Patent Number: 051679051
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS A typical nuclear power plant includes a nuclear reactor core for producing heat and a steam generator in fluid communication with the nuclear reactor core 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 seal or block the inlet and outlet primary nozzles of the steam generator. Disclosed herein is a foldable nozzle dam having a foldable extrusion-resistant seal or gasket for sealing the primary nozzles of the nuclear 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 houses a nuclear reactor core 40 for producing heat. Reactor vessel 10 is disposed in a reactor cavity 50 that is defined by a reactor vessel enclosure 60. Reactor cavity 50 is partitioned into an upper cavity portion 70 for enclosing closure head 30 and a lower cavity portion 80 sealingly isolated from upper cavity portion 70 for enclosing lower portion 20. Reactor core 40 is surrounded by a primary fluid (e.g., demineralized borated water) circulating through reactor vessel 10 for removing the heat produced by reactor core 40. In addition, the exterior of closure head 30 is surrounded by a refueling pool 90 (e.g., demineralized borated water) substantially filling upper cavity portion 70 for providing a biological shield so that nuclear radiation from reactor core 40 is attenuated during refueling of reactor core 40. Of course, during refueling operations, 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 100 is disposed near reactor vessel 10 for generating steam, as described more fully hereinbelow. Interconnecting reactor vessel 10 and steam generator 100 is a pipe 110 that is in fluid communication with the primary fluid surrounding reactor core 40 and through which pipe 110 the primary fluid is pumped, as shown by the direction of the arrows in FIG. 1. In this regard, a reactor coolant pump 120 is interconnected with pipe 110 for pumping the primary fluid through pipe 110, through reactor core 40 and through steam generator 100. Referring to FIG. 2, there is shown steam generator 100 with parts removed for clarity. Steam generator 100 comprises a vertically-oriented shell 130 defining a cavity 140 therein. Shell 130 has a generally dome-shaped upper shell portion 150, a generally cylindrical hull portion 160 integrally attached to upper shell portion 150 and a generally bowl-shaped lower shell portion or channel head 170 integrally attached to hull portion 160. Disposed in cavity 140 are a plurality of vertically-oriented inverted U-shaped heat transfer tubes 180 for conducting the radioactive primary fluid therethrough. Each U-shaped tube 180 defines a pair of vertical tube leg portions 190a and 190b interconnected by a U-bend tube portion 200 integrally formed therewith. Each tube leg portion 190a and 190b has a pair of open tube ends 210a and 210b, respectively, for passage of the primary fluid therethrough. Moreover, disposed in cavity 140 near lower shell portion 170 is a horizontal tube sheet 220 having a plurality of apertures for receiving and for vertically supporting each open tube end 210a/210b, which open tube ends 210a/210b are suitably attached to tube sheet 220, such as by welding. Referring yet again to FIG. 2, disposed in lower shell portion 170 is a vertical divider plate 230 for dividing lower shell portion 170 into an inlet plenum chamber 240 and an outlet plenum chamber 250. Moreover, formed through lower shell portion 170 and in communication with inlet plenum chamber 240 and outlet plenum chamber 250 are a plurality of relatively small diameter access ports or manway openings 260 (only two of which are shown) for providing access to inlet and outlet plena 240/250 so that maintenance can be performed in steam generator 100. Such maintenance may be to inspect any of the tubes 180 for degradation and/or to repair any degraded tubes 180. Of course, manway openings 260 are capable of being sealingly covered by suitable manway covers and seals (not shown) during operation of steam generator 100. As illustrated in FIG. 2, integrally attached to lower shell portion 170 is a conduit, such as inlet primary nozzle 270, and another conduit, such as outlet primary nozzle 280, in fluid communication with inlet plenum chamber 240 and with outlet plenum chamber 250, respectively. Each of the primary nozzles 270 and 280 has an inside surface 283 defining an open end 286 of diameter larger than the diameter of manway openings 260, the open end 286 defining an annular depending shoulder or rim 288 therearound (see FIG. 4). As shown in FIG. 2, integrally attached to upper shell portion 150 is a feedwater nozzle 290 for passage of non-radioactive secondary fluid or feedwater (i.e., demineralized water) into cavity 140 of steam generator 100. In addition, integrally attached to the top of upper shell portion 150 is a main steam line nozzle 300 for passage of steam therethrough. Moreover, attached to hull portion 260 and horizontally disposed in