Mars 2011: Kärnkraftverket i Fukushima - utbruten och sammanfogad tråd

The calculations published by Austria's Central Institute for Meteorology and Geodynamics may add to growing concern in Japan and elsewhere over the contamination of food products such as milk and vegetables in areas near the Japanese reactor site.

The Austrian institute's Dr Gerhard Wotawa stressed the two isotopes from Fukushima he had sought to estimate -- iodine-131 and caesium-137 -- normally make up only one tenth of total radiation.

Based on measurements made at monitoring stations in Japan and the United States, Wotawa said the iodine released from Fukushima in the first three-four days was about 20 percent of that released from Chernobyl during a ten-day period.

For Caesium-137, the figure could amount to some 50 percent.

The International Atomic Energy Agency said on Wednesday Japanese authorities had told the Vienna-based agency that radiation dose rates at the plant were decreasing, although the overall situation remained serious.

One U.N. study has estimated Chernobyl, in Ukraine, may over time cause 4,000 to 9,000 extra deaths from cancer.

And there are big differences in the handling of the crises.

"At Chernobyl, the population was not generally aware that the accident had happened," said Malcolm Crick, Secretary of the U.N. Scientific Committee on the Effects of Atomic Radiation.

"People in the nearby town of Pripyat were watching the fire from just a kilometre or so away. They were evacuated a day or so later," he said, adding that children kept drinking milk despite risks of contamination.

"In Japan, there was a precautionary evacuation early on," he said, adding "it's too early to make a real assessment of the overall impact."

Japanese authorities also distributed units of stable iodine which can help protect against radioactive iodine.