Radionuclide Accumulation in Fine Fractions: Implications for Occupational and Environmental Exposure

Radiological screening of construction materials is conventionally performed on bulk samples, yet occupants are exposed to fine, abradable, and respirable fractions. This study examines how particle-size distribution (PSD) governs the redistribution of 226Ra, 232Th, 40K, and anthropogenic 137Cs in construction materials and in ash from domestic biomass heating. In total, 27 materials were analysed by high-resolution HPGe gamma spectrometry, with granulometric segregation at coarse (PSD3) and fine (PSD7) resolution. PSD3 showed no monotonic effect, whereas PSD7 revealed a systematic rise of the activity concentration index toward the finest fractions. 40K and 137Cs showed the strongest particle-size dependence, indicating preferential accumulation in the fines; 137Cs was detected only in biomass-ash pellets, where it was enriched in the fines (EF = 2.73 relative to bulk; a single-material case study). Two models are proposed: a physically grounded surface-controlled model and a descriptive granulometric enrichment factor for material ranking, both fitted to the present dataset. A fraction-resolved dose assessment showed that the internal hazard index (Hin) rises toward the fines and exceeds unity (≈1.1) in the respirable ash and fireclay fractions even though their bulk values remain acceptable. Bulk screening, therefore, underestimates the dose-relevant burden carried by the fine fractions that workers and households actually inhale.