News FocusTohoku-Oki Earthquake

Crippled Reactors to Get Cooled and Wrapped

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Science  20 May 2011:
Vol. 332, Issue 6032, pp. 910
DOI: 10.1126/science.332.6032.910

The crisis at the stricken Fukushima Daiichi nuclear power plant is far from over. Some 100,000 residents who were evacuated will not return home until the reactors are firmly under control.

TOKYO—The crisis at the stricken Fukushima Daiichi nuclear power plant may have faded from the headlines, but it's far from over. To cope with the loss of reactor cooling systems knocked out by the 11 March earthquake and tsunami, the plant's owner, Tokyo Electric Power Co., has installed jury-rigged cooling setups that have cut radiation emissions dramatically. But some 100,000 residents who were evacuated will not return home until the reactors are firmly under control. Last month, Tokyo Electric unveiled a two-stage plan to build more robust cooling systems and reduce radiation leaks within 3 months, then, 3 to 6 months later, achieve a cold shutdown in which fuel is cooled by water below the boiling point at atmospheric pressure.

Off limits.

Robots are finding radiation too high for humans.

CREDIT: TOKYO ELECTRIC POWER CO.

Nuclear fuel in four of the plant's six reactors overheated, with extensive core damage in three units. Last week, a robotic inspection increased suspicions that the fuel in unit 1 may have melted through the bottom of the pressure vessel and pooled at the base of the containment structure. Hydrogen explosions completely blew the upper walls and roofs off two units and severely damaged a third; vessels and piping are leaking contaminated water. “There are many challenging tasks ahead,” says Tony Irwin, a nuclear technology expert at Australian National University and the University of Sydney. Workers must reduce radiation levels, plug leaks, and decontaminate water—all while the threat of aftershocks persists.

In the early days, Tokyo Electric hoped to restart Fukushima's original cooling systems. But the company was forced to explore alternatives, says Hidehiko Nishiyama, deputy director of Japan's Nuclear and Industrial Safety Agency. The utility is now planning to build heat exchangers that will circulate fresh water through the reactors to cool the fuel. To seal off reactor buildings, engineers are planning to wrap them in polyester sheets stretched over steel frames.

The biggest challenge, Nishiyama says, is protecting workers. Some have been entering the unit 1 building to prepare for construction. But so far, only robots have entered the unit 2 and 3 reactor buildings, where radiation levels top 50 millisieverts per hour. Tokyo Electric may expand the use of robots, which so far have been limited to taking radiation measurements and videos. Because integrated circuits can be affected by radiation, these probes must be primarily mechanical or have hardened electronics. Once new cooling systems and enclosures are in place, work could start on the semipermanent buildings needed for recovering nuclear fuel and decommissioning the reactors, a process that could take a decade or longer.

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