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.