Unfavorable conditions for nuclear criticality following disposal of transuranic waste at the Waste Isolation Pilot Plant

Modeling of nuclear criticality was omitted from performance assessment cal culations for the Waste Isolation Pilot Plant (WIPP), a repository for wast e contaminated with transuranic radioisotopes, located in southeastern New Mexico, based on arguments of low probability and low consequence. Low-prob ability arguments are presented here. Guidance provided by the Environmenta l Protection Agency (EPA)-the regulator of WIPP-allowed either qualitative "credibility" arguments or quantitative probability estimates when screenin g features, events, and processes such as criticality. Although information to quantitatively evaluate the probability of a criticality event was most ly lacking, qualitatively reasoned discussion of the inability to assemble a critical configuration of fissile material was accepted by the EPA. Speci fically, after disposal and prior to an inadvertent human intrusion into th e repository, there is no credible mechanism to move radioisotopes (and par ticularly, fissile material) since only small amounts of brine enter the re pository, as adequately demonstrated in calculations over the years. An ina dvertent human intrusion (an event that must be considered because of safet y regulations) might allow a large pressure gradient to move more brine thr ough the repository, but there is still no credible mechanism to counteract the natural tendency of the material to disperse during transport. Unfavor able physical conditions on concentrating fissile material include low init ial solid concentration of fissile material, small mass of fissile material transported over 10 000 yr, and insufficient physical compaction; unfavora ble hydrologic conditions include the limited amount of brine available to transport fissile material. Unfavorable geochemical conditions on concentra ting the fissile radioisotopes include lack of sufficient adsorption and wa ter chemistry conducive to precipitation.