Analysis on factors generating shipper/receiver difference of nuclear material. Effects of adjacent condition in core based on three-dimensional depletion calculation

As a factor causing Shipper/Receiver Differences of nuclear material mass between nuclear power plants and reprocessing plants, effects of adjacent fuel assemblies in depletion calculations are focused in this report. In an equilibrium core, a fuel assembly is surrounded by fuel assemblies with different nuclear property every cycle. The amount of nuclear material, however, is evaluated by depletion calculations in single assembly geometry. In the present study, depletion calculations are carried out in 3 by 3 assembly geometry considering typical fuel loading patterns in a 3 loop PWR equilibrium core with the MVP-BURN code. Here, the detailed geometries of core structures are neglected for simplicity. These calculations are done for each node with axial burnup distribution evaluated by three-dimensional single assembly geometry. Depletion calculations in single assembly geometry are also performed in the same calculation condition for comparison. The results showed that the effects considering the adjacent fuel assemblies are -2.1 ∼ -3.9% for ²³⁵U, +0.046 ∼ +0.077% for ²³⁸U, +0.016 ∼ +0.031% for U-Total, -0.52 ∼ -2.4% for ²³⁹Pu, -0.40 ∼ -2.1% for ²⁴¹Pu and -1.2 ∼ -2.3% for Pu-Total in the end of cycle 4. These results show detailed modeling of adjacent fuel assemblies and boundaries are required for accounting fuel material mass. (author)