Strongback for remotely installing tie rod assembly in annulus below core spray piping in boiling water reactor

A strongback for lowering a tie rod into the downcomer annulus of a boiling water reactor during a shroud repair operation. The tie rod strongback is suspended from a cable via a cable adaptor at its upper end. The lower end of the strongback is coupled to a tie rod adaptor, which in turn couples to the top of the tie rod. The strongback is a welded assembly of square tubes, channels for reinforcing the joints of the welded tubes, and upper and lower couplings. In particular, the strongback has mutually parallel first and second rigid linear members which are disposed vertically when the strongback is suspended from a plumb cable. The second rigid linear member is connected to the first rigid linear member by a relatively obliquely disposed third rigid linear member. The first and second rigid linear members lie in a vertical plane which is offset from the axis of a plumb cable to allow the strongback assembly to circumvent the core spray downcomer piping when the tie rod/lower spring assembly is in its final position in the annulus. The first rigid linear member is further offset from the second rigid linear member cable axis to allow the strongback assembly to circumvent the feedwater sparger and the core spray header. This facilitates proper positioning of the bottom of the tie rod/lower spring assembly relative to the gusset plate to which the assembly will be anchored.

FIELD OF THE INVENTION 
This invention relates to tooling which is useful in installing hardware in 
a nuclear reactor. In particular, the invention relates to tooling for 
installing hardware for stabilizing the core shroud of a nuclear reactor 
to resist deflection in response to a seismic event and/or loss-of-coolant 
accident (LOCA). 
BACKGROUND OF THE INVENTION 
A conventional boiling water reactor (BWR) is shown in FIG. 1. Feedwater is 
admitted into a reactor pressure vessel 10 via a feedwater inlet 12 and a 
feedwater sparger 14, which is a ring-shaped pipe having suitable 
apertures for circumferentially distributing the feedwater inside the 
reactor pressure vessel (RPV). The feedwater from sparger 14 flows 
downwardly through the downcomer annulus 16, which is an annular region 
between RPV 10 and core shroud 18. In addition, a core spray inlet 11 
supplies water to a core spray sparger 13 (located inside the shroud 18) 
via core spray header 15, core spray downcomer piping 17 and core spray 
elbow 19 (which penetrates the shroud wall). The core spray header 15 has 
a circular section that occupies space directly underneath feedwater 
sparger 14. 
Core shroud 18 is a stainless steel cylinder surrounding the nuclear fuel 
core. The core is made up of a plurality of fuel bundle assemblies 22 
(only two 2.times.2 arrays of which are shown in FIG. 1). Each array of 
fuel bundle assemblies is supported at the top by a top guide 20 and at 
the bottom by a core plate 21. The core top guide 20 provides lateral 
support for the top of the fuel assemblies and maintains the correct fuel 
channel spacing to permit control rod insertion. 
The water flows through downcomer annulus 16 to the core lower plenum 24. 
The water subsequently enters the fuel assemblies 22, wherein a boiling 
boundary layer is established. A mixture of water and steam enters core 
upper plenum 26 under shroud head 28. Vertical standpipes 30 atop shroud 
head 28 are in fluid communication with core upper plenum 26. The 
steam-water mixture flows through standpipes 30 and enters steam 
separators 32, which are of the axial-flow centrifugal type. The separated 
liquid water then mixes with feedwater in the mixing plenum 33, which 
mixture then returns to the core via the downcomer annulus. The steam 
passes through steam dryers 34 and enters steam dome 36. The steam is 
conducted from the RPV via steam outlet 38. 
The BWR also includes a coolant recirculation system which provides the 
forced convection flow through the core necessary to attain the required 
power density. A portion of the water is pumped from the lower end of the 
downcomer annulus 16 via recirculation water outlet 42 and forced by a 
centrifugal recirculation pump (not shown) into jet pump assemblies 44 
(only one of which is shown) via recirculation water inlets 46. The BWR 
has two recirculation pumps, each of which provides the driving flow for a 
plurality of jet pump assemblies. The jet pump assemblies are 
circumferentially distributed around the core shroud 18. 
