Abstract
The fuel handling operating rules exclude any accidental risk. However in the framework of the PRECCI R&D project, the bending of a spent fuel assembly resulting from its locking during a translation displacement is taken into account. This enabled us to develop an approach based on experiments and calculations that allows us to simulate the behaviour of an assembly under such loading. This study was carried out in CEA laboratories with the funding and the technical support of EDF. A three points bending test on a spent fuel rod segment was performed at the Laboratory for Mechanical Behaviour of Irradiated Materials (LCMI). From the experimental strength-displacement curve, a maximum failure strain, a maximum failure curvature and an equivalent constitutive equation were determined. CAST3M modelling of the fuel rod taking into account the elasto-plastic behaviour of the clad and the cracking of the UO2 fuel pellets was verified by the experimental results. Consequently, the identification of the respective contributions of the clad and of the pellets to the rod global behaviour was made possible. A two dimensional assembly with beam elements was modelled with CAST3M. The properties of the beams modelling the different parts of the assembly (top and bottom nozzle, grids) were chosen and adjusted according to their materials (zirconium alloys, steel) in order to obtain stiffness, tensile and shear behaviour, sliding and holding functions close to the experimental ones. Assembly bending calculations were performed. In order to obtain a rod integrity estimator, their maximum calculated strains and curvatures as a function of the bending angles can be compared to the failure experimental ones.
Highlights
We developed an approach based on experiments and calculations that allows us to simulate the behaviour of a spent fuel assembly under a mechanical load leading to its bending
CAST3M modelling of the rod bending was performed taking into account the elasto-plastic behaviour of the clad
The experimental results for fuel rod were satisfactorily compared to experimental data
Summary
Three points bending tests were carried out on a sixcycle fuel rod segment at the LECI Facility in Saclay: laboratory for studies on irradiated materials. When the pellets are taken into account, the experimental results on the spent rod (dark blue curve with symbols) are bounded by the calculations taking into account the two different boundary conditions between the pellets and the clad. When the bending displacement is weak, the experimental behaviour is better reproduced by taking into account a gap between the pellets and the clad (blue curve) since the cooling of the rods after irradiation has caused the opening of the gaps between the fuel fragments and between the fuel and the cladding. The fragments lock and the fuel rod behaviour is better reproduced by the merging of the point of internal clad surfaces and external pellet diameter ones (pink curve)
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