Patent Number: 061730262
Section: claims

1. A power oscillation monitoring system for detection and indication of instability of a nuclear reactor having a plurality of fuel bundles and a plurality of neutron flux detectors for generating a plurality of respective output signals indicative of thermal-hydraulic oscillation frequencies of the fuel bundles, said system comprising a processor programmed to: (a) determine an instability threshold confirmation count based on the neutron flux detector output signals utilizing a period-based algorithm, determining the instability threshold confirmation count comprises generating an instability threshold flag S.sup.i.sub.th determined in accordance with the following function: ##EQU5##  (b) determine a confirmation density based on the instability threshold confirmation count utilizing a confirmation density stability algorithm; and  (c) generate a thermal-hydraulic instability signal when the confirmation density reaches a predetermined level.  where M is the number of active neutron flux detectors in the core and S.sub.th.sup.i is the instability threshold flag.  where E is an axial loss of period based algorithm efficiency, and F.sup.max.sub.CD is a bounding maximum fraction wherein the denominator is the number of active neutron detector output signals and the numerator is the number of active detector output signals that equal or exceed the instability threshold confirmation count.  (a) determining an instability threshold confirmation count based on the neutron flux detector output signals utilizing a period-based algorithm;  (b) determining a confirmation density based on the instability threshold confirmation count utilizing a confirmation density stability algorithm, determining the confirmation density comprises the step of generating an instability threshold flag status S.sup.i.sub.th for each of the neutron flux detector output signals in accordance with the following function: ##EQU7##  (c) generating a thermal-hydraulic instability signal when the confirmation density reaches a predetermined level.  where M is the number of active neutron flux detectors in the core and S.sup.i.sub.th is the instability threshold flag status for each of the neutron flux detectors.  where E is an axial loss of period based algorithm efficiency, and F.sup.max.sub.CD is a bounding maximum fraction wherein the denominator is the number of active neutron detector output signals and the numerator is the number of active detector output signals that equal or exceed the instability threshold confirmation count. 2. A power oscillation monitoring system in accordance with claim 1 wherein the period-based algorithm is tuned based on a successive confirmation count model. 3. A power oscillation monitoring system in accordance with claim 1 wherein the neutron flux detectors are local power range monitors. 4. A power oscillation monitoring system in accordance with claim 1 wherein the confirmation density is a fraction wherein a denominator is the number of active neutron detector output signals and a numerator is the number of active neutron flux detector output signals that exceed a target successive oscillation period confirmation count. 5. A power oscillation monitoring system in accordance with claim 1 wherein the instability signal has a first state and a second state, and the signal remains in the first state until the confirmation density exceeds the a confirmation density setpoint. 6. A power oscillation monitoring system in accordance with claim 1 wherein the confirmation density is determined in accordance with the following function: ##EQU6## 7. A power oscillation monitoring system in accordance with claim 1 wherein the instability threshold confirmation count N.sub.th is equal to 11. 8. A power oscillation monitoring system in accordance with claim 1 wherein a maximum confirmation density setpoint is determined in accordance with the following function: EQU S.sup.max.sub.CD =(1-E) * F.sub.CD.sup.max 9. A power oscillation monitoring system in accordance with claim 8 wherein the axial loss of period based algorithm efficiency is equal to 0.25. 10. A power oscillation monitoring system in accordance with claim 8 wherein the maximum confirmation density setpoint is in the range of approximately 0.2 to 0.33. 11. A method for indicating instability of a nuclear reactor using a power oscillation monitoring system, the reactor having a core including a plurality of fuel bundles and a plurality of neutron flux detectors, the flux detectors distributed throughout the core contiguous the fuel bundles, each of the neutron detector providing an output signal being indicative of thermal-hydraulic oscillation frequencies of the fuel bundles, the system having a processor, said method comprising the steps of: 12. A method in accordance with claim 11, wherein the periodic based algorithm is tuned based on a successive confirmation count model. 13. A method in accordance with claim 11, wherein the step of generating a confirmation density is determined in accordance with the following function: ##EQU8## 14. A method in accordance with claim 11, wherein generating an instability signal further comprises the step of comparing the confirmation density to a predetermined level in accordance with the following function:. EQU CD&gt;=S.sub.CD.sup.max 15. A method in accordance with claim 11, wherein the maximum confirmation density setpoint S.sub.CD.sup.max is in the range 0.2 to 0.33. 16. A method in accordance with claim 11 wherein the instability threshold confirmation count N.sub.th is equal to 11. 17. A method in accordance with claim 11 wherein determining a confirmation density further comprises the step of determining a maximum confirmation density setpoint in accordance with the following function: EQU S.sup.max.sub.CD =(1-E) * F.sub.CD.sup.max 18. A method in accordance with claim 17 wherein the axial loss of period based algorithm efficiency is equal to 0.25.