TECHNOLOGICALLY ENHANCED NATURAL RADIOACTIVITY OF FLY-ASH PRODUCED IN LIGNITE-FIRED POWER PLANTS AND THE ASSOCIATED RADIOLOGICAL IMPACT

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 highly 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 may, in some cases, exceed the 1kBqkg-1 level, which is 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 4kBkg-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, enrichment factors of natural radionuclides in the ashes and radon exhalation measurements from fly-ash collected at different stages along the emission control system. Enrichment factor results are being interpreted in relation to the physical properties of the investigated nuclides and the temperature and flow parameters in the flue gas pathway. Discarded ash deposition fields in the vicinity of the power plants are also investigated in terms of surface soil radon exhalation and soil gas radon concentration.