Fukushima’s underground ice wall keeps nuclear radiation at bay

The intricate network of small metal pipes, capped off by six-foot-high metal scaffolding, shouldn’t stand out amid the numerous pieces of industrial equipment littered throughout the Fukushima Daiichi Nuclear Power Plant. After all, it’s a power plant.

I take a closer look, and notice spheres of ice perched upon the smaller pipes, which line the center of the structure. The facility sits at the water’s edge, and there’s a brisk breeze blowing through.

Fixing Fukushima

Fixing Fukushima is a CNET multi-part series that explores the role technology plays in cleaning up the worst nuclear disaster in history.

But not that brisk.

It turns out, coolant is running through the pipes, freezing the soil below and creating an impermeable ice wall that’s nearly 100 feet deep and a mile long, encircling the reactors.

It’s like a smaller-scale subterranean version of the Wall in Game of Thrones, but instead of keeping out White Walkers and wights, this line of defense keeps in a far more realistic danger: radioactive contaminants from melted-down reactors that threaten to spill into the water by Fukushima Daiichi.

Daiichi is the site of the worst nuclear disaster, which happened after an earthquake hit on March 11, 2011, triggering a tsunami that devastated the facility. Two 50-foot-high waves knocked out the power generators that were keeping three of the six reactors’ fuel rods cool, triggering explosions and meltdowns that forced more than 160,000 people to flee their homes. Many of them still haven’t returned.

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Inside Fukushima: Standing 60 feet from a nuclear disaster



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I came to Fukushima to check out the robots tasked with the near-impossible task of cleaning up Fukushima Daiichi.  While here, I encountered this underground wall of ice.

The structure, which cost roughly $300 million, paid for by public funds, serves as critical protection, defending the Fukushima area from one of the most radioactive hotspots in the world. While Tokyo Electric Power Co., also known as Tepco, struggles to find a way to remove radioactive material from the facility – a process the government estimates could take more than four decades — the more immediate concern is what to do with the contaminated water leaking out from the facility.

One of the solutions has been to put up (down?) this underground ice wall, which prevents much of the surrounding groundwater from getting in. And while the practice of freezing soil to create a barrier has been around for more than 150 years, the magnitude of the application that stands before me is quite literally groundbreaking.

“Nobody has taken on a project of this scale,” Hideki Yagi, general manager of Tepco’s Nuclear Power Communications Unit, tells me through an interpreter.

Ice cold

While the term “ice wall” has a colorful ring to it, engineers use the more academic-sounding term Artificial Ground Freezing. The technique came out of France in 1862 as a way to help with the construction of mine shafts before German engineer F.H. Poetsch patented it. Since then, it’s been used to aid in building underwater tunnels or vertical shafts, as well as to cut off groundwater or redirect contaminated materials.

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Little balls of ice line the pipes that go underground and form the ice wall.


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At Fukushima, my eyes follow the path of the pipes, which stretch around the reactor building. A Tepco employee tells me that a calcium chloride solution is pumped down through a smaller inner pipe, and circulated back up a large outer pipe.

The coolant brings down the temperature of each pipe to -30 degrees Celsius, or -22 degrees Fahrenheit, and the pipes are spaced about three feet apart. The cold emanating from each one hardens the soil around it.

The point of the ice wall is to keep the groundwater that runs down from the mountains to the west from entering Fukushima Daiichi and mixing with the toxic water leaking out of the Unit 1, 2 and 3 reactors. That is,  keep the clean water on the outside of the wall, while the contaminated water stays inside.  

Tepco and manufacturing partners, such as Toshiba and Mitsubishi, are working on robots to identify and determine how to clear out the radioactive materials in each of the reactors’ primary containment vessels, essentially the heart of each facility.

Until then, they need a way to slow or stop the flow of water into the facility. At least initially, Tepco wasn’t even sure if the project was feasible.

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The coolant is -22 degrees Fahrenheit — cold enough to solidify the surround soil.


James Martin/CNET

“One of the challenges was how they would inject the pipes into the earth at such a deep level without impacting the other operations around it, and whether it would work,” Yagi says.

With the wall in place, Tepco says it has been able to reduce the level of contaminated water generated from Daiichi. But a Reuters report in March 2018 found that the wall still let a fair amount of clean water in, adding to the volume of toxic water the company needs to deal with. Tepco, however, says it’s been effective in reducing the volume.

“We know this is not the end of our effort,” says a company spokesman. “We will be continuously working hard to reduce the amount of  generation of contaminated water.”

The leaky bucket

Imagine a leaky bucket that constantly needs to be filled with water. At the same time, the water from the leak needs to be collected and stored. And there’s no end in sight to this cycle.

