WO1997021956A2

WO1997021956A2 - System and method for stabilizing a heat exchanger tube

Info

Publication number: WO1997021956A2
Application number: PCT/US1996/019893
Authority: WO
Inventors: Edward P. Belizar; Pedro C. Olivera; Jayne Boynton; Thomas M. Frick; Lawrence A. Nelson; Jeffrey M. Hall; Robert M. Wepfer; John Houtman; Albert R. Vaia
Original assignee: Westinghouse Electric Corporation
Priority date: 1995-12-15
Filing date: 1996-12-13
Publication date: 1997-06-19
Also published as: WO1997021956A3

Abstract

System and method for stabilizing a nuclear heat exchanger heat transfer tube (90) against lateral movement thereof, which tube is received in a plate member (40) for supporting the tube, which includes a sleeve (340) disposed therein. A mandrel (140) is disposed in the sleeve (340), the mandrel (140) having a flow channel (160) formed therethrough. A pair of spaced-apart resilient bladders (150A, 150B) surround the mandrel. A pressurizer (170) in communication with the flow channel (160) supplies a pressurized fluid into the flow channel to radially expand the bladders. A controller (180) operates the pressurizer (170) to controllably supply the fluid to the bladders. Even if complete separation of the tube were to occur, the installed sleeve prevents lateral movement of the severed tube ends.

Description

SYSTEM AND METHOD FOR STABILIZING A NUCLEAR HEAT EXCHANGER HEAT TRANSFER TUBE

BACKGROUND OF THE INVENTION

This invention generally relates to apparatus and methods for stabilizing structures and more particularly relates to a system and method for stabilizing a nuclear heat exchanger heat transfer tube against lateral movement thereof.

In a pressurized water nuclear reactor, heat generated by a nuclear reaction in a reactor core is absorbed by a primary coolant that circulates through the reactor core and that is then utilized to generate steam in a steam generator. A transverse tubesheet divides the steam generator into a primary side below the tubesheet and a secondary side above the tubesheet. Below the tubesheet, a vertical wall (i.e., divider plate) bisects the primary side into an inlet section and an outlet section. An array of thousands of U-shaped heat transfer tubes is disposed in the secondary side of the steam generator. One end of each U-shaped tube is inserted into a hole in the tubesheet and communicates with the inlet section of the primary side and the other end of each tube is inserted into another hole in the tubesheet and communicates with the outlet section of the primary side. As an aid in securing the end portions of each tube in the tubesheet, the tube end portions may be roll expanded into engagement with the tubesheet. The secondary side of the steam generator also includes a plurality of spaced-apart transverse tube support plates having holes for passage of the tubes therethrough, which tube support plates laterally support the tubes.

The heated primary coolant is introduced under pressure into the inlet section of the primary side, circulates through the U-shaped tubes and exits through the outlet section of the primary side. Water introduced into the secondary side of the steam generator circulates around the U-shaped tubes and is transformed into steam by heat given up by the primary coolant. The steam is conveyed to a turbine-generator through a pipe interconnecting the steam generator and the turbine-generator in order to produce electricity in a manner well known in the art.

Occasionally, some of the tubes may degrade and experience circumferential cracking in the roll expanded portion of the tube. Such cracking, if severe enough, may allow leaking of the primary coolant through the crack. A leaking tube is undesirable because the primary coolant is radioactive and any leakage of primary coolant into the secondary side of the steam generator will radioactively contaminate the steam produced by the steam generator. Any tube exhibiting severe degradation is either sleeved at the site of the degradation or plugged to remove the tube from service so that the primary coolant will not leak into the secondary side. When plugging is used, it is necessary to demonstrate that, with further degradation of the tube resulting in complete separation of the tube at the crack location, damage to surrounding tubes is avoided. More specifically, it has been postulated that, if complete separation of the tube occurs, at least one of the separat¬ ed tube ends may laterally move and impact neighboring tubes and possibly damage the neighboring tubes, a highly undesirable result. Such lateral movement of a separated tube end may occur, for example, during a steam generator transient event (e.g., steam generator "blowdown"), which may be caused by a steam line break producing rapid depressurization of the steam generator interior. Such rapid depressurization of the steam generator may in turn give rise to undesirable lateral hydraulic forces acting on the heat transfer tube. Alternatively, lateral movement of a separated tube end may occur during steam generator service operations such as during sludge lancing, wherein the hydraulic forces generated by the sludge lance fluid laterally impacts the tube. It is therefore desirable to stabilize the tube, such that even if complete separation of the tube were to occur, the separated tube end will not laterally move to damage neighboring tubes during transient events and normal servicing. Nuclear heat exchanger tube stabilizers are known.

