In-Service Degradation and Life Extension of Nuclear Reactor Vessels: Combining Experiments and Modelling

Abstract

The reactor pressure vessel (RPV) is the primary containment in a commercial nuclear power plant and represents the first line of defense in the case of an accident. Thus, the structural integrity of the RPV, constructed of ferritic low-alloy steels, is of paramount importance to the overall safe operation of the plant. The RPV operates at relatively high pressure and temperature and is subjected to neutron irradiation. As part of an overall investigation of RPV aging, the Heavy-Section Steel Irradiation Program at Oak Ridge National Laboratory (ORNL), sponsored by the U.S. Nuclear Regulatory Commission, includes irradiation and testing of mechanical property and fracture toughness specimens, as well as a task to determine the microstructural basis for radiation-induced property changes in RPV steels. This modeling and microstructural characterization task aids in understanding and applying the results obtained from the experiments through the development of predictive models used in forecasting the aging behavior of RPVs. A series of experiments have been conducted over the past three decades to determine the effects of neutron irradiation on the fracture toughness and crack-arrest toughness of RPV steels with fracture toughness specimens up to 100 mm and 250 mm thick in the irradiated and unirradiated conditions, respectively. Furthermore, experiments have also been conducted to investigate the effects of thermal annealing on mitigation of the irradiation-induced embrittlement as well as on the effects of reirradiation. The modeling research focuses on the development of an improved description of primary damage formation in irradiated materials and the further development and use of predictive models of radiation-induced microstructural evolution and its impact on the mechanical behavior of RPV materials. Molecular dynamics cascade simulations have been extended to higher energies and temperatures and have revealed significant observations regarding defect survival, interstitial clustering dependencies and vacancy cluster formation. The microstructural characterization research focuses on the methods of atom probe-field ion microscopy (APFIM) and small-angle neutron scattering (SANS) to determine the matrix copper content and the chemical composition of radiation-induced precipitates. The new ORNL energy-compensated optical position-sensitive atom probe has been used for atom probe tomography (APT) studies of irradiated and thermally aged RPV welds. The results from SANS experiments are compared with those of the APFIM and APT investigations to provide detailed characterization of the features responsible for the observed changes in mechanical properties. Results from the modeling studies and the experiments are compared so that the models can be evaluated for modification and experiments can be planned with a view to providing the information needed for continued evolution of the models.