cavity 140 are a plurality of circular spaced-apart tube support plates 310 (only four of which are shown) having holes therethrough for receiving each tube leg portion 190a/190b so that each tube 180 is laterally supported thereby. Each support plate 310 also has a plurality of unobstructed additional holes therethrough for passage of the non-radioactive secondary fluid. During operation of steam generator 100, the primary fluid, which is heated by reactor core 40, is pumped from reactor core 40, through one segment of pipe 110, through inlet primary nozzle 270 and into inlet plenum chamber 240. The primary fluid then flows through open tube end 210a, through tube 180, out the other open tube end 210b and into outlet plenum chamber 250, whereupon the primary fluid exits steam generator 100 through outlet primary nozzle 280 and is returned by another segment of pipe 110 to reactor core 40. As the primary fluid flows through tubes 180, secondary fluid simultaneously enters cavity 140 through feedwater nozzle 290 to surround tubes 180. It will be appreciated that as the primary fluid flows through tubes 180, it gives up its heat to the secondary fluid surrounding tubes 180. A portion of the secondary fluid surrounding heat transfer tubes 180 is converted to steam which rises upwardly to exit steam generator 100 through main steam line nozzle 300. The steam is then transported to a turbine-generator (not shown) for producing electricity in a manner well known in the art of steam-powered electricity production. Such a typical nuclear steam generator is more fully disclosed in U.S. Pat. No. 4,079,701 entitled "Steam Generator Sludge Removal System" issued Mar. 21, 1978 to Robert A.l Hickman et al., the disclosure of which is hereby incorporated by reference. Referring briefly to FIG. 3, there is shown the subject matter of the present invention, which is a circular foldable nozzle dam 320 having an extrusion-resistant seal or gasket, nozzle dam 320 being disposed in inlet plenum chamber 240 and across open end 286 of inlet primary nozzle 270 to block or seal inlet primary nozzle 270, as more fully described hereinbelow. It will be appreciated that although nozzle dam 320 is shown disposed in inlet plenum chamber 240 and across the open end of inlet primary nozzle 270, nozzle dam 320 may also be disposed in outlet plenum chamber 250 and across the open end of outlet primary nozzle 280 to block or seal outlet primary nozzle 280. Thus, although the description of the invention refers to sealing or blocking inlet primary nozzle 270, it will be understood that the invention may be used to seal or block outlet primary nozzle 280 as well. Turning now to FIGS. 4 and 5, a generally annular ring member or bracket 330 is permanently sealingly attached, such as by welding, to rim 288 of inlet primary nozzle 270, for reasons described more fully hereinbelow. Bracket 330 is permanently attached to rim 288 and remains attached to rim 288 as steam generator 100 is operated (i.e., generates steam) in the manner disclosed hereinabove. For this purpose, annular bracket 330 defines a generally circular opening 340 centrally transversely therethrough for passage of the primary fluid during operation of steam generator 100. Bracket 330 has a top surface 343 thereon having a multiplicity of grooves 346 (only two of which are shown in FIG. 5) formed therein for reasons provided hereinbelow. Grooves 346 provide a roughened top surface 343 for reasons more fully described hereinbelow. Moreover, bracket 330 may be "INCONEL", stainless steel or the like, to resist stress corrosion cracking during operation of steam generator 100. Bracket 330 is preferably fabricated of stress corrosion resistant material because stress corrosion cracking of bracket 330 may lead to creation of fluid flow paths therethrough such that inlet primary nozzle 270 is not sealed when nozzle dam 320 is installed in the manner more fully described hereinbelow. More specifically, the "INCONEL" material may comprise by weight approximately 76.0% nickel, 0.08% carbon, 0.5% manganese, 8.0% iron, 0.008% sulfur, 0.25% copper and 15.5% chromium. Bracket 330 also has an integral periphery portion 350 extending circumferentially therearound. The bottom of periphery portion 350 of bracket 330 is sealingly attached to rim 288, such as by welding. Bracket 330 may also have a slot 355 (see FIG. 8) extending around the bottom thereof for pressure testing the soundness of the weldment that sealingly attaches the bottom of periphery portion 350 to rim 288. In this regard, a source of pressurized gas (not shown) is connected to a channel 357 that is in gas communication with slot 355. The pressurized gas (e.g., nitrogen) is passed through channel 357 and flows into slot 355. If the weldment is not sound, the gas will leak from slot 355 and across the bottom of bracket 330 and will be detected by means known in the art. Alternatively, if the weldment is not sound, the gas will leak from slot 