A stabilizer for heat exchanger tubes is disclosed by U.S. Patent 4,590,991 titled "Flexible Stabilizer For Degraded Heat Exchanger Tubing" issued May 27, 1986 in the name of Frank W. Cooper, Jr., et al . This patent discloses a flexible vibration stabilizer and method for reducing vibration in a tube in a nuclear heat exchanger. The stabilizer, which is disposed in the tube, includes an elongate flexible cable or chain which may have a plurality of rigid members mounted thereon. A tube plug may be used for simultaneously supporting the stabilizer in the tube and for sealing the tube. However, the stabilizer dis¬ closed by this patent is relatively long. Such long stabilizers are only partially remotely installable. Therefore, longer stabilizers take more time to install because they are only partially remotely installable and require some manual manipulation. Applicants have observed that the installation rate of such long stabilizers is approximately 20 stabilizers a day, depending on the length of the stabilizer. Moreover, the level of radiation dose exposure to maintenance personnel, although within safe limits, is higher than desirable because the relatively long stabilizers are only partially remotely installable. Further, stocking the cable stabilizers in inventory is costly because of the wide range of sizes and lengths required and the uncertain demand for a stabilizer of a specific size and length. In addition, applicants have discovered that reliable sources of supply of prior art cable stabilizers are not abundant. Therefore, what is needed is a suitable cost- effective system and method for stabilizing a nuclear heat exchanger heat transfer tube against lateral movement thereof. SUMMARY Disclosed herein is a system and method for stabilizing a nuclear heat exchanger heat transfer tube against lateral movement thereof. Each of the tubes is received in a plate member for supporting the tube. The system includes a sleeve disposed in a predetermined one of the tubes, which sleeve has a distal end portion thereof extending above the top surface of the plate member and a proximal end portion disposed between the top surface and the bottom surface of the plate member. In order to stabilize the tube, a mandrel is disposed in the sleeve, the mandrel having a flow channel formed therethrough terminating in a pair of spaced-apart ports on the exterior surface of the mandrel. A pair of spaced-apart resilient bladders surround the mandrel and covers respective ones of the ports for radially expanding the sleeve into intimate engagement with the tube as the bladders radially expand. A pressurizer in communication with the flow channel formed through the mandrel supplies a pressurized fluid into the flow channel, through the pair of ports and to the bladders to radially expand the bladders. A controller operates the pressurizer to controllably supply the fluid to the blad¬ ders. When the bladders expand, a first expanded region is formed in the sleeve and the tube above the top surface of the plate member and a second expanded region is formed in the sleeve between the top surface and the bottom surface of the plate member. Even if complete separation of the tube were to occur, the installed sleeve prevents lateral movement of the severed tube ends.. In its broad form the invention is, for use in a nuclear heat exchanger having a plurality of heat transfer tubes disposed therein, the tubes received in respective ones of a plurality of holes formed through a plate member having a top surface and a bottom surface, a system for stabilizing a predetermined one of the tubes, comprising a sleeve disposed in the predetermined tube and expansion means connected to the sleeve for radially expanding the sleeve into intimate engagement with the tube, so that a first expanded region is formed in the sleeve and the tube above the top surface of the plate member and so that a second expanded region is formed in the sleeve between the top surface and the bottom surface of the plate member.

In its broad form, the invention is also, in a nuclear heat exchanger having a plurality of heat transfer tubes disposed therein, the tubes received in respective ones of a plurality of holes formed through a plate member having a top surface and a bottom surface, a method for stabilizing a predetermined one of the tubes, comprising the steps of disposing a sleeve in the predetermined tube and forming a first expanded region in the sleeve and the tube above the top surface of the plate member and forming a second expanded region in the sleeve between the top surface and the bottom surface of the plate member by radially expanding the sleeve into intimate engagement with the tube by operating an expansion unit connected to the sleeve.

