Patent Number: 054065984
Section: summary

BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a system for monitoring power of a nuclear reactor and a power distribution in a nuclear reactor core especially for monitoring the stability of the state of the reactor core in accordance with a neutron flux distribution in the reactor core. Description of the Related Arts A boiling water reactor (hereinafter called a "BWR") is equipped with a nuclear instrumentation system provided with a plurality of neutron flux detection devices which are arranged in the core to monitor the power distribution of an operating power level. The nuclear instrumentation is called a "local power regional monitor (LPRM)", which has four neutron flux detectors disposed along the vertical direction in the core. For example, a 1100 MEe class BWR has, in the reactor core thereof, 43.times.4=172 (channels) neutron detectors. Signals (LPRM signals) from each neutron flux detector are, in each group of about 20 signals, averaged into an average power range monitor (APRM). For example, a 1100 MWe class BWR has 8 channels of the average power range monitors, and therefore, APRM signals from 8 channels are monitored. All of the APRM signals and the LPRM signals are analog signals. In the current BWR, the APRM signals are monitored to cause the operation of the nuclear reactor to be performed stably and safely. A nuclear reactor core condition is extremely stable at a rated operational point. However, in the case of flow down to a natural circulation state due to a trip of the recirculation pump, the reactor power decreases along with the flow down. However, since the reactor power is reduced to only about 50% as contrasted with the fact that the reactor flow decreases to about 30% of the normal rated flow, the core condition becomes unstable. In the unstable condition, there is a possibility that the reactor power oscillates with a cycle of about 2 to 3 seconds. Although the oscillations of the reactor power dumps quickly in the stable condition, the oscillations of the reactor power can be sustained in the unstable condition. In order to maintain the fuel integrity during the reactor power oscillation, the following counter-measures have been taken at present. One of the countermeasures is arranged in such a manner that APRM signals, each of which has been obtained by averaging the LPRM signal supplied from the neutron flux detectors, are monitored if oscillations have been generated in the reactor power and all control rods are inserted (made scram) if the value of the APRM signal is larger than a predetermined limit value so that the operation of the nuclear reactor is shut down. Although the insertion of the control rod is a very effective means in terms of the safety operation of the nuclear reactor, it has been considered that the foregoing method is not the best method in terms of efficiently operating the nuclear reactor. Another method is a method in which the nuclear reactor is stabilized while preventing the oscillation of the reactor power even if the unstable reactor state has been realized. The foregoing method is arranged in such a manner that an upper limit of the stable nuclear reactor power in a low flow state has been evaluated, and that a portion of the control rods is selectively inserted as to make the reactor power smaller than the upper limit. The method in which a portion of the control rods is selectively inserted is called "selected rod insertion" (hereinafter called an "SRI"), the foregoing method being a safety and efficient operation method because the operation of the nuclear reactor can be stabilized while preventing the operation shutdown of the nuclear reactor. Since the SRI is arranged so that the control rods mounted on the reactor core are selectively used, it is necessary that the control rods for use must be previously determined. The selection of the control rod is so performed that the control rods are selected so as to exclude the unstable region and the reactor power distribution is sufficiently flattened. Since the radial directional power distribution in the nuclear reactor is high in the center of the reactor core and low in its periphery, control rods adjacent to the central portion are employed and inserted as the selected control rods in order to reduce the reactor power sufficiently. The positions of the control rods for use at the time of the execution of the SRI are previously registered in a process control computer disposed in a site. The SRI realizes a flat reactor power distribution in the reactor core of the nuclear reactor. Although the uniform or flattered reactor power distribution avoids the core-wide power oscillations, it undesirably generates oscillations of the power in a partial region of the core or enhances the oscillations. The power oscillations in the partial region of the core are called "regional oscillations". The reason why the regional oscillations are generated will now be described. It has been generally known that the neutron flux distribution in the core of the nuclear reactor meets the following equation: [Numerical Formula 1] EQU (L+A).phi.0=1/.lambda..multidot.F.multidot..phi.0 (1) where .phi.0: neutron flux L: neutron leakage cross section PA0 A: neutron absorption cross section PA0 F: neutron fission cross section PA0 .lambda.0: critical eigenvalue PA0 .phi.n: n-th harmonics PA0 .lambda.n: eigenvalue of n-th harmonics As indicated below, these neutron fluxes are in the following orthogonal relationship with each other. PA0 An: magnitude of n-th harmonics Usually, the neutron flux .phi.0 meeting Equation (1) is called a neutron flux in the fundamental mode, while critical eigenvalue .lambda.0 is called an "eigenvalue" in the fundamental mode. Actually, neutron special harmonics exist which satisfy the relationship expressed by Equation (1) are present as expressed by the following formula: [Numerical Formula 2] EQU (L+A).phi.n=1/.lambda.0.multidot.F.multidot..phi.n(n=0,1,2 . . . )(2) where n: harmonics order [Numerical Formula 2'] ##EQU1## Here, it is assumed that the integration is done over the entire reactor core and the harmonics are normalized. Among the neutron fluxes .PHI.n expressed by Equation (2) only the neutron flux in the fundamental mode (corresponding to n=0) is always present in the core, while the residual modes (corresponding to n=1, 2, 3, . . . , usually called "higher modes") dump instantaneously, although they are present temporarily if a certain disturbance, such as insertion of a control rod, takes place in the reactor core. The degree of the "short life" can be known from the subcriticality of the neutron higher harmonics. The subcriticality can be expressed by the difference .DELTA.n between the critical eigenvalue .lambda.0 (.lambda.0 is necessarily 1.0) in the fundamental mode and the harmonics eigenvalue .lambda.n in the higher mode. [Numerical Formula 3] EQU .DELTA.n=.lambda.0-.lambda.n(n=0, 1, 2, . . .) (3) Since the order of the mode is given in proportion to the harmonics eigenvalue, the relationship expressed by Equation (4) is held. [Numerical Formula 4] EQU 0.0=.DELTA.0&lt;.DELTA.1&lt;.DELTA.2&lt;. . . (4) Further, the neutron flux .PHI. is expressed by the following equation if the core of the nuclear reactor is in a transient state: [Numerical Formula 5] EQU .phi.=Sum An.multidot..phi.n(n=0,1,2, . . .) (5) where .phi.: neutron flux at the time of transient The harmonics magnitude can be obtained in the following equation (5') using the inter-mode orthogonal condition given by the equation (2'). [Numerical Formula 5'] ##EQU2## In Equation (5), magnitude An of the n-th harmonics shows the degree of contribution of each harmonics mode to the neutron flux, the magnitude An being a function of the subcriticality and time. That is, the neutron flux in the reactor core in the transient state is expressed by the superposition of the respective modes while using the magnitude An of the mode as a weight at this time. Therefore, even if the distribution form of the higher mode locally takes a negative value, the neutron flux distribution in the reactor core does not actually take a negative value. If the subcriticality of the higher mode is large, the magnitude of the mode decreases as time passes, resulting in Equation (5) to be as follows as described above in a stationary state after the transient state has been realized: [Numerical Formula 6] EQU .PHI.=.PHI.0 (6) However, if the subcriticality in the higher mode is small for some reason, the dumping of the first mode, the subcriticality of which is the smallest among the higher modes is particularly slow, resulting in that the neutron flux .PHI. in the reactor core is temporarily expressed by the sum of the fundamental mode and the first mode. [Numerical Formula 7] EQU .PHI.=A0 .phi.0+A1 .PHI.1 (7) If a certain disturbance exciting the first mode of the neutron flux takes place in the foregoing core state of the nuclear reactor, the first mode is changed in accordance with the fundamental mode, and therefore, a possibility arises that oscillations are excited if the core is unstable. Even if the oscillation has been excited, the reactor power does not oscillate in the whole core region because the fundamental mode is not changed. However, the power distribution is oscillated in the form of the distribution of the first mode. Although the subcriticality is changed depending upon, for example, the size of the reactor core or the fuel instrumentation pattern, it considerably depends upon the power distribution of the reactor core. FIGS. 17A, 17B, FIGS. 18A and 18B respectively show the radial neutron flux distribution in the fundamental mode and the first mode in two different states of a 1,100,000 kwe class nuclear reactor. The axis of ordinate of each of FIGS. 17 and 18 indicates the neutron flux distribution (unit is arbitrary), while two axes of abscissa indicate the positions of the fuel assembly. The states of the reactor core shown in FIGS. 17A and 17B are characterized in that the fundamental mode of the neutron flux distribution is sufficiently flattened but the subcriticality of the first mode of the neutron flux is small as compared with the states shown in FIGS. 18A and 18B. As can be understood from the foregoing examples, the subcriticality of the first mode of the neutron flux distribution in the state of the reactor core in which the power distribution is flat. Therefore, it