Risk follow-up by probabilistic safety assessment: Experience from a Finnish pilot study

Risk follow-up by probabilistic safety assessment (PSA) provides a systematic method to analyze incidents. Events can be evaluated from the safety point of view to get feedback from operating experience, for the identification of risk contributors and for the verification of PSA models. This paper is concerned with the risk follow-up methodology, which is based on a marked point process framework. This framework provides a theoretically rigorous method for retrospective risk evaluations. Basic concepts for the modelling and an evaluation of the operating history by PSA are presented, and a Finnish pilot study on risk follow-up is summarized. We recommend the ‘total memory approach’ for the assessment of the unavailability of the standby safety systems. According to our experience, accurate evaluations with time-dependent component models are not necessarily needed but simplifying approximations can be used. The modelling of common cause failures remains a problem in practice, however, because there are not enough data to distinguish them properly.