Degrading by aging of passive structures and components implied in NPP safety

The structure, systems and components of a nuclear power plant are subject to a degrading process that is unavoidable due to exposure to environmental conditions (temperature, moisture, cyclic loadings, etc). Deterioration rate depends on structural design of the components, materials employed, execution quality, preservation conditions and environmental aggressiveness. Various programs for evaluation of aging phenomenon were launched (e.g. the US-NRC / US-Nuclear Regulatory Commission). The objectives of such programs are: 1. Developing the technical bases for validation and improvement of analytical methods and acceptance criteria usable in decision making; 2. Approaching the technical aspects relating to passive structures and components ageing. A first step is identification and evaluation of the degradation events due to aging. The data obtained are introduced into a computerized database organized according to the parameters characterizing the degradation phenomena. On this basis one identifies the passive structures and components most sensitive to aging, the most frequent ageing mechanisms, the aging effects, degrading rate, and other relevant characteristics. This step is then followed by an analysis of the requirements, guides and programs launched by Regulating Organizations, of industrial programs, and of world wide gathered information concerning degradation, all these allowing the identification of the pertinent aspects of passive structures and components with virtual impact on NPP risk. Among these components on finds primarily building elements, reservoirs, anchorages, concrete structures (other than confinement buildings for which special programs exist). Regarding the renewing the licence of operation for a NPP, of greatest importance are the concrete buildings where nonexistence of cracks is a major criterion of acceptance. In conclusion, one stresses the necessity of improving the analytical methods of assessing the effects of ageing degradation upon the performances of the passive structures and components, including fragility analyses for probabilistic risk assessment. These methodologies are to be applied for quantifying the impact of degradation of passive structures and components on the Cernavoda NPP safety and for use as decision tool in the NPP re-licensing as well