UCSD Press Release
Atmospheric chemists at UCSD have reported the first quantitative measurement of the amount of radiation leaked from the damaged nuclear reactor in Fukushima, Japan, following the devastating earthquake and tsunami earlier this year.
Their estimate, reported August 15 in the early, online edition of the
Proceedings of the National Academy of Sciences
, is based on a signal sent across the Pacific Ocean when 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 senior author Mark Thiemens, dean of the Division of Physical Sciences at UCSD. “We were able to say how many neutrons were leaking out of that core when it was exposed.”
On March 28, 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. They recognized that the signal came from the crippled power plant.
Over a four-day period ending on March 28, 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.
Neutrons and other products of the nuclear reaction leak from fuel rods when they melt. Seawater pumped into the reactor absorbed those neutrons, which collided with chloride ions in the saltwater. Each collision knocked a proton out of the nucleus of a chloride atom, transforming the atom to a radioactive form of sulfur.
When the water hit the hot reactors, nearly all of it vaporized into steam. To prevent explosions of the accumulating hydrogen, operators vented the steam, 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 UCSD’s Scripps Institution of Oceanography where Thiemens’ group continuously monitors atmospheric sulfur.
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. Gerardo Dominguez, another member of Mark Thiemens’ research group, is also an author of the report.