Radiation levels at Fukushima reactor puzzle nuclear experts

It is unclear why there is less radioactivity under the reactor vessel, when it is where there should be the most.

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A robot was expected to solidify ways to clean up the No. 2 reactor at the Fukushima No. 1 nuclear plant, but its short-lived mission raised puzzling questions that could derail existing decommissioning plans.

The robot, Sasori, was abandoned in the melted-down reactor after it became stuck in deposits and other debris that are believed to have interfered with its drive system.

But it did take radiation measurements that indicate Tokyo Electric Power Co., operator of the plant, was too optimistic about the state and location of the melted fuel within the reactor. The melted fuel, in fact, may be spread out all over the reactor’s containment vessel.

Scientists had believed the melted nuclear fuel fell through the reactor’s pressure vessel and landed on metal grating and the floor of the containment vessel.

The results of Sasori’s investigation, coupled with previous data taken from possible images of the melted fuel, show the situation within the reactor is much worse than expected. And a fresh investigation into the reactor is now nowhere in sight.

A remote-controlled video camera inserted into the reactor on Jan. 30 took what are believed to be the first images of melted fuel at the plant, which suffered a triple meltdown after the March 2011 Great East Japan Earthquake and tsunami.

Based on the images, TEPCO estimated 530 sieverts per hour at a point almost halfway between the metal grating directly beneath the pressure vessel and the wall of the containment vessel. Black lumps on the grating are believed to be melted fuel.

A different robot sent in on Feb. 9 to take pictures and prepare for Sasori’s mission estimated 650 sieverts per hour near the same spot.

Both 530 and 650 sieverts per hour can kill a person within a minute.

Sasori, equipped with a dosimeter and two cameras, on Feb. 16 recorded a reading of 210 sieverts per hour near the same location, the highest figure measured with instruments in the aftermath of the disaster.

Sasori was supposed to travel along a rail connecting the outer wall of the containment vessel with the metal grating to measure radiation doses and shoot pictures inside, essential parts of work toward decommissioning the reactor.

After traveling only 2 meters, the robot became stuck before it could reach the metal grating.

TEPCO at a news conference repeatedly said that Sasori’s investigation was not a “failure” but had produced “meaningful” results.

However, an official close to TEPCO said, “I had great expectations for Sasori, so I was shocked by how it turned out.”

Hiroaki Abe, professor of nuclear materials at the University of Tokyo who has studied TEPCO’s footage, tried to explain why high doses were estimated between the pressure vessel and the containment vessel.

Instead of directly landing on the rail, the melted nuclear fuel may have flown off after it reacted violently with the concrete, which had a high moisture content, at the bottom of the containment vessel, just like what happens when lava pours into the sea,” Abe said.

But he said this scenario raises a puzzling question, considering the estimated radiation readings near the area below the pressure vessel were down to 20 sieverts per hour, according to an analysis of the video footage.

If nuclear fuel debris had splattered around, the radiation levels at the central area below the pressure vessel must be extremely high,” he said. “In addition, deposits on the rail would have taken the shape of small pieces if they were, in fact, flying nuclear fuel debris. The findings are puzzling.”

Images by the remote-controlled camera also showed that equipment in the lower part of the pressure vessel was relatively well preserved, indicating that the hole at the bottom of the vessel is not very large.

How to remove nuclear fuel debris will all depend on how much remains inside the pressure vessel and how much fell out,” Abe said.

Toru Obara, professor of nuclear engineering at the Tokyo Institute of Technology, stressed the need to retrieve substances from the bottom of the robots or elsewhere.

We could get clues as to the state of the melted nuclear fuel and the development of a meltdown if we could figure out which materials mixed with the nuclear fuel,” he said.

The surveys by the camera and robots were conducted from a makeshift center at the No. 2 reactor. The center’s walls are made from radiation-blocking metal.

TEPCO and the government plan to determine a method to remove nuclear fuel debris in fiscal 2018 before they proceed with the actual retrieval process at one of the three destroyed reactors.

