Abstract: The normal residual heat removal (RHR) system is used to remove the decay heat generated in the reactor core when a nuclear power plant (NPP) is in Mid-loop condition. Once the RHR system is lost without taking any action, it could eventually lead to core damage following core boiling and uncovery. The RELAP5/Mod3.2 code was used to analyze the reflux condensation when RHR system was lost in Mid-loop condition for a 300 MW NPP. The result shows that when RCS is closed, the core may remain covered for 24 h if two SGs are filled with water, or one SG is filled with water and one auxiliary feedwater is available. Reflux condensation can be used as one of successful mitigation measures.