What event in history prevented a disaster?

An avoidable disaster

On March 11, 2011, Japan experienced a threefold catastrophe: First, an earthquake with a magnitude of 9.0 and a tsunami devastated large parts of the north east coast of Honshu, then it became clear that the Fukushima Daiichi nuclear power plant was also affected - fatally. The natural events, coupled with serious safety deficiencies at the power plant and wrong decisions by the operator Tepco, trigger a reactor disaster. It will go down in history as the second worst nuclear disaster after Chernobyl.

What happens during the crucial minutes and hours in Fukushima Daiichi and what consequences this has for the reactor cores and the fallout is in some cases not revealed until years later.

Quake and tsunami

What is clear, however, is that reactor units 1-3 will initially respond to the earthquake with an emergency shutdown as planned. But the 15 meter high tsunami, which hit the coast a good half an hour later, flooded the protective wall of the power plant and flooded the reactor buildings, control centers and twelve of the 13 emergency diesel generators in the basement. The lines to other generators outside are cut.

As a result, the power supply to the reactors and the cooling circuit breaks down, and the entire electronics in the control rooms fail. The power plant employees no longer have any insight or control over what is happening in their reactors.

Dangers ignored

But the tsunami flooding was neither unpredictable nor inevitable. "One of the main factors that contributed to the accident was the widespread belief in Japan that nuclear power plants are so safe that a nuclear accident of this magnitude seemed unthinkable," stated a report by the International Atomic Energy Agency in 2015 of the construction of the system - both by the operator Tepco and by the authorities.

"There was discussion, but no action was taken," reports the World Nuclear Association. As early as 2008, scientists from the power plant operator Tepco presented a study according to which tsunami heights of up to ten meters must be expected in this region. During this time, other reports also come to the conclusion that protective walls and other safety measures near nuclear power plants must be designed for levels of at least ten meters.

Authorities looked the other way

While other power plants, including the neighboring Fukushima Daini nuclear power plant, are being built or converted accordingly, Fukushima Daiichi remains unchanged. Instead, Tepco published an internal counter-report in 2010 in which a maximum tsunami height of 5.7 meters is assumed - this is revealed by evaluations of hundreds of documents, some of them internal. The placement of the diesel generators in the flood-prone basement of the facility and the lack of water protection encapsulation are neither objected to nor changed.

"The Fukushima disaster could have been prevented if internal standards had been followed, if there had been international reviews and if common sense had been used to evaluate existing geological and hydrodynamic facts," stated Costas Synolakis of the University of Southern California and his team in their 2015 study.

The consequences can be seen on March 11, 2011. While Fukushima Daini survived the earthquake and tsunami largely unscathed, a catastrophe occurred in Fukushima Daiichi.

Meltdown and hydrogen explosion

It is now known that the cooling water level in reactor block 1 dropped to the level of the fuel rods just three hours after the earthquake due to the failure of the power supply. Four hours after the quake, temperatures in the reactor core reached 2,800 degrees and the fuel rods made of uranium dioxide pellets with a zirconium coating began to melt. The zirconium not only loses its shielding effect, it also reacts with the water vapor and generates hydrogen. This causes the pressure in the reactor vessel to continue to rise.

On March 12, around 3:36 p.m. local time, the first hydrogen explosion occurred, which blasted off the roof and the upper part of reactor block 1 and released radioactive decay products. Deep down in the reactor core, the hot fuel mass has now burned through the bottom of the pressure vessel and sunk around 65 centimeters into the 2.60 meter thick concrete foundation.

In reactor blocks 2 and 3, thanks to a remaining diesel generator and batteries, the cooling lasts 70 and 36 hours respectively longer before the cooling fails and overheating occurs there too. Overheating, a hydrogen explosion and a core meltdown also occur in these reactor cores.

This escalation could only have been prevented by immediately flooding the reactors with seawater. But the power plant operator Tepco is delaying this because this measure would have meant the irretrievable end of the power plant. The cooling with sea water only takes place under pressure from the Japanese authorities - and too late. In reactor block 1, the pumping in of seawater does not start until 28 hours after the quake - four hours after the core began to melt and after the first hydrogen explosion. The other blocks are also flooded too late.

March 11, 2021

- Nadja Podbregar