Investigations on the source term of the May 2023 detection event: the most comprehensive one over the last decade in Northern Europe

The detection of traces of radionuclides in Northern Europe has become more frequent in recent years, although the origin of these emissions has not been officially confirmed by any authorities. IRSN has undertaken investigations to determine the source of the past detections and to understand the origin and the mechanisms that may have led to these releases into the environment (J.J. Ingremeau, O. Saunier, Investigations on the source term of the detection of radionuclides in North of Europe in June 2020, EPJ Nuclear Sci. Technol. 8, 10 (2022), O. Saunier, J.J. Ingremeau, I. Homan, P. Mekarski, J. Yi, A. Botti, Methodology for the investigation of undeclared atmospheric releases of radionuclides: Application to recent radionuclide detections in Northern Europe from 2019 to 2022, Ann. Nucl. Energy 192, 109907 (2023)). Recently, in May 2023, a new detection event was recorded in the same part of Europe, but this is the first time that such a high number of isotopes has been reported. This paper presents the analysis carried out by IRSN to identify the origin of this new release, the most comprehensive one in the last ten years. Using inverse atmospheric dispersion modelling methods, the most likely geographical origin was identified between Estonia and the western part of the Russian Federation, in line with previous releases. The key feature of this event is the simultaneous detection of 46 Sc alongside low volatile fission products and actinides, which prompted further investigations. About the origin of 46 Sc, it has been shown that it is an activated corrosion product specifically produced in WWER reactors. Although there is no certainty with so few data, this finding reinforces the interpretation from previous studies assuming the release is likely to have originated from a spent primary ion exchange resin of a WWER reactor, possibly involving a fuel cladding failure leading to fuel dispersion within the primary circuit. Finally, a scenario is proposed to explain the atmospheric release which is consistent with all available detection data. However, this scenario is based on highly unlikely assumptions and remains speculative.