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

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. Standard Westinghouse VANTAGE+ 16x16 fuel assembly was used in the calculation. The first approach is to smear uniformly all centrally located spacer grids along 120 in of fuel assembly and perform 2D transport calculation. Another approach is to smear spacer 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. Last analyzed option is to do additional localization of spacer grid and perform homogenization within 2 inches of fuel assembly height. The additional subdivision is then performed of closest regular axial core subdivision in nodal core calculation. The core calculation was performed using PARCS code for NPP Krsko cycle 29. The normalized axial power distributions calculated by PARCS for three different spacer grid homogenizations are then compared to quantify the influence of spacer modeling. 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. (author).