An object of the present invention is to provide a cost effective system and method for stabilizing a nuclear heat exchanger heat transfer tube against lateral movement thereof.

A feature of the present invention is the provi¬ sion of a sleeve disposed in the tube and expansion means connected to the sleeve for expanding the sleeve, so that a first expanded region is formed in the sleeve and the tube above the top surface of the plate member and so that a second expanded region is formed in the sleeve between the top surface and the bottom surface of the plate member. An advantage of the present invention is that more tubes may be stabilized in a given period of time when compared to prior art methods of tube stabilization.

Another advantage of the present invention is that use thereof reduces radiation exposure to maintenance personnel when compared to use of prior art cable stabiliz¬ ers for stabilizing the tube.

These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described illustrative embodi¬ ments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter of the invention, it is believed the inven¬ tion will be better understood from the following descrip¬ tion taken in conjunction with the accompanying drawings wherein: Figure 1 is a view in partial vertical section of a typical nuclear steam generator with parts removed for clarity, the steam generator having a plurality of heat transfer tubes disposed therein supported by a plurality of support plates and also supported by a tubesheet, this view showing the system of the invention connected to the steam generator for stabilizing a predetermined one of the tubes;

Figure 2 is a view in partial vertical section of the system of the invention in operative condition to stabilize the predetermined tube;

Figure 3 is a view in vertical section of an expansion mandrel and sleeve connected thereto disposed in the predetermined tube prior to stabilizing the tube;

Figure 4 is a view in vertical section of the tube after stabilization thereof;

Figure 5 is a view in elevation of an alternative embodiment of the sleeve disposed in the tube before the expansion process is begun;

Figure 6 is a view of the alternative embodiment of the invention taken along section line 6-6 of Figure 5; and

Figure 7 is a fragmentation view in elevation of the sleeve (exaggerated for clarity) belonging to the alternative embodiment of the invention, the sleeve being disposed in the tube after the expansion process is com¬ pleted.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig. 1, there is shown a typical nuclear heat exchanger or steam generator, generally referred to as 10, for generating steam. Steam generator 10 comprises a cylindrical body portion 20 enclosed at its lower end by a hemispherical shell 30. A transverse plate or tubesheet 40 divides steam generator 10 into a primary side 50 below tubesheet 40 and a secondary side 60 above tubesheet 40. Tubesheet 40 has a top surface 42 and a bottom surface 45. The primary side 50 is divided by a divider plate 70 into an inlet section 72 and an outlet section 75. Still referring to Fig. 1, tubesheet 40 has a plurality of holes 80 therethrough. A plurality of U-shaped heat transfer tubes 90 (only two of which are shown) have ends received in respective ones of the holes 80 so that one end of each tube 90 communicates with inlet section 72 and the other end of each tube 90 communicates with outlet section 75. Each tube 90 is laterally supported on secondary side 60 by a plurality of spaced-apart parallel transverse plate members, such as support plates 100. Each support plate 100 has a top surface 105 and a bottom surface 110. In addition, each support plate 100 has a plurality of holes 108 formed therethrough for passage of each tube 90. A plurality of manways 120 (only one of which is shown) provide access to inlet section 72 and outlet section 75 to allow servicing of steam generator 10. Steam generated by steam generator 10 is transported to a turbine-generator (not shown) by means of a pipe (not shown) for generating electricity in a manner well known in the art. Referring to Figs. 1 and 2, there is shown expansion means, which may be a hydraulic expansion unit generally referred to as 130, for controllably expanding tubular members, such as a sleeve 340 (see Fig. 3) disposed in a predetermined tube 90a which may have a circumferentially extending crack (not shown) in the wall thereof above top surface 42 of tubesheet 40.

