SEALING COLD LEG NOZZLES OF NUCLEAR REACTOR PRESSURE VESSELS

Abstract: Cold leg nozzles (32) of a nuclear reactor pressure vessel are sealed during plant outages for reactor vessel inspection and core support maintenance by inserting a sealing plug (20) into each of the cold leg nozzle (32) by means of an installation tool (16) suspended within the vessel, to permit drainage of downstream piping and inspection and mainte­ nance of associated steam generators and pumps.

Description: SEALING COLD LEG NOZZLES OF NUCLEAR REACTOR PRESSURE VESSELS This invention relates to apparatus for and methods of sealing cold leg nozzles of nuclear reactor pressure vessels, during maintenance and inspection of associated steam generators and pumps. In a typical nuclear steam supply system, the nuclear reactor vessel, as well as steam generators and reactor coolant pumps, must be periodically inspected. During the inspection and maintenance of the reactor vessel itself, a closure head is removed from the top of the reactor vessel and the internals are removed from the interior of the vessel. The vessel and refuelling canal are filled with water to act as a radiological shield during servicing. In a once through steam generator of a type manufactured by The Babcock & Wilcox Company, heated water travels through the top of the steam generator during operation and leaves at the bottom thereof. Reactor coolant pumps pump the water back into the reactor through an entrance nozzle, known as a cold leg nozzle, which is situated in the same horizontal plane as the outlet nozzle. Thus, the introduction of water into the reactor vessel during inspection and maintenance procedures would result in flooding of the lower portion of the steam generators, the reactor coolant pumps, and associated piping leading up to the cold leg piping to permit maintenance and inspection of the reactor coolant pump and lower portions of the steam generator during the inspection period in which the reactor vessel and refuelling canal are filled with water. Such arrangements pose the threat of loosening or dislodgement of such a plug and flooding of the associated piping as indicated above. Means for sealing or plugging pipelines in general are well known in the art, as shown for example by US Patent No. 4 262 702 (Streich). However, such plugs are not suitable for a specialised application required by nuclear reactor maintenance and inspection. The present invention provides apparatus for and a method of sealing a cold leg nozzle of a nuclear reactor pressure vessel from a remote location during# maintenance and inspection of reactor coolant pumps, steam generators, and associated piping while the pressure vessel and a refuelling canal are filled with water. The apparatus includes a sealing plug for mechanically sealing the cold leg nozzle from the inside of the reactor pressure vessel. The sealing plug include primary and secondary O-rings. An installation tool is suspended within the reactor vessel and carries the sealing plug. The tool telescopes to insert the sealing plug within the cold leg nozzle, and subsequently to remove the sealing plug after the completion of maintenance operations. Hydraulic means are used to activate the sealing plug, and support means serve to suspend the installation tool within the reactor vessel during installation and removal of the sealing plug. According to one aspect of the present invention there is provided apparatus for sealing a cold leg nozzle of a nuclear reactor pressure vessel during maintenance and inspection of reactor coolant pumps, steam generators, and associated piping while the pressure vessel and a refuelling canal are filled with water, the apparatus being characterised by: a. a sealing plug for mechanically sealing the cold leg nozzle from the inside of the reactor pressure vessel, the sealing plug including a primary O-ring and a secondary O-ring; b. an installation tool suspended or suspendable with the reactor vessel, the installation tool including a telescoping means arranged within a horizontal plane and carrying or capable of carrying the sealing plug for insertion into the cold leg nozzle, and for subsequent removal of the sealing plug after completion of maintenance operations; c. latching means for attaching the sealing plug to the installation tool; d. hydraulic means to activate the sealing plug; and e. support means to suspend the installation tool within the reactor vessel during installation and removal of the sealing plug. According to another aspect of the present invention there is provided a method of sealing a cold leg nozzle of a nuclear reactor pressure during maintenance and inspection of associated steam generators and reactor coolant pumps while the pressure vessel and a refuelling canal are filled with water, the method being characterised by: a. attaching a sealing plug to an end of an essentially horizontally disposed installation tool located above and in alignment with the reactor pressure vessel; b. lowering the installation tool into the reactor pressure