Patent Number: 055240305
Section: summary

FIELD OF THE INVENTION This invention relates generally to maintenance of control rod drives in boiling water reactors. In particular, the invention relates to tools for uncoupling a control rod from a control rod drive prior to a maintenance operation. BACKGROUND OF THE INVENTION Control rod drives (CRDs) are used to position control rods in boiling water reactors (BWRs) to control the fission rate and fission density, and to provide adequate excess negative reactivity to shutdown the reactor from any normal operating or accident condition at the most reactive time in core life. Referring to FIG. 1, each CRD is mounted vertically in a CRD housing 10 which is welded to a stub tube 8, which in turn is welded to the bottom head of the reactor pressure vessel 4. The CRD flange 6 is bolted and sealed to the flange 10a of the CRD housing 10, which contains ports for attaching the CRD hydraulic system lines 80, 81. Demineralized water supplied by the CRD hydraulic system serves as the hydraulic fluid for CRD operation. As shown schematically in FIG. 1, the CRD is a double-acting, mechanically latched hydraulic cylinder. The CRD is capable of inserting or withdrawing a control rod (not shown) at a slow controlled rate for normal reactor operation and of providing rapid control rod insertion (scram) in the event of an emergency requiring rapid shutdown of the reactor. A locking mechanism in the CRD permits the control rod to be positioned at 6-inch (152.4 mm) increments of stroke and to be held in these latched positions until the CRD is actuated for movement to a new position. A spud 46 at the top of the index tube 26 (the moving element) engages and locks into a socket at the bottom of the control rod. Once coupled, the CRD and control rod form an integral unit which must be manually uncoupled by specific procedures before a CRD or control rod may be removed from the reactor. When installed in the reactor, the CRD is wholly contained in housing 10. The CRD flange 6 contains a withdraw port 70 and an insert port 66 with an integral two-way check valve (with a ball 20). For normal drive operation, drive water is supplied via an associated hydraulic control unit (HCU) to the insert port 66 for drive insertion and/or to withdraw port 70 for drive withdrawal. For rapid shutdown, the check valve directs external hydraulic pressure or reactor pressure to the underside of drive piston 24. When higher than the external hydraulic pressure, reactor pressure is admitted to the two-way check valve from the annular space between the CRD and a thermal sleeve (not shown) through passages in the CRD flange, called scram vessel ports. Referring to FIG. 2, the CRD further comprises an inner cylinder 57 and an outer tube 56, which form an annulus under the collet piston 29b through which water is applied to the collet piston (see FIG. 1) to unlock index tube 26. The internal diameter of inner cylinder 57 is honed to provide the surface required for expanding seals 65 on the drive piston 24. Returning to FIG. 1, welded pipes 80 and 81, installed in the CRD housing, port water to the insert port 66 and the withdraw port 70 respectively. A port 69 below outer tube 56 connects to withdraw port 70 in CRD flange 6 so that water is applied through the under-the-collet-piston annulus to collet piston 29b when a withdraw signal is given. The CRD is secured to the CRD housing flange 10a by eight mounting bolts (not shown). A pressure-tight seal is effected between the mated flanges by O-ring gaskets (not shown) mounted in a spacer 7 secured to the CRD flange face. Insert port 66 contains a ball check valve which consists of check-valve ball 20, ball retainer 21, and retainer O-ring 22. This valve directs HCU accumulator pressure or reactor pressure to the underside of drive piston 24 during scram operation. Port 66 is connected internally to the annulus under the drive piston 24 and serves as the inlet for water during normal insertion or scram. Water enters this port for a brief period in response to a withdraw signal to move the index tube 26 upward so that collet fingers 29a (see FIG. 1) are cammed out. Following this brief unlocking period, water from below drive piston 24 is discharged through port 66 and through the under-piston hydraulic line for the duration of the withdraw signal. The withdraw port 70 serves as the inlet port for water during control rod withdrawal and as the outlet port for water during normal or scram insertion. It connects with internal porting and annuli to the area above drive piston 24. During a withdraw operation, water is supplied from port 70 through a small connecting port in CRD flange 6 to the annular space between outer tube 56 and inner cylinder 57 for application to the bottom of collet piston 29b. The locking mechanism comprises collet fingers 29a, collet piston 29b and collet spring 31. This mechanism is the means by which index tube 26 is locked to hold the control rod at a selected position. The collet mechanism requires a hydraulic pressure greater than reactor pressure to unlock for CRD-withdraw movement. A preload is placed on collet spring 31 at assembly and must be overcome before the collet can be moved toward the unlocked position. For control rod withdrawal, a brief insert signal is applied to move index tube 26 upward to relieve the axial load on