A study by the Norwegian Institute for Air Research found twice as much radioactive substance than Japanese authorities estimated.
The report states that the Japanese estimate came only from data in Japan and missed emissions blown out to sea.
The journal Atmospheric Chemistry and Physics posted the report online, but the study has not yet completed a formal review by experts in the field or been accepted for publication.
The health implications of the radiation are still unclear. 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.
The Japanese government said it cannot comment on the study until it has a chance to review it.
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 estimates about 36,000 terabecquerels through April 20. That's about 42 percent of the estimated release from Chernobyl, the report says.
The Associated Press contributed to this story