Abstract: A wide range flux monitoring assembly is provided for in-core neutron flux monitoring in a nuclear reactor vessel. The assembly comprises a fixed in-core power range monitor and a fixed in-core extended range start-up monitor which are connected to signal processing circuitry means for generating signals representative of neutron flux in the reactor vessel. The extended range start-up monitor includes a fission chamber including an anode, a cathode, a neutron sensitive material interposed between the anode and the cathode and a cable assembly for communicating electrical signals between the anode and the cathode to the signal processing circuitry means. An insulating oversheath is disposed about the extended range start-up monitor for insulating the fission chamber from reactor vessel potential. The oversheath comprises a casing having an inner wall and an outer wall and includes an insulating layer interposed between the fission chamber and the inner wall.