A massive atmospheric release of radioactive 106 Ru occurred in Eurasia in 2017, which must have been caused by a sizeable, yet undeclared nuclear accident. This work presents the most compelling monitoring dataset of this release, comprising 1,100 atmospheric and 200 deposition data points from the Eurasian region. The data suggest a release from a nuclear reprocessing facility located in the Southern Urals, possibly from the Mayak nuclear complex. A release from a crashed satellite as well as a release on Romanian territory (despite high activity concentrations) can be excluded. The model age of the radioruthenium supports the hypothesis that fuel was reprocessed ≤2 years after discharge, possibly for the production of a high-specific activity 144 Ce source for a neutrino experiment in Italy.

In October 2017, most European countries reported unique atmospheric detections of aerosol-bound radioruthenium ( 106 Ru). 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 106 Ru 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 106 Ru 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 144 Ce source for a neutrino experiment.

Nuclear accidents are serious threats due to their immediate and perceived consequence for both health and environment. The lay public thus relies on the responsibility of their leaders to provide information on radioactive releases and their impact on human and environment health. Early in the 1960s, and even more after the Chernobyl accident, European radioprotection authorities 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. The Ro5 was founded in the mid-1980s by 5 member countries: Sweden, Federal Republic of Germany, Finland, Norway, and Denmark. Today, the memberships have grown to laboratories in 22 countries (while the name was kept), and the Ro5 is still an informal arrangement on a laboratory level and between scientists. In January 2017, the Ro5 alerted its members regarding the widespread detection of airborne 131I in Europe (1). In October 2017, an unprecedented release of ruthenium-106 (106Ru; T 1/2 = 371.8 d) into the atmosphere was the subject of numerous detections and exchanges within the Ro5. The goal of this report is to give an overview of the global spreading of this fission product through airborne concentrations observed in Europe and beyond, its forensic history, and chemistry.

After 2 d (and further detection reports from Poland, Austria, Switzerland, Sweden, and Greece), official information notes were published by national radioprotection authorities, for example, in Switzerland, Austria, and Norway. On October 7, 2017, the International Atomic Energy Agency (IAEA) requested data and possible known sources of radioruthenium from all 43 European member states. On October 9, 2017, Chelyabinsk and Sverdlovsk regional authorities ruled out any possible 106 Ru release from their region (Russian Federation). On November 21, 2017, the Russian Federal Service for Hydrometeorology and Environmental Monitoring (Roshydromet) declared to have measured 106 Ru in the southern Urals in the late September ( 2 ). However, one possible source in the region, the Federal State Unitary Enterprise “Production Association Mayak” in Ozersk, immediately declared that it was not the source of increased 106 Ru ( 3 ). On November 23, 2017, the IAEA addressed the release of 106 Ru in a press conference. All members submitted the requested data, but none declared an accident and none declared being aware of any source. On December 8, 2017, Russian officials once again claimed that Mayak could not be the source because of the lack of any radioruthenium traces in the soil around the facility ( 4 ). Instead, the officials pointed at the possibility of a radionuclide battery of a satellite that had burned during its reentry into the atmosphere. On January 22, 2018, the Nuclear Safety Institute of the Russian Academy of Sciences invited radiation protection experts from Germany, France, Finland, Sweden, the United Kingdom, and Russia to aid in the elucidation of the release. Two commissions of inquiry were held: on January 31, 2018 and on April 11, 2018. The second meeting concluded by emphasizing that not enough data were yet available to point out any verified hypothesis of the origin of the 106 Ru ( 5 ). The present article aims at closing this gap.

