A three dimensional CFD-based approach for the dispersion of radioactive cloud in urban environment

The presence of buildings and obstacles in urban environment can modify the velocity and spatial concentration fields of a radioactive cloud emitted by a source, therefore affecting the dispersion of the plume. A simplified model for estimating the enhanced dispersion due to the presence of obstacles and buildings has been presented by Huber, by introducing modified parameters in the framework of the Gaussian Plume Model (GPM). The Gaussian Plume Model (GPM) represents the most adopted implementation for submersion dose evaluations from an emission stack. The radioactive cloud dispersion is obtained by calculating the Brigg’s coefficients that varies with the meteorological conditions, mainly the wind speed and the atmosphere stability. In the model developed by Huber dispersion parameters and Brigg’s coefficients have been properly modified by including looping movement of the plume as a result of its interaction with vortices induced in the flow filed surrounding the obstacles. The limit of the model is represented by the lack of detailed geometrical information of the obstacles, resulting in a simplified description of the spatial distribution of the concentration and velocity of the radioactive cloud in the surrounding of the radioactive source. A more accurate estimation of the dispersion of a radioactive plume in an urban environment can be obtained by recurring to computational fluid-dynamics (CFD) models. In the work, similarity Theory (MOST) for the entire vertical Atmospheric Boundary Layer (ABL) profile under non-neutral stability conditions has been included in the framework of the Reynolds-Averaged Navier-Stokes (RANS) approach, which is a well-established method in CFD, known for its effectiveness across different applications, showing satisfactory results in similar applications of pollutant transport in urban areas. Subsequently, the radionuclide dispersion can also be implemented in the Monte Carlo code FLUKA to make more accurate dose evaluations. CFD models can be particularly useful for evaluations at short distances in urban areas where the hypothesis or the GPM plume cannot be applied. This is particular important for nuclear medicine and hadrontherapy centers situated in populated areas in which GPM models can excessively overestimate submersion doses. In the work, comparisons between Gaussian plume and fluid dynamic models are performed in order to make comparison at short and long distances. The case study geometry used in the numerical simulation was inspired by the actual urban agglomeration similar to the one surrounding the CNAO (National Oncological hadrontherapy Center, in Pavia-Italy), structure of international excellence in the field of oncological treatments with hadrontherapy techniques using a synchrotron for the particle acceleration. A sample chimney was modeled on the CNAO building, emitting exhaust. The domain for the CFD-simulation has been discretized by an hybrid type mesh, with refinement regions set to accurately solve the flow field and the plume transport near the obstacle surfaces, close to the ground, and in the surrounding of the chimney. Numerical results have been obtained by considering different stability atmospheric conditions and comparisons and differences with Huber approximation are presented and discussed.