Diffusion of H in Zircaloy-2 and Zr-2.5%Nb rolled plates between 250 °C and 350 °C by off-situ neutron imaging experiments

Abstract

Zirconium alloys in nuclear power plants operate in high-pressure water at temperatures between 250 and 350 °C. Hydrogen (or deuterium) ingress due to waterside corrosion and if the solubility is exceeded H precipitates as a brittle hydride phase. Degradation mechanisms involve the accumulation of these brittle hydrides at cold spots or crack tips, as a result of H redistribution in response to thermal and stress gradients, respectively. Knowledge of H diffusion coefficients at operating temperatures is central to evaluating the rate of hydride accumulation and crack growth velocity. We determine the diffusion coefficients of H in Zircaloy-2 and Zr-2.5%Nb rolled plates at 250 °C, 300 °C and 350 °C along the rolling and normal directions by neutron imaging experiments with sensitivity of 5 wt ppm H for a spatial resolution 0.04 mm × 2 mm. These values were evaluated from H concentration profiles measured at room temperature on specimens of dimensions 10×10×4 mm3 containing a hydride layer on one face, after annealing treatments between 60 and 600 min. This allowed the identification of a transition zone of 200–300 μm between the hydride layer and the Zr alloy material, composed by large, sparsely distributed hydrides. In Zircaloy-2 plates, no substantial differences were observed in H diffusion along different directions or metallurgical conditions, and diffusion coefficients (0.6 ± 0.1 10−10 m2/s at 300 °C). By contrast, in hot rolled Zr-2.5%Nb plates the diffusion along the rolling direction (5.5 ± 0.5 × 10−10 m2/s at 300 °C) was much faster than along the normal direction (2.5 ± 0.7 10−10 m2/s at 300 °C), very likely due to H diffusing along the continuous network of β filaments. After a thermal treatment of 3 h at 860 °C the plate microstructure changed generating radically changed H diffusion coefficients, resulting in H diffusion being much faster along the normal direction (4.0 ± 0.5 10−10 m2/s at 300 °C) than along the rolling direction (1.4 ± 0.5 10−10 m2/s at 300 °C).