Patent Number: 061223395
Section: summary

BACKGROUND OF THE INVENTION Field of the Invention The invention relates to a method of operating a boiling-water reactor which is an unstable state as a result of local oscillation of a physical variable (in particular of the power or of the neutron flux associated therewith). In addition, the invention relates to a device for carrying out this method and to a method and a device for monitoring the unstable reactor state. The nuclear fission determining the power of a nuclear reactor is controlled by moving absorber elements into the reactor core in order to attenuate the neutron flux. In this arrangement, measuring lances having sensors for the flux of thermal neutrons are distributed over the reactor core, in order to register the current state. In order to adjust a desired operating state, it is also necessary for the throughput of coolant (cooling water), which serves at the same time as a moderator, to be adapted to the respective state. The coolant enters in liquid phase into the reactor core from below, flows through the fuel elements, in which it partially evaporates, and emerges from the core as a vapor phase/liquid phase mixture, as a result of which the fuel/moderator ratio in the various parts of the fuel elements is changed. At the same time, however, the flow conditions are changed, in particular the location at which the single-phase flow, with which the liquid coolant enters the fuel elements, changes into the two-phase flow of the liquid/vapor mixture. In this case, at high power and low coolant throughput, unstable conditions have been observed in which this phase boundary goes into an oscillating motion, which results in a pulsation of the moderator density and the power, which has a bearing on the cooling capacity and the movement of the phase boundary. In this case, periodic temperature fluctuations with considerable peak values may occur in the fuel elements. The permissible power maximum of the fuel elements is mainly limited by the temperature resistance of the materials used in the fuel elements. If an upper temperature limit is exceeded, the materials loose their mechanical, chemical and physical properties and can undergo irreversible changes, which can force an exchange of the fuel elements. Therefore, care must be taken that this thermal-hydraulic upper power threshold (and hence a thermal-hydraulic threshold value A.sub.th, of the neutron flux) in the reactor is not exceeded. Safety provisions in the reactor operation therefore call for a rapid shutdown of the reactor (so-called "SCRAM"), in the event the threshold value is exceeded. In such an emergency program, all the control rods are rapidly moved in and the corresponding cooling capacity is set. Following such a SCRAM, the reactor is restarted according to a predetermined startup program, so that there is a considerable disturbance to the reactor operation. In addition, the fuel elements have to be changed for safety reasons, if the thermal-hydraulic threshold value has been reached many times or over a relatively long period of time. The art is therefore concerned with detecting and damping an unstable state of this type as early as possible, before the power pulsations reach the vicinity of the thermal-hydraulic threshold value. It has been shown that these pulsations always occur in a frequency range between about 0.3 and 0.7 Hz and have a very constant frequency. The method described in U.S. Pat. No. 5,174,946 to Watford et al. (=EP 0 496 551) for monitoring the power fluctuation band for nuclear reactors is based on that fact. That process utilizes the flux as a measured variable for the unstable state caused by the local oscillation of a physical variable, the measuring lances mentioned ("local power range monitor-strings", LPRM strings) being used for this flux measurement. Each such lance normally contains four sensors, whose signals are observed anyway for power control purposes, then further processed and documented. Each of these four sensors in each measuring lance is used, two sensors being assigned to a first monitoring system, the two remaining sensors being assigned to a redundant second monitoring system. Each monitoring system thereby contains two monitoring channels, each sensor signal of a measuring lance being assigned to a different monitoring channel. Different subdivisions of the reactor into individual regions ("monitoring cells") are in this case based on the two monitoring channels of a system, each cell being bounded by four measuring lances in order to form a corresponding region signal. Depending on the location of the measuring lance in the core (in the interior of the core or at the edge of the core), a sensor signal in each monitoring channel belongs to two, three or four cells. As a result of this multiple use of the sensor signals, it is intended to achieve the situation where virtually the state of each individual fuel element can be monitored and identified by means of the influence which it has on the sensor signals of the individual cells. To this end, provision is made that an alarm is set in a system only when both monitoring channels respond. Although it is sufficient for the alarm to be