Is it safe to restart disaster-hit Onagawa Nuclear Power Station?

By Shinohara Hironori, Sendai Nuclear Problem Research Group

The Nuclear Regulation Authority (NRA) has approved the inspection draft for the Unit 2 reactor at Tohoku Electric Power Co.’s Onagawa Nuclear Power Station (ONPS) in Miyagi Prefecture, thus recognizing that it had passed the pre-start safety inspection. The nuclear watchdog announced this on November 27, 2019, and commenced procedures for the one-month-long public comment.

The Tohoku utility filed an application for improvement of the plant’s facility installations with NRA on December 27, 2013, aiming to pave the way for the reactor to go back online. This means it took nearly six years for NRA to complete the inspection. The authority held as many as 176 meetings, far more than in the case of the inspection of Units 6 and 7 of Tokyo Electric Power Co. (TEPCO)’s Kashiwazaki-Kariwa Nuclear Power Station in Niigata Prefecture. In this case, they held 151 meetings.

Of all nuclear power stations in Japan, ONPS was situated closest to the epicenter of the Great East Japan Earthquake in March 2011. At the time of the mega earthquake, the Onagawa plant was therefore hit by severe shaking, measuring 567.5 Gal. This was more severe than the 550-Gal shaking measured at the Fukushima Daiichi Nuclear Power Station (FDNPS) in 2011. On the Oshika Peninsula, where ONPS is located, the mega earthquake caused land subsidence of around 1 meter, pushing down the level of the plant premises to 13.8 meters above sea level. Then the 13-meter-high tsunami hit the plant, and the earthquake-proof walls of the reactor building became riddled with 1130 cracks of less than one-millimeter width each. This indicates that the stiffness and rigidity of the walls declined by 70% at most, some experts said. The damage from the severe earthquake and tsunami is a major reason why it took so many years for NRA to complete their inspection.  

Since the nuclear regulators started the inspection to examine whether the plant conforms to the new regulatory requirements and regulations, Tohoku Electric have dispatched about 200 engineers to Tokyo, where they were stationed to explain about the nuclear plant situation to NRA. The total personnel costs of sending the engineers must have been quite expensive. The nuclear authority conducted various analyses, assessments, experiments and research studies for the inspection, and the utility was required to prepare large amounts of data and reference documents in advance. The discussions held between the Tohoku Electric engineers and the nuclear regulators and inspectors, based on these data and references, gave the impression that they were competing with each other on the basis of their expertise and competence. NRA, however, is currently using the new regulations and regulatory requirements by making revisions and additions to the conventional ones, and even if the Onagawa plant’s conformity to the new regulations was confirmed by the nuclear watchdog, it is not certain if the plant is really safe. Since the whole truth of the nuclear accident at the Fukushima plant has yet to be uncovered, there is a possibility that the new regulations are not sufficient for assessing the safety of nuclear plants.

The most important point of the authority’s inspection was the plant’s tsunami barrier, measuring 29 meters above sea level, and around 800 meters long. This was built to be the symbol of the plant’s safety measures and the utility has staked its prestige on it. When Tohoku Electric designed ONPS Unit 1, it assumed that the nuclear plant might be hit by tsunami of up to three meters high, and when it designed Unit 2, it revised the assumption to up to 9.1 meters high. The nuclear plant, however, was hit by much higher tsunami, up to 13 meters, in 2011. This time, therefore, the utility designed the 29-meter-high breakwater on the assumption that the nuclear plant would be hit by an even higher tsunami of up to 23.1 meters. In September 2017, the company announced that the barrier was virtually complete and opened the facility to the mass media. Despite this effort, NRA said at its meeting with the utility in January 2018 that the barrier failed to meet the new requirements.

The new requirements demand that the foundation of the nuclear plants’ major facilities should reach hard bedrock. In the case of the ONPS breakwater, 168 steel-pipe piles were driven into the ground as a foundation wall. But NRA has pointed out that some of the piles do not reach the bedrock because there is some facility existing underneath the breakwater, and this may cause irregular subsidence of the breakwater when the plant is hit by a severe earthquake. The nuclear authority thus demanded that the utility reinforce the ground underneath the breakwater.

Tohoku Electric decided to comply with this demand one month later, in an attempt to shorten the inspection period and restart the plant as soon as possible. This construction work to be done on the sea side of the plant premises, however, may trigger an influx of water from the mountain side and also cause the underground-water level to rise. Discussions are now being held on how to deal with these problems. But the question is whether or not it is really effective to conduct this ground reinforcement work for preventing the irregular subsidence of the breakwater.

In the Onagawa plant’s premises, the underside of the sea side breakwater is largely comprised of earth fill. For this reason, it seems extremely difficult to reinforce the strength of the ground uniformly underneath the completed breakwater. The total cost of the utility’s security measures, including this additional construction work and the work to build earthquake-proof facilities, is expanding enormously. Tohoku Electric previously estimated the combined costs of additional security measures for ONPS Unit 2 and the Higashidori nuclear plant at 300-350 billion yen. The utility, however, announced in March 2019 that the costs for ONPS Unit 2 alone may total 340 billion yen.

