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 […]