Most of the melted nuclear fuel inside the No. 2 reactor at the disaster-hit Fukushima No. 1 power plant is likely located at the bottom of its pressure vessel, Tokyo Electric Power Company Holdings Inc. has revealed.
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, announced on Thursday, 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.
Tepco plans to decide how to start removing the debris in two years, with work to start in 2021.
The cosmic-ray 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 the 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 routine maintenance work and all of its fuel was stored in the spent fuel pool, avoiding a meltdown. The utility removed all fuel rod assemblies from the pool and transported them to a more stable building in 2014.
Also on Thursday, Tepco said it plans to seek additional financial support from the government to cover soaring costs for dealing with the disaster.
Tepco specifically asked the government to clarify its views on how the costs for providing compensation to affected residents and the decommissioning of the plant should be shouldered.
Each request for additional aid will be carefully scrutinized by the government out of concern it could come under fire from taxpayers, sources said.
The government has already agreed to provide up to ¥9 trillion in loans to Tepco to cover ¥5.4 trillion in compensation to be paid to affected residents and ¥2.5 trillion in decontamination costs that were projected in January 2014.
But Tepco said compensation payments have already topped ¥6 trillion and that decontamination costs are increasingly likely to exceed estimates.
The utility also said it is concerned about massive costs to be incurred when it starts full-fledged decommissioning work at the damaged nuclear plant.
If the situation is left unaddressed, Tepco’s market value will suffer, Executive Officer Keita Nishiyama said.
Tepco President Naomi Hirose said that although overall compensation costs have not been fixed, his company will consult with the government on how those costs will be funded.