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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