Zircaloy is commonly used for the cladding or pressure tubes in commercial nuclear reactors because of its strength, corrosion resistance, and low absorption of thermal neutrons. Fracture toughness test techniques using small samples fabricated from archival materials from the N Reactor pressure tubes of Zircaloy-2 were developed to study the factors affecting tube fracture toughness. Compact tension specimen thickness was limited by the wall thickness (7 mm) of the tubes. Specimens (5 mm thick) were prepared for fracture toughness testing, and results were analyzed using the J-integral approach. To reduce the high cost of irradiated specimen testing and to more easily precrack specimens remotely, single-specimen potential drop techniques were employed to evaluate the fracture toughness of Zircaloy-2.
The initiation fracture toughness was determined from J-R curves, which were constructed by plotting values of J as a function of crack extension computed from the electric-potential calibration curve. The J-R curves obtained from the calibration curve equation fit the initial portion of the blunting line and heat-tint data.
Results showed that at room temperature, the toughness of unirradiated N Reactor pressure Tube 1054 was the lowest and that of Tube 2755 was the highest. As the temperature was increased the fracture toughness decreased slightly, while the tearing modulus increased with temperature. The fracture resistance in the circumferential orientation was inferior to that for the longitudinal orientation. The effects of neutron fluence and hydrogen content on the fracture toughness of N Reactor pressure tubes were evaluated. Neutron irradiation substantially degraded fracture toughness. Increasing fluence decreased the fracture toughness of the alloy. Hydrogen also decreased fracture toughness, but this effect was insignificant for the pressure tubes tested.