The transloc numerical model for the simulation of atmospheric dispersion processes and its use for reactor accident consequence analysis

Together with experimental data confirming its validity, a numerical diffusion model is presented for simulating the dispersion of tracers in the atmosphere. The model is based on a solution of the three-dimensional non-stationary equation for turbulent diffusion using a difference method. The advection-diffusion equation is broken down using the Fractional Step Method into five one-dimensional differential equations. The CARLSON scheme is used to separate out the advection component. The diffusion component is estimated using a generalized CRANK-NICOLSON procedure. The main advantage of the model is that it takes account of the time-dependent nature of all the processes involved, and that it gives the vertical structure of the windfield and of the turbulent diffusion. Deposition and sedimentation processes, rain-out, wash-out and radioactive decay can be simulated. In view of these properties the model is also particularly suitable for calculating the dispersion of radioactive substances released in the event of a hypothetical reactor accident. The accuracy of the model was checked by means of data from the HANFORD Tracer Experiments and from dispersion experiments carried out at the Karlsruhe Nuclear Research Centre in which various conditions were obtained in respect of ground roughness, height of release and time pattern of the discharge. The results show good agreement between measured and calculated values and represent a significant improvement on the customary dispersion models