The estimate of much higher levels of radioactive cesium-137 comes from a worldwide network of sensors. Study author Andreas Stohl of the Norwegian Institute for Air Research says the Japanese government estimate came only from data in Japan, and that would have missed emissions blown out to sea.
The study did not consider health implications of the radiation. Cesium-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 of measuring radiation amounts people received.
In a telephone interview, Stohl said emission estimates are so imprecise that finding twice the amount of cesium isn't 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 has not yet completed a formal review by experts in the field or been accepted for publication.
Last summer, the Japanese government estimated that the March 11 Fukushima accident released 15,000 terabecquerels of cesium. Terabecquerels are a radiation measurement. The new report from Stohl and co-authors estimates about 36,000 terabecquerels through April 20. That's about 42 percent of the estimated release from Chernobyl, the report says.
An official at the Nuclear and Industrial Safety Agency, the Japanese government branch overseeing such findings, said the agency could not offer any comment on the study because it had not reviewed its contents.
It also says about a fifth of the cesium fell on land in Japan, while most of the rest fell into the Pacific Ocean. Only about 2 percent of the fallout came down on land outside Japan, the report concluded.
Experts have no firm projections about how many cancers could result because they're still trying to find out what doses people received. Some radiation from the accident has also been detected in Tokyo and in the United States, but experts say they expect no significant health consequences there.
Still, concern about radiation is strong in Japan. 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. And former prime minister Naoto Kan has said the most contaminated areas inside the evacuation zone could be uninhabitable for decades.
Stohl also noted that his study found cesium-137 emissions dropped suddenly at the time workers started spraying water on the spent fuel pool from one of the reactors. That challenges previous thinking that the pool wasn't emitting cesium, he said.