Finding and removing melted fuel rods at Fukushima No. 1 — Nikkei Asian Review, The Japan Times

Nikkei Asian Review, “Survey fails to find melted rods at Fukushima reactors”:

” TOKYO — A remote survey of the Fukushima Daiichi nuclear plant’s No. 1 reactor was unable to locate and photograph melted nuclear fuel, Tokyo Electric Power Co. Holdings said Thursday, complicating efforts to remove that material as part of an extensive cleanup.

Tepco on Saturday sent a robot equipped with a camera into the containment vessel for the No. 1 unit. The majority of fuel rods have melted through the unit’s pressure vessel since the plant was struck by the March 11, 2011, earthquake and tsunami. The prevailing view has been that those melted fuel rods are now sitting under 2.5m of water at the bottom of the containment vessel.

The plan was to explore the bottom section by dipping a camera into the pool of water for the first time. But unexpected barriers such as pipes kept the camera around 1 meter from the bottom in most of the 10 positions surveyed instead of the intended depth of about 40cm from the bottom. While the camera was able to capture sand-like sediment, there was no trace of the melted fuel rods. Adding a fifth day to the investigation turned up no further evidence.

Yuichi Okamura, acting general manager of Tepco’s onsite nuclear power division, offered few comments at the utility’s Thursday news conference, saying only that “photographs and radiation data will need to be evaluated in conjunction with one another.”

The timeline set by Tepco and the government for decommissioning the Fukushima plant aims to begin extraction of melted-down material from the No. 1, No. 2 or No. 3 reactor in 2021 or earlier. An extraction plan is to be decided this summer. But the fact that the status of the melted rods still remains unknown underscores the seriousness of the accident.

The results of the robot survey were “limited,” according to Masanori Naitoh, director of nuclear safety analysis at the Institute of Applied Energy’s Nuclear Power Engineering Center. “It would be difficult to set a plan for extraction based on the information from this survey alone.”

An investigation of the No. 2 reactor also fell short, with the survey robot unable to reach the targeted spot right under the unit’s pressure vessel. ”

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The Japan Times, “Tepco’s biggest hurdle: How to remove melted fuel from crippled Fukushima reactors”:

” Six years after the triple meltdown at the Fukushima No. 1 nuclear power plant, recent investigations underneath the damaged reactor 2 using cameras and robots came close to identifying melted fuel rods for the first time.

Experts say getting a peek inside the containment vessel of reactor 2 was an accomplishment. But it also highlighted how tough it will be to further pinpoint the exact location of the melted fuel, let alone remove it some time in the future.

The biggest hurdle is the extremely lethal levels of radiation inside the containment vessel that not only prevent humans from getting near but have also crippled robots and other mechanical devices.

Safely removing the melted fuel would be a best-case scenario but the risks and costs should be weighed against the option of leaving the melted fuel in the crippled reactors, some experts said.

“The work to probe inside the containment vessels and remove the fuel debris will be extremely tough because of the high radiation levels,” said Hiroshi Miyano, who heads a panel of the Atomic Energy Society of Japan, which is discussing ways to decommission the Fukushima plant and making recommendations to the government.

The government and Tokyo Electric Power Company Holdings Inc. are trying to find a way to remedy the situation but existing methods and technologies may not be sufficient, Miyano said.

In search of melted fuel

The world’s attention turned to the melted fuel rods in late January when Tepco inserted a 10-meter-plus tube equipped with a camera into the containment vessel of reactor 2 to capture images under the pressure vessel that housed the fuel rods.

The images showed black lumps scattered beneath the pressure vessel.

When the March 11, 2011, Great East Japan Earthquake and monstrous tsunami hit, the plant suffered a blackout and lost its key cooling system, triggering meltdowns in reactors 1, 2 and 3. The melted nuclear fuel rods penetrated the pressure vessels and fell into the containment vessels.

Tepco had put cameras inside the containment vessels several times in the past six years but January’s probe was the first to apparently find melted fuel debris.

“We understand that this is a big milestone. We could finally get to see what it was like underneath the pressure vessel,” said Yuichi Okamura, general manager of Tepco’s nuclear power and plant siting division.

“This is critical information in order to remove the fuel debris.”

Radiation barrier

But Tepco hasn’t confirmed that the black lumps are melted fuel, saying they could be paint or cable wrappings, and further investigation is needed.

Capturing the images may be progress but the robot and camera forays have not provided enough information about how to deal with the melted fuel.

Last month, Tepco sent a remote-controlled, scorpion-shaped robot in to further probe inside the reactor 2 containment vessel. But the robot failed before it reached under the pressure vessel after a tire became stuck.

