Study of the spatial discretization and temperature distribution approximation effects on BWR assembly calculations

The effects of the spatial discretization and temperature distribution approximation on the BWR assembly calculation were studied using the continuous energy Monte Carlo burn-up calculation under several conditions. The radial-division, temperature-distribution, and azimuthal-division effects of a fuel pellet for BWR fuel (8 × 8, wide-water-rod) were investigated. The calculation results indicate that the effect of the temperature distribution approximation on the assembly calculation is negligible, even if a pin-averaged temperature was assigned to all fuel pins. The effect of the spatial discretization of fuel pellets in the azimuthal direction brings about a 0.15%dk/k prediction error in k-infinity. This effect is caused by the change of the burn-up characteristics in the Gd-rods. Therefore, the azimuthal-division of Gd-rods is desirable to reduce the spatial discretization effect for BWR assembly with higher Gd fuel rods.