For 2 weeks in September and October last year, traces of the humanmade isotope ruthenium-106 wafted across Europe, triggering detectors from Norway to Greece and Ukraine to Switzerland. The radioactive cloud was too thin to be dangerous, containing no more than a few grams of material, but its origin posed an outsize mystery.
Now, scientists at the French Institute of Radioprotection and Nuclear Security (IRSN) in Paris say the isotope may have been released from the Mayak nuclear facility near Ozyorsk in southern Russia. IRSN argues that the leak could have taken place when Mayak technicians botched the fabrication of a highly radioactive component for a physics experiment at the Gran Sasso National Laboratory in L’Aquila, Italy.
The Russian government and state nuclear operator Rosatom have vehemently denied that an accident took place, however. Meanwhile, an international committee set up by the Russian Academy of Sciences’s Nuclear Safety Institute (IBRAE) in Moscow that met on 31 January is divided over the origins of the pollution.
Based on a computer model that used the air-sampling data and weather patterns, IRSN concluded in early October 2017 that the ruthenium most likely originated in the southern Urals; its German counterpart agreed. The French team went on to rule out a number of potential sources, including a mishap at a nuclear reactor. Such an incident would have spewed many other radioactive pollutants besides ruthenium.
The southern Urals are home to the secretive Mayak facility, the scene of one of the world’s worst nuclear accidents 60 years ago, and speculation soon turned to a possible accident at its reprocessing plant, which extracts isotopes from spent nuclear fuel. The IRSN report, made public on 6 February, says Mayak’s attempt to manufacture a capsule of cerium-144 destined for Gran Sasso “should be investigated” as a possible cause. Scientists at Gran Sasso needed the cerium for a search—now called off—for hypothetical particles called sterile neutrinos.
The estimated amount of radioactive ruthenium released could only have come from processing several tons of spent nuclear fuel, IRSN says. What’s more, the ratio of ruthenium-106 to the faster-decaying isotope ruthenium-103, detected in smaller amounts last autumn, reveals that the fuel must have been removed from its reactor only a year or two earlier. Spent fuel is normally cooled for up to a decade before it is reprocessed, so it seems the plant was preparing material for an application requiring high levels of radioactivity, IRSN says.