Development of Novel Small Scale Mechanical Tests to Assess the Mechanical Properties of Ex-Service Inconel X-750 CANDU Reactor Component

Inconel X-750 is a precipitation hardened nickel superalloy designed to have high strength and creep resistance at elevated temperatures. In Canada Deuterium Uranium (CANDU) reactors, this material serves as tight fitting fuel channel annulus spacers in the form of springs separating the cold calandria tube from the hot pressure tube. Unlike light water reactors (LWRs), CANDU reactors have an elevated thermal neutron flux spectrum which amplifies nickel transmutation reactions, producing more radiation damage and increased amounts of internal hydrogen and helium compared to all other current generation reactors. Bulk component testing performed over the last few years indicates that these spacers are losing ductility and strength after time in service, and that the quantity of this loss is dependent on the irradiation temperature and dose. In addition, observations of fracture surfaces reveal intergranular failure and transmission electron microscopy (TEM) shows that helium bubbles preferentially align along grain boundaries. However, a dearth of knowledge on the mechanical properties of these components at high dose exists because only one to three component tests are performed at each irradiation condition inside hot cells, and these component tests produce complicated stress states, making the extraction of yield stresses and failure stresses challenging. Thus, two first of their kind, in-situ, small scale mechanical tests (SSMTs) employing scanning electron microscopy (SEM) and focused ion beam (FIB) techniques were developed to test components irradiated to doses of 53, 67, and 81 dpa in CANDU reactors at average irradiation temperatures of 180 °C and 300 °C. The first, a lift-out, three-point bend test quantified the yield stresses of the components as a function of irradiation temperature and dose. Material irradiated at the higher temperature undergoes significant yield strength increases up to 1 GPa, whereas this is negligible for material irradiated at the lower temperature (≤ 310 MPa). Grain boundary cracking after yielding was observed in specimens irradiated to 67 dpa at 180 °C. The second new SSMT is a push-to-pull, micro-tensile test quantifying the yield strengths, failure strengths, and total elongations of the components as a function of irradiation temperature and dose. For specimens irradiated to 81 dpa, intergranular failure was directly observed, so failure stresses quantify grain boundary strengths. Both novel SSMTs also revealed significant regional differences in mechanical properties between the inner diameter, center, and outer diameter due to variations in the cold working and grinding of the component that went undetected in bulk component testing. The conclusion of this work is a detailed analysis of the degradation of an Inconel X-750 CANDU component in terms of its quantitative mechanical properties through the invention of two novel SSMTs. These new testing methods and analyses are applicable for assessing the mechanical ageing of other reactor core components.