Much of northeastern Japan is well along in recovering from the magnitude 9 earthquake and 40-meter tsunami of 11 March 2011. However, at the Fukushima Daiichi Nuclear Power Plant, “We’re still just very near the starting line” for cleaning up after the meltdowns and explosions triggered by the natural disasters, Fukushima prefecture Governor Masao Uchibori said at a 17 February press briefing.
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The plant owner, Tokyo Electric Power Co. (TEPCO), envisions roughly 30 more years of work to retrieve undamaged fuel, remove resolidified melted fuel debris, disassemble the reactors, and dispose of contaminated cooling water. The fuel debris and contaminated water pose especially thorny problems that could threaten that timetable. The government puts the cost of decommissioning the four reactors at 8 trillion yen ($76 billion); but the Japan Center for Economic Research, a think tank, estimates the bill could be much more.
The earthquake cut the plant’s electrical lines, moving it to emergency generators, which the tsunami swamped 50 minutes later. The power failures disabled pumps that circulate cooling water. As the reactor cores overheated and melted, radioactive steam and hydrogen escaped from the reactor pressure vessels, accumulated in the upper floors of three of the buildings, and then exploded, blowing out roofs and wall panels and spewing fallout over nearby communities.
Though the four reactors are similar, the meltdowns and explosions damaged them differently. So, four unique structures have been or will be built to support cranes needed to safely retrieve undamaged fuel stored on upper floors of the units. Removing the fuel is expected to take until 2031.
Past, present, future
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Removing the fuel debris is a tougher task, with no target completion date yet. TEPCO believes some of the debris is still in the reactor cores; some dropped to the bottoms of the reactor pressure vessels, which surround the cores; and some burned through the vessels and landed on the concrete at the base of the reactors. Remote-control devices have been used to try to verify the location and condition of the fuel within the highly contaminated reactors. But, “We do not yet have an accurate grasp of what has happened to the molten fuel,” Uchibori said. In 2022, workers will test a remotely operated mechanical arm to retrieve small amounts of fuel debris believed to be at the bottom of the Unit 2 reactor.
The other major challenge is disposing of water that gets contaminated as it circulates through the reactors to remove residual heat from the fuel debris. Initial treatment removes many radioactive elements but not tritium, an isotope of hydrogen that is extremely difficult to capture. Over the past decade, more than 1.24 million tons of tritium-contaminated water have accumulated, filling more than 1000 tanks that occupy nearly every available nook on the Fukushima Daiichi campus.
Because it gives off only low-energy beta particles, tritium poses a modest health risk. It is also present in seawater and the atmosphere, something Japan’s Ministry of Economy, Trade and Industry points out in defense of proposals to gradually discharge the accumulated water into the ocean or atmosphere. But neighboring nations, environmental groups, and the fishing industry fiercely oppose both options. Government officials say no final decision has been made.
Traces of ruthenium, cobalt, strontium, and plutonium isotopes in the treated water also raise concerns, says Ken Buesseler, a marine chemist at the Woods Hole Oceanographic Institution. TEPCO says it will reduce such material “as much as possible.” A reckoning approaches: TEPCO says it will run out of room for water by summer 2022.
Source: Science Mag