluate the relative merits of {sup 236}U and {sup 238}U as fertile isotopes, four cases (two oxide and two metal, with {sup 238}U and {sup 236}U as the fertile isotope for each of the two fuel types) were evaluated with respect to core safety and performance parameters. Because this is the first such attempt in the liquid-metal reactor program to evaluate the characteristics of the {sup 236}U isotope in either oxide or metal fuels, a formal core assessment is necessary. This paper describes the model and presents the results of the evaluation. The results of this study indicate that replacement of part of the {sup 238}U with {sup 236}U in an enriched uranium fuel supply for the FFTF would have minimal effect on the operating characteristics of the reactor.