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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Nuclear fuel debris that penetrated reactor pressure vessel possibly found at Fukushima No. 1 — The Japan Times

” Tepco on Monday found what may be melted nuclear fuel debris that penetrated the reactor 2 pressure vessel at the Fukushima No. 1 plant.

Tokyo Electric Power Company Holdings Inc. said more analysis and investigation is needed to confirm that the black lumps detected in the reactor’s containment vessel are indeed fuel debris.

The steel pressure vessel houses the nuclear fuel rods and is set up inside the surrounding containment vessel.

“At this point, it’s difficult to clearly identify what they are,” said Yuichi Okamura, general manager of Tepco’s nuclear power and plant siting division, during an evening news conference at the utility’s Tokyo headquarters.

Video footage from Monday’s probe showed black lumps that looked like something that had melted and then congealed, sticking to parts of a steel grating area at the base of the containment vessel.

The material could be melted paint, cable covers or pipe wrappings, Okamura said.

Still, this is the first time Tepco has detected anything in any of the facility’s three wrecked reactors that might be melted fuel rods since the outbreak of the crisis in March 2011. Okamura described the finding as “valuable information.”

The location of the debris and what form it is in are critical to eventually recovering the fuel.

Tepco plans next month to send in a remote-controlled robot equipped with a thermometer and dosimeter. Analyzing the temperature and radiation level will help identify whether the lumps are fuel debris, Okamura said.

The fuel melted after the March 11, 2011, earthquake and tsunami knocked out Fukushima No. 1’s power supply, including the vital cooling functions.

It is believed that reactor No. 2’s fuel rods melted and penetrated the bottom of the 20-cm-thick pressure vessel and fell in to the containment vessel.

Tepco has been conducting an investigation to check the interior of the containment vessel since last week.

In a previous try, workers inserted a rod equipped with a small camera as a precursor to sending in the remote-controlled robot.

The first attempt turned up nothing of note, but the utility then tried a longer rod — 10.5 meters long — on Monday that could capture images of the area beneath the pressure vessel.

The video footage also showed that water droplets were falling, which Tepco said must be cooling water being injected into the damaged pressure vessel.

Reactor 2 is one of three reactors, including 1 and 3, that experienced fuel meltdowns. ”

by Kazuaki Nagata

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Near miss at Fukushima is a warning for U.S., panel says — Richard Stone, Science

” Japan’s chief cabinet secretary called it “the devil’s scenario.” Two weeks after the 11 March 2011 earthquake and tsunami devastated the Fukushima Daiichi Nuclear Power Plant, causing three nuclear reactors to melt down and release radioactive plumes, officials were bracing for even worse. They feared that spent fuel stored in the reactor halls would catch fire and send radioactive smoke across a much wider swath of eastern Japan, including Tokyo.

Thanks to a lucky break detailed in a report released today by the U.S. National Academies, Japan dodged that bullet. The near calamity “should serve as a wake-up call for the industry,” says Joseph Shepherd, a mechanical engineer at the California Institute of Technology in Pasadena who chaired the academy committee that produced the report. Spent fuel accumulating at U.S. nuclear reactor plants is also vulnerable, the report warns. A major spent fuel fire at a U.S. nuclear plant “could dwarf the horrific consequences of the Fukushima accident,” says Edwin Lyman, a physicist at the Union of Concerned Scientists, a nonprofit in Washington, D.C., who was not on the panel.

After spent fuel is removed from a reactor core, the fission products continue to decay radioactively, generating heat. Many nuclear plants, like Fukushima, store the fuel onsite at the bottom of deep pools for at least 5 years while it slowly cools. It is seriously vulnerable there, as the Fukushima accident demonstrated, and so the academy panel recommends that the U.S. Nuclear Regulatory Commission (NRC) and nuclear plant operators beef up systems for monitoring the pools and topping up water levels in case a facility is damaged. It also calls for more robust security measures after a disaster. “Disruptions create opportunities for malevolent acts,” Shepherd says.

