Patent Number: 051715147
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) A nuclear power plant includes a nuclear reactor for producing heat and a steam generator in heat transfer communication with the nuclear reactor for generating steam. The steam generator includes inlet and outlet primary nozzles attached to the steam generator. At times it is necessary to perform maintenance in the steam generator. To safely and satisfactorily perform this maintenance, it is prudent first to sealingly block the inlet and outlet primary nozzles of the steam generator. Disclosed herein is a nozzle dam having a unitary plug for sealingly blocking the inlet and outlet primary nozzles of the steam generator so that maintenance activities can be safely and satisfactorily performed in the steam generator. Before describing the subject matter of the present invention, it is instructive first to generally describe the structure and operation of a typical nuclear reactor and associated steam generator. Therefore, referring to FIG. 1, there is illustrated a nuclear reactor vessel, generally referred to as 10, having a lower portion 20 and a closure head 30 mounted atop lower portion 20. Lower portion 20 of reactor vessel 10 houses a nuclear reactor core, generally referred to as 40, for producing heat. Reactor vessel 10 is disposed in a reactor cavity 50 defined by a reactor vessel enclosure 60. Reactor cavity 50 is partitioned into an upper cavity portion 52 sealingly isolated from a lower cavity portion 54. Moreover, reactor core 40 is surrounded by a radioactive primary fluid (e.g., demineralized water) for removing the heat produced by reactor core 40 and for moderating neutrons produced by reactor core 40 so that the fissioning process of nuclear fuel contained in reactor core 40 is enhanced. In addition, the closure head 30 is surrounded by a refueling pool 70 (e.g., demineralized water) substantially filling upper cavity portion 52 for providing a biological shield during refueling operations so that nuclear radiation from reactor core 40 is attenuated during refueling of reactor core 40. Of course, before refueling operations commence, closure head 30 is removed from lower portion 20 in a manner well known in the art to provide access to reactor core 40. Still referring to FIG. 1, a steam generator, generally referred to as 80 is disposed near reactor vessel 10 for producing steam, as described more fully hereinbelow. Interconnecting reactor vessel 10 and steam generator 80 is a conduit or pipe 90 that is in fluid communication with the primary fluid surrounding reactor core 40 and through which conduit 90 the primary fluid flows. In fluid communication with conduit 90 is a reactor coolant pump 100 for pumping the primary fluid through conduit 90, through reactor core 40 and through the primary side of steam generator 80. Referring again to FIG. 1, as coolant pump 100 operates, the primary fluid circulates through reactor vessel 10 and then flows from reactor vessel 10 through a portion of conduit 90 and into steam generator 80, wherein the heat of the primary fluid is removed for producing steam in the manner described hereinbelow. The primary fluid then exits steam generator 80 and flows through pump 100 and through the remaining portion of conduit 90 to return to reactor core 40 which is housed in reactor vessel 10. In this manner, the primary fluid continuously circulates through the closed loop defined by reactor vessel 10, steam generator 80 and conduit 90. The path of primary coolant flow is generally illustrated by the arrows shown in FIG. 1. Turning now to FIG. 2, there is shown steam generator 80 with parts removed for clarity. Steam generator 80 comprises a vertically-oriented shell 110 defining a cavity 120 therein. Shell 110 has a dome-shaped upper shell portion 130, a frusto-conical transition portion 140 integrally attached to upper shell portion 130, a cylindrical hull portion 150 integrally attached to transition portion 140, and a bowl-shaped lower shell portion or channel head 160 integrally attached to hull portion 150. Formed through lower shell portion 160 are a plurality of relatively small diameter manway openings 170 (only two of which are shown) for reasons provided hereinbelow. Of course, manway openings 170 are capable of being sealingly covered by suitable manway covers (not shown) during operation of steam generator 80. Still referring to FIG. 2, disposed in cavity 120 are a plurality of inverted vertically-oriented U-shaped steam generator tubes 180 for conducting the radioactive primary fluid therethrough, the plurality of tubes 180 defining a tube bundle 190. Moreover, as shown in FIG. 2, each U-shaped tube 180 has a pair of vertical tube leg portions 200 interconnected by a