This spring's nuclear disaster at the Fukushima Daiichi power plant released almost double the amount of radiation the Japanese government has claimed, according to a new analysis. The authors say the boiling pools holding spent fuel rods played a role in the release of some of the contaminants, primarily cesium-137 - and that this could have been mitigated by an earlier response.
Researchers at the Norwegian Institute of Air Research examined radiation monitoring stations throughout Japan and the rest of the globe, extrapolating their findings from initial radiation-release estimates. They say the amount of cesium-137, a long-lived isotope that persists in the atmosphere, was about twice as high as the Japanese government's official estimate. That number (3.5 × 1016 bequerel, for those of you keeping track) is about half the emission from Chernobyl.
The researchers also say about 20 percent of the total fallout landed over Japan, but the vast majority fell over the Pacific Ocean. (The effects of this fallout on fisheries and aquatic wildlife are still being determined.)
The Fukushima Daiichi power plant, you'll recall, shut down after the devastating earthquake and tsunami that wracked Japan March 11. The tsunami knocked out backup power needed to keep the plant's six reactors cool, and the active reactors overheated, venting hydrogen gas in a series of explosions. These explosions started fires and also exposed a pool containing spent fuel rods from reactor No. 4. The rods started heating up, releasing a radioactive isotope called cesium-137, among other radionuclides.
Cesium-137 emissions peaked three or four days after the quake and tsunami, remaining high until March 19, according to this new study. That's the day authorities started spraying water on the spent-fuel pool at reactor unit 4, the researchers note. "This indicates that emissions were not only coming from the damaged reactor cores, but also from the spent-fuel pool of unit 4 and confirms that the spraying was an effective countermeasure," they say. This contradicts Japanese government reports claiming the pools released no radiation, as Nature News points out.
Nature News says the disparity between the Japanese government's totals and this new study, led by atmospheric scientist Andreas Stohl, can be explained at least in part by the data set. Japanese researchers used monitoring stations in their country, whereas Stohl used monitoring stations throughout the world, which captured much of the radiation that blew over the Pacific and toward North America.
The paper was just posted online for peer review in the open-source journal Atmospheric Chemistry and Physics.