Wims-AECL multicell calculations for burnup heterogeneity

The burnup heterogeneity effect in the ACR™-1000 was investigated using the WIMSAECL multicell (MC) calculation capability, which allows the calculation of lattice properties taking into account the effects of neighbour cells. The MC calculations were performed with WIMS-AECL version 3.1.1, based on a 3×3 group of cells with the ACR-1000 fuel at various burnup values. The MC results for the central cell were then compared to those obtained with the standard single-cell (SC) calculation. While the MC model does not reproduce exactly the SC values, the reactivity differences are bounded by the values; -4.82 mk and +1.45 mk, which are small relative to the absolute SC reactivity values; +143 mk for fresh fuel and -38 mk for exit-burnup fuel, respectively. Considering that these differences tend to cancel out over the bundle residence time in the core, it is concluded that there is no strong need to replace the SC calculation with the MC calculation for routine time-average or core-tracking simulations.