When the reactor of underground nuclear power plant is arranged inside the mountain, the impacts of possible groundwater pollution on outside environment under the condition of extremely serious accident must be evaluated in the site se-lection stage. Based on the characteristics of rock mass in underground nuclear power cavities, we use an equivalent continuum model characterized as 3D permeability tensor to simulate the seepage features of fractured rock mass and set up a connotative fault seepage model to calculate the cavities’ seepage field, and further nuclide migration in fractured rock mass is computed. For a preliminarily selected plant site, the calculations show that the surrounding rock mass of underground cavern is in a drain-aged condition under the protection of seepage control measures, and even under the extremely serious accident, the nuclides will not migrate to the external environment.%地下核电站将反应堆等涉核建筑物布置于稳定的山体中，在厂址选择时必须评价极端严重事故工况下可能的地下水污染对外界环境的影响。结合地下核电站洞室群赋存岩体特征，提出采用以三维渗透系数张量表示的等效连续介质模型模拟裂隙岩体渗流特性，建立“隐含断层”渗流模型，求解洞室群三维渗流场。并以该渗流场为基础，进行核素运移计算。对某初选的地下核电站厂址进行模拟计算表明：在渗控措施作用下，地下洞室群工程区处于“疏干”状态；即使在严重事故工况下，向反应堆洞室内注入冷却水后，洞室内外渗流场不连通，核素不会随消防水渗入外界。
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