Aging Management of Nuclear Power Plant Reactor Internals

As U.S nuclear power plants seek to extend operation beyond their 40 year license, aging management of reactor vessel internals is of prime concern for continued safe and reliable plant operation. A framework for aging management has been developed by screening all components for potential degradation, performing detailed finite element analyses of critical components, identifying components that are most likely to fail, and developing a strategy to monitor and inspect these components. Aging of reactor internals components can be attributed to specific damage mechanisms that include stress corrosion cracking, irradiation-associated stress corrosion cracking, wear, fatigue, thermal embrittlement, irradiation embrittlement, void swelling, irradiation-induced stress relaxation and irradiation creep. An understanding of the relationships among these damage mechanisms is the basis for developing a tiered approach to inspection and evaluation, i.e., evaluation of primary components with the earliest expected manifestations of damage followed, if necessary, by evaluation of expansion components that have been categorized to be subsequently susceptible to similar damage mechanisms. Sixty-year aging finite element simulations are conducted on the most highly irradiated components to identify trends in the predicted component behavior. They are used to develop broad-based recommendations for inspection and evaluation by considering the complex interactions between the structure, loading, and aging degradation in the reactor internal components. The results can be used to identify critical locations within the structure and indicate the time frame and scale of degradation in the component. This paper examines the aging management strategy, the detailed finite element analyses of highly irradiated components, the categorization of inspection components, and the inspection recommendations for life extension beyond 40 years for the Westinghouse and CE-designed reactor internals.