U-bend tube portion 210 integrally formed therewith. In addition, each tube leg portion 200 has an open tube end 220 for passage of the primary fluid therethrough. Disposed in cavity 120 near lower shell portion 160 is a horizontal tube sheet 230 having a plurality of apertures 240 therethrough for receiving and for vertically supporting each tube end 220, which tube ends 220 are suitably attached to tube sheet 230, such as by welding. Referring again to FIG. 2, disposed in lower shell portion 160 is a vertical divider plate 250 for dividing lower shell portion 160 into an inlet plenum chamber 260 and an outlet plenum chamber 270. It will be appreciated that manway openings 170 allow access to inlet plenum chamber 260 and outlet plenum chamber 270, so that maintenance can be performed in steam generator 80. Such maintenance may be to plug and/or sleeve any degraded tubes 180 or to decontaminate steam generator 80. Integrally attached to lower shell portion 160 is an inlet primary nozzle 280 and an outlet primary nozzle 290 in fluid communication with inlet plenum chamber 260 and outlet plenum chamber 270, respectively. Each of the nozzles 280 and 290 has a circumferential inside surface 300 which may define an annular depending shoulder 295 therearound. Disposed in cavity 120 above tube sheet 230 and interposed between shell 110 and tube bundle 190 is a cylindrical wrapper sheet 310 defining an annular downcomer region 320 between shell 210 and wrapper sheet 310. Wrapper sheet 310 is open at its bottom end and partially closed at its top end. That is, formed through the top end of wrapper sheet 310 are a plurality of holes (not shown) in its top end for passage of a steam-water mixture therethrough. Mounted atop wrapper sheet 310 is a moisture separator assembly, generally referred to as 330, for separating the steam-water mixture into liquid water and relatively dry saturated steam. Moisture separator assembly 330 also has holes (not shown) in the bottom portion thereof for receipt of the steam-water mixture from the interior of wrapper sheet 310 and holes (not shown) in the top portion thereof for passage of the dry saturated steam flowing upwardly through moisture separator assembly 330. In addition, integrally attached to the top of upper shell portion 130 is a main steam line nozzle 340 for passage of the dry saturated steam therethrough after the dry saturated steam is separated from the steam-water mixture and passes upwardly from moisture separator assembly 330. As shown in FIG. 2, integrally attached to upper shell portion 130 is a feedwater nozzle 350 for passage of nonradioactive feedwater (i.e., secondary fluid) into a toroidal feedring 360 which is in fluid communication with feedwater nozzle 350. Feedring 360 surrounds wrapper sheet 310 at the upper portion of wrapper sheet 310 and has a plurality of nozzles 370 attached thereto for passage of the feedwater from feedring 370 downwardly into downcomer region 320. Disposed inwardly of wrapper sheet 310 are a plurality of horizontal spaced-apart tube support plates 380 (only four of which are shown) having holes 390 therethrough for receiving each tube 180 so that each tube 180 is laterally supported thereby. Each support plate 380 also has a plurality of additional holes for passage of the nonradioactive secondary fluid upwardly therethrough. It will be appreciated that primary nozzles 280/290, inlet plenum chamber 260, the interior of tubes 180 and outlet plenum chamber 270 together define the primary side of steam generator 80. It will be further appreciated that the portion of cavity 120 above tube sheet 230 and externally of tubes 80 defines the secondary side of steam generator 80. During operation of steam generator 80, the primary fluid, which is heated by reactor core 40, flows through the primary side of steam generator 80. In this regard, the primary fluid flows from reactor core 40 through conduit 90, through inlet nozzle 280 (which is connected to conduit 90) and into inlet plenum chamber 260. The primary fluid then travels through the open tube ends 220 that are in fluid communication with inlet plenum chamber 260, through tubes 180, out the other open tube end 220 and into outlet plenum chamber 270, whereupon the primary fluid exits steam generator 80 through outlet nozzle 290 and is returned by conduit 90 to reactor core 40. As the primary fluid flows through the primary side of steam generator 80, secondary fluid flows through the secondary side of steam generator 80. That is, as the primary fluid flows through tubes 180, feedwater (i.e., the secondary fluid) simultaneously enters steam generator 80 through feedwater nozzle 350, flows through nozzles 370 and then flows downwardly through downcomer region 320 until the feedwater