Analysis of VIP-BWR reactor core physics experiments on UO₂ and MOX mockup fuel assemblies with CASMO5

The VIP-BWR program comprises reactor core physics experiments on three core configurations each consisting of BWR mockup, reference, and driver fuel assemblies. The mockup fuel assemblies are an all-UO₂, an island-type MOX and a full-MOX-type 8 × 8 fuel assembly each including Gd₂O₃-UO₂ fuel rods. The measurement results include the critical water levels and fission rates for large numbers of fuel rods. Analysis of the experiments was performed in a two dimensional geometrical model with the lattice physics code CASMO5 coupled with the JENDL-4.0-based nuclear library. The axial leakage of neutrons from the core was taken into account adopting the measured axial buckling. The effective multiplication factors of the critical cores ranged from 0.9940 to 0.9976, with the measurement uncertainties of 0.0003 to 0.0004 (1 σ). The calculated results of core radial fission rate distributions were compared with the measurements. For the cores loaded with the MOX fuel assemblies, the calculations overestimated the fission rates of MOX fuel rods by 4 to 5% relatively to those of UO₂ and Gd₂O₃-UO₂ fuel rods. It agreed with the trends in the other preceding analysis studies, and biases were confirmed in the calculation results of power distributions in MOX-fueled light water reactor cores.