Risk measures in living probabilistic safety assessment

Risk measures in various applications of probabilistic safety assessment (PSA) are defined and their use to support operational safety management of nuclear power plants is discussed. The basic risk measures represent the plant risk frequency (core damage frequency) either as a static value - nominal risk frequency or reference risk frequency - or as a plant configuration dependent, dynamic measure - instantaneous risk frequency. Generated risk measures are used in applications. The most significant application, or rather an objective of PSA, is the risk contributor identification in which risk importance measures are practical. Other applications as well as generated risk measures can be seen as advanced forms of risk contributor identification and risk importance measures, respectively. In the living use of PSA, plant status knowledge is used to represent actual plant safety status in monitoring or follow-up perspective. The PSA model must be able to express the risk given a time and plant configuration. This requires a development of time-dependent basic event models. The basic events can be divided to evident and hidden events, and the hidden events can be further divided to time-independent and time-dependent events. One problem is to develop realistic common cause failure models in this context. The applicability of the measures should be tested in order to find a practical set of quantities for the result presentation. There might be a danger that too many measures and results are presented. On the other hand, while some measures do not seem to be applicable at core damage frequency level evaluations, they might be descriptive at system and component level evaluations. The risk measures do not only characterize the plant safety status, but they reflect the structure of the PSA model. A proper use of the measures requires that decision making criteria are established. Probability or frequency based criteria are not sufficient in complex decision making situations. They might, however, give guidance or first indication about the acceptability of the decision alternative. An absolute criterion or measure is perhaps the correct quantity from the rationality point of view, but a relative criterion can be justified due to uncertainties in PSA.