The design concepts of in-service inspection and maintenance are very important for the reactor system design of the nuclear power plant. The strategy of in-service inspection and maintenance should be reflected in the mechanical system design for the verification of the operability of liquid metal reactor KALIMER. In this paper the fundamental approaches of the in-service inspection and maintenance of the KALIMER are established to ensure the safety and reliability of the reactor system. The general method and requirement of the in-service inspection and maintenance for the reactor system and components are proposed and described to satisfy the intents of the ASME Section XI Division 3 and the design characteristics of KALIMER.