Deuterium diffusion in steam-corroded oxide layer of zirconium alloys

Abstract

In situ diffusion experiments of the hydrogen isotope deuterium in the oxide layer formed on zirconium alloys were carried out to clarify the hydrogen diffusion mechanism in the layer. The experiments were done in deuterium plasma for the temperature range from 523 to 673 K by using a nuclear reaction analysis for D( 3He,p) 4He. The zirconium alloys used were GNF-Ziron (high iron Zircaloy-2 type alloy) and VB (high iron and chromium alloy), which had been corroded in 673 K H 2O- or D 2O-steam for 10–15 days. The oxide thickness ranged from1.4 to 1.7 μm of pre-transition condition. The results showed that the steam oxides had a double-layer structure composed of the outside non-protective oxide with faster diffusivity and the inside barrier layer with slower diffusivity. The barrier layer thickness was about 0.8–0.9 μm and unchanged for the two alloys. For the in situ deuterium plasma diffusion experiments, the diffusion coefficient of deuterium in the barrier layer of GNF-Ziron was given as, D (cm 2/s) = 4.5 × 10 −13exp (−17,000/RT). The diffusion coefficient in the VB oxide at 573 K was approximately half of that in the GNF-Ziron oxide. This factor for the diffusivity was roughly consistent with their hydrogen absorption performance. For the deuterium release experiments in a vacuum subsequent to the in situ deuterium diffusion experiments, the diffusion in the barrier oxide layer was further retarded, suggesting lower diffusivity than for the case of the in situ deuterium plasma atmosphere.