For the purpose of cooling off their reactors, nuclear power plants are located near rivers, ponds or in coastal areas. In temperate regions and during fall and winter, they are frequently exposed to fogs. Operational models
of atmospheric dispersion and deposition used to estimate radioactive contamination of the environment after a nuclear accident, consider dry deposition and wet deposition by rain. Should we take into account the deposition by fog droplets as an additional process in term of deposition? To answer this question, an experimental study has been setup on several sites often exposed to fogs or clouds. Fogwater collection and measurement of
fogwater deposited on plants were realized with string collectors and precision balances, respectively. Different plant types were used for this study: small conifers, cabbage, grass plus a bare soil as a reference deposition surface. By measuring the liquid water content (LWC) and the mass of water deposited on plants, fog droplet deposition velocities can be calculated. In the case of fog composed mainly of big droplets (mean median volume diameter of 20 μm), deposition velocities of these droplets can reach several tens cm.s-1. These high deposition velocities highlight the fact that gravitational but also turbulent processes are both contributing to the deposition on plants especially with a tri-dimensional structure such as conifers. The edge effect linked to a strong density of trees has also been identified and quantified during our experiments. The radionuclides found in trace amounts in fogwater showed that fogwater could be up to twenty times more concentrated in radionuclides than rain
water. It is due to the activity dilution during the condensation phase of the droplets, when the droplets are growing. Based on measurements of the mass of water deposited by fog and concentrations of radionuclides in fog droplets on the site of Houdelaincourt, occult deposition that cannot be quantify by rain gauges or meteorological radar can be estimated. Over a whole season of fog, occult deposition can represent up to 25% of the total deposition (dry and wet by rain) of radionuclides. In case of an accidental release, fog could contribute to 14% of the total deposition if no rain event occurs. Those results highlight the fact that fog deposition should be considered to better quantify radioactive fallouts in areas embedded by fog (or clouds for high altitude sites), particularly in post-accident situation. A simple model of dry deposition to simulate the
droplets deposition by sedimentation can be developed, using the median diameter of droplets derived from the visibility. This work still needs to be completed to quantify the turbulence contribution induced by the air flow
near obstacles such as large plants and to implement a specific scheme into deposition model.