Ahead of the 10-year anniversary of the March 2011 accident at Tokyo Electric Power Company Holdings Inc.'s Fukushima No. 1 nuclear power plant, a panel of the Nuclear Regulation Authority on Tuesday released a draft interim report on the accident investigation, which resumed in September 2019. The on-site investigation found that there were areas with extremely high radiation levels near the fifth floor of the reactor buildings of Units 2 and 3. This may lead to a delay in the decommissioning process of the plant.
According to the draft report, high levels of contamination were found at the bottom of a concrete lid called a shield plug, located at the top of the reactor containment vessel. Radioactive cesium there was estimated to be giving off about 20 to 40 petabecquerels of radiation at Unit 2 and about 30 petabecquerels at Unit 3. (The prefix peta indicates 1,000 trillion.)
In terms of radiation dosage, this is likely to be equivalent to several sieverts per hour. If a person were to enter the area, a fatal dose would accumulate in just a few hours.
TEPCO plans to first remove the nuclear fuel debris, which is a mixture of melted nuclear fuel and other materials, from the Unit 2 reactor. Depending on the removal route, it may be necessary to remove the shield plug. The removal of this structure is expected to take a long time, not only because of the high radiation levels but also because the shield plug weighs a total of 465 tons.
Looking back at what had happened during the crisis at the plants in 2011, the panel also examined the effects of venting, or releasing steam containing radioactive materials into the atmosphere to reduce pressure, which was done to prevent damage to the containment vessels. As a result of examining the piping and other parts, the panel found that steam flowed back into the reactor buildings of Units 1 and 3, where the venting was successful.
They also analyzed the TV footage from that time and noted that there had been multiple hydrogen explosions at the Unit 3 reactor.
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