Occupational radiation exposure assessment in tin smelting: an integrated evaluation of external and internal doses from NORM in Indonesia

The tin industry in Indonesia is a major global producer, and mining and smelting processes can mobilize and concentrate Naturally Occurring Radioactive Materials (NORM), particularly radionuclides from the 238U and 232Th decay series, potentially leading to occupational radiation exposure. We conducted a comprehensive assessment of annual effective doses to workers by integrating external and internal exposure pathways at two tin smelters using raw materials with different tin contents. External exposure was evaluated using ambient dose equivalent rate measurements, while internal exposure was assessed through simultaneous measurements of radon (222Rn), thoron (220Rn), and thoron progeny concentrations with passive discriminative detectors. Activity concentrations of 226Ra, 232Th, and 40K in raw materials and by-products were determined by HPGe gamma spectrometry to identify dominant contributors to gamma radiation fields. The total annual effective dose for most workers was <2 mSv, but reached 20.72 mSv for slag handling at one site, exceeding the recommended IAEA occupational limit. Radionuclides from the 232Th decay chain were the dominant contributors to external gamma exposure, particularly in slag, with activity concentrations increasing from raw materials to slag by approximately 28-fold for 226Ra and 79-fold for 232Th at Smelter A. Internal exposure was mainly influenced by thoron progeny in smelting areas, whereas radon contributed more in office and laboratory environments. These results identify slag storage as a critical radiological hotspot and highlight the need to evaluate both exposure pathways to support ALARA-based protection strategies and regulatory frameworks for NORM management in industrial settings.