Efforts Toward Enhancing Seismic Safety at Kashiwazaki Kariwa Nuclear Power Station

Efforts toward Enhancing Seismic Safety at Kashiwazaki Kariwa Nuclear Power Station Kazuhiko YAMASHITA General Manager, Niigataken Chuetsu-oki Earthquake Restoration Management Center, Nuclear Asset Management Department, TOKYO ELECTRIC POWER COMPANY, JAPAN 1. Introduction It has been three years since the Niigata-ken Chuetsu-oki Earthquake (NCOE) occurred in 2007. The earthquake brought a major disaster for Kashiwazaki, Kariwa, and the neighboring areas. First of all, we would like to give condolences to people in the devastated area and to pray for the immediate recovery. Our Kashiwazaki Kariwa Nuclear Power Station located in the same area (Fig.1) was naturally caught up in the earthquake. The station was hit by a big tremor more than its intensity assumed to be valid at the station design stage. In spite of unexpected tremor, preventive functions for the station safety worked as expected as it designed. Critical facilities designed as high seismic class were not damaged, though considerable damages were seen in outside-facilities designed as low seismic class. We currently make efforts to inspect and recover damages. While we carefully carry out inspection and assessment to make sure the station integrity, we are also going forward restoration as well as construction for seismic safety enhancement in turn. This report introduces details of the following accounts, these are an outline of guidelines for seismic design evaluation that was revised in 2006, a situation at Kashiwazaki Kariwa Nuclear Power Station in the aftermath of the earthquake, and efforts toward enhancing seismic safety that the Tokyo Electric Power Company (TEPCO) has made since the seismic disaster, and our approach to evaluation of facility integrity. Fig.1 Kashiwazaki-Kariwa Nuclear Power Station 2. Responsive Actions to Guidelines for Seismic Design Evaluation (1) About seismic design for nuclear power plant A nuclear power plant facility including a nuclear power reactor depends upon a high seismic safety standard. When encountering such a big earthquake as extremely rare, it is critical to prevent the community from radiation exposure by controlling three important safety functions, shutting down, cooling off the power reactor, and sealing off radioactive materials. In Japan, when a nuclear reactor installation is planned, a law requires to get the permission from the government in advance. The nuclear power operator must submit the application including explanation of safety design for the facility. The regulatory agency (the Nuclear and Industry Safety Agency under the Ministry of Economy, Trade and Industry) then reviews on safety to make sure that the location, the structure and facility of the nuclear power plant and so forth are well contemplated for disaster prevention. Subsequently, the process is finalized alone the reviewing report and consultation with the Nuclear Safety Commission of Japan and the Japan Atomic Energy Commission. For the safety review, the Nuclear Safety Commission of Japan has set out the guidelines for seismic design evaluation (referred as guideline for seismic design) as a basis to check adequacy for the seismic design plan. According to the guidelines for seismic design, a facility considered to be important on seismic design must be built in the way that its safety functions are not damaged by the seismic ground motion that is assumed against a possible earthquake in the neighboring area of the facility site. That is, the nuclear power operator must specify the standard seismic ground motion that is a seismic motion in the seismic design standard based on a geological survey, and needs a design for the crucial facility to have sufficient tolerance to a force by seismic ground motion. (2) A revision of the guidelines for seismic design evaluation In September 2006, the guideline for seismic design was revised to improve further the seismic safety’s reliability for a nuclear power plant facility that was to reflect the recent knowledge in science and technology associated with seismology and earthquake engineering, and improvement and progress in the seismic design technology. The main items of the revision include as follows: geological survey and advance on active fault evaluation advancement on a methodology to define a standard seismic ground motion revision of importance classification on seismic safety efforts toward using a methodology of probabilistic safety analysis In accordance with the revision of the guidelines for seismic design, the Nuclear and Industry Safety Agency directed for the nuclear power plant operators to evaluate seismic safety anew even on the existing nuclear facility based on the revised guidelines. Although the new guidelines for seismic design are mainly targeted at the evaluation of a new case, it is also to help improve further seismic safety for the existing nuclear power facility. Along the new guidelines, we now undertake a so-called seismic back-check, that is to make sure that crucial safety functions, shutting down, cooling off, and sealing off radioactive materials are also satisfied with the new standard seismic ground motion in the new guidelines. The NCOE occurred in the middle of working on the seismic back-check, which was influenced to some extent. 