Survey on Challenging Issues of Multi-Unit PSA

The major difference of Fukushima accident comparing with the other severe accidents is that this was the first event affecting multi-units by a single event. The consequences of the accident included core damage on three of the operating reactor units, containment breach on at least one of the reactors, and a large release of radioactive material. After this accident, multi-unit risk on site was highlighted and studied internationally. Most of nuclear power plant sites in the world have more than two reactor units. However, there are still no detailed guidelines and comprehensive methods for risk assessment for sites, especially those housing multi-unit nuclear power plants. The purpose of this paper is to review two technical issues related with multi-unit PSA. First, multi-unit initiating events such as Fukushima Daiichi accident have been screened out due to low frequency. The other one is back-up or shared systems. In single-unit PSA, these systems had been considered to one of systems of single unit. However, dependencies and interaction between these systems should be considered to perform multi-unit PSA. Multi-unit risk issue is well-defined, but should be addressed many aspects of plant safety in the view point of multi-unit accident management such as potential source terms, impact of shared components and system dependencies, emergency planning against simultaneous multi-unit accidents, human and organizational factors, relationship between hazard intensity and design margin, and so on. Therefore, first of all, it is important to the understanding the potential risks associated with multi-unit event affected multi-units within a site. In addition, the issue can have significant regulatory impact on current operating multi-unit sites and new plants, if it is proved that the multi-unit site operations have significant risk insights. It is reason that multi-unit sites have been licensed since the birth of commercial nuclear plant plants in the world