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fallout forensics walk radiation cool

Global data on Fukushima defies Japanese estimates.

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The Fukushima accident prompted mass evacuations from nearby towns such as Minamisoma.
Credit: AP Photo/S. Ponomarev

The disaster at the Fukushima Daiichi nuclear plant in March released far more radiation than the Japanese government claims. This concludes a study1 which combines radioactivity data from around the world to estimate the scale and fate of emissions from the shattered plant.

The study also suggests that, contrary to government claims, the pools used to store spent nuclear fuel played a significant role in releasing the long-lived environmental pollutant caesium-137, which could have been prevented with immediate action. The analysis has been published online for open peer review by the journal. Chemistry and Atmospheric Physics.

Andreas Stohl, an atmospheric scientist at the Norwegian Institute for Air Research in Kjeller, who led the research, believes 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 modeler at the Swedish Defense Research Agency in Stockholm, who was not involved in the study.

The reconstruction is based on data from dozens of radiation monitoring stations in Japan and around the world. Many are part of a global network to monitor nuclear weapons tests run by the Comprehensive Nuclear Test Ban Treaty Organization in Vienna. The scientists aggregated data from independent stations in Canada, Japan and Europe, then combined it with large European and US caches of global weather data.

Stohl cautions that the resulting model is far from perfect. Measurements were scarce in the immediate aftermath of the Fukushima accident, and some monitoring posts were too contaminated by radioactivity to provide reliable data. More importantly, what exactly happened inside the reactors – a crucial part of understanding what they emitted – remains a mystery that may never be solved. “If you look at the estimates for Chernobyl, you still have a lot of uncertainty 25 years later,” says Stohl.

However, the study provides an overview of the accident. “They really took a big picture and used all the data available,” says De Geer.

Japanese researchers had already developed a detailed timeline of events following the March 11 earthquake that precipitated the disaster. Hours after the earthquake rocked all six of Fukushima Daiichi’s reactors, the tsunami struck, destroying crucial backup diesel generators designed to cool the reactors in an emergency. Within days, all three reactors that were operating at the time of the accident overheated and released hydrogen gas, leading to massive explosions. Radioactive fuel recently removed from a fourth reactor was in a storage pool at the time of the earthquake, and on March 14 the pool overheated, possibly starting fires in the building for the next several days.

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But accounting for the radiation that came from the plants has proven much more difficult than reconstructing this chain of events. The latest Japanese government report, released in June, says the plant emitted 1.5×10sixteenbecquerels of caesium-137, an isotope with a half-life of 30 years that is responsible for most of the plant’s long-term contaminationtwo. A much larger amount of xenon-133, 1.1×1019Bq, was released, according to official government estimates.

The new study challenges those numbers. Based on their reconstructions, the team claims that the accident released around 1.7×1019Bq of xenon-133, greater than the estimated total radioactive release of 1.4 × 1019Chernobyl bq. The fact that three reactors exploded in the Fukushima accident explains the huge amount of xenon, says De Geer.

Xenon-133 does not pose serious health risks because it is not absorbed by the body or the environment. Radioactive fallout from caesium-137, however, is of much greater concern because it will remain in the environment for decades. New model shows Fukushima released 3.5×10sixteenBq caesium-137, about double the official government figure and half of the Chernobyl release. The higher number is obviously concerning, De Geer says, although ongoing field studies 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 may be partly explained by the larger data set used. The Japanese estimates are based primarily on data from monitoring posts within Japan.3, which never recorded the large amounts of radioactivity that erupted over the Pacific Ocean and eventually reached North America and Europe. “Taking into account the radiation that has drifted into the Pacific is essential to get a true 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 the ground around Fukushima. .

Click to see the full image

Stohl adds that he sympathizes with the Japanese teams responsible for the official estimate. “They wanted to get something out fast,” he says. The differences between the two studies may seem large, says Yukio Hayakawa, a volcanologist at Gunma University who also modeled the accident, but uncertainties in the models mean the estimates are quite similar.

The new analysis also states that the spent fuel stored in the Unit 4 pool emitted large amounts of caesium-137. Japanese officials have maintained that virtually no radioactivity leaked from the pool. However, Stohl’s model clearly shows that spraying the pool with water caused the plant’s caesium-137 emissions to be markedly reduced (see ‘Radiation crisis’). The finding implies that much of the consequences could have been avoided by flooding the pool earlier.

Japanese authorities continue to maintain that the spent fuel was not a significant source of contamination, because the pool itself did not appear to be significantly damaged. “I think the release of unit 4 is not important,” says Masamichi Chino, a scientist at the Japan Atomic Energy Authority in Ibaraki, who helped develop the official Japanese estimate. But De Geer says the new analysis implying the fuel pool “looks compelling”.

The latest analysis also presents evidence that xenon-133 began to emanate from Fukushima Daiichi immediately after the earthquake and before the tsunami inundated the area. This implies that even without the devastating flood, the earthquake alone was enough to cause damage to the plant.

The Japanese government report has already acknowledged that the tremor at Fukushima Daiichi exceeded the plant’s design specifications. Anti-nuclear activists have long been concerned that the government has failed to adequately address geological hazards when licensing nuclear plants (see Nature 448, 392-393; 2007), and the xenon whiff could prompt a major rethink of reactor safety assessments, Yamauchi says.

The model also shows that the accident could easily have had a much more devastating impact on the people of Tokyo. In the first days after the accident, the wind blew out to sea, but on the afternoon of March 14 it returned to the coast, bringing clouds of radioactive caesium-137 over a large part of the country (see ‘Radioisotope Reconstruction’). Where precipitation fell, along the country’s central mountain ranges and to the northwest of the plant, higher levels of radioactivity were subsequently recorded in the soil; Fortunately, the capital and other densely populated areas experienced dry weather. “There was a period where there was a fairly high concentration in Tokyo, but it didn’t rain,” says Stohl. “It could have been much worse.”

Additional Information

Additional reporting by David Cyranoski and Rina Nozawa.

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Cite this article

Brumfiel, G. Fallout Forensics Increases Radiation Casualty Toll.
Nature 478, 435–436 (2011).

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