UCSD first to estimate radiation release from Japan
Scripps pier monitoring station tracked flow from Fukushima plant
Atmospheric chemists at UCSD on Monday published what they called the first quantitative measurement of radiation that leaked from the damaged nuclear reactor in Fukushima, Japan, following the earthquake and tsunami in March.
Their estimate is based on traces of radioactive sulfur in air samples collected at Scripps Pier at the University of California San Diego. The substance showed up at levels that were unprecedented the past two years of air monitoring at the site - but far below what air experts said would be a threat to human health.
The modeling shows that the amount of radioactive sulfur in the atmosphere above Fukushima was 365 times higher than normal, according to Monday's paper in the Proceedings of the National Academy of Sciences. Less than 1 percent of the total radioactive sulfate at Fukushima reached San Diego.
Public health officials said repeatedly during and after the meltdown that radiation levels weren't high enough to harm residents in California.
Mark Thiemens, dean physical sciences at UCSD, said the analysis was useful to scientists who are modeling air movements and trying to understand how sulfates contribute to climate change. He and his co-authors took local readings of airborne sulfur and used a series of mathematical models to calculate conditions 5,500 miles away at the one of the world's largest nuclear plants.
They already had been using sulfur to track air movements from Asia to California, but Thiemens said naturally occurring levels are so low that they are hard to measure. Radioactive sulfur leaves a signature that's much easier to track. "We easily and clearly saw the whole event," said Thiemens.
His team could do that because neutrons and other products of the nuclear reaction leak from fuel rods when they melt. Seawater pumped into the crippled Fukushima 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, it vaporized into steam. To prevent explosions of the accumulating hydrogen, operators released 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 east across the Pacific Ocean on prevailing westerly winds to an instrument at the end of the Scripps pier.
"It provided a completely new way to track an air mass and the particles and gas within it," Thiemens said. "There are other ways, but a radioactive tracer, much as is used in medical studies, provides a tracer that is extremely sensitive and makes visible the invisible."
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