Natural radionuclides from coal fired thermal power plants – estimation of atmospheric release and inhalation risk

Abstract

Coal, bottom ash and fly ash samples were collected from three different coal-fired thermal power plants in India and subjected to gamma spectrometry analysis for natural radioactivity contents. The results of present study show that fly ash and bottom ash contains two to five times more natural radionuclides than feed coal. None of the fly-ash and bottom ash samples had radium equivalent activities and external hazard index values more than 370Bq kg -1 and unity respectively. However the absorbed dose rate at 1m above the ash pond was 79.19nGyh -1 (average of 3 plants) higher than the global average value of 55nGyh -1 . The corresponding annual external effective dose is estimated to be 0.68mSvy -1 , which is also more than that (0.46mSvy -1 ) in areas of natural background radiation. The 5th percentile, 95th percentile and mean values for total inhalation risk arising from radionuclides (Ra 226 ,T h 228 ,P b 210 and Nat-U) were found to be 3.83 × 10 −9 , 6.50 × 10 −8 and 2.08 × 10 −8 respectively. In India, the rapid increase in thermal generation capacity and at the same time a deterioration of the quality of coal with increased ash content resulted in huge quantity of fly ash and bottom ash of varying properties. The coal contains naturally occurring primordial radionuclide thus the use of coal as a heat source for electric power generation will result in the emission of a variety of natural radioactive elements into the environment. The trace elements in coal that are naturally radioactive are uranium (U), thorium (Th), and their decay products, including radium (Ra) and radon (Rn). Thus coal fired power plant is a major contributor towards the Technologically Enhanced Natural Radiation (TENR) as it results in generation of huge amounts of fly ash and bottom ash containing natural radionuclides. Indian coal used in power plants generally has high ash content and is of lower quality (Mathur et. al., 2003). Presently about 110 million tonnes of coal-ash is generated in India from more than 70 thermal power plants (Sarkar et. al., 2005). By the year 2012 this is predicted to increase to 170 million tonnes per annum (Rajamane, 2003). Thus, fly ash and bottom ash are significant sources of exposure to the naturally occurring radionuclides that affect especially the population in the vicinity of industrial areas. In India, the major portion of fly ash produced goes for disposal in ash ponds and landfills; and a small fraction of it is utilized in construction material (taking advantage of its cementitious or binding characteristic) or as soil amendment (mainly because of its high alkalinity). The openly dumped fly ash poses radiation hazard due to the leachability of radionuclides into the ground water stream ultimately contaminating the drinking waters. The disposal and even the utilization in construction work also leads to exposure to radiation doses to the personals in vicinity. Therefore it is necessary to characterize and quantify the natural radioactivity content in fly ash and bottom ash for subsequent evaluation of the associated environmental and biological risks. The feed coal used in power plants contains various elements, minerals and organic constituents. Upon burn up the elements tend to get enriched in the ashes. Radionuclides which are normally found in the coals get enriched into the ashes after burn up. The concentration of most radioactive elements in