Implicit local void model for cross section generation in the APOLLO2-A - ARTEMISTM BWR core simulator methodology

In this paper, we present a new approach to account for the local heterogeneity in the moderator density for Boiling Water Reactor cross section generation The so-called Implicit Local Void Model (ILVM) functionalizes the water density distribution as a function of lattice type, average void fraction, and burnup The reference water density distributions that are used in the ILVM are calculated from the spectral code APOLLO2-A coupled with the subchannel code F-COBRA-TFTM Then the resulting local variations are implicitly captured in the neutronics data (cross sections, pin form factors, etc) generated by APOLLO2-A The ILVM is tested on the newly developed APOLLO2-A/HERMES/ARTEMISTM BWR computational chain for two different equilibrium cycles (12 months and 24 months, respectively) based upon AREVA's ATRIUMTM 11 assembly The effects of the ILVM are found to be very dependent on the assembly neutronic design For the first equilibrium cycle, eigenvalue trends, peaking factors, and projected thermal limits during the cycle are found to be mostly unaffected by the ILVM, while the second one had a decrease in most of the thermal limits and peaking factors The change in the neutron flux spectrum introduced by the functionalization of the radial moderator density in the ILVM model causes an increase in burnable poison worth, and as a result, the impact of the model is stronger on assembly designs with high gadolinium content (authors)