Evaluation of Missing Pellet Surface Geometry on Cladding Stress Distribution and Magnitude

Missing pellet surface (MPS) defects are local geometric defects in nuclear fuel pellets that result from pellet mishandling or manufacturing. The presence of MPS defects can cause clad stress concentrations that are substantial enough to produce through-wall cladding failure for certain combinations of fuel burnup, and reactor power level or power change. Consequently, the impact of potential MPS defects has limited the rate of power increase, or ramp rate, in both pressurized and boiling water reactors (PWRs and BWRs, respectively). Improved three-dimensional (3-D) fuel performance models of MPS defect geometry can provide better understanding of the probability for pellet clad mechanical interaction (PCMI), and correspondingly the available margin against cladding failure by stress corrosion cracking (SCC). The Bison-CASL fuel performance code has been developed within the Consortium of Advanced Simulations of Light Water Reactors (CASL) to consider the inherently multi-physics and multidimensional mechanisms that control fuel behavior, including cladding stress concentrations resulting from MPS defects. This paper evaluates the cladding hoop stress distributions as a function of MPS defect geometry and the presence of discrete pellet cracks for a set of typical operating conditions in a PWR fuel rod. The results are a first step in a probabilistic approach to assess cladding failure during power maneuvers. This analysis provides insight into how varying pellet defect geometries affect the distribution of the cladding stress, as well as the temperature distributions within the fuel and clad; and are used to develop stress concentration factors for comparing 2-D and 3-D models.