Evaluation of the canister spacing and criticality safety of spent plutonium-thorium nuclear fuel

The Canadian conceptual design of the Super-Critical-Water Reactor (SCWR), known as Pressure-Tube –SCWR (PT-SCWR) is a GEN-IV design that employs the Pu/Th direct fuel cycle. The spent Pu/Th fuel is expected to be stored into Deep Geological Repository (DGR). In this study, a thermal analysis for a repository with Pu/Th spent fuel was performed using the computer code CODE_BRIGHT. The maximum temperature of the bentonite was obtained, and the minimum canister spacing was calculated for different cooling times and different numbers of channels per canister. The size of the repository needed for Pu/Th fuel was found to be three to four times larger than that for uranium for the same electrical energy output. This increases the cost of the long-term management of the spent fuel. The nuclear criticality safety aspect of storing Pu/Th spent fuel in the DGR was evaluated as well using MCNP. The effective multiplication factor, for the most reactive situations was 0.858, which is less than the acceptance criterion of 0.95.