UC San Diego researchers found radioactive sulphur (sulphur-35) in the atmosphere that probably originated from Fukushima I Nuclear Power Plant in the attempt to cool the reactors by injecting seawater. Chlorine in the seawater reacted with neutrons from the reactors to produce sulphur-35, which was then transported by a strong westerly wind to southern California, the researchers say.
Their findings were published in the electronic version of the Proceedings of the National Academy of Sciences of the United States of America on August 16.
Here's the abstract from the PNAS (emphasis is mine, in blue):
The amount is so tiny there's nothing to worry about, says one of the researchers at UCSD:
A recent earthquake and the subsequent tsunami have extensively damaged the Fukushima nuclear power plant, releasing harmful radiation into the environment. Despite the obvious implication for human health and the surrounding ecology, there are no quantitative estimates of the neutron flux leakage during the weeks following the earthquake. Here, using measurements of radioactive 35S contained in sulfate aerosols and SO2 gas at a coastal site in La Jolla, California, we show that nearly 4 × 1011 neutrons per m2 leaked at the Fukushima nuclear power plant before March 20, 2011. A significantly higher activity as measured on March 28 is in accord with neutrons escaping the reactor core and being absorbed by the coolant seawater 35Cl to produce 35S by a (n, p) reaction. Once produced, 35S oxidizes to and and was then transported to Southern California due to the presence of strong prevailing westerly winds at this time. Based on a moving box model, we show that the observed activity enhancement in is compatible with long-range transport of the radiation plume from Fukushima. Our model predicts that , the concentration in the marine boundary layer at Fukushima, was approximately 2 × 105 atoms per m3, which is approximately 365 times above expected natural concentrations. These measurements and model calculations imply that approximately 0.7% of the total radioactive sulfate present at the marine boundary layer at Fukushima reached Southern California as a result of the trans-Pacific transport.
"The levels we observed are in no way harmful in California," Thiemens said.