Influence of Spacer Grids Homogenization on Core Reactivity and Axial Power Distribution

Abstract

The paper presents the influence of spacer grid homogenization during cross section generation on core reactivity and axial power distribution. Homogenization calculation was performed at fuel assembly level using FA2D code. The first approach is to smear uniformly all centrally located spacer grids along 120 inches of fuel assembly and carry out 2D transport calculation. The second approach is to smear spacer grid within 6 inches of fuel assembly and perform homogenization calculation. That composition is then assigned to closest 6 in axial subdivision of the core calculation. The last analysed option is to do additional localization of spacer grids and carry out homogenization within 2 inches of fuel assembly height. The additional subdivision is afterward performed of the closest regular axial core subdivision in nodal core calculation. The core calculation was performed using modified PARCS 2.5 code for NPP Krško cycle 29. The normalized axial power distributions obtained by PARCS for three different ways of spacer grid homogenization are then compared to quantify the influence of modelling. Similar comparison was performed for critical boron concentration. As expected larger influence is present for axial power distribution (more details for fine localization), with some influence on axial power offset and global reactivity.