Formal handling of the level 2 uncertainty sources and their combination with the level 1 PSA uncertainties

As an essential part of the Level 2 PSA, a probabilistic treatment of complex phenomenological accident pathways inevitably leads to two sources of uncertainty: (a) an incomplete modeling of these accident pathways and their subsequent impacts on the Level 2 risk, and (b) an expert-to-expert variation in the their probabilistic estimates. While the former type of uncertainty is epistemic in nature from the viewpoint that we deal with an uncertainty addressed in the deterministic events, the latter type is a random/aleatory uncertainty. The impacts of the preceding sources of uncertainty on the Level 2 risk measures are different for each other, thus leading to a different conclusion in the decision-making process. An important aspect of the foregoing distinction of uncertainty is that it plays an essential role in identifying what sources of uncertainty impact more on the Level 2 risk and what sources among them should be focused on for a greater reduction of the overall Level 2 uncertainty. Another aspect of the importance is closely related to its role in combining the Level 1 and Level 2 uncertainties. A primary objective of this paper is to explore the aforementioned sources of the Level 2 uncertainty and provide the corresponding approaches for handling them formally. An additional purpose is to provide an approach for combining consistently aleatory uncertainties addressed in the Level 1 PSA with the Level 2 epistemic uncertainties, so that the Level 1 and 2 uncertainties are finally represented as an integrated measure. (authors)