Patent Number: 059563818
Section: summary

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The invention relates to a method for detecting the dropping of one or more control elements into a reactor core. The invention also relates to a configuration for carrying out the method. Nuclear reactors of power stations have so-called control elements which can be moved into and out of a reactor core in order to control the reactor. A particularly critical malfunction situation occurs if one or more such control elements drop by mistake into the reactor core (in contrast to an intentional drop, for example for an emergency shutdown). The reactor can then no longer be controlled optimally. In a malfunction situation of that type, power densities and distributions that are unfavorable to the burnup of the fuel elements arise in the reactor core. In that case, the fuel elements are put at particular risk when peaks occur. Malfunction situations of that type are very critical in terms of the operation of the reactor and of the safety of the power station. The dropping of control elements by mistake into a reactor core must therefore be detected quickly so that suitable counter measures can be initiated in good time. In principle, the dropping of a control element in the reactor core can be detected by using effects on signals of reactor instrumentation. However, detection is not straightforward on account of the large number of control elements and the fact that the effects differ according to position. A suitable number of vertically disposed detector groups are provided inside or outside the reactor core in order to determine the power and/or power density. As a rule, those detectors each output a signal which has an edge when one or more control elements pass them. To date, the signals from one detector group (per control element or control element group) in practice have firstly been differentiated (in order to obtain a pulse) and then added, so as to provide a total signal pertaining to the dropping of a control element. However, it has been found that such a sum signal was not suitable for reliable fault detection in all cases, in particular due to signal noise. In connection with controlling the power of a nuclear reactor, U.S. Pat. No. 4,548,784 discloses a method for detecting the dropping of at least one control element in a reactor core, in which signals from detectors disposed along the fall path of the control element are added and fed to a respective monitoring or evaluation device, with the addition being followed directly by averaging. SUMMARY OF THE INVENTION It is accordingly an object of the invention to provide a method and an apparatus for detecting the dropping of a control element, which overcome the hereinafore-mentioned disadvantages of the heretofore-known methods and apparatuses of this general type and which permit better fault detection than the prior art for the dropping of control elements in a reactor core, on the basis of existing detector configurations. With the foregoing and other objects in view there is provided, in accordance with the invention, a method for detecting the dropping of at least one control element into a reactor core, which comprises placing detectors supplying signals along a fall path defining a fall time of a control element; delaying each of the respective signals from the detectors by a delay time proportional to the fall time; then coherently adding the signals to form a sum signal; and feeding the sum signal to a monitoring device. A better detection of a drop than in the prior art is made possible in this way. This is because the signals are added coherently due to the delays, and interference or noise components are partly eliminated or averaged out through the use of this method. The amplitude of the sum signal is then equal to the sum of the individual signals, so that the noise component is comparatively small as a ratio. The fault detection is therefore particularly reliable. Temporal coincidence is thus produced for the related signals pertaining to an event. In accordance with another mode of the invention, the respective delay times are fixed in advance. The delay components are therefore particularly simple to produce, and the total expenditure is low. In accordance with a further mode of the invention, the delay is a function of the respective time difference between the occurrence of the respective signal and that of the last signal. This provides a delay which is dependent on the dropping of the control element. The fault detection is therefore very precise. The delay time may, for example, be determined according to the relationship: EQU Ti=Xn/v-Xi/v in which i=1, . . . , n-1 Xi and Xn indicate positions of respective detectors 1 and n along the fall path of the control elements, PA1 Ti indicates the delay times of the respective signals Xi and Xn, and PA1 v denotes a fall rate of the control elements. This provides a simple rule, according to which the delay times in the delay components can be calculated. In accordance with an added mode of the invention, the delay times are determined by using a correlation analysis, through which exact coincidence of the signals is achieved. In accordance with an additional mode of the invention, it is further beneficial if the respective delay time is determined as a function of the starting position of the control element before it drops. This makes it possible to have coincident monitoring in every operating position of the control element. The instantaneous operating state is thus taken into account. The different fall times are thereby taken into account exactly, and a high degree of accuracy in the fault detection is achieved through the use thereof. In accordance with yet another mode of the invention, before they are added, the signals are fed to a thresholding process so that only signals exceeding a predetermined threshold are added. Therefore, only signals with a predetermined signal amplitude are evaluated, through the use of which, for example, it is possible to discriminate between fast insertion of the control element and dropping by mistake. In accordance with yet a further mode of the invention, in the thresholding process, the signals are converted into binary signals, which makes them simpler to evaluate in comparison with an analog signal waveform (even in digitized form). In accordance with yet an added mode of the invention, the signals produced by the detectors are differentiated first, before they are processed further. Instead of an edge signal, this provides a pulse which is beneficial in terms of signal processing. With the objects of the invention in view, there is also provided a method for detecting the dropping of at least one control element into a reactor core, which comprises placing detectors supplying signals along a fall path of a control element; adding the signals from the detectors to form a sum signal; integrating the sum signal; differentiating the integrated sum signal; and feeding the differentiated integrated sum signal to a monitoring device. This permits high-precision fault detection which, in particular, is suitable for digital signal processing using a computer, for example in a control automation device with a microcomputer. With the objects of the invention in view there is additionally provided a method for detecting the dropping of at least one control element into a reactor core, which comprises placing detectors supplying signals along a fall path of a control element; feeding the signals from the detectors to thresholding components having binary output signals time-delayed to be approximately simultaneous; feeding the delayed binary output signals to a coincidence monitoring device for registering or identifying the dropping of the control element; and recording and feeding the signals from the coincidence monitoring device to a monitoring device. This manner of achieving the object is very simple and provides better fault detection and signal evaluation than the prior art. With the objects of the invention in view there is also provided a configuration for detecting the dropping of at least one control element into a reactor core, comprising detectors disposed along a fall path of at least one control element of a reactor core defining a fall time, the detectors having signal outputs; delay components having a delay time proportional to the fall time, the delay components each connected to the signal output of a respective one of the detectors; a common adder component connected to the delay components, the common adder component having an output side; and a monitoring device connected to the output side of the common adder component. With this configuration, in contrast to the prior art, coincidence monitoring of the signals is possible, which leads to reliable fault monitoring in the reactor core. In accordance with another feature of the invention, a differentiator component is connected in each case between the respective detectors and the adder component. Pulses which permit beneficial coincident signal processing are therefore available as signals. In accordance with a further feature of the invention, there is provided a thresholding component in each case connected between the respective detectors and the adder component. Therefore, only signals which are unambiguously due to the dropping of a control element are processed. This is because control elements drop at a high fall rate, which results in a higher signal amplitude than under normal conditions for the control elements. Further alternative configurations may also be provided for alternative methods. In accordance with a concomitant feature of the invention, the respective signal processing components and/or the monitoring device are formed by one or more programmed computers. This provides more reliable monitoring of malfunctions, which is fast and, where appropriate, can be effected by using an existing computer with all of the advantages of digital signal processing. 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 an apparatus for detecting the dropping of a control element, 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.