vessel until the installation tool and attached plug are aligned with the cold leg nozzle; c. inserting the sealing plug into the cold leg nozzle; and d. mechanically expanding the sealing plug to effect a seal within a cylindrical portion of the cold leg nozzle. A preferred embodiment of the present invention described below provides a sealing plug which is reliable in operation and suitable for nuclear reactor inspection and maintenance procedures. The preferred embodiment utilises the hydrostatic pressure created by water within the reactor pressure vessel to ensure a fail safe plug which will not dislodge in the event of failure of the primary plug sealing means. The preferred method and apparatus, for plugging the cold leg nozzles of a nuclear reactor pressure vessel during maintenance and inspection of same, permits inspection and maintenance of the reactor coolant pumps, steam generators, and associated piping during the same period of time. The invention will now be described, by way of illustrative and nonlimiting example, with reference to the accompanying drawings, in which like reference numerals designate like or corresponding parts throughout, and in which: Figure 1 is a perspective view of a nuclear steam supply system; Figure 2 is a schematic side view of apparatus embodying the invention for installing a sealing plug; Figure 3 is an end view of the sealing plug; Figure 4 is a sectional view of the sealing plug taken along a line IV IV of Figure 3; Figure 5 is a plan view of an installation tool of the apparatus embodying the invention; Figure 6 is a side view of the installation tool and an attached reaction beam; and Figure 7 is an end view of a latch assembly of the apparatus embodying the invention. Figure 1 shows a nuclear steam supply system including a nuclear reactor pressure vessel 2, a once through steam generator 4 and a reactor coolant pump 6. In the system as illustrated, each of two reactor coolant loops contains two reactor coolant pumps, i.e. a total of four pumps, each feeding into the vessel 2 through a separate cold leg nozzle. The reactor coolant pump 6 is connected to the reactor vessel 2 by piping 8. Referring now to Figure 2, a chain hoist 10, scales 12, a sling 14, and an installation tool 16 are attached to a chain hook 18. The chain hoist 10 is situated above the sling 14, and the scales 12 are situated above the chain hoist, as illustrated. A sealing plug 20 is attached to each end of the installation tool 16. This is accomplished on a fuel handling bridge walkway 22. Because the operator is remote from the actual site of insertion of the sealing plug 20, a television camera 24 and an underwater light 26 are utilised to facilitate insertion and removal of the sealing plug. A first drive socket extension 28 is attached to a reaction nut 30 in the centre of the installation tool 16. The installation tool 16, now carrying a sealing plug 20 at each end thereof, is aligned with a diametrically opposite cold leg nozzle 32 of the vessel 2. The tool 16 is then lowered into the water-filled vessel 2 until it is in elevational alignment with the diametrically opposite cold leg nozzles. At this point in the procedure, the sealing plugs 20 will be in proper horizontal and vertical alignment for engagement in the cold leg nozzles 32. The use of the scales 12 serves to establish a predetermined submerged weight for the installation tool 16, both with and without attached sealing plugs 20. Thus, an aberration in apparent weight of the installation tool 16 during the installation and subsequent removal of the sealing plugs 20 would indicate to an operator that one or both plugs either rests on a lower conical section of a cold leg nozzle 32 (abnormally low weight) or is pressed against an upper conical section of a cold leg nozzle (abnormally high weight). This would indicate to the operator that he should take appropriate action to correct the alignment of the installation tool 16. Tumbuckles or other adjusting means well known in the art may be used to adjust the sling 14, if necessary, to maintain the tool 16 in an essentially level condition. Before submergence of ¯ the installation tool 16 with the attached sealing plugs 20, the first drive socket extension 28 is lowered until a socket end rests on an upper flange 35 of the vessel 2. The socket extension 28 is then marked with tape at a suitable reference elevation on the fuel handling bridge walkway 22. A second tape mark is added to the socket extension at a distance of 2.007 m (6ft, 7in) above the first mark, and the first mark is then removed. The distance of 2.007 m (6ft, 7in) is the distance between the upper flange 35 and the centreline of the cold leg nozzle 32 in this preferred embodiment. Therefore, when the installation tool 16 is lowered until the tape mark on the first drive socket extension 28 coincides with the reference elevation on the fuel handling bridge walkway 22, the sealing plugs 20 will be at the correct elevation for engagement into the nozzles 32. The chain hoist 10 may be used to effect