DISTURBANCE OF RADIOACTIVE EQUILIBRIUM IN THE ASHES PRODUCED IN LIGNITE FIRED POWER PLANTS

Coal- and lignite-fired power plants produce significant amounts of ashes, which are quite often being used as additives in cement and other building materials. In many cases, coal and lignite present high concentrations of naturally occurring radionuclides, such as 238U, 226Ra, 210Pb, 232Th and 40K. Moreover, during the combustion process the produced ashes are enriched in the above radionuclides, thus characterized as Technologically Enhanced Naturally Occurring Radioactive Materials (TENORM). The most important naturally occurring radionuclide in such power plant ashes is 226Ra, since it produces radon, which when exhaled contributes significantly to the dose received by workers and the public. An extensive research project for the determination of the natural radioactivity of lignite and ashes from Greek lignite-fired power plants is in progress in the Nuclear Engineering Section of the National Technical University of Athens (NES-NTUA) since 1983. From the results obtained so far, it may be concluded that 226Ra radioactivity of fly-ash in some cases exceeds 1kBqkg-1, which is very high compared to the mean 226Ra radioactivity of surface soils in Greece (25Bqkg-1). Furthermore, the radioactivity of 210Pb in fly-ash may reach the value of up to 4kBqkg?1, depending on the sampling location inside the power plant. This paper presents: natural radioactivity results from the analysis of the lignite fed to and the ashes produced in Greek lignite-fired power plants, and enrichment ratios of natural radionuclides in the ashes collected at different stages along the emission control system of the power plant. Enrichment ratio results are being interpreted in relation to the physical properties of the investigated nuclides and the temperature in the flue gas pathway. The different enrichment of the various nuclides of uranium series along emission control system, results to a disturbance of their radioactive equilibrium, which is more pronounced towards the end of the flue gas pathway. Furthermore, the fly-ash collected in several points along the emission control system of the power plant was investigated, in terms of radon exhalation rate.