will be said that regional oscillations can easily be excited. As described above, the regional oscillations can easily be excited if the subcriticality of the first harmonics is small. Therefore, by monitoring the subcriticality, the possibility of the onset of the regional oscillations can be estimated. Further, a certain countermeasure for preventing the onset of the regional oscillations must be taken. However, in the operation of the reactor, a method for evaluating the subcriticality of the first mode by solving the Equations (2) and (3) to the direct first mode involves difficulty, thus being not practical. Since the subcriticality of the first mode considerably depends upon the core condition even if the nuclear reactor and the operational cycle are specified, it must always be reevaluated to be adaptable to the change of the state of the reactor core. However, solving Equation (2) for the first mode encounters a problem that the calculations take a long time because the reactor power is converged slowly as contrasted with the fundamental mode. The nuclear reactor is operated in such a manner that the APRM signal obtained by averaging the LPRM signals is used to monitor the distribution of the neutron fluxes to avoid the operation in an unstable core condition. Although the APRM signal can detect the core-wide power change because the APRM signal is obtained by equally averaging the LPRM signals, in the use of the APRM signal, there is a possibility of making difficult the detection of the reactor power distribution, if the core of the nuclear reactor is locally changed or if the same is changed while spatially having a phase difference because the quantity of the change is set off due to averaging of the LPRM signals. As an example of the local change in the reactor core, a so-called "channel oscillations" can be considered in which a thermal-hydraulically severe fuel assembly generates an oscillation phenomenon called "density wave oscillations". Although the oscillation phenomenon can be diffused by the oscillations of the neutron fluxes, there is a possibility that the change is limited in only a relatively narrow range. As an example of the change taking place while having the spatial phase difference, there is an oscillation phenomenon called a regional oscillation occurring at symmetrical positions in the core while having a phase difference of 180.degree.. The foregoing oscillation phenomenon has been observed in some overseas plants. For example, a regional oscillation observed in CAORSO plant in Italy showed the maximum oscillation of APRM of 10% or less. On the other hand, oscillations reaching to 60% were observed in the LPRM that shows the largest oscillation. The reason for this is considered that the fact, that oscillations symmetrically are generated at a phase difference of 180.degree. in the core, causes the maximum value and the minimum value of the LPRM to be simultaneously averaged, and therefore, cancelling takes place during this. When the stability of the reactor core is monitored, the decay ratio, the period of the oscillation and the amplitude denoting the stability are calculated from the APRM signal to estimate usually the stability of the state of the core. However, there is a possibility that the stability of the reactor core cannot accurately be detected by simply monitoring the APRM signal. SUMMARY OF THE INVENTION The present invention has been directed to overcome the foregoing problems encountered in the prior art, and therefore, an object of the present invention is to provide a system for monitoring a nuclear reactor which detects the change of the power of the reactor core by using a conventional LPRM signal or the like and which is capable of improving the safety of the reactor core and the availability of a nuclear reactor. Another object of the present invention is to provide a system for monitoring a nuclear reactor which provides a filter for peculiarly extracting the oscillation mode of the neutron flux distribution and which is able to monitor and discriminate the stability of the reactor core in accordance with a signal processed by the filter. Another object of the present invention is to provide a system for monitoring a nuclear reactor capable of quickly discriminating a possibility of generation of the regional oscillations and enabling the nuclear reactor to be operated safely and efficiently. Another object of the present invention is to provide a system for monitoring a nuclear reactor which quickly discriminates the possibility of the regional oscillations occurring at the time of selected rod insertion and that enables the nuclear reactor to be operated safely and efficiently. These and other objects can be achieved according to the present invention by providing, in one aspect, a system for monitoring power of a nuclear reactor comprising: a plurality of neutron flux measuring means disposed in a core of the nuclear reactor for measuring neutron flux in the core and generating neutron flux signals; PA1 means for calculating a neutron flux distribution in the core in response to the neutron flux detection signals from said neutron flux measuring means; PA1 means for calculating a higher mode of