impinges tube sheet 230. The feedwater then turns upwardly to surround tube bundle 190. As the primary fluid flows through tubes 180 it gives up its heat to the secondary feedwater fluid surrounding tube bundle 190. A portion of the secondary fluid surrounding tube bundle 190 is converted into a steam-water mixture that flows upwardly to moisture separator assembly 330 which separates the steam-water mixture into liquid water and relatively dry saturated steam. The liquid water returns downwardly to bundle 190 (i.e., the plurality of tubes 180) as the dry saturated steam continues upwardly to exit steam generator 80. The dry saturated steam exits steam generator 80 through main steam line nozzle 340 and is transported to a turbine-generator (not shown) for producing electricity in a manner well known in the art of nuclear-powered electricity production. Such a steam generator is disclosed in U.S. Pat. No. 4,079,701 entitled "Steam Generator Sludge Removal System" issued Mar. 21, 1978 to Robert A. Hickman et al., the disclosure of which is hereby incorporated by reference. Referring to FIGS. 2, 3, 4, and 5, a nozzle dam, generally referred to as 400, is disposed in primary inlet nozzle 280 for sealingly blocking primary inlet nozzle 280 so that maintenance can be safely and suitably performed in inlet plenum chamber 260. Of course, it will be appreciated that nozzle dam 400 may also be disposed in primary outlet nozzle 290 for blocking outlet nozzle 290 so that maintenance can be safely and suitably performed in outlet plenum chamber 270. However, for the sake of conciseness, the description of the invention herein will be made primarily with reference to blocking primary inlet nozzle 280. Moreover, it will be understood that nozzle dam 400 may also be disposed in conduit 90 for sealingly blocking conduit 90, if desired. For reasons provided immediately hereinbelow, it is important that nozzle dam 400 suitable block inlet nozzle 260 during steam generator maintenance activities. Because the elevation of refueling pool 70 is typically above the elevation of inlet nozzle 280 during refueling, the hydraulic head experienced by the primary fluid in conduit 90 would force the primary fluid through inlet nozzle 280 and into inlet plenum chamber 260, if primary nozzle 280 were not blocked. Primary fluid in inlet plenum chamber 260 will interfere with maintenance activities being performed in inlet plenum chamber 260. Therefore, according to the present invention, nozzle dam 400 prevents the primary fluid from entering inlet plenum chamber 260 while steam generator 80 is being serviced. Referring to FIGS. 6, 7, 8, and 9, nozzle dam 400 is there shown disposed in inlet nozzle 280 for sealingly blocking inlet nozzle 280. In this regard, nozzle dam 400 comprises a generally cylindrical bracket 410 mounted on shoulder 295 defined by inside surface 300 of inlet nozzle 280. Bracket 410 has a circumferential outside surface 420 for matingly engaging the circumferential inside surface 300 of inlet nozzle 280. Bracket 410 may be permanently sealingly attached to inside surface 300, such as by one or more weldments 430 extending around bracket 410. Bracket 410 also has a plurality of transverse slots 425 therein in communication with each opening 440 for reasons disclosed hereinbelow. Bracket 410 also has a plurality of generally triangular or generally arcuate-shaped openings 440 therethrough sized to matingly receive means disposed in each opening 440 for sealingly plugging or closing each opening 440, the plugging means operable to engage slots 425 of bracket 410 for connecting the plugging means to bracket 410. In the preferred embodiment of the invention, the plugging means is a generally arcuate-shaped unitary plug, generally referred to as 450, matingly disposed in each opening 440 for sealably plugging each arcuate-shaped opening 440, plug 450 being operable to engage slots 425 of bracket 410 in the manner disclosed hereinbelow so that each plug 450 is connected to bracket 410. As best seen in FIGS. 7, 8, and 9, each plug 450 comprises a generally triangular or generally arcuate-shaped first plate 460 matingly disposed in each opening 440. First plate 460 may define a cavity 465 therein and a drain hole 467 therethrough. Drain hole 467 allows fluid to enter and leave cavity 465. First plate 460 has a generally cylindrical threaded first bore 470 longitudinally therethrough for reasons disclosed more fully hereinbelow. First plate 460 may also have a plurality of longitudinal generally triangular or generally arcuate-shaped holes 475 therethrough for providing strength to first plate 460 while simultaneously decreasing the weight of first plate 460. Moreover, first plate 460 has a plurality of substantially smooth