On October 2, 2017, an informal alert by an Italian laboratory was issued to the Ro5 network, reporting the detection of airborne 106 Ru in the millibecquerel per cubic meter (mBq·m −3 ) range in Milan, Italy. Limits of detection (LOD) in laboratories connected to the Ro5 are typically in the range of 0.1 to 10 microbecquerels per cubic meter (μBq·m −3 ). This first report occurred on a Monday, when most European laboratories usually exchange their aerosol filters, which are operated on a weekly basis. Later that day, 106 Ru detections were reported from Czech Republic, Austria, and Norway in the 1- to 10-mBq·m −3 range. This widespread detection in such range immediately suggested a considerable release.

Results and Discussion

Monitoring Results. Information sources: The entire airborne concentration dataset and deposition dataset are available as SI Appendix, Tables S1–S4 and were mainly compiled through Ro5 exchanges, personal exchange, and data already published. Valuable information are also available on the Roshydromet website (6, 7), on the Typhoon Association website (6), and on the website of the Unified State Automated Monitoring System of the Radiation Situation in the Russian Federation (8). Data from the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) (9) are not part of the dataset except for those already published (10). The International Monitoring System (IMS) data supporting the CTBTO (9) are available directly from the CTBTO upon request and signing a confidentiality agreement to access the virtual Data Exploitation Platform. This study provides 106Ru observations (>1,120 data points related to airborne activity and about 200 data points for deposited contamination, from about 330 sampling locations) that can be used for the purpose of atmospheric dispersion and deposition model validation. With the exception of the westernmost parts of Europe, most monitoring stations reported detections of 106Ru in the range between some tenths µBq·m−3 meter and more than a 100 mBq·m−3. Fig. 1 illustrates the maximum activity levels per country. Activity concentrations in the millibecquerel per cubic meter range were reported between September 29, 2017 and October 7, 2017, exhibiting a short build-up and rapid decline behavior. The last traces of the plume (microbecquerel per cubic meter range) were measured in sampling periods ending between October 12, 2017 and the end of October by laboratories equipped with high-volume samplers and low-level γ-ray spectrometry. The eastern and southeastern parts of Europe, including western Russia, exhibited the highest reported levels. The maximum level in Europe was reported from Romania (176 ± 18 mBq·m−3). Even at this level, the plume did not represent any threat for human and environmental health. Fig. 1. Compiled maximum 106Ru airborne “uncorrected” activity concentrations (in mBq·m−3; sampling period in parentheses) in Europe. However, it is important to note that these compiled data were obtained with different sampling durations, which limits their comparability without further correction. Outside Europe, 106Ru was also detected east of the geographical border between Europe and Asia in the Urals region (Russian Federation) with activity levels of some tens mBq·m−3. Tiny amounts of 106Ru were also pointed out elsewhere in the northern hemisphere by aerosol stations belonging to the IMS supporting the CTBTO: in Guadeloupe, Kuwait, Florida (United States), Russia (central and eastern parts), and Mongolia. 106Ru is a nuclide that may be released upon detonation of a nuclear weapon, and is therefore a “CTBT-relevant radionuclide.” 106Ru had not been detected in the global atmosphere since the Chernobyl accident (11) [estimated release <73 PBq (12)], not even after the Fukushima accident on the Japanese territory (13, 14), because of the different accident and release characteristics. As a result, there is no usual background or reference level, which could be used to define an increasing factor. As a matter of fact, this radionuclide is usually not detected in the atmosphere. Besides aerosol filtration, gaseous sampling was conducted at some locations (Austria, Sweden, Italy, and Poland), thus allowing checking for the presence of gaseous Ru species. Ruthenium may be present in volatile forms, especially in the form of ruthenium tetroxide, RuO 4 (15). Since gaseous RuO 4 is a highly reactive and strong oxidizer, it is expected to rapidly nucleate into particulate and low volatile RuO 2 . No 106Ru was detected in gaseous form. In addition to 106Ru, the anthropogenic ruthenium isotope 103Ru (T 1/2 = 39.3 d) was detected at a limited number of high-performing stations (SI Appendix, Table S2): Austria, Czech Republic, Poland, and Sweden (10) with activity levels ranging from 0.04 to 7.3 µBq·m−3 (average 2.6 ± 0.1 µBq·m−3). The average ratio 103Ru/106Ru was about (2.7 ± 0.9)·10−4, and the minimum 106Ru activity concentration associated with a 103Ru detection was about 4 mBq·m−3 (SI Appendix, Table S4). Several organizations in Europe analyzed the filters for the occurrence of other γ-emitters, as well as difficult-to-measure radionuclides, such as Pu, Am, Cm, or 90Sr by low-level radiochemical analyses. No unusual traces were found that would have been indicative of a release of any of these radionuclides concurrently with the 103,106Ru. This excludes an accidental release from a nuclear reactor as the source, as this would have resulted in an emission of a great multitude of fission products. Instead, the origin of 103,106Ru is rather associated with nuclear fuel reprocessing or with (medical or technical) radioactive sources. In addition, no unusual (stable) element contamination was found on a 106Ru-containing filter from Vienna (Austria) (16).

