Nuclear reactor with in-vessel ex-core neutron detectors and corresponding control method

A pressurized water reactor comprising:

• a vessel having a central axis;
• a core positioned in the vessel, the core comprising a plurality of nuclear fuel assemblies, a primary water layer separating the core from the vessel radially from the central axis and surrounding the core;
• an instrumentation and control system configured to provide an instrumentation and control of the reactor in a predetermined power range of the reactor, the instrumentation and control system comprising a measuring device for continuous measurement of a neutron flux emitted by the core;
• at least one neutron absorbing member;
• and
• a movement device configured for inserting the or each neutron absorbing member into the core,
• the measuring device including a plurality of neutron detectors continuously measuring the neutron flux, the plurality of neutron detectors being positioned in the primary water layer radially between the core and the vessel, the plurality of neutron detectors each being radially positioned for obtaining, owing to an attenuation of the primary water layer, one or several counting rates configured for the instrumentation and control system of the reactor for an entirety of the predetermined power range, the predetermined power range being from a residual power upon stopping of the reactor to a nominal power of the reactor,
• the instrumentation and control system comprising a controller configured to control the movement device by using measurements done by the plurality of neutron detectors,
• wherein each of the plurality neutron detectors is positioned at a same fixed radial distance from the core as each other neutron detectors and is circumferentially spaced apart from each other neutron detector around the central axis of the vessel.