any change in tool elevation. A second drive socket extension 33 is attached to a traverse nut 34 (see Figure 5) in the centre of the installation tool 16. Anticlockwise rotation of the traverse nut 34 results in insertion of the sealing plugs 20 into the cold leg nozzles 32. To prevent the installation tool 16 and attached sealing plugs 20 from rotating in an anticlockwise direction in a horizontal plane during rotation of the traverse nut 34, the first drive socket extension 28 is attached to the reaction nut 30 during this procedure. After insertion of the sealing plugs 20 into respective cold leg nozzles 32, a hydraulic jack 36 is inserted into a sealing plug and pressurised by means of a hydraulic pump 38. The hydraulic jack 36 is supported by a pole 39 made of fibreglass or other suitable material, the jack being connected to the hydraulic pump 38 by lengths of hydraulic hosing. The jack 36 is pressurised while holding some torque on the traverse nut 34. After pressurisation of the hydraulic jack 36 is achieved, torque on the traverse nut 34 is released. The hydraulic jack 36 serves to activate the sealing plug 20 and bring it in sealing relationship with the inside surface of the cold leg nozzle 32, as more fully described below. First, the drive socket extnesion 28 is removed from the reaction nut 30 and placed on a nut runner nut 40. The nut runner nut 40 is then turned clockwise to effectively lock the sealing plug 20 into a sealing relationship with the cold leg nozzle 32. The hydraulic jack 36 is retracted, removed, and inserted in the diametrically opposite sealing plug, and the procedure as described above is repeated. After both plugs 20 have been brought in sealing relationship with the inside diameters of respective cold leg nozzles 32, a nylon cord is used to release latches on each plug. The installation tool 16 may then be removed. After completion of maintenance and inspection, the installation tool 16 is again lowered into the vessel 2, and telescoped into latching relationship with the installed plugs 20. The hydraulic jack 36 is again inserted into each plug 20 in turn to allow release of sealing relationship of each plug 20 with the inside surface of its respective cold leg nozzle 32. The nut runner nut 40 is turned anticlockwise as part of this operation. The diametrically opposite plugs 20 are then withdrawn from their respective nozzles 32 by rotating the traverse nut 34 using the second drive socket extension 33. The installation tool 16, with the plugs 20 now reattached to the ends thereof, is now lifted out of the vessel 2. Figure 3 is an end view of the sealing plug 20. A hexagonal (hex) nut 52 is in communication with the nut runner nut 40 while the latter is being turned during -the activation as well as the deactivation of the sealing plug. Referring now to Figure 4, the sealing plug includes a primary O-ring 42, a secondary O-ring 44, an outer plate 46, a spring retainer 48, and a spring 50. Upon pressurising the hydraulic pump 38, the outer plate 46 is pulled in a direction towards the centre of the vessel 2. Directional control is provided by an outer pipe 56, an inner pipe 57 and a guide bar 49. Because a continuing pressure is being applied to the traverse nut 34 in the centre of the installation tool, no significant net movement of a cone assembly 54 can occur in a direction towards the centre of the vessel 2. The primary O-ring 42 therefore expands radially outwardly between bevelled edges of the outer plate 46 and cone assembly 54. The hex nut 52 is then turned, through communication with the nut runner nut 40, to lock the primary O-ring 42 into position. The spring 50 is thus in a compressed state during the period in which the primary O-ring 42 is in sealing relationship to the inside surface of the cold leg nozzle 32. A locater ring 51 and an end ring 53 remain in a stationary position during this procedure. When the sealing plug is to be removed, the hydraulic pump 38 is again inserted into the plug and pressurised. The nut runner nut 40, in communication with the hex nut 52, is turned in an anticlockwise direction. The hydraulic jack 36 is retracted and removed. The spring 50 will force back the outer plate 46, thereby relieving pressure on the primary O-ring 42 and allowing it to return to its normal state. The sealing plug may then be removed from the cold leg nozzle 32 prior to resumption of operation. As shown in both Figures 3 and 4, the secondary O-ring 44 is mounted near the widest edge of the cone assembly 54. The secondary O-ring 44 is designed to act as a sealing element in case of failure of the primary O-ring 42. Hydrostatic pressure would force the cone assembly 54 and the secondary O-ring 44 into tight communication with a bevelled section of the cold leg nozzle 32. As best shown in Figures 5 and 6, the installation tool 16 utilises a continuous chain drive operated by rotation of the traverse nut 34 by means of the second drive socket extension 33. Thus, by turning the traverse nut 34 anticlockwise, the two principal members of the