Discussion of a Possible Source Melting. Melting of radioactive sources already occurred in the past, leading to detection of radionuclides in several European countries. Indeed, at the end of May 1998, a 137Cs source estimated 0.3 to 3 TBq was incidentally melted in a steelworks near Algeciras (Spain) and led to detections in several European countries (17). 106Ru in ophthalmic radiotherapy sources have typical activities less than 10 MBq, which is clearly insufficient to explain the observed concentrations on a wide scale, as emphasized by the IAEA (18), as it would have required the melting of numerous ophthalmic sources at once.

Discussion of a Possible Satellite Reentry. The possibility of the disintegration of a satellite equipped with a radioisotope thermoelectric generator (RTG) operated with a 106Ru source during its reentry into the atmosphere, as vaguely indicated previously (19), warrants investigation. Generally, such a source appears rather unlikely because of the rather short half-life of 106Ru compared with the expected or desired satellite life span and the low power level (∼33 W·g−1) generated by a 106Ru-powered RTG and radiation protection issues during its manufacture and handling. In addition, several space organizations concluded that no satellite went missing during the 106Ru episode (SI Appendix). Other arguments are also not in favor of the satellite disintegration hypothesis. If a satellite had burned during its reentry into the atmosphere, it would have caused a vertical distribution of 106Ru in the air: the higher the altitude, the higher the concentration. However, 106Ru at high-altitude locations was either below LOD or significantly lower than 106Ru registered above LOD at low altitude (Table 1). The very low level (below LOD) at the station on Zugspitze mountain (Germany) is also very indicative of low concentrations at high altitude. In addition, the levels of 7Be (a cosmogenic radionuclide produced in the upper troposphere/lower stratosphere and used as a tracer of atmospheric movement) remained close to usual range, thus indicating that no downdraft from the lower stratosphere or upper troposphere occurred at that time. Therefore, the 106Ru release has likely happened in the lower tropospheric layers and cannot be linked to a satellite disintegration. Moreover, the concomitant detections of infinitesimal traces of 103Ru and traces of 106Ru at some locations definitely outmoded the satellite reentry hypothesis due to the short 103Ru half-life (T 1/2 = 39.3 d). Table 1. Comparison of airborne 106Ru at high-altitude and at closest low-altitude sampling locations (with similar time stamps)

Sequence of Airborne 106Ru Detections in Europe. Most early 106Ru detections occurred on the aerosol filters sampled during week 39 (September 25 to October 2, 2017), regardless of the location in Europe, including Russia (September 26 to October 1, 2017). In Ukraine, 106Ru was first detected in the sampling period from September 22 to September 29, 2017. Within the framework of national monitoring programs of airborne radionuclides, most aerosol samplings in Europe are performed either on a weekly basis for γ-counting or on a daily basis (in the vicinity of nuclear facilities), first for the purpose of gross β-counting by plant operators, or for γ-counting, for example, within the IMS network. Aerosol filters sampled in Romania were acquired with the highest temporal resolution (down to 5 h plus 1-h shutdown intervals). After sampling, filters of 2 or 4 sequences were compiled before measurement, thus reflecting 10-h sampling of 12 to 20 h of 24 for each composite sample. In addition, the Romanian network consists of several tens of aerosol sampling stations, which made it possible to reconstruct the pattern of the 106Ru plume. The duration of the episode proved to be rather short: at more than 30 Romanian sampling locations that detected 106Ru, this radionuclide was detectable on exclusively 1 d (30% of sampling locations), on 2 consecutive days (45%) and on 3 consecutive days (25%). Detections over 2 or 3 consecutive days indicate