Strategies to Ensure Safety Measures for ABWRs in Kashiwazaki-Kariwa Nuclear Power Station

This paper describes strategies to ensure safety measures for ABWRs in Kashiwazaki-Kariwa Nuclear Power Station reflecting the lessons learned from the Fukushima Daiichi accident. The accident and response actions were analysed to extract lessons. Based on the lessons policies to enhance safety and containment integrity were derived. Firstly with considerations of multiple failures of the Fukushima Daiichi accident, defense in depth (DID) was enhanced by applying more diverse safety measures. For this purpose, in addition to refurbishing safety measures for beyond design base events, safety measures for each DID layer was enhanced not only by strengthen robustness to single failure but also by strengthen diversity and by physical separation. Especially capacities for ensuring containment integrity have been drastically improved by installing diverse safety measures for the third layer and the fourth layer of DID including the alternative coolant circulation system. Secondly phased approach was introduced in choosing mitigation measures considering timing of the response actions and required reliabilities. The basic concept of phased approach is that we have to select safety measures based on the assumption that possible and available measures and their required reliability are different depending on time to spare. Lastly performance requirements was clarified for containment vessel and its auxiliary systems after core damage. The basic policy was to define clearly twice design pressure of PCV as its upper limit pressure and to define 200 degree C as PCV upper limit temperature, and then to define function requirements for other auxiliary systems in order to contain fission products in the PCV and to make them decay as long as possible not only by enhancing PCV capacity but by utilizing auxiliary systems to ensure PCV integrity after core damage. Various safety measures were implemented based on these policies and applied at ABWRs in Kashiwazaki-Kariwa Nuclear Power Station.