Patent Number: 059995831
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention provides a method for analyzing operation of an electromagnetic drive mechanism 22 for nuclear control rods 24, shown generally in FIG. 1, in a manner that permits detection of performance problems including not only failure of components, but also deterioration of performance over time. By collecting and storing data on the performance of the components and comparing present performance to previously stored historical data, the invention permits detection of impending problems before a failure actually occurs. The control rods as shown in FIG. 1 are attached in clusters 26, each cluster being commonly driven by a drive rod 28 disposed in a vertical extension housing 32 of the reactor core pressure vessel 34 containing the fuel rod assemblies 36 into which the control rods 24 are advanced or from which the control rods are retracted for variable damping of nuclear flux. The moving parts of the mechanism are within the pressure envelope of the reactor and the electromagnetic coils 42 for driving the movable parts are disposed around and about each of the extensions. FIG. 2 shows one drive rod mechanism with the extension housing partly cut away, and FIG. 3 is a detailed view, partly in section, showing exemplary grippers that or operable in sequences to engage, lift and/or lower the drive rod when the associated coils of the drive mechanism are energized. This arrangement is substantially as disclosed in U.S. Pat. No. 5,009,834--Tessaro, which is hereby incorporated with respect to the mechanical and electromagnetic aspects of the control rod drive. The drive mechanism comprises stationary grippers 44 and movable grippers 46 for engaging the drive rod 28, and a lifting armature 48 by which the movable grippers are lifted or allowed to fall along the longitudinal axis of the drive rod 28. Each is operated by a corresponding electromagnetic coil 54, 56, 58. The grippers are arranged normally to release the drive rod 28, for example being mounted to pivot clear of the drive rod or spring biased to retract, when not electromagnetically forced to engage. Thus, when neither of the stationary and movable gripper coils 54, 56 is energized, the corresponding grippers 44, 46 release their hold on the drive rod, which falls by gravity, allowing the control rods 24 to drop into the nuclear core. At other times the coils are energized for either holding drive rod 28 and the associated control rods 24 in position or for stepping them up or down in response to signals from a controller (not shown) that regulates the output level of the reactor. FIG. 3 shows an exemplary mounting of a movable gripper, in particular one of three grippers that are spaced circumferentially around drive rod 28 for bearing radially inwardly to engage ridges or grooves 68 on rod 28 or outward to clear the ridges or grooves. The same reference numbers are used throughout the drawings to identify corresponding elements. The drive mechanism has at least one stationary gripper coil 54, at least one movable gripper coil 56, each having an independently driven gripper 44, 46, and at least one lifting armature 48 driven by a lift coil 58 for displacing the movable gripper relative to the stationary gripper. Each coil is coupled to a coil current driver controlled by a timing circuit that is wired or programmed to effect a series of switching operations for achieving coil current sequences as shown in FIG. 4 (for lowering the drive rod) and FIG. 5 (for lifting the drive rod) by one step at a time. The current drivers 62 and timing circuit 64 are shown generally in FIG. 6. Current drivers 62 provide current to coils 54, 56, 58 for effecting sequences of holding and moving operations that cause the control rod drive rod 28 to be raised or lowered by one increment during the sequences shown in FIGS. 4, and 5, each step taking about 1.7 s to complete and any number of sequences being executed one after another depending on the needs of the reactor for more or less nuclear flux. The holding and moving operations include phases in which the coils are energized individually, and other phases in which the coils are energized in combinations. The coil currents having nominal amplitudes at different times in the cycle (e.g., full-on to initially engage, part-on for holding, and off) and nominal timing relationships. FIG. 4 is a time plot showing the nominal current levels and nominal timing relationships for steps in which drive rod 28 and associated control rods 24 are lowered or advanced into the spaces between the nuclear fuel rods in the core; and FIG. 5 represents raising or retracting control rods 24 from the core. Lifting (FIG. 5) is accomplished beginning from a situation in which stationary gripper 54 is energized for holding drive rod 28, and movable gripper 46 resides in its lower position but is disengaged from drive rod 28 because movable gripper coil 56 and lift coil 58 are both de-energized. During the raising or "out" step, movable gripper 46 is latched onto drive rod 28 by energizing movable gripper coil 56; stationary gripper coil 54 is then de-energized and stationary gripper 44 releases rod 28; lift armature 48 is operated to lift movable gripper 46 together with the drive rod by an increment equal to the armature's span of movement; stationary gripper 44 is again engaged; and finally the movable gripper coil and armature coil are de-energized, and the mechanism returns to its start position. The timing circuit 64 has suitable gating, timing and/or state circuits that move through this sequence with each step, in particular by sending a signal to the appropriate current driver(s) 62 at the appropriate times. Current drivers maintain the required coil current, for example, using a current sensing resistor 72 coupled in a feedback loop for maintaining the current level indicated by the "send" signal from the timing circuit. Each gripper preferably comprises a plurality of circumferentially spaced toothed bodies that are brought radially inwardly by electromagnetic force to engage in annular slots or ridges 68 of