Five years after the meltdown, is it safe to live near Fukushima? — Science

” A  long, grinding struggle back to normal is underway at the Fukushima Daiichi Nuclear Power Plant in Japan. As workers make progress in cleaning up contaminated land surrounding its infamous reactor, evacuees are grappling with whether to return to homes sealed off since the accident there 5 years ago. The power plant itself remains a dangerous disaster zone, with workers just beginning the complex, risky job of locating the melted fuel and figuring out how to remove it.

The magnitude 9.0 earthquake that struck northeastern Japan on 11 March 2011 and the 40-meter tsunami that followed left 15,893 dead and 2572 missing, destroyed 127,290 buildings, and damaged more than a million more. It also triggered the meltdowns at Fukushima and the evacuation of 150,000 people from within 20 kilometers of the nuclear plant as well as from areas beyond that were hard hit by fallout.

Now, the nuclear refugees face a dilemma: How much radiation in their former homes is safe? In a herculean effort, authorities have so far scooped up some 9 million cubic meters of contaminated soil and leaves and washed down buildings and roadways with the goal of reducing outdoor radiation exposure to 0.23 microsieverts per hour. Last September, the government began lifting evacuation orders for the seven municipalities wholly or partly within 20 kilometers of the plant. As the work progresses, authorities expect that 70% of the evacuees will be allowed to return home by spring 2017.

But evacuees are torn over safety and compensation issues. Many claim they are being compelled to go home, even though radiation exposure levels, they feel, are still too high. “There has been no education regarding radiation,” says Katsunobu Sakurai, the mayor of Minamisoma, where 14,000 people were evacuated after the accident. “It’s difficult for many people to make the decision to return without knowing what these radiation levels mean and what is safe,” he says. Some citizen groups are suing the national government and Tokyo Electric Power Company (TEPCO), the Fukushima plant’s owner, over plans to end compensation payments for those who choose not to return home. Highly contaminated areas close to the nuclear plant will remain off limits indefinitely.

Conditions at the plant are “really stable,” the plant manager, Akira Ono, recently told reporters. Radioactivity and heat from the nuclear fuel have fallen substantially in the past 5 years, he says. But cleanup is off to a slow start, hampered by sketchy knowledge of where the nuclear fuel is located. Last year managers agreed to a road map for decommissioning the site over the next 30 to 40 years that calls for removing melted nuclear fuel masses and demolishing the plant’s four reactor halls at a cost that could top $9 billion. TEPCO intends to start removing nuclear debris from the reactors in 2021.

Ono puts the decommissioning at “around 10%” complete. One big hurdle was cleared in December 2014, when crews removed the last of 1535 fuel rods stored in the Unit 4 spent fuel pool. At the time of the accident, some feared that cooling water had drained out of the pool and exposed the fuel to air, which might have led to overheating and melting. It hadn’t, but the fuel remained a threat.

The biggest challenge at present, Ono says, is contaminated water. Cooling water is continuously poured over the melted cores of units 1, 2, and 3 to keep the fuel from overheating and melting again. The water drains into building basements, where it mixes with groundwater. To reduce the amount of contaminated water seeping into the ocean, TEPCO collects and stores it in 10-meter-tall steel tanks. They now fill nearly every corner of the grounds, holding some 750,000 tons of water. The government is evaluating experimental techniques for cleansing the water of a key radioisotope, tritium. Ono says a solution is sorely needed before the plant runs out of room for more tanks.

TEPCO has found ways to divert groundwater from the site, cutting infiltration to about 150 tons per day. Now it’s about to freeze out the rest. Borrowing a technique for making temporary subsurface barriers during tunnel construction, a contractor has driven 1500 pipes 30 meters down to bedrock, creating something akin to an underground picket fence encircling the four crippled reactor units. Brine chilled to –30°C circulating in the pipes will freeze the soil between the pipes; the frozen wall should keep groundwater out and contaminated water in. TEPCO was planning to start the operation shortly after Science went to press.

The most daunting task is recovering the fuel debris. TEPCO modeling and analyses suggest that most, if not all, of the fuel in the Unit 1 reactor melted, burned through the reactor pressure vessel, dropped to the bottom of the containment vessel, and perhaps ate into the concrete base. Units 2 and 3 suffered partial meltdowns, and some fuel may remain in the cores.

To try to confirm the location and condition of the melted fuel, the International Research Institute for Nuclear Decommissioning, set up by TEPCO and other entities, has been probing the reactors’ innards with muons. Wispy cousins of the electron, muons are generated by the trillions each minute when cosmic rays slam into the upper atmosphere. A few muons are absorbed or scattered, at a rate that depends on a material’s density. Because uranium is denser than steel or concrete, muon imaging can potentially locate the fuel debris.

In February 2015, a group at Japan’s High Energy Accelerator Research Organization in Tsukuba supplied two van-sized muon detectors, which TEPCO placed adjacent to the Unit 1 reactor at ground level. After a month of collecting muons, the detectors confirmed there was no fuel left in the core. Because they were positioned at ground level, the devices could not image the reactor building basements and so could not pin down where the fuel is or its condition. TEPCO plans to use robots to map the location of the fuel debris so it can develop a strategy for removing it (see story, right).

A second team has developed detectors that observe muons before and after they pass through an object of interest, promising a more precise picture of reactor interiors. For Fukushima, the researchers—from Los Alamos National Laboratory in New Mexico and Japan’s Toshiba Corp.—built mammoth detectors, 7 meters across, which they intended to place outside Unit 2. That work has been postponed because TEPCO decided to first send a robot into the containment vessel; high radiation levels have delayed that plan. “Our current task is to reduce that exposure,” Ono says, using robotic floor and wall scrubbers in the area workers need to access to deploy the robot.

While the authorities struggle to clean up the site and resettle residents, some locals are judging safety for themselves. In 2014, a group of enterprising high school students in Fukushima city, outside the evacuation zone, launched an international radiation-dosimetry project. Some 216 students and teachers at six schools in Fukushima Prefecture, six elsewhere in Japan, four in France, eight in Poland, and two in Belarus wore dosimeters for 2 weeks while keeping detailed diaries of their whereabouts and activities. “I wanted to know how high my exposure dose was and I wanted to compare that dose with people living in other places,” explains Haruka Onodera, a member of Fukushima High School’s Super Science Club, which conceived the project. The students published their findings last November in the Journal of Radiological Protection. Their conclusion: “High school students in Fukushima [Prefecture] do not suffer from significantly higher levels of radiation” than those living elsewhere, Onodera says.

That’s good news for Fukushima city residents, perhaps, but cold comfort to displaced people now weighing the prospect of moving back to homes closer to the shattered nuclear plant. ”

by Dennis Normile



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