Patent Number: 054266807
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A conventional position indicator probe has 53 position switches (see FIG. 5), usually designated S00 to S52. Forty-nine of the reed switches are spaced at equal 3-inch intervals on the switch support 103 (FIG. 4), providing an indication signal at each locking position and at the halfway point between each locking position. These switches are closed one at a time to transmit signals to the rod position information system (not shown), which in turn energizes the corresponding rod position display in the control room. The control room digital displays resulting from the closing of these 49 switches range from "00" (control rod fully inserted into the core) to "48" (control rod withdrawn to the backseat position). The switches in the position indicator probe are assigned corresponding number designations, S00 to S48. The even-numbered switches correspond with the locking positions and the backseat position of the index tube 26 (FIG. 1); the odd-numbered switches correspond with the intermediate positions on the index tube. Switches S00, S51 and S52, located at the top of position indicator probe 12a (FIG. 2A), provide the control room with a signal indicating "rod full in". Switch S49, located near the bottom of the probe and on the opposite side of switch support 103 from switch S48 (see FIG. 4), closes simultaneously with switch S48 to provide the control room with a signal of "rod full out". Switch S50 is installed 2 inches below the normal full-out position of the CRD, i.e., at the so-called "overtravel" position. Since the limit of drive piston down travel is provided by the backseat position of the control rod in the reactor vessel guide tube (not shown), switch S50 is closed only when the control rod and CRD are uncoupled and, when closed, provides the control room with an annunciation of this condition. The present invention utilizes switches S47, S49 and S50, during CRD removal, to monitor CRD uncoupling, as described in detail hereinafter. In accordance with a preferred embodiment of the invention depicted in FIG. 6, the means for supporting the CRD during removal comprise a generally circular cylindrical bucket adaptor 100 coupled to an extension tube 110 by a pin 113. The adaptor has a first circular cylindrical chamber 115 of diameter slightly greater than that of the ring flange 17 and a second circular cylindrical chamber 117 of diameter less than that of the ring flange 17 and greater than that of position indicator probe housing 14. These chambers are connected by way of a seat 101 on which ring flange mounting bolts 9 bear. Seat 101 is preferably an annular horizontal surface having an inner diameter which is less than the diameter of a pitch circle circumscribing the heads of ring flange mounting bolts 9. Extension tube 110 supports the CRD by way of bucket adaptor 100 after the mounting bolts 2 are unscrewed using a conventional de-torquing tool 112. Before the bucket adaptor/extension tube assembly is installed, an electronic monitoring tool 102 is connected to receptacle 14 of the position indicator probe assembly (see FIG. 6) via a plug 106 and shielded cables 104. Plug 106 fits snugly into receptacle 14, allowing tool 102 to be suspended via the shielded cables. Bucket adaptor 100 has a chamber for receiving the suspended monitoring tool 102. A window 111 is provided to allow observation of indicator lights 123-125 which are visible through a transparent enclosure (e.g., made of LEXAN.TM. pipe) of electronic monitoring tool 102. In accordance with the CRD removal method of the present invention, the position indicator probe 12a can be left in place inside the position indicator tube 61 and used to monitor the state of CRD coupling/uncoupling. A green LED 124 lights when ring magnet 67 is aligned with switch S50, indicating that the index tube is in the overtravel position, i.e., the CRD is uncoupled from the control rod. This condition indicates that the CRD can be safely removed from the CRD housing. A red LED 123 lights when ring magnet 67 (FIG. 2A) is aligned with either of switches S48 or S49, indicating that the index tube is in the backseat position or the adjacent intermediate position, respectively. In either case, the CRD should be pushed back into the housing and then uncoupled from the control rod. Switch S47 indicates that the index tube is extended just short of the first locking position (corresponding to switch S46) and also lights the red LED 123. Extension to the first locking position is to be avoided since the CRD cannot be simply pushed back in to retract the index tube. Finally, a flashing red LED 125 flashes when the ring magnet 67 is between adjacent switches, i.e., all position switches S00 to S52 are open. For example, green LED 124 might be lit, indicating that the CRD can be removed, but upon lowering of the CRD, the flashing red LED is activated, along with a pulsed horn 129 (see FIG. 7). The flashing red light and pulsed horn warn the crew that the CRD is not uncoupled and that CRD removal should be halted. The monitoring circuit in accordance with the preferred embodiment of the invention is shown in FIG. 7 connected to pins 3, 4, 6 and 8 of plug 106. Pin 8 is connected to ground; pins 3, 4 and 6 are respectively connected to switches S49 (or S48), S47 and S50 (see FIG. 5). A first circuit loop is formed by a closed ON/OFF switch 120, a dc voltage source 121 (e.g., a 9-volt battery), red LED 123, a closed switch S47 or S49 connected to pin 3, and a resistor 126 (470 .OMEGA.). Red LED 123 is activated when switch S47 or S49 is closed. Alternatively, green LED 124 is activated when switch S50, connected to pin 6, is closed. The light-emitting diodes 123 and 124 can be tested by pressing test pushbuttons 108 and 108', respectively. A switching transistor 122 (e.g., 2N1711 or its equivalent) has its base coupled to the positive terminal of the voltage source via resistors 126 and 127 (both 470 .OMEGA.) connected in series. The emitter of transistor 122 is connected to the positive terminal of the voltage source via a parallel network: one line consisting of a resistor 128 (1 k.OMEGA.) and flashing red LED 125 connected in series, and the other line consisting of a pulsed horn 129. The collector of transistor 122 is connected to the negative terminal of the voltage source via a series of diodes 130 (e.g., 1N914). When switches S47, S49 and S50 are all open, transistor 122 is biased to switch on the flashing red LED 125 and the pulsed horn 129. Using the monitoring circuit shown in FIG. 7, a CRD can be removed from a CRD housing with its position indicator probe in place. A signal is activated to indicate that the index tube occupies a position corresponding to CRD uncoupling from the control rod, i.e., the CRD is ready to be removed from its housing. In the event that the CRD is not uncoupled, another signal will be activated as the index tube, which is still coupled at its end to the control rod via the spud, is extended relative to the descending CRD. The preferred embodiments of the bucket adaptor and electronic monitoring tool have been disclosed for the purpose of illustration. Variations and modifications of the disclosed structure which do not depart from the concept of this invention will be readily apparent to mechanical engineers skilled in the art of monitoring devices. Also it should be borne in mind that different nuclear reactors may have switch or pin numbering schemes different than those disclosed herein. All such variations and modifications are intended to be encompassed by the claims set forth hereinafter.