Probabilistic Assessment of Peak Cladding Hoop Stress and Hydrogen Content of PWR SNF Rod Cladding

This study presents two statistical models that were developed to estimate the expected peak cladding hoop stress (CHS) and the amount of hydrogen in pressurized water reactor (PWR) spent nuclear fuel (SNF) rod cladding. Peak CHS is caused by high rod internal pressure during vacuum drying performed when transferring SNF to dry storage. During in-reactor operation of PWR fuel, the rod cladding tends to corrode and uptake hydrogen. The hydrogen content and CHS control hydride-related cladding embrittlement at low material temperatures. The two methodologies developed in this study were used to create a generic rod database with information on PWR SNF conditions. This database provides information on 100 000 randomly selected rods that form part of the current U.S. SNF inventory. According to the statistical results, the expected hydrogen content of PWR rod cladding is in a sensitive interval that may facilitate hydride reorientation. However, the modeling results suggest that the expected peak CHS of the selected rods is significantly below 90 MPa, which is the estimated lower bound stress necessary to trigger significant radial hydride embrittlement in cladding after being cooled to room temperature. Further, the results indicate that hydride embrittlement due to excessive hydrogen in cladding is unlikely. Therefore, a low probability of hydride-related embrittlement of PWR SNF cladding currently stored in the U.S. inventory is anticipated, even under consideration of low cladding temperatures after long-term SNF dry storage.