Probabilistic approach to ageing management of structures, systems and components important to safety

Ageing of components (the general term commonly used in PSA studies for structures, systems and components in a plant) needs to be managed effectively for assuring operational safety of the plant. A number of ageing-related degradation mechanisms such as fatigue, stress corrosion cracking, irradiation embrittlement and flow-induced corrosion and vibration can cause component failure affecting safety function, thereby increasing the plant risk (i.e. system unavailability, core damage frequency etc.) Deterministically there are detailed surveillance and maintenance programme for taking corrective action to the ageing components, as necessary for their reliable functioning. A complementary Probabilistic Safety Assessment (PSA) methodology can also be used to assess risks resulting from ageing of structures, systems and components (SSCs) and replacement, repair or overhaul strategy can be developed and action implemented as appropriate with risk informed decision approach. Probabilistic approach can be used to find out the failure probability of the component considering all possible failure modes. This is known as Probabilistic Fracture Mechanics (PFM). These failure probabilities can then be used for the calculation of the risk to the systems and plant as a whole at any point of time to determine the residual life of the component and thereby find out the ageing effect of the component. In this paper, different ageing models (to calculate the 'ageing rate') have been considered to incorporate the ageing effect of the component important to safety. Using this, a general probabilistic approach (PSA) has been addressed for evaluating ageing impacts of the component important to safety on plant risk (i.e. core damage frequency) by ranking them with importance measures. A case study for the demonstration of this probabilistic approach to the ageing management of some components important to safety has also been presented whereby suitable repair/replacement strategy has been established for acceptably very low risk to plant operation. (author)