Deuterium Ingress at Rolled Joints in CAmU Reactors

Abstract

In CANDU nuclear reactors, the fuel bundles are contained in a lattice of horizontal Zr-2.5Nb pressure tubes. The pressure tubes, through which the heavy water coolant passes, are connected to out-of-core piping by roll expanding them into 403 stainless steel end fittings. The deuterium ingress into the pressure tube at the rolled joint is enhanced relative to the ingress in the main body of the tube. Because of the potentially deleterious effects of excessive deuterium buildup in the pressure tube, an understanding of the mechanisms of deuterium ingress at the rolled joint and a capability to predict deuterium buildup in the pressure tube is essential. Deuterium ingress at rolled joints depends on a number of factors including: 1) the characteristics of the crevice between the end fitting and pressure tube, 2) chemical reactions in the crevice, 3) diffusion properties of deuterium in 403 steel, Zr-2.5Nb tubing and their oxides, and 4) mechanical aspects of the rolling process. When a joint is rolled, some of the oxide on the outside surface of the pressure tube is removed and areas of metal-to-metal contact with end fittings are created. During reactor operation, the oxide damage is repaired except at these contact areas. The cathodic deuterium released during oxidation reactions in the crevice is the primary source for ingress; these reactions are enhanced both by galvanic coupling and unique crevice chemistry effects. Early in the life of the rolled joint, while oxide repair is occurring, deuterium can enter the pressure tube directly through unrepaired areas in the oxide. When only areas of intimate contact remain unrepaired, deuterium ingress occurs predominantly by diffusion through the end fitting to these contact areas. A diffusional model was developed to predict deuterium buildup in the rolled joint region of pressure tubes. The model incorporates deuterium ingress at the rolled joint as well as pickup along the pressure tube due to corrosion on the coolant side and takes into account hydride precipitation effects. Deuterium analyses in pressure tube ends from 35 rolled joints removed from power reactors are used to determine deuterium ingress rates at rolled joints. The results, which show that the ingress rate is declining with time, are extrapolated to predict deuterium buildup in the pressure tubes still in the reactors.