The core shroud 18 (shown in more detail in FIG. 2) in one type of BWR 
comprises a shroud head flange 18a for supporting the shroud head 28; a 
circular cylindrical upper shroud wall 18b having a top end welded to 
shroud head flange 18a; an annular top guide support ring 18c welded to 
the bottom end of upper shroud wall 18b; a circular cylindrical middle 
shroud wall comprising three sections 18d, 18e and 18f welded in series, 
with a top end of section 18d being welded to top guide support ring 18c; 
and an annular core plate support ring 18g welded to the bottom end of 
middle shroud wall section 18f and to the top end of a lower shroud wall 
18h. The entire shroud is supported by a shroud support 50, which is 
welded to the bottom of lower shroud wall 18h, and by annular shroud 
support plate 52, which is welded at its inner diameter to shroud support 
50 and at its outer diameter to RPV 10. 
In the event of a seismic disturbance, it is conceivable that the ground 
motion will be translated into lateral deflection relative to the reactor 
pressure vessel of those portions of the shroud located at elevations 
above shroud support plate 52. Such deflections would normally be limited 
by acceptably low stresses on the shroud and its weldments. However, if 
the shroud weld zones have failed due to stress corrosion cracking, there 
is the risk of misalignment and damage to the core and the control rod 
components, which would adversely affect control rod insertion and safe 
shutdown. 
Stress corrosion cracking in the heat affected zone of any shroud girth 
seam welds diminishes the structural integrity of shroud 18, which 
vertically and horizontally supports the core top guide 20 and the shroud 
head 28. In particular, a cracked shroud increases the risks posed by a 
loss-of-coolant accident (LOCA). During a LOCA, the loss of coolant from 
RPV 10 produces a loss of pressure above the shroud head 28 and an 
increase in pressure inside the shroud 18, i.e., underneath shroud head 
28. The result is an increased lifting force on shroud head 28 and on the 
upper portions of the shroud to which the shroud head is bolted. If the 
core shroud has fully cracked girth welds, the lifting forces produced 
during a LOCA could cause the shroud to separate along the areas of 
cracking, producing undesirable leaking of reactor coolant. 
A known repair method for vertically restraining a weakened core shroud 
utilizes tensioned tie rods 54 coupled to the shroud flange 18a and to the 
shroud support plate 52, as seen in FIG. 2. The lower end of the tie 
rod/lower spring assembly hooks underneath a clevis pin 60 inserted in a 
hole machined into gusset plate 58, which plate is in turn welded to 
shroud support plate 52 and RPV 10. In addition, the shroud 18 is 
restrained laterally by installation of wishbone springs 56a/56b and 72, 
which are components of the shroud repair assembly. 
Referring to FIG. 2, the shroud restraint tie rod/lower spring assembly 
comprises a tie rod 54 having a circular cross section. A lower end of tie 
rod 54 is anchored in a threaded bore formed in the end of a spring arm 
56a of a lower spring 56. Tie rod 54 extends from the end of spring arm 
56a to a position adjacent the outer circumferential surface of the top 
guide support ring 18c. The upper end of tie rod 54 has a threaded 
portion. 
The lower spring 56 is anchored to a gusset plate 58 attached to the shroud 
support plate 52. The lower spring 56 has a slotted end which straddles 
gusset plate 58 and forms a clevis hook 56c. The clevis hooks under 
opposite ends of a clevis pin 60 inserted through a hole machined in the 
gusset plate 58. Engagement of the slotted end with the gusset plate 58 
maintains alignment of lower spring 56 under the action of seismic motion 
of the shroud, which may be oblique to the spring's radial orientation. 