One possible method involves filling the containment vessels with water to prevent radioactive substances from escaping.

http://www.asahi.com/ajw/articles/AJ201702190042.html

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Robot stuck in Fukushima No. 2 reactor on 1st try, abandoned. Damage inside No. 2 reactor building at Fukushima plant greater than expected

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The Sasori robot is stuck inside the containment vessel of Fukushima No. 1 nuclear power plant’s No. 2 reactor on Feb. 16. (Provided by International Research Institute for Nuclear Decommissioning)

Robot stuck in Fukushima No. 2 reactor on 1st try, abandoned

In the latest hitch in efforts to decommission reactors at the Fukushima No. 1 nuclear power plant, a robotic surveyor became mired in deposits and was lost on its maiden journey on Feb. 16.

Tokyo Electric Power Co., which operates the plant in Fukushima Prefecture, had to abandon the Sasori (scorpion) robot after it became stuck inside the containment vessel of the power station’s No. 2 reactor that morning.

The highly touted probe was specially developed for the important task of surveying the interior of the crippled reactor and collecting data to assist in removing the melted fuel.

But with the environment inside too treacherous for a key component in the process, TEPCO’s decommissioning project seems to have come to a standstill.

According to the utility, the robot entered the containment vessel around 8 a.m. It traveled along a 7.2-meter-long rail connecting the outer wall of the containment vessel with its central portion immediately beneath the pressure vessel.

But about 5 meters into its mission, the robot’s controls started to become less responsive. TEPCO believes it was due to deposits and other debris that are blocking the rail entering its drive system.

The operator tugged on the electrical cable connected to the robot and had it pull back to an area along its path with less obstacles, but it ultimately became stuck there.

The robot measured the radiation levels in the area at 210 sieverts per hour, which is lethal enough to kill a human in two minutes. Earlier, the company had estimated the level in the area at 650 sieverts per hour from video footage captured on Feb. 9 by another robot that paved the way for the Sasori.

With the robot completely immobilized, TEPCO gave up on retrieving it around 3 p.m. The operator cut the electric cable and closed the tunnel bored into the wall of the containment vessel, entombing the robot inside.

The probe was cast aside to the edge of the 0.6-meter-wide rail so that it would not impede future surveyor robots.

Had everything gone according to plan, TEPCO would have sent the Sasori onto the grating in the heart of the containment vessel, which is covered in black chunks believed to be melted fuel rods that fell from the pressure vessel above.

The utility had hoped to measure the dosage of these radioactive lumps, as well as capture images of the underside of the pressure vessel, which contains holes from when the nuclear fuel burned through it in the meltdown that was triggered by the Great East Japan Earthquake and tsunami in 2011.

http://www.asahi.com/ajw/articles/AJ201702170048.html

 

Damage inside No. 2 reactor building at Fukushima plant greater than expected

Damage within the No. 2 reactor building at the Fukushima No. 1 Nuclear Power Plant was found to be greater than expected, based on images sent back by a robot sent into the structure by plant owner Tokyo Electric Power Co. (TEPCO) in a mission that concluded Feb. 16.

The operation used a self-propelled scorpion-shaped robot, with the goal of investigating the inside of the reactor’s containment vessel and the area directly beneath the reactor, but the area under the reactor was not reached. The No. 2 reactor building was thought to be comparatively undamaged compared to the No. 1 and No. 3 reactor buildings, where hydrogen explosions occurred. Worse damage than expected was discovered, however, such as holes in the grating foothold inside the containment vessel.

At a press conference, TEPCO official Yuichi Okamura stressed, “This investigation was the first of its type in the world and uncovered information about the debris inside. The mission wasn’t a failure.”

The robot’s camera also took footage of the condition of pipes in the structure, and image processing could make these pictures clearer. However, the robot’s treads stopped moving after it proceeded over 2 meters along a rail, and TEPCO was not able to use it to check the melted nuclear fuel.

