Abstract: In a pressurized water reactor (PWR) nuclear power plant, the risk of loss of residual heat removal system at mid-loop operation is very significant. Thermal-hydraulic behaviors of mid-loop operation with loss of residual heat removal were analyzed. When reactor coolant pump and steam generator are under inspection and maintainance, the core losses coolant and temperature increases, which results in core melton. But when primary system is closed, the pressure of the reactor coolant system increases quickly and the system overpressures.