DisastersFirst quantitative measurements of Fukushima leakage
Atmospheric chemists report the first quantitative measurement of the amount of radiation leaked from the damaged nuclear reactor in Fukushima; 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
Atmospheric chemists at the University of California, San Diego, report 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 15 August in the early, online edition of the Proceedings of the National Academy of Sciences (PNAS), 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 UC San Diego. “We were able to say how many neutrons were leaking out of that core when it was exposed.”
A University of California – San Diego release reports that on 28 March 2011, fifteen 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 power plant.
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 UC San Diego’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 ten 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