355 and a maximum stable or equilibrium value for the gas pressure will not be obtainable. If the weldment is sound, the gas will not leak from slot 355 and a plug (not shown) will be welded into channel 357 to permanently seal channel 357. As shown in FIGS. 4 and 5, formed through periphery portion 350 are a plurality of spaced-apart threaded transverse holes 360 disposed circumferentially around periphery portion 350 at predetermined intervals. In addition, bracket 330 may be formed of a plurality of mating sections, such as half section 370a and 370b, sealingly joined, such as by welding, at their interface 380. In one embodiment of the invention, bracket 330 is formed of section 370a and 370b in order to pass each section 370a/370b separately through the relatively small diameter manway 260 and yet dispose the assembled sections 370a/370b that comprise bracket 330 completely circumferentially around open end 286 of inlet primary nozzle 270. Bracket 330 provides a foundation or support means for supporting nozzle dam 320, as described in more detail hereinbelow. Referring to FIGS. 6, 7, 17 and 18, mounted atop bracket 330 is foldable nozzle dam 320 for sealingly covering opening 340 of bracket 330. As described in more detail hereinbelow, nozzle dam 320 is foldable for passing through the relatively small diameter manway 260 and unfoldable for being disposed completely across opening 340 of bracket 330. In this regard, nozzle dam 320 comprises a generally arcuate-shaped first wing or first side section 390a hingedly connected by a pair of hinge assemblies, generally referred to as 400, to a generally arcuate-shaped second wing or second side section 390b. A handle 395, which is adapted to be grasped by hand or by a suitable tool (not shown), is attached to each side section 390a and 390b for outwardly and inwardly pivoting side sections 390a and 390b about hinge assembly 400. In use, side sections 390a and 390b are capable of being pivoted about hinge assembly 400 such that they are deployable in the same plane with respect to each other in order to cover opening 340. Each side section 390a and 390 b defines a generally rectangular cut-out 405 (best seen in FIG. 16) open on one side for maneuvering nozzle dam 320 through manway 260. As shown in FIGS. 6, 7, 8 and 8A, each side section 390a and 390b defines an integral semi-circular periphery portion 410 having a plurality of spaced-apart smooth bores 420 therethrough. Bores 420 are formed such that they are capable of being coaxially aligned with threaded holes 360 of bracket 330, for reasons provided immediately hereinbelow. Each bore 420 is capable of matingly receiving therethrough clamping means, such as an externally threaded elongated shank portion 430 belonging to a fastener or bolt 440, for removably fastening or bolting nozzle dam 320 to bracket 330. Each bolt 440 has a bolt head 450 integral therewith so that bolt 440 may be turned to threadably engage hole 360 for tightly connecting nozzle dam 320 to bracket 330. Moreover, interposed between periphery portion 410 and bolt head 450 is an adjustable positioning block 460 connected to periphery portion 410 by a shoulder screw 470 and having a threaded bore therethrough for threadably receiving bolt 440 to connect nozzle dam 320 to bracket 330. The adjustable feature of positioning block 460 allows it to align bolt 440 with bore 420 and hole 360. Each positioning block 460 is adjustable in the sense that it can be rotated in a 360.degree. circle about shoulder screw 470, as shown by the dotted circular arrow in FIG. 8A, and moved laterally with respect to shoulder screw 470, as shown by the straight solid arrow in FIG. 8A. In addition, as best seen in FIG. 8, disposed between positioning block 460 and bolt head 450 may be a set of leaf springs 480 for longitudinally tensioning bolt 440 so that bolt 440 snugly intimately engages the threads of hole 360. Referring again to FIGS. 6 and 7, a generally rectangular center section 490 is matingly disposed in opening 405 defined by side section 390a and 390b for completely covering opening 405, center section 490 having integral arcuate-shaped end portions 500 capable of being bolted to bracket 330 by a plurality of spaced-apart bolts 505. Each arcuate-shaped end portion 500 has the same radius as the radius of side sections 390a and 390b. When center section 490 is bolted to bracket 330 and to side sections 390a and 390b, nozzle dam 320 assumes a rigid generally circular and planar configuration for completely covering opening 350 of bracket 330. Thus, nozzle dam 320 functions as a cover plate for covering opening 350 of bracket 330. The configuration of nozzle dam 320, when fully assembled, is sufficiently structurally sound to block, the column of water present in nozzle 270 or 280 during reactor refueling operations. Moreover, in the preferred embodiment of the invention, a handle 510 having a hole 520 therethrough extends along each longitudinal side of