Dryout Methodology with New Steady-State Criterion and More-Accurate Statistical Treatment of Channel Bow

Abstract

An overview is given of existing design criteria to prevent fuel cladding dryout and the methods used in boiling water reactor reload analysis to evaluate the impact of channel bow on margins in the critical power ratio (CPR). Potential weaknesses in today’s methodologies are discussed. Westinghouse in collaboration with KKL and Axpo - operator and owner of the Leibstadt NPP - has developed an enhanced CPR methodology based on a new criterion to protect against dryout during normal operation and with a more rigorous treatment of channel bow. The new steady-state criterion is expressed in terms of an upper limit of 0.01 for the dryout failure probability per year. This is considered a meaningful and appropriate criterion that can be directly related to the probabilistic criteria setup for the analyses of anticipated operation occurrences and accidents.In the Monte Carlo approach, a statistical modeling of channel bow and an accurate evaluation of CPR response functions allow the associated CPR penalties to be included directly in the plant safety limit minimum CPR and operating limit minimum CPR in a best-estimate manner. In this way, the treatment of channel bow is equivalent to all other uncertainties affecting CPR. Emphasis is put on quantifying the statistical distribution of channel bow throughout the core using measurement data.The enhanced CPR methodology has been implemented in the Westinghouse Monte Carlo code McSLAP. The methodology improves the quality of dryout safety assessments by supplying more-valuable information and better control of conservatisms in establishing operational limits for CPR.The methodology is demonstrated with application examples from the introduction at KKL.