Lattice physics sensitivity studies for thorium-based fuels in pressure tube heavy water reactors

Lattice physics sensitivity studies have been performed to quantify the impact of various design and operating parameters on the performance characteristics of thorium-based fuel concepts in pressure-tube heavy water reactors (PT-HWR). Lattice calculations were performed with WIMS-AECL. Fuels modeled included 35-element bundles of mixed oxide with thorium and reactor-grade plutonium or ²³³U. Calculations were also performed with 37-element bundles with natural uranium oxide for comparison. Key performance parameters evaluated included the lattice reactivity, exit burnup, coolant void reactivity (CVR), and fissile concentration. The effects of various design/operational parameters were evaluated, including moderator purity, coolant purity, zirconium enrichment, and temporary out-of-core fuel storage at zero power. Results demonstrated that increasing the moderator purity beyond the nominal value (99.83 at% D₂O) can increase the discharge burnup by 6%, while increasing the coolant purity beyond the nominal value (99.0 at% D₂O) can reduce the CVR by up to 1 mk (100 pcm, 0.001 Δk/k). Increasing the enrichment of ⁹⁰Zr in zirconium to 100% for the all the zirconium alloys used in the clad (Zircaloy-4), pressure tube (Zr-2.5Nb) and calandria tube (Zircaloy-2) can increase the discharge burnup by nearly 40%, while reducing the CVR by as much as 1.6 mk. Temporary out-of-core storage of partially burned thorium-based fuel for a single refueling period (~70 to 90 days) to allow ²³³Pa to decay to ²³³U before core re-insertion could increase the lattice reactivity by 71 mk (7,100 pcm) and the discharge burnup by as much as 7%. (author)