The tie rod 54 is supported at its top end by an upper support assembly 62 
which hangs on the shroud flange 18a. A pair of notches or slots are 
machined in the shroud head ring 28a of shroud head 28. The notches are 
positioned in alignment with a pair of bolted upper support plate segments 
64 of upper support assembly 62 when the shroud head 28 is properly seated 
on the top surface of shroud flange 18a. These notches facilitate coupling 
of the tie rod/lower spring assembly to the shroud flange. 
The pair of notches at each tie rod azimuthal position receive respective 
hook portions 64a of the upper support plates 64. Each hook 64a conforms 
to the shape of the top surface of shroud flange 18a and the shape of the 
steam dam 29. The distal end of hook 64a hooks on the inner circumference 
of shroud dam 29. 
The upper support plates 64 are connected in parallel by a top support 
bracket (not shown) and a support block 66 which forms the anchor point 
for the top of the tie rod. Support block 66 has an unthreaded bore, 
tapered at both ends, which receives the upper end of tie rod 54. After 
the upper end of tie rod 54 is passed through the bore, a threaded 
tensioning nut 70 is screwed onto the upper threaded portion 54a (see FIG. 
4) of tie rod 54. 
As seen in FIG. 2, the assembly comprised of support plates 64 with hooks 
64a, support block 66, tie rod 54, lower spring 56, clevis pin 60 and 
gusset plate 58 form a vertical load path by which the shroud flange 18a 
is connected to the shroud support plate 52. In the tensioned state, the 
upper support plates 64 exert a restraining force on the top surface of 
shroud flange 18a which opposes separation of the shroud 18 at any assumed 
failed circumferential weld location. 
Lateral restraint at the elevation of the top guide support ring 18c is 
provided by an upper spring 72 having a double cantilever "wishbone" 
design. The end of the radially outer arm of upper spring 72 has an upper 
contact spacer 74 rotatably mounted thereon which bears against the inner 
surface of the wall of RPV 10. 
A spring arm 56a of lower spring 56 laterally supports the shroud 18 at the 
core plate support ring 18g, against the vessel 10, via a lower contact 
spacer 76. The top end of spring arm 56a has a threaded bore to provide 
the attachment for the threaded bottom end 54b (see FIG. 4) of tie rod 54. 
The member 56d connecting the upper wishbone spring 56a, 56b to clevis 
hook 56c is offset from the line of action between the lower end of tie 
rod 54 and clevis pin 60 to provide a vertical spring compliance in the 
load path to the tie rod. 
A middle support 80 is preloaded against the vessel wall at assembly by 
radial interference which bends the tie rod 54, thereby providing improved 
resistance to vibratory excitation failure of the tie rod. The middle 
support also provides a lateral motion limit stop for the shroud central 
shell, in the event of complete failure of its girth welds. To facilitate 
mounting of the middle support 80, a mid-support ring 82 is secured to the 
tie rod 54, as shown in FIG. 4. The middle support 80 has a section of an 
annular recess counterbored in its bottom which form fits on ring 82, 
thereby preventing lateral shifting of middle support 80 relative to tie 
rod 54. The middle support 80 is latched to midsupport ring 82 by a 
wishbone spring latch (not shown), which blocks upward vertical 
displacement of middle support 80 relative to tie rod 54. 
During installation of the shroud repair hardware shown in FIG. 2, the tie 
rod/lower spring assembly comprising tie rod 54 screwed into lower spring 
56 is suspended from a cable and lowered into the annulus to the desired 
elevation. Only after clevis hook 56c has been hooked under clevis pin 60 
and the tie rod/lower spring assembly has been braced in the hooked 
position will the upper support assembly 62 be installed, followed by 
upper spring 72. 
