| Molten salt reactor(MSR) is the only one of Generation IV Reactors which adopts liquid fuel. It possesses many advantages such as inherent safety, good economy, sustainability, nuclear nonproliferation and so on. Chinese Academy of Sciences launched the Thorium Molten Salt Reactor nuclear energy system(TMSR) research program in 2011, and 2MW liquid-fueled molten salt experimental reactor(TMSR-LF1) is a part of the program. During reactor operation, the release of radioactive effluent is inevitable. Since molten salt reactor is not cooled by water, the main products of its effluent are gaseous radionuclides. Airborne radioactive effluents can dispersion through environmental medium and be taken in human bodies by breathing, immersing and eating. As a consequence, the public will suffer the radiation effect injury. Therefore it is necessary to make radiation effect analysis for the reactor. According to the characteristics of the fourth generation advanced reactors and “Safety Classification of Research Reactor(trial out) ", the airborne radioactive radiation environmental impact assessment of TMSR-LF1 focused on the near field range. Combining with the characteristics of TMSR-LF1, we first calculated the environmental release under normal operation and hypothetical accident cases. Then a calculation of radioactive activity concentration was performed by using the result of first step. In this step, we used computational fluid dynamics(CFD) method and Gaussian plume model to simulate respectively near-field and far-field airborne radioactive effluent dispersion. Finally, the exposure doses for the staff and the public were calculated based on the recommended dose calculation model by ICRP. This study can provide references for site evaluation, radiation protection and nuclear emergency plan. The main chapters of this paper are as follows:Chapter I: Introduction. This chapter introduces an overview of the history and development status of molten salt reactor and the method of environment impact analysis for airborne radioactive effluents. Emphatically, various kinds of atmospheric dispersion models and their merits and drawbacks were showed in detail.Chapter II: Analysis for environmental release of airborne radioactive radionuclides. Source term is the foundation and prerequisite of environmental impact assessment, which directly affects the accuracy and reliability of result. According to the online dispose character of MSR and core structure and physical parameters of TMSR-LF1, the SCALE 6.1 program which added online processing function was used to calculate the yields of radionuclides in the core of TMSR-LF1. Then combining with the migration behaviors of airborne radionuclides in TMSR-LF1 system, we analyzed environmental release under the conditions of normal operation and hypothetical accident.Chapter III: Numerical simulation to far-field dispersion of airborne radioactive effluents. This chapter is about an analysis for far-field dispersion of airborne radioactive effluents from TMSR-LF1 by using the modified Gaussian plume model. We analyzed the influence degree and range of different parameters and correction method of Gaussian plume dispersion model. The correction methods included source strength loss, index wind profile and surface roughness. Then we calculated radionuclide activity levels in both normally and hypothetical accident cases. The calculation results can provide conference for dose assessment of subsequent different groups.Chapter IV: Numerical simulation to near-field dispersion of airborne radioactive effluents. Since the complex terrain within the near-field range, the traditional Gaussian plume model has some limitations. The method of CFD was used to simulate the near-field effluent dispersion, which was widely used on the pollutant dispersion in street canyons. We compared the results of atmospheric dispersion factors calculated by the following two methods in the case of no buildings: CFD method and Gaussian plume model. Because of the difference of two kinds plume rise height and the spread of the horizontal and vertical direction, the results of the same location of Gaussian plume model are all less than those obtained by CFD method and they shows the limitation of the Gaussian plume model in the near field further more. Through the analysis of influences due to different factors on the distribution of atmospheric dispersion factors around buildings, we conclude that the greater the wind speed is, the higher the concentration is in near-field area; the pollutant accumulation areas will be prone to present at upwind side and leeward side of buildings. Base on the above, we calculated the ground-level activity concentration under different situations in site.Chapter V. Analysis of exposure dose of airborne radioactive effluents. Based on the calculations of near field and far field concentration distribution of radionuclide under normal and accident situations and the recommended dose calculation mode by ICRP, we calculated the exposure doses for staff and the public, and analyzed main exposure pathways and radionuclides. Under normal operation, the exposure pathways of workers include internal exposure of inhalation, external exposure of immersion and ground deposition. The exposure pathways of the public are plus internal exposure of ingestion on the ways of workers. Under hypothetical accident, we calculated cumulative exposure doses of different groups in two ways: inhalation and immersion.Chapter VI. Summary and prospect. Summarized the main work of this research and prospected the possible improvements. |
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