collet fingers 29a , camming them outward against the sloping lower surface of index tube locking notch 55. Immediately thereafter, withdraw pressure is applied. In addition to moving index tube 26 downward, this pressure is at the same time applied to the bottom of collet piston 29b to overcome the spring pressure and cam the fingers 29a outward against a guide cap (not shown). When the withdraw signal ceases, the spring pressure forces the collet downward so that fingers 29a slip off the guide cap. As index tube 26 settles downward, collet fingers 29a snap into the next higher notch and lock. When collet fingers 29a engage a locking notch 55, collet piston 29b transfers the control rod weight from index tube 26 to the outer tube 56. Unlocking is not required for CRD insertion. The collet fingers are cammed out of the locking notch as index tube 26 moves upward. The fingers 29a grip the outside wall of index tube 26 and snap into the next lower locking notch for single-notch insertion to hold index tube 26 in position. For scram insertion, index tube 26 moves continuously to its limit of travel during which the fingers snap into and cam out of each locking notch as index tube 26 moves upward. When the insert, withdraw or scram pressures are removed, index tube 26 settles back, from the limit of travel, and locks to hold the control rod in the required position. The drive piston 24 and index tube 26 are the primary subassembly in the CRD, providing the driving link with the control rod as well as the notches for the locking mechanism collet fingers. Drive piston 24 operates between positive end stops, with a hydraulic cushion provided at the upper end only. Index tube 26 is a nitrided stainless-steel tube threaded internally at both ends. The spud 46 is threaded to its upper end, while the head of the drive piston 24 is threaded to its lower end. Both connections are secured in place by means of bands 25, 25' with tab locks. There are 25 notches machined into the wall of index tube 26, all but one of which are locking notches 55 spaced at 6-inch intervals. The uppermost surfaces of these notches engage collet fingers 29b, providing 24 increments at which a control rod may be positioned and preventing inadvertent withdrawal of the rod from the core. The lower surfaces of the locking notches slope gradually so that the collet fingers cam outward for control rod insertion. The component parts of drive piston 24 are shown in FIG. 4 and include magnet housing 24a, seal cups 24b, piston coupling 24c and piston head 24d. Drive piston 24 is provided with internal (62, 71, 72) and external seal rings (65), and is operated in the annular space between piston tube 15 and inner cylinder 57. Internal (63) and external (64) bushings prevent metal-to-metal contact between drive piston 24 and the surface of piston tube 15 and the wall of inner cylinder 57 respectively. The magnet housing 24a contains a ring magnet 67 which actuates the switches of the position indicator probe 12a to provide remote electrical signals indicating control rod position. The piston tube assembly forms the innermost cylindrical wall of the CRD. It is a welded unit consisting of piston tube 15 and a position indicator tube 61 (see FIG. 2). The position indicator tube 61 is a pressure-containing part which forms a drywell housing for a position indicator probe 12a. Piston tube 15 provides for the porting of water to or from the upper end of the piston head portion of drive piston 24 during rod movement. The tube section 15a and head section 15b of piston tube 15 provide space for position indicator tube 61, which is welded to the inner diameter of the threaded end of head section 15b and extends upward through the length of tube section 15a, terminating in a watertight cap near the upper end of the tube section. A threaded end 15c of piston tube 15 is secured by a nut 16 at the lower end of the CRD. The position indicator probe 12a, which is slidably inserted into indicator tube 61, transmits electrical signals to provide remote indications of control rod position and CRD operating temperature. Probe 12a is welded to a plate 12b, which plate is in turn bolted to housing 12. Housing 12 is secured to CRD ring flange 17 by cap screws 13. A cable clamp (not shown), located at the bottom of a receptacle 14, secures a connecting electrical cable to receptacle 14. Ring flange 17 is in turn secured to the CRD housing by screws 9. Thus, probe 12a, housing 12 and the cable clamp (with the cables passing therethrough) can be removed as a unit. Probe 12a includes a switch support with 53 reed switches and a thermocouple for transmitting electrical signals to provide remote indications of control rod position and CRD operating temperature. The reed switches are connected by electrical wires to receptacle 14, which receives a plug (not shown). Housing 12 serves as a protective covering for the electrical wires. The reed switches are normally open and are closed individually during CRD operation by ring magnet 67 installed in the bottom of drive piston 24. As seen in FIG. 3, spud 46, which connects the control rod 90 and the CRD, is threaded onto the upper end of index tube 26 and held in place by locking band 25'. The coupling arrangement will accommodate a small amount of angular misalignment between the CRD and the control rod. Six spring fingers permit the spud