Lattice physics evaluation of 35-element mixed oxide thorium-based fuels for use in pressure tube heavy water reactors

A series of 2-D lattice physics calculations with depletion were carried with WIMS-AECL Version 3.1 out as part of exploratory scoping studies to evaluate various thorium-based fuel bundle concepts for potential application in pressure tube heavy water reactors (PT-HWRs). Fuel bundles concepts investigated consisted of a cluster of 35 fuel elements arranged in two rings (14 + 21), and surrounding a central graphite displacer rod. The fuel is comprised of thorium dioxide mixed with a fissile driver of reactor-grade plutonium (∼67 wt% Pufissile/Pu; 3.5-4.5 wt% PuO2/(Pu,Th)O2), low enriched uranium (5 wt% 235U/U; 40-50 wt% LEUO2/(LEU,Th)O2) or uranium-233 (1.8 wt% 233UO2/(233U,Th)O2). Estimates of burnup-averaged fuel temperature coefficients (FTC) and coolant void reactivity (CVR) were found to be lower than those for conventional natural uranium dioxide (NUO2) PT-HWR fuel in a 37-element bundle. A low-burnup option for using (LEU,Th)O2 fuel in a PT-HWR is found to be attractive as a means for extracting energy from thorium, while also generating stockpiles of 233U, and demonstrating enhanced safety characteristics with reduced CVR and FTC relative to NUO2.