center section 490 to provide means for removably disposing center section 490 in opening 405. As best seen in FIGS. 7, 8, 8B and 8C, interposed between top surface 343 of bracket 330 and nozzle dam 320 is extrusion-resistant seal means, such as a foldable extrusion-resistant seal member or gasket 530, for providing a fluid-tight seal between nozzle dam 320 and bracket 330 so that fluid will not pass through opening 340 of bracket 330. Of course, it will be appreciated that sealing opening 340 in this manner also seals inlet primary nozzle 270. As disclosed in more detail hereinbelow, seal member 530 has an integral periphery portion 540 therearound having a plurality of spaced-apart transverse apertures 550 therethrough for receiving shank portion 430 of each bolt 440 and 505. As disclosed hereinabove, sections 390a, 390b and 490 comprising nozzle dam 320 are drawn toward top surface 343 of bracket 330 as bolts 440 and 505 pass through bore 420 and threadably engage hole 360. Bolts 440 and 505 may be torqued to a value of approximately 175.+-.25 foot-pounds force so that bolts 440/505 are neither undertorqued nor overtorqued. As sections 390a, 390b and 490 are drawn toward bracket 330, a compressive force will act perpendicularly on each opposing face or side of seal member 530 because seal member 530 is interposed between sections 390a, 390b, 490 and bracket 330. This compressive force acting perpendicularly against each side of seal member 530 will tend to cause aperture 550 of seal member 530 to extrude laterally outwardly away from shank portion 430 of bolt 440 and bolt 505 and assume a generally oval shape. Such extrusion of aperture 550 laterally away from shank portion 430 as sections 390a, 390b and 490 are tightly connected to bracket 330 will tend to enlarge the annular gap or fluid flow path defined by aperture 550 that surrounds shank portion 430 (see FIG. 8C). Such enlargement of the gap defined by aperture 420 will tend to decrease the surface area "A" to a smaller area "A'", which area "A" is available for sealing, between bracket opening 340 and aperture 550 (see FIGS. 8B and 8C). Excessive extrusion may result in a portion of aperture 550 overlapping opening 340 such that any fluid present in opening 340 will easily flow through that portion of aperture 550 overlapping opening 340. This is undesirable because such enlargement of the gap or flow path will compromise the ability of seal member 530 to perform its intended function of providing a nozzle dam 320 that is fluid-tight. Thus, according to the invention, seal member 530 is configured to be extrusion-resistant so that seal member 530 will not excessively laterally extrude away from bolts 440 and 505 in a manner that excessively enlarges the fluid flow path defined by apertures 550 surrounding bolts 440 and 505, as described in more detail hereinbelow. It should be understood that aperture 550 may experience some limited extrusion laterally away from shank portion 430 as sections 390a, 390b and 490 are tightly connected to bracket 330. However, such limited extrusion will not be sufficient to cause any portion of aperture 550 to overlap opening 340. Turning now to FIGS. 9, 10, 11, and 12, seal member 530 comprises a plurality of layers laminated or bonded together. In the preferred embodiment of the invention, seal member 530 comprises a generally annular first layer 560a. First layer 560a is sealingly bonded, such as by a suitable adhesive, to the underside of a generally circular second layer 560b. Second layer 560b is sealingly attached, such as by a suitable adhesive, to periphery portion 410 of first side section 390a and to periphery portion 410 of second side section 390b of nozzle dam 320. In addition, second layer 560b is circular and extends across the diameter of nozzle dam 320 for covering opening 405 so that fluid cannot pass through opening 405. First layer 560a, which may be EPDM (ethylene propylene diene monomer) rubber, has a Shore A durometer hardness of between approximately 40 and 60, and preferably a durometer hardness of approximately 50. Second layer 560b, which also may be EPDM rubber, has a Shore A durometer hardness of between approximately 60 and 80, and preferably a durometer hardness of approximately 70. The dual hardness of seal member 530 allows it to be extrusion-resistant and also allows it to intimately engage top surface 343 of bracket 330 for creating a seal between nozzle dam 320 and bracket 330. It will be appreciated that the relatively harder material of second layer 560b resists extrusion and the relatively softer material of first layer 560a assists in maintaining seal member 530 in intimate sealing engagement with top surface 343. As best seen in FIG. 12, first layer 560a is relatively soft for intimately engaging the grooves 346 formed in top surface 343 of bracket 330. The intimate engagement of first layer 560a with grooves 346 provides a multiplicity of obstructions or ridges that oppose the