A Comparative Risk Assessment for Sites with Single and Double Units

Abstract

A majority of nuclear power generating sites in the world houses more than one nuclear power plant. Traditionally, a PSA is carried out to evaluate the risk associated with single unit NPP taking into account the defence in depth features and postulating combination of potential accident initiators for different hazards. The objective of PSA is to quantify risk metrics such as core damage frequency and LERF and identify weak links in the system to strengthen and ensure that safety targets are met. Post Fukushima accident, it is evident that for a site consisting of multiple units, a single reactor specific metric is not adequate and there is a need to estimate the risk arising from events affecting multiple units in the site. Our earlier work presents an approach to estimate the risk from a multi-unit nuclear power plant site. In the present paper, an attempt is made to compare the risk for a single and double unit site using the same approach. The integrated risk at a multi-unit site is estimated against various external hazards and internal events and the risk metric used is ‘Site Core Damage Frequency’ which is defined as the sum of all possible single and multiple combinations of core damage per site per year, with consideration of various inter-unit dependencies. The study when extended, through sensitivity analysis can form the basis to optimize the shared resources effectively at the multi-unit sites. The spin-off from such a study carried out during the design stage will provide input to decide the optimum number of units at a site, the optimal distance between two units, etc.