Neutronics Analysis of Full MOX BWR Core Simulation Experiments FUBILA

A part of the experimental program FUBILA was dedicated to the study of the core physics characteristics related to in-channel void fractions of BWR MOX assemblies at power operation. The present paper describes analysis results of the experimental data with deterministic analysis codes and a continuous-energy Monte Carlo code coupled with major nuclear data libraries. It also discusses the accuracy of the adopted analysis methods. The major findings are summarized. The calculated critical keff's with the Monte Carlo calculations range from 0.999 to 1.006. Those of the transport calculations with sixteen energy groups are close to those of the Monte Carlo calculations; those of diffusion calculations with the sixteen energy groups are systematically smaller by -0.4 to -0.5%Δk than those of the Monte Carlo calculations. The RMS's of differences between the calculated and measured core radial fission rates are from 1.1 to 2.2%, which are from about one to two times larger than the measurement errors. The axial fission rate distributions measured using integral gamma scanning and 235U and 237Np fission chambers are well reproduced by the diffusion calculation. The analysis results well reproduce the measured modified conversion ratios for the 40% void core; however, they overestimate those of the 70% void core. They tend to underestimate the measured spectrum indexes σf(239Pu)/σf(235U) and σf(237Np)/σf(239Pu).