By NORIHIKO KUWABARA
Exceedingly high radiation levels found inside crippled reactor buildings at the Fukushima No. 1 nuclear plant were labeled by nuclear regulators as an “extremely serious” challenge to the shutdown process and overall decommissioning of the site.
The Nuclear Regulation Authority (NRA) said a huge amount of radioactive materials apparently had attached to shield plugs of the containment vessels in the No. 2 and No. 3 reactors.
Radiation levels were estimated at 10 sieverts per hour, a lethal dose for anyone who spends even an hour in the vicinity, according to experts.
The finding would make it exceptionally difficult for workers to move the shield plugs, raising the prospect that the plan to decommission the reactors will have to be reassessed.
A shield plug, made of reinforced concrete, is circular in shape and measures about 12 meters in diameter.
It has a triple-layer structure, with each layer about 60 centimeters thick. It is placed above the containment vessel like a lid on the top floor of a reactor building.
The shield plug blocks radiation from the reactor core at normal times.
When nuclear fuels need to be replaced, workers remove a shield plug to gain access to the interior of the containment vessel.
The NRA’s study found that the amount of radioactive cesium 137 was estimated at 20-40 petabecquerels between the space between the top and middle layers of the shied plug of the No. 2 reactor.
That works out to more than 10 sieverts per hour based on readings of radiation levels nearby. Radiation at such levels can kill a person if they are exposed for an hour, according to experts.
The estimated figure was 30 petabecquerels for the No. 3 reactor.
As larger amounts of cesium 137 leaked from the No. 1 reactor through the damaged plug, the amount of the radioactive material attached to its shield plug was estimated at 0.16 petabecquerels, considerably lower than for the No. 2 and No. 3 reactors.
In contrast, the shield plugs for the No. 2 and No. 3 reactors remained relatively unscathed, blocking a huge amount of radioactive substances that leaked from their containment vessels from escaping into the atmosphere, according to the NRA.