That essentially is the problem that Tepco faces at Daiichi. The fuel rods stored in the three radioactive units constantly have to be cooled with fresh water, but leaks mean the company needs to be vigilant about keeping the tainted liquid from getting out of the facility’s grounds. 

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The pipes go nearly 100 feet deep underground, and contain a coolant that freezes the soil into a wall.


James Martin/CNET

Since the accident nearly eight years ago, Tepco has collected 1.1 million tons of contaminated water in 900 tanks stored on the grounds at Daiichi. The company estimates it has enough space in the 37.7-million-square-foot facility to house an additional 270,000 tons of water, which means it would run out sometime in 2020.

“We’re conscious of the fact that we can’t keep storing more and more water,” Kenji Abe, a spokesman for Tepco’s decommissioning and decontamination unit, says through an interpreter.

Tepco has worked on several solutions to decrease the level of contaminated water generated by the facility. The company has switched from tanks sealed with bolts to welded tanks, which offer greater storage capacity and less risk of leaks. There’s a steel wall by the water to keep the contaminants from flowing into the ocean. Tepco has also covered 96 percent of the surface of most of the facility with concrete, preventing rainwater from seeping in.

Then there’s the ice wall, which has done its share of lowering the amount of contaminated water generated from the facility by keeping out most of the groundwater.

Over the past three and a half years, Tepco has seen the amount of polluted water generated fall by a quarter to just under 3,900 cubic feet of water per day, with occasional spikes during periods of rainfall.

The final element

I’m in full protective gear, including a Tyvek coverall, hardhat and full-face respirator mask, walking through one of three water treatment facilities at Daiichi. I move hastily, trying to keep up with my Tepco guides, when my suit gets snagged on an exposed bolt.

Did the suit rip? My eyes shoot back at my photographer and widen with fear. This is usually the part in an outbreak movie that dooms a key character. I look down and see the suit is still intact, and breathe a sigh of relief.

Fukushima Daiichi

Massive tanks of water litter the grounds of Fukushima Daicchi. 


James Martin/CNET

It turns out, I didn’t need to panic. The facility, called the Advanced Liquid Processing System, isn’t radioactive, although it’s designed to remove radioactive elements from the collected water. There are three such facilities, which can process a total of 70,630 cubic feet of water a day.

So far, treatment technology from partner companies like Kurion and Sarry have enabled Tepco to remove 62 of the 63 radioactive elements from the water, but one, tritium, remains.

An inside look at Fukushima Daiichi’s Ice Wall

It’s this one element, which is bonded to the water at an atomic level, that means Tepco needs to keep collecting and storing the water.

Lake Barrett, a senior adviser to Tepco who previously served as acting director of the Office of Civilian Radioactive Waste Management at the US Department of Energy, notes that reactors in China and Canada already discharge water with tritium.

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Surveying the scene by the Fukushima Daiichi nuclear reactors.


James Martin/CNET

“It’s fundamentally safe,” Barrett says.

But organizations such as Greenpeace have called for Tepco to keep storing the water, noting that much of the early batches of treated water far exceed safety limits for radioactive elements.

Given the sensitivities around Fukushima, Tepco must continue to store the water. A spokesman said the company isn’t planning to disperse the water. But it is one option being considered by the Japanese government, which ultimately makes the decision. 

“Resolving the issue of the contaminated water is something we haven’t yet reached a final solution on,” Yagi says.

Analyzing the data

Underneath the building housing the restaurant and employee rest area is a water treatment analysis center, a super-clean area that requires us to go through numerous radiation tests and four sets of boot changes.

There are glass beakers containing sea water, groundwater and water from the ALPS facilities. Scientists walk around in silence, moving beakers from one machine to another. A dozen machines in a second room measure the gamma ray levels.

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Inside the water treatment center.


James Martin/CNET

The facility was originally built underground in 2014 because it needed to be on the Daiichi site, but couldn’t be exposed to radiation because of the nature of the tests. The walls are 8 inches thick, with the more sensitive labs hardened with an additional 20 inches. The facility has grown by 16 times over the past four years as it expanded the number of workers and machines.

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Inside the water treatment analysis facility underneath Fukushima Daiichi. 


James Martin/CNET

“No other facility in Japan can handle the amount of data and work we do here,” says a Tepco scientist working at the facility who preferred not to identify himself.

He adds that all of the data is released publicly. “That’s because society demands work with a high level of trust,” he says.

The scientist explains that Japan has set a legal radioactivity limit of 60,000 becquerel per liter of tritium. But the treated water is still at 1.7 million Bq per liter, or roughly 30 times what’s deemed safe.

So, for now, Tepco must continue collecting the water. And the ice wall continues to stand, invisible to onlookers, as one of the most important lines of defense. 

The story originally published at 5 a.m. PT on March 5.

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