At 14:46 on March 11, 2011, a tremendous earthquake of a 9.0 magnitude occurred undersea off the coast of the Sanriku region of Japan, triggering a massive tsunami on an unprecedented scale that hit the northeastern coast 50 minutes later.
The earthquake caused the lose of all off-site power supplies of the power station, but we succeeded in cooling the reactors by using emergency power. Units 1 to 3, which were in operation when the earthquake struck, shut down safely as designed.
However, this emergency power was also lost due to flooding from the tsunami, causing the cooling equipment to become inoperable, thereby resulting in the water in the reactor pressure vessels of Units 1 to 3 evaporating into steam.
It is supposed that hydrogen, produced by the chemical reaction between fuel rods sticking out of the water and steam, accumulated in the upper part of the reactor buildings and triggered explosions in Units 1 and 3. For Unit 4, it is supposed that hydrogen that flowed in through the joint part of the exhaust stack accumulated when the air in the primary containment vessel of Unit 3 was vented to the outside, leading to the explosion.

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