Fukushima Daiichi Nuclear Power Plant: What caused the accident?

The accident that took place at the Fukushima Daiichi Nuclear Power Plant following the earthquake and tsunami posed a tremendous challenge to all researchers engaged in the field of nuclear power. What insight can the academic community offer to help the plant regain stable control and recover from the accident?

Tsuyoshi Takada
Professor, Graduate School of Engineering

Accident at the Fukushima Daiichi Nuclear Power Plant and future challenges

It has already been several months since the accident at the Fukushima Daiichi NPP owned by the Tokyo Electric Power Company in the wake of the March 11 earthquake and tsunami. Despite the strenuous efforts made to bring the situation under stable control, it is still unclear when this will be achieved. Until now, under such uneasy circumstances I have thought it best to refrain from expressing any opinion on the matter. However, as an expert in the field I have a responsibility to contribute to the discussion and have therefore agreed to pen an article for this magazine. Specifically, my expertise lies in seismic engineering of buildings and I have long been engaged in seismic design and earthquake risk assessments for nuclear power plants. In light of the seismic damage caused to the Kashiwazaki Kariwa Nuclear Power Plant in 2007, I was of the quiet opinion that power plants were highly resistant against earthquakes. But the accident at the Fukushima Daiichi NPP has demonstrated nuclear power plants’ vulnerability to earthquakes in a way that has both surprised and frustrated me. Regardless of the true cause of the accident, as a researcher working in the area of nuclear power I feel extreme regret at the serious impact felt by local communities from the radioactive contamination resulting from the accident.

In the following, in my capacity as an expert in seismic engineering I will outline the accident and also describe the future challenges and the direction of improvements that should be made.

Outline of the accident at the Fukushima Daiichi NPP

The Pacific Ocean side of the eastern part of Japan is home to 15 nuclear reactors at four nuclear power plants. All four of the plants were struck by the large earthquake and tsunami that followed. The Fukushima Daiichi NPP subsequently fell into a state of crisis but reactors at the three other plants were brought to cold shutdown by March 15. (“Cold shutdown” means the temperature within the reactor dropped to below 100°C and the reactors was stopped safely). For those of us engaged in engineering, it is important to correctly understand the cause of accident at the Fukushima Daiichi NPP and how it progressed. Moreover, in order to effectively plan future measures we need to clarify the differences between that power plant and the other three where no such accident occurred. Below, I outline the accident in reference to various damage investigation reports and other information currently available.

At the Fukushima Daiichi NPP, Units 1 through 3, which were operating at the time, were automatically shut down when shaking that exceeded the emergency stop level was detected. The damage caused to the entire plant at that time is a subject for future study. But we already know that the off-site power supply facilities, which were important to ensure the safety of the plant, were disabled and that the six independent power supply lines were subsequently cut off as a result of damage to the breakers, etc. and collapse of transmission towers. Then, about 40 minutes after the earthquake, a huge tsunami as high as nearly 14 meters (against the design height of 5.7 meters) struck the entire power plant, flooding and disabling both sea water pump facilities for component cooling installed at each of the reactors. Moreover, the emergency diesel generators and switchboards installed on the basement floor of the turbine buildings were also submerged, depriving the plant of all AC power supply. Although the nuclear fission chain reactions in the reactor cores were automatically stopped, the total loss of power made it impossible to inject water into the pressure vessels to remove the remaining heat and cool the cores for a certain period of time. The nuclear fuel in the reactor cores was subsequently exposed to the air, leading to core meltdowns. The overheated fuel generated steam, which in turn increased the pressure inside the pressure vessels. To suppress the rising pressure, venting was performed, but hydrogen leaked into the building, causing an explosion that released a vast amount of radioactive material into the air. Roughly one month after the disaster, based on the total airborne emissions of radioactive substances, the accident was rated as level 7, which is characterized as a major accident according to the International Nuclear Event Scale.

Factors that shaped the outcome

Although other nuclear power plants located in the afflicted areas were also hit by the large earthquake and tsunami, they, fortunately, did not experience any crisis. Even at the Fukushima Daiichi NPP, Units 5 and 6, which lost off-site power supply due to the tsunami, were successfully brought to cold shutdown because power was supplied by the sole surviving air-cooled emergency diesel generator at Unit 6. At the Fukushima Daini NPP, four reactors were in operation when the earthquake occurred. Operations were automatically stopped and of the four off-site power supply lines, two were available to cool the reactors and bring them to cold shutdown. At the Onagawa Nuclear Plant operated by Tohoku Electric Power, two reactors were in operation and one was being started up at the time of the earthquake. All three were automatically stopped. The on-site power supply for Unit 1 was lost but power continued to be supplied by the emergency diesel generator and the reactor was brought to cold shutdown on the day after the disaster. Units 2 and 3, for which the off-site power supply was available, were brought to cold shutdown on the following day.

The three principles to be followed at nuclear power plants in case of emergency are “stop, cool, and confine.” The aforementioned facts imply that it was the loss of power, which made it impossible to cool the reactors and that caused the disastrous accident at the Fukushima Daiichi NPP. Radioactive substances that could not be confined inside the reactors in Units 1 through 3 were emitted, incurring serious damage to the surrounding areas.

Importance of ensuring safety in engineering

The violent earthquake and huge tsunami, which exceeded the design specifications of the power plant, caused the tragic accident by imposing extremely external severe conditions on the plant. In order to help prevent the reoccurrence of such an accident, I analyze the challenges posed to nuclear power plants and improvements needed by focusing on the following two aspects. First, I focus on natural phenomena, which is an external factor. Second, I focus on the extremely large and complex engineering systems of nuclear power plants.

The large earthquake that triggered the tragic accident had the largest magnitude of any earthquake previously recorded in Japan and also had a vast source area. It was certainly impossible for anyone to predict—or even speculate on—such a large earthquake based on the knowledge and experience accumulated before the disaster. The Kobe earthquake that hit the southern part of Hyogo Prefecture in 1995 and the one that occurred off the coast of Chuetsu in Niigata in 2007 provided us with new findings and knowledge but seismologists still need to conduct further studies to elucidate unknown factors.

For engineers, however, it is common to set a sufficient design margin even against unknown risks. It is essential for engineers to determine how great a margin should be set in the future.

Specifically, it is necessary to conduct additional studies and surveys on tsunamis, which have been somewhat neglected by nuclear power researchers, to fully assess the impacts of earthquakes on nuclear power plants. In Japan, countermeasures against tsunamis had been taken from the following two approaches: construction of coastal embankments based on past tsunami experience and the issuance of emergency tsunami evacuation warnings. The tsunami in March was the first large tsunami to affect a nuclear power plant. In the future, technological improvements need to be made to predict the potential height and power of tsunamis that may strike plants in the future as part of efforts to assess the impacts of large earthquakes.

It is also necessary to improve the safety of the entire power plant system against earthquakes and large tsunamis. At the Fukushima Daiichi NPP, the power supply system, which had been thought to have sufficient backups, was totally cut off. Measures to avoid the loss of off-site power and ensure alternate power supplies need to be considered. At minimum, the power supply system must have more diverse, robust and independent backups to be more resistant against earthquakes and tsunamis.

Last but not least, the concept of “defense in depth” must further be expanded for accident management programs in case an accident occurs, and disaster prevention measures should be further enhanced in the areas surrounding nuclear power plants.

In closing, I pray for the souls of all those who lost their lives in the Great East Japan Earthquake and express my heartfelt sympathy to all victims of the tsunami and accident at the nuclear power plant, who are still suffering from the aftermath. I hope that the nuclear accident will soon be brought under stable control and wish for the earliest possible recovery of the affected areas.