As the cable is lowered, the tie rod/lower spring assembly must be guided 
into the narrow space between adjacent jet pump assemblies. However, in 
some BWRs this installation site lies below the feedwater sparger, core 
spray header and core spray downcomer piping, which lie in the path of a 
descending tie rod suspended from an overhead crane. To protect the 
feedwater sparger and core spray header from damage due to impact by the 
descending tie rod/lower spring assembly, which weighs in excess of 1,000 
pounds, a cover is hooked onto the feedwater sparger to deflect the tie 
rod away from the feedwater sparger and core spray header. However, the 
cover obstructs the cable so that the tie rod/lower spring assembly does 
not hang plumb from the crane. This makes it difficult to maneuver a 
suspended tie rod/lower spring assembly into the correct position in the 
downcomer annulus. In particular, unless appropriate steps are taken, the 
cover will obstruct the taut cable from becoming oriented vertical and 
limit radially outward movement of the cable at the point of contact and 
tie rod/lower spring assembly suspended therefrom. Also the friction 
between the taut cable and the cover impedes tangential movement of the 
suspended tie rod/lower spring assembly. As a result, the azimuthal and 
radial positions of the tie rod/lower spring assembly cannot be controlled 
by moving the crane to a corresponding position overhead, preventing 
placement of the suspended tie rod/lower spring assembly at the precise 
position required for coupling to the gusset plate. 
SUMMARY OF THE INVENTION 
The present invention is a strongback for lowering a tie rod into the 
downcomer annulus of a boiling water reactor during a shroud repair 
operation. The tie rod strongback is suspended from a cable via a cable 
adaptor at its upper end. The lower end of the strongback is coupled to a 
tie rod adaptor, which in turn couples to the top of the tie rod. The 
strongback is designed to circumvent the piping obstructions so that the 
tie rod/lower spring assembly is freely suspended from the end of the 
cable and the cable remains plumb. 
In accordance with the preferred embodiment of the invention, the upper 
coupling of the strongback is an apertured plate which can be attached to 
an apertured clevis of the cable adaptor by means of a first clevis pin, 
and the lower coupling of the strongback is an apertured clevis which can 
be attached to an apertured plate of the tie rod adaptor by means of a 
second clevis pin. The first and second clevis pins are preferable 
mutually parallel. In this case, a line perpendicular to the clevis pins 
and intersecting the axes of both clevis pins defines a reference axis, 
which will be disposed generally collinear with the cable when the cable 
is plumb and the strongback is suspended from the end of the cable. In 
other words, when the strongback is freely suspended from the end of a 
cable which is plumb, the reference axis will be vertical. 
In accordance with the preferred embodiment of the invention, the 
strongback is a welded assembly comprising: a plurality of rigid tubes, 
each tube having a square cross section; a plurality of channels for 
reinforcing the joints of welded tubes; and the aforementioned upper and 
lower couplings. In particular, the strongback in accordance with the 
preferred embodiment comprises mutually parallel first and second rigid 
linear members which are disposed parallel to the reference axis. The top 
of the second rigid linear member is connected to the bottom of the first 
rigid linear member by a relatively obliquely disposed third rigid linear 
member. The first, second and third rigid linear members lie in a vertical 
plane which is offset from the reference axis, to allow the strongback to 
circumvent the core spray downcomer piping. In addition, the first and 
second rigid linear members are offset from each other to allow the 
strongback to circumvent the feedwater sparger and the core spray header. 
