An evaluation of the feasibility of disposal of nuclear waste in very deep boreholes

Abstract

Deep boreholes, 3 to 5 km into igneous rock, such as granite, are evaluated for next- generation repository use in the disposal of spent nuclear fuel and other high level waste. The primary focus is on the stability and solubility of waste species, waste forms, and canister materials in saline, anoxic water, which is the most severe potential downhole environment given the sparse data available. Pourbaix (Eh-pH) diagrams and solubility products were calculated for 20 materials of interest. In general, extremely low dissolved concentrations were estimated. Copper was identified as the best canister material. Wall-to-far-field temperature increases were estimated to be about 20⁰ C for canisters containing two PWR assemblies, which is quite tolerable. Aspects requiring further work in the near term are detailed canister interior design to withstand crushing under a 1 km stack of same, and development of a borehole plug concept having a comparable or better impermeability and radionuclide holdup than the surrounding granite bedrock.