Modeling and criticality evaluation of the voloxidation process

Highlights: → It is new and informative to perform criticality evaluation of pyroprocessing. → A model and an evaluation procedure were built. → No criticality risk without H2O intrusion was observed. → With H2O intrusion, subcritical mass is 40.1 kg for UO2 and 19,155 kg for spent fuel. → Contributions of key nuclides were evaluated quantitatively. - Abstract: Voloxidation is a necessary process in the dry reprocessing of spent nuclear fuels. The criticality evaluation plays a considerable role in the design of voloxidation apparatus. As conservative results are always preferred in a criticality evaluation, an optimized model was built in consideration of both the geometry of voloxidation apparatus and the occurring forms of evaluation material. The criticality evaluation of fresh UO2 fuel and PWR spent fuel were then performed by employing Monte Carlo techniques, respectively. It is demonstrated that there is no criticality risk concerning the voloxidation process dealing with fresh UO2 or PWR spent fuel if water does not intrude into the cell. However, if water intrudes and mixes with the fuels, the subcritical mass limit is 40.1 ± 0.1 kg for fresh UO2 and 19,155 ± 50 kg for spent fuel. The contributions of 1H and 235U were analyzed quantitatively by the TSUNAMI code to clarify the competition between 1H moderation effect and its dilution effect on the concentration of 235U.