Assessment of the mode of occurrence and radiological impact of radionuclides in Nigerian coal and resultant post-combustion coal ash using scanning electron microscopy and gamma-ray spectroscopy

走査型電子顕微鏡とガンマ線分光法を用いたナイジェリアの石炭および燃焼後の石炭灰中の放射性核種の発生様式と放射線学的影響の評価

Okeme, I. C.*; Scott, T. B.*; Martin, P. G.*; 佐藤 志彦   ; Ojonimi, T. I.*; Olaluwoye, M. O.*

Okeme, I. C.*; Scott, T. B.*; Martin, P. G.*; Satou, Yukihiko; Ojonimi, T. I.*; Olaluwoye, M. O.*

Natural radionuclide concentrations in coal and associated fly ash can occur at levels well in excess of those observed in other materials; enough to raise potential human and environmental health concerns when (re)suspended or incorporated into construction materials. To evaluate such concerns, this study characterised coal and fly ash samples obtained from two Nigerian coal mines (Okaba and Omelewu) using high resolution gamma spectroscopy combined with scanning electron microscopy and energy dispersive spectroscopy. Discrete uraninite particles were observed dispersed within the fly ash samples, alongside U and Th containing mineral grains (monazite, xenotime and zircon) with monazite the most abundant radioactive mineral particles. The pitted and cracked surface morphologies of these radioactive particles (with sizes between 10m and 80m) indicates their susceptibility for disintegration into more dangerous and readily inhalable PM2.5 aerosol particles, with the potential to deliver localised dose and cause chronic respiratory diseases. While the results of activity concentrations and radiological hazard indices (radium equivalent, annual effective dose equivalent, total radium, and excess lifetime cancer risk) from Okaba and Omelewu coal samples were of no concern, results for the corresponding fly ash samples from both mines were between 3 and 5 times higher than internationally recommended safe limits. These results therefore imply that such coal-derived fly ash is not safe for incorporation in the construction of residential houses and should be properly disposed of or contained to prevent hazard due to increased risk of prolonged indoor exposure to gamma radiation, radon gas and inhalation of liberated radioactive particles.