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.
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.
Hydrogen migration has been followed through the thermally-grown oxide on a Zr-2.5% Nb alloy. A secondary ion mass spectrometer, calibrated for deuterium, was used to measure the concentration as a function of depth into the film. Thin flm oxide specimens, grown in steam to -1 pm thickness, were heated to 350°C and exposed to deuterium gas at pressures ranging from 6 x lo-' to 6 Pa and times from 30 min to 870 min. Some irreversible uptake was detected for all exposures using SIMS. At low exposures, the shape of the deuterium concentration profile is Fickian and diffusion coefficients have been calculated. In this lowexposure regime, the effect of temperatures between 280°C and 350°C on the dimion equation has been measured. At longer exposures, the rate of deuterium ingress was sharply curtailed and a more complex difhrsion profile was obse~ed. Using Raman spectre scopy during a depth profile, the ZrO, component of the oxide is found to change from a monoclinictetragonal mixture at the oxide surface to a primarily tetragonal oxide near the oxidelmetal interface. Such changes in phase probably contribute to the complex deuterium diffusion kinetics observed during the longer exposures of the oxide film to deuterium gas.
The microstructure of Zr-2.5Nb nuclear reactor pressure tube material has been studied using electron microscopy. X-ray diffraction, and energy dispersive X-ray spectroscopy techniques. The materials studied contained a range of impurity contents with iron ranging from 300 to 1500 ppm in both the cold-worked and cold-worked + annealed (24 h/560°C) conditions. Cold-worked tubes consisted of deformed α-Zr grains with a network of β-Zr (∼20wt%Nb) surrounding the α-Zr grains. Annealing produced a decomposition of the β-Zr phase into α-Zr and β-Nb (∼83 wt% Nb). Cold-worked tubing with Fe contents ⩾500 ppm also contained a very small volume fraction of Zr2Fe precipitates having a tetragonal structure, β-quenching during fabrication of the tubing appeared to suppress the precipitation of Zi2Fe. On annealing, both Zr(FeCr)2 and Zr2Fe precipitates were formed. The Zr(FeCr)2 precipitates had either the C-15 cubic or C-14 hexagonal Laves phase structure. It is proposed that on annealing at 560°C the β-Zr phase (containing Fe and Cr in solution) transforms to α-Zr + β-Nb + Zr(FeCr)2 phases.
Understanding the mechanisms of hydrogen ingress into pressure tubes fabricated from zirconium-25% (w/w) niobium alloy requires knowledge of the hydrogen concentration in the surface oxides, of the oxidelmetal interface and in the alloy phase beneath the interface. Secondary ion mass spectrometry (SIMS) has attractive capabilities for detecting hydrogen isotopes in such surface films, but its quantitative response and spatial resolution are controversial for hydrogen because of the strong tendency for the element to migrate, owing to thermal and sputtering effects. High sputter rate conditions have been used here, which result in an improved capability to detect hydrogen in Zr-Nb alloy and ZrO, down to concentrations of < 2 x lo'' atoms cm-' (~0 . 1 ppm, w/w). Quantitation of deuterium concentrations has been accomplished by SIMS calibrated using ion implants and, in some cases, verified by nuclear reaction analysis (NRA).
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