Referring now to Figs. 2 and 3, expansion unit 130 comprises a mandrel, generally referred to as 140, inserta- ble into tube 90a and having a pair of spaced-apart resil¬ ient and expandable bladders 150a and 150b theresurrounding for reasons disclosed presently. Connected to mandrel 140 and in communication with bladders 150a/150b is a flexible conduit 160 for reasons disclosed hereinbelow. Conduit 160 is connected to a pressurizer 170 for supplying a pressur¬ ized fluid (e.g., gas and/or liquid) through conduit 160 and to mandrel 140 to radially expand bladders I50a/150b in the manner disclosed in more detail hereinbelow. Control means, such as a controller or computer 180, is connected to pressurizer 170 for controllably operating pressurizer 170, so that pressurizer 170 controllably supplies the pressurized fluid to mandrel 140 in order to controllably pressurize bladders 150a/150b to a predetermined pressure. Of course, the pressure (e.g., approximately 25,000 psia) required to expand tube 90a is determined by the yield strength of the tube material (e.g., "INCONEL 690" materi¬ al) . The expansion process includes pressurizing bladders 150a/150b until tube 90a yields and then continuing to apply pressure at a constant pressurization rate for a predetermined time (e.g., approximately nine seconds) to produce a desired predetermined outside diameter of tube 90. In addition, translation means, such as a conduit driver 190, engages conduit 160 for driving or translating conduit 160 and the mandrel 140 connected thereto along the longitudinal axis of tube 90a. Pressurizer 170 may include a motor-driven piston arrangement 200 therein electrically connected to and controlled by controller 180 for control¬ lably pressurizing the fluid supplied to mandrel 140 by pressurizer 170. Pressurizer 170 may also include a fluid reservoir 210 in fluid communication with piston arrange¬ ment 200 for providing fluid to piston arrangement 200, which fluid is then pressurized (or depressurized) by piston arrangement 200. As best seen in Fig. 3, mandrel 140 comprises a pair of elongate central bodies 220a and 220b each having an externally threaded distal end portion 230a/230b and an externally threaded proximal end portion 240a/240b. A channel 250, which is in communication with bladders 150a/150b, extends longitudinally in central bodies 220a/220b for conducting the fluid to and away from blad¬ ders 150a/l50b in order to pressurize and depressurize bladders 150a/150b, respectively. Channel 250 terminates in a pair of ports 260a and 260b on the exterior surface of central bodies 220a and 220b, respectively. Ports 260a and 260b are in communication with the interior surfaces of bladders 150a and 150b, respectively, for conducting the fluid to the interior surfaces of bladders 150a/l50b in order to pressurize bladders 150a/150b and for conducting the fluid away from the interior surfaces of bladders 150a/l50b in order to depressurize bladders 150a/150b. Threadably connected to respective ones of distal end portions 230a/230b is a threaded nose member 270a and 270b. Each nose member 270a/270b has an internally threaded step bore 280a/280b therein for threadably engaging the external threads of distal end portions 230a/230b. In addition, threadably connected to each of the proximal end portions 240a/240b is an internally threaded end fitting 290a/290b. Each end fitting 290a/290b has an internally threaded step bore 300a/300b therein for threadably engaging the external threads of proximal end portions 240a/240b belonging to central bodies 220a/220b. Distal end portions 310a/310b of bladders 150a/150b are sealingly captured, such as by a press fit, between nose members 270a/270b and central bodies 220a/220b. Similarly, proximal end portions 320a/320b of bladders 150a/l50b are sealingly captured, such as by a press fit, between proximal end portions 240a/240b and end fittings 290a/290b. Moreover, end fitting 290b has a bore 330 therein for receiving an end portion of conduit 160, the bore 330 being in communication with channel 250 formed in central bodies 220a/220b. In this manner, fluid can be conducted from conduit 160 and into channel 250 to pressurize bladders 150a/l50b and conducted from channel 250 and into conduit 160 to depressurize bladders 150a/150b. It will be appreciated from the description hereinabove that the