Startup Time for Fukushima's Frozen Wall. Here’s Why it Should Work

Photo: TEPCO
Readying refrigeration lines at Fukushima to create a 30-meter-deep frozen barrier against groundwater

Japan's TEPCO is about to flip the switch on the infamous ‘ice wall’ intended to divert flowing groundwater around its crippled reactors at Fukushima and thus help stem the contamination of fresh groundwater at the site. The widely mischaracterized and maligned installation—which is a barrier of frozen soil rather than a wall of ice—has every chance of delivering the hoped for results, say radiation cleanup experts at U.S. national laboratories and feedback from initial system tests.

"The frozen barrier is going to work,” predicts Brian Looney, senior advisory engineer at the U.S. Department of Energy's Savannah River National Laboratory in South Carolina and co-author of an independent assessment of TEPCO’s frozen barrier. The report, produced in collaboration with researchers at Looney's lab and at Pacific Northwest National Laboratory, was completed in February but only released late last month; it found the system’s design to be sound and within the bounds of prior practice. 

Since the 2011 meltdowns at Fukushima, TEPCO has been fighting a losing battle with groundwater that flows downhill towards the sea, permeates Fukushima’s fractured reactor buildings, and contacts their melted-down nuclear fuel. Until recently, TEPCO was sucking 300-400 tons of contaminated water out of the buildings every day, adding continuously to the site’s ballooning fields of radioactive water storage tanks. The frozen barrier is one of a suite of measures intended to stem that tide. 

In September, TEPCO started operating a system of ‘subdrains’ to capture groundwater around the reactors before it enters the buildings. After months of negotiations, fishermen’s groups agreed to allow TEPCO to treat the lightly contaminated water and then discharge it to the sea. The subdrain could cut groundwater inundation of the reactor buildings in half, leaving “only” about 150 tons per day according to World Nuclear News.

The frozen barrier is a more definitive solution, intended to completely isolate the reactor buildings from groundwater by encircling them with a 1.5-kilometer-long, 30-meter-deep wall of frozen soil. It was designed by Japanese engineering and construction firm Kajima Corp., which began piecing together its infrastructure in June 2014. Since then, Kajima has perforated the ground surrounding Fukushima’s four reactors with 1571 bore holes, lined them with chiller pipes, and hooked up those pipes to refrigeration plants that pump out brine at an icy -30 degrees Celsius.

This groundwater isolation technology has been applied hundreds of times since the 1950s at mines and at deep excavations for tall buildings. But at Fukushima, it has been a magnet for scorn. The negativity stems from an understandable dearth of confidence in TEPCO and misreporting by media outlets. 

Many news outlets accidentally conflated Kajima's frozen barrier with a distinct and ill-fated TEPCO effort to freeze 5,000 to 6,000 tons of contaminated seawater in a utility trench adjacent to one of the reactors. CleanTechnica’s August 2014 report, "TEPCO Concedes Failure of Fukushima Ice Wall”, was one of many to describe the failed trench freeze as a section of the frozen barrier. 

And this confusion persists. Al Jazeera opined in March that the frozen barrier had “turned out to be another of the cleanup’s dramatically costly and utterly ineffective schemes.” It is also one of many new outlets to erroneously assert that Kajima's design was beyond the scale or applications of prior frozen walls.

The U.S. national labs’ analysis is as complimentary of Kajima’s design as the media’s opinions have been skeptical. According to Looney, their report found that Kajima’s design is “within the envelope of experience for successful barriers.” The team identified installations larger than Kajima’s design, such as a 3.66-km-long freeze wall at an open pit gold mine in Ontario that was four times deeper than Fukushima's, as well as urban projects that had more buried structures to work around (or through) than is the case at Fukushima. 

Site-specific analysis by Looney et al, meanwhile, projects that the freezing energy to be deployed by Kajima’s system will be equal to the task of managing Fukushima’s hydrological conditions. That finding is affirmed by initial testing of about 60 meters of the barrier in May 2015; the ground chilled as predicted.

The national labs study did identify small areas that could potentially resist freezing. But it also identified 10 to 12 available fixes, and also concluded that small leaks would be of little consequence given the suite of other groundwater control systems TEPCO has in place. “The goal of the barrier is to minimize flow to the reactors. You don’t actually need 100 percent effectiveness to reach that goal,” says Looney. 

While skepticism over the barrier’s water-stopping capability reigns in the popular media, Japanese regulators have been fretting over whether it might prove overly effective. Japan’s Nuclear Regulation Authority (NRA) has been holding back TEPCO from beginning the freeze while it assesses the risk of groundwater levels plummeting within the perimeter as the freeze kicks in, drawing highly radioactive water out of the reactors and contaminating the soil below.

Looney argues that this scenario is unlikely. It will take 1-2 months of refrigeration to establish each section of the barrier and at least six months of refrigeration before it has a measurable impact on groundwater levels within the frozen wall's perimeter, according to Looney. He adds that extensive monitoring should give TEPCO several months' notice of potential groundwater imbalances. 

But to minimize the risk of a groundwater crash, the NRA has requested that TEPCO first freeze the barrier’s side and downhill segments, saving the uphill segment (which Kajima finished first) for last. According to Looney and recent media reports, the final piping should be complete and the system should be ready to switch on within weeks.

If the barrier kicks in next year, concluding what will have been a five-year battle against groundwater, TEPCO will then be in a position to attack a still-tougher foe: its shattered reactors’ melted fuel. Naohiro Masuda, president of TEPCO’s cleanup subsidiary, Fukushima Daiichi Decommissioning Company, told Japanese state broadcaster NHK in March that TEPCO has “no idea” what the physical state of the fuel is (though it's got robots on the hunt) and no idea how to get it out. As Masuda put it: “We have to remove it remotely from 30 meters above. But we don’t have that kind of technology yet. It simply doesn’t exist.”


Newsletter Sign Up

Sign up for the EnergyWise newsletter and get biweekly news on the power & energy industry, green technology, and conservation delivered directly to your inbox.