Inverse modelling of the Chernobyl source term using atmospheric concentration and deposition measurements
Nikolaos Evangeliou1, Thomas Hamburger1,a, Anne Cozic2, Yves Balkanski2, and Andreas Stohl11NILU – Norwegian Institute for Air Research, Department of Atmospheric and Climate Research (ATMOS), Kjeller, Norway 2CEA-UVSQ-CNRS UMR 8212, Laboratoire des Sciences du Climat et de l'Environnement (LSCE), Institut Pierre et Simon Laplace, L'Orme des Merisiers, F-91191 Gif-sur-Yvette Cedex, France anow at: BfS – Bundesamt für Strahlenschutz, Section SW 2.2, Decision Support Systems, 14 D-85764, Oberschleißheim, Germany
Abstract. The present paper describes the results of an inverse modelling study for the determination of the source term of the radionuclides 134Cs, 137Cs and 131I released after the Chernobyl accident. The accident occurred on 26 April 1986 in the Former Soviet Union and released about 1019 Bq of radioactive materials that were transported as far away as the USA and Japan. Thereafter, several attempts to assess the real magnitude of the emissions were made that were based on the knowledge of the core inventory and the levels of the spent fuel. More recently, when modelling tools were further developed, inverse modelling techniques were applied to the Chernobyl case for source term quantification. However, because radioactivity is a sensitive topic for the public and attracts a lot of attention, high quality measurements, that are essential for inverse modelling, were not made available except for a few sparse activity concentration measurements far from the source and far from the main direction of the radioactive fallout. For the first time, we apply Bayesian inversion of the Chernobyl source term using not only activity concentrations, but also deposition measurements from the most recent public dataset. These observations refer to a data rescue attempt that started more than 10 years ago, with a final goal to give such kind of measurements into anyone interested. As regards to our inverse modelling results, emissions of 134Cs were estimated to be 80 PBq or 30–50 % higher than what was previously published. From the released amount of 134Cs, about 70 PBq were deposited all over Europe. Similar to 134Cs, emissions of 137Cs were estimated as 86 PBq, in the same order with previously reported results. Finally, 131I emissions of 1365 PBq were found, which are about 10 % less than the prior total releases. The inversion pushes the injection heights of the three radionuclides to higher altitudes (up to about 3 km) than previously assumed (≈ 2.2 km) in order to better match both concentration and deposition observations over Europe. The results were of the present inversion were confirmed using an independent Eulerial model, for which deposition patterns were also improved when using the estimated posterior releases. Although the independent model tends to underestimate deposition in countries that are not in the main direction of the plume, it reproduces country levels of deposition very efficiently. The results were also tested for robustness against different set-ups of the inversion through sensitivity runs. The source term data from this study are made publically available.
Evangeliou, N., Hamburger, T., Cozic, A., Balkanski, Y., and Stohl, A.: Inverse modelling of the Chernobyl source term using atmospheric concentration and deposition measurements, Atmos. Chem. Phys. Discuss., doi:10.5194/acp-2017-330, in review, 2017.