Far more radioactive caesium released into the atmosphere than previously estimated, according to study

The Fukushima nuclear disaster released twice as much radioactive caesium into the atmosphere as Japanese authorities estimated, reaching 40% of the total from Chernobyl, according to a preliminary report.

The levels of caesium-137 were reported by a worldwide network of sensors. Andreas Stohl, author of a study by the Norwegian Institute for Air Research, said the Japanese government's estimate came only from data in Japan and would have missed emissions blown out to sea.

The study did not consider health implications but caesium-137 is dangerous because it can last for decades in the environment, releasing cancer-causing radiation. The long-term effects of the nuclear accident are unclear because of the difficulty in measuring radiation doses that people received.

In a telephone interview Stohl said emission estimates were so imprecise that finding twice the amount of caesium was not considered a major difference. He said some previous estimates had been higher than his.

The journal Atmospheric Chemistry and Physics posted the report online for comment but the study is still under formal review by experts in the field and has not been accepted for publication.

Last summer the Japanese government estimated that the 11 March Fukushima accident released 15,000 terabecquerels of caesium. The terabecquerel is a radiation measurement. The new report from Stohl and co-authors estimates about 36,000 terabecquerels through to 20 April. That is about 42% of the estimated release from Chernobyl, the report says.

An official at Japan's Nuclear and Industrial Safety Agency said it could not offer any comment on the study because it had not reviewed the contents.

The report says about a fifth of the caesium fell on land in Japan, while most of the rest fell into the Pacific Ocean. About 2% came down on land outside Japan, it concludes. Some radiation from the accident has been detected in Tokyo and the United States but experts say they expect no significant health consequences there.

Many parents of small children in Tokyo worry about the discovery of radiation hotspots even though government officials say they don't pose a health risk. The previous prime minister, Naoto Kan, has said the most contaminated areas inside the evacuation zone could be uninhabitable for decades.

Stohl noted that his study found caesium-137 emissions dropped suddenly at the time workers started spraying water on the spent fuel pool from one of the reactors. That challenged previous thinking that the pool had not been emitting caesium, he said.

(AP / The Guardian, 28/10/2011)