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Spent fuel transport and handling operating rules exclude any accidental risk. However, in the framework of the PRECCI R&D project, the impact on a spent fuel rod is taken into account. This study was carried out in CEA laboratories with the funding and the technical support of EDF. Since fuel rods are slender structures, it is of particular interest to model them with one dimensional finite elements. While the zirconium alloy elasto-plastic law can be directly used for the plugs and plenum regions, an equivalent stress-strain law is necessary for the fuel column level, especially in the case of high burnups since the contact between the pellets and the clad reinforce the contribution of the fuel to the rod mechanical behavior. The equivalent law as well as the correlation between the clad and the equivalent plastic strain were established from CAST3M finite element modelling of four point bending of fuel rods segments. This model, meshed with three dimensional elements, takes into account elasto-plastic behavior for the clad and cracking of the UO2 fuel pellets. Bending experiments on spent fuel rods performed at LCMI laboratory allowed to validate this modelling as reported in a previous article. The axial dynamic load impact tests performed by Hirose et al. on PWR fuel rods specimens were modelled with CAST3M with one dimensional elements using the determined equivalent law. The maximum loads calculated with the equivalent law are in excellent agreement with the experimental results. The deformed shapes as well as the failure modes are also in good agreement. The maximum calculated strains reach the fracture strain of the rods at the location where the failures are observed experimentally. This study reinforces the confidence in the equivalent law. It moreover indicates that zircaloy behavior for dynamic loadings can be modelled with the laws established at lower strain rates, as suggested by the moderate variation of the mechanical properties as a function of strain rates determined by dynamic tensile tests from literature.

CAST3M modelling of dynamic experiments on PWR high burn-up fuel rods equivalent fuel rod modelling approach validation