Tokyo Electric has said that a remotely controlled robot investigating the inside of reactor 3 at the Fukushima No. 1 power plant has finally spotted objects that could be fuel debris — a potential milestone in the effort to clean up one of the worst atomic disasters in history.
This is the first time Tepco has found something likely to be melted fuel. When the utility sent a different robot into reactor 2 in January, it found black lumps sticking to the grating in the primary containment vessel but said they were difficult to identify.
The objects spotted this time look like icicles hanging around a control rod drive attached to the bottom of the pressure vessel, which holds the core, Tokyo Electric Power Company Holdings Inc. said at an evening news conference Friday.
Enclosed by the huge primary containment vessel, the pressure vessel originally contained the fuel rod assemblies. But the rods melted into a puddle and burned through its bottom once the plant lost power after being swamped by the monstrous tsunami of March 11, 2011.
The robot also captured images of lumps of material that appear to have melted and resolidified near the wall of the pedestal, a concrete structure that supports the pressure vessel.
“From the pictures taken today, it is obvious that some melted objects came out of the reactor. This means something of high temperature melted some structural objects and came out. So it is natural to think that melted fuel rods are mixed with them,” said Takahiro Kimoto, a Tepco spokesman.
“In that sense, it is possible that the melted objects found this time are melted fuel debris or probably around it,” he said, adding the utility will think about how they can be analyzed to determine if they are the former fuel rods.
Fuel from a nuclear meltdown is known as corium, a mixture of fuel rods and other structural materials.
“It is important to know the exact locations and the physical, chemical, radiological forms of the corium to develop the necessary engineering defueling plans for the safe removal of the radioactive materials,” said Lake Barrett, a former official at the U.S. Nuclear Regulatory Commission who was involved with the cleanup at the Three Mile Island nuclear power plant in the U.S. “The recent investigation results are significant early signs of progress on the long road ahead.”
Because of the high radioactivity in the reactor, only specially designed robots can probe the unit. And the unprecedented nature of the Fukushima disaster means that Tepco is pinning its efforts on technology not yet invented to get the melted fuel out of the reactors.
The utility aims to decide on the procedure for removing the melted fuel from each unit as soon as this summer. And it will confirm the procedure for the first reactor during fiscal 2018 ending in March 2019, with removal slated to begin in 2021.
Decommissioning the reactors will cost ¥8 trillion ($72 billion), according to an estimate in December from the Ministry of Economy, Trade and Industry. Removing the fuel is one of the most important steps in the cleanup, which may take as long as 40 years.
The significance of Friday’s finding “might be evidence that the robots used by Tepco can now deal with the higher radiation levels, at least for periods of time that allow them to search parts of the reactor that are more likely to contain fuel debris,” M.V. Ramana, professor at the Liu Institute for Global Issues at the University of British Columbia, said by email.
“If some of these fragments can be brought out of the reactor and studied, it would allow nuclear engineers and scientists to better model what happened during the accident,” Ramana said.
The utility began probing reactor 3 on Wednesday. Since the PCV has 6 meters of water in it, which is higher than in reactors 1 and 2, the 30-cm robot will have to go deep under water.
The robot has two cameras — one on the front that can pivot 180 degrees vertically, and another on its back.