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Kashiwazaki- Kariwa NPP
Assessment of Impact of Chuetsu-Oki Earthquake Runs into Trouble

Since the July 2007 Chuetsu-oki earthquake, all seven reactors at the Kashiwazaki- Kariwa Nuclear Power Plant (KK) have been out of operation. The Governor of Niigata Prefecture said that, when considering the fate of the plant, he would keep the option of permanent closure on the table. He has not changed that position. Meanwhile, it was revealed in an internal document leaked to CNIC that Tokyo Electric Power Company (TEPCO) hopes to restart the plant from January next year. The residents of Niigata Prefecture are very worried about operating the plant again, so TEPCO is developing a lobbying campaign to overcome local resistance.

Cartoon by Shoji Takagi

Niigata Prefecture has established two investigation committees. The first committee has eight members and is considering equipment integrity and earthquake resistance and safety. The second committee has six members and is considering the earthquake itself and the condition of the ground on which the plant is located. Each committee has members who are opposed to reopening the plant. As of May 16, each committee had met three times.

KK was hit by an earthquake far bigger than predicted. The following issues need to be addressed:
1) the nature of the damage incurred;
2) the location and seriousness of the damage;
3) KK's ability to withstand another earthquake, including after-shocks;
4) the condition of the ground on which KK stands;
5) the question of whether the judgments made by TEPCO and the government when the plant was approved were correct;
6) the connection between the fault plane which caused the Chuetsu-oki earthquake and the surrounding active faults.
Until these issues have been clarified, no decision should be made about whether or not to restart the plant. The first three of these issues are being considered by the first of the above committees, while the last three issues are being considered by the second committee.

TEPCO has submitted an interim report about Unit 7 (ABWR, 1,356 MW, commenced operation in 1997). The report said that visual checks revealed no major damage. The report also claimed that calculations showed that the stress incurred by important equipment and the reactor itself as a result of the earthquake ground motion was within the allowed limits. It concluded that the integrity of the plant was maintained. TEPCO explained that this was because multiple "safety margins" were incorporated at the design stage. The report says that currently a portable Vickers Hardness Tester is being used to check for plastic deformation in representative locations and that so far no abnormalities have been found.

However, it is becoming increasingly clear that these tests and calculations are grossly inadequate. Visual inspections cannot be conducted in areas where there are high levels of radiation. Furthermore, it is not possible to check every nook and cranny for problems like plastic deformation, which cannot be assessed through visual checks. TEPCO intends to make do with representative checks, but who knows whether small defects that are not discovered in these checks will withstand the next shock? Non-destructive tests such as ultrasound tests and permeability tests have not yet been carried out. Even if they were, it is unclear how thorough such tests would be and how reliable the results would be.

The location of maximum stress derived from spectrum response analysis of pipes in the residual heat removal system was incorrect and the stress was grossly underestimated. This emerged as a result of cross-checks by Japan Nuclear Energy Safety Organization (JNES). It is clear that TEPCO's assessment is suspect.

The issue of "safety margin" has been discussed, but no agreed position has been reached. Those who support the restart of KK argue that several "safety margins" were built into the seismic design process and that this is the reason why KK survived the Chuetsu-oki earthquake. However, while "tolerance" is a precise concept meaning "the margin between the permitted stress and the stress incurred", committee members opposed to the restart of KK point out that it is incorrect to say that "margin in the calculation of the stress incurred" and "margin in determining the permitted stress" are also precise concepts. It is these fuzzy concepts that underlie the arguments of those who want to restart KK.

The debate in the second committee revolves around assessments of (a) the strength of the ground on which KK is built and (b) active faults. It began with the question of whether the ground is "soft as a piece of tofu" and whether a fault directly under the plant moved during the earthquake.

Fig. 1 Major active faults in the vicinity of the Kashiwazaki-Kariwa Nuclear Power Plant
The star shows the epicenter of the 2007 Chuetsu-oki earthquake. (Underground rupture along the seismic fault plane began directly beneath this point.) Slip on the fault plane spread to almost the whole area where aftershocks occurred (ellipse). Thick lines show active faults. (Broken lines are inferred ones.) Black triangles on the lines show dip-directions of the faults. The sea area is based on Watanabe et al.. F-A, F-B and F-C are faults after TEPCO's application for a license variation for Units 6 & 7. The Madogasaka fault is after Watanabe et al.. The Jorakuji fault is after Nihon no Katsudanso (New Edition) (Research Group for Active Faults of Japan (Ed.), University of Tokyo Press, 1991).

A conclusion has more or less emerged amongst researchers concerning the location and shape of the earthquake plane where the Chuetsu-oki earthquake originated. However, opinions still vary concerning the location and length of the submarine active fault that connects with that fault plane. This fault is called the "F-B fault" (see Fig. 1). The question is, how far does it go? In 1979 TEPCO assessed that it was 8 kilometers long and that it was not active. In 2003 TEPCO realized that it was a 20 kilometer-long active fault, but did not publicly announce this. The government also knew, but remained silent. In December 2007 TEPCO admitted that it was 23 kilometers. In March 2008 it revised this to 30 kilometers and on 28 April 2008 it increased its estimate again to 34 kilometers. The committee has begun to debate Mitsuhisa Watanabe's theory that the 50-60 kilometer Eastern Boundary Fault of Sado Basin and the F-B Fault are connected.

The KK problem is related to back-checks being carried out on all Japan's nuclear power facilities. New seismic design guidelines were established in September 2006. Checks are continuing to assess whether nuclear power plants and other nuclear facilities meet the standards established in the new guidelines. In March this year interim reports were submitted for 15 nuclear power plants. In addition, a final report was submitted for the Monju prototype fast breeder reactor. No report was submitted for KK, while interim reports had been submitted previously for Hamaoka NPP and the Rokkasho Reprocessing Plant. No interim report is planned for the Tokai Reprocessing Facility, but a final report is scheduled for July 2009.

Based on the new guidelines, the interim reports determined a design basis earthquake ground motion (Ss). In all cases Ss exceeded the "extreme design earthquake" (S2) under the old guidelines. Also, the active faults to be taken into consideration were assessed to be longer than before for some facilities. Nevertheless, it cannot be said that the assessments prioritized safety more than in the past. For example, the assessment of earthquake ground motion is particularly lax in those cases where no seismic center is specified. In those cases, the maximum ground motion is set at 450 Gal. However, on this basis the ground motion at the "free surface of the base stratum" for KK was set at 450 Gal, whereas in fact it is believed to have exceeded 1,000 Gal during the Chuetsu-oki earthquake. Clearly the alibi that there was a "sufficient safety margin" offers no way out.

On May 12, a Magnitude 7.9 earthquake struck Sichuan Province in China. The count of the dead and injured continues to grow and it is unclear when the full extent of the devastation will be known. China stated that the nuclear weapons facilities in Sichuan Province were "safe and secure", although it has admitted that more than 30 sources of radiation were buried by debris. The Sichuan earthquake was another reminder of the threat posed by nuclear facilities in the event of earthquakes. In order to avoid a nuclear-earthquake disaster in Japan, we must not allow the assessment of the implications of the Chuetsu-oki earthquake for the Kashiwazaki-Kariwa nuclear power plant to be carried out in a slipshod manner.

Yukio Yamaguchi (CNIC Co-Director)

1. Watanabe Mitsuhisa, Suzuki Yasuhiro, Nakata Takashi: Programme and Abstracts, Japan Association for Quaternary Research, No. 37, Suppl., 4(2007).

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