The Robot Probe Cannot Confirm Where is the Melted Fuel of Unit 1

24 03 2017

 

Tokyo Electric Power Company announced on February 23 that it had completed a robot probe survey lasting five days in the reactor containment vessel of Fukushima Daiichi nuclear power plant Unit 1.

Its goal was to confirm the whereabout of the melted nuclear fuel, but it was blocked by piping and could not put the camera in athe place where nuclear fuel could be seen.

Information necessary for taking out the nuclear fuel to decommission the reactor remains inadequate, and some voices began to question the robot conducted investigation method.

During the 5-day survey, there was also a point where the measuring instrument with an camera and a radiation dosimeter integrated together was hung up in a range from 0 to 3 meters from the bottom of the containment vessel, pipes and debris blocking its path in many points. The radiation dose in the water is from 3.0 to 11 Sv. Per hour. It was not possible to directly check the melted nuclear fuel.

TEPCO and the country are facing the decommissioning of a furnace …

http://www.asahi.com/articles/photo/AS20170323005483.html

Tepco robot failed to capture images of melted fuel in reactor 1

tepco-reactor1-20170324

A photo taken by a robot on Wednesday shows an underwater image of water pool on the bottom of the containment vessel of the reactor 1 at the Fukushima No. 1 plant

Tokyo Electric said Thursday that it failed to get any photos of potential fuel debris during a five-day probe of the primary containment vessel at reactor 1 of the Fukushima No. 1 power plant.

Tokyo Electric Power Company Holdings Inc., however, stressed that the investigation was worthwhile because its robot was able to take underwater images in the pool of water at its bottom and gauge its radiation level, which will help it estimate where the melted fuel lies.

The monstrous tsunami of March 11, 2011, tipped reactors 1, 2 and 3 into core meltdowns. The molten fuel rods then penetrated their pressure vessels before apparently dropping to the bottom of the giant containment vessels.

There is about a 2.5-meter deep water pool at the bottom of the primary containment vessel of reactor 1, and Tepco believes most of its melted fuel rods fell into it. Thus the main mission of the robot investigation this time was to capture underwater images.

The robot traversed gratings set up several meters above the vessel’s bottom and lowered a wire with a camera and dosimeter on its tip at 10 locations in the water.

Yet none of the images disclosed by Tepco showed anything resembling fuel debris, while parts of machinery, such as a valve, were captured.

When the robot dangled the camera on spots where Tepco thought there was a higher probability of locating the fuel, it instead found a 90-cm pile of sediment.

Tepco spokesman Yuichi Okamura said the sediment is probably not fuel debris, given the relatively low radiation readings, which ranged from 5.9 to 9.4 sieverts per hour.

Although the readings indicate extreme danger to people, Okamura said the readings would have been much higher had they been melted fuel rods. He said Tepco had no idea what the sediment is but added that there was a possibility it was covering the fuel.

According to Okamura, radiation readings get weaker by a hundredth if blocked by a meter of water. Since the robot detected readings from 5.9 to 9.4 sieverts per hour about 90 cm above the pool’s bottom, there might be something down there emitting strong radiation.

Tepco plans another investigation this month to pick up samples of the sediment.

While no fuel debris was recognized, Okamura said Tepco would review the data and analyze it further. By comparing radiation readings from various locations, the utility might be able to roughly pinpoint where the melted rods lay, he said.

He added that it was an achievement that the robot lasted for five days in the deadly radiation and that Tepco was able to retrieve it.

http://www.japantimes.co.jp/news/2017/03/24/national/tepco-robot-failed-capture-images-melted-fuel-reactor-1/#.WNQ_hBh7Sis

Lethal radiation levels but no melted fuel found in Fukushima reactor water

58d2ccd5c36188542e8b4646

The Unit 2 reactor building at Tokyo Electric Power Co.’s Fukushima Daiichi nuclear power plant.

The level of radiation was measured by a special robot on Sunday at a point about 30cm (one foot) from the bottom of the containment vessel of Reactor 1, the Japan Times reported on Tuesday.

The current radiation level is 11 sieverts per hour, the highest detected in water inside the containment vessel. A person exposed to this amount of radiation would likely die in about 40 minutes, the Japan Times reports.

