Radiological dose assessment to marine biota from NORM-contaminated oil spills in the Strait of Malacca.

The Strait of Malacca is a major global shipping corridor, yet the potential radiological impact of Naturally Occurring Radioactive Materials (NORM) in crude oil spills remains unquantified for this region. This study extends existing static, single-scenario dose modeling of NORM-contaminated oil slicks into a systematic, multi-parameter Monte Carlo framework using the TOPAS code, applying 226Ra and 228Ra activity concentrations, oil compositions, and environmental conditions specific to the Strait of Malacca. Absorbed dose rates were evaluated in seawater and to ICRP Reference Animals and Plants (RAPs) for both surface slicks and contaminated seabed sediments, representing short- and long-term exposure phases. Absorbed dose scales linearly with spill thickness but saturates with increasing lateral extent, reflecting water-mediated photon attenuation, and up to 25% of the integrated dose is deposited beyond the visible slick boundary, indicating a measurable peripheral exposure pathway. Despite higher per-decay photon energies in the 228Ra (thorium series) decay chain, 226Ra dominates total dose under realistic activity concentrations because of its higher assumed fraction in the radium mixture. Predicted dose rates (~ 10- 1 µGy/day) remain several orders of magnitude below established radiological benchmarks for marine biota. These findings provide a quantitative framework for interpreting NORM-related exposure in marine oil spills, support the prioritization of chemical over radiological impacts in spill response, and highlight the relevance of sediment-associated pathways for long-term assessment.