It said working on or near damaged asbestos-containing materials, or breathing in high levels of asbestos fibres, could increase the chance of contracting an asbestos-related disease.

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What is left is a damaged core with heavy radioactive materials that cannot go airborne unless there is some additional motive force.

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Certain OLED materials also are damaged by prolonged exposure to sunlight.

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Tuesday's explosion at the No. 2 reactor in the Fukushima complex, 150 miles north of Tokyo, for the first time raised the possibility that the key containment structure of the unit, which protects the reactor vessel and keeps dangerous radioactive materials from leaking out, had been damaged.

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Besides clean-up of materials, the company plans to remove unused utilities in the shaft, repair any damaged structures, install a water ring to control entry of water into the shaft and prevent ice from forming in winter, install a new power cable and a metal brattice between the eastern and western half of the shafts to provide a physical barrier, improving safety and functionality, company officials said.

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