Consideration of Criticality when Directly Disposing Highly Enriched Spent Nuclear Fuel in Unsaturated Tuff: Bounding

Although idealized calculations of the potential for an atomic explosion within a repository can make headlines, a more technically useful assessment is a systematic, multidisciplinary, integrated analysis that uses a set of consistent assumptions of disposal system performance. The analysis described here, called a performance assessment, employs the same general approach to study the potential of a critical mass assembly as has been used to examine other potentially disruptive sce- narios in a nuclear waste disposal system. This report presents one of two approaches—bounding calculations-which were used in a major study in 1994 to examine the possibility of a criticality in a repository. The bounding probabilities in this study are rough and do not entirely dismiss the pos- sibility of a critical condition; however, they do point to the difficulty of creating conditions under which a critical mass could be assembled (i.e., corrosion of containers, separation of neutron absorbers from the fissile material, and collapse or precipitation of the fissile material) and, more important, how significant the geochemical and hydrologic phenomena are in examining this criti- cality issue. Furthermore, the study could not conceive of a mechanism that was consistent with conditions under which an atomic explosion could occur, i.e., first, the manner in which. fissile material could be collected and, then, how it would be assembled (or diffused outward) within microseconds. In addition, should a criticality occur in or near a container in the future, the bound- ing consequence calculations in this study showed that fissions from one critical event (<-l&O fis- sions, if similar to aqueous and metal accidents and experiments) are quite small compared to the amount of fissions represented by the spent nuclear fuel itself. Also, if it is assumed that the con- tainers necessary to hold the highly enriched spent nuclear fuel in this study went critical once per day for 1 million years, creating an energy release of about l02Ofissions, the number of fissions equals about l028, which corresponds to only 1% of the fission inventory in a repository containing 70,000 metric tons of heavy metal (MTHM) (the expected size for the proposed repository at Yucca Mountain, Nevada).