In October 2017, European officials reported that a cloud of the radioactive isotope ruthenium-106 had mysteriously wafted over the continent. Its likely source was in the southern Ural Mountains, near Mayak, the Russian nuclear facility involved in a deadly nuclear disaster in 1957. Strangely, Russia at first denied — but then acknowledged — there had been a surge of radiation. However, it rejected the idea that the surge was the source of the cloud. Now, scientists report in a new PNAS paper that they’ve narrowed down where it actually came from.
There were a number of hypotheses to explain the source of the cloud, which spread over most of Europe and even reached Florida, Guadeloupe, Kuwait, and Mongolia in tiny amounts. Fortunately, it was deemed non-hazardous.
Russian officials denied that Mayak, one of the country’s largest nuclear facilities, was the source of the ruthenium. In November 2017, Rosatom, the state company that runs Russia’s nuclear industry, pointed to high radiation in Italy, Romania, and Ukraine, suggesting they might have been the source. And in December of that year, Russian officials, reasserting that Mayak was not the source of the cloud, suggested the cloud might have come from a satellite that had burned up in the atmosphere. Other hypotheses arose, but scientists have lacked the evidence to support or reject them until now.
The numerous authors of the new paper, led by Olivier Masson, Ph.D., of the French Institute for Radiation Protection and Nuclear Safety (IRSN), write: “The present article aims at closing this gap.”
What the New Study Rules Out
The team compiled the biggest data set to date on observations of ruthenium across Europe, comprising over 1,120 data points related to airborne activity and about 200 data points regarding deposited contamination, collected over 330 locations. They used this data to reconstruct the location and concentration of the plume at any given point in time.
Ruthenium-106, a non-natural isotope usually used for medical purposes like treating cancer, hasn’t been detected in the atmosphere since the Chernobyl accident of 1986, so the team reports it had no issues with background contamination. Here are the hypotheses they were able to exclude, based on their analysis.
A nuclear reactor leak. Because their data largely show only traces of ruthenium in the atmosphere and on the ground (and not other radionuclides like plutonium, americum, curium, or strontium-90 radionuclides), they were able to rule out a nuclear reactor leak. “This excludes an accidental release from a nuclear reactor as the source,” the authors write, “as this would have resulted in an emission of a great multitude of fission products.”
Their data confirms the assessment of the IRSN and a statement from University of Surrey nuclear physicist Patrick Regan, Ph.D., who was not involved with the study, to the Science Media Centre in 2017: “If it was a reactor leak or nuclear explosion, other radioisotopes would also be present in the ‘plume’ and from the reports, they are not.”
A Romanian source. Though concentrations of ruthenium in Romania were high, as the Russian statement pointed out, the data suggest the presence of the cloud there was “rather short” — too short for it to be a feasible source point for the cloud, given the country’s geography and wind patterns during that period.
Melting of a medical radioactive source. Previously, radioactive plumes have resulted from melting a medical radioactive source, like the scrap metal containing cesium-137 melted in a Spanish steelworks plant in 1998. However, the team says that the radioactivity of ruthenium-106 used in radiotherapy is too low for it to have melted and caused a cloud big enough to spread over Europe, explaining “it would have required the melting of numerous opthalmic sources at once.”
Satellite reentry. Contrary to the suggestion of the Nuclear Safety Institute of the Russian Academy of Sciences in December 2017 that the radiation might have come from a ruthenium-powered satellite burning up in the atmosphere, the new study suggests this cannot have been the case. If that happened, the team writes, the concentration of ruthenium-106 would have been highest in regions high up in the atmosphere. But the data show that high-altitude locations had lower concentrations of the isotope than low-altitude locations.
“Therefore, the 106Ru release has likely happened in the lower tropospheric layers and cannot be linked to a satellite disintegration,” they write. Besides, as the IRNS pointed out previously, there aren’t any reports of satellites crashing during that time.
The Likely Source of the Cloud
Having ruled out release from a nuclear reactor, melting of a radioactive source, and satellite reentry, the team concludes that the source of the ruthenium-106 cloud likely has to do with nuclear fuel reprocessing — which may well have occurred at the Mayak site.
“The data suggest a release from a nuclear reprocessing facility located in the Southern Urals, possibly from the Mayak nuclear complex,” they write, noting that the Mayak complex was known to have a nuclear fuel reprocessing facility in 2014. Their analysis of the ruthenium suggests that the fuel was reprocessed around two years after it was first discharged.
Nuclear reprocessing refers to a way to extract leftover plutonium and uranium from used nuclear rods in order to reduce the volume of nuclear waste left over. (This claim is contested by science and environmental advocacy groups.)
Ruthenium is a common product of nuclear fission of uranium or plutonium, explained Regan to the Science Media Centre in 2017, noting that its detection in isolation “suggests a leak from a fuel/reprocessing plant of somewhere they are separating the Ru, possibly for use as a medical radiopharmaceutical/diagnostic material.”
The paper resurfaces an idea that the reprocessing took place to supply “a high-specific activity 144Ce source for a neutrino experiment in Italy,” as the authors write, referring to the radioactive isotope cerium-144. Others had previously suggested this might be the case.
As Science reported in February 2018: “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.”
As other researchers have argued, Mayak is the only facility with the chemical capability to produce the cerium-144 required for such a project.
So far, Russia has not issued a response to the findings. “Even though there is currently no official statement,” said co-author University of Hanover Georg Steinhauser, Ph.D., in a release, “we have a very good idea of what might have happened.”
In October 2017, most European countries reported unique atmospheric detections of aerosol-bound radioruthenium (106Ru). The range of concentrations varied from some tenths of µBq·m−3 to more than 150 mBq·m−3. The widespread detection at such considerable (yet innocuous) levels suggested a considerable release. To compare activity reports of airborne 106Ru with different sampling periods, concentrations were reconstructed based on the most probable plume presence duration at each location. Based on airborne concentration spreading and chemical considerations, it is possible to assume that the release occurred in the Southern Urals region (Russian Federation). The 106Ru age was estimated to be about 2 years. It exhibited highly soluble and less soluble fractions in aqueous media, high radiopurity (lack of concomitant radionuclides), and volatility between 700 and 1,000 °C, thus suggesting a release at an advanced stage in the reprocessing of nuclear fuel. The amount and isotopic characteristics of the radioruthenium release may indicate a context with the production of a large 144Ce source for a neutrino experiment.