Wide range flux monitor assembly

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. The insulating layer preferably comprises aluminum oxide. The insulating material is intimately fit to the ion chamber assembly whereby thermal resistance is minimized between the ion chamber and the oversheath to provide a good heat flow out of the chamber assembly.