Russia’s Mayak PA reprocessing plant in the southern Urals region (part of state nuclear corporation Rosatom) is the most likely source of a release of  ruthenium-106 detected across Europe in late 2017, according to a study led by Olivier Masson from France's Institute for Radiological Protection and Nuclear Safety.

Some 70 experts from all over Europe provided data and expertise for the study and some 176 measuring stations in 29 countries were involved. The paper, 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.  However, no incident has been reported at the Mayak facility or any other Russian nuclear facility.

Very small quantities of ruthenium-106  were detected in several European countries between late September and early October 2017. Initial 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 that the  data available did not provide sufficient information to verify the emission originated from any of the activities assumed to have caused the release.

In the latest study, more than 1300 readings from across Europe and other regions of the world were analysed.

The study notes that 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.

"Based on times series of detections at various locations in Central Europe, the event was characterised as a short release," the study said. "The plume duration lasted about 1-3 days on average, depending on the location, with the exception of a few areas." It noted that the release of the ruthenium-106 was "too substantial" to be associated with the incineration of a medical radionuclide source and that a satellite crash could also be excluded as the source.

"The readings indicate the probably largest single release of radioactivity from a civilian reprocessing plant," said Professor Georg Steinhauser of the University of Hannover, who evaluated the data. "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."

During the investigation, scientists analysed several possibilities that could lead to the release but eventually rejected most of them. For example, the commission excluded the possibility of an accident at the plant’s reactor because, in addition to ruthenium-106, that would also  have resulted in an increased concentration of other radioactive elements, which were not detected in the atmosphere.

The study also ruled out the release of ruthenium-106 as a result of leakage of used radioactive fuel. Experts found that the age of radioactive particles in the atmosphere was less than would have been the case for isotopes contained in used fuel. The disintegration of a satellite containing a battery with a radioactive source  was also excluded. Experts noted that the data on the vertical distribution of ruthenium-106 in the atmosphere did not correspond to data that would be expected in this case – the ruthenium-106 would have been detected at higher layers of the atmosphere.

The commission said the most likely cause of the leakage of ruthenium-106 was an unsuccessful attempt to produce another radioactive substance – the cerium-144 isotope which can be extracted from used fuel. Apparently, cerium-144 supplied by a Russian company was to be used in the Italian Borexino experiment to search for particles of the “sterile neutrino”. This experiment was canceled in December 2017, two months after the possible leak at Mayak. Science News, citing Italian scientists, reported that some unexpected problems have occurred during extraction of the material. “Of course, these are all hypotheses, but we know that the Borexino project required large quantities of cerium-144. To fulfill the obligations to the Italian partners, its manufacturer had to reduce the time that it usually spent on cooling spent fuel, which led to unexpected consequences,” said Steinhauser.

The SOX-Borexino experiment was expected to take place in 2018 at the Italian laboratory of Gran Sasso but was reportedly cancelled in January 2018 because the Russian supplier was unable to provide a sufficient amount of cerium-144, a powerful source of anti-neutrinos. Mayak was one of a very few possible suppliers. Used fuel requires at least three years of cooling before any processing, but the longer the fuel cools, the less cerium-144 remains it contains. Steinhauser noted that the Italian experiment required a huge amount of cerium-144, about 2.5kg of ultra-pure metal. The European scientists assume that their Russian colleagues had to risk using more active material to accelerate the production of the neutrino sources. The final stages of reprocessing produce gaseous compounds of tetroxide and ruthenium dioxide, which are extremely high corrosive. High concentrations of these gases could damage the filters in the processing plant or leak through them in other ways.

However, Steinhauser’s Russian colleagues refuted these conclusions. For example, Academician Leonid Bolshov , one of the commission’s members, said that the theories “ignore measurements of radionuclide fractions in the soil next to Mayak, where almost no ruthenium-106 and other traces of pollution were recorded”.

In November 2017, 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."

In response to the latest report, Rosatom said the production of a source based on cerium-144 could under no circumstances lead to an increased release of ruthenium-106. From August to November 2017, the production of a source of cerium-144 was carried out only with a concentrate of rare-earth elements, in which ruthenium-106 could be found only in "trace" amounts. All the liquid solutions formed at the preliminary stage, containing ruthenium isotopes as well, were sent back  in July 2017 for storage in a special storage facility. Mayak’s information service also said ruthenium-106 was not used in 2017 and, in general, the production of ionising radiation sources   had taken place at the plant for many years.

Mayak, in the city of Ozersk in the Chelyabinsk region, was founded in 1948 and produces components of nuclear weapons, isotopes of radioactive elements, as well as reprocessing and storing used fuel from nuclear power plants and submarines, as well as other types of radioactive waste. In 1957 it was the source of the Kyshtym disaster, one of the world’s worst nuclear accidents, a radioactive waste tank exploded forcing the evacuation of 10,000 people in the area and causing long-term consequences for residents’ health.