This standard applies to systems used for monitoring the vibratory behaviour of the internal structures of pressurized water reactors (core barrel, thermal shield, upper and lower core support, etc.) and fuel assemblies on the basis of neutron fluctuations observed outside the vessel and vessel vibrations. The main objective of monitoring described in this standard is to detect degradation of internal structures. Primary circuit measurements can be considered together with internal monitoring to provide further information and make it possible to detect degradation of primary circuit structural supports. This standard covers the system characteristics and gives recommendations for monitoring.