IWAKI, Japan – An explosion at a nuclear power plant on Japan's devastated coast destroyed a building Saturday
and added leaking radiation, or even outright meltdown, to the threats menacing a nation just beginning to grasp the scale of a catastrophic earthquake and tsunami.
The Japanese government said radiation emanating from the plant appeared to have decreased after the blast, which produced an intensifying cloud of white smoke that swallowed the complex. But authorities did not say why, and the precise cause of the explosion and the extent of the ongoing danger were not clear.
...
The explosion at the nuclear plant, Fukushima Dai-ichi, 170 miles northeast of Tokyo, appeared to be a consequence of steps taken to prevent a meltdown after the quake and tsunami knocked out power to the plant, crippling the system used to cool fuel rods there.
The blast destroyed the building housing the reactor, but not the reactor itself, which is enveloped by stainless steel 6 inches thick.
Inside that superheated steel vessel, water being poured over the fuel rods to cool them formed hydrogen.
When officials released some of the hydrogen gas to relieve pressure inside the reactor, the hydrogen apparently reacted with oxygen, either in the air or the cooling water, and caused the explosion.
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Officials have said that radiation levels at Fukushima were elevated before the blast:
At one point, the plant was releasing each hour the amount of radiation a person normally absorbs from the environment each year.
izvor
Cunami Opustošio Obalu Japana
(hint 2004-a je bila pre 7 godina 
)