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The pressure tubes that contain the fuel bundles and the primary coolant within the core of a CANDU heavy-water reactor are fabricated from a Zr-2.5Nb alloy with a complex microstructure. During reactor operation the pressure-tube interior surface is slowly oxidized by heavy water and a fraction of the deuterium that is released through this process enters the underlying alloy and can reduce its fracture toughness. Considerable variability in deuterium ingress has been observed among the pressure tubes within a single reactor as well as between different reactors. These differences are thought to be due not only to metallurgical variables, such as alloy microstructure and composition, but also to variations in the primary coolant chemistry, including pH and dissolved impurities. In the present study, a combination of surface analytical methods has been employed to characterize the microchemistry and oxidation history of the waterside oxide layers grown on two pressure tubes that were removed from different CANDU reactors. The presence of varying concentrations of iron, manganese and uranium, derived mainly from corrosion of feeder pipes and fuel failures, has been found throughout the oxide layers. An increase in the oxidation rate of one pressure tube later in life could be correlated with evidence of greater open porosity, provided by the depth distributions of impurities, extending deep within the oxide layer.
The pressure tubes that contain the fuel bundles and the primary coolant within the core of a CANDU heavy-water reactor are fabricated from a Zr-2.5Nb alloy with a complex microstructure. During reactor operation the pressure-tube interior surface is slowly oxidized by heavy water and a fraction of the deuterium that is released through this process enters the underlying alloy and can reduce its fracture toughness. Considerable variability in deuterium ingress has been observed among the pressure tubes within a single reactor as well as between different reactors. These differences are thought to be due not only to metallurgical variables, such as alloy microstructure and composition, but also to variations in the primary coolant chemistry, including pH and dissolved impurities. In the present study, a combination of surface analytical methods has been employed to characterize the microchemistry and oxidation history of the waterside oxide layers grown on two pressure tubes that were removed from different CANDU reactors. The presence of varying concentrations of iron, manganese and uranium, derived mainly from corrosion of feeder pipes and fuel failures, has been found throughout the oxide layers. An increase in the oxidation rate of one pressure tube later in life could be correlated with evidence of greater open porosity, provided by the depth distributions of impurities, extending deep within the oxide layer.
The pressure boundary of a CANDU® fuel channel is composed of a cold-worked Zr-2.5Nb pressure tube, which has each end rolled into a stainless-steel end fitting. Heavy-water (D2O) coolant (250–310°C) flows over and through twelve or thirteen fuel bundles contained in each pressure tube. During operation, some deuterium generated by aqueous corrosion of the tube surface enters the metal. Additional deuterium also enters through the rolled joint between the tube and the end fitting. Predictive models for deuterium ingress are required for fitness-for-service assessments for operating pressure tubes and for the development of new reactor designs. A predictive model for assessing the long-term oxidation of, and deuterium ingress into, the body of the pressure tubes has been developed from in-reactor tests of samples which had been pre-oxidized to obtain oxide thickness values representative of long-term behavior. Deuterium ingress is modeled based on a fraction (2–10 %) of the corrosion-freed deuterium entering the metal. The current version of the model contains relationships describing the oxidation rate as a function of oxide thickness, temperature, concentration of dissolved oxygen in the water, and fast neutron flux and fluence. It can successfully predict the observed deuterium-uptake history of pressure tubes in existing CANDU reactors. The model projects a slight increase in the rate of oxidation and deuterium ingress over time. This increase is much less than for Zircaloy-2, a material used in early CANDU units. In parallel with model development, there are experimental programs involving detailed surface analysis of removed pressure tubes and irradiation tests focused on elucidating the mechanisms of oxidation and deuterium ingress. As the results of these programs become available, they will be incorporated into the predictive model. This presentation will focus on the model and recent results from the supporting experimental programs.
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.
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