The robot’s dosimeter measured radiation levels of 210 sieverts per hour — enough to kill humans instantly.

While 210 sieverts per hour indicate the melted fuel was nearby, the radiation crippled the robot’s electronics, including its semiconductors and cameras, indicating that the further use of robots to pinpoint the melted fuel will be difficult, robotics experts said.

There are computer chips “designed to withstand a certain level of radiation, but the level inside the containment vessel is totally different,” said Satoshi Tadokoro, a professor at Tohoku University who is an expert on disasters and rescue robots.

The radiation can damage a robot’s chips that serve as their brains, causing the devices to lose control, said Tadokoro, whose robots have also been used at the Fukushima plant.

“On top of the high level of radiation, the entrance (to the containment vessel) for the robot is very small,” restricting what types of robots can be used to hunt for the melted fuel, he said.

Tepco said the opening it created on the side of the reactor 2 containment vessel is about 11 cm in diameter.

Fuel removal strategy

Tepco is set to conduct internal probes of the reactor 1 containment vessel this month and is preparing similar missions for reactor 3.

The government and utility then plan to adopt a basic fuel removal strategy this summer and fine-tune the plan next year, with the actual fuel removal taking place in or after 2021.

There are essentially three options for the strategy, according to the Tokyo-based International Research Institute for Nuclear Decommissioning (IRID), which is developing technologies for the Fukushima plant decommission.

One option is to flood the containment vessels with water and use a crane above the reactors to hoist up the melted fuel. The second option is to carry out the same process but without water. The third is to install removal equipment through the side of the containment vessel.

There are merits and drawbacks to each option, said Shoji Yamamoto, who heads the team developing technologies to create the fuel removal devices at IRID.

The flooding option can block radiation using water, but if the fuel melts into the water, it could pose a risk of recriticality. The debris may need to be cut into pieces for removal, but this process would enable water to get between multiple pieces, creating the condition for recriticality. For nuclear chain reactions to happen there needs to be a certain distance between nuclear fuel and water.

If there is no water, the recriticality risk is minimal but the massive radiation levels cannot be blocked, Yamamoto said.

Tepco’s Okamura said being able to block radiation with water is a huge plus, but noted the reactor 2 containment vessel had cracks and holes that could let injected coolant water escape.

With the Three Mile Island nuclear accident in the U.S., the flooding option was used to retrieve the melted fuel in the 1980s. But the key difference was that all of the melted fuel stayed inside the pressure vessel, so it was easier to flood the reactor.

Because the melted fuel in reactors 1, 2 and 3 at the Fukushima plant all penetrated the pressure vessels and fell into the containment vessels, extracting it from the top or the side was a tough call, Yamamoto said, noting it was important to know the exact location of the melted fuel.

The distance between the top of the pressure vessel and the bottom of the containment vessel is about 45 meters and some parts inside the pressure vessels will need to be removed if Tepco tries to remove the debris inside the containment vessels from the top.

“If we know that the melted fuel is concentrated in the containment vessels, it will be more efficient to remove it from the side” because the entry point is closer, Yamamoto said.

Whatever option is decided, Yamamoto stressed that maintaining the fuel removal device will be difficult because the radiation will probably cripple it.

“The fuel removal device will be controlled remotely … it will be broken somewhere down the line and the parts will have to be replaced, considering its (ability to withstand) radiation,” he said.

“Given that, maintenance will have to be done remotely, too, and that will be a big challenge.”

To remove or not

Another option altogether is for Tepco to leave the melted fuel where it is.

During a media tour of the Fukushima No. 1 plant last month, Okamura of Tepco said the utility intended to collect the melted fuel because leaving it was “not an appropriate way” to manage nuclear fuel.

Miyano of the Atomic Energy Society of Japan said the debris must be removed because radioactive materials, including nuclear fuel, must be strictly controlled under international rules requiring strict monitoring.

Domestic nuclear power plant operators have to report the amount of nuclear fuel they have to the Nuclear Regulation Authority, which then reports to the International Atomic Energy Agency.

“There is the question of whether the government and Tepco decide not to remove the fuel debris. That would be an international issue,” said Miyano, adding that a consensus from the international community would be needed.

