A nuclear reactor (1) includes a vessel (3) has a central axis (X); and a core (5) arranged in the vessel (3). The core (5) includes a plurality of nuclear fuel assemblies, a primary water layer (7) separating the core (5) from the vessel (3) radially from the central axis (X) and surrounding the core (5); and a system (9) for controlling and protecting the nuclear reactor. The control and protection system (9) includes a device (11) for continuously measuring the neutron flux emitted by the core (5). The measuring device (11) includes at least one neutron detector (12) arranged in the primary water layer (7) separating the core (5) from the vessel (3).