Apparatus for monitoring the neutron flux of a nuclear reactor

Apparatus for monitoring neutron flux of a nuclear reactor located in biological protection concrete (6) surrounding a vessel (4) containing a reactor core (2) and having, in vertical shafts (8 to 22) located in the concrete in the vicinity of said vessel (4), a certain number of neutron detectors, each being associated in the form of assemblies, cascades or chains with electronics and connections, the chains being of three different types in order to cover a neutron fluence range extending over several decades, namely "source" chains for the low reactor power levels, "intermediate" chains for the medium power levels and "power" chains when the reactor is operating at high power, characterized in that the three chain types include detectors, at least one of which is a fission chamber (30, 32, 34, 36, 38, 40) able to operate in the pulse mode, fluctuation mode or current mode, and each of the shafts is equipped with identical neutron detectors, comprising a sensitive detector (26) and a certain number of the fission chambers 30, 32, 34, 36, 38, 40) distributed over the reactor height, each detector being located in a tube (24, 28) extending over the shaft height and extending outside the reactor to permit its introduction and/or removal independently of the other detectors.