Radiological characteristics of coal-to-ash transformation: a paired coal-ash assessment of natural radionuclides

Abstract Naturally occurring radioactive materials (NORM) in coal and its combustion residues represent a potential source of environmental contamination, particularly under large-scale energy production and waste reuse scenarios. However, the radiological transformation of radionuclides during coal combustion remains insufficiently quantified. This study applies a paired coal–ash approach to assess systematic changes in the activity concentrations of 40 K, 226 Ra, and 232 Th and associated radiological hazard indices. Nine hard coal samples from Poland and international sources, together with their corresponding bottom ashes produced under controlled combustion conditions, were analysed using gamma-ray spectrometry. All coal samples exhibited low radionuclide activities, with concentrations below global average values. In contrast, combustion resulted in a highly consistent and approximately six-fold increase in radionuclide activity and all radiological indices, accompanied by complete directional consistency and very large effect sizes (r = 0.889). This increase reflects the concentration of naturally occurring radionuclides within the mineral residue, as the organic fraction of coal is removed during combustion. These findings demonstrate that combustion acts as a predictable process of radionuclide enrichment and radiological amplification. Enrichment factors ranged from approximately 3.8 to 9.9 across the analysed coal–ash pairs, and the external gamma radiation dose rate exceeded the recommended reference level of 54 nGy h⁻ 1 in all nine ash samples. Although most ash samples remained within recommended radiological limits, the magnitude and uniformity of enrichment indicate a potential risk of secondary environmental contamination and increased environmental exposure—particularly where combustion residues are reused in construction materials or land applications. It should be noted, however, that this study assessed the potential for such impact based on elevated radioactive activity of the ash, rather than on direct observation of radionuclide migration into the environment. The results highlight the need to treat coal combustion residues as radiologically distinct materials—at least under the combustion conditions investigated in the present study—and to incorporate transformation processes into environmental risk assessment and management frameworks.