This International Standard applies to in-core neutron detectors and instrumentation which are designed for purposes important to safety: protection, control and information. It is restricted to characteristics and test methods for self-powered neutron detectors (SPNDs). Self-powered neutron detectors can be used for neutron fluence rate (flux) measurements and spatial power measurements in nuclear reactors. This standard gives requirements, recommendations and guidance on selection of the type and characteristics of SPNDs for different possible applications of SPNDs. For the principles of overall plant and I&C system design and the purpose of neutron fluence rate measurements, reference should be made to general principles of nuclear reactor instrumentation according to IAEA Codes and Safety Guides and IEC 61513.