Simulations of Pressure-Tube-Heavy-Water Reactor Cores Fueled with Thorium-Based Mixed-Oxide Fuels

Thorium, a fertile nuclear fuel which is nearly three times as abundant as uranium, represents a long-term energy resource that could complement uranium by making the current nuclear fuel cycle more sustainable. With the expected refurbishment and new construction of pressure-tube-heavy-water reactors (PT-HWRs) within the international community, there is a good opportunity to gain experience with thorium-based fuels and to start the transition phase toward use of thoriumas part of the nuclear fuel cycle. Previous studies have shown that in the near term (10 to 15 years), small amounts of thorium could be introduced into conventional 37-element fuel bundles made with natural uranium (NU) or slightly enriched uranium for use in PT-HWRs, which could help improve performance and safety characteristics relative to the use of NU alone. This work is an evaluation of various types of thorium-based oxide fuels in 35-element bundles that could potentially be implemented in current PT-HWRs to enable the transition toward the thorium fuel cycle to harness the energy potential from thorium and to conserve uranium resources. Such fuel concepts could have additional benefits, including enhanced reactor performance and safety characteristics, a reduction in the production of fissile plutonium, and the ability to consume stockpiles of plutonium. Thorium (in the form of ThO2) is mixed with a fissile driver of either reactor-grade plutonium (67 wt% Pu-fissile/Pu, 3.5 to 4.5 wt% PuO2), low-enriched uranium (LEU) (5 wt% U-235/U, 40 to 50 wt% LEUO2) or U-233 (1.8 wt% 233UO(2)). The 35-element fuel bundle has a central graphite displacer rod to help maintain a low coolant void reactivity (CVR), while also flattening the radial power distribution in the fuel bundle due to supplementary neutron moderation. Analysis suggests that these thorium-based fuel types have lower fuel temperature coefficients (FTCs) and CVR values and higher fissile utilization (FU) compared to conventional 37-element natural uranium oxide (NUO2) fuel used currently in PT-HWRs. Lower values of CVR and FTCs will lead to lower and more negative values of the power coefficient of reactivity (PCR), which will help increase the safety margin for PT-HWRs further. Achieving higher FU will help better conserve nuclear fuel resources (both uranium and thorium), and should lead to lower fuel costs in a once-through fuel cycle. The use of a higher-fissile content fuel (in the form Pu, LEU, or U-233 mixed with thorium) enables both higher fuel burnups and FU relative to NU, while also breeding and burning U-233. The low-burnup (LEU, Th) O-2 core configuration shows the most promise as a feasible and practical means of producing energy from thorium (up to 33% of total energy produced) while also generating stockpiles of U-233. PT-HWR cores using (Pu, Th) O-2 fuels may have some challenges in regard to the power balance in the bundle and reactor operation. Without reactivity control devices, some (Pu, Th) O-2 cores may require power reductions to avoid exceeding bundle power limits, if amaximum linear element rating of 54 kW/m (which applies to uranium-based fuels) is assumed. PT-HWR cores fueled with (Pu, Th) O-2 also demonstrate significant benefits as a means of consuming stockpiles of fissile plutonium, reducing inventories by 84% to 90% in a single pass through a PT-HWR core, while also producing substantial quantities of U-233 (up to 314 kg/year/reactor) in the spent fuel for potential reprocessing and recycling in a closed fuel cycle.