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الثلاثاء، 5 أغسطس، 2014

Radioactive Hazards Phosphate Mines

Author        : Lazaro Hussein Meza
 Degree        : M.Sc. Nuclear
Title: Radioactive Hazards Phosphate Mines  
Abstract:
The specific activities of terrestrial radionuclides 238U, 232Th, 40K and the man - made 137Cs in 45 phosphate rock samples collected from Minjingu phosphate mine were studied and evaluated. Minjingu is an open cast lacustrine-biogenic phosphate mine deposit located in the Northern part of Tanzania about 110 km southwest of Arusha. This work aims to establish radioactivity working levels after some decades of escalated mine activities. Experimental work was carried out at the Neutron Activation Analysis (NAA) Laboratory of the Egyptian Training Research Reactor, (ETRR-2), in Cairo, Egypt. Measurements were carried out under the assumption of secular equilibrium of the radionuclides within the measured samples and results were obtained via high-resolution gamma-ray spectrometry using a hyper-pure germanium detector situated in a low-background environment with a copper inner-plated passive lead shield. Representative samples from various locations across the mine were collected and analyzed in order to establish activity concentrations associated with the 238U/226Ra and 232Th natural decay chains, the long-lived naturally occurring radionuclide 40K and the artificial man-made radionuclide 137Cs. The results show that the specific activity ranges from 122.5±6.9 Bq kg-1 to 8060.0±806 Bq/kg for Radium-226 (226Ra), 13.2±0.8 Bqkg-1 to 1201.3±77.3 Bqkg-1 for Thorium-232 (232Th) and 12.9±0.7 Bq kg-1 to 1827.6±93.9 Bq kg-1for Potassium-40 (40K) and 0.2±0.01 Bq kg-1 to 70.6±4.8 Bq kg-1 for Cesium-137 (137Cs) with arithmetic mean values of 1832.3±91.6 Bq kg-1, 447.6±22.4 Bq kg-1, 399.5±19.9 Bq kg-1 and 22.9±1.2 Bq kg-1 respectively.
In order to evaluate the radiological hazards of the natural radioactivity, the radium equivalent activity (Raeq), the outdoor external gamma absorbed dose rate (DR), the annual effective dose rate (AEDR) and the external (Hex) and internal (Hin) hazard indices were calculated and compared to the world average values. The radium equivalent activity (Raeq) ranges from 219.3±10.9 Bq kg-1 to 9127.1±456.4 Bq kg-1 with mean value of 2433.6±121.7 Bq kg-1. As the obtained maximum value of radium equivalent activity is almost 25 times higher than the safe limit (370 Bq kg-1), the calculated arithmetic mean value is almost 7 times much greater than the said safe limit as recommended by OECD (1979).
The total absorbed dose in the study area varies from 102.1 to 3967.8 nGy h-1with an average value of 1055.9 nGy h-1. The average value is 18 times higher than the recommended world average of 59 nGyh-1. The corresponding indoor and outdoor annual effective doses range from 0.5 to 19.5 and 0.1 to 4.9 mSv with an average value of 5.2 and 1.3 mSv respectively while the world wide average annual effective dose is approximately 0.5 mSv and the results for individual countries being generally within the 0.3 - 0.6 mSv range for indoors.

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