High Temperature Aqueous Corrosion and Deuterium Uptake of Coupons Prepared from the Front and Back Ends of Zr-2.5Nb Pressure Tubes

Abstract

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.

A multi-technique study of “barrier layer” nano-porosity in Zr oxides during corrosion and hydrogen pickup using (S)TEM, TKD, APT and NanoSIMS

“…The Zr-2.5Nb samples were pressure tube material used in Canada Deuterium Uranium (CANDU) reactors. These samples are the back of Tube F from a recent study [35]. They were processed by β-quenching followed by hot extrusion, cold working and stress relief at 400ºC for 24 hours.…”

“…During the stress relieving stage, β-Zr can decompose to β-Nb. This material was exposed to heavy water with the addition of lithium oxide at 335 °C [35], a slightly lower temperature than the Zr-1.0Nb samples. The samples selected for characterisation include 150 and 1040 day samples, primarily to study by NanoSIMS the distribution of deuterium in the oxide.…”