that the plume presence in Romania was rather short and characterized by a narrow peak (Fig. 2). Fig. 2. Airborne 106Ru concentrations (mBq·m−3) at Romanian locations (values have been attributed to the midsampling date of the composite samples). The connecting lines between data points are only meant to guide the eye. The shapes of the Romanian time series match with a short release (i.e., typically less than 1 d), subject to the wind direction did not vary a lot during transport and that the plume border did not undulate while passing at the sampling locations. The detection pattern also provides vague distance-related information on the release point of 106Ru, as multiple Romanian stations detected the plume simultaneously. This is only possible if the plume originated at a sufficiently remote release point to have time to widen to the width of Romania (approximately 600 km) (Fig. 3). Although highest activity concentrations of this 106Ru episode (>100 mBq·m−3) have been reported for Romania, the width of the plume supports excluding a release point on the Romanian territory. Fig. 3. Daily maps of above-LOD airborne 106Ru (red dots) in Romania from September 28 to October 5, 2017. Gray dots indicate sampling locations with 106Ru levels below the respective limits of detection at the given time. All eastern Romanian stations reported airborne 106Ru on September 29, 2017. From September 30, 2017, the general trend indicated a 106Ru front traveling westwards. Peak values were noticed between September 29 and October 1, 2017, depending on the location. On October 1, 2017, eastern stations ceased their detections. From October 4, 2017, no more detection was reported from Romania. In Bulgaria, the 106Ru plume was also assumed to be present only 3 to 4 d (mostly from October 2 to October 4, 2017) (20), about 3 d in Austria and Czech Republic, and over 4 d in Hungary (21). These observations clearly confirm both the shortness of the plume length and the eastern origin of the plume. The discussion of the plume duration exemplifies that in many cases, the sampling duration was longer than the plume duration (21). As a result, a significant fraction of uncontaminated air was pumped through the filter, thus “diluting” the 106Ru activity concentration in most cases. To encompass the entire plume duration regardless of the location, we chose a 7-d integration period. In practice, this decreases the average airborne 106Ru concentration for locations where the 106Ru plume was detected over a period of <7 d (because the absolute amount of 106Ru was mathematically “diluted” with more clean air), while it consequently increases the 106Ru concentration for sampling periods >7 d (as the plume is mathematically “concentrated” with a smaller amount of air) (Fig. 4). This mathematical unification of the sampling periods lets that the corrected values obtained in Romania no longer stand out as the highest, while it can be observed that they are in the same range from the Urals to southern-central Europe as a result of the conservation of the absolute amount of 106Ru transported along the route of the air masses. Fig. 4. (Left) Map of uncorrected average concentrations at European stations, and (Right) map of 7-d corrected average concentrations (based on average plume duration of 7 d at each location). At the Romanian laboratory of Zimnicea—that is, the location with the highest uncorrected value (176 ± 18 mBq·m−3, detected on September 30, 2017 between 3 AM and 2 PM local time)—a Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT) back-trajectory analysis shows that air masses came from Russia and then traveled across Ukraine (Fig. 5). Fig. 5. HYSPLIT-based 240-h backward trajectories ending at the Romanian monitoring station in Zimnicea (black star) (43.666 N, 25.666 E), every 3 h on September 30, 2017, from 2 AM to 11 PM UTC. The National Oceanic and Atmospheric Administration (NOAA) HYSPLIT model uses Global Data Assimilation System (GDAS) meteorological data. The Model Vertical Velocity was used as vertical motion calculation method. The green circle indicates