Device for gripping and detaching a top nozzle subassembly from a reconstitutable fuel assembly

A device for gripping and detaching a top nozzle subassembly from the upper end portions of the guide thimbles in a reconstitutable fuel assembly includes a central spider assembly disposable in overlying relation to an upper hold-down plate of the top nozzle subassembly, locating lugs arranged for insertion into coolant flow openings in the hold-down plate, collars interconnected to the spider and being disposed on the locating lugs and bearing on the hold-down plate when the locating lugs are inserted in the flow openings, and elongated studs received through and rotatable within respective central bores defined in the locating lugs. Upon rotation of the studs in one direction, compression of the hold-down springs disposed between the plates relieves the tension on the guide thimble upper end portions whereby the upper end portions are thereby prepared to be severed at locations immediately below the upper limit. After severing, rotation of the studs in the opposite direction allows the hold-down plate to be released from the unsevered guide thimble upper end portions at the upper limit. In case the hold-down plate does not release, positive release is obtained through the use of an impact-transmitting rod which is mounted in the central spider and attachable at its lower end to the hold-down plate.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to fuel assemblies for nuclear 
reactors and, more particularly, is concerned with a device and method for 
gripping and detaching a top nozzle as an assembled unit from a 
reconstitutable fuel assembly. 
2. Description of the Prior Art 
In most nuclear reactors, the reactor core is comprised of a large number 
of elongated fuel assemblies which receive support and alignment from 
upper and lower transversely extending core support plates. Conventional 
designs of these fuel assemblies include a plurality of fuel rods and 
control rod guide thimbles held in an organized array by a plurality of 
grids spaced along the fuel assembly length and attached to the control 
rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel 
assembly are secured to the guide thimbles to thereby form an integral 
fuel assembly. The guide thimbles extend slightly above and below the ends 
of the fuel rods. Additionally, fuel assemblies have also employed a 
hold-down device to prevent the force of upward coolant flow from lifting 
the fuel assembly into damaging contact with the upper core support plate, 
while allowing for changes in fuel assembly length due to core induced 
thermal expansion and the like. One such hold-down device is illustrated 
and described in the first application cross-referenced above. 
During operation of such fuel assembly in a nuclear reactor, a few of the 
fuel rods may occasionally develop cracks along their lengths resulting 
primarily from internal stresses, thus establishing the possibility that 
fission products having radioactive characteristics may seep or otherwise 
pass into the primary coolant of the reactor. Such products may also be 
released into a flooded reactor cavity during refueling operations or into 
the coolant circulated through pools where the spent fuel assemblies are 
stored. Since the fuel rods are part of the integral assembly of the guide 
thimbles attached to the top and bottom nozzles, it is difficult to detect 
and remove the failed rods. 
In view of the high costs associated with replacing fuel assemblies, 
considerable interest has arisen in reconstitutable fuel assemblies in 
order to minimize operating and maintenance expenses. The general approach 
to making a fuel assembly reconstitutable is to provide it with a 
removable top nozzle. One fuel assembly construction adapted to be 
reconstitutable is the fuel assembly with the hold-down device as 
disclosed in the first application cross-reference above. The method of 
reconstituting this fuel assembly is illustrated and described in the 
third application cross-referenced above. 
The top nozzle subassembly of the fuel assembly disclosed in the first 
cross-referenced application, which includes the hold-down device as an 
integral part thereof, is basically made up of a coil spring disposed 
about the upper end of each guide thimble and sandwiched between a lower 
adapter plate and an upper hold-down plate. The lower adapter plate is 
slidably mounted on the guide thimble and its downward movement is limited 
by a lower retainer attached on the guide thimble. The upper hold-down 
plate has a passageway matched with each guide thimble with an internal 
ledge defined in the lower portion of the passageway. The guide thimble is 
received in the passageway so as to mount the hold-down plate on the 
thimble for slidable movement therealong. An upper retainer is attached on 
the upper end of the guide thimble and disposed within the passageway of 
the hold-down plate. The upper retainer cooperates with the internal ledge 
of the passageway to limit the upward movement of the hold-down plate 
along the guide thimble. The construction is such that the upper end of 
the guide thimble, along with the upper retainer, are permitted to 
reciprocate within the passageway of the hold-down plate, thus allowing 
for thermal growth of the guide thimble. The coil springs are held in a 
state of compression between the lower adapter plate and the upper 
hold-down plate. 
