210Pb and 238U in Estonian fuel products and ashes

Concentrations of 210Pb and 238U(234Th) in local fuels (oil shale, wood and wood-peat mixtures) and in their combustion products were determined using low-background low-energy HPGe gamma spectrometry. The samples were collected from a few operating power plants. In the spectrum analysis, self-attenuation corrections were modelled for each sample. The model took into account the measurement geometry and the determined attenuation coefficients of the sample. An analysis of the radionuclide balance in the combustion process of oil shale showed that approximately 30% of 210Pb became volatile and its behaviour differed significantly from that of other natural radionuclides, e.g. 238U. While depleted in bottom ash in comparison to 238U, 210Pb showed a considerable enrichment (up to 120 Bq kg-1 or 7 times relative to fuel) in the electric filter ash fractions. Near the power plants enhanced 210Pb concentrations were observed in the surface soil layers, as a result of atmospheric dispersion and deposition of oil shale fly-ash. Even higher enrichment factors (up to 20) were found in ashes of local bio-fuels (wood, wood-peat mixtures), where the 210Pb concentrations reached values of 270 Bq kg-1. Annual population doses in the range of 20 .. 60 μSv a-1 caused by technologically dispersed radionuclides were evaluated using a compartmental modelling method near the oil-shale-fired power plants. (au)