Formal qualification of the various sources of an uncertainty employed in the level 2 PSA and the underlying issues

In the Level 2 PSA, a probabilistic treatment of complex phenomenological accident pathways inevitably leads to two sources of an 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. The impacts of the foregoing sources of an uncertainty on the Level 2 risk measures are different from each other, thus leading to a different conclusion in the decision-making process. An important aspect of the foregoing distinction of an uncertainty is that it plays an essential role in identifying what sources of an 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 is closely related to its importance in combining the Levels 1 and 2 uncertainties. A primary objective of this paper is to qualify formally the typical sources of an uncertainty that would often be employed in the Level 2 PSA and the underlying issues for a further clarification.