Frozen soil wall nearly complete; NRA still doubts effect — The Yomiuri Shimbun

” A construction project to create frozen soil walls that encircle the ground beneath Tokyo Electric Power Company Holdings Inc.’s disaster-hit Fukushima No. 1 nuclear power plant is nearly finished.

Although TEPCO insists that the inflow of groundwater beneath the reactor buildings has been reduced, some members of the Nuclear Regulation Authority are skeptical about the project’s effectiveness. With ¥34.5 billion of public funds being spent on this project, the centerpeice of countermeasures for contaminated water, its cost-effectiveness is being carefully watched.

The project entails building a 1.5-kilometer-long frozen soil wall encircling the Nos. 1 to 4 reactors, with 1,568 pipes buried to a depth of about 30 meters below ground and coolant running through the pipes at minus 30 C to chill the soil.

The process is expected to prevent groundwater from flowing into the contaminated, highly radioactive underground water at such sites as the reactor buildings, and to avoid an increase of contaminated water.

The project began in March last year, and operations to freeze the final section, about seven meters wide, on the mountain side began in August this year.

The temperature of the underground soil has remained below zero, except for a part close the surface that is affected by outdoor air, meaning the project to create the 30-meter-deep walls is almost complete.

According to TEPCO’s assessment, before the project started, about 400 tons of groundwater was flowing into the ground underneath the reactor buildings and other sites daily.

TEPCO had initially calculated that the daily inflow of groundwater could decrease to dozens of tons once the walls were installed. However, between April and September the inflow per day was between 120 tons and 140 tons, and in October it was around 100 tons. That the amount of inflow has decreased in stages as the soil freezing progressed seems to prove that the project has been effective to a certain extent. However, it is unclear if the inflow will decrease further in the future.

In parallel with the frozen soil wall project, TEPCO dug about 40 subdrain wells to pump up groundwater before it flows into the reactor buildings. It also reinforced measures to prevent rainwater from soaking into the ground by paving 1.33 million square meters of surface.

In the NRA view, those measures must also contribute greatly to reducing the inflow, casting doubt on the frozen soil walls project by saying the effect of them alone may be limited. The agency has become distrustful of TEPCO and urged the company to verify the effects.

Hiroshi Miyano, visiting professor at Hosei University specializing in system safety, said: “There is sure to be a part that doesn’t freeze completely, and it’s impossible to reduce the inflow to zero. TEPCO must continue applying this measure in tandem with draining the nearby wells for a while.” ”

by The Yomiuri Shimbun

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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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Updated 3/23/15: Fukushima unit 1 muon scan results: No fuel in reactor vessel — Daily Kos; Muon scan gives detailed, but incomplete, look at meltdown of No. 1 reactor — The Japan Times

Daily Kos, updated March 23, 2015: ” So begins an in-depth article in the March 6th edition of the Journal Science entitled Muons probe Fukushima’s ruins. In February of this year two muon detectors from the High Energy Accelerator Research Organization [KEK] in Tsukuba, Japan were installed outside the Fukushima Daiichi unit-1 ruins at reactor vessel height for the purpose of finding that ‘missing’ reactor fuel. Muons – particles somewhat akin to electrons – are generated as cosmic rays slam into atoms in the upper atmosphere and rain down on Earth to the tune of 10,000 or so per square meter every minute. They pass right through solid objects, but a few do get absorbed or deflected by massive objects, so these can generate an X-ray like image of the reactor vessel and what’s in it despite the thick concrete and steel of the containment vessel and assorted ruins/debris all around.

[View scan images in this] TEPCO handout (in Japanese). As expected, the Fuku scans revealed no fuel in the reactor vessel. More scans are available here.

Now the plan is to go ahead and insert the new shape-changing robot in April to see if there is enough left of the control rod drive rail to get that robot onto the containment catwalk, where it should be able to circle inside the containment itself to collect more data about the location of the corium (melted fuel). Hopefully it’s still in the containment drywell, not having melted through the base pad into the lower level basement or ground below. If it exited the drywell it may have melted through the downcomer vents and into the torus in the first level basement, and some may have found its way into drains and drainpipes as one of the flows at Chernobyl did to produce the corium formation known as the “elephant’s foot.”

The geology at Chernobyl is quite different from Fukushima, having been built atop a solid granite bedrock rather than rock and gravel fill. The Chernobyl “elephant’s foot” formation exits a large drainpipe in what was the basement of that plant and melted 3 meters (~9 feet) into the granite. If the Fukushima unit-1 corium made it to the ground underneath the plant it is likely to have spread much further through the fill and be much more difficult to retrieve, even as the ‘underground river’ of groundwater that runs beneath the facility picks up contamination and takes it on out to the ocean. Decommissioning requires that all nuclear fuel – in whatever state – be removed from direct contact with the environment and safely isolated.

