Research on the Influence of Complex Terrain on Atmospheric Dispersion After Accident

Emergency condition should be evaluated during the siting of Nuclear Power Plants (NPP). One of the criteria is the capability of emergency evacuation for the purpose of protecting the NPP staff and public. Exposure dose received by evacuees is a key factor to evaluate the effectiveness of emergency evacuation, which can be characterized by the atmospheric dispersion factors over the evacuation paths. Gaussian plume model is usually applied in the atmospheric dispersion factor calculation for NPPs. However, for some NPP sites with complex terrain like mountain and lake which has significant influence on the local flow field and the nuclides transportation, the calculations based on Gaussian model will not be accurate enough for the evaluation of the evacuation capability. In such cases, Computational Fluid Dynamics (CFD) method with fine calculation grids and rich physical models can be used in the atmospheric dispersion modeling of small-scale region, to get much more accurate results. This paper is devoted to the research on the impact of these complex terrain on atmospheric dispersion with CFD method. The local atmospheric dispersion around a NPP site located on an island is modeled, where there is a mountain on the way of evacuation. For the limitation of the geographic barrier, two evacuation paths, which are located closely with small difference in direction, are planned along different sides of the mountain. Utilizing STAR-CCM+ CFD software, local atmospheric dispersion factors are fine simulated under six worse wind directions. Based on the simulation results, the impact of the mountain on the flow field and of the radiological plume on the evacuation paths is analyzed. Results show that due to the blocking effect of the mountain, those two evacuation paths have little probability to be simultaneously covered by the radiological plume. By the choosing of suitable routes during the evacuation, the radiological risk of public is acceptable and emergency evacuation can be carried out effectively. These results also demonstrate that CFD modeling method is effective in the evaluation of local atmospheric dispersion over complex terrain, which can be applied in the analysis of emergency evacuation of NPP sites with complex terrain.