terminology "proximal end portion" is defined to mean that end portion disposed nearer divider plate 70 and the terminology "distal end portion" is defined herein to mean that end portion farther away from divider plate 70. A suitable hydraulic expansion unit for pressurizing and depressurizing bladders 150a/150b may be of the type disclosed in U.S. Patent Application Serial No. 08/192,536 titled "Apparatus And Method For Expanding Tubular Members" filed February 7, 1994 in the name of David A. Snyder and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated by reference. Turning now to Figs. 3 and 4, hydraulic expansion unit 130 is used to stabilize tube 90a in the manner disclosed hereinbelow. In this regard, an elongate sleeve 340 is slidingly placed around bladders 150a/150b. Blad¬ ders 150a/150b are then prepressurized (e.g., to a pre- pressurization pressure of approximately 2000 psi) to hold sleeve 340 thereon by force of friction without radially expanding sleeve 340. Mandrel 140 and the sleeve 340 held thereby are then disposed through manway 120 and into tube 90a such that a distal end portion of sleeve 340 projects above top surface 42 of tubesheet 40. More specifically, mandrel 140 is positioned and coaxially aligned with tube 90a by suitable positioning means, which may be a remotely operated robot positioner device 350. A robot positioner suitable for this purpose may be a ROSA (Remotely Operated Service Arm) robot available from the Westinghouse Electric Corporation located in Pittsburgh, Pennsylvania. There¬ after, conduit driver 190 is operated to translate mandrel 140 in tube 90a such that the distal end portion of sleeve 340 projects a predetermined distance above top surface 42 of tubesheet 40. In this regard, conduit 160 may have a stop 360 connected thereto, the stop 360 being adapted to abut bottom surface 45 of tubesheet 40, so that mandrel 140 is axially translated in tube 90a only the predetermined distance. Alternatively, at least one marking (not shown) may be present on the exterior surface of conduit 160 to indicate to the operator of system 130 that conduit 160 and mandrel 140 connected thereto have been translated in tube 90a the desired predetermined distance. Still referring to Figs. 3 and 4, operation of expansion unit 130 causes bladder 150a to pressurize and radially expand in order to radially expand tube 90a and sleeve 340 at a first location above top surface 42 of tubesheet 40, so as to define a first expanded region 370 thereat. Moreover, as bladder 150a radially expands, bladder 150b will simultaneously radially expand in order to radially expand sleeve 340 at a second location between top surface 42 and bottom surface 45, so as to define a second expanded region 380 thereat. It will be appreciated from the description provided immediately hereinabove, that as bladder 150b and sleeve 340 radially expand, tube 90a will not substantially radially expand because radial expansion of tube 90a at t s second location is con¬ strained by the surrounding structure of tubesheet 40. Bladders 150a/150b are then depressurized by operation of expansion unit 130 and mandrel 140 is withdrawn from tube 90a in substantially the reverse order of its insertion into tube 90a. The first expanded region 370 and the second expanded region 380 coact to stabilize tube 90a against lateral movement thereof in the event that tube 90a separates adjacent top surface 42 of tubesheet 40 (i.e., between first expanded region 370 and second expanded region 380) . A tube plug 390 may then be sealingly in- stalled into each end of tube 90a for preventing primary fluid from entering tube 90a in order to protect sleeve 340 from further corrosive attack by the primary fluid. A tube plug suitable for this purpose may be of the type disclosed in U.S. Patent 4,502,511 titled "Tube Plug" issued March 5, 1985 in the name of Harvey J. Kucherer and assigned to the assignee of the present invention, the disclosure of which is hereby incorporated by reference. It will be understood from the description hereinabove, that although the inven¬ tion is described for stabilizing tube 90a in the region of tubesheet 40, the invention also can be used to stabilize tube 90a in the region of any one of support plates 100 in substantially the same manner it is used to stabilize 90a in the region of tubesheet 40.

