While a strict PDS operation requires that all the Level 1 ET core damage sequences are assigned to all the relevant PDS categories and that their sum is equal to the total CDF, the current practices of the Level 1 ET/FT combinations causes an inevitable approximation of the total PDS frequency (PDSF) for the total CDF value. The main objective of this paper is to estimate the type of uncertainty bound generated for the total PDSF (or a degree of a PDSF approximation for the CDF) due to a propagation of the Level 1 ET sequence cut sets into the relevant PDSs, consequently leading to a change of the corresponding Level 2 risk metrics (such as containment failure frequencies or large early release frequency). The plant-specific analysis shows that when the cutoff value of 1.0E-11 is applied, the PDS approximation from the Level 1/2-bridge trees may vary the PDSF value from -1.2% to 2.5%, when compared to the corresponding CDF value (i.e., a CDF value estimated from the Level 1 ET/FT models). A detailed investigation shows that the forgoing uncertainty is mainly caused by a mixed impact of a decrease of the PDSF from a truncation limit and its increase from the potential extra cut sets employed in the PDS model when the Level 1 cut sets are propagated into the PDS model (subsequently the other Level 2 models).
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