Fueling Study of a CANDU Reactor Using Fuels Containing Burnable Neutron Absorbers

Trace amounts of burnable neutron absorbers (BNAs) were used to tailor the reactivity of the 37-element, natural uranium (NU) fuel bundle used in CANDU reactors. The BNAs of interest included Gd₂O₃ and Eu₂O₃, which were added to the fuel in variable quantities and combinations. The fuel lattice was modeled using the WIMS–AECL 3.1 code, and core simulations were conducted using the Reactor Fuelling Simulation Program (RFSP). The fuel model assumes an equivalent and uniform distribution of BNAs in the CANLUB layer of each fuel element.

The incorporation of BNAs is designed to improve CANDU reactor operating margins during on-power refueling by eliminating the fueling transient (FT) and reducing the magnitude of the plutonium peak (PP) that is characteristic of NU fuels. By adding an optimal combination of “fast-burning” and “slow-burning” BNAs, the FT and PP can be selectively reduced, and a significantly flatter trend in the burnup-dependent evolution of fuel reactivity can be achieved.

The results of the study indicate that by adding ~150 mg [~8 parts per million (ppm)] of Gd₂O₃ and ~300 mg (~15 ppm) of Eu₂O₃ per fuel bundle, the best gain in the operating margins of a 2650-MW(thermal) (480-channel) model CANDU reactor can be achieved. Based on the simulation of refueling events, it was shown that the magnitude of average postrefueling channel power ripples can be reduced by an average of 100 kW and a maximum of 220 kW for powers observed immediately after refueling. This reduction in postrefueling powers was also shown to allow the average liquid zone controller level to decrease from ~48% to 10%. This decrease implies a potential relief on overpower protection (an operating margin).