Heterogeneous deposition of radioactive particles on urban facades for enhanced exposure assessment in nuclear emergencies.

Radioactive particles released during nuclear accidents rapidly disperse and deposit heterogeneously across urban environments, with building facades becoming significant yet underrecognized secondary radiation sources. Conventional models typically approximate urban surfaces as uniform roughness layers, neglecting the influence of three-dimensional architecture and localized airflow. In this study, we developed a coupled Computational Fluid Dynamics-Discrete Element Method (CFD-DEM) and Geant4 Monte Carlo framework to quantify the dry deposition of Iodine-131 (I-131) particles on building facades and evaluate the resulting ground-level radiation risks from first principles. Parametric analysis reveals that deposition velocity is strongly influenced by particle size, wind speed, surface orientation, and windward angle. Windward surface exhibits the highest deposition velocity compared to side/top ones, reaching up to 1.31 m·s-1, over two orders of magnitude above conventional assumptions. An inflection near a 20° windward angle marks a regime shift from Brownian diffusion to inertia-dominated deposition. Radiation simulations show that facade-deposited I-131 contributes more substantially to ground-level dose rates than flat ground deposition alone, particularly near tall or wide buildings. These findings highlight the need to incorporate facade-specific deposition and 3D urban morphology into nuclear emergency response frameworks, offering a refined scientific basis for exposure assessment, evacuation planning, and public health protection.