EVALUATION OF BURNUP CREDIT FOR FUEL-STORAGE ANALYSIS - EXPERIENCE INSPAIN

Several Spanish light wafer reactor commercial nuclear power plants ar e close to maximum spent-fuel pool storage capacity. The utilities are working on the implementation of state-of-the-art methods to increase the storage capacity, including both changes in the pool design (rera cking) and the implementation of new analysis approaches with reduced conservatism (burnup credit). Burnup credit criticality safety analyse s have been approved for two pressurized wafer reactor plants (four un its) and one boiling wafer reactor (BWR); another BWR storage analysis is being developed at this moment. The elimination of the ''fresh fue l assumption'' increases the complexity of the criticality analysis to be performed, sometimes putting into question the capability of the a nalytic toots to properly describe this new situation and increasing t he scope of the scenarios to be analyzed. From a regulatory perspectiv e, the reactivity reduction associated with burnup of the fuel can be given credit only if the exposure of each fuel bundle can be known wit h enough accuracy. Subcriticality of spent-fuel storage depends mainly on the initial fuel enrichment, storage geometry, fuel exposure histo ry, and cooling time. The last two aspects introduce new uncertainties in the criticality analysis that should be quantified in an adequate way. In addition, each and every fuel bundle has its own specific expo sure history, so that strong assumptions and simplified calculational schemes have to be developed to undertake the analysis. The Consejo de Seguridad Nuclear (CSN), Spanish regulatory authority on the matter o f nuclear safety and radiation protection, plays an active role in the development of analysis methods to support burnup credit, making prop osals that may be beneficial in terms of risk and cost while keeping t he widest safety margins possible.