This Safety Guide provides recommendations on how to meet the requirements established in IAEA Safety Standards Series No. SSR-2/1(Rev. 1) in relation to the reactor coolant system and associated systems for nuclear power plants. It is a revision of IAEA Safety Standards Series No. NS-G-1.9 which it supersedes. The publication takes into account developments, experience and practices in the design of nuclear power plants throughout their lifetime. It references and considers other IAEA safety standards that are relevant and related to the design of the reactor coolant system and associated systems for nuclear power plants. Recommendations to achieve the required reliability of the capabilities designed to transfer residual heat to the ultimate heat sink in the different plant states are also included. As those systems are dependent on specific reactor technologies, more appropriate recommendations have been developed for pressurized light water reactors, boiling water reactors and pressurized heavy water reactors respectively.