Neutron instrumentation of the EPR reactor

The EPR is a third-generation reactor featuring enhanced safety features. These devices enable the three main safety functions to be carried out with a high level of performance and redundancy:

• reactivity control ;

• core cooling ;

• containment of radioactivity.

For this purpose, the power characteristics in the core, e.g. average power level, internal power distribution or power peak factors, need to be known and monitored. This monitoring is carried out continuously or on request.

In all nuclear power reactors, since power generation is based on neutron-induced nuclear fission, neutron detectors are generally used. This nuclear instrumentation is added to the conventional instrumentation commonly used to measure thermal-hydraulic quantities, i.e. flow measurement by pressure variation around depressurizing elements, temperature measurement by variable resistance probes or thermocouples, pressure measurement by diaphragm deformation.

In particular, on the EPR reactor, nuclear power characteristics are monitored using the nuclear instrumentation described below. These are located either outside the vessel or in the core:

• instrumentation outside the tank, i.e. excore :

  • • Source Level Chambers (SLC),

  • • intermediate-level rooms (CNI),

  • • power level rooms (CNP) ;

• heart instrumentation, i.e. incore :

  • • fixed: collectrons or Self Powered Neutron Detectors with cobalt (Co-SPND),

  • • mobile: vanadium beads as part of the Aeroball Measurement System (AMS).

The purpose of these detectors is to convert the neutron flux into conventionally measurable information, such as an electrical signal. The following sections describe the location of each instrument, the mechanism used to generate the useful signal, and the functional uses of this signal. On the EPR reactor, these uses give rise to the following graduated responses:

• control actions to return a monitored parameter to its normal operating band around a control setpoint;

• triggering of alarms, when the parameter leaves its normal operating range defined on the basis of margins on physical safety criteria. These alarms can be accompanied by passive or active actions to facilitate return to the normal operating range, with limited impact on reactor operation;

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