A used fuel reprocessing plant in the southern Urals region of Russia is the most likely source of a release of the isotope ruthenium-106 detected across Europe in late 2017, a study has concluded. No incident has, however, been reported at the Mayak facility or any other Russian nuclear facility.
Map of 7-day corrected average ruthenium-106 concentrations at European measuring stations
Very small quantities of ruthenium-106 - a fission product from the nuclear industry and a radionuclide used in medicine - were detected in several European countries between late September and early October 2017. Early investigations suggested the contaminated air masses measured in Europe originated from the southern regions of the Urals. However, an international commission appointed by Russia to investigate the release met for the second time in April 2018 and concluded available data did not provide sufficient information to verify the emission originated from any of the activities assumed to have caused the release.
Researchers have now published a study in which over 1300 readings from across Europe and other regions of the world were analysed to determine the source of the release. Around 70 experts from all over Europe provided data and expertise for the study, which was led by Olivier Masson from France's Institute for Radiological Protection and Nuclear Safety. A total of 176 measuring stations from 29 countries were involved. The paper - titled Airborne concentrations and chemical considerations of radioactive ruthenium from an undeclared major nuclear release in 2017 - was published in the Proceedings of the National Academy of Sciences of the USA on 26 July.
The study notes that early in the 1960s, and even more so after the Chernobyl accident in April 1986, European radioprotection authorities had established or strengthened radionuclide monitoring networks on a national scale.
"Today most of these European networks are connected to each other via the informal 'Ring of Five' (Ro5) platform for the purpose of rapid exchange of expert information on a laboratory level about airborne radionuclides detected at trace levels," it says. In October 2017, an unprecedented release of ruthenium-106 into the atmosphere was the subject of numerous detections and exchanges within the Ro5.
"Based on times series of detections at various locations in Central Europe, the event was characterised as a short release," the study says. "The plume duration lasted about 1 to 3 days on average, depending on the location, with the exception of a few areas."
The release was "too substantial", it says, to be associated with the incineration of a medical radionuclide source. A satellite crash can also be excluded as the source of the ruthenium-106, it adds.
"It is much more likely that the Ru-106 escaped during reprocessing of spent nuclear fuel, possibly in the course of the miscarried production of a highly radioactive cerium-144 source for research applications in the Gran Sasso Laboratory in Italy," it says.
"According to detection time series, a back-trajectory analysis, and chemical considerations, the Mayak nuclear complex in southern Urals should be considered as a likely candidate for the release," the study concludes.
Professor Georg Steinhauser of the University of Hannover, who evaluated the data, said: "The readings indicate the probably largest single release of radioactivity from a civilian reprocessing plant. We were able to show that the accident happened in the reprocessing of spent fuel, at a very advanced stage of reprocessing, shortly before the end of the process chain. Even though there is no official statement yet, we have a pretty detailed idea of what might have happened."
In a 22 November 2017 statement, Russian state nuclear corporation Rosatom insisted: "The recent release of ruthenium-106 that has been detected across Europe did not come from a Rosatom facility. All Rosatom nuclear facilities, including nuclear power plants and nuclear fuel reprocessing sites, are operating safely and normally. There were no incidents at any of these sites during the period of September-October 2017."
