Thermo-Hydro-Mechanical Evaluation of Critical Mass in Repository Far-Field

摘要

A repository for used nuclear fuel (UNF) must be engineered to ensure that post-closure criticality events do not compromise the natural barrier. In a scenario where canisters are extensively compromised, the impact of a critical mass formation in the far-field must be assessed given the transport and re-concentration of fissile nuclides. In particular, if heat transfer to the surrounding bedrock is inadequate, heating from fission may eventually degrade the surrounding rock via thermal creep. To evaluate the potential for system failure with the long-term release of energy from sustained chain reactions, this study evaluates a critical deposition in bedrock by coupling a neutronics analysis and thermo-hydro-mechanical (THM) simulation of heat and mass transport. Results are intended to provide insight on the criticality concern for direct disposal. Canisters for UNF can be engineered to preclude in-situ criticality events. However, if the waste package materials undergo catastrophic failure, radionuclides from the fuel may undergo dissolution, hydrological transport, and subsequent re-concentration in a localized reducing environment. The scenario of interest in this study is based on a precipitate forming in the far-field of a granitic repository resulting from many compromised canisters.