At Fukushima, the earthquake and tsunami cut power to pumps that circulated coolant through the reactor cores and cooled water in the spent fuel pools. The pump failure led to the core meltdowns. In the pools, found in all six of Fukushima’s reactor halls, radioactive decay gradually heated the water. Of preeminent concern were the pools in reactor Units 1 through 4: Those buildings had sustained heavy damage on 11 March and in subsequent days, when explosions occurred in Units 1, 3, and 4.

The “devil’s scenario” nearly played out in Unit 4, where the reactor was shut down for maintenance. The entire reactor core—all 548 assemblies—was in the spent fuel pool, and was hotter than fuel in the other pools. When an explosion blew off Unit 4’s roof on 15 March, plant operators assumed the cause was hydrogen—and they feared it had come from fuel in the pool that had been exposed to air. They could not confirm that, because the blast had destroyed instrumentation for monitoring the pool. (Tokyo Electric Power Company, the plant operator, later suggested that the hydrogen that had exploded had come not from exposed spent fuel but from the melted reactor core in the adjacent Unit 3.) But the possibility that the fuel had been exposed was plausible and alarming enough for then-NRC Chairman Gregory Jaczko on 16 March to urge more extensive evacuations than the Japanese government had advised—beyond a 20-kilometer radius from the plant.

Later that day, however, concerns abated after a helicopter overflight captured video of sunlight glinting off water in the spent fuel pool. In fact, the crisis was worsening: The pool’s water was boiling away because of the hot fuel. As the level fell perilously close to the top of the fuel assemblies, something “fortuitous” happened, Shepherd says. As part of routine maintenance, workers had flooded Unit 4’s reactor well, where the core normally sits. Separating the well and the spent fuel pool is a gate through which fuel assemblies are transferred. The gate allowed water from the reactor well to leak into the spent fuel pool, partially refilling it. Without that leakage, the academy panel’s own modeling predicted that the tops of the fuel assemblies would have been exposed by early April; as the water continued to evaporate, the odds of the assemblies’ zirconium cladding catching fire would have skyrocketed. Only good fortune and makeshift measures to pump or spray water into all the spent fuel pools averted that disaster, the academy panel notes.

At U.S. nuclear plants, spent fuel is equally vulnerable. It is for the most part densely packed in pools, heightening the fire risk if cooling systems were to fail. NRC has estimated that a major fire in a U.S. spent fuel pool would displace, on average, 3.4 million people from an area larger than New Jersey. “We’re talking about trillion-dollar consequences,” says panelist Frank von Hippel, a nuclear security expert at Princeton University.

Besides developing better systems for monitoring the pools, the panel recommends that NRC take another look at the benefits of moving spent fuel to other storage as quickly as possible. Spent fuel can be shifted to concrete containers called dry casks as soon as it cools sufficiently, and the academy panel recommends that NRC “assess the risks and potential benefits of expedited transfer.” A wholesale transfer to dry casks at U.S. plants would cost roughly $4 billion. ”

by Richard Stone, Science

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Editorial: New technology to help resolve Fukushima nuclear crisis needs gov’t backing — The Mainichi; South China Morning Post

” The Japanese government has once again revised the work schedule for decommissioning reactors at the triple-meltdown-stricken Fukushima No. 1 nuclear plant. The last major change was in June 2013, and this one pushes back the removal of spent fuel rods from the fuel pools of the No. 1-3 reactors by as much as three years. The delay is due to unexpected difficulties preventing the escape of airborne radioactive contaminants during decontamination and wreckage clearing work.
Decommissioning reactors at the heart of one of the world’s worst nuclear disasters is of course bound to be extremely difficult, and this reality is coming into sharp relief.

