Spent nuclear fuel dynamic performance under normal condition of transport

The objective of this project is to evaluate the spent nuclear fuel (SNF) vibration integrity under normal condition of transport. The SNF fatigue strength data has been collected from pressurized water reactors, including H. B. Robinson (HBR) and North Anna (NA) Zircaloy-4 cladding and NA and Catawba M5 cladding, and the Limerick Generating Station boiling water reactor under simulated transportation environments using the Cyclic Integrated Reversible-Bending Fatigue Tester (CIRFT). CIRFT is an enabling hot-cell testing technology developed at Oak Ridge National Laboratory (ORNL). This data will be used to support on-going SNF modeling activities, in addition to addressing licensing issues associated with SNF transport. CIRFT results have provided insight into the fuel/clad system response to transportation related loads. The CIRFT HBR test result is shown to the right. The major findings of CIRFT test on the high burn-up SNF are as follows: • SNF system interface bonding plays an important role in SNF vibration performance, • Fuel structure contributes to the SNF system stiffness, • There are significant variations in stress and curvature of SNF systems during vibration cycles resulting from segment pellets and clad interaction, and • SNF failure initiates at the pellet-pellet interface region and appears to be spontaneous. High burn-up spent nuclear fuel cladding has a significant amount of microcracks and hydrides, which will reduce the stress intensity required for crack growth. Also, the predominance of the hydride platelets that do exist in the cladding after the fuel being discharged from reactors is oriented in the circumferential direction. At elevated temperatures during drying-transfer, some of the hydrogen may go into solution (up to 200 wppm at 400°C). The pressure-induced cladding tensile hoop stress during dryingtransfer operations is high relative to in-reactor and pool-storage conditions. During cooling under tensile hoop stress, some of the dissolved hydrogen will precipitate in the radial direction across the cladding wall. Further cooling during storage may result in radial-hydride-induced embrittlement. Recent testing to understand the effects of hydride reorientation on SNF vibration integrity is also being evaluated. Because of the non-homogeneous composite structure of the SNF system, finite element analyses (FEA) are needed to translate the global moment-curvature measurement into local stress-strain profiles. The detailed mechanisms of the pellet-pellet and pellet-clad interactions and the stress concentration effects at the pellet-pellet interface cannot be readily obtained directly from a CIRFT system measurement. Therefore, detailed FEA is used to understand the global test response, and that data will also be presented.