installation tool 16 will simultaneously telescope radially outwardly, as for example during insertion of the sealing plugs 20 into respective cold leg nozzles 32. Conversely, clockwise rotation of the traverse nut 34 results in simultaneous retraction of the telescoping members of the installation tool 16 in a radially inward direction, as for example during removal of the sealing plugs 20 from the respective nozzles 32. The installation tool 16 is also provided with a latch assembly 58, shown in more detail in Figure 7, which includes a latch plate 62 containing slots for the introduction of lugs (not shown), two of which lugs are arranged on a sealing plug 20. A latch 64 engages the sealing plug lug to provide attachment of the sealing plug 20 to each end of the installation tool 16 before insertion of the plugs into respective cold leg nozzles 32. After insertion, a nylon cord tied to the latch 64 is pulled up, releasing the lug from the latch and thereby permitting withdrawal of the installation tool 16 from the positioned plug 20. For removal of the plugs 20, the latch 64 is reconnected to the lug, if necessary with the aid of a drive socket extension. Figure 6 is a side view of the installation tool, including a reaction beam 60. During removal of the two oppositely disposed sealing plugs 20, if one of the plugs releases from the nozzle 32 while the oppositely disposed plug still maintains some resistance to the installation tool 16, it is necessary to have a means to react against the force exerted by the tool. The reaction beam 60 is slightly less in diametric length than the inside diameter of the vessel 2. In the event of a situation as described above, the reaction beam 60 will come in contact with the inner surface of the side wall of the vessel 2, and in arcuate alignment with the respective nozzle, so as to provide the reactive force. The entire procedure described above relates to the positioning of two sealing plugs 20, one at each end of an installation tool, the plugs being inserted into diametrically opposite cold leg nozzles 32 and subsequently removed. In a typical vessel 2 such as illustrated in Figure 1, it is necessary to seal all four cold leg nozzles before maintenance and inspection may be started. Therefore, the procedures described above are repeated with respect to a third and fourth sealing plug and corresponding diametrically opposite cold leg nozzles. Ancillary equipment may be used in conjunction with the illustrative form of practice of the invention described above. For example, a suitable lubricant may be used on the surfaces of the installation tool 16 that interface with the sealing plug 20 to facilitate the insertion and removal of the plug into the cold leg nozzle 32.

Claims: CLAIMS 1. Apparatus for sealing a cold leg nozzle (32) of a nuclear reactor pressure vessel (2) during maintenance and inspection of reactor coolant pumps, steam generators, and associated piping while the pressure vessel and a refuelling canal are filled with water, the apparatus being characterised by: a. a sealing plug (20) for mechanically sealing the cold leg nozzle (32) from the inside of the reactor pressure vessel (20), the sealing plug including a primary O-ring (42) and a secondary O-ring (44); b. an installation tool (16) suspended or suspendable with the reactor vessel (2), the installation tool including a telescoping means arranged within a horizontal plane and carrying or capable of carrying the sealing plug (20) for insertion into the cold leg nozzle (32), and for subsequent removal of the sealing plug after completion of maintenance operations, c. latching means (58) for attaching the sealing plug to the installation tool (16); d. hydraulic means to activate the sealing plug (20); and support means to suspend the installation tool (16) within the reactor vessel (20) during installation and removal of the sealing plug. 2. Apparatus according to claim 1, wherein the hydraulic means comprises a hydraulic jack (36) for expanding the primary O-ring#(42), and a hydraulic pump (38) for pressurising the hydraulic jack. 3. A method of sealing a cold leg nozzle (32) of a nuclear reactor pressure (2) during maintenance and inspection of associated steam generators and reactor coolant pumps while the pressure vessel and a refuelling canal are filled with water, the method being characterised by: a. attaching a sealing plug (20) to an end of an essentially horizontally disposed installation tool (16) located above and in alignment with the reactor pressure vessel (2); b. lowering the installation tool (16) into the reactor pressure vessel (2) until the installation tool (16) and attached plug (20) are aligned with the cold leg nozzle (32); c. inserting the sealing plug (20) into the cold leg nozzle (32); and d. mechanically expanding the sealing plug (20) to effect a seal within a cylindrical portion of the cold leg nozzle (32).

Inventors: DOOLEY, RAY A.
Applicants: THE BABCOCK & WILCOX COMPANY
Application year: 1983
Main classification: G21C13
IPC codes: F16L55, G21C13, G21C19