TOKYO — A record radiation level has been detected inside the No. 2 reactor at the damaged Fukushima Daiichi nuclear complex, with the estimated reading of up to 530 sieverts per hour, the plant operator said Thursday.
The reading means a person could die from even brief exposure, highlighting the difficulties ahead as the government and Tokyo Electric Power Company Holdings Inc. grope their way toward dismantling all three reactors that melted down in the March 2011 nuclear disaster.
The plant operator also announced that based on an image analysis, a 1-square-meter hole has been found on a metal grating beneath the reactor pressure vessel, likely caused by melted nuclear fuel that fell through the vessel.
The new radiation level, described by some experts as “unimaginable,” far exceeds 73 sieverts per hour, the previously highest radiation reading monitored in the interior of the reactor.
An official of the National Institute of Radiological Sciences said medical professionals have never considered dealing with this level of radiation in their work.
According to TEPCO, the extremely high radiation level was detected inside the containment vessel, in the space around 2.3 meters away from the base of the reactor pressure vessel.
According to the institute, 4 sieverts of radiation exposure would kill one in two people.
Experts say 1,000 millisieverts, which equals 1 sievert, could lead to infertility, loss of hair and cataracts, while exposure to radiation doses above 100 millisieverts increases the risk of cancer.
The latest discovery spells difficulty in removing the fuel debris as part of decommissioning work at the plant. The government and TEPCO hope to locate the fuel and start removing it from a first reactor in 2021.
The debris is believed to have been created as nuclear fuel inside the reactor pressure vessel overheated and melted due to the loss of reactor cooling functions.
In the coming weeks, the plant operator plans to deploy a remote-controlled robot to check conditions inside the containment vessel, but the utility is likely to have to change its plan.
For one thing, it will have to reconsider the route the robot is to take to probe the interior because of the hole found on the grating.
Also, given the extraordinary level of radiation inside the containment vessel, the robot would only be able to operate for less than two hours before it is destroyed.
That is because the robot is designed to withstand exposure to a total of up to 1,000 sieverts of radiation. Based on the calculation of 73 sieverts per hour, the robot could have operated for more than 10 hours, but 530 sieverts per hour means the robot would be rendered inoperable in less than two hours.
The latest analysis follows TEPCO’s discovery Monday of a black mass deposited on the grating directly beneath the pressure vessel, possibly melted fuel after the unit suffered a meltdown along with two other reactors at the six-reactor plant.
Images captured using a camera attached to a telescopic arm on Monday also showed part of the grating has gone.
If the deposits are confirmed as fuel debris, it would be the first time the utility has found any at the three units that suffered meltdowns.
Following the world’s worst nuclear disaster since the 1986 Chernobyl catastrophe, the plant’s No. 1 to 3 reactors suffered fuel meltdowns.
Portions of the fuel in the reactors are believed to have accumulated at the bottom of the containment vessels. But the actual condition of the melted fuel remains unknown due to high radiation levels.