IEC 61468-2021 Nuclear power plants – Instrumentation systems important to safety – In-core instrumentation: Characteristics and test methods of self-powered neutron detectors

IEC 61468-2021 Nuclear power plants – Instrumentation systems important to safety – In-core instrumentation: Characteristics and test methods of self-powered neutron detectors. 7 Application recommendations 7.1 General For each specific task, an SPND shall be selected with optimal characteristics for the solution of this problem. At the same time, for a solution of practically all problems, a small burn-up rate of the emitter is essential. High burn-up may be acceptable if a calibration system is available or burn-up is accounted for by the calculation method. 7.2 Fluence rate mapping – Core monitoring and surveillance SPNDs may be used for fluence rate mapping and core monitoring and surveillance with or without an associated calibration system. 7.3 Power regulation – Feedback control SPNDs may be used for feedback control purposes, however, acceptability of a signal delay will depend on the following limitations: – axial power shape control may tolerate moderate delay; – integral power control shall not permit signal delay. 7.4 Core protection The characteristics required for SPNDs that are used for local core protection purposes shall be determined by consideration of the transient behaviour of the fluence rate levels. Prompt signal response or adequate dynamic compensation to achieve prompt response is required. Low sensitivity typical for prompt response SPND designs may be accepted by using amplifiers with low noise characteristics. 7.5 Reactor noise analysis SPNDs with a large prompt signal response may also be used to analyse the reactor noise, this providing additional information about the reactor core and its elements. 7.6 Classification SPNDs may be used as sensors in measurement channels that provide safety functions having different degrees of importance for safety. These safety functions should be categorised according to IEC 61 226 and the SPNDs shall be designed to meet the appropriate safety class of the system. 8 Design recommendations 8.1 General SPNDs shall withstand the effects of radiation, temperature, pressure, vibration, seismic and chemical corrosion caused by the environment present in the reactor core. The design of the detector should, if necessary, provide for the possibility of fixing it in the core, and also provide the necessary resistance to bending and stretching during transportation and installation. 8.2 Reproducibility of SPND characteristics The design of the SPND shall ensure the reproducibility of the SPND characteristics, which may be achieved due to the high accuracy of manufacturing of the SPND elements and taking into account the effect of changing the...