Inverse modeling of the Chernobyl source term using atmospheric concentration and deposition measurements

Abstract. This paper describes the results of an inverse modeling 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 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 modeling tools were further developed, inverse modeling 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, which are essential for inverse modeling, 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 data set. These observations refer to a data rescue attempt that started more than 10 years ago, with a final goal to provide available measurements to anyone interested. In regards to our inverse modeling 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, on the same order as 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 of the present inversion were confirmed using an independent Eulerian 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 setups of the inversion through sensitivity runs. The source term data from this study are publicly available.