migration or leakage of the primary fluid along the interface of first layer 560a and top surface 343 of bracket 330. Such ridges will tend to increase the pressure drop of any fluid that would tend to migrate or leak across the interface, thereby reducing the rate of or eliminating such leakage. That is, if the primary fluid should tend to migrate along this interface, the pressure of the fluid at the interface will tend to drop as it traverses grooves 346. Such a pressure drop will tend to further limit or eliminate the migration or leakage at the interface because the pressure driving the fluid along the interface will decrease as the fluid migrates or leaks along the interface. Moreover, the intimate frictional engagement of first layer 560a and grooves 346 assists in resisting extrusion and in maintaining seal member 530 in sealing intimate engagement with top surface 343 of bracket 330 so that the fluid path defined by aperture 550 surrounding shank portion 430 is not enlarged as nozzle dam 320 is bolted to bracket 330. Referring now to FIGS. 13 and 14, there is shown an alternative embodiment of seal member 530, referred to as seal member 580, comprising a plurality of molded regions 590a/590b rather than laminated layers 560a and 560b. In this alternative embodiment of the invention, seal member 580 comprises a generally annular first region 590a. First region 590a is sealingly molded, such as by a press-cure process, to the underside of a generally circular second region 590b. Second region 590b is sealingly attached, such as by a suitable adhesive, to periphery portion 410 of first side section 390a and to periphery portion 410 of second side section 390b of nozzle dam 320. In addition, second region 590b extends across the diameter of nozzle dam 320 for covering opening 405 defined by side sections 390a, 390b so that fluid cannot pass through opening 405. First region 590a, which may be EPDM rubber, has a Shore A durometer hardness of between approximately 40 and 60, and preferably a durometer hardness of approximately 50. Second region 590b, which may be EPDM rubber, has a Shore A durometer hardness of between approximately 60 and 80, and preferably a durometer hardness of approximately 70. Referring briefly to FIG. 15, yet another embodiment of seal member 530 is shown. In this alternative embodiment, seal member 530, including layers 560a and 560b associated therewith, has truncated sides 600 to easily accommodate hinge assembly 400 (see FIGS. 17 and 18). Referring to FIGS. 16, 17, and 18, side sections 390a and 390b, which are pivotably hingedly connected together, are there shown in a folded state to pass through the relatively small diameter of manway opening 260 of steam generator 100. As disclosed hereinabove, the diameter of open end 286 of inlet primary nozzle 270 may be larger than the diameter of manway opening 260. Thus, nozzle dam 320 is foldable to pass through manway opening 260 and unfoldable to be disposed completely across opening 340 of bracket 330, such that it also covers open end 286 of inlet primary nozzle 270. Moreover, seal member 530 or 580 is also foldable because seal member 530 or 580 is sealingly permanently attached to foldable nozzle dam 320 in the manner previously described. As best seen in FIGS. 17 and 18, hinge assembly 400 includes a pivot pin 610 for allowing first side section 390a and second side section 390b of nozzle dam 320 to pivot thereabout. As disclosed hereinabove, first side section 390a, second side section 390b and center section 490 are capable of being securely locked in the same transverse plane with respect to each other to cover opening 286 of bracket 330 (as best seen in FIG. 7) when center section 490 is bolted to side sections 390a and 390b by bolts 505. By way of example only and not by way of limitation, steam generator manway opening 260 may have a diameter of approximately 16 inches and open end 286 of inlet primary nozzle 270 (or outlet primary nozzle 280) may have a diameter of approximately 38 inches. Thus, the diameter of manway opening 260 is substantially smaller than the diameter of open end 286 of inlet primary nozzle 270. Moreover, the outside diameter of bracket 330 may be approximately 41.5 inches and the inside diameter of bracket 330 may be approximately 38.5 inches. Thus, in the preferred embodiment, bracket 330 has an inside diameter slightly greater than the diameter of open end 286 for surrounding open end 286. When mounted atop bracket 330, the diameter of nozzle dam 320 may be roughly 42 inches in its unfolded state. However, in its folded state, each side section 390a/390b generally defines a half-circle having a radius of roughly 21 inches and has cut-out 405 for allowing nozzle dam 320 to be maneuvered through manway opening 260. The annular first layer 560a of laminated seal member 530, which intimately engages top surface 343 of bracket 330 and which is adhesively attached to second layer 560b, has an inside