Releases of natural radionuclides from oil-shale-fired power plants in Estonia

In the vicinity of two large oil-shale-fired power plants in northeast Estonia, depth-dependent activity concentrations of natural radionuclides in soil were determined by gamma spectrometry. In the surface soil considerably higher (or lower) concentrations of 40K, 226Ra and 232Th were found than in deeper soil layers. The observed increase or decrease of the enrichment of radionuclides for different sampling sites was dependent on the relative concentrations of radionuclides in fly-ash and in deep soil layers. The fraction of the radionuclides deposited onto the ground was characterized by a mean 226Ra/232Th activity concentration ratio of 2.2, approximately equal to the one (2.1) found for oil-shale filter ash. The atmospheric deposition rates of fly-ash radionuclides onto the ground were estimated and compared to other relevant published data. The migration of the deposited fly-ash radionuclides into soil was satisfactorily described assuming an exponential depth distribution with the relaxation length value, α−1 = 2.9 ± 0.6 cm, for both 226Ra and 232Th.