As the 9.0 magnitude earthquake hit the Japanese shore, the reactors of the Fukushima Daiichi nuclear power plant shut down automatically to control the nuclear fission. The electrical lines collapsed, but the plant responded as designed, and the earthquake itself did not cause any other problems. The tsunami it triggered, however, did.
“The reactors were robust, seismically speaking,” said Gustavo Caruso, Director of the IAEA’s Office of Safety and Security Coordination. “But they were vulnerable to the high tsunami waves.” When the flooding hit, the ‘tsunami walls’ made to protect the plant from such events were too low to prevent the sea water from entering the plant. The water’s strength destroyed some of the structures, and entered the diesel generator room — which was built lower and at a closer distance to sea level than other plants in Japan — affecting Units 1, 2 and 3. “The diesel generators are essential for maintaining the plant’s electrical supplies in emergency situations,” said Pal Vincze, Head of the Nuclear Power Engineering Section at the IAEA. “They were drowned.”
If the diesel generator is affected, special batteries can be used to generate electricity, but these have a limited capacity, and, in the case of Fukushima Daiichi, some were also flooded. “In Japan, they put up a heroic fight to get the electrical systems up and running again, but it wasn’t enough,” Vincze added.
Without functioning instrumentation and control systems, or electrical power or cooling capabilities, the overheated fuel melted, sank to the bottom of the reactors, and breached the reactor vessels, leading to three meltdowns. In addition, data logs and vital systems operated by safety parameters were also flooded, which meant that there was no way for the operator to monitor what was going on inside the reactors.
As stated in the IAEA report on the Fukushima Daiichi accident, “a major factor that contributed to the accident was the widespread assumption in Japan that its nuclear power plants were so safe that an accident of this magnitude was simply unthinkable. But…When planning, designing and constructing the plant, experts did not properly take into consideration past tsunami experiences… “It must be noted that the combination of an earthquake of this magnitude and a tsunami is extremely rare, but unfortunately this is what happened.”…
Excerpt from Laura Gil Fukushima Daiichi: The Accident, IAEA Bulletin, Mar. 2021