The method for reconstituting the top nozzle subassembly without its 
hold-down device coming apart, as disclosed in the third cross-referenced 
application, includes the following operative steps. First, a force is 
applied to the upper hold-down plate to move it downwardly which 
compresses the springs. Second, with the force being maintained on the 
hold-down plate, an in-tube cutter is used to sever the upper end of each 
guide thimble at an axial location below where the thimble is bulge fitted 
or connected to the upper retainer. Third, after severing is completed, a 
pair of container fingers are applied to the top nozzle so as to extend 
above the upper surface of the hold-down plate and below the lower surface 
of the adapter plate. Then, the force is slowly removed allowing the 
springs to expand and push the hold-down plate upwardly into abutting 
contact with the upper ends of the fingers. Finally, with the container 
fingers in place, the top nozzle subassembly is lifted, generally 
vertically, upwardly and off the severed guide thimbles as a unit. The 
severed upper ends of the thimbles and the upper retainers are carried 
away in the passageways of the hold-down plate of the removed top nozzle 
subassembly. By reversing the above steps, the top nozzle subassembly can 
be remounted as a unit back on the guide thimbles, except that now instead 
of the severing step, new upper retainers similar to the old ones are 
inserted into the passageways and attached to the upper ends of the guide 
thimbles. 
Notwithstanding the generally acceptable performance of the above-described 
top nozzle subassembly and method of removing the same as a unit during 
reconstitution of the fuel assembly, certain problems point to the need 
for an improved device for removing the top nozzle. First, the prior 
method of removing the top nozzle fails to provide a mechanism to overcome 
the slight interference fit formed between the outside surfaces of the 
guide thimble upper ends and the inside surfaces of the upper retainers 
due to the bulging operation for attaching the retainers to the guide 
thimbles. In other words, some means is necessary to apply a force on the 
retainers to pull them free of the portions of the guide thimbles 
immediately below the location at which the thimble upper ends were 
severed. Second, the prior method fails to provide a single device capable 
of imposing the required force on the hold-down plate, holding the force 
so as to keep all of the hold down springs in a compressed state, and then 
gradually releasing the force. Third, the prior method does not provide a 
single device useful to both grip the top nozzle subassembly and remove it 
from the fuel assembly. In summary, a need exists for a single device 
which would provide all of the above-described functions in removing the 
top nozzle subassembly from the reconstitutable fuel assembly. 
SUMMARY OF THE INVENTION 
The present invention provides a device capable of gripping and detaching 
the top nozzle subassembly from the reconstitutable fuel assembly in a 
manner designed to satisfy the aforementioned needs. The device overcomes 
all of the shortcomings cited in connection with the prior method. First, 
the device is capable of pressing the hold-down plate downwardly and 
holding a level of force necessary to depress all of the hold-down springs 
until all of the guide thimbles are severed. Then, the device can be 
adjusted to allow the springs to gradually expand to their free length in 
a controlled manner. Second, the device incorporates a mechanism which 
ensures that all of the upper retainers can be easily removed from the 
guide thimbles after severance and substantially at the same instance. 
Third, the device is adapted to grip the top nozzle subassembly and remove 
it from the fuel assembly as a unit. Finally, the device is relatively 
easy to manipulate remotely and takes advantages of pre-existing features 
in the top nozzle hold-down plate and adapter plate to achieve its 
capabilities. 