given by one of the two systems, only simple redundancy is provided thereby. A further disadvantage is that virtually all the monitoring channels are affected by an erroneous measurement or a complete failure of a measuring lance, it being possible in the case of an edge position of the measuring lance, for example, that simultaneously a plurality of cells are no longer being monitored properly. The state of the individual cells (regions) is monitored by initially monitoring in a plausibility control whether the individual sensor signal exceeds a specific lower threshold value and is operating properly. In the case of a sensor defect, the signals belonging to this cell are not evaluated further. By means of summing all the sensor signals of a region, a current region signal is formed which is suppressed, however, if (for example as a result of an erroneous measurement) a plausibility monitoring yields the fact that the region signal does not achieve a predefined minimum value. The region signal is then filtered and related to an average over time, the time constant of which is greater than a period of the oscillation, so that a relative current region signal is produced which indicates by how many percent the current power of the region lies above or below the average. If this current value exceeds a power limit (for example 120%), a check is then made as to whether this is a once-off transition state (so-called "transient") which for example constitutes only an aperiodic transition to a new operating state predefined by the control, without exciting an oscillation. In this case, this is not therefore a critical oscillation in the frequency band from 0.3 to 0.7 Hz, so that no intervention is carried out as long as a threshold value A.sub.max, lying in the vicinity of the thermal-hydraulic threshold value A.sub.th, is not reached. In order to detect the critical oscillation, instead an examination is made to see whether, in a time interval corresponding to this critical frequency band, the value does not also fall below a corresponding threshold value (e.g. 80%) following the exceeding of a limiting value A.sub.o as is necessary for an oscillation. If it is determined in this way that--corresponding to an oscillation--a lower extreme value follows an upper extreme value of the flux, a check is further made as to whether another upper extreme value follows this lower extreme value, and whether this following upper extreme value exceeds an alarm value which lies above the extreme value detected first by a predefined factor (e.g. 1.3). If this is so, then after this one oscillation period it is already concluded that there is a growing, i.e., increasing oscillation, in which the exceeding of A.sub.th is threatened, and the SCRAM is initiated even before the value A.sub.max is reached. With an eye to the present invention, reference is made at this point that, although the above-described prior art monitors whether the oscillation is growing at a rate lying above the predefined factor (here 1.3), the growth (rate of increase) of the extreme values is not itself measured. This factor (1.3) is also relative in as much as it is related to the extreme value detected first, but it independent of the rate of increase. In addition, reference is made to the fact that although it is checked whether the time interval between the detected extreme values corresponds to the critical frequency band of 0.3 and 0.7 Hz, no check is made as to whether the next extreme value A.sub.n+1, follows in practice at the same interval DT.sub.n, which is given by the previously detected upper extreme value (denoted A.sub.n-1, point in time T.sub.n-1) and the presently detected lower extreme value (A.sub.n, point in time T.sub.n), after this point in time T.sub.n. Those skilled in the art of reactor control and monitoring will appreciate that the usual techniques for the monitoring and documentation of the sensor signals apply and they will therefore readily be able not only to register the extreme values A.sub.n-1, A.sub.n, A.sub.n+1 . . . but also the points in time T.sub.n-1, T.sub.n, T.sub.n+1 . . . at which these extreme values occur. The person in charge of monitoring could therefore readily suppress the corresponding region signal if the time interval DT.sub.n =T.sub.n -T.sub.n-1 deviates significantly (for example 0.1 seconds) from the time interval DT.sub.n+1 =T.sub.n+1 -T.sub.n. However, U.S. Pat. No. 5,174,946 contains no advice on this point. In the state of that prior art, therefore, no attention is initially paid to an oscillation whose (unmeasured) rate of increase lies below the set factor (1.3); rather, intervention is considered in the reactor operation only when its extreme values exceed the threshold value A.sub.max. Only rapidly increasing oscillations cause this extremely critical state to be recognized in good time and to the initiation of suitable countermeasures. Apparently, it is assumed that slowly increasing oscillations inherently decay by themselves and normally do not require a SCRAM. To be specific, that prior art provides as counter-measure only to damp the oscillation by means of rapidly moving in virtually all the control rods (total SCRAM). That is to say, apart