Impact of Symmetry Reduction and Boundary Conditions on Fuel Assembly Neutronics in Monte Carlo Simulations

Symmetry-based domain reduction is widely applied in Monte Carlo reactor physics to reduce computational cost using reflective, white, and periodic boundary conditions. In lattice physics analyses, simulations based on minimum symmetric domains are often assumed to provide the best balance between accuracy and computational efficiency. However, this assumption depends on the validity of the imposed symmetry boundaries, which represent mathematical approximations rather than physical interfaces. This study investigates the effects of different boundary conditions on fuel assembly neutronics calculations using two representative geometries: a rectangular PWR fuel assembly and a hexagonal VVER fuel assembly. Infinity multiplication factors (k-inf) and pin-wise power peaking factors (PPPFs) are compared between full-assembly models and reduced-symmetry configurations employing reflective, white, and periodic boundary conditions. Results show that reflective and white boundary conditions provide strong agreement between full and reduced models, with R2 values up to 0.995 and PPPF differences below 1.8% for the PWR assembly, and R2 values up to 0.930 with maximum PPPF differences of 5.0% for the VVER assembly. In contrast, the symmetric 1/8 PWR domain with periodic boundary conditions exhibits lower agreement (R2 of 0.543 and PPPF differences of 12.5%), reflecting the stringent surface-matching requirements imposed by periodic boundaries in highly reduced computational domains. These results demonstrate that the predicted lattice physics parameters are sensitive to the choice of boundary condition and the adopted symmetry reduction level, even in highly regular lattices commonly idealized as infinitely repeated systems.