Hydrogen Transport in the Oxide and Hydrogen Pickup by the Metal During Out- and In-Reactor Corrosion of Zr-2.5Nb Pressure Tube Material

Abstract

A knowledge of the relation between hydrogen in the corrosion film and hydrogen pickup by the metal is important for understanding the hydriding behavior of zirconium alloys. The results obtained from isotope (deuterium, lithium, and boron) incorporation and exchange studies on oxides grown out- and in-reactor on Zr-2.5wt%Nb alloy are presented here. It is shown, using fourier transform infrared (FTIR) spectroscopy and secondary ion mass spectrometry (SIMS), that hydrogen in the corrosion film exists as surface hydroxyls and adsorbed water on oxide crystallites. These hydrated pathways on the oxide crystallite surfaces are random in occurrence, resulting from the nucleation and growth of oxide crystallites. The hydrogen ingress sites are the terminals of the pathways, and hydrogen pickup by the metal is a localized phenomenon, controlled by electron transfer at the oxide-alloy interface. Hydrogen in the bulk of the oxide exists as surface hydroxyls and adsorbed water in the pathways, which serve as fast transport routes for protons, lithium, and boron. Post-irradiation examinations on hundreds of pressure tubes removed during the Pickering Unit 3 large scale fuel channel replacement (P3LSFCR)-project included FTIR measurements. The pressure tubes were in service for 13.4 effective full power years (EFPY) of operation, and corrosion for the most part had been in the so-called post-transition regime of linear oxide growth. The spectra of oxide on the inside surfaces of the tubes near the outlets showed an absorption peak at 2600 cm-1, which is characteristic of deuteroxyl stretching vibration and hence deuterium in the oxide. The intensity of the absorption peak is found to be proportional to the oxide thickness, suggesting that the transport of deuterium through the bulk oxide had been similar for the oxides with different thermal and irradiation histories. However, the histograms of percent pickup for the axial locations 4.69 to 5.85 m near the outlet were asymmetric with slightly more than half of the channels showing 3 to 7% and the rest showing a wide range up to 40% pickup. Such a wide range in pickup cannot simply be attributed to the inherent property of the oxide-alloy interface. It is proposed that the corrosion of the Zr-2.5Nb alloy is normally associated with the generation of deuterium ingress sites whose density and capacity for pickup do not vary much with oxide thickness, i.e., the pickup shows a reasonable proportionality, varying over a small range of 3 to 7%, to oxide thickness. In the case of some tubes, other processes contribute to additional deuterium pickup. A comparison of the deuterium pickup at different axial locations indicates that the additional pickup is likely to be material related. The results from preliminary investigations on microstructural aspects suggest impurity inclusions as one of the main variables among offcuts from tubes showing high and low deuterium pickups. On the coolant side, the inclusions may function as localized windows for the entry of radiolytic deuterium produced in the hydrated pathways. On the annulus side, the inclusions may behave as defects or weak spots in the oxide, growing in nitrogen containing low partial pressures of water and deuterium on the outside surface. Additional slow deuteriding may occur from deuterium entry at the inclusion sites through the degraded oxide. The percent pickup for these tubes is likely to have been higher than the normal 3 to 7%, almost throughout their life in service.

A multi-technique study of “barrier layer” nano-porosity in Zr oxides during corrosion and hydrogen pickup using (S)TEM, TKD, APT and NanoSIMS

“…Although very limited experimental data has been published about the observation of pores via APT, the feature in Figure 9(b) is likely to be a pore contains significant amount of D that is probably in gaseous form or D-containing species chemisorbed on the surfaces of the pore, consistent with other reports [47,48]. In Analysis 2, next to the high density line that has been interpreted as a grain boundary, there is a highly localised region with very high atomic density (Figure 9 (b)), corresponding to the concentration of D-containing ions at 2Da and 18Da.…”

3D-characterization of deuterium distributions in zirconium oxide scale using high-resolution SIMS

“…4. With this detailed understanding of the experimental conditions, we have been able to show that the deuterium is highly concentrated in the fossil oxide near the water/oxide interface, and there is a gradual decrease in trapped deuterium in the denser oxide towards the oxide/metal interface with abrupt peaks in 2 Dsignal that are evidence for local trapping sites like small cracks and porosity [13,14] linked by diffusion paths through which the deuterium can penetrate through to the metal matrix.…”

“…Second Phase Particles (SPPs) could provide a diffusion pathway [9][10][11], although they are usually too well separated to offer a continuous permeation path. Cracks and pores [12], interconnected porosity [13,14] and oxide grain boundaries [15,16] have all been described as preferential paths for both hydrogenic and oxidising species to reach the reacting metal surface. However, there is a lack of direct experimental data, especially on the 3D hydrogen distribution in the oxide, to support these mechanisms, partly because of the experimental difficulty of mapping hydrogen isotopes with high spatial resolution.…”

Change of chemical states of niobium in the oxide layer of zirconium–niobium alloys with oxide growth

“…We also found that the hydrogen diffusivity in the barrier layer increased when the hydrogen pickup fraction increased near the kinetic transition in the oxide layer of the Zry-2 type alloy [19]. The relationship between the barrierness of the oxide layers and the hydrogen diffusivity has been also pointed out in some studies [20][21][22][23], so this kind of hydrogen diffusion measurements on Zr-2.5Nb will discover the relationship between the barrierness to oxidation and that to hydrogen pickup.…”