Axial Burnup Profile Influence on Criticality Safety of WWER Spent Fuel

Criticality safety assessment for WWER-440 NUHOMS^® cask with spent nuclear fuel from Armenian NPP has been performed. The cask was designed in a such way that the neutron multiplication factor k_eff must be below 0,95 for all operational modes and accident conditions. Usually for criticality analysis, fresh fuel approach with the highest enrichment is taken as conservative assumption as it was done for ANPP. Nuclear and Radiation Safety Centre of Armenian Nuclear Regulatory Authority (NRSC ANRA) in order to improve future fuel storage efficiency, initiated research with taking into account burn up credit in the criticality safety assessment. Axial burn up profile (end effect) has essential impact on criticality safety justification analysis. However this phenomenon wasn’t taken into account in the Safety Analysis Report of NUHOMS^® spent fuel storage constructed on the site of ANPP. Although ANRA doesn’t yet accept burn up credit approach for ANPP spent fuel storage, assessment of impact of axial burn up profile on criticality of spent fuel assemblies has important value for future activities of ANRA. This paper presents results of criticality safety analysis of spent fuel assemblies with axial burn up profile. Horizontal burn up profile isn’t taken account since influence of the horizontal variation of the burn up is much less than the axial variation. The Actinides and Actinides + Fission Products approach are discussed. The calculations were carried out with STARBUCS module of SCALE 5.0 code package developed at Oak Ridge National laboratory. SCALE5.0 sequence CSAS26 (KENO-VI) was used for evaluation the k_eff for 3-D problems. Obtained results showed that criticality of ANPP spent fuel cask is very sensitive to the end effect. Using Burn up profiles of Control Assemblies in both approaches leads to much more increasing than in case of Working Assemblies. Usually increasing burn up leads to decreasing Δk_eff, hence decreasing end effect. However for WWER-440 Control Assemblies that worked only within 6^th (operative) group increasing burn up leads to increasing of the end effect.