Patent Number: 052251497
Section: claims

1. A method for detecting thermal hydraulic oscillation in a nuclear fission reactor core through which a reactor coolant passes while the fission reactions in the core generate neutrons and gamma radiation, comprising the steps of: positioning a first plurality of sensors, which are primarily responsive to neutron flux, at a respective plurality of first locations in the core;  positioning a second plurality of sensors, which are primarily responsive to gamma radiation flux, at a respective plurality of second locations in the core;  generating an output signal from each of said neutron sensors, commensurate with the neutron flux at each neutron sensor;  generating an output signal from each of said gamma sensors, commensurate with the gamma radiation flux at each gamma sensor;  comparing each neutron sensor output signal with a respective gamma sensor output signal and generating a comparison value signal for each of said comparisons; and  in response to the comparison value signals generating output data indicative of core thermal hydraulic oscillations.  generating intermediate output data indicative of the spatial distribution within the core, of said comparison values;  monitoring the intermediate output data while coolant flows through the core; and  in response to the monitored intermediate output data, generating said output data as final output data.  the step of positioning includes supporting each neutron sensor in fixed spatial relationship to one gamma sensor, thereby defining a plurality of sensor pairs, and  said step of comparing includes the step of comparing the neutron output signal with the gamma sensor output signal for each of said pairs.  the step of comparing includes determining the difference of the signal amplitudes for each pair.  said first plurality of sensors includes at least four neutrons flux sensors spaced apart axially in the core, and  said second plurality of sensors includes at least four gamma flux sensor spaced apart axially in the core.  sensing at each of a plurality of axially spaced apart zones within the core, changes in the local neutron flux and gamma flux; and  from said sensed changes, generating output data indicative of incipient thermal hydraulic oscillations in the core.  a plurality of a first type of sensor spatially distributed in the core, said first type of sensor having a first time dependent output signal, commensurate with the density of reactor coolant over a short range during the generation of power in the reactor core;  a plurality of a second type of sensor spatially distributed in the core, said second type of sensor having a second time dependent output signal which, relative to the first type of sensor, is commensurate with the density of the coolant over a long range during power operation of the reactor core;  means for associating each of said first type of sensors with one of said second type of sensors, to define a plurality of sensor pairs;  means for generating a third output signal commensurate with a quantitative relationship between the first and second output signals from the sensors in each pair, thereby defining a plurality of paired time dependent measurement values; and  means responsive to the paired measurement values, for generating output data indicative of thermal hydraulic oscillations in the core. 2. The method of claim 1, further including the steps of, 3. The method of claim 1, wherein 4. The method of claim 2, wherein the step of generating intermediate output data includes generating a comparison value in the form of a symbolic code indicative of whether the location of each neutron sensor is experiencing an increase or decrease in coolant voids. 5. The method of claim 3, wherein the step of positioning each sensor in each sensor pair at respective first and second locations, includes supporting the sensors in each pair no farther apart than about ten percent of the core axial dimension. 6. The method of claim 3, wherein the step of positioning each sensor in each sensor pair at respective first and second locations, includes the step of supporting the sensors no farther apart than about one inch. 7. The method of claim 3, wherein the step of positioning includes supporting each sensor in the sensor pair at respective first and second locations within a common housing. 8. The method of claim 3, wherein the neutron sensor output signal includes a time-dependent amplitude commensurate with the neutron flux and the gamma sensor output signal includes a time-dependent amplitude commensurate with the gamma flux and the step of comparing includes determining a time phase difference between the signal amplitudes for each pair. 9. The method of claim 3, wherein the neutron sensor output signal amplitude is commensurate with the neutron flux and the gamma sensor output signal is commensurate with the neutron flux, and 10. The method of claim 3 wherein, 11. The method of claim 8 wherein said plurality of sensor pairs include at least one pair in each of at least two axial zones in the core. 12. The method of claim 10 wherein each of said gamma sensors is located within a distance from one of said neutron sensors, equivalent to no more than about ten percent of the axial dimension of the core. 13. The method of claim 10, wherein each of said gamma sensors is located within about one inch from one of said neutron sensors. 14. A method for detecting thermal hydraulic oscillations parallel to the axis of the core of a boiling water nuclear reactor, comprising the steps of: 15. The method of claim 14, wherein the step of sensing includes supporting in each zone a gamma flux sensor in fixed spatial relation to a neutron flux sensor at substantially the same location in the zone. 16. The method of claim 15, wherein the step of sensing changes includes comparing the sensed neutron flux to the sensed gamma flux at each location and generating a comparison value signal commensurate with said comparison. 17. The method of claim 15, wherein the step of generating output data includes generating symbolically coded values for each location, commensurate with a quantitative relationship between the neutron and gamma fluxes as sensed at each location. 18. A system for monitoring thermal hydraulic oscillations in a boiling water nuclear reactor, comprising: 19. The system of claim 18, wherein the second type of sensor is responsive to gamma radiation intensity. 20. The system of claim 18, wherein the first type of sensor is responsive primarily to thermal neutron flux and the second type of sensor is responsive primarily to gamma radiation intensity. 21. The system of claim 20, including means for supporting the first and second sensors of a given pair within a common housing in the core. 22. The system of claim 21, wherein the housing is an in-core instrument tube passing through the full longitudinal dimension of the core, and wherein each tube contains a plurality of axially spaced neutron flux sensors, and at least one gamma flux sensor.