A blast ripped through Number One reactor of the plant Saturday, destroying the building, but leaving the nuclear reactor itself undamaged, according to official Japanese reports.
The blast occurred after the massive earthquake of magnitude-9 rattled the country's northeast Friday, killing more than 1,000 people. The authorities Sunday increased the magnitude of the quake to 9 on the the richter scale from earlier measurement of 8.9.
A steel container covering the reactor protected it from the blast, the authorities said.
Following the explosion, however, the authorities expanded the evacuation area around the plant from 10-km radius to 20 km and began distributing iodine, which helps protect the body from radioactive exposure, to the residents of the locality.
The Fukushima prefecture authorities urged the people to close windows, turn off air conditioners and stay at home.
The emergency cooling system has failed at another reactor at the Fukushima plant, raising fears of a possible blast due to overheating, the Kyodo news agency reported Sunday, quoting Japanese Chief Cabinet Secretary Yukio Edano.
The Number Three reactor is in process of releasing radioactive steam, Edano said.
It was the sixth reactor at the Fukushima plant to undergo cooling failure since the earthquake and tsunami hit Japan Friday.