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.
Overview and history of the accident:
Other related information:
The Tokyo Electric Power Company recognizes ourselves as the main party involved in the nuclear accident triggered by the Tohoku-Chihou-Taiheiyo-Oki Earthquake on March 11, 2011. We have established the "Fukushima Nuclear Accident Investigation Committee" and "Nuclear Safety and Quality Assurance Meeting Accident Investigation Verification Committee" comprised of external experts, and have been engaging in a thorough investigation and verification of the accident. An interim report that summarizes the results of our investigation and verification efforts was released on June 20, 2012 entitled the "Fukushima Nuclear Accidents Investigation Report".
With the release of this report, we are once again made keenly aware of our responsibility for the accident and are determined to engage in business operations with safety as the top priority in order to prevent future recurrence. At the same time, we will continue thoroughly implementing our mid to long-term measures toward decommissioning of the reactors at Fukushima Daiichi Nuclear Power Station.
We identified unconfirmed and unresolved issues on the detailed development mechanism after the March 11, 2011 accident occurred at Fukushima Daiichi, and are compiling reports in order to make progress on the decommissioning work as well as make continuous safety improvements at nuclear power stations.
TEPCO will continue working to gain a thorough understanding of what happened, such as the performance of the nuclear reactors at the time of accident, through planned investigation of the site and simulation analyses. By doing so, we will also enhance the obligation of nuclear power stations to operate safely, utilize the results to carry out decommissioning work, and continue to make progress in our nuclear safety reform initiative.