Effect of depleted-uranium dioxide particulate fill on spent-nuclear-fuel waste packages

The use of depleted uranium dioxide (DUO2) particulates as fill material fo r repository waste packages (WPs) containing light-water reactor (LWR) spen t nuclear fuel (SNF) was investigated A repository WP would be loaded with SNF, and small DUO2 particulates (0.5 to 1.0 mm) would be added to fill the void space inside the WP-including the coolant channels inside SNF assembl ies. The DUO2 fill slows release of radionuclides from the SNF by (a) creat ing a local chemically reducing environment that slows degradation of the S NF UO2 and (b) reducing groundwater flow through the WP. The depleted urani um (DU) fill minimizes the potential for long-term criticality in the repos itory by isotopic dilution of U-233 and U-235. The potential for criticalit y is primarily determined by U-235 (a) originally in the SNF and (6) from d ecay of Pu-239. The use of DU consumes excess DU from the production of enr iched uranium. The mechanisms for improvements in repository performance wi th DUO2 fill are defined, but additional work is required to fully quantify the benefits and costs of such an approach.