TEPCO plans to decide as early as this summer on how to remove the melted fuel from the No. 1 through No. 3 reactors and start the decommissioning process in earnest. The results of the investigation were to be used as base data for the decommissioning, but with it having not produced an overall understanding of the No. 2 reactor building’s interior, a new investigation will probably be sought.

However, no plan for the next investigation has been decided, and it may begin with the development of a new robot. TEPCO plans to send in a different robot to the No. 1 reactor building next month. For the No. 3 reactor building, a robot capable of moving in water is being developed because there is a large amount of contaminated water at the bottom of its containment vessel.

http://mainichi.jp/english/articles/20170217/p2a/00m/0na/011000c

Fukushima Unit 2 Scorpion Probe Dies But Sends Back Some Data

 

While the press reported Scorpion’s mission as a failure, it provided useful data before being abandoned. It collected some radiation readings and a number of useful images.

The robot seems to have become stranded on a pile of debris on the rail. Radiation data from along this inspection route provided only one radiation reading, no telemetry as other videos had. Tepco’s video is heavily edited but still provides some useful information.

A reminder, these readings are the result of venturing into the more deadly areas of the reactor where they have been unable to previously, no resulting from an increase of radiation. While this is much lower than the earlier camera estimates of radiation it is still extremely high and quite deadly.

Arond the same area where the high radiation source was found, TEPCO stated they found a 210 Sv/h reading with the on board radiation sensor.

New images from inside the pedestal were obtained as were some images looking up into the containment structure.

Image below from TEPCO. White ghosting on the image is likely due to radiation levels rather than steam. The existing melt hole in the pedestal floor grate can be partially seen in the upper mid section of the image. A very thick amount of fuel debris can be see in the lower right section of the image. The mark “clean” on this image with an arrow indicates an area where the floor grate may have failed after the molten fuel had splattered on the area. Further below, more fuel debris and structures can be seen.

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The red circle shows an area where it appears fuel debris was moved or blocked by a fallen piece of sheet steel.

In both images, sections of light colored piping can be seen below the area where the grate is missing. On the far left of the image a partially melted section of flexible conduit can be seen.

This appears to indicate that high temperatures within the pedestal were very localized.

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Sources:

TEPCO handout for this work

http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2017/images/handouts_170216_01-e.pdf

Reactor 2’s Platform has 3 Holes

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Tepco released a new image of the reactor 2: at least three holes in the platform and still no corium.

The platform is made of metal (grating), just below the tank, intended to access the control rods. The bottom of the containment is a little over 3 m below.

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Tepco’s document in Japanese:

http://www.tepco.co.jp/nu/fukushima-np/handouts/2017/images1/handouts_170215_08-j.pdf

 

 

Latest probe of reactor 2 fails after Fukushima robot blocked by obstacles

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A robot shown in this photo by the International Research Institute for Nuclear Decommissioning was put inside reactor 2 at the Fukushima No. 1 nuclear plant on Thursday

Robot stops working in Fukushima reactor

The operator of the crippled Fukushima Daiichi nuclear power plant says it suspended a survey by a robot at one of its reactors after the device stopped working.
Tokyo Electric Power Company, or TEPCO, sent the scorpion-shaped robot into the containment vessel of the plant’s No. 2 reactor on Thursday.
The company believes fuel in the reactor melted through its core during the 2011 accident and accumulated at the bottom of the facility’s containment vessel.
The survey was aimed at getting a close look at what could be fuel debris — a mixture of nuclear fuel and melted parts of the reactor.
The robot was also expected to measure radiation and temperatures there to gather data for scrapping the reactor.
TEPCO officials say the device was advancing on a metal rail leading to a central area below the reactor’s core, but stopped moving before it could reach the center.
The officials say they decided to give up the robot and cut its remote-control cable.
TEPCO plans to analyze data collected by the robot and figure out how to carry out future probes.

https://www3.nhk.or.jp/nhkworld/en/news/20170216_34/

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Robot survey of crippled Fukushima reactor ends in failure

The operator of the crisis-hit Fukushima Daiichi nuclear power plant said Thursday its attempt to take a close look at the crippled No. 2 reactor using a scorpion-shaped robot ended in failure due to a technical flaw.