the neutron flux distribution in accordance with results of calculations performed by the neutron flux distribution calculating means; PA1 a filter calculating means for obtaining a filter for extracting characteristics of change of the neutron flux detection signal in response to the neutron flux detection signal; and PA1 an input/output means for transmitting the neutron flux detection signal filtered by the filter obtained by the filter calculating means. PA1 a plurality of neutron flux measuring means disposed in a core of the nuclear reactor for measuring neutron flux in the core and generating neutron flux signals; PA1 means for calculating the fundamental mode distribution of the neutron flux in response to the neutron flux detection signal measured by the neutron flux measuring means; PA1 a subcriticality evaluating means for estimating a subcriticality of a state of the core in accordance with the neutron flux distribution in the calculated fundamental mode; and PA1 an input/output means for transmitting a result of an evaluation made by the subcriticality evaluation means. PA1 a core present state data measuring means for measuring an operational state of a core of the nuclear reactor and generating a core operational state signal; PA1 means for calculating a neutron flux distribution in a basic mode in response to the core operational state signal from the core present state data measuring means; PA1 means for calculating a higher mode of the neutron flux in a state of the core realized when insertion of a selected rod is executed in accordance with the calculated neutron flux distribution and discriminating whether or not a subcriticality of the higher mode is smaller than a predetermined limit value; and PA1 an input/output means for transmitting results of calculations performed by the higher mode calculating means. PA1 a plurality of neutron flux measuring means disposed in a core of the reactor for measuring neutron flux in the core and generating a signal representing a local power range monitor enumerated data from the neutron flux measuring means; PA1 means for calculating neutron flux distribution in response to the signal from the neutron flux measuring means; PA1 a higher mode calculating means for calculating neutron higher modes in accordance with the calculation results of the neutron flux distribution calculating means; and PA1 an input/output means for outputting calculation results from the neutron flux distribution calculating means and the higher mode calculating means. In a preferred mode, the filter calculating means is operatively connected at one side to the neutron flux measuring means through a data sampler and at another side to the higher mode calculating means. Then, the filter calculating means obtains a filter reflecting a state of the core realized due to change of an operational state in accordance with the higher mode of the neutron flux distribution calculated by the higher mode calculating means. Thus, a filter is obtained in accordance with differences in amplitudes and phases between signals occurring due to change of the neutron flux detection signal measured actually. The system further comprises a stability monitoring means connected to an output side of the filter calculating means and the stability monitoring means has a structure for evaluating a core stability index in response to a power signal filtered by the filter calculating means to monitor the stability of the state of the core. The neutron flux distribution calculating means is constituted by a process control computing means which is provided in association with the higher mode calculating means. The process control computing means includes the higher mode calculating means. A power distribution monitoring means connected at input side to the process control computing means and at output side to a display means. In another aspect, there is also provided a system for monitoring power of a nuclear reactor comprising: In a preferred mode, the apparatus further comprises a higher mode calculating means for calculating a higher mode of the neutron flux distribution in accordance with results of calculations performed by the neutron flux distribution calculating means and a filter calculating means for obtaining a filter for extracting characteristics of change of the neutron flux detection signal in accordance with the neutron flux detection signal and the results of calculations performed by the filter calculating means is transmitted to the input/output means. The neutron flux distribution calculating means is constructed by a process control computing means connected at input side to the neutron flux measuring means through a data sampler and at output side to the subcriticality evaluation means. The process control computing means is further connected at output side to the high mode calculating means. The system further comprises a filter calculating means operatively connected to the neutron flux measuring means for obtaining a filter for extracting characteristics of change of the neutron flux detection signal in response to the neutron flux detection signal and a stability monitoring means connected to an output side of said filter calculating