In a reactor, the containment structure may melt.

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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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This rectangular secondary containment building is the structure that most people have seen in pictures of the reactor.

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