Probabilistic investigations on pinching failure of PWR SNF rods considering hydride-related cladding embrittlement

This study assesses the probability of hydride-related PWR SNF rod pinching failure under a hypothetical, accidental SNF cask transportation event. A 9-m SNF transportation cask drop scenario is investigated, considering a horizontal impact of the unprotected outer cask shell on an unyielding surface, after SNF dry storage of 300 years. Such an event potentially causes high pinching loads that act on the rods at the spacer grid locations of the SNF assemblies. A set of 3,000 pinching scenarios is evaluated, with each scenario representing a unique combination of geometrical, material, and loading parameters. The parameter combinations are obtained using Latin Hypercube Sampling (LHS). The offset strain is used to evaluate cladding failure, defined as the plastic, radial cladding deformation under pinching loading, normalized with respect to the outer cladding diameter. The cladding deformation capacity is estimated from regression and interpolation models that are based on ring compression tests (RCTs) of PWR cladding. The computed offset strain capacity represents the achievable cladding offset strain before failure. The regression and interpolation models consider effects related to hydrogen content, peak cladding hoop stress during SNF vacuum drying, and cladding temperature at the moment of load application on the offset strain capacity. The cladding offset strain demand in each of the evaluated pinching scenarios was computed from load-displacement curves obtained with finite element (FE) simulations. These simulations assume boundary conditions similar to those of RCTs and account for the supporting effect provided to the cladding by a degraded fuel pellet. The investigations indicate a probability of hydride-related ductile pinching failure for the considered 9-m cask drop below 0.2%.