The unit-1 muon scans apparently also found some evidence that some fuel fragments may have been relocated from the reactor vessel to the spent fuel pool and refueling floor, though the precise nature of this evidence and how the fuel managed to get to these locations is not explained. If there is corium/fuel debris in these locations it will make cleanup in preparation for defueling the SFP more complicated, especially in light of the recontamination of rice fields downwind during the cleanup of the unit-3 refueling floor in 2013. Now that the glorified ‘tent’ over unit-1 has been removed, cleanup of that mess is scheduled to start sometime in the next week.

According to the article in Science, TEPCO thinks some fuel may remain in the cores at units 2 and 3. Muon scanning of unit-2 is scheduled to begin in May of this year. ”

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The Japan Times, posted March 22, 2015: ” Confirmation this week that all the fuel inside one of the Fukushima No. 1 plant’s broken reactors has long since melted leaves its operator with the tricky task of eventually scooping it all out, experts say.

Tokyo Electric Power Co. said Thursday it had performed a sophisticated scan of the plant’s No. 1 reactor core, giving the most detailed picture so far of what is going on in the high-radiation environment.

Nuclear experts said Friday that the test showed the unit’s fuel rods had melted beyond recognition.

“The results reaffirmed our previous understanding that a considerable amount of fuel had melted inside the nuclear pressure vessels,” said Hiroshi Miyano, a visiting professor at Hosei University in Tokyo.

“But there has been no evidence that the fuel has melted through the nuclear containment buildings and reached the outer environment,” Miyano said.

However, the scan — based on tomography imaging that made use of elementary particles called muons — did not look at the bottom part of the reactor, where the molten fuel would have pooled. So some experts suggested that it was not possible to tell whether the fuel had indeed been contained.

The fuel rods are installed [in] the reactor’s pressure vessel, which is in turn enclosed by the primary containment vessel. These rods generate the heat used to drive steam turbines and produce electricity but must be submerged at all times to avoid melting.

“Eventually, Tepco is aiming to scoop out the melted fuel little by little, rather than burying it in concrete” as was done at Chernobyl in the former Soviet Union, Miyano said.

Muons, which continually shower the earth from space, penetrate solid objects to a greater depth than x-rays. The rate at which they pass through a material indicates its density and helps scientists to identify it.

Since muons move more slowly through relatively dense plutonium and uranium fuel than through the reactor vessel itself, mapping their trajectory can reveal exactly where the fuel is — or isn’t.

The data from this test should help Tepco’s effort to decommission the plant, which lost all power in March 2011 after the Pacific coast of Tohoku was swamped by huge tsunami. The blackout triggered a triple core meltdown.

The decommissioning process at Fukushima is expected to take three or four decades.

Experts say the latest results and the operator’s assessment of them were in line with earlier expectations.

“We presume that despite the meltdown, the fuel is still in the containment vessel,” said Tomohisa Ito, a spokesman for the International Research Institute for Nuclear Decommissioning, a special research unit involved in dismantling the troubled plant.

“But we still need to directly check the situation one day using remote-controlled robots,” he said.

Last month the International Atomic Energy Agency said Japan had made “significant progress” in its cleanup efforts but warned the situation “remains very complex” due to the growing amounts of contaminated water being generated by the process.

While the quake and tsunami that triggered the man-made nuclear crisis killed almost 19,000 people, mostly by drowning, no one is officially recorded as having died as a direct result of the meltdowns at Fukushima, though indirect deaths related to the disaster continue to climb.

However, tens of thousands of people remain displaced because of radioactive contamination around the plant, and scientists warn that some settlements may have to be abandoned forever. ”

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Leaks confirmed in Fukushima Daiichi Unit 1 containment vessel — Enformable Nuclear News

” After two and a half years, two leaks have been found in the containment vessel of the Unit 1 reactor at the Fukushima Daiichi nuclear power plant.

Workers used a robotic boat equipped with cameras and radiation detectors to investigate the suppression chamber of the Unit 1 reactor.  The suppression chamber is filled with contaminated water, which was injected to cool the melted reactor fuel.  Extremely high radiation levels have prevented workers from investigating these critical areas of the reactor buildings.

Tokyo Electric engineers were not able to locate the actual location of the leaks, or to measure the amount of water which is leaking, but described the flow as gushing out into the basement of the reactor building.

“Part of the containment vessel is damaged, and water leaking from there is likely to be flowing down into the ground via the pipe,” one TEPCO official said.

One leak was discovered near a rupture in a sand-cushioned drain pipe which was used to direct condensation which formed on the surface of the vessel.

The other leak was located just above the suppression chamber, in a vent pipe which connects to the suppression chamber.

TEPCO engineers assume that there is similar damage to the bottom of the Reactor 2 and Reactor 3 containment vessels which are also allowing the leakage of highly contaminated water.

Hiroshi Miyano, a professor at Hosei University, said the volume of leaking water suggests a significantly large amount of damage to critical connections between the containment vessel and suppression chamber.  Miyano suggested that the damage could have been due to the impact from the hydrogen explosion in March of 2011. ”

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