The latest CNIC Statement is ready to view online

October 29, 2024

The Onagawa Nuclear Power Plant Unit 2 reactor has serious safety problems, but Tohoku Electric Power is heading towards a restart.

First of all, let us remind ourselves that the Onagawa Nuclear Power Plant was the plant that was hit by the strongest ground tremor among all of Japan’s nuclear power plants at the time of the 2011 Great East Japan Earthquake and tsunami. It narrowly escaped a major direct hit from the massive tsunami and consequent major destruction, but suffered severe damage due to seawater flowing backwards through a culvert, causing the failure of emergency diesel generators. More than 1,000 cracks appeared in the concrete wall of the Unit 2 reactor building, causing a significant decrease in strength. However, it is doubtful if repair work has been carried out to ensure sufficient strength against a large earthquake that might occur in the future.

There are also problems with countermeasures for seismic safety. The magnitude of the earthquake tremor that could conceivably reach the nuclear power plant has not been set at a realistic level, hence it is possible that evaluation of the plant equipment has been underestimated. It is highly possible that there is a problem with the calculation method, based on the fact that the recorded seismic shaking at the Shika Nuclear Power Plant during the 2024 Noto Peninsula earthquake was more than twice the estimate previously calculated by the power company. Since Tohoku Electric Power uses a very similar method to estimate the magnitude of seismic shakes at Onagawa Unit 2 (standard ground motion), it is very likely that they are underestimating the impact. Because of the error in estimating seismic tremors, it seems that after all the seismic reinforcement work on the suppression pool at the bottom of the containment vessel, it may be unable to withstand even an earthquake of the expected scale.

Another concern is a hydrogen explosion. This could be a serious problem not only for Onagawa Unit 2 but also for all nuclear power plants, and especially, because of their structure, for boiling water reactors. At the Fukushima Daiichi Nuclear Power Station, Units 1, 3 and 4 experienced explosions powerful enough to cause serious damage to their buildings. However, the mechanism of these explosions has not yet been fully understood and countermeasures against hydrogen explosions are yet to be developed. Thus, of course, countermeasures to deal with hydrogen generated in the event of meltdown accident and prevent hydrogen explosions at the Onagawa Nuclear Power Plant have not been formulated. Should a loss of cooling accident occur at Onagawa Unit 2, it is inevitable that the entire accident series of fuel meltdown → hydrogen explosion → collapse of the reactor building → release of radioactivity into the environment could occur.

The attempt to restart the nuclear power plant after more than 13 years in a state of shutdown after March 2011 is also a major safety concern. The occurrence of minor accidents in recent months, such as water leaks from the control rod hydraulic drive system in the reactor building and malfunctions in the emergency gas treatment system, indicate that hidden safety issues exist.