Some of the wreckage at the Fukushima Daiichi plantFLICKR, M1K3Y

Scientists have constructed local and nation-wide maps of the radioactive particles that rained down when the Fukushima Daiichi nuclear power plant failed after a massive earthquake and tsunami rocked Japan last March. The fallout maps, published today (November 14) in Proceedings of the National Academy of Sciences, will help direct decontamination efforts to the worst affected areas.

The temblor and tsunami that hit Japan on March 11 this year damaged the Fukushima plant, releasing a large volume of radioactive particles (radionuclides). In the immediate aftermath, estimates of the contamination, and thus the impact on humans, wildlife, and agriculture, were highly uncertain. (Read The Scientist's coverage of the disaster.)

To obtain a more accurate picture of the contamination situation, Nori Kinoshita of the University of Tsukuba and colleagues measured levels of radioactive iodine, tellurium, and cesium...

They found the highest levels of all three radionuclides in samples taken from the Iitate and Naki-Dori regions, approximately 50 to 70 kilometers northeast of Fukushima. Heavy rain fell here on March 15, and this, together with the local geography—a basin valley surrounded by hills—appears to have contributed to the soils from these areas containing the worst of the fallout.

Distribution patterns further afield varied depending on the radionuclide. Levels of tellurium were low in most of the study area, iodine was homogenously distributed, and cesium was at low levels in most places but high in the regions of southern Ibaraki and Chiba, where more rain fell on March 21.

"Iodine-131 migrates as a gas, cesium and tellurium isotopes migrate as dust," wrote Kinoshita in an email. "I guess that physical form affects the distribution."

Nick Beresford, a radioecologist at the Centre for Ecology & Hydrology in the United Kingdom, agreed. "The importance of wet versus dry deposition may vary," said Beresford, who was not involved on the study. "In the UK, following the Chernobyl accident, cesium deposition was more related to rainfall than iodine deposition."

The radionuclides' boiling points also differ, Kinoshita explained, so as emergency workers brought the power plant's disabled cooling system under control, the proportions of the different nuclides released may have varied.

Although the distribution of all three radionuclides are of interest for determining the overall environmental impact of the fallout, only cesium, with a half-life of 30 years, poses a continuing health threat. Iodine-131 and Tellurium-129m have half-lives of 8 and 33 days, respectively. For this reason, Teppei Yasunari of the Universities Space Research Association in Maryland and colleagues focused a separate project on mapping cesium distribution across all of Japan.

Yasunari's group used a model in which radioactivity emission values were assumed and distribution was determined by weather patterns together with reference soil sample data.

The model showed that western Japan largely escaped the fallout thanks to the sheltering effect of mountain ranges and easterly winds that blew the radionuclides out over the Pacific Ocean. A large area of eastern Japan received at least some of the fallout, however.

For the majority of the east, cesium estimates were low, but in the immediate vicinity of Fukushima, levels were calculated to be between 10,000 and 100,000 megabecquerels (MBq) per square kilometer—figures that correspond with the soil samples taken by Kinoshita's team.

Kinoshita's group also calculated the annual radiation dose likely to be received by a person living in the areas around Fukushima. In Naki-Dori the annual dose would be 10 millisieverts (mSv)—about the amount a person would get from a CT scan, explained Kinoshita. In Iitate, it would be 40 mSv.

"No incidence of cancer has been observed in annual doses of less than 100 mSv," said Kinoshita, "However, I wouldn't recommend living in more than 40 mSv.... People who live in Fukushima prefecture should be tested in the future."

"Concentrations are such that, in the area close to the reactors, studies on potential effects on wildlife should be initiated," said Beresford. On the other hand, Ward Whicker of Colorado State University, said that the radiation level "does not appear to exceed levels in areas like Colorado where the cosmic radiation and natural uranium and thorium are unusually high." He added, "I don't think these numbers should be too alarming for the health of people, animals, or ecology. But researchers should do a careful, scientifically credible analysis of the data to be sure."

Given the remaining uncertainty, Yasunari said, "we strongly suggest the Japanese government immediately carries out soil samplings in all prefectures to validate our results and assess actual soil contamination....and to make plans for decontamination."

N. Kinoshita et al., "Assessment of individual radionuclide distributions from the Fukushima nuclear accident covering central-east Japan," PNAS, doi/10.1073/pnas.1111724108, 2011.

T.J. Yasunari et al., "Cesium-137 deposition and contamination of Japanese soils due to the Fukushima nuclear accident," PNAS, doi/10.1073/pnas.1112058108, 2011.

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