| Description |
This dissertation presents a study on the structural integrity of spent nuclear fuel (SNF) transportation casks, considering hydride-related material degradation of pressurized water reactor (PWR) fuel rod cladding after long-term dry storage of up to 300 years. The thin-walled cladding is highly sensitive to mechanical loading, and hydride-related embrittlement mechanisms due to circumferential or radial hydrides decrease the cladding ductility. These conditions may trigger cladding failure when mechanically stressed, especially when the fuel rods are loaded perpendicularly to their longitudinal axes, i.e., rod pinching loading. A probabilistic assessment is developed in this study to compute the probability of PWR SNF pinching failure under accidental conditions of transport. A Monte Carlo Simulation (MCS) was used to obtain a statistically representative set of fuel rod conditions in the current U.S. PWR SNF inventory. The research focused on cladding hydrogen content, and the peak cladding hoop stresses during SNF vacuum drying while transferring the fuel from wet to dry storage. These two parameters are considered to control hydride-related cladding embrittlement. Ring compression tests (RCTs) on unirradiated, artificially hydrogen-charged PWR fuel cladding were conducted to better understand the plastic deformation capability of the degraded cladding. Some of the ring samples were subjected to high cladding hoop stresses at high material temperatures before testing to investigate hydride iv reorientation in the cladding ductility. The cladding hydrogen content and the cladding hoop stress amplitude were in a representative interval for U.S. PWR SNF. The ring samples included cladding-only and cladding-stainless steel pellet specimens, to understand the pellet supporting effect on cladding pinching capacity. To estimate the probability of cladding failure under pinching loading, a Latin Hypercube Sampling was used to generate a representative set of 3,000 SNF rod pinching scenarios. Each scenario is a unique combination of geometrical, material, and loading parameters. The combinations are used to create finite element (FE) models that simulate fuel rod pinching due to a 9-m cask drop or more realistic train accident scenarios, considering dry storage periods of 30 and 300 years, and to define the expected SNF cladding deformation capability. The study indicates a low probability of hydride-related cladding embrittlement in PWR SNF, and that the cladding is unlikely to fail under extreme pinching loads if fuel pellets provide sufficient support. The likelihood of ductile pinching failure is mainly controlled by modeling assumptions used to estimate the cladding hoop stresses during vacuum drying. Only highly embrittled cladding is at risk of failing, and the storage duration has an insignificant influence on the probability of failure. The cladding deformations under rod pinching are controlled by the pellet-cladding gap width. However, the amplitude of the pinching load is less important for cladding failure. |
