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
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.
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
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 geometry of the SPND elements and the purity of the materials used for sensitivity.
A high degree of reproducibility of the sensitivity of the SPND makes it possible to provide high accuracy in relative measurements.
Confirmation of the identity of the geometric dimensions of the SPNDs and the materials used in their production can be used as an indirect calibration method.
8.3 Background signal
A background signal is generated primarily by neutron and gamma induced currents in the signal cable. Compensation may be achieved by using a separate background SPND (identical device without an emitter) and subtracting its signal from the SPND signal. Another technique involves the use of twin-wire cables (with twisted or not wires) in the SPND construction, with one wire connected to the emitter and the second wire used as the background wire.IEC 61468 pdf download.