means and the stability monitoring means has a structure for evaluating a core stability index in response to a power signal filtered by the filter calculating means to monitor the stability of the state of the core. In a further aspect, there is also provided a system for monitoring power of a nuclear reactor comprising: In a still further aspect, there is also provided a system for monitoring power of a nuclear reactor comprising: In a preferred mode, the higher mode calculating means is provided with a magnitude variation calculating means for calculating a variation in magnitude in each mode on the basis of the higher mode modes and the local power range monitor enumerated data. The neutron flux distribution calculating means is constructed by a process control computing means operatively connected at input side to the neutron flux measuring means through a data sampler and at output side to the higher mode calculating means. The system for monitoring power of a nuclear reactor according to one aspect of the present invention comprises the filter calculating means in addition to the conventional APRM signal obtained by averaging the analog signals to monitor the reactor power and the reactor power distribution by using each neutron flux detection signal. The filter calculating means obtains the filter corresponding to the state of the reactor core or obtains the same corresponding to the change characteristics of the signal in response to each neutron flux detection signal, the filter for extracting the characteristics of the signal change being used to fill each neutron flux detection signal so that the decay ratio, the period of the oscillations and the amplitude showing the stability of the state of the reactor core and the like are obtained at the time of monitoring the stability of the reactor core. The calculation of the filter performed by a filter calculating means by a calculating step for periodically calculating the filter in accordance with the change of the spatial distribution characteristics of the reactor power whenever the operational state is changed and by a sequential calculating step for calculating the same in accordance with the amplitude difference or the phase difference between the signals. The former is calculated in accordance with information from the neutron flux distribution calculating means, which is a process control computer, i.e. process computer, and that from a higher mode calculating means, while the latter is calculated in response to the neutron flux detection signal, which is an actually measured signal that is sequentially detected. The power signal filtered by the filter calculated by the filter calculating means is received by the stability monitoring means to obtain sequentially the decay ratio and the oscillation period showing the stability of the core and the amplitude showing the power change. The obtained values are used to monitor the stability of the reactor core to be evaluated in an on-line manner. The system for monitoring power of a nuclear reactor is able to accurately detect the power change phenomenon, and, in particular, the power oscillation phenomenon due to the regional oscillations, which has been difficult to be detected by using the conventional APRM signal. Therefore, the apparatus is able to contribute to improve the stability of the core and the availability of the nuclear reactor. The system for monitoring power of a nuclear reactor according to another aspect of the present invention is able to discriminate the possibility of the generation of the regional oscillations from the subcriticality of the state of the core obtained by the subcriticality evaluation means, to estimate the easiness of occurring the regional oscillations, to monitor the stability of the state of the core, to control the core while preventing the generation of the regional oscillations and to operate the nuclear reactor safely and efficiently. The system for monitoring power of a nuclear reactor according to a further aspect of the present invention calculates the higher mode of the neutron flux in a state of the reactor core when the selected rod insertion (SRI) is executed, and discriminates whether or not its subcriticality is smaller than a predetermined limit value. Therefore, the possibility of the generation of the regional oscillations at the time of the execution of the SRI can quickly be discriminated. Therefore, the nuclear reactor can safely and efficiently be operated. In a still further aspect, a neutron flux distribution is calculated in accordance with the local power range monitor (LPRM) enumerated data and the higher modes of the neutron flux are calculated in accordance with the calculation results. The variation in strength of each mode is calculated on the basis of the higher modes and the LPRM enumerated data. The calculation results are outputted and reported to the operator. Thus, unlike the conventional method using the APRM value, the method of the present invention makes it possible to quickly detect any regional oscillation. The nature and further features of the present invention will be made further clear from the following descriptions made with reference to the accompanying drawings.