Patent Number: 063109298
Section: claims

1. An in-core fixed nuclear instrumentation system for a reactor, comprising: a plurality of in-core nuclear instrumentation assemblies each having a nuclear instrumentation tube, a plurality of fixed neutron detectors housed in the nuclear instrumentation tube and adapted to detect neutron flux of a local power distribution of a power range in a core of the reactor and a gamma-ray thermometer assembly housed in the nuclear instrumentation tube, said gamma-ray thermometer assembly having a plurality of fixed .gamma.-ray heating detectors for detecting .gamma.-ray heating values and a heater built therein and adapted to calibrate the fixed .gamma.-ray heating detectors, said fixed .gamma.-ray heating detectors being arranged at least close to the fixed neutron detectors;  means for processing a neutron flux detection signal based on the detected neutron flux by each of the fixed neutron detectors;  means for processing a gamma-ray thermometer signal based on the detected .gamma.-ray heating value by each of the fixed .gamma.-ray heating detectors of each of the gamma-ray thermometer assemblies;  means for electrically energizing the heater in each of the gamma-ray thermometer assemblies;  means for storing a plurality of predetermined time intervals therein; and  means for selecting one of the predetermined time intervals for specified .gamma.-ray thermometer assemblies respectively,  wherein said energizing means is adapted to control an electrical energy supplied to the heater according to the selected one of the predetermined time intervals so as to heat the heater, thereby executing a heater calibration of output voltage sensitivities of the fixed .gamma.-ray heating detectors of the gamma-ray thermometer assembly.  said selection means has means for storing at least one of an in-core mounted time of each of the gamma-ray thermometer assemblies and an in-core irradiation quantity of each of the fixed .gamma.-ray heating detectors thereof, which are calculated while the reactor operates, said in-core mounted time of each gamma-ray thermometer assemblies representing an operating time of the reactor after each of the gamma-ray thermometer assemblies is mounted in the core, and means for selecting one of the predetermined time intervals according to at least one of the in-core mounted time and the in-core irradiation quantity corresponding to the gamma-ray thermometer assembly.  in a case where the change curve of the output voltage sensitivity exceeds a predetermined judgment value present with respect to a predetermined future time, set another one of the predetermined time intervals prior to the selected time interval, said another one of the time intervals being shorter than the selected time interval or being a maximum time interval of the predetermined time intervals satisfying the predetermined judgment value, and said energizing means executes the heater calibration of the output voltage sensitivity by heating of the heater at a reset future time in accordance with the set another one of the time intervals.  in a case where the change curve of the output voltage sensitivity is less than a first predetermined judgement value preset with respect to a second predetermined future time interval, set the heater calibration interval with the second predetermined time interval in the stored predetermined time intervals, said second predetermined time interval being longer than the first predetermined time interval so as to execute the heater calibration of the output voltage sensitivity after the second predetermined future time interval by heating of the heater, in a case where the change curve of the output voltage sensitivity exceeds the first predetermined judgement value preset with respect to the second predetermined future time interval, execute the heater calibration of the output voltage sensitivity by heating of the heater in accordance with the first predetermined time interval,  when executing the heater calibration at the second predetermined time interval, in a case where the change curve of the output voltage sensitivity is less than a first predetermined judgement value preset with respect to a third predetermined future time interval, set the heater calibration interval with the third predetermined time interval in the stored predetermined time intervals, said third predetermined time interval being longer than the second predetermined time interval, so as to execute the heater calibration of the output voltage sensitivity after the third predetermined future time interval by heating of the heater, in a case where the change curve of the output voltage sensitivity exceeds the first predetermined judgement value preset with respect to the third predetermined future time interval, execute the heater calibration of the output voltage sensitivity by heating of the heater in accordance with the second predetermined time interval.  in a case where the change curve of the output voltage sensitivity exceeds a predetermined judgment value preset with respect to a predetermined future time of current selected time interval, set another one of the predetermined time intervals, said another one of the time intervals being a maximum time interval which is capable of being selected in the predetermined time intervals or of being an adjacently shorter than the current selected time interval, and said energizing means executes the heater calibration of the output voltage sensitivity at the predetermined future time by heating of the heater in accordance with the set another one of the time intervals. 2. An in-core fixed nuclear instrumentation system according to claim 1, wherein said means for processing a gamma-ray thermometer signal has means for measuring increases of the output voltages of the fixed .gamma.-ray heating detectors of the gamma-ray thermometer assemblies, and said energizing means has means for measuring current and voltage corresponding to the supplied electrical energy with the energizing means so as to execute the heater calibration according to the measured increases of the output voltages of the .gamma.-ray heating detectors and the measured currents and voltages, said increases of the output voltages being caused by the heating of the heaters, and 3. An in-core fixed nuclear fixed nuclear instrumentation system according to claim 2, wherein said means for processing a gamma-ray thermometer signal is adapted to store time series data of the output voltage sensitivity of the fixed .gamma.-ray heating detector of each of the gamma-ray thermometer assemblies, estimate a change curve of each output voltage sensitivity thereof by sampling latest two and over time series data points from a present point of time by using the time series data, 4. An in-core fixed nuclear instrumentation system according to claim 2, wherein said means for processing a gamma-ray thermometer signal comprises means for automatically judging whether or not the selected time interval at a present point of time is changed to another one of the predetermined time intervals, said another one of the predetermined time intervals being adjacently longer than the selected time interval. 5. An in-core fixed nuclear instrumentation system according to claim 2, wherein said predetermined time intervals includes a first time interval, a second time interval, a third time interval and so on in an order from a shortest time interval, and wherein said energizing means is adapted to execute, at first, the heater calibration at the first shortest predetermined time interval in the stored predetermined time intervals so as to store time series data of the output voltage sensitivity of the fixed .gamma.-ray heating detector of each of the gamma-ray thermometer assemblies according to each execution of the heater calibration, estimate a change curve of the output voltage sensitivity thereof by sampling latest two and over time series data points from a present point of time by using the time series data, 6. An in-core fixed nuclear instrumentation system according to claim 2, wherein said means for processing a gamma-ray thermometer signal is adapted to store time series data of the output voltage sensitivity of the fixed .gamma.-ray heating detector of the gamma-ray thermometer assembly according to each execution of the heater calibration, estimate a change curve of the output voltage sensitivity thereof by sampling latest two and over time series data points from a present point of time by using the time series data,