A draft investigation report into the Fukushima Dai-ichi nuclear power plant meltdown, adopted by Japanese nuclear regulators yesterday, said that dangerously high levels of radioactive contamination have been detected at two of the three reactors, adding to concerns about decommissioning challenges.
Data collected by investigators showed that the sealing plugs sitting atop the No. 2 and No. 3 reactor containment vessels were as fatally contaminated as nuclear fuel debris that had melted and fallen to the bottom of the reactors following the March 2011 tsunami and earthquake, the report said.
Experts said that the bottom of the sealed plug, a triple-layered concrete disc-shaped lid 12m in diameter sitting atop the primary containment vessel, is coated with high levels of radioactive cesium-137.
Photo: AP
The No. 1 reactor lid was less contaminated, presumably because the plug was slightly knocked out of place and disfigured due to the impact of the hydrogen explosion, the report said.
The experts measured radiation levels at multiple locations inside the three reactor buildings, and examined how radioactive materials moved and safety equipment functioned during the accident. They also said that a venting attempt at Unit 2 to prevent reactor damage never worked, and that safety measures and equipment designs still need to be examined.
The lid contamination does not affect the environment as the containment vessels are enclosed inside the reactor buildings.
The report did not give further details about if or how the lid contamination would affect the decommissioning progress.
Japanese Nuclear Regulation Commission Chairman Toyoshi Fuketa called the findings “extremely serious,” and said that they would make melted fuel removal “more difficult.”
Figuring out how to remove the lids would be a major challenge, he said.
Removing an estimated 810 tonnes of melted fuel debris from three reactors is a daunting task expected to take decades, and officials have not been able to describe exactly when or how it might end.
The Fukushima plant was to start removing melted fuel debris from Unit 2, the first of three reactors, later this year ahead of the 10th anniversary of the accident.
However, plant operator Tokyo Electric Power Co and the government last month announced a delay until 2022.
They said that the development of a robotic arm for the debris removal — a joint project with the UK — had been delayed due to the COVID-19 pandemic.
Under the current plan, a remote-controlled robotic arm would be inserted from the side of the reactor to reach the molten fuel mixed with melted parts and concrete floor of the reactor. Eventually the lids also would have to be removed, but their contamination is a major setback.
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