Emergency workers struggling to control Japan's damaged nuclear plant fled from one of the troubled reactors after a huge radioactive measurement sparked alarm.
The complex's second reactor showed radiation levels had reached 10 million times higher than normal in the cooling system.
Officials later said that figure was inaccurate, but the level had still been very high.
Concerns have been raised about a possible meltdown at the Fukushima Dai-ichi plant since the country was rocked by a 9.0-magnitude earthquake and tsunami on March 11.
The death toll now stands at over 10,000, while more than 16,500 are still missing. Hundreds of thousands of people have been left homeless by the twin disaster.
A spokesman for the Tokyo Electric Power Company said the radiation figure of 10 million was "not credible".
The worker taking the measurements withdrew before making a second reading because of the initial findings.
"We are very sorry," the spokesman added.
Malcolm Grimston, associate fellow at Chatham House's energy, environment and development programme, believed anomalous readings of this type were expected because of the effects of the tsunami.
He added that the readings did not themselves "represent a significant environmental issue" because the radiation was iodine, which he said had a short half-life.