Viscoplastic behavior of a porous polycrystal with similar pore and grain sizes: application to nuclear MOX fuel materials

International audienceThis study deals with the viscoplastic behavior of a porous polycrystal with pores and grains of similar sizes.Such a microstructure can be encountered in irradiated nuclear Mixed OXide (MOX) fuel materials. MIcronizedMASter blend (MIMAS) MOX are multi-phase materials mainly composed of two or three phases dependingon their fabrication process. One of these phases corresponds to plutonium-rich agglomerates which stronglyevolve during irradiation. The large Pu-rich agglomerates become highly porous due to the accumulation offission gases and to the apparition of irradiation bubbles. In a past study, Wojtacki et al. (2020) showedthat pores distributed inside the Pu-rich clusters have a strong impact on the overall viscoplastic behaviorof MOX fuel, when considering a purely isotropic behavior for the Pu-rich clusters. In the present study,the impact of pores similar in size to the surrounding anisotropic grains on the overall viscoplastic behavioris studied in details through numerical full-field simulations. A crystal plasticity model recently developedby Portelette et al. (2018) is used to describe the anisotropic behavior of the polycrystalline matrix withdislocation glide mechanisms. Three-dimensional full-field simulations are performed by a method based onFast Fourier Transforms (FFT) to compare the behavior of porous materials with that of dense materials. Thesesimulations show that, in the case of spherical pores, their relative size with respect to that of the grains playsa minor role in the overall viscoplastic behavior. However, in the case of polyhedral pores, the relative sizeeffect is more pronounced. With fixed porosity, decreasing the relative size of the cavities with respect to thesize of grains leads to a softening of the material and a decrease of the viscoplastic flow stress