核能是我国能源战略中的重要组成部分，核能的安全利用是核电发展的前提，一旦发生事故，造成的后果对环境和人员将带来巨大危害。为了估计核事故后放射性物质对周边环境和人员造成的危害，首先需要计算放射性物质在大气扩散、输运和沉降作用下的时空分布。目前在放射性物质大气扩散模拟的研究中，缺乏放射性衰变、干沉降、湿沉降等多种物理过程耦合的放射性物质大气扩散模型，气象参数的计算多与扩散过程分离，可用的物理过程参数化方法有限，不利于开展研究和实时预测。此外，对气象条件、地形条件、核素特征等要素对扩散和沉降的影响研究比较缺乏。本文建立了放射性物质大气扩散数值模型，使用观测数据对模型进行了验证，基于该模型，研究了放射性物质的大气扩散规律及环境辐射剂量的评估方法，主要内容如下：（1）放射性物质大气扩散模型的建立。在WRF/Chem的基础上，建立了用于计算放射源排放、放射性衰变过程、干沉降过程及湿沉降过程的模型（包含多种干、湿沉降参数化方法），从而形成了可用于模拟多物理过程耦合作用的放射性物质大气扩散数值模型。使用2011年福岛核事故的观测数据对模型进行了验证，模型总体上可以很好地模拟放射性物质的大气扩散、沉降。（2）放射性物质的大气扩散规律研究。基于建立的放射性物质大气扩散模型，研究风速、风向、降水、干沉降参数、湿沉降参数、源项排放率、源项排放形式、放射性物质的气体/粒子形态比例、粒子粒径分布以及城市冠层模型对放射性物质大气扩散的影响，基于误差计算，展开了敏感性分析，并提出了相对优化的模拟参数方案。结果表明，放射性物质的大气扩散和沉降对风场和降水等气象参数，源项的排放率和排放形式，以及气体/气溶胶粒子形态的比例等要素比较敏感；放射性物质的大气扩散和沉降对于湿沉降参数比干沉降参数更为敏感，在绝大部分区域，湿沉降量大于干沉降量，在沉降中占主要地位；放射性物质的粒子粒径分布及城市冠层模型参数化方案的选择对放射性物质的大气扩散和沉降影响不大。（3）基于放射性物质大气扩散模拟的辐射剂量评估研究。提出了基于放射性物质大气扩散模拟的环境辐射剂量估算方法，提出了基于剂量标准划分应急处置区域的方法。针对空气浸没外照射，考虑城市建筑物的空间屏蔽作用，提出了屏蔽因子的计算方法；针对吸入内照射，考虑城市区域的结构、风场特征，结合建筑物表面的风压分布特征，提出了室内吸入内照射剂量的估算方法。

In order to assess the harm and risk to the surrounding environment and the public, it is definitely necessary to calculate the spatial and temporal distribution of the air concentration and ground depositions of the radionuclides. Most of the previous studies on the numerical models did not take the hybrid processes of radioactive decay, dry deposition and wet deposition into account. Moreover, the meteorological fields and the atmospheric transport are always separated, which can sometimes cause a loss of important information about atmospheric processes, thus it is not suitable for a high-time-resolution prediction. Furthermore, the parameterization schemes for some important physical or chemical processes are limited. In addition, the influence of the meteorological fields and the terrain and the emission characteristics on the atmospheric dispersion and deposition has not been thoroughly studied.In this paper, a numerical model of atmospheric dispersion of radionuclides is established and it is validated by the observed data. Based on the model, some important laws of the atmospheric dispersion of radionuclides and the assessment methodologies of the radiation dose to the people and environment are investigated. The main research contents of this paper are as follows:(1) The establishment of the atmospheric dispersion model of radionuclides. Based on WRF/Chem, an upgraded model is developed by implementing a radioactive decay term into the advection-diffusion solver and adding the emission setting module, the dry deposition module and the wet deposition module, which can be used for the simulation under the hybrid effect of multiple processes. In addition, the forecasting of the transport and meteorology is coupled, allowing the transport simulations to exploit the full spatial and temporal resolutions of the meteorological simulations. The model is validated by the observed data from the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident (March 2011). The results show that the model can accurately predict the air concentration and ground deposition of the radionuclides.(2) Study on the laws of the atmospheric dispersion of radionuclides. Based on the model, the influence on the simulated transport and deposition of the wind speed, the wind direction, the precipitation, the dry deposition parameterizations, the wet deposition parameterizations, the emission rate, the emission type, the gas partitioning, the size distribution and the urban canopy parameterizations, is examined. According to the error calculations, the sensitivity of the model’s performance to various parameterizations is assessed to obtain relatively better parameterizations for a specific simulation. The results show that the deposition is sensitive to the wind fields, precipitation, emission rate and type and gas partitioning; while it is less sensitive to the dry deposition parameterizations than to the wet deposition parameterizations, in addition, deposition is dominated by wet deposition over most of areas; while it is not significantly influenced by size distribution and urban canopy parameterizations.(3) Study on the assessment of radiation dose based on the model of atmospheric dispersion of radionuclides. A method of the assessment of radiation dose based on the model of atmospheric dispersion of radionuclides is proposed and a method for deciding the evacuation zones based on the dose limitation standard is also developed. In terms of the dose from immersion in the atmospheric discharge plume, a method is proposed to calculate the shielding factor considering the shielding effect by the urban buildings; In terms of the dose from inhaled radionuclides, a method for the assessment of the inhalation dose of indoor radionuclides is developed, which takes the urban structure, wind fields and wind pressure distribution on the building surfaces into account.