Sunday’s probe also revealed sandy substances building up at the bottom of the vessel. Tokyo Electric Power Company Holdings (TEPCO) officials, however, dismissed the idea that it might be melted nuclear fuel.

Experts have been looking for the melted fuel, which they believe has been accumulating in tainted water.

In March 2011, a 9.1 earthquake and the 15-meter tsunami that followed disabled the cooling system of Fukushima’s three reactors, causing the worst nuclear incident since the 1986 Chernobyl incident in Ukraine.

TEPCO, which operates the crippled power plant, has been obliged to deal with the consequences of the incident.

In February, a robot sent to explore Reactor 2 broke down because of the “unimaginable” levels of radiation, close to 650 sieverts per hour. This was the first time a robot entered this reactor since the plant’s meltdown in 2011.

Previously, the highest radiation level was recorded one year after the disaster and went up to 73 sieverts per hour.

TEPCO has promised extract the hazardous material stuck in the plant’s second reactor, its president Naohiro Masuda said, according to the Japan Times.

In December, TEPCO nearly doubled the estimated cost for the Fukushima clean-up to $188 billion.

A zone of more than 300 square miles around the plant is currently uninhabitable due to the continuing radiation.

https://www.rt.com/news/381879-fukushima-reactor-radiation-lethal/

 

Clearer water inside reactor 1 should help find melted fuel at Fukushima plant

ughljkl.jpg

A robot on March 18 took this image of a valve and a pipe in cooling water at the bottom of the containment vessel of the No. 1 reactor at the Fukushima No. 1 nuclear power plant.

Cooling water in the No. 1 reactor of the Fukushima No. 1 nuclear plant has improved in transparency, which should make it easier to pinpoint the location of melted nuclear fuel, the plant’s operator said.

The improved transparency, compared with the level two years ago, was confirmed on March 18, when a research robot took an image that clearly showed a valve and a pipe in the water at the bottom of the reactor’s containment vessel, Tokyo Electric Power Co. said March 19.

Devices on the robot measured radiation levels of 7.8 sieverts per hour on a metal stage for workers and 1.5 sieverts per hour in the water.

The research robot on March 20 and 21 will study areas where the melted nuclear fuel could exist.

http://www.asahi.com/ajw/articles/AJ201703200028.html

Radiation Spikes At Fukushima

MAKHIJANI-Man_in_suit.jpg

Juan Carlos Lentijo of the International Atomic Energy Agency looks at tanks holding contaminated water and the Unit 4 and Unit 3 reactor buildings during a February 2015 tour of the tsunami-stricken Fukushima Daiichi nuclear power plant.

Almost six years after a tsunami caused a meltdown at the Fukushima Daiichi Nuclear Power Plant, the facility’s operator, Tokyo Electric Power (Tepco) faces overwhelming problems to clean up the site. Tepco now reports radiation in reactor 2 that would kill a worker in thirty seconds, and even destroys robots. Arjun Makhijani, the President of the Institute for Energy and Environmental Research and host Steve Curwood discuss the implications of this new report and the challenges of cleanup.

MAKHIJANI--Makhijani.png

Arjun Makhijani is the President of the Institute for Energy and Environmental Research.

Transcript

CURWOOD: It’s Living on Earth, I’m Steve Curwood.

Six years after an earthquake and resulting tsunami devastated Fukushima, Japan and led to the meltdown of three nuclear power reactors there on the coast, radiation levels have reached a staggering 530 sieverts an hour, many times higher than any previous reading. Tepco, the plant’s operator, claims that radiation is not leaking outside reactor number two, site of these readings, but concedes there’s a hole in the grating beneath the vessel that contains melted radioactive fuel.

Joining us now to explain what it all means is Arjun Makhijani, President of the Institute for Energy and Environmental Research. Welcome back to Living on Earth Arjun.

MAKHIJANI: Thank you, Steve. Glad to be back.

CURWOOD: So, this report from TEPCO seems serious, maybe even ominous. What what exactly is going on?

MAKHIJANI: Well, they are exploring the molten core of the reactor in reactor number two with robots, and the robot called Scorpion went farther into the bottom of the reactor in an area called “the pedestal” on which the reactor kind of sits and measured much higher levels of radiation than before. The highest level was 73 Sieverts per hour before and this time they measured a radiation level more than seven times higher. It doesn’t mean it’s going up. It just was in a new area of the molten core that had not been measured before.

