A 3-D Lagrangian (Monte Carlo) Method for Direct Plume Gamma Dose Rate Calculations

Abstract

A fully 3-D Lagrangian particle method has been presented for calculating the direct gamma dose rates due to a radionuclide plume in the atmosphere. A continuous release of radionuclides into the atmosphere was simulated by liberating a large number of Lagrangian particles whose trajectories were tracked for about 25 h in a turbulent atmosphere with a known wind field. The atmosphere turbulent/stability characteristics like wind velocity fluctuations, eddy lifetime, etc., were obtained from the reported data in the published literature. For calculating the direct plume gamma dose rates a point isotropic source formula has been used with appropriate attenuation and build-up factors for the air medium. Each Lagrangian particle represented a point source of radioactivity with a known strength. The dose rates at ground due to the radionuclide cloud were calculated by adding the contribution from each Lagrangian particle in the domain. The numerically calculated dose rates were compared with the already reported results. An excellent comparison was observed for a uniform atmosphere with the Gaussian plume model predictions. However, if the wind shear (change in wind direction with height) is taken into account, we observed that for distances exceeding 20 km, the numerical data were below the reported Gaussian Plume Model (GPM) results. This indicated the need of using a modified GPM for extended distances.