There was a nuclear accident in Mayak

Cloud radioactive ruthenium in autumn 2017 came from Russian nuclear plant

The Russian reprocessing plant Mayak has already repeatedly led to the release of radioactivity. In the fall of 2017, it could have been the source of a cloud of radioactive ruthenium that swept across Europe. © Ecodefense / Heinrich Boell Foundation Russia / Slapovskaya / Nikulina
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Source limited: In the fall of 2017, a cloud of radioactive ruthenium-106 swept across Europe. Now researchers have specified the source of this contamination. The Ruthenium probably comes from the Russian reprocessing plant Mayak - which Russia has denied so far. New analyzes now confirm the suspicion. The ruthenium must therefore have been released in the processing of relatively fresh fuel rods.

At the end of September 2017, measuring stations in many European countries raised the alarm: they registered radioactive ruthenium-106 in the air. This radionuclide with a half-life of 372 days does not occur naturally in the atmosphere, but it can be released in the event of a nuclear accident or a nuclear bomb blast. In autumn 2017, the concentrations of ruthenium-106 at some measuring points briefly increased to values ​​of up to 176 millibecquerels per cubic meter of air - not a harmful value, but since Chernobyl the highest ever measured over Europe.

Ruthenium concentrations at various European measuring points. © Masson et al. / PNAS

Nobody wanted to be

But what was the source of this radioactive ruthenium cloud? "Nobody reported back then about a nuclear accident and no member state of the International Atomic Energy Agency (IAEA) admitted to know anything about a possible source, " report Georg Steinhauser from the University of Hannover and his colleagues. Even then, scientists suspected that the source of ruthenium might be somewhere in the southern Urals due to the distribution of readings, possibly in the Russian Mayak reprocessing plant.

"Russian officials said at the time that Mayak could not be the source because they did not find any radio-ruthenium traces in the ground around the plant, " Steinhauser and his team said. "Instead, the officials pointed out that even the radionuclide battery of a satellite as it enters the atmosphere could be the source."

To get to the root of the ruthenium cloud, researchers have since analyzed more than 1, 300 readings from 179 monitoring stations across Europe. display

A satellite reactor was not

The result: contrary to Russian declarations, the radioactive ruthenium does not come from a satellite. "If a satellite were to collapse on reentry, this would have resulted in a vertical distribution of ruthenium-106 in the air: the greater the altitude, the greater the higher the concentration ", explain Steinhauser and his team. However, measuring stations in higher altitudes, such as on the Zugspitze, registered only low values.

According to the researchers, the combustion of a medical or technical source of radionuclides is also not the cause, given the amount released. According to their calculations, around 250 terabecquerels must have been released at the source at one go. This is more than two orders of magnitude above that possible from such sources. Even breakdowns at windscale recycling plants in the UK and La Hague in France released only about 0.37 terabecquerel or 0.05 terabquerel of ruthenium-106, the researchers report.

No nuclear reactor, but a reprocessing plant

To gain more information about the possible source, the scientists also looked for other radionuclides in the radioactive air samples. If an accident in a nuclear power plant were the cause, traces of other fission products such as americium, ciumium or strontium in the air would have to be detected in addition to ruthenium-106. But that was not the case, as the scientists report.

"This excludes the accidental release from a nuclear reactor as a source, because this would have led to the emission of a large number of split production, " Steinhauser and his team state. Instead, the measurements suggest that ruthenium was released during the reprocessing of nuclear fuel.

From the southern Urals

But where? The reassessment confirms that the radioactive ruthenium cloud was blown from the area of ​​the southern Ural over Europe. One indication of this is the fact that monitoring stations in Romania detected the highest and earliest contaminations, the researchers report. From the weather conditions at the end of September 2017, they conclude that the contaminated air masses had previously crossed the area of ​​the Russian reprocessing plant of Mayak.

"Detection of the ruthenium cloud in Zimnicea, Romania, on September 30, 2017, indicates a Mayak release between September 25 at 6:00 pm and September 26 at noon, " Steinhauser and his team report. Mayak soil samples taken by a French team in December 2017, contrary to what the Russian authorities said, indicate increased ruthenium contamination west of the plant.

Ruthenium from young fuel rods

And there is one more indication: from the share of a second ruthenium isotope, the ruthenium-103, in the contaminated air, the researchers were able to draw conclusions about how old the fuel rods were at the time of the accident have to be. Accordingly, the radionuclide comes from relatively young Brennst ben, which must have been active in a reactor core two years earlier. At the same time, however, ruthenium was released at the end of the reprocessing process.

But that means: The fuel rods were processed after an unusually short cooldown. "Western reprocessing plants like La Hague will not start processing for at least four or even ten years, " Steinhauser and his team report. Such a short wait could indicate that the highest possible yield of Cerium-144 was available. Cerium-144, however, is a radionuclide known to have been supplied by Mayak to the Borexino Neutrino Detector in the Gran Sasso Laboratory.

Released in the production of Cerium-144?

Remarkable also: "The order for the delivery of Cerium-144 was canceled by the plant in Mayak, shortly after the release of ruthenium had attracted attention, " the researchers report. Although their analyzes can not clearly demonstrate the relationship between ruthenium release and Cerium-144 production in Mayak. However, Steinhauser and his team consider it quite likely that an accident in this process has led to the release of the radioactive cloud from Mayak.

Although the contamination was not harmful to the health of the European population, it was significant: "The measurements suggest that this was the largest single measured release of radioactivity from a civilian facility, " says Steinhauser. "And even if there is no official statement, we have a pretty good idea of ​​what might have happened." (Proceedings of the National Academy of Sciences, 2019; doi: 10.1073 / pnas.1907571116)

Source: PNAS, Vienna University of Technology

- Nadja Podbregar