The Miyagi prefectural government set up a panel to discuss the safety of ONPS Unit 2 in 2019, when NRA was expected to announce that the nuclear plant was about to pass the pre-restart safety checks before the end of the year. At a meeting of the panel, Tohoku Electric explained about its measures to prevent damage to the containment vessel, and maintained that the risk of a steam explosion was extremely small. They used data to substantiate this prospect, but some experts pointed out that the data seemed to have been “falsified.” Large-scale steam explosion experiments have been held four times so far, of which Tohoku Electric used TROI experimental data in the explanation reference. TROI is a facility dedicated to steam explosion experiments operated by the Korean Atomic Energy Research Institute (KAERI, South Korea). The data, however, were not quoted directly from the TROI experiment report but from a thesis written by a foreign researcher. This means that it consisted of “quotations within quotations.” The measured-temperature data which was submitted to the nuclear authority’s inspection committee was also inaccurate, and this inaccurate data seems to be a serious problem.

The current global trend is to install a core catcher in the containment vessel and ensure that it is effectively cooled. This safety measure was worked out based on the bitter experience of the 1986 Chernobyl NPS accident. A core catcher is a device designed to catch the molten core material (corium) of a nuclear reactor in case of a nuclear meltdown and prevent it from escaping the containment vessel. NRA’s new regulations, however, do not require installment of this device. Electric power companies have claimed that steam explosions can be prevented by creating a water pool in the bottom part of the containment vessel or by pouring water into the bottom part in case of a nuclear meltdown. Even though an expert pointed out in a public comment that the TROI experiment report referred to spontaneous steam explosions, the nuclear regulators paid no attention to this and accepted the utilities’ claim before approving restarts of the reactors.

Each electric power company is taking different steps. In the case of Units 6 and 7 of the Kashiwazaki-Kariwa nuclear power plant, the maximum depth of the water pool is set at 2 meters, while in ONPS Unit 2, it is set at 4.2 meters. In view of this situation, there is a need for the authority to determine which measure is the most appropriate.

Tohoku Electric has insisted that the Onagawa nuclear plant suffered merely minor damage in its 61 major facilities when it was hit by seismic acceleration much greater than that which hit the Fukushima Daiichi plant in 2011.

The utility has announced that the repair work was completed in July 2015 after it injected epoxy resin into the cracks on the outer walls of Unit 2’s turbine building. It is hard to believe that the damage was so small. Indications are that NRA’s verification of the damage and repair work at its inspection meetings was insufficient and inappropriate. In the turbine building, deformation of or damage to the mounting bolts of the bearing brackets, and abrasive marks on the turbine rotor blades were discovered. To repair these parts, a large-scale operation was carried out to cut off all the steam pipes leading to the turbine and to lift up the turbine cylinder. If this big operation is taken into consideration, is it possible to say that the damage was minor? Thus it would be necessary to clarify details of the repair work done in other parts of the nuclear plant to assess the seriousness of the damage to the plant.

Although it was originally believed that Japanese nuclear power plants were so firmly protected by 1) the quintuple barrier of nuclear reactors and 2) the multiple protective structure, that a nuclear reactor core meltdown would never occur, FDNPS suffered just such a nuclear accident. NRA’s new regulations therefore require nuclear plants to reinforce their multiple-layer defense in depth, from the conventional three layers to five layers. The fourth layer calls for alleviation of the negative impacts of a severe accident, and the fifth layer, reduction of negative impacts of a massive release of radioactive substances on local residents (their evacuation). Despite this revision, the effectiveness of the evacuation plan is not examined by NRA inspections.

The Secretariat of the Nuclear Regulation Authority says the evacuation plan is under the jurisdiction of the Nuclear Emergency Disaster Prevention Section of the Cabinet Office, while the Cabinet Office says the existing laws do not require the state to formulate evacuation plans for local residents. The Cabinet Office, however, says the Disaster Countermeasures Basic Law calls on the state to provide advice, guidance, and recommendations to local governments concerning their evacuation plans. This means that the government is evading its responsibility by refusing to be involved in the formulation of evacuation plans.

In the case of the Onagawa plant, there are many problems in the local government evacuation plans, and taking this into consideration, we can hardly approve the restart of Unit 2 of the plant. The residents of Ishinomaki City, Miyagi Prefecture, filed an application with the Sendai District Court for a temporary injunction order against Miyagi Prefectural Governor Murai Yoshihiro and Ishinomaki City Mayor Kameyama Hiroshi, demanding that the two leaders refuse consent to the restart of ONPS Unit 2.

Tohoku Electric has already announced a plan to complete its construction and repair work for ensuring safety of the Onagawa plant before the end of Fiscal 2020 (March 2021) and resume the plant’s operations shortly thereafter. Nevertheless, restarting the nuclear power plant after its operations have been suspended for as many as ten years may mean that some problems will emerge due to the long off-line period. There are suspicions that the plant has many invisible cracks and other damage from the massive earthquake, and it remains doubtful whether it is really safe to restart the crippled nuclear plant even if it has cleared the NRA pre-start safety checks.