By Geoff Brumfiel of Nature magazine

Scientists in California are reporting raised levels of radioactive chemicals in the atmosphere in the weeks following the disaster at Japan's Fukushima Daiichi nuclear power plant. The measurements are the latest evidence that the reactors melted down catastrophically.

Researchers at the University of California, San Diego (UCSD), say that radioactive sulfur from the stricken power plant reached California in late March, two weeks after the crisis at Fukushima began. The sulfur is a by-product of emergency procedures taken immediately after the accident. The work is published in the Proceedings of the National Academy of Sciences.

On 11 March, the Fukushima Daiichi plant was shaken by a magnitude-9 earthquake and slammed with a 13-metre-high tsunami. The disaster knocked out emergency generators designed to back up systems that cooled the plant's three operating reactors.

In a desperate attempt to slow heating and avert a total meltdown, operators flooded the reactor cores with boric acid and sea water. But it didn't work: in May, the Tokyo Electric Power Company, which oversees the plant, announced that despite their best efforts, the reactors at Fukushima Daiichi had melted down completely.

Chemical corroboration

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 sulfur, 35S, which is usually generated by cosmic rays striking argon atoms in the atmosphere. On 28 March, the team detected levels of radioactive sulfur dioxide gas (35SO2) and sulphate aerosols (35SO4-2) that were well above the natural background.

The chemicals posed "no risk" to residents in San Diego, says Thiemens. 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 sulfur 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 35S. They then escaped the reactor in both gas and aerosol form and were spread across the ocean by strong westerly winds.

Although 400 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.On the basis of models, the team estimates that around 400 billion neutrons per square metre 'leaked' from the reactor cores at the time of the meltdowns.

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".

This article is reproduced with permission from the magazine Nature. The article was first published on August 15, 2011.