Reuters says that:
"A nuclear industry body official said on Saturday he believed a blast at a Japanese atomic power plant was due to hydrogen igniting, adding it may not necessarily have caused radiation leakage. 'It is obviously an hydrogen explosion ... due to hydrogen igniting,' Ian Hore-Lacy, communications director at the World Nuclear Association, a London-based industry body, told Reuters after reports of the explosion in Japan."
And the AP adds that: " 'meltdown' is not a technical term. Rather, it is an informal way of referring to a very serious collapse of a power plant's systems and its ability to manage temperatures. It is not immediately clear if a meltdown would cause serious radiation risk, and if it did how far the risk would extend. Yaroslov Shtrombakh, a Russian nuclear expert, said a Chernobyl-style meltdown was unlikely. 'It's not a fast reaction like at Chernobyl,' he said. 'I think that everything will be contained within the grounds, and there will be no big catastrophe.' "