Performing accurate doses assessments are an essential part of the nuclear emergency preparedness and response phases. To improve the dose estimation at the near-range and assess the dynamic behavior, a model is developed in this work using CFD. The evolution of the concentration is formulated as a transient convection-diffusion problem in which the instantaneous profiles for velocity and eddy viscosity are produced by a large eddy simulation of the atmospheric boundary layer. The beta and gamma dose rates are computed from the local concentration and using the point-kernel method with buildup factors, respectively. The model is applied to the dispersion of the unstable isotopes Argon-41 and Xenon-133, released over an open field under neutral atmospheric stratification. It is observed that even under constant pollutant emission rate, strongly fluctuating beta and dose rates in time are registered, regardless the isotope. Furthermore, the results point out that the gamma dose assessment at the near-range is not representative for the external beta dose rate.