the drive rod when engaging. When the respective coils are de-energized, the toothed grippers move radially outwardly to release rod 28. For a lowering step, again starting from a time when stationary gripper 44 is holding the drive rod, the first step is to energize lift coil 58 to cause armature 48 to raise movable gripper 46 (which is de-energized at the time) to a higher point along the drive rod 28; movable gripper coil 56 is then energized and movable gripper 46 engages the drive rod while held by armature 48 at this higher point; stationary gripper 44 is released, then armature coil 58 is de-energized. The drive rod 28 and the control rods 24 coupled to it, drops by gravity as movable gripper 46 falls back to its original position. Stationary gripper 44 is then energized, and finally movable gripper 46 is de-energized to resume the situation preceding the step. Activation and deactivation of the grippers and the armature require a discrete time interval to permit the coil current to be established or cut off or for a mechanical motion to be completed, before the next motion is commenced. The necessary times can be nominally established, allowing for a safety margin, and conventionally are programmed into timing circuit 64. However, with wear and potentially with electrical failures, the mechanism may deteriorate to the point that it cannot operate correctly or operate at nominal timing. Wear and electrical failure can cause sudden failure or operation can deteriorate over time. According to the invention, the operational status of the control rod drive mechanism is monitored by sensing current signals in coils 54, 56, 58 during the sequence of lifting and lowering operations, and analyzing the current signals both against nominal operational thresholds and also against historical data that is accumulated, stored, and compared with data as it is collected during present operations. In this manner, the operational status of the mechanism can be assessed, and impending failure modes can be identified or even predicted. The invention thus provides ongoing diagnostic checking as well as data that can be used, for example over a plant-wide network, for assessing and planning maintenance and engineering. During stepping operations, and preferably continuously, the coil current levels to each of the gripper coils 54, 56 and the armature coil 58 are sampled, digitized and stored to provide coil current data samples over time. This is accomplished as shown in FIGS. 6 and 7 using current sensing means to develop a signal representing coil current, which is digitized by an analog to digital converter 74, operated repetitively to collect samples. The coil current data samples can be stored in raw form, and preferably are processed to generate coil current data representing a measure of at least one of: a current amplitude, a time of current variation indicating a change in coil inductance, and an ac ripple. The coil current data is compared to the nominal current amplitudes and timing relationships, and deviations therefrom are identified and indicated to the operators. For obtaining the samples, each of the coils is coupled to an associated current sensor such as a resistor in series with the coil, an inductor responsive to the coil field, or an inductive loop around the coil, etc. For ac operation, the signal can be coupled through a rectifier and filtered by an RC combination to obtain a voltage signal representing the level of current in the coil. The voltage signal can be sampled via an analog to digital converter triggered by a clock signal from an oscillator, or responsive to a logic device such as a processor 78 programmed repetitively to select and sequentially sample signals developed from a plurality of coils. In the event that a separate clock oscillator (not shown) is associated with the analog to digital converter, or if control computer 78 controls sampling, then a failure of the clock oscillator 76 of the initial timing circuit can be detected by deterioration of timing of the resulting switching operations, detected by computer 78 (or vice versa). Sampling is preferably accomplished at a frequency of at least ten times the ac power frequency to enable assessment of the extent of ripple in the current signal, which is indicative of the operational status of the power supply (not shown) and the current drivers 62 to the coils (e.g., loading). The coil current data can be stored in a raw form or processed to develop factors representative of signal attributes, which are then stored in memory 82, or both. The raw data and/or processed attribute factors, generally termed the coil current data, are stored in the computer memory as a historical log, for example on a disk drive. During the collection and processing of data in later cycles, the most recent coil current data is compared to the previous historical coil current data. By sensing for and indicating variations, e.g., exceeding a predetermined threshold of variation, trends occurring over time can be monitored, and impending problems can be predicted. Referring to FIG. 4, there are certain particular attributes that are advantageously monitored and compared to their historical values. These include the average level of the current signals maintained by the switching controller, the timing of the switching cusps in the signal upon turn-on and turn-off of coil driving current, the extent of ac ripple, the slope of the rising and falling current signal traces, and the occurrence of characteristic notches. The current waveforms for the stationary and movable gripper coils 54, 56 each contain a readily detectable notch 84 occurring with the mechanical pull-in of the associated gripper, due to increase in inductance, until current drive regulation by driver 62 re-establishes the nominal current level due to feedback control. At the bottom of notch 84, the slope of the current waveform changes direction, which is readily detectable when comparing sample values to immediately preceding sample values, or by calculating a running average or slope of a predetermined number of immediately preceding sample