First quantitative measurements of Fukushima leakage
reacted with chlorine to make radioactive sulfur,” said Antra Priyadarshi, a post-doctoral researcher in Thiemens’ lab and first author of the paper. Gerardo Dominguez, another member of Mark Thiemens’s research group, is also an author of the report.
After accounting for losses along the way as the sulfate particles fell into the ocean, decayed, or eddied away from the stream of air heading toward California, the researchers calculated that 400 billion neutrons were released per square meter surface of the cooling pools, between 13 March, when the seawater pumping operation began, and 20 March 2011.
The trace levels of radiation that reached the California coast never posed a threat to human health. “Although the spike that we measured was very high compared to background levels of radioactive sulfur, the absolute amount of radiation that reached California was small. The levels we recorded aren’t a concern for human health. In fact, it took sensitive instruments, measuring radioactive decay for hours after lengthy collection of the particles, to precisely measure the amount of radiation,” Thiemens said.
Concentrations a kilometer or so above the ocean near Fukushima must have been about 365 times higher than natural levels to account for the levels they observed in California.
The radioactive sulfur that Thiemens and his team observed must have been produced by partially melted nuclear fuel in the reactors or storage ponds. Although cosmic rays can produce radioactive sulfur in the upper atmosphere, that rarely mixes down into the layer of air just above the ocean, where these measurements were made.
Over a four day period ending on 28 March, they measured 1,501 atoms of radioactive sulfur in sulfate particles per cubic meter of air, the highest they’ve ever seen in more than two years of recordings at the site.
Even intrusions from the stratosphere – rare events that bring naturally produced radioactive sulfur toward the Earth’s surface – have produced spikes of only 950 atoms per cubic meter of air at this site.
The nuclear reaction within the cooling seawater marked sulfur that originated in a specific place for a discrete period of time. That allowed researchers to time the transformation of sulfur to sulfur dioxide gas and sulfate particles, and measure their transport across the ocean, both important factors for understanding how sulfate pollutants contribute to climate change.
“We’ve really used the injection of a radioactive element to an environment to be a tracer of a very important process in nature for which there are some big gaps in understanding,” Thiemens said.
The event also created a pulse of labeled sulfur that can be traced in the streams and soils in Japan, to better understand how this element cycles through the environment, work that Thiemens and colleagues in Japan have already begun.
