Physics characteristics of internally cooled annular fuel for potential application in pressure tube heavy water reactors

This paper summarizes the results of exploratory lattice physics studies of alternative, advanced fuel bundle concepts that could potentially be implemented in pressure tube heavy water reactors (Pt-HWR s). WIMS-Aecl was used to analyze the performance and operational characteristics of an 18-element and a 12-element internally cooled annular fuel (ICAF) fuel bundle, made with (Leu,Th)O₂ fuel, with both low-burnup and high-burnup options. Such fuel bundles with annular fuel elements may be able to operate at higher bundle power levels and with higher linear element (LER) ratings than fuel bundles with conventional solid cylindrical fuel elements. The use of thorium mixed with Leu can help extend uranium resources, exploit the energy potential in thorium, and also reduce the production of plutonium and minor actinides, due to the smaller fraction of ²³⁸U in the fuel. To improve the neutron economy of these lattices, higher-purity heavy water moderator and coolant (99.90 at % D2O) were used, along with highly enriched zirconium (95 wt % ⁹⁰Zr/Zr) for the zirconium alloys used in the structural components. The results were compared with those for a more conventional 37-element Pt-HWR fuel bundle using natural uranium (NU) fuel. Results show that such fuels could be very attractive, being able to achieve high burnup, comparable or better fissile utilization, reduce coolant void reactivity, and comparable or more negative fuel temperature coefficients. (Author)