Patent Number: 059636106
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention generally relates to the field of diagnostic techniques for nuclear reactor subsystems in a nuclear steam supply system. More particularly, the present invention is directed to a control element drive mechanism (CEDM) in such nuclear power plants. 2. Description of the Related Art In a typical pressurized water nuclear power reactor, a plurality of control element assemblies with associated drive mechanisms (CEDMs) are supported on a nuclear reactor vessel for moving the control element assemblies into and out of the reactor core, for the purpose of controlling the gross power level, or the power distribution in the core. Typically, each control element assembly (CEA) has a shaft which is driven by a CEDM. Such CEDMs each include a plurality of electromagnetic coils having latches which engage grooves on the shaft of the CEA, whereby the shaft may be moved in step-wise fashion. The shaft terminates in a web or similar structure, which supports a plurality of individual control elements or rods sized to pass through openings within or between fuel assemblies which define a reactor core. A particular nuclear reactor may have dozens of CEAs, which are typically grouped or ganged in sets or four or eight, so as to move in unison when actuated by respective control element drive mechanisms. In the event of an emergency, the CEAs can be disengaged from their respective drive mechanisms, thereby falling under force of gravity into the reactor core to shut down the reactor power as quickly as possible. This free fall of a CEA is also known as a rod-drop. The position and movement of each CEA and, hence, its associated control rods, is typically sensed by a reed switch position transmitter which produces an analog position output signal that is sent to a safety control system during normal operation of the power plant. This signal can be used within the safety control system as part of a calculation to determine if the reactor should be tripped, thereby causing the rods to drop. Nuclear steam supply system plants such as those described above routinely perform post-refueling rod testing prior to power ascension. The testing normally consists of a CEDM operational test, withdrawal and insertion of the rods, a rod coupling verification test, a rod-drop test from a small height, and a full height rod-drop, on each CEDM. Such testing is normally performed at reactor operating temperature and pressure. Data is acquired during the CEDM operational tests and analyzed to verify that the nuclear steam supply system is fully operational before normal power ascension occurs. This data can then be used for troubleshooting or to generate outage reports according to governmental regulations. Further due to governmental regulations, the data acquired during CEDM operational testing must be archived for possible review in the future. In the related art, electromechanical data collection apparatus (typically analog strip chart recorders such as the Honeywell Visicorder) are utilized to record coil-current within the control element drive mechanisms and, optionally, the signal outputted from the reed switch position transmitters. Typically, the test data is collected in a "cable spreader" room in which the CEDM control cabinet is located. Since testing occurs on a periodic basis, test points for electrically connecting the data collecting apparatus are permanently provided in the control cabinet. However, data recorded in such a manner results in signal traces being displayed on long continuous rolls of paper which is expensive, bulky in storage and extremely difficult to manipulate when manual trace analysis occurs. Furthermore, such electromechanical systems are prone to mechanical failure thereby further increasing testing delays and increasing expense. Still another deficiency of the above-described electro-mechanical systems is that, due to the limited recording capability of such systems, CEDM operational testing must be performed on one CEA at a time. In particular, this is due to the fact that each drive mechanism typically employs four or five electromagnetic coils to position the associated CEA and one reed switch position transmitter is associated with each CEA. Thus, each CEDM operational test entails recording up to six parameters of data per CEDM tested. Because the known mechanical recording apparatus can only record six parameters at a time, all CEAs in a group (consisting of four or eight CEAs) cannot be tested simultaneously. Rather, multiple tests must be run (four or eight as the case may be) to verify performance of all of the drive mechanisms associated with the CEAs of the group. The above-described deficiencies of the related data acquisition systems, thus, include increased testing expenses, prolonged testing operations, difficulty in analyzing and storing recorded data and a likelihood of mechanical failure of the test equipment. Accordingly, there remains a need in the art for a CEDM data acquisition system which overcomes the aforementioned deficiencies by simplifying data gathering, storage, and manipulation for, e.g., troubleshooting, generation of outage reports, and data archiving. There remains an additional need in the art for an improved CEDM data acquisition system which overcomes the aforementioned deficiencies by permitting simultaneous recordation of data pertaining to all of the CEDMs in a CEA group while not requiring any modification of the nuclear power plant. There remains a further need in the art for a CEDM data acquisition system which overcomes the aforementioned deficiencies by providing continuous and interactive CEDM test monitoring on an easy to read and analyze