A probabilistic assessment of the likelihood of PWR SNF rod failure considering hydride-related cladding embrittlement after dry storage cool-down of 300 years is presented. The investigated rod failure mechanism is cladding cracking caused by rod pinching at the SNF assembly spacer-grids due to transportation cask impacts. The goal of this study is to understand the effect of the pinching load amplitude on the diametric SNF deformations and the probability of failure. The offset strain is used to predict cladding cracking. The offset strain is defined as the diametric, plastic cladding deformation under pinching loading, normalized by the outer cladding diameter. The offset strain capacity OSC of the cladding is the achievable offset strain before cladding failure, and it is estimated from regression and interpolation models that consider hydrogen content, peak cladding hoop stress during SNF vacuum drying, and cladding temperature at the moment of load application. The OSC models are based on results of ring compression tests (RCTs) on empty, irradiated and un-irradiated PWR cladding, and reflect the effects of hydride embrittlement (HE) and radial hydride embrittlement (RHE) on the material ductility. The offset strain demand OSD was computed with finite element (FE) models, assuming boundary conditions similar to those of RCTs, but including fuel pellet support. In this study, a representative set of 3,000 pinching scenarios, developed by Eidelpes et al. (2019) using Latin Hypercube Sampling (LHS), was reassessed. The LHS populations were defined based on available SNF data, and a pseudo rod database that provides information such as cladding geometry, alloy type, burnup, hydrogen content, and vacuum drying peak cladding hoop stress of the PWR SNF in the U.S. inventory. Fuel pellet degradation like pellet cracking or porosity was modeled in the FE simulations by reducing the elastic pellet stiffness by a generic fuel pellet crumbling factor. Unlike the original assessment of Eidelpes et al. (2019), which evaluated a 9-m cask drop scenario followed by a horizontal impact, the pinching load amplitudes in this reassessment were estimated by adapting the pinching load amplitudes of the 3,000 original scenarios, considering data on the expected cask impact velocity and angle in more realistic train accident scenarios. Cladding failure was predicted for OSC<OSD. The probability of failure was computed as the number of predicted failures divided by the total number of realizations. The results of this study indicate that the consideration of more realistic accident scenarios has a small effect on the probability of cladding pinching failure and that the diametric cladding deformations are controlled by the pellet-cladding gap width, and to a smaller extent, by fuel pellet degradation.
A Probabilistic Assessment of PWR SNF Rod Pinching Failure Considering Hydride-Related Cladding Embrittlement
September 13, 2019
Publication: 25th Conference on Structural Mechanics in Reactor Technology