Validation of the French-German model for the treatment of atmospheric dispersion in accidental release situations with experimental data.

M. Montfort, O. Isnard, R. Martens, H. Schnadt, Seventh International conference on harmonisation within atmospheric dispersion models for regulatory purposes, 28-31/05/2001, Beligrate, Italy.

Some years ago the French-German Commission for the Safety Problems of Nuclear Installations (DFK, Deutsch-Französische Kommission) initiated the investigation of the problems that may occur due to the use of different dispersion models within the emergency preparedness procedures of different countries in the case of an accident occurring in a nuclear power plant near a border. The predicted concentration distributions may differ substantially leading to different countermeasures to be considered in each country. Actually, the models used for assessment of atmospheric dispersion in Germany and in France yield different results although being of the same Gaussian type as different approaches were used for the distribution parameters σ y and σ z (function of transfer time in France and of distance in Germany). It was then decided to establish a new model based on general and widely accepted theory of the atmospheric boundary layer. With respect to the horizontal parameter σ y his model is based on considerations of spectra of turbulent energy in the atmosphere and their relation to the standard deviation of the Gaussian distribution (spectral approach). The approach proposed by Monin and Yaglom has been retained. It separates the diffusion process into the diffusion of the puff center of mass of a pollutant (standard deviation of the distribution of particles denoted σ yc) responsible for movement of the puff as a whole and the relative diffusion of the pollutant in the puff around its center of mass (standard deviation denoted σ yb) causing an enlargement of the puff together with a decrease of the concentration of pollutant. The approach used for σ yb modeling is the one proposed by Smith and Hay who assumed an isotropic, homogeneous turbulence and a spatial Gaussian distribution of the pollutant. The diffusion of the puff centers σ yc is derived from the original Taylor’s formulation that concerns the diffusion of all particles simultaneously leaving a plane of infinite dimensions meaning that, at any instant, even the greatest eddies participate in the diffusion. In the vertical direction, the problem is simpler as the range of existing turbulent eddies is restricted to the small scale, even if a part of the turbulence does not follow the similarity theory. With respect to the parameter σ z the similarity approach was then used. This approach is based on the hypothesis that turbulence and its time scale can be described as function of the atmospheric boundary layer variables friction velocity u*, convective velocity scale w*, mixing layer height zi and height above ground z. This hypothesis together with the results of atmospheric boundary layer experiments and some parameterizations are used for estimates of σ z. As for sampling times between 10 and 30 minutes it is not necessary to distinguish between the growth of the puff and the meandering of the puff center, the original Taylor’s formulation is applicable. An approximation of this formulation has been used applying an empirical law that leads in accordance to Taylor’s result to an evolution of σ z proportional to the travel time t for small values of t and to t 1/2 for large travel times: with standard deviation of the vertical velocity fluctuations, This work has been done with the collaboration of the GRS and TUV (Germany).