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Model Estimates Neutron Leak From Japanese Nuclear Plant

Last Updated: August 16, 2011.

A model based on measurements of radioactive 35S in sulfate aerosols and SO2 gas estimates that 35SO42− concentrations at the Fukushima marine boundary are around 365 times higher than normal, and that the radioactive sulphate reached Southern California, according to a study published online Aug. 15 in the Proceedings of the National Academy of Sciences.

TUESDAY, Aug. 16 (HealthDay News) -- A model based on measurements of radioactive 35S in sulfate aerosols and SO2 gas estimates that 35SO42− concentrations at the Fukushima marine boundary are around 365 times higher than normal, and that the radioactive sulphate reached Southern California, according to a study published online Aug. 15 in the Proceedings of the National Academy of Sciences.

Antra Priyadarshi, Ph.D., from the University of California in San Diego, and colleagues measured the long-range transport of harmful radiation from the Fukushima nuclear power plant in Japan, over the Pacific, by quantitatively estimating neutron flux leakage during the weeks following the earthquake and tsunami on March 11, 2011. Measurements of radioactive 35S contained in sulfate aerosols and SO2 gas were carried out at a coastal site in California, and a moving box model was used to calculate activity enhancement of 35SO42−.

The investigators found that nearly 4×1011 neutrons per m² leaked at the power plant before March 20, 2011. On March 28, significantly higher 35SO42−activity was measured, which is consistent with neutrons escaping the reactor core and being absorbed by the coolant seawater 35Cl to produce 35S. The 35S oxidized to 35SO2 and 35SO42−, and due to the presence of strong prevailing westerly winds at the time, was transported to Southern California. The moving box model demonstrated enhanced 35SO42− activity which was compatible with long-range transport of the radiation. The model predicted that the concentration of 35SO42− in the marine boundary layer at Fukushima was around 365 times above the likely natural concentration, measuring 2×105 atoms per m³.

"These measurements and model calculations imply that approximately 0.7 percent 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 authors write.

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