Accordingly, the present invention sets forth for use with a 
reconstitutable fuel asssembly including a top nozzle subsassembly and a 
plurality of guide thimbles, wherein the top nozzle subassembly has a 
lower adapter plate, a plurality of hold-down springs and an upper 
hold-down plate with a plurality of coolant flow holes defined 
therethrough and wherein the guide thimbles have upper end portions 
slidably mounting the lower adapter plate and upper hold-down plate for 
movement therealong between lower and upper limits, a device and method 
for gripping and detaching the top nozzle subassembly from the guide 
thimble upper end portions. 
The device for gripping and detaching the top nozzle subassembly from the 
guide thimble upper end portions includes: (A) a central spider disposable 
in overlying relation to the upper hold-down plate; (b) a plurality of 
locating lugs disposed radially outwardly from the spider and arranged for 
alignment with and insertion into the plurality of coolant flow openings 
in the upper hold-down plate, each of the locating lugs having an 
elongated central bore; (c) a plurality of collars interconnected to the 
spider, each collar connected to one of the locating lugs and bearing on 
the hold-down plate when the locating lug is inserted in its respective 
flow opening; and (d) a plurality of elongated members received through 
and rotatable within the respective central bores of the locating lugs, 
each member being couplible with the lower adapter plate such that upon 
rotation in one direction the lower adapter plate and upper hold-down 
plate are moved toward one another which increases the compression of the 
hold-down springs and concurrently relieves the tension of the guide 
thimble upper end portions at the upper limit thereof in preparation for 
severing of the guide thimble upper end portions at locations immediately 
below the upper limit, whereas upon rotation in an opposite direction the 
compression of the hold-down springs is decreased which causes the lower 
adapter plate and upper hold-down plate to move away from one another and 
allows the hold-down plate to release from the unsevered guide thimble 
upper end portions at the upper limit. 
More particularly, the central spider includes an elongated central hub 
having upper and lower ends, a plurality of spokes having inner and outer 
ends with each spoke being connected at its inner end to the lower end of 
the central hub, extending radially outwardly therefrom and connected at 
its outer end to one of the collars, and a handle connected to the upper 
end of the central hub. 
Further, the gripping and detaching device includes an impact-transmitting 
rod mounted in the central spider hub for movement toward and away from 
the upper hold-down plate with the rod being attachable at its lower end 
to the hold-down plate, and means carried on the rod being actuatable to 
deliver an upwardly directed driving force on the upper hold-down plate 
via the rod in order to cause positive release of the hold-down plate from 
the unsevered guide thimble upper end portions. More specifically, the 
actuatable means includes an impact generating slug having a central bore 
through which the impact-transmitting rod extends for mounting the slug 
for slidable movement along the rod, and an impact-receiving anvil fixed 
on the impact transmitting rod across the path of slidable movement of the 
slug along the rod. 
The method for gripping and detaching the top nozzle subassembly from the 
guide thimble upper end portions includes the operative steps of: (a) 
inserting a plurality of interconnected locating lugs into a plurality of 
coolant flow openings in the upper hold-down plate until upper collars on 
the lugs overlie and bear on the hold-down plate; (b) rotating a plurality 
of elongated studs, being received through respective central bores of the 
locating lugs and coupled with the lower adapter plate, in one 
predetermined direction for causing movement of the lower adapter plate 
and upper hold-down plate toward one another which increases the 
compression of the hold-down springs and concurrently relieves the tension 
on the guide thimble upper end portions at the upper limit thereof in 
preparation for severing of the guide thimble upper end portions at 
locations immediately below the upper limit; and (c) after severing of the 
guide thimble upper end portions, rotating the studs in an opposite 
predetermined direction for decreasing the compression of the hold-down 
springs which causes the movement of the lower adapter plate and upper 
hold-down plate away from one another and allows the release of the 
hold-down plate from the unsevered guide thimble upper end portions at the 
upper limit. Further, the gripping and detaching method, in case that the 
upper hold-down plate fails to release from the unsevered guide thimble 
upper end portions, includes the operative step of delivering an upwardly 
directed driving force on the upper hold-down plate in order to cause 
positive release of the hold-down plate from the unsevered guide thimble 
upper end portions.

