On the diffusion of radioactive materials in the atmosphere of nuclear facilities environs

It is common practice in the Federal Republic of Germany to calculate the diffusion of radioactive materials by means of the Gaussian diffusion model. Influences due to buildings and orography, as a rule, are covered by wind tunnel tests. Deposited activity is calculated separate from diffusion by means of the deposition speed in dry weather including foggy occasions, and with the help of the washout and snowout coefficients in case of rain and snow, respectively. In order to be able to calculate diffusion and deposition together, the wind profiles in and above the vegetation are studied. The various statements for the exchange coefficient above vegetation are assessed. Simplified analytical and numerical diffusion calculations are made with the exchange coefficients. The radionuclide loss of aerosol-bound radionuclides by the source depletion method, and the particle size by means of plume lowering are taken into account. An analytical, combined method (source depletion together with plume lowering) is compared for large particles with numerical calculations. Short-term dispersion factors for calms or light wind diffusion situations are considered by means of the Puff model. Diffusion within one terrain level is considered theoretically and practically. The flow field within one terrain level is calculated by means of the potential flow theory. That theory is applied to the environs of the Muelheim-Kaerlich nuclear power plant and to the formerly planned Wackersdorf reprocessing plant. (orig./HP)