The Fukushima nuclear disaster began on 11 March, with a series of equipment failures and nuclear meltdowns, following the magnitude 9 Tohoku earthquake and tsunami earlier in the day. It was the largest nuclear accident since the Chernobyl disaster and was assessed as level 7 (the highest level) on the International Nuclear Event Scale – a major accident with widespread health and environmental effects.

So just how far did the radioactive plume travel? On the 27 March Romul Mărgineanu, from the Horia Hulubei National Institute of Physics and Nuclear Engineering (IFIN-HH) in Romania, and colleagues, began to collect samples of rainwater from Braşov and Slănic-Prahova. And on 5 April they also started to collect samples of sheep and goat's milk from the same regions.

The samples were all taken to the IFIN-HH's underground laboratory at the Unirea salt mine in Slănic-Prahova, for analysis. Inside this ultra-low radiation environment the levels of iodine131 and caesium137 were measured, using a high resolution gamma-ray spectrometer.

None of the samples contained caesium137 at detectable levels. However, iodine131 was present at up to 0.75 Bq per litre in rainwater, and up to 5.2 Bq per litre in milk. "The level of I-131 in sheep and goat milk was higher than in rain water due to bioaccumulation," explains Mărgineanu. "Bioaccumulation occurs when an organism absorbs a toxic substance – chemical or radioactive – at a rate greater than that at which the substance is lost."

Such levels were two to three orders of magnitude below any intervention limits, for example the limit set for drinking water in Japan was 300 Bq per litre for adults and children and 100 Bq per litre for infants. In this case weather conditions played a role in keeping the radiation levels low. In the weeks following the accident Romania only experienced very light rain; had the rain been heavier more radiation may have been precipitated out. The results are published in Environmental Research Letters.

Without a doubt, the findings demonstrate that the Fukushima radioactive plume travelled over 10,000 km, and suggest that detectable levels of radiation reached almost all parts of the northern hemisphere. "Obviously, the lesson we have to learn from nuclear accidents, either minor or major, is that we have to improve safety conditions and to come up with new solutions which have to cope with the identified new risks," said Mărgineanu. And indeed this is exactly what seems to be occurring now, with most countries reassessing the safety of their nuclear power plants in light of the accident.