Fixed in-core calibration devices for bwr flux monitors

Abstract

In the core of a boiling water nuclear reactor, local power range monitor strings typically including four vertically spaced monitoring sites are modified. Each monitoring site includes a conventional local power range detector in which fissionable material exposed to thermal neutron radiation produces fission fragments, ionizes a gas and produces a current between the anode and cathode. In the improvement herein, each conventional local power range detector is provided with one or two adjacent gamma thermometers; each gamma thermometer including an interior mass to be heated by gamma radiation, a thermocouple for measuring the heated mass and a reference thermocouple connected in series. Both the conventional local power range detector and the gamma thermometer(s) are all geometrically arranged in a string with individual connecting cables leading to external connectors, one for each detector and gamma thermometer. When the sring is inserted into the core and the reactor operated at a steady state, the gamma thermometers can be utilized to calibrate the local power range detectors in conjunction with a convetional reactor heat balance. By the expedient of referencing the gamma flux to the output of the heat balance, calibration of the local power range monitors occurs over their useful in-service life.