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The latest measurements seem to confirm that. For several years, Mark Thiemens, a chemist at UCSD, and his group have been measuring atmospheric levels of a radioactive isotope of sulphur, 35 S, which is usually generated by cosmic rays striking argon atoms in the atmosphere.
Atmospheric Radioactivity and Its Variations | SpringerLink
On 28 March, the team detected levels of radioactive sulphur dioxide gas 35 SO 2 and sulphate aerosols 35 SO 4 -2 that were well above the natural background. The chemicals posed "no risk" to residents in San Diego, says Thiemens.
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In fact, it took a year to even develop equipment sensitive enough to measure levels as low as these, he says. Thiemens and his colleagues believe that the radioactive sulphur was produced from chlorine in the sea water used to flood the reactors. The chlorine atoms probably absorbed neutrons from the ruined nuclear fuel, and were transmuted into 35 S.
They then escaped the reactor in both gas and aerosol form and were spread across the ocean by strong westerly winds. On the basis of models, the team estimates that around billion neutrons per square metre 'leaked' from the reactor cores at the time of the meltdowns. Although billion may sound like a lot, it's tiny in comparison with the normal flux of neutrons inside a reactor, says Patrick Regan, a nuclear physicist at the University of Surrey in Guildford, UK. Regan says that the neutrons do not indicate that the melted reactors restarted after the emergency began, but are a clear by-product of sea water inside the reactors.
Thiemens says that the most significant contribution of the measurement may be in helping researchers to better understand how sulphates and other aerosols travel through the atmosphere after a nuclear accident. Fukushima provided a single, well defined source of traceable radiation, he says. Follow-up studies with Japanese colleagues "will be very significant in uniquely addressing how, and how fast, radioactivity spreads".