It was the most serious release of radioactive material since Fukushima 2011, but the public took little notice of it:
In September 2017, a slightly radioactive cloud moved across Europe.
France’s nuclear safety institute, IRSN, picked up faint traces of ruthenium 106, a radioactive nuclide that is produced when atoms are split in a nuclear reactor and which does not occur naturally, in three of its 40 monitoring stations late September.
At the source of the leak, the quantity of ruthenium 106 released was “major”, between 100 and 300 teraBecquerels, it said, adding that if an accident of this magnitude had happened in France it would have required the evacuation or sheltering of people in a radius of “a few kilometres around the accident site”.
But it said that the probability of importation into France of foodstuffs, notably mushrooms, contaminated by ruthenium 106 near the site of the accident was extremely low.
Now, a study has been published, analyzing more than 1300 measurements from all over Europe and other regions of the world to find out the cause of this incident.
The result: it was not a reactor accident, but an accident in a nuclear reprocessing plant.
The exact origin of the radioactivity is difficult to determine, but the data suggests a release site in the southern Urals.
This is where the Russian nuclear facility Majak is located.
The incident never caused any kind of health risks for the European population.
Among the 70 experts from all over Europe who contributed data and expertise to the current study are Dieter Hainz and Dr. Paul Saey from the Institute of Atomic and Subatomic Physics at TU Wien (Vienna).
The data was evaluated by Prof. Georg Steinhauser from the University of Hanover (who is closely associated with the Atomic Institute) together with Dr. Olivier Masson from the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) in France.
The team has now published the results of the study in the journal Proceedings of the National Academy of Sciences (PNAS).
Unusual Ruthenium Release
“We measured radioactive ruthenium-106,” says Georg Steinhauser.
“The measurements indicate the largest singular release of radioactivity from a civilian reprocessing plant.”
In autumn of 2017, a cloud of ruthenium-106 was measured in many European countries, with maximum values of 176 millibecquerels per cubic meter of air.
The values were up to 100 times higher than the total concentrations measured in Europe after the Fukushima incident.
The half-life of the radioactive isotope is 374 days.
This type of release is very unusual.
The fact that no radioactive substances other than ruthenium were measured is a clear indication that the source must have been a nuclear reprocessing plant.
The geographic extent of the ruthenium-106 cloud was also remarkable — it was measured in large parts of Central and Eastern Europe, Asia and the Arabian Peninsula. Ruthenium-106 was even found in the Caribbean.
The data was compiled by an informal, international network of almost all European measuring stations.
In total, 176 measuring stations from 29 countries were involved.
In Austria, in addition to TU Wien, the AGES (Austrian Agency for Health and Food Safety) also operates such stations, including the alpine observatory at Sonnblick at 3106m above sea level.
No Health Hazard
As unusual as the release may have been, the concentration of radioactive material has not reached levels that are harmful to human health anywhere in Europe.
From the analysis of the data, a total release of about 250 to 400 terabecquerel of ruthenium-106 can be derived.
To date, no state has assumed responsibility for this considerable release in the fall of 2017.
The evaluation of the concentration distribution pattern and atmospheric modelling suggests a release site in the southern Urals.
This is where the Russian nuclear facility Majak is located.
The Russian reprocessing plant had already been the scene of the second-largest nuclear release in history in September 1957 — after Chernobyl and even larger than Fukushima.
At that time, a tank containing liquid waste from plutonium production had exploded, causing massive contamination of the area.
Olivier Masson and Georg Steinhauser can date the current release to the time between 25 September 2017, 6 p.m., and 26 September 2017 at noon — almost exactly 60 years after the 1957 accident.
“This time, however, it was a pulsed release that was over very quickly,” says Professor Steinhauser.
In contrast, the releases from Chernobyl or Fukushima lasted for days. “We were able to show that the accident occurred in the reprocessing of spent fuel elements, at a very advanced stage, shortly before the end of the process chain,” says Georg Steinhauser. “Even though there is currently no official statement, we have a very good idea of what might have happened.”
The laboratory of the CRIIRAD (Commission for Independent Research and Information on RADioactivity), based in Valence (France) carried out soil analyzes taken in December 2017 between 14 and 20 km west of Mayak.
These analyzes confirmed a significant ruthenium 106 contamination, which can reach 580 to 1,200 Bq/m2, which is appreciably higher, but compatible with those found in September 2017 by the Russian agency Rosguidromet (a few hundred Bq/m2).
These results have not led to a conclusion on the question of the origin of ruthenium 106. In fact, they do not correspond to the very strong theoretical fallout expected locally in the event of an estimated release of 100 000 billion of Becquerels.
It should be noted, however, that if the releases lasted only a short time, the locally most contaminated areas may be in a small angular sector that has not been sampled.
In addition, the feedback from uncontrolled releases of ruthenium 106 that occurred in 2001 at the La Hague site (France) shows that, very close to the installation, there can be much more significant fallout than estimates given by the usual models of dust deposition.
More than 4 months after the releases, the Russian authorities have still not published detailed mapping of the level of soil contamination and bioindicators around Mayak.
It is therefore essential to conduct an independent environmental analysis program around Mayak, with a concentric circle sampling plan of increasing radius, and taking into account the weather conditions in the anticipated period of discharges.
It is imperative to add sampling stations, especially in the North sector, but also closer to Mayak. Since a large part of the study area is closed to access by the Russian authorities, part of these samples can only be taken if the authorities lift these prohibitions.
The carrying out of independent and in-depth controls is all the more justified since CRIIRAD has identified contamination of very long-lived radionuclides in the west of Mayak, americium 241 (433 years) and iodine 129 (15.7 million years). The presence of americium 241 in the surface layer of soils indicates that of plutonium.
CRIIRAD calls on the Russian authorities to make public the exhaustive list and quantity of radioactive substances released to the atmosphere by the Mayak site as well as the complete results of analyzes carried out on soil, air, water, fauna, flora and the food chain.
It also calls for the authorization of independent soil radioactivity and terrestrial bioindicator controls closer to the nuclear site.
Materials provided by Vienna University of Technology. Note: Content may be edited for style and length.