Assessment of Natural Radionuclides in Fly Ash Produced at Orji River Thermal Power Station, Nigeria and the Associated Radiological Impact

Abstract

Coal fired power plants produce significant amounts of ashes, which are quite often being used as additives in cement and other building materials. Coal contains trace quantities of 226 Ra, 232 Th and 40 K. The concentrations of these radionuclides are usually low in the coal, but enriched in fly ash. The activity concentrations of natural radionuclides in fly ash and soil samples in the vicinity of Orji River thermal power station in Nigeria was determined by gamma ray spectroscopy method using NaI(Tl) detector. The mean activity concentrations of 226 Ra, 232 Th and 40 K in the fly ash were 40.8 ± 11.6, 49.1 ± 9.3 and 321 ± 17 Bq·kg – 1 , respectively. Coarse fly ash collected from the dump site had mean activity concentrations of 28.2 ± 8.3, 37.6 ± 5.0 and 335 ± 32 Bq·kg –1 , respectively for 226 Ra, 232 Th and 40 K. Soil samples collected at about distances of 10 m from the dump site had 32.7 ± 4.3, 40.0 ± 4.2 and 298 ± 15 Bq·kg -1 , respectively. Those collected at about distances of 100 m from the dump site had 39.1 ± 11.2, 34.1 ± 5.2 and 257 ± 19 Bq· kg - 1 , respectively. Occupational dose received by workers due to exposure to the fly ash ranged between 33.0 and 61.2 μSv· y -1 with a mean value of 47.1 ± 8.4 μSv· y -1 , which is below the intervention exemption level of 1 mSv· y -1 . The radium equivalent activity concentration, external and internal hazard indices of the fly ash were below the recommended maximum values for building materials. The mean outdoor absorbed dose rate and the annual effective dose obtained in the vicinity of the plant were 49.7 ± 4.0 nGy·y -1 and 0.30 ± 0.02 mSv (dump s ite), 51.7 ± 3.6 nGy· y -1 and 0.32 mSv (soil 10 m from dump site), 49.4 ± 4.9 nGy·y -1 and 0.30 ± 0.03 mSv (soil 100 m away from dump site), which are lower than the world average. The results obtained in this study show that there is no significant radiological impact of the fly ash on both the workers and the public from radiation protection point of view.