However, applicants have discovered that a relatively small gap (e.g., approximately 0.002 inch) may exist between sleeve 340 and tube 90a in the lower expan¬ sion region 380. In this regard, analysis performed by applicants indicates that the presence of the gap (not shown) may limit the operating life of steam generator 10. Hence, it is preferable to remove such a gap. More specif¬ ically, applicants' analysis indicates that in the event tube 90a severs above top surface 42 of tubesheet 40, the presence of such a gap between sleeve 340 and tube 90a in the lower expansion region 380 may allow sleeve 340 and the severed upper portion (i.e., above top surface 42) of tube 90a to vibrate during plant operation. Such vibration may cause the wall of sleeve 340 to wear against the inner wall of the severed portion of tube 90a causing sleeve 340 to circumferentially crack and possibly fail its intended function as a stabilizer. Failure of sleeve 340 in this manner may allow the severed portion of tube 90a to further vibrate and impact surrounding tubes 90. On the other hand, if the gap is not present, then sleeve 340 will not vibrate, thereby precluding wear and subsequent failure of sleeve 340.

Therefore, referring to Figs. 5, 6 and 7, there is shown an alternative embodiment of sleeve 340 having at least one "flat" or shallow cut 400 on the exterior surface thereof for removing the previously mention gap. In this alternative embodiment of the invention, sleeve 340 prefer¬ ably has a plurality of shallow cuts, such as four "flats" or shallow cuts 400, at locations spaced 90 degrees apart around the circumference of sleeve 340. Each of these shallow cuts 400 is spaced-apart from its neighbor circumferentially and axially along sleeve 340. Shallow cuts 400 may be machined by an end mill, grinding wheel, or other method commonly used in the machining arts. It will be understood, however, that shallow cuts 400 do not extend through the wall of sleeve 340; rather, shallow cuts 400 are formed by removal of a relatively small amount of material (e.g., approximately 0.020 inch in the radial direction) from the exterior wall of sleeve 340. Shallow cuts 400 alter the bending moment of inertia in sleeve 340 which, when sleeve 340 is expanded as described hereinabove and radially plastically deformed at high pressure (about 22,000 psi), sleeve 340 produces bending in sleeve 340 along it longitudinal axis. Such bending forms at least one contact point 410 radially opposite the corresponding shallow cut 400, as is shown in Figs. 5, 6 and 7, exagger¬ ated for clarity. Intimate contact is maintained between sleeve 340 and the inside surface of tube 90a after radial expansion of sleeve 340 because of sleeve's 340 elasticity in the longitudinal direction. Moreover, it will be appreciated from the description hereinabove, that modifi¬ cations may be made to this alternative embodiment of sleeve 340. For example, shallow cuts 400 may be rela¬ tively axially longer or machined radially deeper into the exterior surface of sleeve 340 than is shown in Figs. 5, 6 and 7. In addition, shallow cuts 400 may be in the form of spiral grooves, extending helically around sleeve 340, or in the form of an arc to cut to a predetermined radial depth. Alternatively, material may be added to the exteri- or surface of sleeve 90a, which added material, for exam¬ ple, may be a plurality of weld build-ups. Another varia¬ tion is to remove an annular ring or groove of material from the exterior surface of sleeve 340 to produce addi¬ tional plastic deformation in sleeve 340 in combination with the addition of a radial elevated portion of built-up material in the exterior surface adjacent to the annular ring or groove. Each of the above modifications will produce the desired bending moments that will close the previously mentioned gap. It will be appreciated from the description hereinabove, that an advantage of the present invention is that use thereof is more cost effective that use of rela¬ tively long prior art cable stabilizers for stabilizing tube 90a. More specifically, use of the present invention results in a greater number of tubes that may be stabilized in a given period of time (e.g., approximately 50 tubes per day) compared to the number of tubes (e.g., approximately 20 tubes per day) that can be stabilized by prior art cable stabilizers because of the automated and rapid installation of the stabilizing sleeves using a robot device. Moreover, the level of radiation dose exposure to maintenance person¬ nel is reduced because the stabilizing sleeves are prefera¬ bly remotely installed by use of the robot device rather than being manually installed. Further, use of the present invention does not require costly stocking in inventory of various sizes and lengths of cable stabilizers. In addi¬ tion, reliable sources of supply of stabilizer sleeves are abundant. Although the invention is illustrated and de¬ scribed herein in its preferred embodiment, 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, although the invention is described herein for stabilizing a nuclear heat exchanger heat transfer tube against lateral movement thereof, the invention is also usable for stabilizing any similar tube whether or not the tube is a nuclear heat exchanger heat transfer tube.

Therefore, what is provided is a system and method for stabilizing a nuclear heat exchanger heat transfer tube against lateral movement thereof.

Claims

What is claimed is:

1. For use in association with a nuclear heat exchanger having a plurality of heat transfer tubes dis¬ posed therein, the tubes received in respective ones of a plurality of holes formed through a plate member having a top surface and a bottom surface, a system for stabilizing a predetermined one of the tubes, comprising:

(a) a sleeve disposed in the predetermined tube; and

(b) expansion means connected to said sleeve for radially expanding said sleeve into intimate engagement with the tube, so that a first expanded region is formed in said sleeve and said tube above the top surface of the plate member and so that a second expanded region is formed in said sleeve between the top surface and the bottom surface of the plate member.

