The Chernobyl nuclear accident 137Cs cumulative depositions simulated by means of the CALMET/CALPUFF modelling system

The widely used dispersion modelling system CALMET/CALPUFF has been applied in order to evaluate its ability to simulate dry and wet depositions at regional scales (up to 1000 km from a source) in the specific case of radionuclides released in the atmosphere, during the 1986 Chernobyl Nuclear Power Plant accident. The 137Cs cumulative deposition data sampled at 410 sites on the entire territory of Ukraine after the accident have been used for model verification. As meteorological input for feeding the CALMET pre-processor, we used a dataset of time series recorded in 211 surface stations, 194 precipitation stations and 14 upper air stations. Two different schemes for the emissions source have been adopted both available from scientific literature on pollutants release during the Chernobyl accident. This work shows that the CALMET/CALPUFF system is able to reproduce the large-scale features of the measured 137Cs deposition pattern, which are the main traces on the territory of Ukraine. However, the fine structure of depositions, which are mainly due to precipitations, are poorly caught. The simulated deposition pattern appears excessively smoothed and an explanation for that is provided. Besides, we have found that the resistant model for dry deposition velocity of 137Cs aerosol particles significantly underestimates depositions and the closest agreement with measurements is achieved with constant deposition velocity of 0.005 m/s. Finally, the strong dependence of the simulated contamination pattern on the emission source parameterization is confirmed.