Thorium-Fuelled CANDU Nuclear Reactor Optimal Fuel Management at Approach to Refuelling Equilibrium

Abstract

A simple mathematical model of a CANDU 600-MWe reactor is used, based on two energy group cell and depletion calculations performed for a thorium-fuelled reactor. Diffusion equations are derived for a two dimension reactor model, which includes a reflector. The numerical equations are then derived, resulting in a non-linear system of equations. The approach to refuelling equilibrium part of the reactor core life is studied in terms of refuelling cycles defined as the time span between successive refuel- lings. The length of these cycles depends on the choice of the channels just refuelled, and also on the number of fresh fuel bundles inserted into these channels at refuelling. However, these choices are subjected to several constraints, such as power peaking and refuelling machine limitations. An optimization problem is written to establish a list of the best choices for the channels to be refuelled during the initial, part of the reactor's life. The objective function is the maximization of the cycle lengths, thus minimizing the number of fresh fuel bundles needed to bring the reactor from initial start-up to refuelling equilibrium. This initial period lasts about a year if the fresh fuel composition is always the same. The constraints mentioned above are accounted for in the optimization problem, and permit a reduction of the number of fuel channels acceptable for the next refuelling. For these channels, their associated cycle lengths are calculated and compared, and the optimum choice is selected , and refuelling is implemented accordingly. The use of perturbation theory is proposed to reduce the number of diffusion calculations to the only one required at each refuelling cycle of the reactor. The method involves non-linear equations and thus applies well to thorium- fuelled CANDU reactors. It is about to be tested on the Gentilly-2 reactor in which thorium-based fuel is used.