diameter of approximately 34 inches and an outside diameter of roughly 42 inches in the preferred embodiment. The second layer 560b of laminated seal member 530, which is adhesively attached to sections 390a and 390b, has a diameter of roughly 42 inches in the preferred embodiment. Moreover, the first layer 560a may have a transverse thickness of approximately 0.188 inches and the second layer 560b may have a transverse thickness of approximately 0.125 inches resulting in a laminated seal member 530 having a total transverse thickness of approximately 0.313 inches. In the alternative embodiment of the invention, the first region 590a of molded seal member 580, which intimately engages top surface 343 of bracket 330, has an inside diameter of approximately 34 inches and an outside diameter of approximately 42 inches. The second region 590b of molded seal member 580 has a diameter of roughly 42 inches in the preferred embodiment. The transverse thickness of molded seal member may be approximately 0.313 inches. Moreover, the threaded holes 360 in bracket 330 may have a diameter of approximately 0.750 inch and the smooth bores in nozzle dam 320 may have a diameter of approximately 0.937 inch. The threaded bore in each positioning block 460 has a diameter of approximately 0.750 inch. The apertures in seal member 530 (or seal member 580) may have a diameter of approximately 0.937 inch. OPERATION In use, bracket 330 cooperates with nozzle dam 320 and seal member 530 or 580 to sealingly block or cover open end 286 of primary nozzles 270 or 280 so that maintenance can be simultaneously performed in steam generator 100 as reactor core 40 is refueled. In this regard, reactor core 40 is first shut down and the level of water in refueling pool 90 is drained, in a manner well understood in the art, to a level that is below the elevation of inlet and outlet primary nozzles 270/280. It will be appreciated that draining refueling pool 90 to a level that is below the elevation of inlet and outlet nozzles 270/280 also drains heat transfer tubes 180 and plenum chambers 240/250. Next, the manway covers (not shown) are removed from the relatively small diameter manway openings 260 for providing access to the steam generator plena (e.g., inlet plenum chamber 240). At this point, first side section 390a and second side section 390b are folded inwardly toward each other about pivot pin 610, which belongs to hinge assembly 400, so that sections 390a/390b can be maneuvered through the relatively small diameter manway opening 260. Once inside inlet plenum chamber 240, sections 390a/390b are unfolded outwardly to an outstretched configuration, using handles 395, for mounting sections 390a/390b on bracket 330 to cover opening 340 of bracket 330. Of course, it will be understood that bracket 330 will have been previously sealingly attached to open end 286 of nozzle 270. In this regard, each section 370a/370b of bracket 330 will have been separately passed through manway opening 260 and matingly attached together, such as by welding, at the interface 380 thereof. Alternatively, bracket 330 may be a unitary one-piece member sealingly attached to open end 286 of nozzle 270 during manufacture of steam generator 100. Sections 390a/390b are mounted atop bracket 330, using handles 395, such that smooth bores 420 of sections 390a/390b are roughly aligned with threaded holes 360 of bracket 330. Adjustable positioning block 460 is caused to pivot about shoulder screw 470 and/or moved laterally with respect to shoulder screw 470 until bolt 440 is coaxially aligned with hole 360. Once bores 420 are coaxially aligned with holes 360, bolts 440 are caused to pass through bores 420 and threadably engage holes 360 for removably clamping or connecting sections 390a/390b to bracket 330. Next, center section 490 is passed through manway opening 260 and matingly disposed, using handles 510, to cover cut-outs 405 defined by side sections 390a/390b. At this point, center section 490 is attached to side sections 390a/390b by bolts 505 that threadably engage holes 360. Once sections 390a, 390b and 490 are properly aligned, all bolts are uniformly torqued in a controlled manner to predetermined torque values. In this manner, side sections 390a/390b and center section 490 form the rigid circular nozzle dam 320 used for covering opening 340 of bracket 330. As described hereinabove, interposed between nozzle dam 320 and bracket 330 is extrusion-resistant seal member 530 or 580, which is adhesively attached to side section 390a/390b, for providing a fluid-tight seal between nozzle dam 320 and bracket 330. As sections 390a, 390b and 490 comprising nozzle dam 320 are drawn toward top surface 343 of bracket 330 when bolts 440 and 505 threadably engage holes 360, a compressive force will act perpendicularly on each opposing face or side of seal member 530 because seal member 530 is interposed between sections 390a, 390b, 490 and bracket 330. This compressive force acting