Assessment of Adequacy of Upper-Shelf Fracture Toughness Model for Zr-2.5Nb Pressure Tubes for Fitness-For-Service Evaluations

Abstract The fracture toughness of Zr-2.5%Nb pressure tubes in CANDU®1 reactors is an important material property for evaluation of protection against fracture and demonstration of leak-before-break. A probabilistic predictive model for fracture toughness of irradiated pressure tubes in the upper-shelf temperature regime was developed on the basis of material surveillance results. The model was subsequently incorporated into the Canadian Standards Association (CSA) Standard N285.8 as the reference model in fitness-for-service evaluations of pressure tubes that use the upper-shelf fracture toughness as an input. As required by CSA Standard N285.4, the model is to be periodically reviewed with material surveillance results of upper-shelf fracture toughness to evaluate its adequacy as a representative model. New burst-test data for upper-shelf fracture toughness have been obtained from ex-service surveillance pressure tubes and other experimental programs since the upper-shelf fracture toughness model was developed. Assessment of the adequacy of the upper-shelf fracture toughness model for Zr-2.5%Nb pressure tubes for continued use in fitness-for-service evaluations was performed, as documented in this paper.