The strongback must have a height sufficient to span the distance between 
a point above the feedwater sparger to a point below the core spray elbow, 
thereby allowing a shorter cable to be used. Because the cable ends at a 
point above the piping obstructions and the strongback circumvents the 
piping obstructions, the tie rod/lower spring assembly can be freely 
suspended from the cable without the cable or the intermediate supporting 
hardware bearing against the piping. Thus, the cable stays plumb and the 
position of the bottom of the tie rod/lower spring assembly relative to 
the gusset plate, to which the assembly will be anchored, can be freely 
adjusted by displacing the cable when the tie rod/lower spring assembly 
reaches its final elevation in the annulus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
During the installation procedure, the tie rod/lower spring assembly (items 
54 and 56 in FIG. 2) is lowered into the downcomer annulus 16. This is 
accomplished using a crane (not shown) on the refueling floor of the 
reactor. First, the tie rod/lower spring assembly must be raised from 
horizontal position on the refueling floor to a vertical position 
suspended from the end of the crane cable. This is accomplished by means 
of a tie rod adaptor which couples the upper end of the tie rod to the end 
of the cable. When the cable is wound, the upper end of the tie rod is 
lifted off the refueling floor into an upright position with all of the 
weight of the tie rod being supported by the cable. The tie rod/lower 
spring assembly can then be lowered into the annulus by unwinding the 
cable. 
Referring to FIGS. 3A and 3B, when vertical access to the downcomer annulus 
16 is limited by internal reactor structures such as the feedwater sparger 
14 and core spray header 15, the tie rod adaptor 100 is coupled to the end 
of the cable 84 via a rigid frame or strongback 90 specially designed, in 
accordance with the present invention, to bypass the obstruction. 
Maneuvering of the tie rod/lower spring assembly must be done with extreme 
care to avoid damaging reactor hardware such as the jet pump sensing 
lines. 
Referring to FIGS. 5A and 5B, the tie rod adaptor 100 comprises a frame 102 
having a hole 104 for receiving a conventional coupling mechanism, such as 
a clevis pin, which must be strong enough to bear the entire weight of the 
tie rod/lower spring assembly. A circular cylindrical shield 106 for 
protecting the threads of the tie rod is connected to the frame 102 by 
means of a mounting plate 108. 
The frame 102 has an axial recess 114 shaped for receiving the upper end of 
the tie rod, and a pair of circular cylindrical holes 116a and 116b which 
communicate with axial recess 114. Each hole 116a and 116b has a 
respective bushing 118a and 118b in which a respective locking pin 120a 
and 120b is slidably mounted. Each locking pin is slidable from a first 
position whereat the locking pin does not interfere with axial recess 114 
to a second position whereat the locking pin interferes with axial recess 
114, as seen in FIG. 5B. Each locking pin 120a, 120b slides from the 
interfering position to the non-interfering position in response to 
actuation of a respective pneumatic cylinder 122a, 122b. The piston of 
pneumatic cylinder 122a is connected to a reduced-diameter end of locking 
pin 120a; the piston of pneumatic cylinder 122b is connected to a 
reduced-diameter end of locking pin 120b. As best seen in FIG. 5B, each 
cylinder is protected against damage by a respective U-shaped cylinder 
shield 126a, 126b attached to frame 102 via screws. 
Each locking pin 120a and 120b is disposed radially relative to the axis of 
the tie rod and is configured to fit with little play inside a respective 
one of circular cylindrical radial holes 58a and 58b formed in the topmost 
portion of the tie rod upper end, as shown in FIG. 4, and inside a 
respective one of the bushings 118a and 118b. The front end of each 
locking pin is chamfered to facilitate entry of the locking pin into the 
radial holes 58a and 58b. In the preferred embodiment, the holes 58a and 
58b are mutually perpendicular, as are the locking pins 120a and 120b. 
Each locking pin is capable of supporting the entire weight of the tie 
rod, which is in excess of 1,000 pounds. 
Each pneumatic cylinder is connected to a separate source of pressurized 
fluid via a respective pneumatic line (not shown). Each piston is 
retracted when pressurized fluid, e.g., air, is supplied to the cylinder 
and extended when the supply of pressurized fluid is cut off. When the 
pistons are extended, they interlock the adaptor to the tie rod via 
locking pins 120a and 120b which extend into tie rod holes 58a and 58b 
(see FIG. 4) respectively. Each cylinder has a spring return which urges 
the locking pins to engage tie rod holes 58a and 58b when pneumatic 
pressure is discontinued. As a safeguard to prevent dropping the tie rod 
into the annulus, each locking pin is latched in the locking position by a 
respective latch 128. The exposed end of each latch shaft is integrally 
joined with a respective eyebolt 124a and 124b. The tie rod cannot be 
disengaged from the lifting apparatus until each latch 128 has been 
manually unlatched by an operator using a handling pole to lift the 
eyebolts. Then pressurized fluid can be supplied to disengage the locking 
pins 120a and 120b from the holes in the tie rod. When both locking pins 
are retracted, the tie rod lifting apparatus can be disengaged from the 
tie rod and removed from the annulus. 