channels 480 formed transversely therethrough for reasons disclosed hereinbelow. Coaxially disposed adjacent first plate 460 and mounted atop bracket 410 (i.e., on the top surface of bracket 410) is a generally triangular or generally arcuate-shaped second plate 490 for sealingly matingly covering each opening 440. Second plate 490 has a generally cylindrical threaded second bore 500 longitudinally therethrough for reasons disclosed hereinbelow. First plate 460 and second plate 490 together may define a space 510 therebetween if plate 460 is disposed such that it is spaced apart from second plate 490. Moreover, as seen in FIGS. 7, 8 and 9, threadably extending through threaded first bore 470 and threaded second bore 500 is an elongated rotatable threaded shaft 520 having a distal end and a proximal end. For purposes of the present description, the term "distal end" is defined to be that end of shaft 520 nearer tubesheet 230 and the term "proximal end" is defined to be that end of shaft 520 which is farther away from tubesheet 230. Shaft 520 also has a handle 530 attached, such as by a set screw 535, to its distal end for turning shaft 520. Threaded shaft 520 has a first threaded portion 540 near the proximal end of shaft 520 for threadably engaging first bore 470 and a second threaded portion 550 near the distal end of shaft 520 for threadably engaging second bore 500. The threads of first threaded portion 540 and the threads of second threaded portion 550 are oriented in the same direction but have a different pitch for reasons provided hereinbelow. That is, if the threads of first threaded portion 540 are "right-hand" threads of a predetermined pitch, then the threads of second threaded portion 550 are also "right-hand" threads but of a different predetermined pitch. Conversely, if the threads of first threaded portion 540 are "left-hand" threads of a predetermined pitch, then the threads of second threaded portion 550 are also "left-hand" threads but of a different predetermined pitch. It will be appreciated that as shaft 520 threadably engages first bore 470 and second bore 500, first plate 460 and second plate 490 are threadably connected thereby. Shaft 540 also has a third threaded portion 560 intermediate first threaded portion 540 and second threaded portion 550 for reasons disclosed hereinbelow. The threads of third threaded portion 560 are oppositely disposed to the direction of the threads of first threaded portion 540 and second threaded portion 550 for reasons disclosed more fully hereinbelow. That is, when the threads of first threaded portion 540 and second threaded portion 550 are "right-hand" threads, the threads of third threaded portion 560 are "left-hand" threads. Conversely, when the threads of first threaded portion 540 and second threaded portion 550 are "left-hand" threads, the threads of third threaded portion 560 are "right-hand" threads. Still referring to FIGS. 7, 8, and 9, an actuator, such as a generally wing-shaped member or wing nut 570, is disposed on shaft 520 and threadably engages third threaded portion 560 of shaft 520. Wing nut 570 is thus interposed between first plate 460 and second plate 490 and coaxially aligned therewith. Wing nut 570 has a threaded bore 580 centrally longitudinally therethrough for threadably engaging third threaded portion 560 of shaft 520. Wing nut 570 also has a plurality of generally wing-shaped extensions or wings 590 integrally attached thereto and extending outwardly upwardly therefrom, each wing 590 having a generally arcuate transverse hole 600 of predetermined contour therethrough defining a cam surface 610. In the preferred embodiment, each wing nut 570 has four mutually perpendicular wings 590, each wing 590 having hole 600 therethrough defining the cam surface 610. Each wing 590 also defines a pair of integral parallel wing tines 590a and 590b having hole 660 therethrough for reasons disclosed more fully hereinbelow. Referring yet again to FIGS. 7, 8 and 9, a generally cylindrical elongated arm 620 is slidably disposed in each substantially smooth channel 480. Thus, each arm 620 is oriented substantially perpendicularly with respect to its associated wing 590. Each arm 620 has a first end portion 630 adjacent wing 590 and a second end portion 640 adjacent bracket 410. First end portion 630 is sized to be interposed between wing tines 590a and 590b, so that first end portion 630 may be securely attached to wing 590. Integrally attached to first end portion 630 of arm 620 and transversely extending therethrough is a rounded cylindrical cam 650 for slidably engaging cam surface 610 of wing tines 590a and 590b. Thus, cam 650 laterally extends through hole 600 of each wing tine 590a and 590b. Integrally attached to second end portion 