Interpretation of the VENUS VIP-BWR program with APOLLO2.8 reference and optimized MOC schemes for 8 × 8 UO2 and MOX BWR assemblies

The interpretation of the VIP-BWR program conducted in the CEN·SCK Mol VENUS critical facility (Belgium), has been performed with the new APOLLO2.8 product and its CEA2005V4.1 library based on the JEFF3.1.1 file. Both reference SHEM-MOC (281groups without equivalence) and Optimized BWR 26G (26 groups with equivalence) schemes are used for UO2 and MOX BWR assembly calculations. The VIP-BWR program was aimed to provide an experimental database for BWR neutronics tools in mixed Gd poisoned configurations with 8 × 8 UO2 and MOX assemblies. The experimental conditions are relatively representative of actual industrial BWR core characteristics, at least in terms of void fraction. Measured pin-by-pin power distributions enable to exact valuable information at various interfaces. For fresh (UO2/UO2–Gd) and recycled UO2 (UO2 only) cores loadings, the information is given through the “UO2” core. In the case of partial MOX loadings (UO2/MOX interface), the power distributions are available through the “T-MOX” core. All critical sizes are predicted within 1 with SHEM-MOC reference calculation scheme. For UO2 core, the (C–E) on keff are (95 ± 266) pcm and (203 ± 266) pcm for SHEM-MOC and Optimized scheme respectively. For MOX core, the results are (87 ± 214) pcm and (283 ± 214) pcm. The uncertainties take into account both measurement uncertainties and technological uncertainties such as enrichment, clad thicknesses, grid pitch or fuel densities.Average (C–E)/E on radial fission rate distributions are in excellent agreement in the UO2 and UO2–Gd assemblies. In mixed loading cores, the calculation shows an over prediction in the MOX fuel pin. This points out a systematic swing between UO2 and MOX. For average fission rates in UO2, the C/E results are within ± 0.2% with SHEM-MOC and OptimizedBWR26G. For average power in MOX, the C/E results are within ±0.5% with SHEM-MOC and ±0.9% with Optimized BWR26G. The PMOX/PUO2 fission rate ratios are (+3.7 ± 1.8%) and (+2.5 ± 1.8%) for SHEM-MOC and Optimized BWR 26G respectively. The peak power in UO2 and MOX cores are reproduced within ±0.8% for both schemes. These results confirm the capability of this product to correctly treat pin-by-pin power distributions in BWR UO2 and island MOX bundles.