Heterogeneous Seed–blanket Cores in Pressure-Tube Heavy Water Reactors for Extracting the Energy Potential From Plutonium/Thorium Fuels

Pressure-tube heavy water reactors (PT-HWR) are advantageous for implementing plutonium/thorium fuels because of their online refuelling capability and high neutron economy. The use of annular seed–blanket core concepts in a PT-HWR where higher fissile-content seed fuel bundles are physically separate from lower fissile-content blanket bundles allows more flexibility in fuel management. The bundle concept modeled was a 35-element fuel bundle made with a mixture of reactor grade PuO2 (~67 wt% fissile) and ThO2, with a central zirconia rod to reduce coolant void reactivity. Eight annular heterogeneous seed-blanket core concepts with plutonium/thorium-based fuels in a 700 MWe-class PT-HWR were analyzed, using a once-through thorium cycle. Blanket region(s) represented approximately 50% of the total fuel volume. There were 1–4 different blanket regions and 1–4 different seed regions. The seed fuel tested was 3 wt% or 4 wt% PuO2, whereas the blanket fuel tested was 1 wt% or 2 wt% PuO2, mixed with ThO2. For comparison, 2 homogeneous reactor cores with either 3 wt% PuO2 fuel or 4 wt% PuO2 fuel were also analyzed. For a number of the core concepts investigated, the fissile utilization was up to 30% higher than what is achieved in a PT-HWR using natural uranium fuel bundles. It was also found among the various core concepts that up to 67% of the Pu was consumed, up to 43% of the energy was produced from thorium, and up to 363 kg/year of fissile uranium (mainly 233U) was produced in the discharged fuel.