A track glitch meant the self-propelled robot was unable to climb over obstacles, Tokyo Electric Power Company Holdings Inc. said. It gave up on retrieving the robot by cutting its remote control cables.

TEPCO, however, said, “We have received new important information about the radiation level and temperature inside the (reactor) containment vessel,” emphasizing it did not view Thursday’s survey as a failure.

http://english.kyodonews.jp/news/2017/02/459048.html

Latest probe of reactor 2 fails after Fukushima robot blocked by obstacles

A renewed attempt to survey reactor 2 at the damaged Fukushima No. 1 nuclear power plant failed Thursday when the latest robot probe became obstructed.

The robot was inserted into the primary containment vessel at around 7:50 a.m. to approach the metal grating directly underneath the pressure vessel, where a black mass has been found.

Tokyo Electric Power Company Holdings Inc. had hoped to take a closer look at what could be melted nuclear fuel, but it was forced to abandon the operation shortly after 3 p.m.

The robot didn’t reach its objective, Tepco said, and the utility eventually severed its controller cable.

Having detected an extraordinarily high radiation level —estimated at 650 sieverts per hour — in a preparatory survey, Tepco had hoped to obtain more precise readings, images and data needed to remove fuel and other debris to decommission the plant.

In previous surveys, the utility found deposits on the grating believed to be nuclear debris and a 1-sq.-meter hole believed to have been created by molten fuel escaping from the pressure vessel.

Challenges dogged the latest attempt from the start. There was little clear surface for the robot to move around, and the radiation could kill the unit as with the preliminary surveys.

Next month, Tepco plans to survey the No. 1 reactor.

http://www.japantimes.co.jp/news/2017/02/16/national/latest-probe-reactor-2-fails-fukushima-robot-blocked-obstacles/#.WKWoEPLraM9

Operation of communication about the “scorpion” robot which will be sent to the confinement enclosure of reactor n ° 2

TEPCO and its partners launched a communication operation about the “scorpion” robot, which will be sent to the containment reactor of reactor n ° 2 in an attempt to locate the corium, ie the highly radioactive molten fuel, mixed with debris. It is not certain that the mission will be a success, the cleaning robot having lasted only two hours in this enclosure because of the extreme radiations, without being able to finish its task.

A press release announces what we already know and insists on the challenges: “every step is a new challenge for TEPCO, but TEPCo welcomes the challenges”. The company would be almost happy with the accident? It is accompanied by a promotional video with a comparison to the kendô fights posted on its Facebook page.

http://www.tepco.co.jp/en/press/corp-com/release/2017/1377951_10469.html

https://www.facebook.com/OfficialTEPCOen/videos/1346869308705698/

The Japanese nuclear industry wants to place itself on the decommissioning market and highlights the technologies being developed. This robot was designed by IRID, Toshiba and TEPCO. IRID benefits from public funds. As for Toshiba, it is almost bankrupt because of its nuclear branch and TEPCO is financially in a bad shape.

The press release and the video do not provide any relevant information and are in complete discrepancy with reality.

1. Current conditions of Unit 2 Primary Containment Vessel (PCV)

Nuclear fuel in the Primary Containment vessel (PCV) was exposed to the air and melted from the impact of March 2011 Great Earthquake.

As a result of the accident analysis, it was found that a portion of melted nuclear fuel might have been fallen inside the pedestal.

To remove fuel debris, it is necessary to investigate the PCV and clarify the conditions of debris and surrounding structures.

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2. Outline of Unit 2 PCV investigation

[Purpose]: To obtain feedback information (deformation of platform, etc.) for the design and

development of next investigation devices inside the pedestal

To inspect conditions on the platform inside pedestal, fuel debris fallen to the CRD housing, and conditions of structures inside pedestal.

