Follow-up IAEA mission in relation to the findings and lessons learned from the 16 July 2007 earthquake at Kashiwazaki-Kariwa NPP - 'The Niigataken Chuetsu-oki earthquake', Tokyo and Kashiwazaki-Kariwa NPP, Japan, 28 January - 1 February 2008. Mission report. V. 1

On 16 July 2007, a strong earthquake, the Niigataken Chuetsu-oki earthquake, with a moment magnitude of 6.6 (MJMA=6.8 according to the Japanese Meteorological Agency), occurred at 10:13 h local time with its hypocentre below the seabed of the Jo-chuetsu area in Niigata prefecture (37 deg. 33' N, 138 deg. 37'E) in Japan, affecting the Kashiwazaki-Kariwa Nuclear Power Plant (NPP) located approximately 16 km south of its epicentre. Kashiwazaki-Kariwa NPP is the biggest nuclear power plant site in the world. It is located in the Niigata prefecture, in the northwest coast of Japan, and it is operated by Tokyo Electric Power Company (TEPCO). The site has seven units with a total of 7965 MW net installed capacity. Five reactors are of BWR type and two reactors are of ABWR type. The five BWR units entered commercial operation between 1985 and 1994 and the two ABWRs in 1996 and 1997, respectively. Following this event, the Government of Japan through the Nuclear and Industrial Safety Agency (NISA) requested the IAEA to carry out a fact finding mission with the main purpose of identifying the preliminary findings and lessons learned from this event in order to share them with the international nuclear community. This first mission took place from 6 - 10 August 2007 and the mission report of the August 2007 mission is available on the IAEA web page http://www.iaea.org. The purpose of the second IAEA mission was to conduct - six months after the event - a follow-up of the preliminary findings of the August 2007 mission on the basis of the results available in January 2008 of the related studies and investigations performed. In accordance with the terms of reference for the follow-up mission and the availability of results from the performed studies and investigations, the scope of the follow-up mission focussed on three subject areas: (1) seismic design basis - design basis ground motions, including the evaluation of the seismic hazard ; (2) plant behaviour - integrity assessment - structures, systems and components response; and (3) fire safety. In general the preliminary findings and lessons learned that were reported in August 2007 were confirmed. Since August 2007 there has been a very significant amount of high quality work performed in all areas that were considered during the follow-up mission including the establishment of required regulations and the participation of recognized institutions in Japan in the area of earthquake engineering and nuclear safety. NISA, JNES, TEPCO and a large number of specialized institutions and universities as well as experts have performed activities relating to the evaluation, regulation and the review aspects of the situation of the plant after the earthquake. The participatory approach that has been chosen by NISA for the review framework provides for a transparent and consensus seeking process. The IAEA Safety Standards relating to seismic safety have already been very useful for the follow-up mission in identifying findings and lessons learned in the areas of the evaluation of seismic hazard and of the seismic response of structures, systems and components. The discussions with the Japanese counterpart confirmed that the IAEA Safety Standards can be used by NISA and TEPCO to address many of the specific findings that are detailed in Section 5 of this report. As example, in the area of seismic hazard evaluation making the adequate use of the IAEA Safety Standards will facilitate the process of integration and synthesis of the vast amount of available data. A change in the design ground motion to be used for the complete safety re-evaluation of the existing facility is to be expected after a strong earthquake that exceeds the original design basis. In that case it appears very important to properly evaluate the relevant capacity reserves of the plant systems, structures and components. That is possible through the use of realistic assumptions, methods, modelling and acceptance criteria in all steps of the post earthquake re- evaluation process as recommended by the IAEA Safety Stand ards. It was confirmed by the Japanese counterpart that the IAEA August 2007 mission and the follow- up mission achieved the objective of sharing the lessons learned with the international nuclear community in many different ways. For NISA, cooperation with the IAEA was essential given the fact that neither international regulatory guidance nor experience was available for dealing with events like this. For TEPCO (the plant operating organization), the missions provided for a clearer road map to characterize the effect of the earthquake on the plant, to update its demonstration and determine the required upgrading. For the international community, it was an invaluable chance to share the experience of Japan and learn from this event through seminars, workshops and site visits organized by the IAEA, NISA and TEPCO. It was well understood that public perception, the need for outreach and a consistent flow of information to the local and international communities are critical components for dealing with a post earthquake situation. The main findings and lessons learned are included in Section 3 of this report while detailed information is provided in the findings sheets in Section 5. (author)