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Fukushima Daiichi Nuclear Power Plant, method to remove debris by submerging the buildings in water: Symbolic of project difficulties after repeated changes, with no prospects for feasibility

October 24, 2022
In order to remove melted nuclear fuel (debris), which is considered the most difficult part of the restoration work at TEPCO’s Fukushima Daiichi Nuclear Power Plant, a method has emerged to submerge the entire reactor building, including its basement, under water. This would require unprecedented large-scale construction work, and there are doubts about its feasibility. The fact that the debris removal plan has been repeatedly changed and a proposal that seems to be grasping at a cloud has emerged is symbolic of the difficulties involved. （The fact that a proposal that seems to be grasping at a cloud has emerged is symbolic of the difficulties that lie ahead.)
Even the experts are not confident.
　The first of its kind in the world,” “Technically quite difficult,”

◆”Not confident” even experts are bearish

At a press conference held on November 11 to explain the proposal that includes a new construction method for the Unit 3 reactor, Mr. Mitsuroku Ikegami, executive director of the Nuclear Damage Liability and Decommissioning Support Organization, who is in charge of providing technical support to help bring about a restoration from the accident, repeated his bearish comments.
　The new construction method is based on the “hull construction method” used for tanker hull construction. The new method is characterized by enclosing the building with a structure that is resistant to water pressure, and ETIC is considering digging a tunnel under the building and enclosing the entire building with a structure consisting of a series of square rooms made of steel.
　If this is realized, water filled with the building will be used to shield it from radiation, thereby increasing the safety of the work. On the other hand, submerging the building in water is expected to generate about 150,000 tons of highly contaminated water that has come into contact with debris. This is equivalent to about 150 tanks storing treated water on the site, and the risk of a leakage accident is immeasurable.

◆”Flooding” first, stop once, flood again.
　At the beginning of the accident in 2011, the government and TEPCO planned to use the “flooding method” to fill the containment vessel with water and remove debris underwater. However, the containment vessels of Units 1 through 3, where debris was located, were all damaged, and even if water was filled, it would leak out of the vessels. The high radiation dose makes it inaccessible to humans, and it is still difficult to determine which parts of the containment vessels are damaged.

In 2005, they switched to the “in-air” method of removing debris without water, and are aiming to begin trial removal of debris from Unit 2 in the latter half of FY2011. However, since the method prioritizes easy access to the debris and uses a robot arm in a confined space, only about 1 gram of debris can be removed in a single operation. It is estimated that there is a total of 880 tons of debris in the three reactors, making it almost impossible to complete the removal using this method.
　Therefore, JAEA has switched to a method to remove a large amount of debris from the Unit 3 reactor. The work is expected to involve the scattering of enormous amounts of radioactive materials, such as by cutting the debris into chunks, and if the debris is not shielded by water, it will be very dangerous. Since the containment vessel cannot be filled with water, the idea of submerging the entire building outside of it has emerged. In other words, it is a flooding method that has been reshaped on a large scale.

◆Even after 11 years, it remains unrealistic.
　Debris from the Unit 1 reactor is believed to be scattered over a wide area of the containment vessel, and there is no way to remove the debris. Eleven and a half years after the accident, debris removal remains unrealistic.
　A spokesperson for TEPCO has refused to go into the feasibility of the proposed new method, saying, “It is still in the idea stage. Even the OIST’s proposal suggests a bleak outlook, concluding with the following words: “If the criteria are not met, we will be forced to take out the debris. If the criteria are not met, we will have to start over from the identification of issues.
https://www.tokyo-np.co.jp/article/209821

October 26, 2022 Posted by | Fuk 2022 | , , , | Leave a comment

Melted Nuclear Fuel Search Proceeds One Dead Robot at a Time

16-fev-2017

 

The latest robot seeking to find the 600 tons of nuclear fuel and debris that melted down six year ago in Japan’s wrecked Fukushima Dai-Ichi power plant met its end in less than a day.

The scorpion-shaped machine, built by Toshiba Corp., entered the No. 2 reactor core Thursday and stopped 3 meters (9.8 feet) short of a grate that would have provided a view of where fuel residue is suspected to have gathered. Two previous robots aborted similar missions after one got stuck in a gap and another was abandoned after finding no fuel in six days.

After spending most of the time since the 2011 disaster containing radiation and limiting ground water contamination, scientists still don’t have all the information they need for a cleanup that the Japanese government estimates will take four decades and cost 8 trillion yen ($70.6 billion). It’s not yet known if the fuel melted into or through the containment vessel’s concrete floor, and determining the fuel’s radioactivity and location is crucial to inventing the technology needed to remove it. 

