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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Fukushima moms don lab coats to measure radiation in food, sand and soil — The Japan Times

” IWAKI, FUKUSHIMA PREF. – At a laboratory an hour’s drive from the crippled Fukushima No. 1 nuclear plant, a woman wearing a white mask over her mouth presses bright red strawberries into a pot, ready to be measured for radiation contamination.

Six years after a massive earthquake off the Tohoku coast triggered tsunami that knocked out the plant’s cooling system, causing three reactor-core meltdowns, local mothers with no scientific background staff a laboratory that keeps track of radiation levels in food, water and soil.

As some women divide the samples between different bowls and handmade paper containers, others are logging onto computers to keep an eye on data — findings that will be published for the public to access.

The women on duty, wearing pastel-colored overalls, are paid a small salary to come in for a few hours each day, leaving them free to care for their children after school.

“In universities, data (are) handled by students, who have taken exams qualifying them to measure radiation. Here, it’s done by mothers working part time. It’s a crazy situation,” laughed Kaori Suzuki, director of Tarachine, the nonprofit organization that houses the mothers’ radiation lab.

“If (university professors) saw this I think they would be completely shocked by what they see.”

Tarachine was set up 60 km down the coast from the Fukushima plant, in the city of Iwaki. After the magnitude-9 quake struck on March 11, 2011, triggering mountainous tsunami, authorities declared a no-go zone around the plant.

Iwaki lay outside its 30 km radius, with lower radiation levels compared to the rest of Fukushima Prefecture.

But with public announcements advising locals to stay indoors in the aftermath of the worst nuclear calamity since Chernobyl, the “invisible enemy” of radiation has continued to worry the mothers working at the lab.

“As ordinary citizens we had no knowledge about radiation. All we knew was that it is frightening,” said Suzuki.

“We can’t see, smell or feel radiation levels. Given this invisibility, it was extremely difficult for us. How do we fight it? The only way is to measure it.”

To supplement readings by the central government and Tokyo Electric Power Company Holdings Inc., which manages the nuclear plant, Tarachine publishes its own findings every month.

With donations from the public that helped them buy equipment designed to measure food contamination, the mothers measure radioactive isotopes cesium-134 and-137, and collect data on gamma radiation, strontium-90 and tritium, all of which were released during the Fukushima disaster.

Strontium-90 gravitates toward the bones when absorbed by breathing it, drinking it in water, or eating it in food. It can remain for years, potentially causing bone cancer or leukemia.

Tritium goes directly into the soft tissues and organs of the human body. Although it is less harmful to humans who are exposed to small amounts every day, it could still be a hazard for children, scientists say.

The mothers say other parents trust the lab’s radioactivity readings in local food more than those from the government.

“This issue is part of everyday life for these mothers, so they have the capability to spot certain trends and various problems rather than just accumulating expert knowledge,” said Suzuki.

To handle potentially dangerous materials, the mothers have had to study for exams related to radiation and organic chemistry.

“At the beginning I was just completely clueless. It gave me so much of a headache, it was a completely different world to me,” said Fumiko Funemoto, a mother of two who measures strontium-90 at the lab.

“But you start to get the hang of it as you’re in this environment every day.”

As the lab only accepts items for testing from outside the exclusion zone, most results show comparatively low radiation levels.

But Suzuki said it was an important process and especially reassuring for the parents of young children. The women also measure radiation levels in sand from the beach, which has been out of bounds to their children.

“If the base is zero becquerels, and there is, say, 15 or 16 becquerels of cesium, that’s still higher than zero. That means there is slightly more risk,” Suzuki said.

“There are also times when you’re like, ‘Oh, I thought levels were going to be high there — but it’s actually OK.’ The importance lies in knowing what’s accurate, whether it’s high or low. Unless you know the levels, you can’t implement the appropriate measures.”

Since official screenings began following the meltdowns, 174 children in Fukushima Prefecture have been diagnosed with — or are suspected of having — thyroid cancer, according to figures from the prefecture.

Despite the International Atomic Energy Agency (IAEA) reporting in 2015 that an increase in thyroid cancer is unlikely, the mothers insist there is value in their work.

The first pictures from inside the nuclear plant were released by Tepco in January, announcing it may have found melted nuclear fuel below the damaged reactor 2 — one of three affected by the 2011 disaster.

“In general, the issue of nuclear power is not really talked about much these days,” Funemoto said. “It was talked about after the (meltdowns) for about a year or so, but today, conversations mentioning words like ‘radiation’ don’t happen anymore.”

However, she said “the reality is different.”

“The radiation isn’t going to go away. That’s why I’m doing this. So many places are still damaged. This idea that it’s safe and that we shouldn’t be anxious doesn’t really add up,” she said.

Ai Kimura, another mother, agreed. “My parents think I’m a bit paranoid. They keep saying, ‘It’s OK isn’t it?’ ” she said.

“But what if there’s a chance that in 10 or 20 years’ time, my own child gets thyroid cancer? And I could have done my bit to minimize the risks. My children are mine and I want to do whatever I can to protect them.” ”

by Mari Shibata, The Japan Times

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