Source reconstruction via deposition measurements of an undeclared radiological atmospheric release

Abstract. Inverse modelling of atmospheric releases of radioactivity consists of reconstructing the release source by combining radiological field measurements with atmospheric transport calculations. This is typically performed with air concentration measurements, although deposition measurements or gamma dose rate measurements could also be used. In this paper, we assess the use of deposition measurements of radioactivity in this context. This is done through a case study of the undisclosed release of the radionuclide ¹⁰⁶Ru in Eurasia during the autumn of 2017. The atmospheric transport model we utilise for this purpose is Flexpart. Inverse modelling is performed with the inverse modelling tool FREAR, which has been modified to work with deposition measurements. The inversion consists of Bayesian and cost function based algorithms to reconstruct the initial source properties. Inverse modelling is applied to both real and synthetic deposition data following the ¹⁰⁶Ru release. We also construct synthetic air concentration data for use in inverse modelling, to make a comparison with the results using deposition data. It is found that source localisation is feasible with both the synthetic and real world deposition data. Synthetic air concentration measurements lead to more precise source localisation than deposition. It is demonstrated that this can be explained by the lower detection limits of air concentration measurements compared to deposition.