A team from the University of California, San Diego, has carried out the first quantitative measurement of the amount of radiation leaked from the damaged nuclear reactor, following the devastating earthquake and tsunami earlier this year.
On March 28, 15 days after operators began pumping seawater into the damaged reactors and pools holding spent fuel, Mark Thiemens and his team observed an unprecedented spike in the amount of radioactive sulfur in the air in La Jolla, California. They established that the signal came from the crippled power plant.
The radioactive sulfur was formed after neutrons and other particles leaked from melting fuel rods. Seawater pumped into the reactor absorbed the 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, which was then vented to prevent explosions. The sulfur reacted with oxygen to form sulfur dioxide gas and then sulfate particles, which blew across the Pacific Ocean.
Using a model based on NOAA's observations of atmospheric conditions, 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," says post-doctoral researcher Antra Priyadarshi.
Over a four day period ending on March 28th, the team 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.
However, the trace levels of radiation that reached the California coast never posed a threat to human health, says the team.
"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," says Thiemens.
"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."