Appraisal of the NRC H-uptake calculation for Swiss boiling water reactors

Abstract

Uptake of hydrogen by the cladding of fuel rods during LWR operation worsens the mechanical properties of the material and can lead to cladding failure. The U.S. NRC has introduced a dependency of the equivalent cladding reacted (ECR) criterion on the H-content in the cladding and has proposed a burnup dependent calculation model of the H-content based on a large database which does not cover high burnup such as those reached in Switzerland. This work addresses how the extrapolation of the proposed H-content calculation is conservative compared to the experimental values on claddings irradiated in Swiss plants. Therefore, a model is proposed to calculate the H-content in Swiss BWR fuels up to high burnup, based on published experimental data. The new correlation leads to less conservative results. This is demonstrated with the simulation of a postulated LOCA in a Swiss BWR, where the high exposure bin in the core is no longer expected to fail.