CURWOOD: Still, it sounds to me like it’s problematic, that six years after this meltdown there’s such a high reading.

MAKHIJANI: It is a very high reading; they may encounter even higher readings. The difficulty with this high reading is that the prospect that workers can actually go there, even all suited up, becomes more and more remote. Robots are going to have to do all this work – That was mostly foreseen – but the radiation levels are so high that even robots cannot survive for very long. So now they’re going to have to go back to the drawing board and redesign robots that can survive longer or figure out how to do the work faster, and it’s going to be more costly and more complicated to decommission the site.

MAKHIJANI-Fukushima-Containment.jpg

The lid of Unit 4’s Primary Containment Vessel lies close to the reactor building. The reactor was shut down for maintenance at the time of the accident.

CURWOOD: Remind us, Arjun, please, of the human impact of this kind of radiation. What’s toxic to humans?

MAKHIJANI: Right. So, if you get high levels of radiation in a short period of time, four Sieverts is a lethal dose for about half the people within two months. So, in 530 Sieverts per hour would give you a lethal dose in less than 30 seconds.

CURWOOD: Wow.

MAKHIJANI: So, it’s a very, very, very high level of radiation. That’s why people cannot go into the reactor and work there. That’s not the end of the bad news, but that’s quite a bit of it.

CURWOOD: OK. All right, there is more bad news. I’m sitting down. Tell me.

MAKHIJANI: Yes, so the bottom of the reactor under the reactor there is a grating and then under the grating there’s the concrete floor, and what this robot discovered — It was supposed to go around the grating and survey the whole area, but it couldn’t because a piece of the grating was deformed and broken. So, now it appears that some of the molten fuel may have gone through the grating and maybe onto the concrete floor. We don’t know because even robotic surveys are now difficult, and a high radiation turns into heat, so the whole environment around the molten fuel is thermally very hot, and so whether it is going through the concrete, whether it is under the concrete, I don’t know that we have a good grip on that issue.

CURWOOD: So, Arjun, what’s going on with the reactors one and three? There have been published reports that TEPCO, Tokyo Electric Power Company that has these reactors, hasn’t really taken a good look at those reactors. What do you know?

MAKHIJANI: Well, they have to develop the robots, and I think that developing them, by looking at reactor two, and they’re finding these surprises, radiation levels much higher than previously measured. It shouldn’t actually be unanticipated. The big surprise here was that a part of the grating was gone, and so that the molten fuel would possibly have gone through the grating. So, I think similar surprises will await reactors one and three because each meltdown will have a different geometry.

MAKHIJANI-Fukushima_Tanks.jpg

Storing contaminated water in tanks at the Fukushima Daiichi site presents an ongoing risk, says Makhijani.

URWOOD: So, now what about the decay products here? We’re starting with the Uranium family, but we wind up with Cesium and Strontium – Strontium 90. What risk is there of Strontium 90 getting into groundwater there?

MAKHIJANI: Yeah, so the peculiar thing about a nuclear reaction is the initial fuel, Uranium, is not very radioactive. It’s radioactive but you can hold the uranium fuel pellets in your hand without getting a high dose of radiation. After it’s gone through the nuclear reaction – Fission, that’s what generates the energy – the fission products which result from splitting the Uranium atom are much more radioactive than Uranium, and Strontium 90 and Cesium 137 are two of the products that last for quite a long time, half-life 30 years, and are quite toxic. So, Strontium 90 is specially a problem when it comes in to contact with water. It’s mobilized by water. It behaves like calcium, so if it gets into like sea water and get into the fish, the bones of the fish, or human beings, of course, it gets into the bone marrow and bone surface, increases the risk of cancer, leukemia. So it’s a pretty nasty substance, and Strontium 90 has been contacted with water. You know, rainwater goes and contacts the molten fuel. Groundwater may be contacting the molten fuel. So, we have had Strontium 90 contamination and discharges into the ocean. They also collect the water. They’ve got about more than 1,000 tanks of contaminated water stored at the Fukushima site. By my rough estimate may be about 100 million gallons of contaminated water is being stored there.

