Proton diffusion in two model grain boundaries of monoclinic zirconia

To bring new insights into the mechanisms governing the hydrogen diffusion within the monoclinic zirconia layer formed on the fuel rods of nuclear pressurized water reactors, we model at the atomic scale two grain boundaries using ReaxFF, an interatomic potential. We study the behavior of protons within these two grain boundaries and find a strong proton stabilization compared to the bulk of zirconia, in accordance with the experimental observations from literature of an overconcentration in the oxide layer grain boundaries. We tackle the proton diffusion mechanisms, considering a substantial number of migration barriers. The obtained migration energies are higher in the grain boundaries than in the bulk. This indicates that the protons diffuse more slowly within the grain boundaries of monoclinic zirconia than within the bulk. Thus, the simulated grain boundaries act more as traps rather than as diffusion short circuits.