Japan earthquake: Nuclear blast radiation explained
14 March 2011 Last updated at 12:06 GMT
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.