“The roadmap for removing the fuel is going to be long, 2020 and beyond,” Jacopo Buongiorno, a professor of nuclear science and engineering at the Massachusetts Institute of Technology, said in an e-mail. “The re-solidified fuel is likely stuck to the vessel wall and vessel internal structures. So the debris have to be cut, scooped, put into a sealed and shielded container and then extracted from the containment vessel. All done by robots.”

Read more: Robots are being utilized to clean up U.K.’s nuclear waste

To enter a primary containment vessel, which measures about 20 meters at its widest, more than 30 meters tall and is encased in meters of concrete, outside air pressure is increased to keep radiation from escaping and a sealed hole is opened that the robot passes through. Three reactors at the plant suffered meltdowns, and each poses different challenges and requires a custom approach for locating and removing the fuel, said Tatsuhiro Yamagishi, Tokyo Electric Power Co. Holdings Inc. spokesman.

The machines are built with specially hardened parts and minimal electronic circuitry so that they can withstand radiation, if only for a few hours at a time. Thursday’s mission ended after the robot’s left roller-belt failed, according to Tokyo Electric, better known as Tepco. Even if it had returned, this robot, like all others so far designed to aid the search for the lost fuel, was expected to find its final resting place inside a reactor.

No. 1 Unit

Hitachi Corp. in the next two months plans to send a machine into the No. 1 reactor core that scientists hope can transmit photos of the fuel and measure radiation levels.

The snake-like robot will lower a camera on a wire from a grate platform in the reactor to take photos and generate 3-D models of the bottom of the containment vessel. This will be the third time Hitachi sends in this robot design.

While the company is hopeful this robot will find some of the fuel, it will likely be unable to find all of it, according to Satoshi Okada, a Hitachi engineer working on the project. The company is already planning the next robot voyage for after April.

“We are gathering information so that we can decide on a way to remove the fuel,” said Okada. “Once we understand the situation inside, we will be able to see the way to remove the fuel.”

No. 2 Unit

On Thursday, Toshiba’s scorpion-like robot entered the reactor and stopped short of making it onto the containment vessel’s grate. While Tepco decided not to retrieve it, the company views the attempt as progress.

“We got a very good hint as to where the fuel could be from this entire expedition” Tepco official Yuichi Okamura said Thursday at a briefing in Tokyo. “I consider this a success, a big success.”

Tepco released images last month of a grate under the No. 2 reactor covered in black residue that may be the melted fuel — one of the strongest clues yet to its location. The company measured radiation levels of around 650 sieverts per hour through the sound-noise in the video, the highest so far recorded in the Fukushima complex.

A short-term, whole-body dose of over 10 sieverts would cause immediate illness and subsequent death within a few weeks, according to the World Nuclear Association.

The Hitachi and Toshiba robots are designed to handle 1,000 sieverts and no robot has yet been disabled due to radiation.

“Radiation levels near the fuel are lethal,” said MIT’s Buongiorno, who holds the university’s Tepco chair, a professorship based on an initial donation by the company 10 years ago. There are no formal affiliations or obligations for the faculty who receive the chair, he said.

Because the No. 2 unit is the only one of the three reactors that didn’t experience a hydrogen explosion, there was no release into the atmosphere and radiation levels inside the core are higher compared to the other two units, according to the utility.

No. 3 Unit

Tepco’s balance sheet has been strapped by ballooning Fukushima cleanup costs and slumping national power demand. All of the company’s nuclear power plants remain shut since it halted the No. 6 reactor at its Kashiwazaki Kariwa station in March 2012. The company is seeking drastic changes in top management in consultation with the Japanese government, TV Asahi reported Friday, without attribution.

The utility has focused on removing spent fuel in the upper part of the reactor building, which Toshiba aims to extract with a claw-like system. This fuel didn’t melt and is still in a pool that controls its temperature.

The used-fuel in No. 3 is scheduled to begin removal before the end of the decade, the first among the three reactors that melted down. Toshiba is developing another robot to search for melted fuel, planned to enter sometime in the year ending March 2018. The company hasn’t announced yet the design or strategy.

https://about.bnef.com/blog/race-for-japans-melted-nuclear-fuel-leaves-trail-of-dead-robots/

February 21, 2017 Posted by | Fukushima 2017 | , , , | Leave a comment