Most of the melted nuclear fuel inside the No. 2 reactor at the disaster-hit Fukushima Daiichi power plant is likely located at the bottom of its pressure vessel, Tokyo Electric Power Company Holdings Inc. said Thursday.
According to a study that used a cosmic ray imaging system, an estimated 130 tons of the so-called fuel debris remains at the bottom of the vessel, the first time the location and amount of the melted fuel have been estimated.
The finding is important as the data could help the operator to narrow down methods to remove the fuel debris, the most challenging task in decommissioning the plant's Nos. 1 to 3 reactors that experienced meltdowns in the nuclear crisis that began in March 2011.
The study was carried out by a team involving Tokyo Electric and the High Energy Accelerator Research Organization in Ibaraki Prefecture.
As high radiation levels are continuing to hamper direct access to the reactors, researchers have tracked muon elementary particles, which are produced as cosmic rays collide with atmospheric particles and change course when coming into contact with nuclear fuel.
The No. 2 reactor was in operation when the nuclear crisis was triggered by a powerful earthquake and subsequent tsunami that devastated Japan's northeast.
About 160 tons of fuel assemblies are estimated to have been present inside the reactor vessel prior to the crisis. Most of the fuel is believed to have fallen to the bottom of the pressure vessel and mixed with nearby structures to form debris.
In the nuclear crisis, massive amounts of radioactive substances were released into the environment, with the Nos. 1, 3 and 4 reactor buildings damaged by hydrogen explosions.
The No. 4 reactor was offline for periodic maintenance work and all of its fuel was stored in the spent fuel pool, avoiding a meltdown.
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