A 3D Lagrangian particle model for direct plume gamma dose ratecalculations

Abstract

A fully 3D Lagrangian particle model has been presentedfor calculating the direct gamma dose ratesdue to a radionuclide plume in the atmosphere.A continuous release of radionuclides into the atmospherewas simulated by liberating a series of puffs (eachcontaining 100 Lagrangian particles).These puffs were released with a constant time lag betweenthe successive puffs. The Lagrangian particle trajectorieswere tracked for about 25 h in a turbulent atmosphere,with a specified wind field.The atmosphere turbulent/stability characteristics likewind velocity fluctuations, eddy lifetime, etc, wereobtained from the reported data in the published literature.For calculating the direct plume gamma dose rates,a point isotropic source formula has been used withappropriate attenuation and build-up factors for theair medium. Each Lagrangian particle representeda point source whose radioactive strength wascalculated from the known release rate.The dose rates at ground due to the radionuclide plumewere calculated by adding the contribution fromeach Lagrangian particle in the domain.The numerically calculated dose rates were comparedwith the numerical results reported in the literature.An excellent comparison was observed fordownwind distances up to about 20 km.However, for distances exceeding 20 km, thenumerical data were below the reported results for the Gaussian plume model.This discrepancy was due to the vertical wind shear.It is concluded that a Gaussian plume modelcan be used for the concentration calculationsprovided the lateral dispersion parameter, σy,includes the effect of wind shear, for distances exceeding 20 km.