Abstract: When the inlet pipe of the residual heat removal (RHR) system ruptures on the downstream of the containment isolation valve, the radionuclides in the primary coolant release to the atmosphere directly. A method to assess the radiological consequence of this accident was given in this paper according to the accident process. In addition, calculation results were given with different assumptions of the isolation time of the broken RHR system. The results show that 300 s can be chosen as the limiting time, as one of the design basis of the RHR isolation system for Chinese generation Ⅲ nuclear power plant, according to the calculation results and the requirement of GB 6249—2011.