Patent Number: 
Section: claims

1. A nuclear reactor power monitor, comprising:a first calculation unit configured to calculate a first stability index of the reactor core based on time series data which indicate a power oscillation in nuclear instrumentation signals outputted from a plurality of nuclear instrumentation detectors which detect neutrons in a reactor core;a first determination unit configured to compare the first stability index and a first reference value and determine whether nuclear thermal hydraulic stability of the reactor core is stable or deteriorated;a second calculation unit configured to calculate a second stability index of the reactor core based on the time series data when the nuclear thermal hydraulic stability is determined to be deteriorated in the first determination unit; anda second determination unit configured to compare the second stability index and a second reference value and determine whether to perform suppressing operation of the power oscillation,wherein in response to determination in the first determination unit and the second determination unit, any of an alarm, oscillation information and an automatic start signal of an oscillation suppression device is issued in stages as a suppression operation of the power oscillation. 2. The nuclear reactor power monitor according to claim 1, comprisinga grouping unit configured to divide the nuclear instrumentation detectors into groups, whereinthe grouping unit performs grouping of the nuclear instrumentation detectors based on information including any of an estimated power distribution of the reactor core, a higher order space mode distribution of neutron fluxes, and a specified fuel assembly position. 3. The nuclear reactor power monitor according to claim 2, whereinthe nuclear instrumentation signals processed in the first calculation unit and are individual signals of the nuclear instrumentation detectors selected from the groups or an average signal of a plurality of nuclear instrumentation detectors in units of the groups. 4. The nuclear reactor power monitor according to claim 2, whereinthe power distribution or the higher order space mode distribution of neutron fluxes is estimated based on a physical model or a data base. 5. The nuclear reactor power monitor according to claim 1, whereinan index indicating variations in oscillation period of a plurality of the time series data is used as the first stability index. 6. The nuclear reactor power monitor according to claim 1, whereinan amplitude or a decay ratio of a plurality of the time series data is used as the second stability index. 7. The nuclear reactor power monitor according to claim 5, whereinthe oscillation period is obtained by applying a statistical method including an autoregression analysis method, an autocorrelation function method, and a spectrum analysis method, to the time series data. 8. The nuclear reactor power monitor according to claim 6, whereinthe amplitude or the decay ratio is obtained by a peak detection method including fitting the time series data set on intervals by a polynomial and searching for a point where a differential value of the polynomial is equal to zero, andthe point where the differential value is equal to zero is first searched within a period of time, composed of half of the oscillation period and a margin, with a switchover point from the first determination unit as an origin, and then the point is searched within a period of time, composed of a half of the oscillation period and a margin, with searched point as an origin in a repeated manner. 9. The nuclear reactor power monitor according to claim 5, whereina sampling period of the time series data converted into digital data or the sampling period multiplied by a coefficient set in consideration of a decay ratio is used as the first reference value, andthe first determination unit executes the determination based on duration time during which the nuclear thermal hydraulic stability is indicated to be in a deteriorated state in comparison between the first stability index and the first reference value. 10. The nuclear reactor power monitor according to claim 8, whereina spline function is used as the polynomial, andout of intervals which are interposed in between data points that constitute the time series data, an interval in which a product of derivatives of the data points placed on both sides of the interval is a minus-sign product is obtained, the obtained interval is further divided, and out of these dividing points, a point where an absolute value of a derivative is minimum is searched as the point where the differential value is equal to zero. 11. The nuclear reactor power monitor according to claim 1, whereinwhile the second calculation unit is in operation, the first calculation unit is concurrently operated, andwhen a determination to execute power control of the reactor core is not made in the second determination unit in predetermined time, operation of the second calculation unit is stopped and operation of the first calculation unit is continued. 12. The nuclear reactor power monitor according to claim 1, whereina spatial distribution of phase difference in the power oscillation is obtained based on phase difference in the time series data from the nuclear instrumentation detectors positioned at a local instability central region and peripheral regions of the local instability central region so as to estimate a power oscillation mode including any of a local oscillation, a regional oscillation and a core-wide oscillation. 13. The nuclear reactor power monitor according to claim 2, whereina power oscillation mode is estimated by calculating phase difference between average signals in units of the groups, and the first reference value and the second reference value are changed into optimum values corresponding to the estimated power oscillation mode. 14. A nuclear reactor power monitor, comprising:a first calculation unit configured to calculate a first stability index of the reactor core based on time series data which indicate a power oscillation in nuclear instrumentation signals outputted from a plurality of nuclear instrumentation detectors which detect neutrons in a reactor core;a first determination unit configured to compare the first stability index and a first reference value and determine whether nuclear thermal hydraulic stability of the reactor core is stable or deteriorated;a second calculation unit configured to calculate a second stability index of the reactor core based on the time series data when the nuclear thermal hydraulic stability is determined to be deteriorated in the first determination unit; anda second determination unit configured to compare the second stability index and a second reference value and determine whether to perform suppressing operation of the power oscillation, further comprisinga third determination unit configured to execute the power oscillation suppression operation if it is determined that the first stability index satisfies a third reference value which is set more severely than the first reference value even when the power oscillation suppression operation has been determined to be unnecessary based on the second stability index. 15. The nuclear reactor power monitor according to claim 1, whereinthe first calculation unit calculates a decay ratio as the first stability index, andthe second calculation unit calculates the second stability index by counting the time series data determined to indicate the deterioration in the first determination unit. 16. The nuclear reactor power monitor according to claim 15, whereinthe nuclear instrumentation detectors are divided into groups, andthe second calculation unit calculates the second stability index in units of the group. 17. The nuclear reactor power monitor according to claim 15, further comprisinga reference value updating unit configured to update the first reference value once the second stability index exceeds the second reference value, whereinwhenever the updating is performed, a plurality of power suppression devices different in suppression level are activated in stages. 18. The nuclear reactor power monitor according to claim 17, whereinthe power suppression devices include those implementing at least three kinds of different effects: alarm issuance; suppression preparation; and suppression. 19. The nuclear reactor power monitor according to claim 15, whereina decay ratio which exceeds the first reference value and a decay ratio which does not exceed the first reference value are distinguishably displayed on a map which displays placement configuration of the nuclear instrumentation detectors. 20. The nuclear reactor power monitor according to claim 19, whereinthe map is automatically displayed in synchronization with instruction to activate the power suppression device. 21. The nuclear reactor power monitor according to claim 16, whereinan activation instruction unit instructs activation of the power suppression device when the second stability index exceeds the second reference value at least in two or more groups. 22. The nuclear reactor power monitor according to claim 21, comprisinga reference value correction unit configured to obtain an average value of the decay ratios in units of the group and to correct the first reference value based on a deviation between the average values. 23. The nuclear reactor power monitor according to claim 15, comprisinga weighting factor setting unit configured to set a weighting factor, whereinthe second calculation unit calculates a plurality of second stability indexes based on the first reference values each having a different value, multiplies the calculated index values by the weighting factors corresponding to respective indexes, adds up the multiplied values, and then outputs the resulting value. 24. The nuclear reactor power monitor according to claim 23, whereinthe weighting factor is normalized to provide a value of 1 when respective values corresponding to the second stability indexes are added up. 25. The nuclear reactor power monitor according to claim 23, further comprisinga reference value updating unit configured to update the second reference value once the second stability index exceeds the second reference value.