At the same time, Miyano said debate and analysis will be required to decide which choice would be best by looking at various factors, including how much it will cost to pick up all the melted fuel and where to store it. ”

by Kazuaki Nagata

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6 Years after Fukushima disaster, robots continue search for radioactive fuel — Bloomberg, Insurance Journal; The Japan Times

” The latest robot seeking to find the 600 tons of nuclear fuel and debris that melted down six years 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 [on Thursday, Feb. 16] 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.” … ”

Continue reading about the fuel-removal status of Fukushima No. 1’s Units 1 through 3.

by Emi Urabe and Stephen Stapczynski, Bloomerg via Insurance Journal

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Read a similar article by The Japan Times

Cleaner robot pulled from Fukushima reactor due to radiation — Beloit Daily News

” TOKYO (AP) — A remote-controlled cleaning robot sent into a damaged reactor at Japan’s Fukushima nuclear plant had to be removed Thursday before it completed its work because of camera problems most likely caused by high radiation levels.

It was the first time a robot has entered the chamber inside the Unit 2 reactor since a March 2011 earthquake and tsunami critically damaged the Fukushima Da-ichi nuclear plant.

Tokyo Electric Power Co. said it was trying to inspect and clean a passage before another robot does a fuller examination to assess damage to the structure and its fuel. The second robot, known as the “scorpion,” will also measure radiation and temperatures.

Thursday’s problem underscores the challenges in decommissioning the wrecked nuclear plant. Inadequate cleaning, high radiation and structural damage could limit subsequent probes, and may require more radiation-resistant cameras and other equipment, TEPCO spokesman Takahiro Kimoto said.

“We will further study (Thursday’s) outcome before deciding on the deployment of the scorpion,” he said.

TEPCO needs to know the melted fuel’s exact location and condition and other structural damage in each of the three wrecked reactors to figure out the best and safest ways to remove the fuel. It is part of the decommissioning work, which is expected to take decades.

During Thursday’s cleaning mission, the robot went only part way into a space under the core that TEPCO wants to inspect closely. It crawled down the passage while peeling debris with a scraper and using water spray to blow some debris away. The dark brown deposits grew thicker and harder to remove as the robot went further.

After about two hours, the two cameras on the robot suddenly developed a lot of noise and their images quickly darkened — a sign of a problem caused by high radiation. Operators of the robot pulled it out of the chamber before completely losing control of it.

The outcome means the second robot will encounter more obstacles and have less time than expected for examination on its mission, currently planned for later this month, though Thursday’s results may cause a delay.

Both of the robots are designed to withstand up to 1,000 Sieverts of radiation. The cleaner’s two-hour endurance roughly matches an estimated radiation of 650 Sieverts per hour based on noise analysis of the images transmitted by the robot-mounted cameras. That’s less than one-tenth of the radiation levels inside a running reactor, but still would kill a person almost instantly.

Kimoto said the noise-based radiation analysis of the Unit 2’s condition showed a spike in radioactivity along a connecting bridge used to slide control rods in and out, a sign of a nearby source of high radioactivity, while levels were much lower in areas underneath the core, the opposite of what would normally be the case. He said the results are puzzling and require further analysis.

TEPCO officials said that despite the dangerously high figures, radiation is not leaking outside of the reactor.

Images recently captured from inside the chamber showed damage and structures coated with molten material, possibly mixed with melted nuclear fuel, and part of a disc platform hanging below the core that had been melted through. ”

by Mari Yamaguchi

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Small robot to probe Fukushima Daiichi — NHK World

Read the latest technical updates on the robot’s ability to probe Unit 2 for a estimated limit of 2 hours, down from its originally estimated 10-hour lifespan.

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” Engineers at Tokyo Electric Power Company decommissioning the Fukushima Daiichi nuclear plant are hoping a tiny camera will show them where melted fuel is located inside the crippled No. 2 reactor.

The camera is the latest bit of technology TEPCO engineers are pinning their hopes on. They want to insert it inside Fukushima Daiichi’s No.2 reactor containment vessel for the first time, and they can finally learn whether the fuel is inside, or whether it has penetrated through.

It’s a question they’ve been asking since 2011. Three of the plant’s reactors melted down that March, following a powerful earthquake and tsunami. High radiation levels have prevented anyone from going inside to find the molten fuel.

Experts believe it may have mixed with structures at the bottom of the containment vessels and formed “fuel debris” and on Tuesday, they hoped to catch a glimpse inside. But early on, they ran into trouble and had to reschedule.

Over the years, various remote-controlled robots have been sent inside the 3 reactors, but they haven’t gotten any clear pictures yet of fuel debris.

TEPCO plans to send in another robot to Reactor 2 next month. The utility hopes to be able to analyze results from this research, and create a plan for removing debris by summer.

But it will be a long road — they’ve estimated it will take as long as 4 decades to dismantle the plant, and this first step is the most difficult. ”

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New robot to map Fukushima reactors — BBC News

” A new robot designed to help decommission reactors at the Fukushima nuclear plant is to undergo tests before entering the harsh radioactive environment, it’s been reported.

