Assessment of atmospheric dispersion and deposition from routine releases at Belgian Nuclear Power Plants

In order to assess the impact from atmospheric releases in the surrounding of a nuclear power plant atmospheric dispersion and deposition calculations are required. In this part these calculations are described and the results obtained presented. Air concentrations at ground level and deposition fluxes for the most important radionuclides released are calculated in detail by using the Immision Frequency Distribution Model (IFDM). IFDM allows for a straightforward, accurate modeling of emissions in the atmosphere. The ambient concentrations and deposition fluxes can be calculated on an hourly or a daily base, while averages, percentiles, minima and maxima can be reported. The inherent flexibility allows modeling results to be presented for most typical applications in a suitable format.
Air concentration at ground level and deposition fluxes for the nuclear power plants of Doel and Tihange are first calculated for a unit source term of 1 TBq/y for specific groups of radionuclides. The different release groups are treated separately because this allows the use of radionuclide specific dispersion parameters. Results of these unit source term calculations are later used for deriving results for release rates corresponding to the release limits and average release rates for the most important radionuclides over several years reported by the nuclear power plants of Doel and Tihange. Although the atmospheric calculations are performed in detail by e.g. using a 1-hour resolution local meteorological data set for a whole year several simplifications are required to reduce the complexity of reality. In general these simplifications are performed in a way that the calculation results are conservative, i.e. higher concentrations and deposition values are found than can be expected in reality. Atmospheric dispersion calculations have an inherent high uncertainty and this should be taken into account when using the results in further evaluations or assessments. Results can have easily an uncertainty of a factor of 2-3 or more even for calculations in the validated domain. For Tihange extra attention has to be paid to the results, especially at somewhat larger distances, due to the fact that topography is not taken into account.
In addition to the calculations of air concentrations at ground level and deposition values also maximal air concentration just above the release chimney are calculated and the calculation method described here is compared with the basic atmospheric calculations used by the nuclear power plants to assess the impact of routine atmospheric release on humans.