The Ruthenium-106 event, September-October 2017: A cold case

In late September 2017, traces of ¹⁰⁶Ru were recorded by air sampling stations in large parts of Europe. In some regions, over 100 mBq/m³ were measured as one-day means. Although resulting exposure was far below radiological concern, and, in addition, below detectability with the EURDEP dose rate network [1], the event aroused considerable interest, because this radionuclide is found only very rarely in environmental media. µThe main exposure pathway was through inhalation, leading to up to about 0.3 µSv regionally. Other pathways are negligible in comparison. The geographical distribution of inhalation dose, inferred from monitoring results, is Two years later, the origin of the Ru is still a mystery. The Ru found in 2017 was not associated with other radionuclides except minute traces of ¹⁰³Ru. Therefore, a reactor accident can be excluded. Meteorological and dispersion calculations performed by several institutes unanimously led to a plausible region of origin in the Southern to Northern Ural region. However, Russian authorities denied any releases by civilian nuclear installations in the region. In this presentation, we focus on summarizing dose calculations and the geographical distribution of dose. We also review documented accidents and incidents involving radioactive ruthenium. Finally, we report on talks with Russian authorities about the incident, and speculate about circumstances of the release and its likely origin as argumentum e silentio. Although the incident has happened about 2 half lives of ¹⁰⁶Ru ago (376 days), we think that with today's possibilities of radiometric monitoring, and appropriate monitoring scheme, it should still be detectable in the environment in the trace of the plume not too far from the point of emission