Measuring thermal neutron flux.

摘要

Local power range monitor strings (Fig. 2A) typically including four vertically spaced monitoring sites are provided in the core of a boiling water nuclear reactor. Each monitoring site includes a conventional local power range detector (M) in which fissionable material (64) exposed to thermal neutron radiation produces fission fragments, ionizes a gas (68) and produces a current between an anode and a cathode. Each local power range detector (M) is provided with one or two adjacent gamma thermometers (T). Each gamma thermometer (T) includes an interior mass (74) heated by gamma radiation, a thermocouple (78) for measuring the heated mass and a reference thermocouple (80) connected in series. Both the local power range detector and the gamma thermometer(s) are arranged in a string with individual connecting cables leading to external connectors, one for each detector and gamma thermometer. When the string is inserted into the core and the reactor operated at a steady state, the gamma thermometers are utilized to calibrate the local power range detectors in conjunction with a conventional 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.