Thermal criticality in a repository environment

This report explores a scenario in which burial containers fail and fissile material is transported through the tuff by water to some location, away from the burial site, where an over-moderated critical mass gradually accumulates. Because of the low solubilities of plutonium and uranium, and the low ground water velocities, the analysis shows that such a scenario with {sup 239}Pu is probably impossible because the time required to accumulate a critical mass is large compared with the half-life of the {sup 239}Pu. In the case of {sup 235}U, the analysis indicates that the accumulation rates are so low that relatively small fission power levels would consume the {sup 235}U as fast as it accumulates, and that the thermal conductivity of the tuff is large enough to prevent a significant increase in temperature. Thus, the conditions for the removal of water by boiling and the associated autocatalytic increase in reactivity are not met in the case of {sup 235}U. An explosive release of energy does not appear to be possible. A simple water voiding model, which allows water removal at about the fastest possible rate, was used to explore a scenario in which the fuel accumulation rate was arbitrarily increased enough to cause water boiling and the associated dryout of the tuff. Calculations for this case indicate that disruption of the tuff, leading to a neutronic shutdown, would probably occur before an explosive energy release could be generated. Additional scenarios, which should be investigated in future work are identified