Simplified models for high burnup spent nuclear fuel rods and their comparison

Comparison of SNF rod failure criteria and corresponding simplified fuel rod models. • Survey and comparison of failure criteria for spent nuclear fuel rods. • Introduction of simplified fuel rod models for each failure criterion. • Evaluation of the applicability of simplified models in dynamic impact simulations. • Discussions on the influence of complex pellet-cladding interactions on fuel rod behavior. Effective management of spent nuclear fuel requires maintaining its structural integrity with safety guidelines emphasizing protection of the cladding from mechanical and physical damage that could lead to significant fuel rod failure. According to NUREG-1864, the plastic strain that can cause cladding failure was observed to be in the range of 1.0 ∼ 4.0 %. The experimental data was obtained by pressurized tube tests with defueled cladding specimens and could not reflect the complicated stress states due to the pellet-clad interaction that occurs in real drop impacts. However, the failure criterion of irradiated cladding under more complicated stress states is not easily available. In this study, we perform a comparative analysis of the applicability of simplified models for an SNF rod based on two new fracture criteria and flexural rigidity modification proposed in NUREG-2224. The two new failure criteria utilize curvature based plastic bending strain and membrane plus bending stress through the thickness of the cladding, respectively. Of the three models, the simplified model based on membrane plus bending stress failure criterion was found to be the most conservative in dynamic impact applications.