Avis d'expert en biologie environnementale

Natural Radioactivity Levels and Environmental Significance at Ngaka Coal Mine

Lazaro H. Meza

This study aims to determine the activity concentrations of naturally occurring radionuclides and technically enhanced levels of radiation in fifteen (15) coal samples collected across the overlying and underlying rock beds from surface and sub-surface sections of northwest part of Ngaka Coal Mine Mbinga, Tanzania. Ngaka (Mbalawala project), is an open cast coal mining project located in the Ngaka coalfields area, Mbinga District in Ruvuma Region, southwest Tanzania. Being the first of its own kind, this work aims to establish the first baseline measurements for radioactivity concentration at Ngaka Coal Mine. Representative samples from various locations across the mine have been collected and analyzed in order to establish the 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 activity concentration was measured via high resolution gamma ray spectroscopy using a hyper-pure germanium detector situated in a low-background environment with a copper inner-plated passive lead shield. The average specific activity of 226Ra, 232Th, 40K and 137Cs (Bq kg-1) in coal samples were 28.3± 1.6, 13.4±0.8, 52.8±2.7, and 5.7±0.4 Bq/kg respectively. These values lie within the expected range relative to the world average values in coal samples of 30, 35 and 400 Bq/kg respectively. The mean value of radium equivalent (51.56 Bq/kg), the external hazard index (0.14), internal hazard index (0.22), gamma dose rate (22.61 nGy/h) and annual effective dose equivalent (0.03 mSv/Y) for the complete set of samples as shown in brackets, are below the published maximal admissible values and indicate that the Ngaka Coal Mine can be regarded as having normal levels of natural background radiation. The value of gamma dose rate obtained for sample no. M3C was found to be higher than the permissible value which is 51 nGy/h. The obtained dose rate at M3C was 83.5 nGy/h. However, the value of the radium equivalent (Raeq) in the said sample was found to be just below the permissible value of Raeq of 370 Bq/kg, which corresponds to an annual effective dose of 1 mSv. It can be concluded that the measured level of the natural radiation background in the present study show that the studied area of the mine have normal levels of background radiation, with exception of sample M3C from Location MC.