The nuclear safety agency said early Saturday that some radiation has also seeped outside the plant, prompting calls for further evacuations of the area. Some 3,000 people have already been urged to leave their homes.
The cooling system for a reactor at the Fukushima Daiichi power plant failed on Friday after a massive earthquake caused a power outage.
The continued loss of electricity has also delayed the planned release of vapor from inside the reactor to ease pressure. Pressure inside one of the reactors had risen to 1.5 times the level considered normal.