Patent Number: 
Section: description

Referring now to FIG. 1 there is generally shown a flange 10 of a removable RPV head 12 supported on a flange 14 of a RPV 16 at a flange interface 18. A commercial RPV may have up to fifty four studs (represented by stud 20) extending upwardly through stud holes in the RPV head flange 10. A stud 20 may have a diameter of up to about seven inches or more and may weigh up to 775 pounds. During on-line power generation operations, large heavy nuts and washers (depicted by nut 22 and washer 24 shown in chain) are engaged with the studs 20 for maintaining a leak tight seal between the flanges 10, 14 at O-rings 26 while the reactor coolant system operates at pressures of up to 2250 psi or more and at temperatures of up to about 600xc2x0 F. or more. During subsequent off-line operations, the RPV nuts 22 and washers 24 and the RPV head 10 must be removed to permit access to the internal portions of the RPV 16 (after the internal pressure and temperature in the RPV 16 are reduced to approximately atmospheric pressure and to below about 150xc2x0 F.). FIG. 1 generally depicts the RPV head 12 after the RPV nuts 22 and washers 24 have been removed from the RPV studs 20. As shown, each of the studs 20 is protected by a stud enclosure 30 of the present invention. The stud enclosure 30 includes a cylindrical can 32 having a longitudinal axis 34 and a cross sectional radius 36 extending at a right angle to the axis 34. The cylindrical can 32 has a capped end 38, an open end 40, an inner surface 42 and an outer peripheral surface 44. The cylindrical can 32 as shown in FIG. 1 preferably includes a fiberglass portion 46 with a stainless steel end piece 48 fit thereto. Alternatively, the cylindrical can 32 may be entirely fabricated of a suitably strong material. The capped end 38 of the cylindrical can 30 shown in FIG. 1 has a structurally re-enforced collar 52 with an axially extending hole 54 for receiving a fastener 56 such as a cap screw. An elastomeric O-ring (not shown) or other sealing device may be employed under the head of the fastener 56 to provide an airtight seal. The fastener 56 is designed to threadedly engage the upper end of the stud 20 for fastening the stud enclosure 30 over the stud 20. A seal ring 60 such as an EPDM (ethylene proplyene dimer monomer) ring is provided adjacent or near the open end 40 of the cylindrical can 32. The seal ring 60 may be adhesively bonded to cylindrical can 32. The seal ring 60 is designed to seat in a countersunk stud hole on the RPV flange 14 and form a substantially water tight seal. Advantageously, the seal ring 60 shown in FIG. 1 may be compressed between the can 32 and the flange 14 as the fastener 56 is screwed into the stud 20. In addition to a square cross section as is shown in FIG. 1, the seal ring 60 may have an xe2x80x9cOxe2x80x9d or any other suitable cross-section for sealingly engaging the RPV flange 14. Also, the seal ring 60 may be carried on the end of the can 32 as shown or in other seal designs on the peripheral cuter surface 44 of the can 32 (and adhesively attached thereto) so long as the seal ring 60 will not readily separate from the can 32 while being transported or while in use. In addition, the stud enclosure 30 may have a second seal ring 62 near the open end 40 and disposed on its peripheral outer surface 44. Advantageously, the second seal ring 62 can also support the sides of the stud enclosure 30 against the sides of a countersunk stud hole. As is shown in FIG. 1, the second seal ring 62 may be an O-ring. In other embodiments of the invention, only one seal ring 60 may be enployed or an inflatable seal ring 60 and/or an inflatable second seal ring may be employed. A gas valve 68 is disposed in the capped end 38 of the stud enclosure 30 for pressurizing the interior portion of the stud enclosure 30. Preferably the valve 68 is an air valve such as an automobile tire valve that permits a gas (such as plant air) to be introduced into and bled from the interior portion of the stud enclosure 30. FIG. 1 depicts an air valve 68 having a longitudinal axis 72 that intersects the fastener 56. Also, the valve 68 shown in FIG. 1 has a distal end 74 that is within the radius 36 of the cylindrical can 32. Advantageously, this design tends to protect the valve 68 from physical contact with objects adjacent the stud enclosure 30. As is shown in FIG. 1 for purposes of illustration, the distal end of the valve may extend outwardly of the surface of the stud enclosure 30. Most preferably, the distal end of the valve does not extend outwardly of the surface of the stud enclosure 30. Advantageously, pressurizing the interior portion of the stud enclosure 30 tends to prevent boron-containing water in the refueling pool in which the RPV 16 would be submerged from contacting the threads of the RPV stud 20. In addition, pressurizing the stud enclosure 30 tends to reinforce the stud enclosure 30 against the weight of the approximately 20 to 25 feet of water above it. Stud covers 30 are particularly useful for protecting the RPV studs 20 during a refueling outage. After the RPV 16 has been taken off-line, cooled down below about 150xc2x0 F. and to atmospheric pressure, the RPV nuts 22 and washers 24 may be removed from threaded engagement with the studs 20 using known detensioners. While the RPV head 12 remains in place, the stud covers 30 may be slid over the studs 20 and downwardly to the point where a gasket or other seal ring 60 physically contacts the RPV flange 14. The stud covers 30 may be about seven inches in diameter by about four feet high by about fifty thousandths of an inch thick in order to fit over the studs 20 and within the countersunk stud holes of a RPV flange 14. The stud enclosure 30 depicted in FIG. 1 may weigh about ten pounds and may be readily handled by a technician. The stud enclosures 30 may be fastened in place by tightening cap screws or other fasteners 56 into the studs 20. Advantageously, the seal rings 60 may be compressed by tightening the fasteners 56 to the studs 20. After the stud enclosures 30 have been fastened to the studs 20, the interior portions of the stud enclosures 30 may be pressurized to about 10 psi or more to reinforce the capped ends 28 of the cylindrical cans 22 against the weight of approximately twenty feet of water above them and to later protect the threads of the studs 16 from the water in the refueling pool. After the stud enclosures 30 have been installed, the reactor cavity and refueling canal above the RPV may be flooded with water in accordance with the industry""s practice to shield workers and the plant from radiation. The RPV head 12 may be raised from the RPV 16 and placed on a remote stand (not shown) in the refueling canal. With the interior portions of the RPV 16 exposed, the fuel (not shown) may be removed and/or the RPV 16 inspected. At the end of the outage, the RPV head 12 may be repositioned on the RPV 16 and the pool water pumped into a refueling tank (not shown). The pressure in the stud enclosures 30 may then be relieved thorough valves 68 and the cap screws or other fasteners 56 unscrewed so that the stud closures 30 can be removed. The RPV nuts 22 and washers 24 may then be replaced and the reactor returned to on-line operations. While a present preferred embodiment of the present invention has been shown and described, it is to be understood that the invention may be otherwise variously embodied within the scope of the following claims of invention.