Repository Criticality Control with Depleted-Uranium-Dioxide Cermet Waste Packages

It is proposed that the structural components and internal basket structures of waste packages (WPs) be constructed of depleted uranium dioxide (DUO2)-steel cermets. The cermet contains DUO2 imbedded in a steel matrix. The WPs are filled with spent nuclear fuel (SNF) and placed 2 in a geological repository. The WP provides a handling container for placement of SNF in the repository and is an engineered barrier to delay SNF degradation and subsequent release of radionuclides. SNF and other fissile wastes contain enriched uranium and transuranic fissile isotopes; thus, the potential for nuclear criticality exists. Most of the transuranic fissile isotopes, such as 239Pu, will have decayed to 233U or 235U before significant fissile-isotope migration from the degraded SNF or other fissile waste forms has occurred. Consequently, post-closure repository criticality issues are primarily from the fissile isotopes of uranium. As the WP degrades, the 238U in the DUO2-steel cermet would mix with the degrading SNF and isotopically dilute 233U and 235U to levels that would ensure that post-closure criticality would not occur.