A typical light water reactor (LWR) fuel pin is composed of enriched uranium dioxide fuel pellets stacked inside a cladding tube made of zirconium alloy. In the pristine state, there exists a gap between the UO2 pellet and the cladding. However, neutron irradiation, fuel swelling and cladding creep down phenomena due to in-the-reactor conditions result in the closure of this pellet-cladding gap. Additionally, as fuel burnup increases, the chemical and mechanical bonding between the fuel and cladding results in the formation of an oxide layer. The objective of the proposed study is to investigate the role of the pellet-cladding interface in mechanical fuel rod degradation, especially under fatigue failure conditions.
Under normal vibration conditions during transport to storage and disposal facilities, the spent nuclear fuel (SNF) rods experience cyclic loads, which can lead to rod failure over time. In this regard, the fatigue performance of SNF has previously been investigated at Oak Ridge National Laboratory (ORNL). The results have shown a clear indication of a non-uniform performance degradation of the fuel rods relative to cladding-only rod performance, where with increasing strain amplitudes, the fatigue performance of SNF rods approaches that of cladding only rods. This dependance on strain amplitude can be attributed to the fuel pellets inside the cladding tube which change the internal stress concentrations on the cladding at regions of pellet-pellet interfaces and intra-pellet cracks. A possible explanation could be that at high values of strain amplitudes, the pellet and the cladding are no longer bonded to each other, which in turn, decreases the stress concentrations at the pellet discontinuities. However, there is currently no qualitative analysis to substantiate this theory and no experimental evidence to show the evolution of pellet cladding bonding as a result of mechanical fuel failures.
Through this proposal, we seek to elucidate the influence of pellet-cladding interface in fatigue failure by characterizing fuel rod cross-sections, failed at different strain amplitudes during fatigue testing. In addition, we propose to study different cross-section sites across the failed sample to study the evolution of pellet-cladding interface as a function of local stress/strain values for the same cycles to failure. We expect the pellet-cladding interface of the fuel rod samples failed at high strain amplitudes to show more crack damage, as compared to the samples failed at lower strain amplitude values under different cycles to failure. Optical microscopic characterization is the most efficient way to study the fuel surfaces as it involves minimal preparation (hot cell) time and yet presenting a direct method to probe fracture surfaces. The imaging steps will take around 6 months and a further 2 months will be required for data analysis.
The US DOE is finalizing the location for a federal consolidated interim storage facility to store around 15,000 metric tons of commercial SNF by removing and transporting it from nuclear power plants across the US, where it is currently stored. This is the first step for the DOE to fulfill its legal obligation to take ownership of commercial SNF and dispose of it. In addition, the industry trend is increasing fuel burnup (i.e., increasing the power generated by each fuel assembly). As burnup increases, the fuel pellet becomes chemically bonded to the cladding, which likely causes stress concentrations at pellet discontinuities like cracks.
The current knowledge on stress concentrations affecting fuel rod degradation, such as fatigue failure, is limited in terms of experimental evidence. The non-uniform fatigue performances at low, medium and high strain amplitudes point towards the involvement of irregular stress concentrations on the inner diameter of the cladding, possibly from a mixture of pellet-pellet and pellet-cladding bonding. Understanding the conditions where stress concentrations no longer occur is as important as understanding when they do occur. There is, however, a dearth in experimental data to corroborate these interactions and their role in failure. We propose to utilize the failed Sister Rod specimens tested under a range of fatigue conditions for fractography analysis. The analysis will focus on studying the fuel surfaces at various local and bulk strain conditions to compare the crack patterns as a function of strain amplitude. The study will evaluate pellet-cladding interface and their evolution leading to failure on these samples, thus opening potential avenues to better understand SNF failure.
Thus, the study's findings will contribute to the safe, long-term management of SNF, ensuring resilience against hypothetical failure scenarios.