Modelling of radioactive particle resuspension after a dirty bomb event

Abstract

The primary hazard of a Dirty Bomb emanates not from the initial detonation but from the subsequent re-suspension of deposited radioactive particles. Although radiological contamination is substantial on surfaces initially, the biological risk intensifies through ingestion or inhalation due to re-suspension. Experimental simulations in a 10 m wind chamber unveiled average bin-by-bin resuspension factors for particle sizes between 0.9 and 6.5 μm downstream from the initial fallout. Calculated values were 4.12E-05 × (1 ± 46.8%) m-1 and 4.56E-05 × (1 ± 79.5%) m-1, indicating the magnitude of the resuspension process. In a prototypical study using data from a full-scale dirty bomb experiment by DRDC Canada, maximum committed effective inhalation radiation doses were calculated as 1.89E+02 μSv for the public and 1.89E-2 μSv for first responders, considering a 35.2 × (1 ± 10%) GBq dirty bomb. Subsequently, Computational Fluid Dynamics (CFD) was applied via FLUENT software, incorporating Regional and Global models to simulate particle resuspension. The unsteady Large Eddy Simulation viscous model with Smagorinsky-Lilly Subgrid-Scale models effectively captured turbulent flow dynamics. CFD resuspension factors at specific locations were computed as 4.14E-04 × (1 ± 13.3%) m-1 and 4.01E-4 × (1 ± 16.3%) m-1 for particle sizes between 0.9 and 6.75 μm. Notably, an order of magnitude difference between CFD and experimental results highlights the intricacies in modelling particle resuspension. Future refinements may include incorporating surface roughness elements in both downstream and transverse directions in the Regional CFD model to capture particle saltation, enhancing resuspension predictions' accuracy, and introducing a multilayer resuspension model. This study underscores the complex nature of Dirty Bomb scenarios, emphasizing the need for a holistic understanding that combines experimental insights with advanced computational modelling for effective risk assessment and response planning.