color display, such monitoring occurring while the CEDM operational tests occur. There remains yet another need in the art for a CEDM data acquisition system which overcomes the aforementioned deficiencies of the related art by providing a test data display which has the capability to automatically change the parameters displayed upon the occurrence of a predetermined triggering event such as a rod-drop event. SUMMARY OF THE INVENTION It is accordingly an object of the present invention to provide an improved CEDM data acquisition system which provides simplified data gathering, storage and manipulation for, e.g., troubleshooting, generation of outage reports, and simplified data archiving. It is a further object of the present invention to provide an improved CEDM data acquisition system which permits simultaneous recordation of data pertaining to all of the CEDMs in a CEA group while not requiring any modification of the nuclear power plant. It is another object of the present invention to provide an improved CEDM data acquisition system which provides continuous and interactive CEDM test monitoring on an easy to read and analyze color display, such monitoring occurring while the CEDM operational tests occur. It is yet another object of the present invention to provide an improved CEDM data acquisition system which displays images of the acquired test data and has the capability to automatically change the parameters displayed upon the occurrence of a predetermined triggering event such as a rod-drop event. It is yet another object of the present invention to provide a CEDM data acquisition system for use in the CEDM operational testing of nuclear steam supply systems which offers an optimal combination of simplicity, reliability, efficiency, and versatility. These and other objects and advantages of the present invention are provided in one embodiment by providing a CEDM data acquisition system which (1) receives analog coil-current signals from control element drive mechanism coils; (2) conditions the analog signals to remove noise induced therein by the electronic circuitry associated with the control element drive mechanisms; (3) digitizes the conditioned analog signals; (4) displays the acquired test data; and (5) records the data for future use. The inventive data acquisition system preferably has the ability to simultaneously measure, display and record coil-current data for all of the five coils associated with each CEDM rod-group with respect to time. Further, the inventive data acquisition system preferably allows the user to monitor, display and record data for up to eight CEDMs simultaneously. It also allows the measurement of rod-drop times during rod-drop testing of the CEDMs. In the preferred embodiment of the inventive data acquisition system, the data from the CEDM is digitized following signal conditioning and is processed and displayed on a display screen in real time (i.e., as the CEDM operates). Preferably the display screen displays five coil-current traces versus time for a particular CEDM. These traces move horizontally through the screen in real time as the CEDM cycles to move the control rod at, e.g., about 0.75 inches (1.9 cm) per cycle. Preferably the operator can select that the traces pass continuously or, alternatively, that a particular display screen be frozen for study. The operator can select any one of, e.g., eight CEDMs (identified at the top of the display screen), which are grouped for simultaneous movement, and display and record all group related data, e.g., up to forty coils. The data from all of the coils of any CEDM can be stored and replayed as desired. At the operator's option, traces indicative of CEDM reed switch position versus time can also be displayed for analysis. These traces originate from reed switch position transmitters (RSPT) in the CEDMs. The operator analyzes the traces for timing, shape or anomalies in a manner similar to what is conventional for the analysis of strip chart traces of the related art. However, since the data is digitally recorded and, thus, easy to manipulate, data analysis, data storage and report generation are greatly simplified. One preferred aspect of the inventive data acquisition system includes the ability of the display to automatically change images upon the occurrence of a rod-drop event. For example, when the coil-current signal for a given CEDM coil deviates from a predetermined value by a predetermined amount, the display automatically switches from displaying five coil-current signals and a position signal, to displaying only the triggering coil-current signal and the position signal. Optionally, the subsequent display image can also include an "acceptance" coil-current trace superimposed on the two aforementioned signal traces to aid a user in analyzing the coil-current trace of the triggering coil. The signal conditioning portion of the inventive data acquisition system includes a noise suppression network preferably consisting of differential amplifies and low pass filters with high common mode rejection in order to suppress unwanted electrical noise originating in the CEDM power supply and to prepare the analog coil-current and position signals for digitization. Accordingly, the signal conditioning portion of the system also provides the electrical interface between a digital processing unit (such as a computer) and the control cabinet test points. Optionally, the signal conditioning portion of the system can include an isolation amplifier to ensure that voltage spikes, or other erroneous electrical signals, which may occur downstream in the system are not fed back to the control element drive mechanisms thereby creating the possibility of inadvertently triggering a rod-drop.