Detection of radioactivity of unknown origin: Protective actions based on inverse modelling.

The detection of radioactivity of unknown origin necessitates the use of models that can quantify unknown corresponding source term parameters. In this work, a method for solving this inverse problem is described. The main goal of the method is that it can be used in emergency response. Therefore, the full modelling chain dealing with the collection and pre-processing of measurement data, source term estimation, (forward) dispersion modelling, and consequence assessment are discussed. Firstly, to verify this inverse model SHERLOC, the part of the modelling chain concerning the source term estimation based on measurement data, is applied to the first episode of the European Tracer Experiment (ETEX). Secondly, the complete model chain is applied to a release that is still unaccounted for; the Ru measured in the atmosphere of Europe in September and October of 2017. It is estimated that during the night of the 25th to the 26th of September 2017 approximately 1.33 PBq (1.33×10 Bq) of Ru was emitted at a location in the region of the Southern Urals in the Russian Federation. Statistical indicators show that the modelled levels of concentration are in good agreement with the measurements. The radiological consequences of the release are estimated to be minor at distances farther than 22 km from the estimated source. However, in the vicinity of the emission the maximum committed dose received by the public may have exceeded 100 mSv. Since the presented approach can be executed within few hours after the collection of measurement data it can be used in the emergency response following the detection of radioactivity of unknown origin.