Fears of nuclear meltdown after plant blast

An explosion sent white smoke rising above a nuclear plant where a massive earthquake and tsunami crippled cooling systems in northeastern Japan, the country's chief Cabinet secretary said Saturday.

Four workers were injured after the blast at the Fukushima Daiichi plant, Chief Cabinet Secretary Yukio Edano told reporters. It was not immediately clear where the blast occurred inside the plant, or what caused it.

The roof of a reactor at the plant collapsed following the explosion around 3:30 p.m., Japan's Kyodo News Agency reported, citing Tokyo Electric Power Company.

One expert said the explosion was "clearly a serious situation," but may not be related to problems inside the plant's nuclear reactor.

Other effects of the tsunami may have caused the blast, said Malcolm Grimston, associate fellow for energy, environment and development at London's Chatham House.

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