The corrosion mechanisms of zirconium alloys in pressurised water reactors are still a subject of debate more than half a century after the first research on this material. The literature reports two distinct mechanisms for the transport of diffusing species in oxide layers: one favours the molecular diffusion of oxygen and hydrogen through interconnected nanopore channels during the pre-transient regime, while the other favours diffusion via short circuits (grain boundaries, etc.) in the oxide layer. In the latter case, the oxide layer is considered to be relatively homogeneous and impermeable to the oxidising medium, in this case the water in the primary circuit. On the other hand, the first interpretation is based on the principle that there is a layer that is permeable to the medium due to an interconnected network of nanopores, even during the pre-transient regime, with the density of percolated nanopores increasing over time.
Technically speaking, how can we decide between these two divergent interpretations in terms of the diffusion mechanism, which consequently leads to different solutions for protection against degradation? What is the reaction mechanism that ultimately leads to the hydration of Zr alloys and their oxidation?
To address this challenge, we will explore diffusion processes by studying the dissociation-recombination rates of molecular species at different temperatures in equi-isotopic gas mixtures such as H2/D2, 18O2/16O2, H218O/D216O, H218O/D2, etc., using an experimental device equipped with a mass spectrometer that tracks the molecular species of interest in real time.