Removal of Hydrogen from Rolled Joints in CANDU Reactors by Yttrium Getters

Abstract

In the core of CANDU (CANada Deuterium Uranium) nuclear power reactors, the fuel and heavy-water heat transfer fluid are contained within Zr-2.5Nb alloy tubes. Outside the core, the heat transfer fluid is carried by steel piping. The connections between the external and internal piping are rolled joints, made by rolling the Zr-2.5Nb tubing into 403 stainless steel hubs. Although the structural integrity of these rolled joints has been excellent, it has been found that hydrogen pickup in the Zr-2.5Nb tubes at the rolled joints is significantly larger than in the remainder of the tube during reactor operation. This buildup of hydrogen at the rolled joints results in the precipitation of brittle zirconium hydrides when the terminal solid solubility for hydrogen is exceeded in the Zr-2.5Nb alloy. At the measured hydrogen buildup rates these hydrides do not pose a problem for the integrity of the rolled joints. However, hydrogen from the rolled joints diffuses inboard along the pressure tubes to the region of the burnish marks at the end of the rolling. Here, the combination of the brittle hydrides, formed when terminal solid solubility is exceeded, and the residual circumferential tensile stresses from the rolling operation can result in cracking of the pressure tubes by the delayed hydride cracking mechanism. The occurrence of such cracks may require replacement of a pressure tube. One approach to prevent the hydrogen concentration from exceeding terminal solid solubility at the burnish mark is to use yttrium to getter hydrogen from the pressure tube. Yttrium has a higher chemical affinity for hydrogen than does zirconium. When the two metals are in metallurgical contact hydrogen diffuses from the zirconium to the yttrium until the hydrogen concentrations in the two metals are in chemical equilibrium. To protect the rolled joint region against hydrogen buildup, an yttrium ring, encapsulated in zirconium alloy, is welded to the end of the pressure tube prior to making the rolled joint. At this position both theoretical calculations and experimental measurements have shown that the yttrium, by gettering hydrogen from the pressure tube, should prevent the hydrogen terminal solid solubility from being exceeded at the burnish mark for up to 30 years of reactor operation.