KK-5 Headed Towards Restart
Hamaoka-5 Confronted by the Difficult Problem of Ground Structure
On October 28 Niigata Prefecture's technical committee, in its third meeting for the year, concluded that it was safe to restart Tokyo Electric Power Company's (TEPCO) Kashiwazaki-Kariwa No. 5 Reactor (KK-5, BWR, 1100MW). If it is restarted, it will be the fourth KK reactor to restart since the Chuetsu-oki Earthquake of July 16, 2007. Besides Unit 5, Units 2, 3 and 4 also have not yet been restarted.
The meeting was scheduled to run for two and a half hours, but it went one and a quarter hours over time. Nevertheless, the only discussion of substance related to whether or not the work to strengthen the seismic resistance of the containment vessel was sufficient to ensure safety. Committee member Motoe Suzuki suggested that it might in fact increase the danger. A report summarizing the main points for discussion was produced by the technical committee's subcommittee dealing with equipment integrity and seismic safety. The subcommittee's report went through nine drafts and involved considerable discussion, but it would be fair to say that the members of the technical committee proffered no opinions on the issues raised in the subcommittee's report. The Chair asked, "In that case may we conclude that there are no safety problems?" His question was greeted with silent consent and the committee moved on to the next item on the agenda. It was truly a weird meeting.
The meeting was punctuated by frequent booing from the concerned residents who observed the proceedings. For this meeting, for the first time residents of Niigata Prefecture submitted 73 questions in advance. Even though the discussion proceeded along the lines of the residents' questions, in the end no clarity emerged on the issues that concerned them, including in relation to penetrating cracks in four places in the reinforced concrete walls of Unit 5, displacement of movement indicators of spring hangers and constant hangers (suggesting possible residual strain - see NIT 138), and rushed seismic reinforcement of the containment building.
The Governor said that he will reserve comment about the technical committee's discussions until after hearing an accurate report. However, the local people are very worried about the situation.
Ground structure: shared problems at KK and Hamaoka
There has been a lot of debate about the seismic movement experienced by the KK Nuclear Power Plant and why it so greatly exceeded predictions. Was the design inadequate, or was there some unknown cause? Similar debates are in progress in regard to Chubu Electric Power Company's Hamaoka-5 Nuclear Power Plant, but the reason for the huge seismic movement resulting from the Suruga Bay Earthquake on August 11, 2009 still has not been clarified (see NIT 132). For both KK and Hamaoka, it appears that the problem relates to the structure of the ground on which the plants are standing and that the problem is of a type not previously encountered at Japan's nuclear power plants.
The Suruga Bay Earthquake was a magnitude 6.5 earthquake (Mj = Japanese scale). Both Units 4 and 5 shut down automatically, but the seismic movement experienced by Unit 5 was extraordinarily strong. At the time, Unit 3 was closed for periodic inspection and Units 1 and 2 were already permanently shut down.
The recorded data showed that the seismic response spectrum for Unit 5 in the 0.3 second to 0.5 second period range was two to three times as great as for Units 3 and 4. Unit 4 is right next to Unit 5. The two plants are only separated by a distance of about 400 meters.
KK was struck by a medium size (Mj6.8) earthquake. The huge seismic movement experienced by the plant was not due to the energy released at the earthquake's seismic center alone. There is an ancient folded stratum at a depth of a few kilometers and beneath that is irregular ground. Although it has not been proved, the theory is that these acted as a lens to concentrate the seismic waves on the nuclear power plant.
In the case of Hamaoka, the amplified seismic movement of Unit 5 at first puzzled researchers. They thought that perhaps there was some totally unknown factor involved, or that something had been missed. They wondered whether there might be a peculiar ground structure in the vicinity of the reactor site at a depth of less than a few hundred meters. Like KK, there is a folded stratum beneath the plant, but judging from the speed at which the seismic waves were transmitted that was not thought to be the main cause. However, as a result of boring and ground surveys, it was discovered that 300 to 500 meters below Hamaoka-5 was a "slow formation" where the S-wave velocity was about 700 meters per second, around 30 percent slower than the surrounding bedrock.
The government's investigation committee is now debating the size and shape of this shallow slow formation. A clear and consistent explanation of the amplification characteristics of the seismic movement is not yet available and there is no immediate prospect of restarting Hamaoka-5.
One thing is clear, namely that pre-existing knowledge was insufficient to explain the seismic response spectrums of earthquakes that have struck Japan's nuclear power plants in recent years. The nuclear industry is now confronted with the difficult problem of ground structure.
Yukio Yamaguchi (CNIC Co-Director)
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