Potential fuel debris hanging as icicles

July 21, 2017

Possible 'icicle' of melted fuel spotted at Fukushima plant

THE ASAHI SHIMBUN

http://www.asahi.com/ajw/articles/AJ201707210036.html

An icicle that appears to be melted nuclear fuel has been seen hanging from the bottom of the pressure vessel of the No. 3 reactor at the crippled Fukushima No. 1 nuclear power plant, sources said.

Tokyo Electric Power Co., the plant's operator, confirmed the presence of what is believed to be fuel debris on July 21 with an underwater robot equipped with a camera, the sources said.

If the icicle is confirmed to be fuel debris, it will become valuable data in the investigation to determine the cause of the accident, triggered by the March 2011 Great East Japan Earthquake and tsunami, and efforts to decommission the reactor.

The survey of the No. 3 reactor conducted so far has found that most of the nuclear fuel apparently melted and fell through a hole at the bottom of the pressure vessel.

On July 21, TEPCO deployed the underwater robot into the No. 3 reactor’s containment vessel, which holds the pressure vessel, from early in the morning to look into the area below the pressure vessel.

When the robot entered the area, its camera showed what seems to be fuel debris hanging like an icicle from a hole at the bottom of the pressure vessel.

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Fukushima robot finds potential fuel debris hanging like icicles in reactor 3

http://www.japantimes.co.jp/news/2017/07/21/national/fukushima-robot-finds-potential-fuel-debris-hanging-like-icicles-reactor-3/#.WXNFQFFpyos

Staff Report, Bloomberg

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.

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July 20, 2017

See also 2-minutes video on

Swimming robot probes Fukushima reactor to find melted fuel

https://mainichi.jp/english/articles/20170720/p2g/00m/0dm/032000c

TOKYO (AP) -- An underwater robot entered a badly damaged reactor at Japan's crippled Fukushima nuclear plant Wednesday, capturing images of the harsh impact of its meltdown, including key structures that were torn and knocked out of place.

Plant operator Tokyo Electric Power Co. said the robot, nicknamed "the Little Sunfish," successfully completed the day's work inside the primary containment vessel of the Unit 3 reactor at Fukushima, which was destroyed by a massive March 2011 earthquake and tsunami.

TEPCO spokesman Takahiro Kimoto praised the work, saying the robot captured views of the underwater damage that had not been previously seen. However, the images contained no obvious sign of the melted nuclear fuel that researchers hope to locate, he said.

The robot was left inside the reactor near a structure called the pedestal, and is expected to go deeper inside for a fuller investigation Friday in hopes of finding the melted fuel.

"The damage to the structures was caused by the melted fuel or its heat," Kimoto told a late-night news conference held nine hours after the probe ended its exploration earlier in the day.

The robot, about the size of a loaf of bread, is equipped with lights, maneuvers with five propellers and collects data with two cameras and a dosimeter. It is controlled remotely by a group of four operators.

The robot was co-developed by Toshiba Corp., the electronics and energy company charged with helping clean up the plant, and the International Research Institute for Nuclear Decommissioning, a government-funded consortium.

It was on a mission to study the damage and find the fuel that experts say has melted, breached the core and mostly fallen to the bottom of the primary containment chamber, where it has been submerged by highly radioactive water as deep as 6 meters (20 feet).

The robot discovered that a grate platform that is supposed to be below the reactor core was missing and apparently was knocked down by melted fuel and other materials that fell from above, and that parts of a safety system called a control rod drive were also missing.

Remote-controlled robots are key to the decades-long decommissioning of the damaged plant, but super-high levels of radiation and structural damage have hampered earlier probes at two other reactors at the plant.

Japanese officials say they want to determine preliminary methods for removing the melted nuclear fuel this summer and start work in 2021.

Scientists need to know the fuel's exact location and understand the structural damage in each of the three wrecked reactors to work out the safest and most efficient ways to remove the fuel.

Robots tested earlier became stuck inside the two other reactors. A scorpion-shaped robot's crawling function failed and it was left inside the plant's Unit 2 containment vessel. A snake-shaped robot designed to clear debris for the scorpion probe was removed after two hours when its cameras failed due to radiation levels five times higher than anticipated.

The robot used Wednesday was designed to tolerate radiation of up to 200 sieverts -- a level that can kill humans instantly.

Kimoto said the robot showed that the Unit 3 reactor chamber was "clearly more severely damaged" than Unit 2, which was explored by the scorpion probe.