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In 2017, a cloud of ruthenium-106 particles set off radiation detectors across Europe. Although scientists gradually honed in on the geographic source, attempts to confirm it have been frustrated. Careful analysis of the cloud's composition has now indicated the source was a civilian site rather than a military one.
In Central and Western Europe, the late 2017 radiation spike was not high enough above background levels to pose a threat to human life. Closer to the source, however, things could have been quite different, sparking a hurried search to identify the origin of the emissions. Equally important is the question of what triggered the release, lest something larger occur in the future.
From the start, suspicion turned to Russia based on the general direction and a poor nuclear safety record. Subsequent research narrowed the location down to the southern Urals. The refusal of Russian authorities to acknowledge responsibility, let alone reveal the source, inspired speculation that the origins might lie in a secret military facility.
However, Professor Thorsten Kleine of the University of Münster thinks that is extremely unlikely. Although radioactive ruthenium-106 drew attention, non-radioactive ruthenium isotopes were also detected, which Klein realized could be the key to identifying the element's source.
"We usually measure ruthenium isotopes to study the formation history of Earth,” Kleine said in a statement. This has given Kleine and colleagues experience in studying samples with tiny isotope concentrations, a skill that came in handy given the low quantities of ruthenium available from the Austrian measuring stations whose filters they could access.
In Nature Communications, Klein reports seven ruthenium isotopes were captured by these filters. Of these, only two are radioactive (Ruthenium-106 and Ruthenium-103, with half-lives of 372 and 39 days respectively).
The ratios of the stable ruthenium isotopes are consistent with a fuel cycle with a high plutonium content, which, surprising as it may seem, actually indicates a civilian site. Leaks from military sites, such as in groundwater around the decommissioned Hanford production complex, have less ruthenium-100 and 102, compared to ruthenium-101 because their fissile source is uranium-235. More specifically, the ratios are what would be expected from reprocessing fuel from a VVER reactor, a design most popular in Eastern Europe.
This points the finger squarely at a facility in Mayak that reprocesses Russian nuclear fuel, including from VVER-440 plants. The site was already considered the prime suspect, probably in a failed attempt to produce cerium-144. The Russian Academy of Sciences continues to reject this theory, however. The paper quotes the Academy as saying: “If the Mayak facility [were] the source, then we would have found concentrations hundreds of thousands of times the norm around it and in the soil.”
At the Austrian sites, radiation never came close to dangerous levels. However, if Kleine is right, the area around Mayak could be a different matter, making denial a threat to all those who work there.