Progress on dismantling the Fukushima reactors has a direct bearing on both overall regional disaster recovery and when local residents will be able to finally return home. As such, we call on both the government and plant operator Tokyo Electric Power Co. to develop a reactor decommissioning strategy with a solid strategic foundation, and to thoroughly release information on the process.
The latest revisions to the decommissioning work schedule were based on the basic principle of putting the safety of locals and plant workers first. The first version of the work schedule was obsessed with speed. The result was a rash of worker injuries and deaths and other problems that ended up causing progress to be delayed. Rather than making speed top priority, it’s more important to carefully and surely reduce the various risks related to the Fukushima plant.

The jobs with the highest priority under the work plan’s latest iteration are the recovery of nuclear fuel rods from the fuel pools, and dealing with the vast quantities of radioactively contaminated water produced at the plant. Though these tasks are certainly important, the most difficult hurdle in the decommissioning process will be extracting the melted fuel from inside the stricken reactor vessels. Under the new schedule, this is set to start on just one of the reactors sometime in the year 2021.

That’s some six years away, but the path from here to there remains foggy at best. First of all, no one knows for sure exactly what state the fuel is in or even where it is in the reactor housings.

The method for getting the fuel out is also up in the air. At first, planners thought it best to fill the reactor vessels with water to suppress the intense radiation when the operation began. This fell by the wayside, however, when it turned out to be difficult to identify damaged spots on the reactor vessels and stop water from escaping. Now, an in-air removal method is being considered, though entirely new equipment will need to be developed to perform the operation in the highly radioactive environment while at the same time preventing contaminants from getting airborne.

There are a number of research institutes and universities across Japan that are receiving government support to invent the technology needed for this reactor decommissioning work. The “control tower” for these efforts is the Nuclear Damage Compensation and Decommissioning Facilitation Corp. (NDF), created by the government in August last year. The corporation is tasked with overseeing each project from basic research through to practical application, and to optimize the development process.

The NDF, however, has just 35 or so technical staff. It’s an open question whether the NDF can exercise effective oversight for such a wide program with so few people. The government is trying to enhance the corporation’s functions, but there have been no concrete measures forthcoming so far. At this rate, might the 30-40 year target to decommission the Fukushima reactors come under serious pressure?

Prime Minister Shinzo Abe has said more than once that “the national government stands on the front lines” of the efforts to deal with the decommissioning work. Then more than ever, the government must create a system to provide full and complete support for the technology research and development projects needed to finally bring the nuclear crisis to an end. ”

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Also read a similar article by the South China Morning Post: “Scale of Fukushima clean-up revealed as decommissioning ‘road map’ is revised

The Horrors of Fukushima — Mitsuhei Murata, April 20, 2015

Document dated April 20, 2015:

” The Fukushima accident gives the impression to the world that it has been brought under control. It is far from it. The following point of view of Dr. Norio Iriguchi, Professor Emeritus of Kumamoto University, impresses us with the potential horrors of the damaged nuclear plant. I wish to convey to you my impressions it has given me.

1. The point of view of Professor Iriguchi

The containment vessels of units 1, 2 and 3 are broken. The containment vessels were the last strongholds to shut radioactive materials in, but now they are now in contact with the external environment. The volume of each vessel is about 15,000 cubic meters. The temperature inside the vessels varies day and night, winter and summer, and we can presume they are breathing, so to speak, accordingly. Within the vessels, there are molten nuclear fuel rods. If the temperature rises for some reasons, radioactive materials are released into the atmosphere.

Cesium 137 contained in the containment vessels of units 1, 2 and 3 is the equivalent of 14 thousand Hiroshima atomic bombs, 7000 of which are spent nuclear fuel rods whose radioactivity will be reduced by one to one thousand in 300 years. During this period, radioactive materials will continue to be scattered bit by bit as stated above, unless a major natural disaster takes place. However, if molten nuclear fuel rods are exposed through cracks to the atmosphere due to a mega earthquake or the liquidization of the site that causes the collapse or the inclination of a nuclear reactor, Japan’s landmass would become uninhabitable to a large extent.

Cesium 137 contained in unspent nuclear fuel rods amounts to about 7000 Hiroshima atomic bombs, and the possibility of it causing a nuclear fission chain reaction persists for one million years.

