Relationship between pressure tube processing, impurity/alloying element concentrations and performance in CANDU reactors

In a CANDU reactor, pressure tubes of cold-worked Zr-2.5 Nb material are used in the reactor core to contain the fuel bundles and the heavy water (D2O) coolant. Under normal operating conditions the pressure tubes are exposed to an operating environment of high fast neutron flux, D2O coolant flow rate and temperature for up to 30 years. To ensure that these tubes will perform acceptably over their design life in such an environment, their behaviour has been monitored in relation two variations in the manufacturing processes used to produce them and their alloying/impurity element concentrations. For example, elongation data are available for all tubes in most reactors, over 2000 deuterium concentration measurements are available from operating tubes, transverse strain profiles have been determined by inspection of approximately 100 tubes and mechanical properties, fracture toughness and microstructural data are available from about 50 tubes removed from reactors, the largest proportion of which were taken from the Pickering 3 and 4 Nuclear Generating Stations when these reactors were re-tubed. Evaluations of the information provided by these measurements of the characteristics of tubes that have seen power reactor service, combined with corresponding results from laboratory experiments of unirradiated pressure tube material and similar material irradiated in research reactors, have yielded relationships between the property changes and the manufacturing process and alloying/impurity concentration levels of the tubes. This information is being exploited to improve the performance of pressure tubes in present and future CANDU reactors. For example, evidence is presented to show that reductions in Cl and, to a lesser extent, P and C, substantially increase the fracture toughness after irradiation, and that increasing the Fe concentration will result in a significant reduction in the elongation rate of the tubes. Some of the relationships found in these studies have already been factored into the pressure tube specification for recent reactors while others will be included in the near future. These changes will allow the pressure tubes to reach their design lives with greater margins