Bayesian inverse modeling and source location of an unintended I-131 release in Europe in the fall of 2011
Ondřej Tichý1, Václav Šmídl1, Radek Hofman1, Kateřina Šindelářová1, Miroslav Hýža2, and Andreas Stohl31Institute of Information Theory and Automation, Czech Academy of Sciences, Prague, Czech Republic 2National Radiation Protection Institute, Prague, Czech Republic 3NILU: Norwegian Institute for Air Research, Kjeller, Norway
Received: 05 Mar 2017 – Accepted for review: 14 Mar 2017 – Discussion started: 16 Mar 2017
Abstract. In the fall of 2011, iodine-131 (I-131) was detected at several radionuclide monitoring stations in Central Europe. After investigation, the International Atomic Energy Agency (IAEA) was informed by Hungarian authorities that I-131 was released from the Institute of Isotopes Ltd in Budapest, Hungary. It was reported that a total activity of 342 GBq of I-131 was emitted between September 8 and November 16, 2011. In this study, we use the ambient concentration measurements of I-131 to determine the location of the release as well as its magnitude and temporal variation. Although the location of the release became eventually known, its temporal variation is still uncertain and only partial information is available. For our source reconstruction, we use no prior knowledge. Instead, we estimate the source location and emission variation using only the available I-131 measurements. Subsequently, we use the information about the source term 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) weather forecast model. Second, we use a recently developed inverse method, least-squares with adaptive prior covariance (LS-APC), to determine the I-131 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 forward calculation 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, while our estimate for the total released activity (490 GBq) is higher than the reported one (342 GBq). Nevertheless, even using our larger source term, dose amounts were very low and never exceeded regulatory limits.
Tichý, O., Šmídl, V., Hofman, R., Šindelářová, K., Hýža, M., and Stohl, A.: Bayesian inverse modeling and source location of an unintended I-131 release in Europe in the fall of 2011, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-206, in review, 2017.