perpendicularly against each side of seal member 530 will tend to cause aperture 550 of seal member 530 to extrude laterally outwardly away from shank portion 430 of each bolt 440 and bolt 505. Such extrusion of aperture 550 laterally away from shank portion 430 as sections 390a, 390b and 490 are tightly clamped to bracket 330 will tend to enlarge the annular gap or fluid flow path surrounding shank portion 430. Such enlargement of the gap will tend to decrease the surface area "A", which is available for sealing, between bracket opening 340 and aperture 420. Excessive extrusion may result in a portion of aperture 420 overlapping opening 340 such that any primary fluid present in opening 340 will easily flow through that portion of aperture 420 overlapping opening 340. This is undesirable because enlargement of such a flow path will compromise the ability of seal member 530 to perform its intended function of providing a nozzle dam 320 that is fluid-tight. Thus, according to the invention, the extrusion-resistant configuration of seal member 530 results in a seal member 530 that will resist lateral extrusion away from bolts 440 and 505 that would otherwise enlarge the fluid flow path surrounding bolts 440 and 505. More specifically, first layer 560a is relatively soft for intimately engaging the grooves 346 formed in top surface 343 of bracket 330. The engagement of first layer 560a with grooves 346 of bracket 330 assists in creating a fluid-tight seal in the manner previously described. Therefore, it will be appreciated that the relatively softer material of first layer 560a (or first region 590a) assists in maintaining seal member 530 in intimate sealing engagement with bracket surface 343 as the relatively harder material of second layer 560b (or second region 590b) resists extrusion of seal member 530 laterally away from bolt 440 (or bolt 505). In addition, as disclosed hereinabove, the intimate engagement of first layer 560a with grooves 346 provides a multiplicity of obstructions or ridges that oppose migration or leakage of the primary fluid along the interface of first layer 560a and top surface 343 of bracket 330. In this regard, such ridges will tend to increase the pressure drop of any fluid that would migrate across the interface, thereby reducing the rate of such leakage. Moreover, it will be appreciated that the relative softness of first layer 560a or first region 590a will allow it to fill any indentations or imperfections in top surface 343 of bracket 330 to further enhance the sealing of nozzle dam 320. In addition, it should also be understood that apertures 550 are punched through seal member 530 or 580 after seal member 530 or 580 is mounted on the nozzle dam assembly such that apertures 550 will precisely align with their associated bolts. After nozzle dam 320 is suitably installed in inlet primary nozzle 270 in the manner disclosed hereinabove, upper cavity portion 70 of reactor cavity 50 is refilled with water and closure head 30 is removed, in a manner well known in the art, to provide access to reactor core 40 for refueling reactor core 40. However, as upper cavity portion 70 is refilled, the primary fluid will not rise into inlet plenum chamber 240 or outlet plenum chamber 250 because nozzle dam 320 seals or blocks openings 286 of primary nozzles 270/280. At this point, maintenance may be simultaneously performed in steam generator plena 240/250 as reactor core 40 is refueled. After reactor core 40 is refueled and after maintenance is performed in steam generator 100, nozzle dam 320 is removed from steam generator 100 substantially in the reverse order of its installation in steam generator 100. 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, bracket 330 may be deleted and nozzle dams 320 removably clamped or connected directly to rim 288 of nozzles 270/280. In such a modification of the invention, rim 288 will have a prepared mating surface and threaded holes therein to receive bolts 440. A further modification to the present invention would be to eliminate the grooves 346 in bracket 330, if desired. An additional modification to the present invention would be to provide a seal member 530 made of extrusion-resistant other than homogeneous EPDM. Yet another modification to the present invention would be to provide a seal member comprising relatively soft EPDM rubber but having a multiplicity of extrusion-resistant "NYLON" or like fibers homogeneously dispersed or specifically located therein. Moreover, although the invention was conceived during an investigation directed towards a foldable nozzle dam having a foldable extrusion-resistant seal or gasket for sealing the open ends of steam generator primary nozzles, the invention may have other uses, such as to seal the open ends of any similar conduit. Therefore, what is provided is a foldable nozzle dam having a foldable extrusion-resistant seal or gasket for sealing conduits, such as the primary nozzles of a nuclear steam generator.