Deuterium diffusion in oxide layers of Zr-2.5Nb alloy

摘要

In situ diffusion measurements of the hydrogen isotope deuterium in the oxide layers formed on Zr-2.5Nb alloy have been carried out at 523 and 573 K by using a combined technique of deuterium plasma exposure and nuclear reaction analysis for the D(3He,p)~4He reaction. The oxide layers were prepared in two environments, 1 M LiOH-containing water at 563 K and steam at 673 K, and their thicknesses ranged from 1.6 to 1.9 μm. The deuterium profile evolution in the oxides showed a combined process of absorption and dissolution in the surface region, and subsequent bulk diffusion in the deeper region. The diffusion coefficients of deuterium were evaluated for the two formation environments from the transient deuterium profiles of the oxide layers. The diffusion coefficients in the LiOH-water oxide were significantly larger than the values in the steam oxide. Compared with previously obtained data for three kinds of Zircaloy-type alloys (Zry-2, GNF-Ziron and VB), both of the Zr-2.5Nb oxides possessed the smallest diffusivities among the four alloys. Moreover, the surface concentrations of deuterium in the Zr-2.5Nb oxides were distinctly lower than those in the other Zircaloy-type alloy oxides. The superior hydrogen absorption performance of Zr-2.5Nb alloy observed in the out-of-pile corrosion tests was attributed to the smaller diffusivity and the smaller concentration gradient. The mechanism for the lower hydrogen diffusion flux of Zr-2.5Nb alloy was discussed in terms of the dissolution effect of niobium with variable valences of Nb~(2+)-Nb~(5+) from the β-Nb precipitates into the ZrO_2 lattice.