3. Incident of the NCOE Approximately at 13 minutes after 10 a.m. July 16, 2007, a major earthquake of 6.8 magnitude occurred at the seismic center located at a depth of 17 kilometers below offshore, Niigata-ken Chuetsu area (Fig. 2). According to Japan Meteorological Agency, the earthquake recorded 6+ seismic intensity in the areas of Kashiwazaki city and Kariwa village in Niigata-ken where Kashiwazaki Kariwa Nuclear Power Station is located, and caused extensive damage of 15 death and devastated houses more than 40,000 in the area. A distance between the seismic center and Kashiwazaki Kariwa Nuclear Power Station site was as short as about 23 km (about 16 km from epicenter). The motion intensity was observed 680 Gal at the base mat of the reactor building. (Gal is a unit to measure acceleration, and 980 Gal is equivalent to gravity acceleration). Most likely, it was such seismic motion as a nuclear power plant would have never gone through before. The ground motion intensity was beyond its expectation from empirical assumption for 6.8 magnitude, and also exceeded a design base seismic ground motion. For this reason, the Nuclear and Industry Safety Agency directed anew for nuclear power operators to reflect the new knowledge learned from the earthquake into the seismic back-check. Below is the implementation status of seismic back-check including a factor analysis of why the seismic motion was beyond its expectation at Kashiwazaki Kariwa Nuclear Power Station. Fig.2 The positions of the epicenter of Niigata-ken Chuetsu Earthquake and the nuclear power station 4. Formulation of a New Standard Seismic Ground Motion (1) Geological survey and active fault evaluation in the neighboring areas In accordance with the new guideline for seismic design, a standard seismic ground motion is determined on evaluating seismic motion assumed to be valid for neighboring areas of the nuclear power plant site. For that reason, a geological survey was carried out carefully for both the neighboring sea and land areas (Fig.3 and Table 1). Assessment of the active fault to cause a possible earthquake is then made. Various research has been carried out using the most recent techniques such as maritime acoustic exploration for sea area, and aerial photograph deciphering, surficial geology survey and subsurface explorations for land. As for active faults to be considered on seismic design, the old seismic design guidelines defined it as what became active from 5 million years downward, whereas the new one extends back to the late pleistocene (before 12-13 million years) where we can not exclude the possibility. As for formulation of a standard seismic motion in case of Kashiwazaki Kariwa Nuclear Power Station, we decided to set conservatively a length of the active fault and also to set for the case where active faults close to each other act simultaneously. Fig.3 Main active faults around the NPS Table 1. Main active faults taken into account upon standard seismic ground motion (2) Analysis of the NCOE There are two symptomatic characters in the NCOE, these are; The actual seismic intensity exceeded, to a great extend, a seismic intensity resulted from an empirical evaluation made for magnitude scale of 6.8, A maximum acceleration observed at the foundation rock of the reactor building (the lowest part of the basement) was in considerable difference between reactors from Unit 1 to Unit 4 (680 - 384 Gal) and Unit 5 to Unit7 (442 - 322 Gal) located 1km away from the others. We analyzed seismic observation data such as geological survey, seismic observation data in aftermath of the NCOE, and that of the previous NCOE in the year 2004. We found out, for an analysis of the particularity of the above symptomatic characters, that there are elements to amplify seismic motion arriving from the direction of sea. The following mechanisms for the amplification (Fig.4) are conceivable for the earthquake occurred this time; 【Element of amplification - 1】 We presumed the seismic motion level at the seismic center based on the seismic motion observed. As a result of comparison between the seismic motion in the center and a seismic scale established empirically, we found out that the earthquake was in stronger motion at the seismic center than that of the usual one. 【Element of amplification - 2】 There is depth ground irregularity of the complicatedly shaped stratum that protrudes massively in the direction from sea to land. This causes a seismic wave to reverberate, at the same time a propagation velocity of seismic motion decreases once a seismic wave reaches to the ground in where propagation velocity is low. This made a seismic wave travelling behind to catches up with the one before, which then created multiplier effect of double amplification.

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