As single-unit PSA (SUPSA) calculates single-unit core damage frequency (SUCDF), multi-unit PSA (MUPSA) calculates site core damage frequency (SCDF) and multi-unit core damage frequency (MUCDF). In this paper, modeling and quantification uncertainties in internal, seismic, and level 3 MUPSAs are discussed, and modeling and quantification methods to minimize these uncertainties are proposed.
Very careful attention and consideration are required to minimize the modeling and quantification uncertainties in MUPSA. Negate modeling, delete-term approximation (DTA), and rare event approximation (REA) affect the modeling and quantification uncertainties in MUPSA. If very little attention is paid to them, inaccurate or very conservative SCDF or MUCDF will be calculated.
SCDF and MUCDF can be highly overestimated depending on negate modeling, DTA application method, and REA. The reasons for SCDF and MUCDF overestimation are explained in this study. Furthermore, modeling and quantification methods for the accurate calculation of SCDF and MUCDF are suggested.
Additionally, current practices for level 3 MUPSA assume that all the NPPs in a nuclear site are located at the same point such as the center of mass (COM) or base NPP position, and all the radionuclides are released from this single point. If NPPs in a nuclear site are not close, the calculated offsite consequences such as radionuclide concentrations and exposure doses will be distorted. In order to overcome this disadvantage of the COM method, the idea of a new multiple location (ML) method was proposed.