Developed by the Universities of Tokyo and Tsukuba, and the International Research Institute for Nuclear Decommissioning (IRID), the remotely-controlled robot will use 360-degree cameras and lasers to create a three-dimensional map of the inside of the reactor buildings. It will help plant operator Tepco to know how much wreckage needs to be cleared before decontamination work can begin, Japanese newspaper The Mainichi reports.

The cameras, which are mounted on top of the robot, will give a panoramic view to scientists who have so far been hampered by a lack of clear information. “We’ve expanded the field of vision, so it should give the workers operating the robot a bird’s-eye view of what they’re doing,” an unnamed project official tells The Mainichi.

The Fukushima plant suffered a meltdown after a massive earthquake and subsequent tsunami in March 2011. Efforts to clean up the site have been repeatedly held up by obstacles blocking remotely-controlled devices, as well as high levels of radiation inside the power plant. In April, a robot failed after three hours inside the reactor building, and while a flying drone has been tested for use at the site, it has not entered the most badly damaged buildings. It’s estimated that decontaminating the Fukushima site will take up to 40 years and cost tens of billions of dollars. ”

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Japan to start Fukushima fuel debris retrieval in 2021 — World Nuclear News

” Japan expects to start the retrieval of fuel debris from the Fukushima Daiichi nuclear power plant in 2021, the executive director of the Nuclear Damage Compensation and Decommissioning Facilitation Corporation (NDF) said yesterday. Three of the plant’s six reactors suffered core meltdowns in the March 2011 accident, leaving melted nuclear fuel debris on the floor of their containment vessels.

NDF was established in September 2011 by the Japanese government and the country’s nuclear power plant operators to manage a fund to support operators in providing compensation to victims of nuclear accidents. Although established following the Fukushima Daiichi accident, NDF will continue to be maintained in the future as part of the Japanese nuclear liability regime.

“We expect to select the retrieval method within the next few years, after which detailed design and mock-up tests will follow,” NDF executive director Yasuharu Igarashi told delegates at the VII Atomexpo conference in Moscow. The event was hosted by Russian state nuclear corporation Rosatom. “For the start of retrieval of fuel debris, we are now thinking 2021,” Igarashi said.

NDF has identified three “priority methods” for retrieving highly radioactive nuclear fuel debris in three reactors at Tokyo Electric Power Company’s (Tepco’s) Fukushima Daiichi plant. They are: submersion; partial submersion top access; and partial submersion side access. The first two rely on the removal first of core internals above fuel debris, while the third requires that the reactor pressure vessel pedestal exterior component inside the primary containment vessel (PCV) and interference have first been removed.

The submersion method would remove the debris in a submerged condition, with the containment vessel filled with water to shield against radiation and prevent the spread of radioactive materials during the retrieval process. The reactor damage would need to be accurately identified and repaired before water is poured in. Partial submersion means that water would only be used when the debris has been separated from the vessel floor. The debris would be removed from either the top or the side of the vessel. This method would require continuous cooling of the retrieved debris and measures to prevent radioactive materials from scattering.

Tepco started inspections of unit 1 of the plant in April using robots mounted with cameras and radiation-measuring equipment. The shape-changing robot began surveying conditions within the PCV of the damaged unit on 10 April. However, the robot stopped working before completing its mission. The robot was developed by the International Research Institute for Nuclear Decommissioning and Hitachi-GE Nuclear Energy to investigate hard-to-access areas of the plant. Tepco later sent in a second robot On April 20. The company said that it had decided not to try to retrieve a second robot dispatched inside the unit to avoid it becoming blocked and impeding future missions.

Igarashi said that NDF is following five “guiding principles” for risk reduction in its strategic plan for decommissioning the Fukushima Daiichi plant. They are that the plan is safe, proven, efficient, timely and “field-oriented”, meaning it is based on the actual conditions of the site and structures.

“The decommissioning of the Fukushima Daiichi nuclear power plant is a continuous risk reduction activity to protect people and the environment from the risk of radioactive materials resulting from the severe accident,” he said. “A risk reduction strategy along a mid- to long-term timeline will be designed in NDF’s strategic plan.”

The structure of NDF’s risk management strategy is to be completed by 2017, he said.

In addition to fuel debris retrieval, the strategic plan will also include a policy for waste management involving storage, processing and disposal methods, he said.

NDF has reviewed international standards for waste management, he said, and the International Atomic Energy Agency’s General Safety Requirements Part 5 and Specific Safety Requirements-5 are “important references”. ”

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