Oxide Characteristics and Corrosion and Hydrogen Uptake in Zr-2.5 Nb CANDU Pressure Tubes
Authors: Warr BD , Van Der Heide PAW , Maguire MA 
Published: March 2009

Oxides on removed pressure tubes from Pickering Unit 3 after 13.4 effective full power years (EFPY) have been examined to investigate the cause of variability in bulk alloy deuterium contents in outlet regions in order to improve predictions and minimize deuterium uptake in operating CANDU reactors. Secondary ion mass spectroscopy (SIMS) and electrochemical impedance spectrometry (EIS) were used for characterization with minimal sample preparation and modification. Two SIMS techniques were used for quantification: (1) the relative sensitivity factor (RSF) method, which requires a reference material and is subject to matrix effects as a result of variation in the secondary ion intensities of a species when different materials are sputtered; and (2) the SIMS infinite velocity (IV) method, which circumvents matrix effects by extrapolating all secondary ion intensity data to infinite velocity. A novel 13C oxide dating technique was used to determine oxide growth kinetics and ensure that oxide spalling had not occurred in the regions examined.

Pressure tubes with high bulk alloy deuterium contents showed characteristics near the metaloxide interface in inside surface oxides that were not present in oxides on tubes with low deuterium contents. In samples with high bulk alloy deuterium content, the inside surface corrosion rate, determined by the 13C dating method, may have increased from ∼0.3 to 1 μm/EFPY about five years before tube removal. A constant rate of corrosion was inferred in samples with low deuterium contents. The inner regions of inside surface oxides in tubes with high deuterium contents, corresponding to the faster growing oxide (up to ∼5 μm from the interface) showed relatively higher porosity (inferred from the 2H profile) and almost constant levels of lithium. These oxides also showed a low value of the electrical resistance term in one of the EIS responses, which has been interpreted as being due to the presence of a larger number of water penetration routes.

In order to investigate possible matrix effects in relatively thick inside surface oxides, through-oxide thickness concentration profiles for 2H and 12C, obtained by SIMS RSF and IV methods, were compared. Reasonable agreement was obtained between these methods for 2H concentration profiles. However, evidence of a significant matrix effect for 12C quantification, up to 5 μm from the inside surface metal-oxide interface, was found in tubes with high bulk alloy deuterium uptake. Further work is required to understand the reason for this matrix effect and its implications with respect to SIMS RSF quantification for other elements and analysis of excess 13C profiles.

Outside surface oxides generally showed similar characteristics for all tubes. Very low constant rates of corrosion of 0.1 μm/EFPY were inferred from excess 13C profiles. Apparent substoichiometry (O:Zr ∼ 1) was found by SIMS IV analysis in outside surface oxides on a tube with high deuterium content that may be related to breakdown of the efficacy of the oxide as a deuterium permeation barrier. Thus, although present results correlate deuterium uptake with inside surface corrosion effects, a contribution from the gas annulus cannot be ruled out.