Abstract
Abstract. In the fall of 2011, iodine-131 (131I) was detected at several radionuclide monitoring stations in central Europe. After investigation, the International Atomic Energy Agency (IAEA) was informed by Hungarian authorities that 131I was released from the Institute of Isotopes Ltd. in Budapest, Hungary. It was reported that a total activity of 342 GBq of 131I was emitted between 8 September and 16 November 2011. In this study, we use the ambient concentration measurements of 131I to determine the location of the release as well as its magnitude and temporal variation. As the location of the release and an estimate of the source strength became eventually known, this accident represents a realistic test case for inversion models. For our source reconstruction, we use no prior knowledge. Instead, we estimate the source location and emission variation using only the available 131I measurements. Subsequently, we use the partial information about the source term available from the Hungarian authorities for validation of our results. For the source determination, we first perform backward runs of atmospheric transport models and obtain source-receptor sensitivity (SRS) matrices for each grid cell of our study domain. We use two dispersion models, FLEXPART and Hysplit, driven with meteorological analysis data from the global forecast system (GFS) and from European Centre for Medium-range Weather Forecasts (ECMWF) weather forecast models. Second, we use a recently developed inverse method, least-squares with adaptive prior covariance (LS-APC), to determine the 131I emissions and their temporal variation from the measurements and computed SRS matrices. For each grid cell of our simulation domain, we evaluate the probability that the release was generated in that cell using Bayesian model selection. The model selection procedure also provides information about the most suitable dispersion model for the source term reconstruction. Third, we select the most probable location of the release with its associated source term and perform a forward model simulation to study the consequences of the iodine release. Results of these procedures are compared with the known release location and reported information about its time variation. We find that our algorithm could successfully locate the actual release site. The estimated release period is also in agreement with the values reported by IAEA and the reported total released activity of 342 GBq is within the 99 % confidence interval of the posterior distribution of our most likely model.
Highlights
In the fall of 2011, 131I was detected in the atmosphere by the European Trace Survey Stations Network for Monitoring Airborne Radioactivity (Ring of 5, Ro5)
We evaluate the probability of the source-receptor sensitivity (SRS) matrices obtained using backward runs of the dispersion models FLEXPART (Stohl et al, 2005) and Hysplit (Draxler and Hess, 1997), which were based on meteorological input data from global forecast system (GFS) meteorological fields with resolution of 0.5◦ × 0.5◦ in the case of FLEXPART and from GFS meteorological fields with resolutions of 0.5◦ × 0.5◦ and 1◦ × 1◦ and European Centre for Medium-range Weather Forecasts (ECMWF) meteorological fields with a resolution of 0.5◦ × 0.5◦ in the case of HYSPLIT
We apply the Bayesian inverse modeling method introduced in Sect. 3 to iodine measurements described in Sect. 2 and computed SRS matrices from Sect. 4 for all four cases: (i) FLEXPART driven with the GFS analyses with the resolution 0.5◦ × 0.5◦ (Flexpart-GFS-0.5), (ii) HYSPLIT driven with the GFS analyses with the resolution 0.5◦ × 0.5◦ (Hysplit-GFS-0.5), (iii) HYSPLIT driven with the GFS analyses with the resolution 1◦ × 1◦ (HysplitGFS-1.0), and (iv) HYSPLIT driven with the ECMWF analyses with resolution 0.5◦ × 0.5◦ (Hysplit-ECMWF-0.5)
Summary
In the fall of 2011, 131I was detected in the atmosphere by the European Trace Survey Stations Network for Monitoring Airborne Radioactivity (Ring of 5, Ro5). The measured values were very low, up to a few tens of μBq m−3, close to the minimum detectable activity of the instruments. After the first findings in Austria and their subsequent confirmation by Czech laboratories, it was clear that these detections could not be explained by local sources. The International Atomic Energy Agency (IAEA) was informed on November 11 and launched an investigation. Detectable concentrations of 131I were afterwards measured by other laboratories, mainly in central Europe (International Atomic Energy Agency, 2011a). Based on the information provided by other Ro5 laboratories and a rough
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