CURWOOD: What happens if there’s an earthquake?

MAKHIJANI: That’s exactly right. So about a week into the accident, I sent a suggestion to the Japan Atomic Energy Commission that they should buy a supertanker, put the contaminated water into the supertanker, and send it off elsewhere for processing. They do have a site in the north of Japan which was supposed to be for plutonium separation, but it could be used to support the cleanup of Fukushima. But they rejected that proposal more than once and decided to build these tanks instead. They have a decontamination process on-site, and there are a very vast number of plastic bags on the site filled with contaminated soil. Nobody wants the stuff and nobody knows what’s going to happen with it.

CURWOOD: It’s six years after the original meltdown. How much of a disaster is Fukushima today?

MAKHIJANI: Well, Fukushima is possibly the longest running, continuous industrial disaster in history. It has not stopped because the risks are still there. This is going to take decades to decommission the site, and then what is going to happen with all this highly radioactive waste, ‘specially the molten fuel? Nobody knows.

CURWOOD: Arjun Makhijani is President of the Institute for Energy and Environmental Research. Thanks for taking time with us today, Arjun.

MAKHIJANI: So good to be back with you, Steve.

http://www.loe.org/shows/segments.html?programID=17-P13-00007&segmentID=6

Tepco’s biggest hurdle: How to remove melted fuel from crippled Fukushima reactors

b.jpg

Tepco’s scorpion-shaped robot. | IRID

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.

http://www.japantimes.co.jp/news/2017/03/09/national/tepcos-biggest-hurdle-remove-melted-fuel-crippled-fukushima-reactors/#.WMFAFKKmnIV

How will melted fuel at Fukushima plant be removed?

Naohiro Masuda, head of decommissioing for the damaged Fukushima 2.jpg

Naohiro Masuda, head of decommissioning for the damaged Fukushima

 

On March 2, 2016, five years after the meltdown caused by the 2011 earthquake and tsunami, Naohiro Masuda the Chief Decommissioning Officer of the Fukushima nuclear plant said that operators have yet to locate where the melted nuclear fuel has gone: “There are melted fuels in units 1, 2 and 3,” Masuda said. “Frankly, we do not really know what the situation is for these (melted fuel), nor where it has gone.”

One year later the melted fuel has not yet been located with certainty. The two major problems are first to find where it is, and if found how to remove it from where it is. Both jobs rendered extremely difficult by high levels radiation frying the robots’ electronic semiconductors….

How will melted fuel at Fukushima plant be removed?

The Mainichi answers common questions readers may have about how disaster-response workers plan to remove melted fuel from the disaster-stricken Fukushima No. 1 Nuclear Power Plant.
Question: What methods are being considered for removing the fuel?

Answer: Innovation will be needed in order to avoid exposing people to radiation, due to the high levels of radiation released from the fuel. One method under consideration is to fill the containment vessels holding the fuel with water, since water has radiation-blocking properties.

Q: Aren’t the containment vessels ruptured?

A: Just like you can’t fill a cup with water if it has a hole in it, the water-filling method won’t work if the containment vessels are ruptured. If they are, then another possible method is removing the fuel from the air.

Q: Which way is better?

A: Both have advantages and disadvantages. The water method could require finding and patching holes in the containment vessels. The air method wouldn’t need this, but could cause dust and other particles containing radiation to be released. The national government and plant owner Tokyo Electric Power Co. (TEPCO) will discuss as early as this summer about these two plans.

Q: What is the fuel like now?

A: At the time of the meltdown, the reactors at the plant were heated to over 2,000 degrees Celsius. The melted fuel is thought to have mixed with equipment in the plant, concrete and other materials, and to have cooled to a rock-like state. It will have to be cut out and removed.

Q: How will the fuel be cut loose?

A: The plan is to use a remotely-controlled robot. However, high-tech electronics using semiconductors are easily broken by radiation. There are ideas to make the robot use hydraulics or springs for its movement, to make it resistant to the radiation. Robot technology will be the key to a successful decommissioning of the reactors.

(Answers by Mirai Nagira, Science & Environment News Department)

http://mainichi.jp/english/articles/20170305/p2a/00m/0na/007000c