Radioactive materials released to the atmosphere will cause consequent radiation exposures to humans through various environmental pathways. JAERI has continued R&D of an atmospheric dispersion model that mathematically simulates the movement of radioactive nuclides released on complex terrain in cooperation with several other national institutes. In this project, JAERI is responsible for the development of the calculation model system for the atmospheric transport and diffusion of radioactive materials released to the environment. The model system consists of three models, an atmospheric model for calculating the spatial and temporal variations of the wind and air temperature, a second-order turbulence closure model for air turbulence, and a particle-type atmospheric dispersion model (See Fig. 4-13). In this program, simulation calculation tests and field tracer experiments for model verification were performed to confirm and improve the reproducibility of practical meteorological conditions and to confirm the effects of a forest on diffusion characteristics. Figure 4-14, which compares the observed meteorological data and the model calculation results, demonstrates that the model can predict well the regional wind fields and their temporal variations. The developed calculation model can be effectively used as a general tool for analysis of natural phenomena, as shown in Fig. 4-15. This is a calculated concentration distribution of natural radon (222Rn) emanated from the ground in a mountain and valley area. The model is to be applied to the prediction calculation for environmentally dispersed radioactive materials released by nuclear reactor accidents.

Reference H. Nagai et al., Examination of Atmospheric Dynamic Model's Performance over Complex Terrain under Temporarily Changing Synoptic Meteorological Conditions, J. Nucl. Sci. Technol., 32(7), 671 (1995).

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