Abstract: Digital reactor protection systems are currently widely used in nuclear power plants (NPP). The reactor protection systems, which perform important safety functions of nuclear power plants, have strict requirements and restrictions on their availability during normal operation of nuclear power plants. During the commissioning or overhaul of NPP, the reactor protection system is always out of service and in service. Due to the large number of equipment and interlock signals controlled by the reactor protection system, in order to ensure that equipment malfunction and personnel injury do not occur, it is necessary to formulate the out-of-service and in-service schemes. In the early reactor protection system using a large amount of analog circuit module and relays, the commissioning and maintenance personnel required a large number of disconnection and wiring work when performing reactor protection system out of service and in service. After using the digital reactor protection system，the workload of field maintenance personnel is reduced, there are still potential human error risks and potential risks in system design. This paper analyzes the current exit out-of-service and in-service schemes for reactor protection systems in domestic nuclear power plants and improves the automation level of maintenance systems by optimizing the design of reactor protection systems and equipment. The research reduces the workload and improves the work efficiency in order to reduce the risk of damage to the nuclear power plant equipment or personal injury caused by the reactor protection system out-of-service and in-service work.

Key words: nuclear power plants (NPP); protection system; digital control system