Abstract: A nuclear reactor monitoring system includes incore and excore neutron detectors for producing neutron noise signals and system data including at least one of reactor coolant pump motor current, temperature and pressure of the reactor coolant system and pressurizer pressure and means for diagnosing conditions of the reactor internals and core support structures by comparing analyzed instrumentation signatures. The signatures may be derived from scale model test data, plant test data, analytical model results and trending analysis results. Analyses performed by the system include time and frequency domain analyses of the neutron noise signals and the system data. The analyzed data are compared with rules in a rules base of an expert system to generate a diagnosis of the condition of the reactor internals and core support structures and action recomendations for maintenance of the nuclear reactor.