DETAILED DESCRIPTION OF THE INVENTION 
In the following description, like reference characters designate like or 
corresponding parts throughout the several views. Also, in the following 
description, it is to be understood that such terms as "forward", 
"rearward", "left", "right", "upwardly", "downwardly", and the like, are 
words of convenience and are not to be construed as limiting terms. 
IN GENERAL 
Referring now to the drawings, and particularly to FIG. 1, there is shown a 
reconstitutable fuel assembly, being generally designated by the numeral 
10, on which a top nozzle subassembly gripping and detaching device, 
generally indicated as 12 in FIGS. 4 through 6, can be employed. 
Basically, the fuel assembly 10 includes a lower end structure or bottom 
nozzle 14 for supporting the assembly on the lower core plate (not shown) 
in the core region of a reactor (not shown) and a number of longitudinally 
extending control rod guide tubes or thimbles 16 projecting upwardly from 
the bottom nozzle 14. Further, an organized array of fuel rods 18 are held 
in spaced relationship to one another by a number of transverse grids 20 
spaced along the fuel assembly length and attached to the guide thimbles 
16. An instrumentation tube 22 is located at the center of the fuel 
assembly 10, while attached to the upper ends of the guide thimbles 16 is 
an end structure or top nozzle 24. The guide thimbles 16 and bottom and 
top nozzles 14,24 together form an integral assembly capable of being 
conventionally handled without damaging the assembly parts. 
To form the fuel asembly 10, the transverse grids 20 are attached to the 
longitudinally guide thimbles 16 at predetermined axially locations and 
the fuel rods 18 are inserted from below through the grids 20. The lower 
nozzle 14 is then suitably attached, such as by machine screws 26, to the 
lower ends of the guide thimbles. Finally, the top nozzle 24 is attached 
to the upper end portions 28 of the guide thimbles 16. to control the 
fission process, a number of control rods (not shown) are reciprocally 
movable in the control rod guide thimbles 16 of the fuel assembly 10. 
While the top nozzle 24 is in the form of a subassembly illustrated and 
described in detail in the first application cross-referenced above, it 
will be described herein to the extent necessary to facilitate a complete 
and thorough understanding of the top nozzle gripping and detaching device 
12 of the present invention. 
TOP NOZZLE SUBASSEMBLY WITH HOLD-DOWN MEANS 
Referring now to FIGS. 1 through 3, the top nozzle subassembly 24 basically 
includes a lower adapter plate 30 and an upper hold-down plate 32. The 
lower adapter plate 30, as seen in FIG. 3, is formed of a plurality of 
cross-laced ligaments or bars 34 defining coolant flow openings 36 and is 
provided with a number of through holes 38 corresponding in number to the 
number of guide thimble upper end portions 28. The bars 34 overlie the 
upper ends of the fuel rods 18 such that the fuel rods are axially 
restrained or captured between the bottom and top nozzles 14,24. The holes 
38 are of sufficient dimensional size and are in a pattern matched with 
the arrangement of the guide thimbles 16 such that the adapter plate 30 
can be slidably mounted on the guide thimble upper end portions 28. 
Downward movement of the adapter plate 30 is limited by a plurality of 
lower ring-shaped retainers 40 mounted to the respective guide thimbles 16 
at a location axially spaced above the upper ends of the fuel rods 18. 
The upper hold-down plate 32 is also slidably mounted on the guide thimble 
upper end portions 28, via a plurality of respective passageways 42, with 
each of the passageways having an internal ledge 44 defined therein in a 
lower portion of the passageway. Each passageway 44 has a generally 
constant larger diameter upper segment and a generally constant diameter 
lower segment, with the latter defining the internal ledge 44 and being 
slidably engageable with the guide thimble upper end portions 28. As seen 
in FIG. 2, the hold-down plate 32 is also provided with a number of 
relatively large coolant flow openings 46 and several smaller coolant flow 
openings 48 positioned at predetermined locations about the plate 32. 
