Monitoring the microstructural evolution of Nd 2 Zr 2 O 7 pyrochlore during dissolution at 90 °C in 4 M HCl: Implications regarding the evaluation of the chemical durability

Understanding the dissolution mechanism at the solid/solution interface of Nd 2 Zr 2 O 7 pyrochlore is a critical step to evaluate its suitability to immobilize radionuclides. In this aim, the chemical durability of Nd 2 Zr 2 O 7 pyrochlore sintered samples in 4 M HCl was examined by several complementary techniques. Microstructural and structural changes were evidenced at the solid/solution interface. Environmental Scanning Electron Microscopy (ESEM), Atomic Force Microscopy (AFM) and X-Ray Reflectivity (XRR) showed that the dissolution occurs heterogeneously at the surface with a complex interplay of several dissolution rate contributors. The most important process observed is the chemo-mechanical corrosion of the pyrochlore as a consequence of the grain boundaries dissolution. The evolution of the crystal structure at the solid/solution interface was also 2 followed by Grazing Incidence X-Ray Diffraction (GI-XRD). It was inferred to the dissolution mechanism as a consequence of Nd and Zr occupancy of non-equivalent cationic positions in the pyrochlore structure. Regarding the chemical durability of the pyrochlore as a conditioning matrix for plutonium, minor actinides or fission products, this work demonstrates the impact of the microstructure. By increasing the grain size, and thus by decreasing the density of grains boundaries, it would be possible to limit the detachment of the grains and thus the subsequent microstructural crumbling.