the position of the Mayak industrial complex. The altitude of the air parcels is given in meters above ground level (AGL). The green circle in the altitude sections of the trajectories ending at 3 PM and 5 PM UTC (maps surrounded by red frames), respectively, indicates the time and altitude (approximately 500 m) the air parcels were in closest proximity to the Mayak area. The trajectory model suggests that air masses arriving at Zimnicea on September 30, 2017 had previously traveled close to the Mayak industrial complex around September 25, 2017 at an altitude of at most about 500 m. According to Roshydromet (2), the meteorological situation in the Southern Urals and central part of European Russia during the period of September 25 to October 6, 2017 was due to a vast anticyclone centered around the White Sea (south of the Kola Peninsula) practically merging with an anticyclone in the central part of Western Siberia. Their report (2) reads, “As a result, conditions for an active transfer of air masses and pollutants from the territory of the Southern Urals and southern Siberia to the Mediterranean region and, then, to the north of Europe, arose in the southern part of Western Siberia, in the Southern Urals, in the Caspian lowland and Ciscaucasia.” Among the different every 3-h back-trajectories, only 2 passed very close to the Mayak industrial complex. The detection of the 106Ru plume at Zimnicea on September 30, 2017 hence indicates a release from Mayak would have occurred between September 25, 2017, around 6 PM coordinated universal time (UTC) and September 26, 2017, around noon (UTC) (Fig. 5). The Mayak Production Association was one of the first and largest nuclear facilities of the former Soviet Union and spearheaded the Soviet nuclear weapons program. In the Cold War era, it hosted a total of 10 nuclear reactors, mainly for the production of weapons-grade plutonium. In 2014, the Mayak complex employed ∼12,000 people and hosted 2 reactors for isotope production, storage facilities for nuclear materials, and a nuclear fuel reprocessing facility (22). On September 29, 1957, a chemical explosion took place in a radioactive waste storage tank at the Mayak nuclear complex, causing a massive release of radionuclides. The accident became known as the “Kyshtym accident.” In the course of this accident, about 2,700 TBq of 106Ru (together with various other radionuclides) were released into the environment, causing a significant contamination in a more than 100-km-long strip that has been termed the East Urals Radioactive Trace (23). More than 10,000 residents had to be evacuated (24). The Kyshtym accident was retrospectively rated at level 6 on the International Nuclear and Radiological Event Scale. Although such incidents have become rare events in recent years, 106Ru was released from nuclear reprocessing facilities in the past on multiple occasions. On September 26, 1973, following an exothermic reaction at the Windscale reprocessing plant (United Kingdom), 35 workers were contaminated through an atmospheric release of 106Ru estimated at 0.37 TBq (25). On April 6, 1993, an explosion at the reprocessing plant of the Tomsk-7 nuclear complex (Siberia, Russian Federation) led to the release of approximately 0.52 TBq of 106Ru among other fission products and actinides (26, 27). About 200 km2 were contaminated. On May 18, 2001 and October 31, 2001, a failure in the vitrification shops at the La Hague reprocessing plant (France) led to an atmospheric release of 106Ru. Based on aerosols sampled at 200 km downwind from the stack and grass sampled in the vicinity, the first release was estimated between 0.005 and 0.05 TBq, while the second was estimated to range between 0.0005 and 0.02 TBq (28). Significant atmospheric releases also occurred from the early Hanford operations that were linked to United States nuclear weapons production, with 106Ru (14 TBq from 1944 to 1972) being a relatively minor constituent (compared with 2.7 EBq 131I in the same time span) (29). For comparison, the present, undeclared accident released an estimated activity of 250 TBq at once.