from the SCRAM, this strategy provides no further measure for damping the oscillation and does not reduce the probability of the SCRAM either, which constitutes a considerable intervention in the reactor operation. Instead, in the event that there is a rapidly increasing oscillation, damping only takes place earlier (i.e., below A.sub.max). As a result, only the thermal loading of the fuel elements is reduced. SUMMARY OF THE INVENTION It is accordingly an object of the invention to provide a method and device for operating a reactor in an unstable state, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which improves the oscillation detection and damping so as to entirely obviate any SCRAM, i.e., to manage without an intervention in the reactor operation, or with an intervention which is the least disturbing. It is a further object to allow the monitoring of the critical state with a system and method which is least susceptible to interference. With the foregoing and other objects in view there is provided, in accordance with the invention, in a reactor operated in accordance with operationally dependent input parameters, a method of operating the reactor which is unstable as a result of an oscillation of an internal physical variable, which comprises: measuring the physical variable during at least two oscillations and calculating at least one measured value for a rate of increase of the oscillation; and PA1 deciding, in dependence on the measured value, whether a stabilization strategy is to be initiated with changed input parameters for damping the instability or a reactor operation is to be continued with unchanged input parameters. PA1 a system selection stage, a plurality of region selection stages connected to the system selection stage, a given number of region monitoring stages connected to each the region selection stage, and a sensor stage connected to each the region monitoring stage with a plurality of sensors strategically disposed in regions of a reactor core of a boiling water reactor, wherein PA1 a) a plurality of sensors disposed in a plurality of regions of a reactor core of a boiling-water reactor, the sensors measuring a physical variable of the reactor core and outputting output signals, the output signals of a plurality of the sensors of a given region being combined into an associated region signal; PA1 b) a plurality of evaluation stages each receiving a respective region signal, the evaluation stages identifying in the region signal an occurrence of extreme values of the physical variable and, given an oscillation of constant frequency, determining a rate of increase of the extreme values in the respective region; and PA1 c) at least one monitoring stage receiving output signals from the evaluation stages, the monitoring stage setting an alarm signal when the extreme values of a predefined number of regions satisfy a local monitoring criterion which depends on the rate of increase of the extreme values. By measuring the physical variable (i.e., the neutron flux, in the case of the thermally-hydraulically induced oscillations) the invention provides for the formation of local measured values in a plurality of regions of the reactor core, the measured values being assigned to the respective regions. The monitoring of the measured values leads to the formation of a current alarm stage from among a hierarchy of alarm stages with associated monitoring criteria, and the selection of the highest alarm stage, whose monitoring criterion is satisfied by the measured values in a predefined minimum number of the regions. (The monitoring criterion can in this case be composed of a plurality of individual conditions, for example the exceeding of separate threshold values for the amplitude and for the rate of increase of the extreme values.) Depending on the current alarm stage, a stabilization strategy is then initiated. As a stabilization strategy which belongs to a low-ranking alarm stage, provision is made to intervene in the operational control and regulation of the reactor only so as to block a removal of the control rods, as is envisaged in the case of an operational increasing of the reactor power: the power of the reactor cannot then be raised by the operating personnel of the reactor; instead only such control commands which correspond to the control of the reactor to a constant or decreasing power become effective in the reactor control system. In at least one higher-ranking alarm stage, provision is made as stabilization strategy for a plurality of control rods to be introduced into the core in the sense of a reduction in the reactor power (alarm stage I). Advantageously, at least two higher-ranking alarm stages (alarm stage II and alarm stage III) are provided, in alarm stage II only a plurality of control rods, corresponding to a fraction of the total number, being moved into the core slowly and in such a way as corresponds to an operational reduction in the power (that is to say the reactor control system performs an operational reduction in the power, even if, for example, a higher power consumption would intrinsically require a higher reactor power and the operating personnel wish to increase the reactor power). In the second