By Dennis Normile
Originally published in Science
Mar. 2, 2017
TOKYO—Six years into a decommissioning effort expected to last into the 2050s, an official leading the work on the stricken Fukushima Daiichi Nuclear Power Plant claims that cleanup crews are making “steadfast progress.” But thorny technical obstacles must be overcome.
The 9.0-magnitude earthquake off Japan’s northeast coast on 11 March 2011 triggered one of history’s most devastating tsunamis. The one-two punch killed nearly 16,000 people, left more than 2500 missing, and wiped out infrastructure in coastal communities.
The tsunami also knocked out Fukushima’s systems for cooling its nuclear reactors, causing core meltdowns in three of the plant’s six reactors. Hydrogen explosions blew out the walls and roofs of the buildings housing units 1, 2, and 3, releasing massive amounts of radiation [editors note: explosions were in units 1, 3, and 4]. Much of the contamination was swept into the Pacific Ocean, but winds deposited fallout over parts of northeastern Japan. Some 160,000 people living near the plant were evacuated or fled on their own.
On the eve of the sixth anniversary of the disaster, officials took pride in what they view as successful efforts to minimize the health threat to surrounding communities. Radiation from the crippled reactors is no longer having an impact outside the plant, Naohiro Masuda, head of decommissioning for Fukushima owner Tokyo Electric Power Co. (TEPCO), said today at a briefing here. He noted that evacuated residents are returning to their homes as decontamination work reduces exposure levels below thresholds. At the power plant, radiation levels are now so low that the 6000 workers slowly demolishing the damaged reactor halls need only wear typical construction site safety gear except when working near the three reactors that suffered meltdowns. And radiation levels just offshore remain below the limit for drinking water set by the World Health Organization, Masuda said. Given the progress, he reiterated that TEPCO is confident they can stick to a previously set roadmap that envisions completing the decommissioning 30 to 40 years after the accident. But doing so won’t be cheap. Last December, Japan’s Ministry of Economy, Trade and Industry revised its estimate of the total cost of decommissioning up to $188 billion.
Stemming ocean contamination has been a thorny challenge. Since early in the crisis, crews have circulated water through the damaged reactors to prevent overheating that could lead to further fuel melting. That water, and groundwater flowing through the site, is heavily contaminated and TEPCO has struggled to keep it from seeping into the Pacific. Schemes to divert groundwater away from the plant and freeze a wall of soil around the reactors down to bedrock—to contain contaminated water—have minimized leaks, Masuda said.
In the meantime, TEPCO has accumulated 960,000 tons of contaminated water stored in 1000 10-meter-tall tanks at the site. TEPCO has removed cesium, strontium, and more than 50 other radionuclides from that water. But they have been stymied by tritium, a radioactive hydrogen isotope in the water. Several experimental approaches to removing the tritium “were judged to be impractical,” Masuda said.
Tritium occurs naturally in water but in minuscule concentrations. Simply releasing the tritium-laden water, perhaps after further dilution, is one disposal option, Masuda said. Another would be to evaporate the water, releasing some tritium into the atmosphere, as was done at the Three Mile Island nuclear plant in Pennsylvania after its 1979 accident. An advisory committee is now studying the problem and will hold discussions with local communities “so TEPCO will be able to act in a responsible manner in dealing with the tritium,” Masuda said.
Another major hurdle is determining the condition and location of the melted fuel, much of which is believed to have dropped to the bottoms of the containment vessels where high radiation levels preclude human entry. Robotic investigations are proving problematic. In January, the camera on a robotic probe sent into the Unit 2 containment vessel was fried by radiation, though it did return important images before its demise. Then last month, a small robot on tanklike treads was sent through a 10-centimeter-diameter pipe into the vessel to investigate the presumed location of the damaged fuel. But it got tangled up in debris and was abandoned.
TEPCO is now thinking it might need a robot able to jump over debris. And they are planning robotic investigations of the units 1 and 3 containment vessels in preparation for a planning session this summer to set a policy for recovering the melted fuel.