ISSUES RELATED TO STRUCTURAL AGING IN PROBABILISTIC RISK ASSESSMENT OF NUCLEAR-POWER-PLANTS

Structural components and systems have an important safety function in nuclear power plants. Although they are essentially passive under nor mal operating conditions, they play a key role in mitigating the impac t of extreme environmental events such as earthquakes, winds, fire and floods on plant safety. Moreover, the importance of structural compon ents and systems in accident mitigation is amplified by common-cause e ffects. Reinforced concrete structural components and systems in NPPs are subject to a phenomenon known as aging, leading to time-dependent changes in strength and stiffness that may impact their ability to wit hstand various challenges during their service lives from operation, t he environment and accidents. Time-dependent changes in structural pro perties as well as challenges to the system are random in nature. Acco rdingly, condition assessment of existing structures should be perform ed within a probabilistic framework. The mathematical formalism of a p robabilistic risk assessment (PRA) provides a means for identifying ag ing structural components that may play a significant role in mitigati ng plant risk. Structural condition assessments supporting a decision regarding continued service can be rendered more efficient if guided b y the logic of a PRA. (C) 1998 Elsevier Science Ltd. All rights reserv ed.