Effect of Application of Surface Depletion Model on Estimated Reactor Accident Consequences

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First, a dispersion model is described in which the vertical mixing is treated with the gradient-transfer (Kz-model) theory and in which the dry deposition is accounted as a surface depletion. In this model the ground scavenges the lowest part of the release plume and the vertical mixing gradually brings new material from the higher parts of the plume. A fast and accurate method employing Green's function technique has been implemented for the calculation of the direct external gamma dose from release plume which has an arbitrary vertical concentration distribution. Second, reactor accident consequences are evaluated with the Kz-model described and a standard Gaussian plume model with source depletion where the dry deposition is assumed to remove material from the release plume at the same relative rate at all the heights of the plume. Near the release source the ground level concentrations and the dose components directly proportional to the concentration are considerably smaller when evaluated with the surface depletion model (Kz-model) than when evaluated with the source depletion model (Gaussian). Far from the source, the plume concentration and doses calculated with surface depletion model are greater because the fraction of the release material remaining in the plume is larger. The lower doses obtained with the surface depletion model near the release source cause an even greater decrease in the estimated number of persons exceeding high doses, that bring about acute health effects.

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