640 of arm 620 is an extended portion 660, which may be finger-shaped, for matingly engaging slot 425 of bracket 410, so that plug 450 is capable of being connected to bracket 410 as extended portion 660 engages slot 425. Moreover, matingly surrounding each arm 620 and interposed between first plate 460 and arm 620 may be a generally cylindrical wear-resistant sleeve or bushing 670 for providing a low-friction sliding surface, in order that arm 620 easily slides in bushing 670 with minimum friction. Integrally attached to second plate 490 and extending therearound is a fluid impermeable seal 680. Seal 680 is interposed between second plate 490 and bracket 410 for forming a seal therebetween so that opening 440 of bracket 410 is suitably sealed thereby. Seal 680 may be an elastomeric rubber of a type resistant to degradation caused by nuclear radiation, so that seal 680 will not lose its sealing ability when repeatedly disposed in the radiation environment of steam generator 80. For this purpose, seal 680 may be ethylene propylene diene monomer (EPDM). Thus, it will be appreciated that plates 460/490, shaft 520, actuator 570, arms 620, bushing 670 and seal 680 together substantially define the unitary plug 450. As illustrated in FIGS. 7, 8, and 10, attached to the exterior surface of second plate 490 may be a vent valve 690 in gas communication with opening 440 for venting entrapped radioactive gas (e.g., air) from opening 440 as the hydraulic head in upper cavity portion 52 forces the gas out of opening 440 when upper cavity portion 52 refills with water. It will be understood that when vent valve 690 is closed, opening 440 is sealed and when vent valve 690 is open, opening 440 is not sealed. It will be appreciated that vent valve 690 may be temporarily connected to a gas vacuum pump 700 for evacuating gas from opening 440. In addition, there may be a circular seal 710 disposed in second plate 490 and surrounding shaft 520 near the distal end thereof for providing increased sealing of opening 440. As best seen in FIG. 10, plug 450 is there shown connected to bracket 410. In this regard, arms 620 are caused to slide in channel 480, in the operative manner disclosed hereinbelow, so that extended portion 660 slidably engages slot 425 to connect plug 450 to bracket 410. FIG. 10A shows a plurality of elongated, generally cylindrical anti-rotation locating pins 715 engaging first plate 460 and second plate 490. Each locking pin 715 may be attached to first plate 460 and extend outwardly therefrom to matingly engage an associated bore formed in second plate 490. Locking pins 715 serve an important function. In this regard, locking pins 715 maintain the correct alignment of first plate 460 and second plate 490 so that first plate 460 will not rotate relative to second plate 490 (or vise versa) as shaft 520 is rotated. In this manner, extended portion 660 will remain aligned with slot 425 for easily slidably engaging extended portion 660 into slot 425. Referring now to FIGS. 11, 12 and 13, there is shown an alternative embodiment of nozzle dam 400. The alternative embodiment of nozzle dam 400 is substantially similar to the preferred embodiment of nozzle dam 400 except that wing nut 570 is replaced by a link assembly or link mechanism, generally referred to as 720, interconnecting shaft 520 and arms 620. Link mechanism 720 comprises a plurality of generally cylindrical links, such as a first link 730a and a second link 730b, interconnecting shaft 520 and arm 620 for sliding arm 620 in channel 480. In this regard, first link 730a is attached at one end thereof to shaft 520 intermediate the distal end and the proximal end of shaft 520. The other end of first link 730a is pivotally connected by a first link pin 733 to one end of second link 730b. The other end of second link 730b is pivotally connected by a second link pin 736 to the first end portion 630 of arm 620. First link 730a and second link 730b define an angle of approximately 110 .degree. or greater therebetween for suitably moving arm 629 in channel 480 in the manner described immediately hereinbelow. As shaft 520 is rotated, each first link 730a will pivot about each first link pin 733 for pivoting second link 730b about second link pin 736 so that arm 629 slides in its associated channel 48 for engaging extended portion 660 into slot 425. FIGS. 14 and 15 show a second alternative embodiment of the invention. In this second alternative embodiment, a generally wedge-shaped seal 740 is interposed between first plate 460 and second plate 490 for providing sealing of inlet primary nozzle 280. As first plate 460 and second plate 490 are brought closer together, due to the rotation of shaft 520, wedge-shaped seal 740 will compress and sealingly engage