Volume 7, Issue 7 (July 2010)
High Temperature Aqueous Corrosion and Deuterium Uptake of Coupons Prepared from the Front and Back Ends of Zr-2.5Nb Pressure Tubes
Pressure tubes for CANada Deuterium Uranium (CANDU) reactors are extruded from billets of Zr-2.5Nb, at a temperature of ∼815°C, and then cold drawn to give a final length of ∼6 m. The manufacturing process often results in a variation of properties along the length of a tube including grain structure, texture, dislocation density, and phase distribution. This variation affects the mechanical and deformation properties as well as the aqueous oxidation and deuterium uptake behavior along the installed pressure tube. The orientation of the installed pressure tube in the reactor, with its axial variation of properties, is an important factor in the effective optimization of its service life. This work reports on the differences in aqueous oxidation and deuterium uptake between the extruded front- and back-end sections of a number of pressure tubes. The corrosion tests were conducted in heavy water in static autoclaves at Chalk River Laboratories and in a heavy water re-circulating loop in the Halden Boiling Water Reactor. The test conditions, such as water chemistry and temperature, were similar to those in the primary heat transport system of a CANDU reactor. The results indicate that under some exposure conditions, the deuterium uptake may be up to 40 % lower for back-end coupons compared to front-end coupons. Several microstructural factors including texture, grain size, and concentrations of alloying elements may cause the observed differences in deuterium uptake. The results will be discussed within the current mechanistic understandings of Zr-2.5Nb corrosion and deuterium ingress.