The amount of cesium-137 is more than twice the official number, and the amount of xenon-133 55% more, according to a new study using a larger data set, says Nature.
The study claims the amount of cesium-137 from Fukushima is half of that from Chernobyl, and the amount of xenon-133 from Fukushima far exceeds Chernobyl.
As the reason why the Japanese government numbers were low: the Japanese government probably only accounted for the radioactive fallout within Japan. So the large amount that went over the Pacific Ocean, reaching North America and Europe was never considered by the Japanese government.
The new study also claims that the Reactor 4 Spent Fuel Pool emitted a large amount of cesium-137.
From Nature News (10/25/2011):
Fallout forensics hike radiation toll: Global data on Fukushima challenge Japanese estimates. (Geoff Brumfiel)
The disaster at the Fukushima Daiichi nuclear plant in March released far more radiation than the Japanese government has claimed. So concludes a study that combines radioactivity data from across the globe to estimate the scale and fate of emissions from the shattered plant.
The study also suggests that, contrary to government claims, pools used to store spent nuclear fuel played a significant part in the release of the long-lived environmental contaminant caesium-137, which could have been prevented by prompt action. The analysis has been posted online for open peer review by the journal Atmospheric Chemistry and Physics.
Andreas Stohl, an atmospheric scientist with the Norwegian Institute for Air Research in Kjeller, who led the research, believes that the analysis is the most comprehensive effort yet to understand how much radiation was released from Fukushima Daiichi. "It's a very valuable contribution," says Lars-Erik De Geer, an atmospheric modeller with the Swedish Defense Research Agency in Stockholm, who was not involved with the study.
The reconstruction relies on data from dozens of radiation monitoring stations in Japan and around the world. Many are part of a global network to watch for tests of nuclear weapons that is run by the Comprehensive Nuclear-Test-Ban Treaty Organization in Vienna. The scientists added data from independent stations in Canada, Japan and Europe, and then combined those with large European and American caches of global meteorological data.
...The latest report from the Japanese government, published in June, says that the plant released 1.5 × 10^16 bequerels of caesium-137, an isotope with a 30-year half-life that is responsible for most of the long-term contamination from the plant. A far larger amount of xenon-133, 1.1 × 10^19 Bq, was released, according to official government estimates.
The new study challenges those numbers. On the basis of its reconstructions, the team claims that the accident released around 1.7 × 10^19 Bq of xenon-133, greater than the estimated total radioactive release of 1.4 × 10^19 Bq from Chernobyl. The fact that three reactors exploded in the Fukushima accident accounts for the huge xenon tally, says De Geer.
Xenon-133 does not pose serious health risks because it is not absorbed by the body or the environment. Caesium-137 fallout, however, is a much greater concern because it will linger in the environment for decades. The new model shows that Fukushima released 3.5 × 10^16 Bq caesium-137, roughly twice the official government figure, and half the release from Chernobyl. The higher number is obviously worrying, says De Geer, although ongoing ground surveys are the only way to truly establish the public-health risk.
Stohl believes that the discrepancy between the team's results and those of the Japanese government can be partly explained by the larger data set used. Japanese estimates rely primarily on data from monitoring posts inside Japan, which never recorded the large quantities of radioactivity that blew out over the Pacific Ocean, and eventually reached North America and Europe. "Taking account of the radiation that has drifted out to the Pacific is essential for getting a real picture of the size and character of the accident," says Tomoya Yamauchi, a radiation physicist at Kobe University who has been measuring radioisotope contamination in soil around Fukushima.
Stohl adds that he is sympathetic to the Japanese teams responsible for the official estimate. "They wanted to get something out quickly," he says. The differences between the two studies may seem large, notes Yukio Hayakawa, a volcanologist at Gunma University who has also modelled the accident, but uncertainties in the models mean that the estimates are actually quite similar.
The new analysis also claims that the spent fuel being stored in the unit 4 pool emitted copious quantities of caesium-137. Japanese officials have maintained that virtually no radioactivity leaked from the pool. Yet Stohl's model clearly shows that dousing the pool with water caused the plant's caesium-137 emissions to drop markedly (see 'Radiation crisis'). The finding implies that much of the fallout could have been prevented by flooding the pool earlier.
(The article continues.)