bracket 410, as best seen in FIG. 15. Moreover, in this second alternative embodiment of the invention, third threaded portion 560 of shaft 520 is modified such that the threads of third threaded portion 560 on shaft 520 are no longer present. In this regard, an annular collar 750 integrally attached to shaft 520 is matingly received in a groove 760 formed in wing nut 570. Thus, wing nut 570 will translate downwardly (or upwardly) as shaft 520 threadably translates downwardly (or upwardly) through threaded bores 470 and 500 due to the engagement of collar 750 in groove 760. OPERATION Maintenance of steam generator 80 is preferably performed during refueling of reactor core 40. In this regard, reactor core 40 is shut down prior to refueling and closure head 30 is removed from atop lower portion 20 of reactor vessel 10 for providing access to reactor core 40. The elevation of refueling pool 70 is lowered to a level lower than inlet primary nozzle 280 and outlet primary nozzle 290. However, for safety reasons well known in the art, the elevation of refueling pool 70 is not lowered to a level that uncovers reactor core 40. After the elevation of refueling pool 70 is lowered to a level lower than nozzles 280 and 290, plugs 450 are installed in openings 440 of bracket 410 in the manner disclosed hereinbelow, which bracket 410 will have preferably been previously permanently disposed in inlet primary nozzle 280 and/or outlet nozzle 290. In this regard, bracket 410 will have been previously permanently attached to inside surface 300 by weldment 430, such as during manufacture of steam generator 80, so that bracket 410 abuts shoulder 295 of inlet primary nozzle 280, as best seen in FIGS. 8 and 10. Of course, it will be appreciated that shoulder 295 need not be present in inlet primary nozzle 280 in order to suitably dispose bracket 410 in inlet primary nozzle 280 because bracket 410 will be in any event welded to inlet primary nozzle 280. A robotic device (not shown) is suitably connected to one of the plugs 450. The robotic device maneuvers plug 450 through manway opening 170 and into one of the openings 440 of bracket 410 for plugging opening 440. The robotic device can maneuver plug 450 through manway opening 170 because the widest outside dimension of plug 450 is less than the diameter of manway opening 170. By way of example only, for a typical nuclear steam generator the widest outside dimension of plug 450 may be approximately 14.0 inches and the diameter of manway opening 170 may be approximately 16.0 inches. Moreover, the diameter of the inlet or outlet primary nozzle may be approximately 26.0 inches to 32.0 inches. The robotic device inserts plug 450 into opening 440 until second plate 490 is mounted atop bracket 410 at which time extended portion 660 will be disposed adjacent slot 425. Disposing extended portion 660 adjacent slot 425 in this manner will result in extended portion 660 being aligned with slot 425. The robotic device then will be operated to suitably rotate handle 530 so that shaft 520 rotates. Shaft 520 is rotated such that first threaded portion 540 of shaft 520 threadably engages threaded first bore 470 of first plate 460 and such that second threaded portion 550 of shaft 520 threadably engages threaded second bore 500 of second plate 490. Rotation of shaft 520 threadably advances shaft 520 in first bore 470 and second bore 500 for bringing first plate 460 and second plate 490 closer together because the threads of first bore 470 and second bore 500 have the same orientation (i.e., either "right hand" or "left hand" threads) and a different pitch. As shaft 520 thusly advances through first bore 470 and second bore 500, wing nut 570 will advance along third threaded portion 560 of shaft 520 because the threads of third threaded portion 560 and the threads of bore 580 (of wing nut 570) are oriented in a direction opposite the direction of the threads of first and second threaded portions 540 and 550. As wing nut 570 threadably translates or advances along third threaded portion 560, cam surface 610 will slidably engage cam 650 such that cam 650 will slide along and follow the contour of hole 600 which defines cam surface 610. As cam surface 610 slides along cam 650, arm 620 will slide in bushing 670 (disposed in channel 480) because cam 650 is attached to first end portion 630 of arm 620. As arm 620 slides in bushing 670, extended portion 660 of arm 620 will slidably engage slot 425 for connecting plug 450 to bracket 410 because extended portion 660 is aligned with slot 425. As shaft 520 is further rotated by rotating handle 530, second plate 490 will further bear against the exterior surface of bracket 410 for further compressing seal 680 into frictional engagement with bracket 410 so that