to enter the mating socket 92 on the control rod. A lock plug 94 then enters spud 46 from socket 92 and prevents uncoupling. Two uncoupling mechanisms are provided. The lock plug 94 may be raised against the return force of a spring 95 by an actuating shaft 96 which extends through the center of the control rod velocity limiter to an unlocking handle (not shown). The control rod, with lock plug 94 raised, may then be lifted from the CRD. The lock plug may also be raised from below to uncouple the CRD from below the reactor vessel. Conventional practice is to remove the position indicator probe from the CRD prior to drive removal. The purpose is to allow access by an uncoupling tool in the space occupied by the probe. The uncoupling tool is used to uncouple the drive from the control rod from beneath the RPV. To accomplish this, an uncoupling tool is attached to the bottom of the CRD and used to raise the piston tube 15. Piston tube 15 supports an uncoupling rod 48 (shown in FIG. 3) which is welded to the flared end of a tube 43, which is in turn slidably supported in the base of spud 46. When the control rod is in its "full-out" position, i.e., backseated position atop the guide tube (not shown), the drive piston is separated from piston head 15b by a distance of 21/8 inches. Raising the piston tube and uncoupling rod 48 by 11/8 inches lifts lock plug 94 out of the spud. The drive piston/index tube/spud assembly 24/26/46 is then withdrawn until the drive piston sits on the piston head 15b (i.e., a distance of 1 inch), thereby disengaging the spud from the control rod coupling socket 92 (i.e., uncoupling the control rod). The uncoupling tool is then lowered by 11/8 inches to lower the control rod, assembly 24/26/46 and piston 15 together until piston head 15b is again backseated on the CRD ring flange 17. This is referred to as the overtravel travel position of the control rod. A conventional uncoupling tool comprises a probe supported on a thrust bearing via a battery housing. A mounting frame has a threaded bore for receiving a drive screw, the end of which is coupled to the thrust bearing. To install the uncoupling tool on a CRD, the mounting frame is bolted to the CRD ring flange (17 in FIG. 2). Then the drive screw is turned using a handle until the thrust bearing bears against the bottom of threaded end (15c in FIG. 2) of the piston tube. Further turning of the handle causes the thrust bearing to push the piston tube assembly up, which in turn lifts the lock plug (94 in FIG. 3) and uncouples the control rod. This uncoupling tool has a mechanical reed switch inside the probe which changes state when the ring magnet in the magnet housing (24a in FIG. 4) moves into proximity therewith, i.e., during withdrawal of the drive piston assembly. This change in state activates an indicator lamp, thereby indicating that the control rod has been uncoupled from the control rod drive. When uncoupling has been verified by activation of the indicator, the control rod drive is ready to be removed from the CRD housing flange 10a. SUMMARY OF THE INVENTION The present invention is an improved multistage uncoupling tool having the capability to indicate if the CRD has drifted in after being uncoupled from the control rod. The uncoupling tool has a two-module design: one module being a driver assembly for developing the uncoupling movement and the other module being a probe assembly for verifying the uncoupled condition. The probe assembly attaches to the threaded end of the piston tube with its probe extending inside the piston tube. The driver assembly attaches to the CRD ring flange. The driver assembly has a drive screw which pushes the probe assembly and the piston tube assembly connected thereto upward to uncouple the control rod. In accordance with a preferred embodiment of the invention, the probe assembly detects the ring magnet in the drive piston using analog position sensors which operate on the principle of the Hall effect and solid state circuitry. There are no moving parts. A plurality of analog position sensors are axially spaced inside the probe to provide a corresponding plurality of position indication signals. In accordance with a preferred embodiment, a top sensor provides a signal indicating that the control rod is in its "full out" position; a bottom sensor provides a signal indicating that the control rod is uncoupled; and an intermediate sensor provides a signal indicating that the control rod is in a transition position between the "full out" and uncoupled positions. The probe incorporates spacers which focus the magnetic flux onto the sensors. To uncouple the control rod, the probe assembly is lifted using a separate driver assembly. The driver assembly includes a thrust bearing which bears against the base of the probe assembly, a mounting frame which is coupled to the CRD ring flange, and a drive screw which is threadably coupled to a threaded bore in the mounting frame and which supports the thrust bearing at one end. As the drive screw is turned, the probe assembly and piston tube assembly coupled thereto are lifted, thereby raising the lock plug to uncouple the control rod. The uncoupling tool of the invention has improved reliability, repeatability and accuracy and is less sensitive to varying magnetic field strengths of magnet housing and to eccentric placement of the tool relative to the CRD.