The hole 104 of tie rod adaptor 100 is coupled by a first clevis pin (not 
shown) to an apertured clevis 90a (see FIGS. 6A and 6B) which forms the 
lower end of the strongback 90. The upper end of strongback 90, in turn, 
has an apertured clevis 90h which is coupled by a second clevis pin (also 
not shown) to a cable 84 by a cable adaptor 86 (see FIGS. 3A and 3B). The 
strongback must have a height sufficient to span the distance between a 
point above the feedwater sparget 14 to a point below the core spray elbow 
19, thereby allowing a shorter cable to be used. Because the cable ends at 
a point above and the strongback circumvents the piping obstructions, the 
tie rod/lower spring assembly 54/56 can be freely suspended without the 
supporting hardware or cable bearing against the piping. Thus, the cable 
stays plumb and the position of the tie rod/lower spring assembly relative 
to the gusset plate 58 can be freely adjusted by displacing the cable 
adaptor, e.g., by displacing the crane or by exerting a lateral force on 
the cable. 
To circumvent the piping obstructions, the strongback 90 is designed to 
have a first rigid linear member 90c which is parallel to and offset from 
the reference axis A (see FIG. 6A). Strongback 90 further comprises a 
second rigid linear member 90e which is also parallel to and offset from 
the reference axis A. The rigid linear members 90c and 90e are mutually 
parallel and define a midsection plane. The bottom end of rigid linear 
member 90e is connected by a welded joint to the top end of an oblique 
rigid linear member 90d; the top end of rigid linear member 90c is 
connected by a welded joint to the bottom end of oblique rigid linear 
member 90d. Similarly, the bottom end of rigid linear member 90c is 
connected by a welded joint to the top end of an oblique rigid linear 
member 90b. The bottom end of rigid linear member 90b is joined to or 
integrally formed with the lower clevis 90a; the top end of rigid linear 
member 90e is connected by a welded joint to the bottom end of an oblique 
rigid linear member 90f. The top end of oblique rigid linear member 90f is 
in turn connected by a welded joint to the bottom end of a rigid linear 
member 90g which is coaxial with reference axis A. The top end of rigid 
linear member 90g is joined to or integrally formed with the upper clevis 
90h. Preferably, each rigid linear member is a tube having a square cross 
section. Each of the welded joints connecting an oblique rigid linear 
member to a vertical rigid linear member is reinforced by a respective 
channel welded to both rigid linear members and spanning the welded joint. 
These reinforcing ribs bear the designations 90i-90m in FIGS. 6A and 6B. 
Finally, a coupling 90n is attached to oblique tube 90f such that the axis 
of a hexagonal socket in the head of the coupling is generally vertical 
and accessible from above by a tool which can be manipulated remotely to 
cause the strongback 90 to rotate about reference axis A during 
positioning of the tie rod/lower spring assembly relative to the gusset 
plate. 
The preferred embodiment of the strongback in accordance with the present 
invention has been disclosed for the purpose of illustration. Variations 
and modifications of the disclosed structure which fall within the concept 
of this invention will be readily apparent to persons skilled in the art 
of tooling design. For example, it will be apparent that not all tubes of 
the welded strongback assembly need to be straight. Nor does the tube 
cross section need to be square. All such variations and modifications are 
intended to be encompassed by the claims set forth hereinafter.