The earthquake that violently shook Japan and crippled the reactor at Fukushima Daiichi
nuclear plant in March caused international concern—and then panic—after operators released radioactive steam to prevent a more catastrophic explosion. Why? Immediately afterword, US TV news and cable networks over-sensationalized the reports, freely speculating worst-case scenarios and causing fear up and down the West coast.
Atmospheric chemists, led by Mark Thiemens
at the University of California, San Diego, recently reported the first measurements of the leaked radiation. The amounts were not dangerous, in fact they were "almost nothing," Thiemens told USA Today
. "It took me three years to figure out the chemistry, to be able to measure things that low," he says.
The estimate, reported August 15 in the online edition of the Proceedings of the National Academy of Sciences
, is based on data collected from particles sent across the Pacific Ocean after operators of the damaged reactor had to resort to cooling overheated fuel with seawater.
“In any disaster, there’s always a lot to be learned by analysis of what happened,” said Thiemens in a press release
. “We were able to say how many neutrons were leaking out of that core when it was exposed.”
Suddenly data pours into sensors in La Jolla, California
On March 28, 2011, 15 days after operators began pumping seawater into the damaged
reactors and pools holding spent fuel, Thiemens’ group observed an unprecedented spike in the amount of radioactive sulfur in the air in La Jolla, California. They recognized that the signal came from the crippled Japanese power plant. Over a four day period, they measured 1501 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.
Measuring and then investigating a long chain of events
The nuclear reaction within the cooling seawater marked sulfur that originated in a specific place for a discrete period of time. That allowed them to time the transformation of sulfur to sulfur dioxide gas and sulfate particles, and measure their transport across the ocean. “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.
Neutrons and other products of the nuclear reaction leaked from the fuel rods when they
melted. Seawater pumped into the reactor absorbed those neutrons, which collided with chloride ions in the saltwater—each collision knocking a proton out of the nucleus of a chloride atom and transforming the atom to a radioactive form of sulfur.
Then when the water hit the hot reactors, it vaporized into steam as it was vented by operators along with the radioactive sulfur into the atmosphere. In air, sulfur reacts with oxygen to form sulfur dioxide gas and then sulfate particles. Both blew across the Pacific Ocean on prevailing westerly winds to an instrument at the end of the pier at UC San Diego’s Scripps Institution
of Oceanography where Thiemens’ group continuously monitors atmospheric sulfur.
Following the data back to Fukushima
Using a model based on NOAA’s observations of atmospheric conditions at the time, the team determined the path air took on its way to the pier over the preceding 10 days and found that it led back to Fukushima.
Then they calculated how much radiation must have been released. “You know how much seawater they used, how far neutrons will penetrate into the seawater and the size of the chloride ion. From that you can calculate how many neutrons must have reacted with chlorine to make radioactive sulfur,” said Antra Priyadarshi, a post-doctoral
researcher in Thiemens’ lab and first author of the paper.
Trace levels no threat
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 March 13, when the seawater pumping operation began, and March 20, 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.
Photo: Fukushima Power Plant-- DigitalGlobe, wikicommons
Photo: Wind graph-- UC researcher Gerardo Dominguez, press release ,UC San Diego
Graph: Particle levels-- Priyadarshi A, Dominguez G, UC San Diego, PNAS