99 Accident Sequence Study for Seismic Event at the Multi-Unit Site (PSAM-0387)

One or more units of a multi-unit nuclear power plant (NPP) could fail simultaneously at seismic event depending on the seismic ground motion and its influence to units of the site. Excellent approaches for quantification of total core damage frequency (CDF) have been proposed for quantifying the pertinent and integrated CDF of the multi-unit site, but the most of these approaches did not stress on the interfacing capability of the Level 2 and Level 3 parts of probabilistic safety assessment (PSA). That is, the individual accident sequence with the associated CDF at a multi-unit site cannot be delineated explicitly by these approaches.

The approach proposed here is to analyze the multi-unit accident sequences with CDF at seismic event explicitly by best applying and improving the existing technology and providing interfacing capability of the Level 2 and 3 parts of PSA.

The identified accident sequence at each unit, the end state of which could be failure (damaged core) as well as success (intact core), is linked mutually and a set of these sequences are conditionally quantified. If all potential accident sequences would be conditioned mutually, quite a number of accident sequences have to be analyzed. To circumvent such an unnecessary and quite resource-intensive burden, a screening process is effective and important for this approach. Therefore two-stage screening method was developed for the approach proposed here. The first and second stage screenings were applied to initiating events and accident sequences respectively. In these screening processes the correlation analysis of seismic-induced component failures was necessary and important, and was performed in use of representative response and capacity correlation factors, in application on which the floor response spectra were used for structures and components. The correlation analysis results were applied to quantify the concurrent seismic-induced failure probability. This calculation process was formulated and integrated into the computer code of the Seismic PSA Analysis code that has been developed in the Nuclear Power Engineering Corporation (NUPEC). The concurrent failure probabilities were applied to the minimal cutsets of accident sequence.

The above-mentioned approach was applied to an example of twin-unit BWR5 site for verification purpose. The preliminary analysis results revealed that the contribution of concurrent core damage accident sequence frequencies at both units was approx. 10 % and that the contribution of combined accident sequences for damaged core at a unit and intact core at another unit was approx. 90%. The contribution of concurrent core damage sequences was very small because earthquake attenuation was large enough that component random failures rather than seismic-induced structure and component failures dominated CDF as the characteristics of the applied BWR5 model. Then, the concurrent core damage sequences will be expected to contribute more and more if damping of soil is small and seismic ground motion affects on a site more strongly.