When the disaster knocked out off- and on-site power supplies on March 11, 2011, three of the cooling systems for the plant’s four reactor units were disabled. But the process is still expected to be a long, expensive slog, requiring as-yet untried feats of engineering-and not all the details have yet been worked out. By ascertaining the ratio of association of the beryllium to the iodine, tracing the beryllium-7 as it moves through the environment then allowed the researchers to track the parallel transport of iodine, and to demonstrate the accumulation of iodine fallout in stream sediments.Seven years after one of the largest earthquakes on record unleashed a massive tsunami and triggered a meltdown at Japan’s Fukushima Daiichi nuclear power plant, officials say they are at last getting a handle on the mammoth task of cleaning the site before it is ultimately dismantled. The Dartmouth researchers have shown that beryllium-7 follows the same transport paths as the iodine isotopes. It’s an easily detected natural radionuclide, and is routinely used by the Dartmouth researchers in their environmental analyses. What became an important off-shoot of their work was the methodology of using the benign radioisotope, beryllium-7, as the tracking indicator. Thus, the group’s research turned toward the development of an innovative alternative approach to measuring and tracking the iodine. However, he explains, “Once the iodine-131 decays, you lose your ability to track the migration of either isotope.” The iodine-131 is going to decay away pretty quickly over the course of weeks, but the iodine-129 is there forever, essentially,” Landis says. “If you have a recent event like Fukushima, you are going to have both present. The two substances travel together, so the presence of the easily detectable isotope also signals the presence of the longer-lived one. The production rate of these two isotopes in a nuclear reactor occurs at a fixed ratio of three parts iodine-131 to one part iodine-129. “Due to its long half-life and continued release from ongoing nuclear energy production, is perpetually accumulating in the environment and poses a growing radiological risk,” the authors point out. It is not as radioactive, which makes it much harder to measure, but it is much longer lasting and, as it concentrates in certain areas over time, it may become more hazardous. This is not the case with another isotope, iodine-129, released concurrently with iodine-131. Its high radioactivity, however, makes it very detectable by the gamma-ray spectroscopy instruments used by the Dartmouth team in its analyses. “It releases a lot of radioactivity, which makes it dangerous, but it’s gone very quickly so there is no long term exposure risk,” he says.
It does have a relatively short half-life, which is both a blessing and a curse, Landis notes. The radioisotope iodine-131, a significant constituent of the fallout, is a by-product of nuclear fission, highly radioactive, acutely toxic, and presents a health risk upon its release to the environment. But even in these concentrations, stream and river transport are expected to result in significant dilution. However, a sampling of Mink Brook stream sediments showed a doubling of iodine concentrations relative to what was found in soils.
Landis comments that “at these levels, it is unlikely that this is going to cause measurable health consequences.” The paper reports that testing in New Hampshire’s Mink Brook watershed during March through May 2011 showed the amount of radioactive iodine deposition in the soil was minimal, with calculations revealing the total amount to be on the order of 6,000 atoms per square centimeter. “We took up this study mostly as concerned scientists in our own right, wondering how much of this contaminant is really coming down, and where the iodine is moving in the landscape.” “Though regrettable, the Japanese catastrophe did provide a unique opportunity to examine the transport and accumulation of radioactive iodine in the environment,” Landis says. Landis and a team drawn from Dartmouth’s Department of Earth Sciences and Department of Geography recently published a paper in the Proceedings of the National Academy of Sciences addressing such concerns. The instrument was used by the Dartmouth research team to analyze radioactive iodine found in the local New Hampshire environment as a direct consequence of the failure of Japan’s Fukushima Daiichi nuclear power facility in 2011. Dartmouth Research Associate Joshua Landis prepares to introduce a radioactive sample into a gamma-ray spectrometer.
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