values. The current traces for all the coils have inductive exponential charging and discharging slopes, but likewise have readily detectable transitions between the respective current levels representing turn-on and turn-off. The timing of switching can be identified by comparing each sample to previous values or averages leading up to the level change. For proper operation, the switch-on edges 86, switch-off edges 88 and gripper pull-in notches 84 must occur at the proper times, namely in the correct sequence and with sufficient spacing in time that earlier needed operations occur and are completed before later operations depending on the earlier ones commence. For example, for stepping upwardly, stationary gripper 44 cannot be released until movable gripper 46 has pulled in, or else the drive rod could drop. Lifting armature 48 cannot be activated until movable gripper 46 has pulled in and stationary gripper 44 is released. By noting the occurrence of the pull-in gripper notches as well as the time spacing between switching operations, the operational status of the control rod drive mechanism can be monitored. FIG. 7 generally indicates the steps undertaken to obtain and analyze current data repetitively. In the event that comparison of present sample data and historical data shows that a gripper pull-in notch 84 has abruptly begun to occur earlier, the gripper mechanical fittings may be getting loose. If the gripper pull-in notch becomes later, the gripper coil drive arrangement is suspect, or the mechanical fittings may be binding. These attributes can be assessed, as well as the ability of the current drive circuits to maintain predetermined current levels, whether a coil has become open circuited (leading to zero current) or shorted across adjacent windings (reduced inductance and electromagnetic power), etc. For making these assessments, the nominal and measured current amplitudes, the presence and occurrence of the gripper pull-in notch 84 said notch at nominal and measured delay times and amplitudes following application of a current to the associated gripper coil, are each assessed. The amplitudes and delays as compared to nominal and historical data are used by processor 78 in analyzing the data according to programmed processing steps, to identify mechanical impairment of the gripper, degradation of magnetic flux of the driven coils and in general to assess whether the control rod drive mechanism continues to operate as designed. Preferably, the current monitoring steps are only one aspect of a monitoring device that is programmed to localize defective or failing components based on the specific results of monitoring. For this purpose, it is advantageous to provide additional measurements apart from the coil currents, for use in conjunction with coil current data to localize problems. By suitably monitoring various status signals, for example using relays or other switched means coupled along the power distribution path from the mains to the coils, problems can be localized to the circuits or elements at which a failure has occurred. For example, a status monitor relay or switching circuit can provide a status input to the apparatus of the invention representing the presence of a signal on each phase of the mains, on the output side of any circuit breakers or similar interrupters coupled to the mains, on the outputs of a regulated power supply that feeds the current regulating circuits and at points in the control and logic circuits that trigger switching. As noted above, a timing fault in one of clock oscillator 76 driving the timing circuit and the timing reference (e.g., clock) of the processor 78 can be detected from changes in timing. Alternatively, the clock oscillator signal can be digitized together with the current signal, or its cycles counted, to determine whether switch-on edges 86 and switch-off edges 88 occur in correct synchronization with the clock signal. Current or voltage can be sensed at a plurality of points between an ac power source and the control rod drive coils to assess the respective switching devices, power regulators, controllers and the like by monitoring for the presence of current or voltage or for comparing current of voltage levels to nominal levels for identifying electrical or mechanical failure points, whereupon the failure point is logged, made the subject of a warning message or used to trigger an alarm or signal to a remote apparatus. By monitoring the presence and/or level of voltages and currents, including not only two or more coil current levels but also other voltages such as power supply outputs, and by monitoring the timing of switching operations and the occurrence of notches due to gripper or armature pull-in, it is possible readily to determine and indicate various fault conditions. In a preferred embodiment, the conditions include: power cabinet thyristor failure; PA1 power cabinet circuit interrupter operation (e.g., fuse); PA1 power cabinet loss of AC phase; PA1 drive mechanism physical impairment of movement; PA1 drive mechanism coil flux degradation; PA1 controller clock oscillator frequency shift; PA1 controller clock counter malfunction; PA1 controller to power cabinet data transmission error/failure; and, PA1 controller decoder malfunction. The apparatus of the invention can be embodied in a portable unit for periodic testing or permanently installed as a part of the control rod drive system. In addition, the invention is operable for online diagnostic testing during operations and/or can be part of a larger diagnostic system, for example wherein the apparatus communicates data to a plant diagnostic system or preventive maintenance system. The invention having been disclosed in connection with the foregoing variations and examples, additional variations will now be apparent to persons skilled in the art. The invention is not intended to be limited to the variations specifically mentioned, and accordingly reference should be made to the appended claims rather than the foregoing discussion of preferred examples, to assess the scope of the invention in which exclusive rights are claimed.