Edano, citing information from the plant operator TEPCO, said the reactor container was likely undamaged and there was a low possibility of major radiation.
Japan`s nuclear safety agency said the blast, at the number 3 reactor of the Fukushima No.1 plant, was believed to be caused by hydrogen.
A hydrogen explosion had hit the number 1 reactor at the same plant on Saturday, a day after an earthquake and tsunami devastated the northeast coast.
Authorities have declared an evacuation zone within a 20km radius of the plant and evacuated 210,000 people.
"We have strongly advised all the people still within the evacuation area to go inside nearby facilities," said nuclear safety agency spokesman Ryo Miyake.
Some 746 people - patients, elderly people and care workers at three hospitals and nursing homes - remained within the 20km area on Monday.
A Fukushima official said 311 had so far been moved out.(*)
A050/H-AK
Editor: Jafar M Sidik
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