Effect of Depleted-Uranium Dioxide Particulate Fill on Spent-Nuclear-Fuel Waste Packages

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.