Mechanisms and Possibility of Autocatalytic Criticality by High-Level Wastes Buried in Water-Saturated Geologic Formation

Abstract

  An analysis of underground autocatalytic criticality has been performed for vitrified high-level radioactive wastes arising from reprocessing of spent nuclear fuel that are placed in a water-saturated fractured geologic formation. We estimate by mass transport analyses the mass and ²³⁵U-enrichment of accumulated uranium in the host rock originated from multiple failed waste canisters, for which static neutronic analyses are performed. Uranium accumulation with 12% enrichment (denoted as U(12)) can be created. With 30% porosity of the host rock, the minimum critical mass of U(12) required for over-moderated criticality is 280 kg. Heterogeneous U(12) depositions can exhibit positive reactivity feedback due to medium temperature increase.