Report On External Criticality of Plutonium Waste Forms In A Geologic Repository

This report presents the analyses and results for the potential occurrence of external criticality events which could result from plutonium waste forms emplaced in a geologic repository similar to the one being developed at Yucca Mountain. The analyses evaluate both the MOX spent fuel and the immobilized plutonium waste forms in a repository if the waste package has degraded and if the fissile material has migrated to the invert and out into the far-field. Previous analyses have already examined the potential for criticality in an intact waste package configuration (Reference 1), and the degraded waste package configuration (Reference 2) for these waste forms. To ensure commonality in analysis, the methodology for the development of scenarios that affect flow and transport of the plutonium wastes into the external repository environment is the same as that being used for the commercial SNF and HLW. The mechanisms for release of the fissile material form the matrix are, however, a function of the waste form characteristic. A systems approach for development of the scenario trees that could result in the accumulation of material to flow, transport, and accumulate were considered. Specific threads for the occurrence of each event were constructed, geochemical analyses conducted to determine the possibility of fissile material/neutron absorber partitioning, absorption/adsorption of fissile materials on clays estimated, natural analogs analyzed to provide insight into the possible long term behavior of these configurations, geometric configurations defined, and neutronic analyses conducted to determine the circumstance under which criticality events can occur. Further, analyses were performed to determine the consequences of any the criticality event which comes the closest to possibility.