CANDU Pressurized Heavy Water Reactor Thorium-²³³U Oxide Fuel Evaluation Based on Optimal Fuel Management

The optimization problem of the in-core fuel management of a thorium-fueled CANDU pressurized heavy water reactor (PHWR) consists of several component actions: the number of fresh fuel bundles inserted in the channels, the choice of the channels to be refueled next, the refueling rate, and the composition of the fresh fuel bundles (the latter relevant to advanced fuel cycles). Several fresh fuel compositions of ²³²Th and ²³³U were investigated and compared to the self-sufficient equilibrium thorium (SSET) cycle fuel, in terms of the objective function of an optimal fuel management problem. This optimization problem consisted of minimizing the total refueling rate at equilibrium with respect to criticality and power peaking constraints. The maximum acceptable value of the form factor was equal to 1.20, the form factor defined as the maximum-to-average power density ratio in the reactor core. The reactor core was divided into two refueling zones, each characterized by a uniform refueling rate for its channels. The control variables of the optimization problem were the average fluences (irradiations) of the bundles discharged from the channels of each of the zones, these variables being directly related to the refueling rates. A computer code, ASTERIX, was written to solve the optimization problem, using a steepest descent method, which required only a moderate number of diffusion calculations. Simulation was performed on simple models of a 600-MW CANDU PHWR. Because of the presence of ²³³Pa in the fuel, the diffusion calculations are nonlinear, needing a more complex solution technique. The cell parameters used were calculated by the Atomic Energy of Canada Limited code LATREP for a two energy-group model. This optimization technique gave optimal results that represent substantial savings in the refueling rates (up to 14%) when compared to nonoptimal feasible cases. The comparison of the various fuel compositions studied revealed that the sensitivity of the refueling rate (and the burnup) to the fresh fuel content is quite large for the SSET fuel and the low enrichment fuels.