Assessment of occupational radiation exposures of artisan miners of columbite-tantalite (coltan) in the Eastern democratic republic of Congo

Abstract

Artisan miners are faced with risks of radiation exposures due to the presence of naturally occurring radioactive materials (NORM) in the earth and in the materials (products, by-products, and wastes) derived from mining operations. The aim of this research work was to assess the potential radiation exposure doses to artisan miners in the Democratic Republic of Congo (DRC) as they mine, mill and handle coltan (Columbite- Tantalite). Samples of coltan were collected from various mines in Masisi, a territory noted for coltan mining in the North Kivu province of the eastern DRC, and analysed in our laboratories at the Institute of Nuclear Science, University of Nairobi. The analyses were in three parts, i.e. elemental analysis using Energy Dispersive X-ray Fluorescence and Neutron Activation Analysis (NAA was Qualitative); measurement of activity concentrations of the naturally occurring radionuclides using gamma ray spectroscopy; and dose estimation based on model calculations. The elements identified, and the range of concentrations encountered were Sn (207 - 464) mg/g, Ta (144.1 - 340) mg/g, Nb (17.2 - 123.7) mg/g, Fe (40.5 - 100.4) mg/g, Mn (31.9 - 86.2) mg/g, and Ca «0.41 - 72.5) mg/g. Other elements also identified at relatively low concentrations are Ti «0.08 - 29) mg/g, Au «0.03 - 11.6) mg/g, Pb (0.2 - 3.6) mg/g, Rb «0.0005 - 0.6) mg/g, Sr «0.004 - 0.3) mg/g, Y «0.003 - 0.5) mg/g, Zr (1.0 - 2.3 mg/g), Th (0.2 - 1.0) mg/g, and U (0.2 - 0.9) mg/g. The activity concentrations vary from 4.0 Bq/g to IV 13.45 Bq/g for 238U and from 0.05 Bq/g to 0.9 Bq/g for 232Th. In most cases 40K was below the minimum detection limit of the gamma-ray detector method used for analysis. The working scenarios considered in the dose calculations include digging to expose the coltan ore, sloshing the ore and soil in washtubs to separate coltan from soil, drying the coltan in open air, and finally grinding and sieving the dried coltan. The exposure pathways considered in the dose calculations include: external exposure due to gamma-rays from bulk materials containing gamma emitting radionuclides and due to submersions in air containing radioactive dusts and internal exposure due to ingestion and inhalation of radionuclides. Among the exposure pathways considered, inhalation resulted in the highest doses. Also, among the working scenarios, grinding and sieving coltan at the mills contributed the highest average effective dose to the workers, about 2.1 E-02 Sv per annum, compared to digging which contributes about 0.5 E-05 Sv per annum. In order to put the findings of this research into perspective, the dose values were compared with the exemption levels and dose limits for workers, as specified by international bodies and national regulatory authorities in other parts of the world. For example, the exemption levels for workers in mining industries specified in the IAEA Basic Safety Standards and the European Atomic directive were based on an individual dose criterion of the order of 10 E-06 Sv per annum. This value is several orders of magnitude lower than the doses experienced by workers in the mining and milling of coltan in the DRC.