Migration of hydrogen through thin films of ZrO₂ on Zr–Nb alloy

The barrier properties of ZrO₂ to inward migration of deuterium have been investigated with a view to understanding the hydriding mechanisms of a Zr‐2.5% Nb alloy used in CANDU nuclear reactors fuel channels. Thin film oxide specimens, grown in steam to ∼1 μm thickness, have been heated to 350 °C and exposed to deuterium gas at pressures ranging from 6×10⁻³ Pa to 101 kPa (1 atm) and times from 10 to 810 min. Some irreversible uptake can be measured for all exposures using secondary ion mass spectrometry. At low exposures, the shape of the deuterium concentration profile is can be fitted to a Fickian relationship. During longer exposures, the rate of deuterium ingress is sharply curtailed, presumably due to passivated outer oxide surface. Reactions between D₂O vapor and the thin film oxide in the 10⁻³ Pa pressure region and above show a sharply higher uptake of deuterium than in the equivalent pressure of D₂ gas. This is ascribed to a more efficient decomposition of D₂O on the ZrO₂ surface compared to D₂.