The use of depleted uranium dioxide (DUO2) particulates as fill material for repository waste packages (WPs) containing light-water reactor (LWR) spent 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 assemblies. The DUO2 fill slows release of radionuclides from the SNF by (a) creating a local chemically reducing environment that slows degradation of the SNF UO2 and (b) reducing groundwater flow through the WP. The depleted uranium (DU) fill minimizes the potential for long-term criticality in the repository by isotopic dilution of 233U and 235U. The potential for criticality is primarily determined by 235U (a) originally in the SNF and (b) from decay of 239Pu. The use of DU consumes excess DU from the production of enriched uranium. The mechanisms for improvements in repository performance with DUO2 fill are defined, but additional work is required to fully quantify the benefits and costs of such an approach.