Patent Number: 055240305
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The driver assembly 100 (see FIGS. 6 and 7) consists of a drive housing assembly 104 and a drive screw assembly 155. Drive housing assembly 104 in turn comprises a pair of lugs or ears 105, 105', a drive housing 106 and a drive base 107, all made of stainless steel, and a bronze bushing 108 having a threaded bore. Drive screw assembly 155 comprises a drive screw 156 threadably coupled to the threaded bore of bushing 108, a thrust plate 157 and a bearing 158 which allows thrust plate 157 to rotate relative to drive screw 156. A plastic washer 159 is captured between the drive base 107 and the larger-diameter portion of the drive screw 156 to provide a cushion for the drive screw when it is lowered to a backseat position. This prevents the drive screw from being locked against the drive base due to overtightening. As best seen in FIG. 5, the driver assembly is attached to the CRD by means of lugs 105, 105' and ring flange cap screws 9. The probe assembly 110 is seated atop the thrust plate 157 and slides inside drive housing 106 along the CRD axis as drive screw 156 is turned. In accordance with one procedure for removing the CRD, the housing 12 and position indicator probe 12a are removed as a unit and then the piston nut 16 is removed. The uncoupling tool 110 is screwed onto the threaded end 15c of the piston tube as shown in FIG. 5. In this way the lock plug 94 (see FIG. 3) and the uncoupling tool are effectively coupled (i.e., via the piston tube 15 and the uncoupling rod 48). The details of the structure of probe assembly 110 are shown in FIGS. 8, 9A, 9B and 10. The probe assembly comprises a probe 118 contained in a probe housing 116 connected to a frame base 115a. Frame base 115a has a threaded bore which screws onto the threaded end 15c of the piston tube 15. The two legs of a U-shaped frame member 115b extend from frame base 115a (see FIG. 8). A base 138 is bolted to the crosspiece of frame member 115b. Base 138 sits on the thrust plate 157 of the driver assembly 100 (see FIG. 5) during lifting of the piston tube assembly. Frame member 115b is the sole member transmitting the load exerted by the driver assembly from base 138 to frame base 115a. Frame member 115b supports a battery holder and a printed circuit board 147, hereinafter referred to as the "CRD position module". A housing (not shown) and a lamp housing 140 are held between frame base 115a and tool base 138. A plurality of indicator lamps are electrically connected to CRD position module 147 and housed in lamp housing 140 in front of a pair of reflectors. The housing is provided with a window for inserting batteries into the battery holder. After battery insertion, a sleeve 135 is slid over the housing and O-ring seals and then locked into place by a threaded locking ring 151. The probe 118 has three analog position sensors 119a-c mounted between spacers such as spacers 123 and 124 shown in FIG. 9A. Sensors 119a-c are Hall effect transistors electrically connected via a five-pin connector 126 to CRD position module 147, as shown in FIG. 10. Each sensor outputs a voltage proportional to magnetic flux. The spacers are made of low-carbon steel and have truncated conical ends which focus the magnetic flux onto the adjacent sensor. Each analog position sensor comprises a hemispherical transistor with three electrodes all mounted on a substrate. These structural details are depicted in FIGS. 9A and 9B for sensor 119b. Sensors 119a and 119c have similar structures. The space surrounding each sensor is filled with a silicon compound and closed with a respective cover 120. Each cover is provided with three holes for passage of the three terminals of the Hall effect transistor. Wire 125 carries the transistor output to the CRD position module 147 via a channel formed between probe 118 and probe housing 116. The output terminal which carries the analog position indication signal is connected to the CRD position module 147 as shown in FIG. 10. The CRD position module is programmed to compare the voltage output by each sensor with a respective threshold voltage and then selectively activate one of three indicators: a pair of green LEDs 130, 130', a yellow LED 131 or a red LED 132. An intermittently illuminated red LED 132' is used to indicate a low battery condition. The LEDs are visible through lamp housing 140, which is made of a polished transparent plastic, e.g., polycarbonate. Module 147 is powered via a two-pin connector 134 by batteries held in battery holder 128. During upward travel of probe 118 relative to the drive piston 24, the top sensor 119a moves into proximity with the ring magnet 67 in magnet housing 24a. The voltage output by sensor 119a increases as the axial distance between the sensor and the centerline of the magnet decreases. When the sensor output voltage exceeds a predetermined threshold, module 147 turns on red LED 132. As the ring magnet is displaced downward during drive piston withdrawal and control rod uncoupling, the voltage output by sensor 119b increases. However, as long as the output of sensor 119a remains above its threshold, module 147 will continue to illuminate the red LED, i.e., if sensors 119a and 119b are both outputting voltages which exceed their respective thresholds, then module 147 is programmed to give the red indicator precedence. Only when the output of sensor 119a falls below its threshold and the output of sensor 119b remains above its threshold will the red LED 132 be switched off and yellow LED 131 be switched on. As the magnet housing is displaced further downward during drive piston withdrawal, the voltage output by sensor 119c will eventually exceed its respective threshold. If sensors 119b and 119c are both outputting voltages which exceed their respective thresholds, then module 147 is programmed to give the green indicator precedence by turning off the yellow LED and turning on a pair of green LEDs 130, 130'. (Two green LEDs are provided in order to increase the intensity of the green indicator.) Thus, the yellow LED is illuminated only if sensor 119b is outputting a voltage above its threshold and sensors 119a and 119c are both outputting voltages below their respective thresholds. A continuously illuminated red LED indicates the control rod is in the "full out" position; the illuminated green LEDs indicate the control rod is in the "overtravel" position, i.e., is uncoupled; and an illuminated yellow LED indicates that the control rod is in a "transition" position therebetween. Since the limit of drive piston down travel is provided by the backseat position of the control rod in its associated reactor vessel guide tube (not shown), switch 119c is closed only when the control rod and CRD are uncoupled and, when closed, provides an annunciation of this condition. This condition indicates that the CRD can be safely removed from the CRD housing. Red LED 132' will illuminate intermittently in response to detecting a low battery condition even when probe assembly 110 is not installed on threaded end 15c of the piston tube. The particular driver assembly disclosed above is not required in order to utilize the probe assembly. Any conventional means for bearing against base 138 and lifting the probe assembly and the piston tube assembly connected thereto can be used in place of the driver assembly. For example, a conventional tool known as a "pogo stick" can be used to uncouple the control rod by lifting the probe assembly. The preferred embodiment has 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 apparent to mechanical engineers skilled in the art of control rod drive tools. All such variations and modifications are intended to be encompassed by the claims set forth hereinafter.