The use of products containing naturally occurring radioactive materials (NORM) such as minerals, quarry, sand, clays, etc is widespread in Rwanda, but they are not subject to radiological quality control. This paper presents the results of studies on the occupational radiation exposure due NORM in the extraction and processing of the Columbite-Tantalite (Coltan) mineral in the Muhanga, Ruli and Ngoma areas of Rwanda, by artisanal miners. Activity concentration of primordial radionuclides, 40K, 238U and 232Th series in coltan ore (extracted, processed), soil, and mine tailing sediment were determined by HPGe-based gamma-ray spectrometry. The average activity concentrations of 238U were 50.2-972.9 Bq kg-1, 64.3-2011.8 Bq kg-1 and 70.7-853.3 Bq kg-1 in Muhanga, Ruli and Ngoma respectively. These values are far higher than the world average of 35 Bq kg-1. In all samples from the three regions, the activity concentrations of 40K were below average of 500 Bq kg-1 with the processed ore being the lowest (< 30 Bq kg-1). This implies that 40K is primarily in soil and not the ore. The average activity of 232Th (44.78-75.5 Bq kg-1) for processed coltan was greater than the world average, while the average activity for 232Th (26.7 Bq kg-1) in the extracted coltan was below the world wide average. This indicates that 232Th is enhanced when processing coltan. Based on these values and the working scenarios in artisanal coltan mining, the occupational doses that may accrue from a variety of exposure pathways were determined by model calculation. The working scenarios considered included digging to exposure the coltan ore, drying the coltan in open air, grinding, and sieving the dried coltan. The exposure pathways considered in the dose calculation included external exposure due to gamma-rays from bulk materials containing gamma emitting radionuclides, external exposure due to sumersions in air containing radioactive dust and the internal exposure due to ingestion and inhalation of radionuclides. Among the exposure pathways considered, inhalation of coltan bearing dust resulted in the highest does while crushing and sieving coltan in the mill; 0.27, 0.52, 0.25 mSv per annum on average in Muhanga, Ruli and Ngoma respectively. These values are below 1 mSv y-1 which are the values recommended by ICRP for the public and occupational exposure respectively. These results are however important in establishing radiological regulatory protocol for occupational exposure in artisanal coltan mining since the area is a HBRA (X 11 the world average).
Key words: Coltan; Processed coltan; Extracted coltan; Background radiation, Naturally Occurring Radioactive Materials (NORM); High Background Radiation Areas (HBRA).