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  • 執筆者の写真奥山 俊宏

Left Open “Protection Door” at Tsunami-Hit Nuclear Power Plant Resulted in Fukushima Catastrophe

Protective door left open on March 24, 2011 (an edited extract from an image released by TEPCO on December 22, 2020)
Protection door left open on March 24, 2011 (an edited extract from an image released by TEPCO on December 22, 2020)

On March 11, 2011, the Fukushima Daiichi Nuclear Power Station, operated by Tokyo Electric Power Company, Inc. (TEPCO), experienced a catastrophic tsunami that triggered a series of meltdowns. Recently, it has come to light that the disaster was exacerbated by an unfortunate incident where a protection door was left open at the large loading entrance, referred to as the "truck bay," of Unit 1 turbine building facing the Pacific Ocean.


The protection doors are known as "physical protection doors" and were installed outside the entrance shutters to safeguard nuclear materials from physical attacks, such as attempts by terrorists to forcefully enter using vehicles. However, when the powerful tsunami struck the plant at 15:36 on March 11, 2011, the open protection door allowed an exceptionally large volume of seawater to enter the Unit 1 buildings, causing a complete loss of electric power. Surprisingly, TEPCO became aware of this critical fact of a protection door being left open only around 2016, more than five years after the initial accident occurred.

Buildings of unit 1 and unit 2 of Fukushima Daiichi Nuclear Power Station on March 20, 2011 (an edited extract from an image released by TEPCO on December 22, 2020)
Buildings of unit 1 and unit 2 of Fukushima Daiichi Nuclear Power Station on March 20, 2011 (an edited extract from an image released by TEPCO on December 22, 2020)

The loss of power in Unit 1 triggered a chain reaction of failures that affected Units 3, 2, and 4. If the volume of water entering the building had been smaller, the DC power (batteries) responsible for controlling the reactor could potentially have been maintained, thereby preventing the subsequent meltdowns and radiation releases.


The revelation about the open protection door was disclosed in 2016 during an accident review conducted by the Niigata Prefectural government. Subsequently, the information was reported to the national Nuclear Regulation Authority (NRA) in 2017. However, it has not been actively communicated to the media or the general public in a manner that garnered significant attention.

“An image of tsunami inflow into Unit 1 turbine building” from an extract for the Attachment Earthquake-tsunami-2 to the TEPCO’s report to the NRA on December 25, 2017
“An image of tsunami inflow into Unit 1 turbine building” from an extract of the Attachment Earthquake-tsunami-2 to the TEPCO’s report to the NRA on December 25, 2017

In a report among a large volume of various reports submitted to the NRA, TEPCO stated, "The entrance had a protection door and shutter. Usually, they both would be closed, but the door of Unit-1 was open at the time of the tsunami arrival. On that day, the protection door had been opened for scheduled work. The door was left open, upon evacuation of the workers immediately after the earthquake, until the time of the tsunami arrival. If the protection door had been closed, the tsunami flooding might have been controlled to a certain degree, but with the door being left open, the shutter was deformed and broken by the tsunami and drifting wreckage, and inflow of a large amount of seawater was allowed. "

“Locations of power equipment of each unit and main paths assumed of tsunami inundation” from an extract for the Attachment Earthquake-tsunami-2 to the TEPCO’s report to the NRA on December 25, 2017
“Locations of power equipment of each unit and main paths assumed of tsunami inundation” from an extract of the Attachment Earthquake-tsunami-2 to the TEPCO’s report to the NRA on December 25, 2017

The situation inside the entrance presented an unobstructed 110 meters path to the control building, which served as the nerve center for reactor management. Surprisingly, there were no walls or doors between the entrance and the area where DC power battery chargers and panels were installed in the Unit 1 control building. While there was a partition wall between the Unit 1 control building and the adjacent Unit 2 control building, a door in that wall was not watertight, as confirmed by a TEPCO spokesperson. Consequently, the invading seawater would have disabled the batteries of the DC power supply in both control buildings, rendering them inoperable. This loss of power have made it impossible to monitor and control the two reactors effectively.


These details have now been disclosed in a report published by the Weekly Economist on July 10, 2023. (Source: Weekly Economist, July 18-25, 2023, pp.74-78, also available at https://weekly-economist.mainichi.jp/articles/20230725/se1/00m/020/048000c)

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