The radiation level near the Fukushima Number One nuclear power station rose to 1,015 mircrosieverts per hour on Saturday, the prefecture said.
Earlier in the day a Japanese nuclear safety panel said radiation levels were 1,000 times higher than normal in a control room and eight times higher than normal just outside the plant.
Japanese authorities are preparing to hand out iodine, which helps protect the body from radioactive exposure, to residents in the area near the nuclear power plants hit by a massive earthquake.
The U.S. and France also said they had plans to distribute doses of stable potassium iodine.
Media reports said earlier on Saturday the cooling system had failed at the Fukushima nuclear plant in northeastern Japan. Pressure was eased and steam was released from the nuclear reactor to prevent any meltdown.
The Fukushima prefectural government has expanded the evacuation area around Fukushima Number 1 Power Station from an earlier established 10-kilometer radius to a 20-kilometer radius.
The prefectural government is working to determine which towns and villages fall under the new evacuation order, NHK Television said.
Naoto Sekimura, a professor at the University of Tokyo, told the Associated Press a major radioactive disaster was unlikely.
"No Chernobyl is possible at a light water reactor. Loss of coolant means a temperature rise, but it also will stop the reaction," he said. "Even in the worst-case scenario, that would mean some radioactive leakage and equipment damage, but not an explosion."
The Fukushima prefecture authorities urged the people in the area to close windows, turn off air conditioners and stay at home.
Northern Japan was hit by a massive 8.9-magnitude earthquake and a ten-meter tsunami wave on Friday. Authorities said the death toll could exceed 1,000 with 784 people still missing. Over 1,130 people were injured.
Rescuers have recovered 400-500 bodies in the Miyagi prefecture.