Preliminary findings and lessons learned from the 16 July 2007 earthquake at Kashiwazaki-Kariwa NPP- 'The Niigataken Chuetsu-Oki earthquake, Kashiwazaki-Kariwa NPP and Tokyo, Japan, 6-10 August 2007. Mission report. V. 1

Upon request from the Government of Japan an IAEA expert mission was conducted at the Kashiwazaki-Kariwa NPP following a strong earthquake that affected the plant on 16 July 2007. The objective, as agreed with the Japanese counterpart, was to conduct a fact finding mission and to identify the preliminary lessons learned that might have implications for the international nuclear safety regime. Although the Niigataken Chuetsu-Oki earthquake on 16 July 2007 significantly exceeded the level of the seismic input taken into account in the design of the plant, the installation behaved in a safe manner, during and after the earthquake. In particular, the automatic shutdown of the reactors of Units 3, 4 and 7, which were operating at full power, and of the reactor of Unit 2, which was in the start up state, were performed successfully. Based on the reports from experts from the Tokyo Electric Power Company (TEPCO) and the limited but representative plant walkdowns and visual observations performed by the IAEA team, safety related structures, systems and components of the plant seem to be in a much better general condition than might be expected for such a strong earthquake, and there is no visible significant damage. This is probably due to the conservatisms introduced at different stages of the design process. The combined effects of these conservatisms were apparently sufficient to compensate for uncertainties in the data and methods available at the time of the design of the plant, which led to the underestimation of the original seismic input. However, important components like the reactor vessels, the core internals and the fuel elements have not yet been examined and in-depth inspections are still to be performed. On the other hand, non-safety related structures, systems and components were affected by significant damage such as soil and anchorage failures and oil leakages. A re-evaluation of the seismic safety the Kashiwazika-Kariwa NPP needs to be done with account taken of the lessons learned from the Niigataken Chuetsu-Oki of earthquake and using updated criteria and methods. Detailed geophysical investigations are foreseen both on land and offshore for defining the new seismic input to the plants. In particular, these investigations should address the issue of the potential existence of active faults underneath the site. Another consideration is the possibility that long term operation of components could be affected by hidden damage from this event. Thus, the potential interaction between large seismic events and accelerated ageing may be an important topic to consider in future inspection programmes. Throughout the mission, the attitude of the Japanese counterpart was open and cooperative. It was felt that this search for openness was shared by all the individuals with whom the team interacted. In addition to the more detailed and comprehensive inspections and analyses to be performed by the Japanese and their commitment to present a report at the Regulator's Meeting during the 51st IAEA General Conference in September 2007, the IAEA will follow this mission with extensive international cooperation, including technical assessments and studies, as well as communicating the findings and lessons learned to nuclear operators, regulators and technical support organisations. (author)