higher-ranking alarm stage (alarm stage III)--in a manner similar to the case of a total rapid shutdown of the reactor (total SCRAM)--control rods are moved in rapidly, however likewise not all thereof but only some of the control rods being involved ("partial SCRAM"). A total SCRAM is then no longer necessary, but an option for an alarm stage IV which triggers the SCRAM can be retained. In particular, during the monitoring of the measured values, at least two periods of the oscillation are evaluated, so that the reactor is therefore initially further operated in an unchanged manner, although an oscillation is already indicated. Furthermore, a method of operating a reactor which is unstable as a result of oscillation of a physical variable occurring in the core makes provision, by measuring the physical variable, for forming a measured value which registers the rate of increase of the oscillation (if appropriate, also further measured values). Depending on this measured value, a decision is made as to whether a stabilization strategy should be initiated in order to damp the instability or the reactor is initially further operated in accordance with measured values entered as a function of operation. In particular, in this case the reactor can continue to be operated for at least two more oscillations during the measurement of the rate of increase, without an intervention being made in the reactor control system--provided that no measured value reaches a threshold value which calls for the initiation of a total SCRAM. Thus, for example, it is possible that when a threshold value A.sub.max for the oscillation amplitudes is exceeded, the SCRAM--corresponding to the highest alarm stage IV--is initiated only at high rates of increase, but at low rates of increase the reactor is still operated with relatively high amplitudes, since in the case of amplitudes which are growing so weakly, a SCRAM which is initiated only later (in the event that the oscillation then does not intrinsically decay) still has sufficient time to become effective before A.sub.th is reached. A threshold value, dependent on the rate of increase, is preferably predefined for the extreme values of the oscillating physical variable, and the stabilization strategy is triggered if the extreme values exceed this threshold value. However, a threshold value for the rate of increase can also be predefined, the stabilization strategy then being initiated when the rate of increase exceeds this threshold value. In a similar embodiment of the invention, a number of oscillations can be predefined, the number depending on the rate of increase, and the stabilization strategy can then be triggered only when the oscillation of the physical variable persists over the duration of these oscillation periods. In accordance with an added feature of the invention, a plurality of stabilization strategies are provided, from which the stabilization strategy to be triggered is selected as a function of the rate of increase. The (unstable) state of the reactor core is monitored with a plurality of sensors which are strategically distributed about the core. The sensor locations are divided into a plurality of regions of the reactor core and the sensors measure the behavior of the physical variable in those regions. The output signals of the sensors are combined into a number Mp of region channels and each region channel is assigned a region and sensors arranged therein for generating a region signal. The region signals are then combined into a number P of system channels, with a plurality of region channels being assigned to a system channel, in that they generate a system signal. The system signals are finally assigned to an output channel and they generate an output signal. By means of monitoring stages and selection stages, in this case an alarm output signal is set in the output signal as soon as, at least in a predefined number N.sub.p of the system channels, particularly in a minimum number N.sub.mp of region channels of the system, a monitoring criterion is satisfied over a plurality of oscillation periods. In this case, the output signal of each sensor influences a maximum of one single region signal and each region signal influences a maximum of one system signal. The region signals of a system channel are in each case formed from the output signals of sensors which are located in regions which are distributed over the cross section of the reactor core in such a way that the regions which are adjacent to such a region contain sensors whose output signals are assigned to region channels of other system channels. The invention thus effectively dispenses with multiple evaluations and region overlaps. Although each individual fuel element is no longer as precisely monitored as in the Watford et al. patent, experience and model calculations with unstable states have shown that it is always relatively large parts of the reactor, but not isolated fuel elements, which begin oscillating. In other words, fine resolution of the measured value registration is not necessary. In addition, the redundancy and interference immunity of the registration is increased. With the above and other objects in view, there is further provided, in