On October 2, 2017, scientists from an Italian laboratory issued an alert: They had detected radioactive ruthenium-106 in the air in Milan. It was not enough to be dangerous, but it was certainly not natural. Other laboratories in the informal Ring of Five network of European nuclear monitoring stations soon confirmed similar observations in Austria, Norway, and the Czech Republic. Within a few days, more than two dozen countries in Europe had confirmed the detections of the radioactive compound.

Ruthenium is a transition metal, similar to platinum, that is used in electronics, solar cells, and some jewelry. The last time anyone had detected the radioactive isotope ruthenium-106 in the atmosphere was after the Chernobyl disaster. It is a byproduct of the fission of uranium-235, a common nuclear fuel. Detecting it was a near-definite indication of a nuclear accident.

The French Institute for Radiological Protection and Nuclear Safety, known as IRSN, quickly determined in an investigation that the plume of ruthenium had originated in the southern Ural Mountains, in Russia, through the reprocessing of spent nuclear fuel. In that area of the world, there is only one nuclear reprocessing plant: the Russian facility Mayak. The Russian state nuclear corporation, Rosatom, has denied in multiple statements that any such release of nuclear material occurred. A new study, however, seems to have definitively established Mayak as the origin of the ruthenium-106.

"For the very first time, all the European monitoring stations, they are now speaking with one voice, and they're putting all the data together," says Georg Steinhauser of the University of Hannover in Germany, one of the main researchers on the study. The study itself had 69 authors from 50 institutions. "The picture is much clearer now."

A novel finding in the study is the young age of the original fuel. Spent fuel, when initially taken from a nuclear reactor, is dangerously radioactive. According to Steinhauser, the policy in France is to let spent fuel sit for four years before trying to reprocess it; in Russia, the policy is three years. Steinhauser and his team found that the spent fuel was "incredibly" young, only between 1.5 to 2 years old.

"It's very, very unusual," Steinhauser says. "The Russians, they're very experienced in how to do fuel reprocessing. They are the world's leading experts in what they're doing. But when something like this happens, it immediately looks like they have deviated from the regular process."

It was also a clue. The young age of the fuel "means that there was an intention to do something special" with it, says Jean-Christophe Gariel, deputy director general for health and environment at the IRSN. "And our hypothesis is that the intention was to create a strong radioactive source of cerium."

After the ruthenium plume was detected in late 2017, IRSN issued a report in which it theorized that the accident occurred when Mayak attempted to create a highly compact, highly radioactive material that could emit a large number of neutrinos (difficult-to-detect fundamental particles) for a physics experiment in Italy called SOX. The experiment called for a sample of cerium-144, a soft and silvery metal with some special properties. It is radioactive enough to emit a large number of neutrinos, and can be small enough that its volume won’t interfere with calculations. Younger nuclear fuel would have a higher concentration of radioactive cerium-144, allowing the sample to be smaller while still being extremely radioactive.

SOX spokesperson Marco Pallavicini, a professor at the University of Genova, says the Mayak Production Association was the only company that could supply a cerium-144 sample that satisfied the needs of the experiment.