2 . The system of claim 1, further comprising control means connected to said expansion means for con¬ trolling said expansion means.

3. The system of claim 1, further comprising translation means connected to said sleeve for translating said sleeve in the tube.

4. The system of claim 1, further comprising positioning means connected to said sleeve for positioning said sleeve in the tube.

5. The system of claim 1, wherein said sleeve has a shallow cut in an exterior wall thereof for generating a bending moment therein to move a portion of said sleeve into contact with the tube.

6. For use in association with a nuclear heat exchanger having a plurality of heat transfer tubes dis- posed therein, the tubes received in respective ones of a plurality of holes formed through a plate member having a top surface and a bottom surface, a system for stabilizing a predetermined one of the tubes against lateral movement thereof, comprising: (a) a sleeve disposed in the predetermined tube, a distal end portion of said sleeve extending above the top surface of the plate member and a proximal end portion of said sleeve disposed between the top surface and the bottom surface of the plate member; (b) a mandrel disposed in said sleeve, said mandrel having an exterior surface and a flow channel formed therethrough terminating in a pair of spaced-apart ports on the exterior surface of said mandrel;

(c) a pair of spaced-apart resilient bladders surrounding said mandrel and covering respective ones of the ports for radially expanding said sleeve into intimate engagement with the tube, so that a first expanded region is formed in said sleeve and said tube above the top surface of the plate member as one of the bladders expands and so that a second expanded region is formed in said sleeve between the top surface and the bottom surface of the plate member as the other one of the bladders expands;

(d) a pressurizer in communication with the flow channel formed through said mandrel for supplying a pres¬ surized fluid into the flow channel, through the pair of ports and to said bladders to radially expand said bladders into intimate engagement with said sleeve; and

(e) a controller connected to said pressurizer for operating said pressurizer to controllably supply the fluid into the flow channel in order to controllably radially expand said bladders.

7. The system of claim 6, further comprising a flexible conduit interconnecting said pressurizer and the flow channel formed in said mandrel for conducting the fluid from said pressurizer and into the flow channel .

8. The system of claim 7, further comprising a conduit driver engaging said conduit for translating said conduit and said mandrel connected thereto axially in the tube.

9. The system of claim 6, further comprising a robot positioner connected to said sleeve for positioning said sleeve in the tube.

10. The system of claim 6, further comprising a fluid supply reservoir for supplying the fluid to said pressurizer.

11. The system of claim 6, wherein said sleeve has a plurality of shallow cuts in an exterior wall thereof for generating a bending moment therein to move a portion of said sleeve into contact with the tube in order to close a gap between said sleeve and the tube, the shallow cuts each being spaced-apart around the circumference of said sleeve and being formed in said sleeve at predetermined axial locations thereof.

12. In association with a nuclear heat exchanger having a plurality of heat transfer tubes disposed therein, the tubes received in respective ones of a plurality of holes formed through a plate member having a top surface and a bottom surface, a method for stabilizing a predeter¬ mined one of the tubes, comprising the steps of:

(a) disposing a sleeve in the predetermined tube; and

(b) forming a first expanded region in the sleeve and the tube above the top surface of the plate member and forming a second expanded region in the sleeve between the top surface and the bottom surface by radially expanding the sleeve into intimate engagement with the tube by operating an expansion unit connected to the sleeve.

13. The method of claim 12 , further comprising the step of controllably forming the first expanded region and the second expanded region by operating a controller connected to the expansion unit.

14. The method of claim 12, further comprising the step of translating the sleeve axially in the tube by operating a translation mechanism connected to the sleeve.

15. The method of claim 12, further comprising the step of positioning the sleeve in the tube by operating a positioner connected to the sleeve.

16. The method of claim 12, wherein said step of disposing a sleeve in the predetermined tube comprises the step of disposing a sleeve having a shallow cut in an exterior wall thereof for generating a bending moment therein to move a portion of the sleeve into intimate contact with the tube.