This underwater robot was used in the recent probe of reactor 3 at the Fukushima No. 1 power plant.
The first images of melted fuel from the crippled Fukushima No. 1 power plant indicate that it did not burn through the pressure vessel of reactor 3, but exited through the holes used to insert the control rods, officials say.
While the landmark robot footage from the primary containment vessel of unit 3 is helping Tokyo Electric grasp the reality of the damaged fuel assemblies, it may also force it to rewrite the road map for decommissioning the meltdown-hit plant.
Tokyo Electric Power Company Holdings Inc., better known as Tepco, sent an underwater robot into reactor 3 earlier this month to confirm its hypothesis that the core — the fuel assemblies in the pressure vessel — broke apart and fell to the bottom, letting molten fuel burn through and drip into the primary containment vessel.
According to Tepco spokesman Takahiro Kimoto, however, the images taken beneath the PCV indicate the pressure vessel probably withstood the heat of the molten fuel. He said the fuel apparently seeped through the holes for the control rods.
“We do not presume that the vessel, which is 14 cm thick, melted and collapsed together with the fuel, but that part of the fuel instead made its way down through holes,” Kimoto said. The control rods are used to moderate the chain reaction and are inserted vertically into the core.
Tepco said it estimates reactor 3 has about 364 tons of fuel debris, and that similar amounts will be found in reactors 1 and 2. Removing the fuel from the reactors is the largest challenge in defueling the aged plant — a process that could take up to 40 years to complete.
The camera on the underwater robot also captured images of rubble around the fuel debris, which could slow the removal process. The rubble includes devices for supporting the control rods at the bottom of the PCV and scaffolding for maintenance workers beneath the pressure vessel.
Minister of Economy, Trade and Industry Hiroshige Seko said the government and Tepco will try to draft a plan for removing the melted fuel in September, with an eye to hammering out the specifics in the first half of fiscal 2018 and starting the work in 2021.
But the findings from reactor 3 may force them to alter the state’s road map for decommissioning Fukushima No. 1, officials said.
An entity providing technical support for the project has urged that efforts be made to remove the melted fuel from the submerged lower part of the PCV by keeping air in the upper part, according to a source familiar with the plan.
Although filling the PCV completely with water would largely reduce the radiation risk to the robot probes, the Nuclear Damage Compensation and Decommissioning Facilitation Corp. is reluctant to do so because it is damaged and the toxic water will just leak out, the source said.
At the other two reactors, Tepco thinks most of the fuel in reactor 1 fell to the bottom of the primary containment vessel, and that some of the fuel in reactor 2 remained in the pressure vessel. The company made the estimates based on cosmic ray imaging analysis and by sending robots and endoscopes into the PCVs of the two reactors.