Consideration of ageing within probabilistic safety assessment models and results

Ageing is a process, where the properties of systems and processes may degrade through the time and age. The objective of the paper is to analyze the possibilities of introduction of ageing directly into the probabilistic safety assessment. Theoretical models of ageing and examples of their application are investigated, which show, how the current models can be upgraded in sense to directly include the effects of ageing into the models of probabilistic safety assessment. This paper shows that consideration of ageing is theoretically a straightforward issue, which can be dealt with in several ways, which are demonstrated in the paper. The most important problem is the lack of data about the effects of ageing, which would suit to the detailed models of ageing. The results, which are obtained, do not show significantly different results compared to the initial models without direct consideration of ageing. The majority of risk measures change slightly or notably, but no major break through has been found. Contribution of ageing may be smaller then contribution of changes due to other parameters. (orig.)