Capacity-associated variability of naturally occurring radionuclides in coal combustion slag: probabilistic radiological risk assessment and multi-scenario exposure modelling.

Coal combustion generates large volumes of mineral residues containing naturally occurring radioactive materials (NORM), whose radiological characteristics vary systematically with plant operational scale and fuel properties. Understanding the factors governing this variability is essential for radiological risk assessment and sustainable management of coal combustion residues. This study investigates the capacity-associated variability of radionuclide activity concentrations (226Ra, 232Th, 40K) in coal combustion slag using a dataset of 251 samples collected from 18 Polish power plants with capacities ranging from 51 to 5091 MW. Gamma spectrometry and multivariate statistical methods, including K-means clustering and PERMANOVA, were applied to identify patterns in radionuclide distribution and their relationship to plant scale as a proxy for facility-scale operational categories. Three distinct clusters were identified, revealing a non-linear relationship between plant capacity and radionuclide concentrations. Slag from medium-capacity plants exhibited the highest activity levels and radiological hazard indices, indicating that plant-scale factors are associated with systematically elevated radionuclide activity concentrations in slag. In contrast, high-capacity plants showed more stable and moderate radiological profiles, while low-capacity units exhibited greater variability linked to fuel characteristics and combustion efficiency. Radiological risk assessment based on radium equivalent activity, activity index, and indoor dose indicators demonstrated that a subset of slag materials may exceed commonly applied screening levels, particularly within the medium-capacity cluster. These findings highlight the importance of incorporating plant-scale operational factors into the radiological risk evaluation of coal combustion residues. The results demonstrate that naturally occurring radionuclides serve as effective indicators of capacity-associated variability in slag radiological composition, while also providing a basis for risk-informed management of coal-derived waste. Integrating plant-scale parameters with radiological assessment offers a practical framework for supporting environmental regulation and sustainable reuse strategies.