In addition to the above-mentioned dangers, there exists a cooling pool for spent nuclear fuel rods on the third floor of each building of the three units. The total of the radioactive materials in the three pools amounts to the equivalent of 16,000 Hiroshima atomic bombs. They are submersed in water in order to confine radioactivity. For 300 years at least, they make us dread mega earthquakes and the associated collapse of the buildings that could take place at any moment.

2. My impressions

The DNA of Japan as a nation has been damaged by the nuclear accident. I have been asserting that nuclear reactors are no less dangerous than nuclear weapons, but it is more correct to say that nuclear reactors are more dangerous than nuclear weapons.

The nuclear accident has obliged many residents of Fukushima to abandon their native places. It has contaminated Japan’s invaluable soil with radioactivity and has brought about a situation that obliges the relevant residents to abandon forever their living and farming land.

The ongoing radioactive contamination of the sea with no prospect for a solution is dishonoring Japan, being criticized as harming the global environment. In spite of all this, attempts are shamefully being made to hide Fukushima.

The Tokyo Olympic Games, the Senkaku Islands problem, the Takeshima issue, all these are just out of the question. ”

*also read Mitsuhei Murata’s letter from May 6, 2015.

Updated 4/13/15: Survey robot breaks down inside Fukushima No. 1 reactor in under three hours — The Japan Times

Updated April 13, 2015, The Japan Times: ” A remote-controlled robot inserted to survey the inside of the No. 1 reactor at the damaged Fukushima No. 1 nuclear power plant has stopped functioning, Tokyo Electric Power Co. said.

As a first step in the utility’s effort to remove melted nuclear fuel from the bottom of the unit’s primary containment vessel, the shape-shifting robot was sent in Friday morning to find the exact location of the highly radioactive debris.

Set to cover some 20 meters of the first floor on the first day, the robot began its trip at around 11:20 a.m. but halted at around 2:10 p.m. after completing two-thirds of the route, Tepco said.

The utility said footage from the robot’s camera shows it passed an opening leading to the vessel’s basement, where the molten fuel is believed to have ended up after the core meltdowns occurred after the March 2011 quake and tsunami. ”

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Posted April 10, 2015, The Japan Times, “Robot enters primary containment vessel of reactor 1 in Fukushima”:

” A robot on Friday crept into the deadly primary containment vessel of reactor 1 of the Fukushima No. 1 power plant to surveil its damaged interior, Tokyo Electric Power Co. said.

It is the first time a robot has entered the PCV of any of the three stricken reactors at the meltdown-hit plant, and the snake-like contraption might give the utility a better idea of what happened to the pressure vessel and its core in the disaster.

Tepco plans to have the robot check half of the first floor of the bulbous PCV on Friday and examine the other half on Monday.

Ultimately, the utility plans to explore the underground portion of the vessel, where the melted fuel rods are believed to have puddled. But that is not yet feasible because the robot isn’t waterproof. A waterproof version is expected to be developed by the end of next March.

The snake-like robot, about 9.5 cm high and 60 cm long, entered the vessel through a pipe 10 cm in diameter. After going down to the first floor, it was to assume a U shape and measure temperature and radiation levels. It was also to photograph the interior and check for obstacles in the area leading to the underground portion.

When a measuring device was snaked into the same vessel in October 2012, the radiation peaked at a deadly 11 sieverts per hour.

The fuel in reactor Nos. 1 to 3 is believed to have melted through their reactor pressure vessels and spread to the primary or other containment vessels. But the exact details are still a mystery more than four years after the crisis began because the site is too hostile to explore.

Developed by Hitachi-GE Nuclear Energy Ltd. and the International Research Institute for Nuclear Decommissioning, the robot is equipped with a camera, a thermometer and dosimeter.

Given the high radiation, it can only function for about 10 hours before the electronics fail, the institute said.

The robot will be remotely guided from a plant building where radiation is lower. About 40 workers will be involved and radiation exposure will be limited to 2.5 millisieverts or lower per person per day. ”

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