A Study of the Hydrogen Uptake Mechanism in Zirconium Alloys

Hydrogen uptake in zirconium alloy CANDU (CANada Deuterium Uranium) pressure tubes and other core components is controlled by the rate of transport of atomic/ionic species across the oxide film. The importance of understanding the mechanism of transport stems from the need to predict and control the rate of uptake. Samples of Zr-2.5Nb and Zircaloy-2 were prefilmed in steam (H2O, 400°C at ̃2 MPa) and subsequently exposed to D2O (10-3 Pa to ̃ 18 MPa) and D2 (̃10-3 Pa) at a temperature range of 250 to 380°C in the laboratory. Samples from Zr-2.5Nb pressure tubes removed from CANDU power reactors were also examined. Hydrogen mobility in oxides was investigated by secondary ion mass spectroscopy (SIMS) following these exposures. Diffusional-type through-oxide-thickness deuterium profiles have been observed adjacent to the oxide-metal interface for samples exposed to environments containing D2O for 4 h out-reactor and up to ̃10 years in-reactor. These profiles probably represent the density of accessible sites on surfaces of intergranular porosity through-thickness. Although, in small regions observed by transmission electron microscopy (TEM) such porosity has not been found. Nevertheless, from observations of grain size, sufficient sites would be available to produce deuterium concentration observed near oxide surfaces. The observed deuterium concentration profiles appear to result predominantly from deuteroxyl groups bonded to such sites. Deuterium content at the oxide-metal interface provides an indication of the extent of interfacial intergranular porosity. High deuterium contents at the interface may imply local regions with absent oxide barrier at the interface. In the presence of sufficient D2O, the oxide is continually healed, and deuterium uptake is relatively low where short-circuit routes such as intermetallics in Zircaloy-2 are not present. In environments with relatively high D2:D2O ratios, deuterium atoms may diffuse through the oxide to the interface and react directly with the metal resulting in high deuterium uptake rates. It is proposed that observed deuterium profiles may be the sum of mainly two components. The predominant component is due to deuteroxyl groups residing on accessible sites on surfaces of intergranular porosity with no direct link to hydrogen uptake by the bulk alloy. The second masked component would be due to another mobile hydrogen species (for example, H) that is diffusing to the bulk alloy. Further work is needed to substantiate the proposed hypothesis that would include exposures with varying D2:D2O ratios and further TEM examination.