The amount of the long-lived radioactive isotope cesium-137, and the shorter-lived iodine-131, which have already escaped from the troubled Fukushima power plant in Japan may be significant, approaching emissions of these isotopes from the Chernobyl accident in 1986. The estimates come from Austria’s weather service, the Central Institute for Meteorology and Geodynamics in Vienna.
Fukushima is no Chernobyl, however, cautions Gerhard Wotawa, a researcher at the centre involved in the study. Crucially, he points out, only a few highly volatile elements, such as iodine, cesium and xenon, have escaped from the Fukushima plant, whereas at Chernobyl the greater damage to the core caused substantial quantities of many other less-volatile isotopes to be released, leading to far higher total radiation levels. It was also “good luck” that prevailing wind patterns pushed much of the radioactive cesium out over the Pacific Ocean away from land, he notes. Even on the days when wind blew material inland, rain close to the plant quickly washed out radioactive material. Little of the plume reached Tokyo, he says, noting also that reported radioactive dose levels in the affected area beyond the immediate area of the plant are low, and falling.
The model’s results were calculated by inputing measurements of radioisotope levels at different distances from the plant and then modelling weather patterns and other factors to estimate the amounts released at the source. This model run used measurements of radioisotopes in detector stations in Takasaki in Japan and in Sacramento, California, part of a sophisticated global network of stations created to monitor for nuclear weapons tests – the Comprehensive Nuclear-Test-Ban Treaty Organization (see my long article last week about the CTBTO network’s role in responding to the accident).
Animation of current I-131 plume.
Source: Austria’s Central Institute for Meteorology and Geodynamics
The researchers estimate that 3×10^15 becquerels of Cs-137 (which has a half-life of 30 years) were released during the first two days following the disaster on 11 March. A further 3×10^16 was released over the next two days, totalling 50% of the Cs-137 emitted in the Chernobyl accident. 4×10^17 becquerels of I-131, with a half-life of 8 days, were likely released over the same period, roughly 20% of I-131 released from Chernobyl. Wotawa says he was “surprised at the cesium levels, but less so with respect to iodine, as its highly volatile.”
Jim Smith, an environmental physicist at the University of Portsmouth, UK, who was not involved in the study, says that the figures are high, and should they prove to be correct “could be of very significant concern”. He adds that the modelling was only for the 137 isotope of cesium, but that he’d expect similar amounts of cesium-134 (which has a half-life of two years) to have been released.
Smith agrees with Wotawa that the fact that the prevailing winds on the first two days blowing the plume away from land was “a best case scenario”. It could cause significant marine contamination, but the ocean’s enormous diluting capacity would reduce any doses, he says, adding that nonetheless fishing bans might be needed near to the Fukushima plant.
Smith says a major caveat to the estimates is that such modelling is “pretty difficult”. He says that he hasn’t seen enough details to assess how accurate the work is, but that he himself has been surprised by the high levels of iodine that have been detected in spinach sampled at significant distances from the site. “This suggests that there was a fairly major release of iodine,” he says. “This is exactly the sort of modelling we should be doing,” he adds. “I’m a bit surprised that we haven’t seen similar modelling results from the Japanese; they ought to have an accident response model in place.”
To confirm whether the Austrian centre’s estimates are accurate Smith says he would like to see the Japanese authorities publishing more radiation measurements, and in particular maps of cesium deposition in the region. “I would have expected that by now they would flying gamma ray spectrometry over the area,” he says, adding that a problem is that there is not sufficient data coming out of Japan, in particular on the immediate 20-30 km zone around the plant.
For full coverage of the Fukushima disaster, go to Nature’s news special.