[Investigation point]: Platform and Control Rod Drive (CRD) will be investigated from the platform inside pedestal

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3. Work steps for Unit 2 PCV investigation

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4. Preparatory investigation results from X-6 penetration to CRD rail

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4. Preparatory investigation results at the entrance of pedestal area

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4. Preparatory investigation results of pedestal area

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5. Additional results expected from the self-propelled investigation device

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6. Investigation by the self-propelled investigation device to the end of CRD rail

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6. Investigation by the self-propelled investigation device to the end of CRD rail

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Reference: Investigation results on the platform inside the pedestal

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Technical information for the media is available here:

In Japanese about the upcoming mission http://www.tepco.co.jp/nu/fukushima-np/handouts/2017/images1/handouts_170215_08-j.pdf

And in English http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2017/images/handouts_170215_01-e.pdf

And about radiation protection measures http://www.tepco.co.jp/nu/fukushima-np/handouts/2017/images1/handouts_170215_09-j.pdf

And in English http://www.tepco.co.jp/en/nu/fukushima-np/handouts/2017/images/handouts_170215_02-e.pdf

Translated from L’ACROnique de Fukushima http://fukushima.eu.org/operation-de-communication-sur-le-robot-scorpion-qui-va-etre-envoye-dans-lenceinte-de-confinement-du-reacteur-n2/

Clean-up robot pulled from Fukushima Reactor 2 due to extremely high radiation

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Cleaner Robot Pulled From Fukushima Reactor Due to Immense Radiation

The camera on the bot was compromised by the high levels of radiation.

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.

http://www.popularmechanics.com/science/energy/a25155/cleaner-robot-pulled-from-fukushima-reactor-due-to-radiation/?src=socialflowTW

 

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Extremely high radiation breaks down Fukushima clean-up robot at damaged nuclear reactor

A clean-up mission using a remotely operated robot at Japan’s crippled Fukushima nuclear plant has had to be aborted, as officials feared they could completely lose control of the probe affected by unexpectedly high levels of radiation.

The robot equipped with a high-pressure water pump and a camera designed to withstand up to 1,000 Sieverts of cumulative exposure had been pulled off the inactive Reactor 2 at the Fukushima Daiichi nuclear complex earlier this week, The Japan Times reported Friday, citing the plant operator, Tokyo Electric Power Company (TEPCO). The device reportedly broke down just two hour into the probe.

The failure led experts to rethink estimated levels of radiation inside the damaged reactor.

While last week TEPCO said it might stand at 530 Sieverts per hour – a dose that can almost instantly kill a human being, following the latest aborted mission a company official has said a reading of up to 600 Sieverts should be “basically correct.”

Even despite the considerable 30-percent margin of error for the revised estimate, the latest probe left no doubt that radiation levels are at record highs within the reactor. Even though it cannot be measured directly with a Geiger counter or dosimeter, the dose is calculated by its effect on the equipment.

Last month, a hole of no less than one square meter in size was discovered beneath the same reactor’s pressure vessel. The apparent opening in the metal grating is believed to have been caused by melted nuclear fuel, TEPCO then said.

The recent mission has demonstrated that the melted fuel is close to the studied area.

 

While extreme radiation levels have been registered within the reactor, officials insist that no leaks or increases outside have been detected.

The failure might force Japan to rethink the robot-based strategy it has adopted for locating melted fuel at Fukushima, according to The Japan Times.

The robot affected by radiation was supposed to wash off thick layers of dirt and other wreckage, clearing ways for another remotely controlled probe to enter the area, tasked with carrying out a more proper investigation to assess the state of the damaged nuclear reactor. Previously, even specially-made robots designed to probe the underwater depths beneath the power plant have crumbled and shut down affected by the radioactive substance inside the reactor.

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The Fukushima Daiichi nuclear power plant suffered a blackout and subsequent failure of its cooling systems in March 2011, when it was hit by an earthquake and tsunami. Three of the plant’s six reactors were hit by meltdowns, making the Fukushima nuclear disaster the worst since the Chernobyl catastrophe in Ukraine in 1986. TEPCO is so far in the early stages of assessing the damage, with the decommissioning of the nuclear facility expected to take decades.

https://www.rt.com/news/377025-fukushima-radiation-breaks-robot/