Efforts toward Enhancing Seismic Safety at Kashiwazaki Kariwa Nuclear Power Station

1. Introduction

It has been two 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.

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.

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.

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.

【Element of amplification - 3】

Due to an old fold structure in the grand, the seismic wave reverberated and the waves diverged into the direction of the1st reactor site, which made the double intensity comparing to that of Unit 5 site.

(3) Formulation of a standard seismic ground motion

As we take into account of knowledge on the seismic motion amplification mentioned earlier based on the result of active fault evaluation, we found out that the impact on the nuclear power station is bigger in the earthquake caused by F-B faults for sea area and by the Nagaoka Plain Western Rim active fault zone for land area, comparing to other areas. For these active faults, we carried out the evaluation that reflected knowledge learned from the NCOE in accordance with the new guidelines for seismic design. In such a way, we formulated a standard seismic ground motion anew in (Table 2).

A standard seismic motion is ruled it out that formulates on the basis of so called “free surface of base stratum” where is a large ground surface in the horizontally flat spread (that assumes no layer and structures). At  Kashiwazaki Kariwa Nuclear Power Station, the free surface of base stratum is set in 146 – 290m depth in the ground, depending on the depth of each reactor. To evaluate the impact on the facility by an earthquake, it is important to take in the seismic motion at the base mat of the reactor building (the lowest part of the basement of the building).  Respectively we evaluated the seismic motion at the base mat of each reactor taking account of phenomenon that seismic motion runs down while it travels from the free surface of base stratum to the reactor building (Table 2).

Table 2. Evaluation of earthquake motions

5. Implementation of Seismic Reinforcement Constructions

The Tokyo Electric Power Company currently carries out seismic back-check and implements seismic reinforcement works for each reactor in turn at Kashiwazaki Kariwa Nuclear Power Station in parallel with a facility integrity evaluation work and recovery works on facilities damaged by the earthquake. For the seismic back-check, it has been verified by a computer analysis that critical safety functions in the facility are protected to the motion which is set in the standard seismic motion conforming with the new guideline for seismic design. In addition, as for seismic reinforcement construction, we are carrying out as needed the constructions so as to tolerate the seismic motion of 1000 Gal, that is equivalent to 1.5 time intensity of maximum acceleration observed on the base mat in all reactors where went through the NCOE. As of August 17, we have already completed both back-check and the seismic reinforcement constructions for Unit 6 and Unit 7. Below are described contents of the seismic reinforcement constructions.

(1) Seismic reinforcement construction for piping

As for the prioritized piping relatively in less tolerance that is shown in the computer analysis, we have managed to reduce seismic response to the piping by providing the additional piping support structure in order to lower the stress added to the piping body (Fig.5) and to improve the strength of a piping support structure itself as well (Fig.6). We consider managing an unexpected stress caused by heat expansion of the piping structure that is arisen as a piping support structure holds tightly the piping structure body.

(2) Seismic reinforcement construction for a roof truss over a reactor building

As for a roof truss above a reactor building, it is found, as result of a computer analysis, that the main truss still have a tolerance, but there is less tolerance in a part of the sub-truss crossing the main truss and in the secondary components such as the lower horizontal-brace. For components in less tolerance, we carried out various seismic reinforcement constructions such as supplementing reinforcement materials, exchanging the component for better tolerance to stress, and so forth (Fig.7). By carrying out the reinforcement construction, a stress added to the main truss reduces, which makes it to improve the safety allowance for a whole of the roof truss.

(3) Seismic reinforcement construction for a stack

As for a stack, a part of the steel tower component is found in less tolerance. After assessing ways to manage the issue, we decided to install a vibration motion control device (Fig.8) due to the condition that needs a large scale construction and takes a longer time to complete if the components are replaced entirely. The device is to control its sway by absorbing vibration energy in consequence of oil fluid resistance force in the device.

(4) Seismic reinforcement construction for a crane under the ceiling of a reactor building

There is a moment that the crane travels over the spent fuel storage pool that is regarded as high importance on seismic. The construction is to secure the crane from the accident that it drops in the pool by seismic motion. The crane is built with the structure that it travels on the rails installed near in the height of the ceiling of the reactor building, and is installed using metal fittings to prevent it from derailing. To prevent the accident, we enlarged the size of derailment prevention device and reinforced the supporting structure for the rail track (Fig.9).

(5) Seismic reinforcement construction for refueling machinery

The refueling machinery is always in standby mode over the spent fuel storage pool. For this reason, the construction is to secure the exchanger not to drop in the pool by seismic motion as is the case of the crane. The refueling machinery body travels on the rails installed near on the pool edges, and a trolley also travels around on a rail installed on the refueling machinery bridge. Derailment prevention device fixed to the body and the trolley. As for the seismic reinforcement construction, we enlarge the size of derailment prevention device, or install partially additional device, and moreover increase the strength of the refueling machinery body by adding structure components partly (Fig.10).

6. Implementation of Facility Integrity Evaluation after the Earthquake

In facility integrity evaluation, we analyzed seismic observation data of the NCOE using a computer on how seismic force was added on the nuclear power facility, in which we confirmed that the stress added to each equipment is about within elasticity allowance. In addition, we made sure that there are no problem in its functions of each equipment in all facilities by doing visual inspection, leak inspection, functional test for each facility. Integrity of the facility is checked out comprehensively by going through verifying integrity of each equipment by the above inspections, functional test as a system integrated with various equipments and further functional test for the station as a whole by pulling control rods and vaporizing water, and so forth. Particularly for the testing of the station as whole, the integrity is checked out carefully for every stage in progress of 20 %, 50 %, 75% and 100% by making sure that there is no irregularity of fluid vibration by steam and water supply, and by fully analyzing station data observed in the station as well.

As of August 17, we completed evaluation of the facility integrity in Unit 7 in 100% output as well as the system functional test for Unit 6, and subsequently we were informed of the safety confirmation from the Nuclear Safety Agency and the Nuclear Safety Commission. Currently the review is under the deliberations of Niigata-prefecture. The methodology of integrity evaluation used at this time is expected to be a standard model for Japan where a restart standard after an earthquake has not been established, and is highly appreciated by the International Atomic Energy Agency (IAEA), as well as the Electric Power Research Institute (EPRI) in America.
(note: As of November 14, 2009, Unit 6 and Unit 7 are in operation in 100% output. Also, Unit 1 and Unit 5 have started the system functional test.)

7. Closing Remark

In our going through with facility integrity evaluation and reconstruction in the aftermath of the earthquake at Kashiwazaki Kariwa Nuclear Power Station, we trust that a disaster tolerable plant is established by making such seismic safety improvements progressively.We were successfully able to restart Unit 6 as well as Unit 7 that was accepted to be safe in understanding and cooperation with people in the local community, the central and local governments, academia, and related associations. We continue to make every effort towards safety as a top priority in embracing lessons learnt form the earthquake. We also strive to open and share the　lessen so as to be useful for safety enhancement of a nuclear facility across the world.