A Nuclear Regulation Authority study group said Tuesday that a high concentration of radioactive cesium is highly likely to have accumulated in the lids of the containment vessels for the No. 2 and No. 3 reactors at the tsunami-hit Fukushima No. 1 nuclear power plant.
The group showed the possibility in a draft of a new interim report on the nuclear accident.
This could affect the decommissioning process for the Tokyo Electric Power Company Holdings Inc. plant. It has been believed that most of the radioactive materials is left at the bottom of the reactors in the form of melted nuclear fuel debris.
The group, which is investigating details of the accident, produced the draft report based on data obtained from on-site inspections of the No. 2 and No. 3 reactors conducted in September 2019 and later following the decline of radiation dose and progress in the decommissioning work. The first interim report was compiled in October 2014.
On the fifth floor of the No. 2 and No. 3 reactor buildings, a very high radiation dose was measured near the three-layered reinforced concrete lids, or shield plugs, at the upper part of the reactor containment vessels, according to the draft report.
An analysis found that 40 to 70 petabecquerels of cesium-137 is highly likely to remain between the first and second layers, counted from the outside, of the No. 2 reactor lid, and 30 petabecquerels between the first and second layers of the No. 3 reactor lid. That radiation dose can kill humans quickly.
The dose measured 0.1 to 0.2 petabecquerel at the No. 1 reactor lid.
The study group also looked into the hydrogen explosion at the No. 3 reactor, which occurred on March 14, 2011, three days after the nuclear plant was damaged by a powerful earthquake and tsunami. The group said that the burning of flammable gas inside the reactor building may have added to the explosion.
The new interim report will be adopted by the end of March.
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