Radionuclides in fly ash and bottom ash: Improved characterization based on radiography and low energy gamma-ray spectrometry

Two radiation-based techniques for determining the distribution and relative abundance of radionuclides are described, and applied to a suite of fly ash and bottom ash samples from a Kentucky power plant. The technique of fission-track radiography provides new observations of the variety of uranium hosts and of uranium distribution in individual particles of fly ash, and thus aids prediction of the leachability of uranium during long-term disposal or utilization of fly ash. Uranium is largely dispersed within glassy components of fly ash particles and shows little evidence for obvious surface enrichment that could be attributed to secondary adsorption. The technique of low energy gamma-ray spectrometry provides simultaneous, non-destructive determination of the relative abundance of ²³⁸U, ²²⁶Ra, ²²⁸Ra and ²¹⁰Pb in representative 150–250 g samples. The measurements provide a means for screening samples to determine if the combustion process causes significant preferential redistribution of radionuclides that could affect their subsequent mobility. Results indicate that radium isotopes are not significantly (within 10–15%) fractionated from parent ²³⁸U and ²³²Th during coal combustion. In contrast, ²¹⁰Pb appears to be preferentially enriched in some samples of fly ash, and depleted in bottom ash relative to ²³⁸U and ²²⁶Ra. In this application ²¹⁰Pb acts as a tracer for elemental lead, and confirms the expected greater volatility of Pb compared to more refractory elements during coal combustion.