opening 440 is suitably sealed thereby. Radioactive gas (e.g., radioactive air) may be entrapped in opening 440 because a gas bubble forms anteriorly or proximally of nozzle dam 400 as upper cavity portion 52 and conduit 90 fills with water during the refueling operation. Therefore, after opening 440 is sealed, vent valve 690 is opened and vacuum pump 700, which is connected to vent valve 690, is operated to evacuate the radioactive gas from opening 440. It is desirable to evacuate opening 440 when any radioactive gas is present therein because evacuation of opening 440 reduces the risk that radioactive gas will be forced into inlet plenum chamber 260 during subsequent manned service operations of steam generator 80. In this regard, the water which displaces the gas in opening 440 also provides radiation shielding for service personnel who may be present in lower shell portion 160 during subsequent steam generator maintenance activities. Moreover, opening 440 will substantially refill with radioactive primary fluid during the refueling operation because refueling pool 70 will be refilled at that time and the elevation of refueling pool 70 will thus rise as it is refilled. That is, as the elevation of refueling pool 70 rises, the level of primary fluid in opening 440 will also rise because the hydraulic head in conduit 90 will increase, thereby increasing the risk that any gas trapped in opening 440 will seep past seal 680 and be forced into inlet plenum chamber 260. Hence, evacuating opening 440 of any gas present therein decreases the risk that such gas will seep past seal 680 and rise into inlet plenum chamber 260. Thus, vent valve 690 is opened to vent opening 400 of any radioactive gas that may be present in opening 400. After opening 400 is suitably evacuated of gas, vent valve 690 is sealingly closed. When vent valve 690 is closed, opening 440 is sealed so that maintenance can be performed in steam generator 80 during refueling of reactor core 40. After reactor core 40 is refueled and after steam generator 80 is serviced, the elevation of refueling pool 70 is again lowered and each plug 450 is remotely removed from each opening 440 in substantially the reverse of its installation into each opening 440. However, before plugs 450 are removed from each associated opening 440, it may be necessary to again open vent valve 690 to permit air from inlet plenum 260 to vent into opening 440 for permitting easier removal of plugs 450. That is, the water in conduit 90 and opening 440 may cause a negative pressure gradient transversely across plugs 450 thereby inhibiting removal of plugs 450. After air is admitted into opening 440, plugs 450 are then removed from steam generator 80 through manway 170. In the preferred embodiment of the invention, bracket 410 which may have been previously permanently attached to inside surface 300 of inlet primary nozzle 280, such as during manufacture of steam generator 80, will remain in steam generator 80 after plugs 450 are removed from bracket 410 and steam generator 80. When steam generator 80 is returned to service, openings 440 of bracket 410 will allow the primary fluid to flow therethrough without substantially altering the flow path of the primary fluid flowing through inlet primary nozzle 280. It will be appreciated that due to the unitary construction of each plug 450, it is not necessary to fold or to disassemble plug 450 in order to pass plug 450 through the relatively small diameter manway opening 170. Moreover, due to the unitary construction of plug 450, it is not necessary to dispose maintenance personnel in steam generator 80 to unfold or reassemble plug 450 in order to dispose plug 450 in inlet primary nozzle 280. Although the invention is fully illustrated and described herein, it is not intended that the invention as illustrated and described be limited to the details shown, because various modifications may be obtained with respect to the invention without departing from the spirit of the invention or the scope of equivalents thereof. For example, openings 440 need not be arcuate-shaped; rather, openings 440 may be any suitable shape, such as circular. A further modification to the present invention would be to eliminate vent valve 690 and the associated vacuum pump 700, if venting gasses from opening 440 is not desired. Moreover, although the invention was conceived during an investigation directed towards a nozzle dam having a unitary plug for sealing the primary nozzles of a nuclear steam generator, it will be appreciated that the invention may have other uses, such as sealing the interior of any similarly-shaped conduit. Therefore, what is provided is a nozzle dam having a unitary plug for sealing the primary nozzles of a nuclear steam generator.