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

This article is more than 8 years old

This article is more than 8 years old

The Fukushima Daiichi nuclear power plant may have released twice as much radiation into the atmosphere as previously estimated, according to a study that contradicts official explanations of the accident.

In a report published online by the journal Atmospheric Chemistry and Physics, experts from Europe and the US estimated that the quantity of the radioactive isotope caesium-137 released at the height of the crisis was equivalent to 42% of that from Chernobyl.

Significantly, the report says the plant, 150 miles north of Tokyo, may have started releasing radiation between being hit by a magnitude-9 earthquake on 11 March and the arrival of a tsunami about 45 minutes later.

"This early onset of emissions is interesting and may indicate some structural damage to the reactor units during the earthquake," the report said.

The operator of the plant, Tokyo Electric Power, and the Japanese government maintain that the facility withstood the quake but was damaged by waves that breached its protective seawall.

The tsunami swamped Fukushima Daiichi's backup electricity supply, causing fuel in three reactors to go into meltdown and sparking Japan's worst nuclear accident.

Andreas Stohl, of the Norwegian Institute for Air Research, said measurements taken from a global network of sensors showed that the plant had released 36,000 terabecquerels of caesium-137 between 11 March and 20 April.

That is more than twice the 15,000 terabecquerels reported in the summer by the Japanese government. (A becquerel is a widely used unit of measurement of radioactivity.)

In a telephone interview with Associated Press, Stohl, who led the study, attributed the discrepancy to the failure of Japanese authorities to include emissions that were blown out to sea.

The report, which has yet to undergo a formal peer review, does not mention the emissions' possible health implications. Caesium-137 has a half-life of 30 years, distributes uniformly throughout the body, and can cause leukaemia and other forms of cancer.

Experts say it will be difficult to gauge the potential impact on public health because it is unclear how much radiation people were exposed to at the time.

The study casts doubt on other official accounts of the accident. It found that caesium-137 emissions dropped dramatically after emergency workers started dousing a spent fuel pool at the No 4 reactor, contradicting earlier claims that the pool had not emitted the radioactive isotope.

Official reassurances that radiation levels outside the 20km (12 miles) exclusion zone around the plant pose no risks to public health have been greeted with scepticism by some residents.

Several civic groups are calling for the assisted evacuation of pregnant women and young children from places where radiation hotspots have been identified, including Fukushima city, 60km (37 miles) from the power plant.

An estimated 100,000 people have been evacuated from in and around the exclusion zone. In August, the government conceded that towns closest to the plant could remain uninhabitable for decades.

Japan's nuclear safety agency declined to comment on the new report, saying that it had yet to review its findings.

The report said that about 20% of the caesium fell on land in Japan and about 2% on land outside the country. The remainder came down in the Pacific Ocean, it added.