In fact, unlike Fukushima, Chernobyl had no containment structure whatsoever to limit the spread of radioactivity.

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They have a primary and a secondary containment structure, both made from thick reinforced concrete, to protect against the release of radioactive materials.

FORBES: Explainer: What Caused The Incident At Fukushima-Daiichi

In a reactor, the containment structure may melt.

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It doesn't have a proper containment structure.

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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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The containment capacity in the new structure, the sealing cap, will draw to two different boats and increase the containment capacity from roughly 15, 000 to between 20, 000 and 30, 000.

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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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