Ruthenium Deposition across Europe. Besides airborne activity determinations, several rainwater, plant, and soil samples attested the deposition of 106Ru across Europe (SI Appendix, Tables S3 and S4). Most deposition arose from rain events that occurred between the last week of September 2017 and the first week of October, as for example at several Scandinavian sampling locations (up to about 50 Bq·m−2 in Sweden and about 50–90 Bq·m−2 in Finland) or, for example, in Greece in the second week of October (30). In Poland (up to about 80 Bq·m−2), a washout ratio ([106Ru rain ]/[106Ru air ]) of at least 4,900 was found. In central Europe, fallout deposition reached 5 Bq·m−2 in Vienna (Austria) between October 3 and October 5, 2017; 40 Bq·m−2 in Ostrava (Czech Republic) from October 2 to October 3, 2017; and 8 Bq·m−2 in Udine (northeastern Italy) the last week of September 2017 and the first week of October (SI Appendix, Tables S3 and S4). The majority of the highest surface-deposition records have been reported from locations within 20 km from the Mayak complex: Khudaiberdinskiy, Argayash, Novogorny, and Metlino (31, 32), where the surface deposition reached up to 343 Bq·m−2. However, the sole accumulation of positive reports from the vicinity of the Mayak facilities, by itself, is not a conclusive indication of the source, as a nuclear facility naturally is more densely monitored than nonnuclear areas. Depending on the official Russian source, levels are highly variable: up to a factor of 10 that can arise from the deposition pattern. For the purpose of clarifying the situation, a soil-sampling campaign was conducted by the French Commission for Independent Research and Information on Radioactivity (CRIIRAD) nongovernmental organization (33) around the Mayak facilities in December 2017 at a closest authorized distance of about 16 km. Among the 8 soils sampled in various directions, only the 1 sampled west-southwest from Mayak indicated a 106Ru deposited activity estimated between 580 and 1,200 Bq·m−2. Inasmuch as abnormal, again, this sole result is not sufficient to clearly demonstrate whether or not the 106Ru originated from Mayak, as deposition levels are not as high as one might expect from a major release. However, the weak soil-sampling density may also be a good reason for the plume deposition to escape from the investigation grid. Moreover, the atmospheric behavior (e.g., the transfer kinetics from volatile RuO 4 into particulate RuO 2 ) and deposition of 106Ru are not well understood, especially when released in its volatile form of RuO 4 . At a longer distance (530 km in Bugulma, Russian Federation) in the same direction, deposition of 106Ru (noticed only on samples collected on September 26 to September 27, 2017 [11.3 Bq·m−2] and September 27 to September 28, 2017 [30 Bq·m−2]) matches with the hypothesis of a release from the Mayak industrial complex on September 25, 2017, which, therefore, has to be considered as a possible candidate for the source of the release of 106Ru. A detailed dispersion analysis using inverse dispersion modeling techniques and field observations using the data from the present study (airborne concentration and deposition) was conducted to assess both the source location and the source term. This modeling work also suggests that a hot-spot of 106Ru deposition occurred in southeastern Bulgaria. Accordingly, pine needles, oak leaves, forest litter, grass, and soil samples from this area confirm that 106Ru deposition originating from a release on September 26, 2017 from the Mayak area was prone to produce these depositions in combination with rain events.* The 106Ru-deposited activity in plants sampled in the southeast area of Bulgaria was up to several tens Bq·kg−1, whereas they remained in the millibecquerel per kilogram (mBq·kg−1) range in the western part of the country where no rain occurred, while the variability of 106Ru volume activity in the atmosphere remained somewhat limited across the country. Other 106Ru detections occurred in early 2018 (until March) in fallout and rainwater samples (in Norway, Poland, Slovenia), and even in March 2019 (Poland), but they were assumed to be induced by the resuspension of previously contaminated soil particles, indicating that 106Ru had not yet completely migrated from the topsoil layer.