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Japan earthquake: Nuclear blast radiation explained

A second explosion has hit a Japanese nuclear plant that was damaged in Friday's earthquake, but officials said the reactor core was still intact.

A huge column of smoke billowed from Fukushima Daiichi's reactor 3, two days after a blast hit reactor 1.

Environment Correspondent, Richard Black, explains what is likely to be happening inside the reactors.

Go to next video: Second blast at Japan's nuclear plant