Deterministic modeling of 100%MOX ABWR lattices with increasing void fraction. Validation of the REL2005 code package on FUBILA experimental program

This paper details the validation work led on the REL2005 code package, applied to the modeling of 100%MOX 9x9 Advanced BWR assemblies with increasing void fraction (0 to 70%). The REL2005 package relies on the deterministic lattice transport code APOLLO2.8 based on the Method of Characteristics (MOC), and its new CEA2005 multigroup library based on the latest JEFF-3.1.1 nuclear data file. We describe the overall results obtained on 3 critical cores of the FUBILA experimental program that took place in the EOLE facility between 2005 and 2006: FUBILA/REF (0% void), NORM (40% void) and 70%VOID, for critical masses, void reactivity coefficients and pin-by-pin power distributions by using the REL2005-BWR optimized scheme (26 energy groups coupled with unstructured 2D geometry). For critical masses, the calculation-to-experimentratio C/E on the keff increases with the void fraction from 200 pcm for the REF core to 500 pcm at 70%void. A reference full 3D calculation made with TRIPOLI-4.5 Monte Carlo confirms this trend, as for the void coefficient. For pin-by-pin radial power distributions, REL2005 gives acceptable results, with maximum discrepancies of the order of 3 to 4%, as void fraction increases. These values are within 2 standard deviations of the experimental uncertainty. Those results lead us to be confident in the capability of the REL2005 code package to accurately predict 100%MOX BWR key-neutron parameters. (author)