This standard applies to in-core (on-line) neutron detectors and instrumentation which is designed for safety, information or control purposes. It also applies to components in so far these components are contained within the primary envelope of the reactor. The detector types usually used are d.c. ionization chambers, fission ion chambers and self-powered neutron detectors. This standard is intended as a code of practice for the design of in-core instrumentation for neutron fluence rate measurements in thermal neutron reactors designed for power production. The major emphasis in this guide is on the general design aspect of on-line systems. Establishes requirements for system design, in-core neutron detectors in general, mechanical characteristics, electrical and nuclear characteristics, range of operating conditions, prototype testing, production testing, testing of system before operation.