Chemistry of Waterside Oxide Layers on Pressure Tubes

Abstract

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.

SIMS and thermal evolution analysis of oxygen in Zr‐1%Nb alloy after high‐temperature transitions

“…The main techniques used for these studies are the SIMS, thermal evolution analysis (TEA), optical microscopy, microhardness and ring compression testing. Special attention will be paid to the SIMS technique, which had been successfully applied to variety of Zr‐alloy research studies 3–9…”