Effects and Modeling of Power History in Thorium-Based Fuels in Pressure-Tube Heavy Water Reactors

Lattice and core physics modeling and calculations have been performed to quantify the impact of power flux levels and power history on the reactivity and achievable burnup for 35-element fuel bundles made with thorium-based fuels, such as (Pu,Th)O-2 and (U-233,Th)O-2. These bundles are designed to produce on the order of 20 MWd/kg burnup in homogeneous cores in a 700-MW(electric) class pressure-tube heavy water reactor, operating on a once-through thorium cycle. Methods have been developed to model time-dependent power histories in lattice physics calculations that are more consistent with core physics analysis results. Results demonstrate that the impact of power/flux level and the modeling of time-dependent power histories on the core power distributions and achievable fuel burnup are modest for Pu/Th fuels but are more significant for U-233/Th fuels. Thus, to reduce the neutron capture rate in Pa-233 and to increase fuel burnup and fissile utilization, there may be an incentive to develop solutions to reduce the time-average specific power in the fuel.