Spent Nuclear Fuel Transport Reliability Study

SNF rods lie horizontally and are supported by the assembly guide tubes and spacer grids inside specially designed transportation casks. During normal conditions of transport (NCT), these fuel rods are subjected to oscillatory bending due to inertial effects. This oscillatory bending is the major vibrational load of SNF rods as discussed in 10 CFR §71.71. Besides the change in composite structure of fuel pellets and cladding that occurs during irradiation, the SNF rods typically have burnup induced damage (pores and micro cracks), oxide and hydride layers, residual stresses, altered interfaces, and trapped fission products. Understanding the impact these changes may have on the strength of the SNF fuel/cladding system is required to accurately simulate the performance of SNF rods during transport.The objective of this research is to collect dynamic experimental data on spent nuclear fuel (SNF) under simulated transportation environments using the Cyclic Integrated Reversible-Bending Fatigue Tester (CIRFT), the hot-cell testing technology developed at Oak Ridge National Laboratory (ORNL). The collected CIRFT data will be utilized to support ongoing spent fuel modeling activities, and support SNF transportation related licensing issues. Recent testing to understand the effects of hydride reorientation on SNF vibration integrity is also being evaluated.CIRFT results have provided insight into the fuel/clad system response to transportation related loads. The major findings of CIRFT on the high-burnup (HBU) 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, andSNF failure initiates at the pellet-pellet interface region and appears to be spontaneous.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.