accordance with the invention, a device for monitoring a reactor core of a boiling-water reactor with regard to local oscillations of a physical variable causing an unstable state of the reactor. The device comprises: a) measured signals supplied by the sensors to a respective the region monitoring stage are combined into a region signal for the physical variable; each the region signal is monitored in the respective the region monitoring stage in accordance with a monitoring criterion, and a region signal containing a region monitoring signal is output by each region monitoring stage; PA2 b) each region signal is connected to at least one of the region selection stages, and the region selection stages forming respective system monitoring signals from a predefined minimum number of region monitoring signals; and PA2 c) the region selections stages each outputting a respective system monitoring signal to the system selection stage, and the system selection stage outputting an output monitoring signal according to a predefined minimum number of systems. There is further provided, in accordance with the invention, a device for monitoring a reactor core of a boiling-water reactor with regard to a state which is unstable as a result of local oscillation of a physical variable in the reactor core, comprising: In other words, there is provided a system selection stage, a number P of region selection stages, for each region selection stage a number Mp of region monitoring stages and for each region monitoring stage a sensor stage having a plurality of sensors which are arranged inside a region of the core and are assigned to this region monitoring stage. The device is constructed in such a way that the sensors which are respectively assigned to a region monitoring stage supply measured signals for the physical variable which are combined into a region signal, and each region signal is monitored in accordance with a monitoring criterion in the region monitoring stage assigned to the sensors. Each region monitoring stage supplies a region signal which contains a region monitoring signal. Each region monitoring signal is connected to at least one region selection stage which forms a system monitoring signal from a predefined minimum number of region monitoring signals. Each system monitoring signal is then fed to the system selection stage; the latter supplies an output monitoring signal by means of a predefined minimum number of system monitoring stages. The system sensors which are strategically distributed about a plurality of regions of the reactor core for measuring the physical variable. The output signals of a plurality of sensors of a region are combined into an associated region signal. Each region signal is assigned an evaluation stage, which identifies in the region signal the occurrence of extreme values of the physical variable (in particular over a plurality of oscillation periods) and, given an oscillation of constant frequency and appropriate duration, determines the rate of increase of the extreme values in this region. The evaluation stages are assigned at least one monitoring stage which sets an alarm signal as soon as the extreme values at least in a predefined number of regions satisfy a local monitoring criterion which depends on the determined rate of increase. With a view to the proposed stabilization criteria, a device for monitoring the local oscillations can contain sensors for measuring the physical variable, which sensors are arranged in a plurality of regions of the reactor core, and the output signals of a plurality of sensors of a region being combined into an associated region signal. Each region signal is then assigned an evaluation stage which identifies the occurrence of an oscillation of constant frequency in the region signal. The evaluation stages are assigned an output monitoring stage which selects an alarm stage from a hierarchy of alarm stages in accordance with predefined monitoring criteria for the oscillations identified in at least a predefined number of region signals. In this case, the output monitoring stage, corresponding to the selected alarm stage, defines a point in time (or at least the criteria for the point in time) at which an emergency instruction is output to initiate a stabilization strategy corresponding to the alarm stage. This point in time can be predefined, for example, by means of a number of oscillation periods which are allowed to elapse before the initiation of a stabilization measure. However, by this means it can also be defined that, depending on the instantaneous current values (for example current values of the rate of increase) a threshold value (for example a threshold value for the amplitude) is defined, which leads to the triggering of the stabilization measure at a later point in time, at which a monitored current value (e.g., the amplitude) then exceeds this predetermined threshold value. Other features which are considered as characteristic for the invention are set forth in the appended claims. Although the invention is illustrated and described herein as embodied in a method and device for operating a reactor in an unstable state, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.