Axially spaced above the lower retainer 40 is another, upper retainer 50 
mounted on each guide thimble upper end portion 28 and disposed within the 
upper segment of the respective passageway 42. The upper retainer 50 is in 
the form of a collar having an internal annular groove 52 in the upper 
portion thereof and is attached to an upper end 54 of the guide thimble 16 
by circumferentially bulging the wall of the guide thimble upper end 
portion 28 into the internal groove 52 of the upper retainer 50. The 
attached upper retainers 50 cooperate with the internal ledges 44 to limit 
the upward movement of the hold-down plate 32 along the guide thimble 
upper end portion 28. 
The top nozzle subassembly 24 further includes a coil spring 56 disposed on 
each of the guide thimble upper end portions 28 and interposed between the 
lower surface of the upper hold-down plate 32 and the upper surface of the 
lower adapter plate 30. Preferably, the coil springs 56 are held in 
compression between the upper hold-down plate 32 and the lower adapter 
plate 30 to preload the fuel assembly 10. For proper alignment of the coil 
springs 56 and to prevent them from contacting their respective guide 
thimbles 16, a spring seat 58 is provided on the lower end of each of the 
coil springs 56, setting in a recess formed in the upper surface of the 
lower adapter plate 30. In defining an enclosure for channeling the 
coolant flow upwardly at the upper region of the fuel assembly 10, 
upstanding sidewalls 60 are formed on the perpheral edges of the lower 
adapter plate 30. The upper hold-down plate 32 is slidable along the guide 
thimble upper end portions 28 within the enclosure sidewalls 60. 
TOP NOZZLE GRIPPING AND DETACHING DEVICE 
Now turning to FIGS. 4 through 6, there is shown the device, generally 
designated 12, for gripping and detaching the top nozzle 24 as a 
subassembly from the guide thimble upper end portions 28 of the 
reconstitutable fuel assembly 10. The device 12 utilizes a plurality of 
internally-threaded anchors 62 (FIGS. 2 and 3) defined in the lower 
adapter plate 30 between the coolant flow openings 36 and guide thimble 
holes 38, and a plurality of large coolant flow openings 46 defined in the 
upper hold-down plate 32 between its passageways 42 and vertically aligned 
with the anchors 62. 
The gripping and attaching device 12 basically includes a central spider 
64, a plurality of locking lugs 66 disposed radially outwardly from the 
spider 64, a plurality of collars 68 attached on the locating lugs 66 and 
interconnected to the central spider 64, and a plurality of elongated 
spring compression studs 70 associated with the locating lugs 66. The 
central spider 64, which during use (as seen in FIGS. 5 and 6) is disposed 
in overylying relation the upper hold-down plate 32, includes an elongated 
central hub 72 having upper and lower ends 74,76 (FIG. 4), a plurality of 
spokes 78 being connected at their inner ends to the lower end 76 of the 
central hub 72, extending radially outwardly therefrom, and being 
connected at their outer ends to the collars 68, and a handle 80 connected 
to the upper end 74 of the central hub 72 for use in handling the device 
12. 
The locating lug 66 are arranged and sized for alignment with and insertion 
into the large coolant flow openings 46 in the upper hold-down plate 32. 
Also, each of the locating lugs 66 and its upper collar 68 have elongated 
central bores 82,84 aligned with one another and with one of the anchors 
62 in the lower adapter plate 30 when the lug 66 is inserted into its 
respective flow opening 46. As clearly seen in FIGS. 5 and 6, each collar 
68 overlying one of the locating lugs 66 has a diameter larger than its 
respective lug 66 and the flow opening 46 into which the lug is inserted 
so that the collar will overlie the flow opening and bear on the upper 
surface 86 of the hold-down plate 32 when its lug is inserted in its 
respective flow opening. 
The elongated spring compression studs 70 of the gripping and detaching 
device 12 are received through and rotatable within the respective aligned 
central bores 82,84 of the locating lugs 66 and collars 68. Each stud 70 
has a socket element 88 on the upper end thereof for grasping to rotate 
the stud. The socket element 88 overlies and bears against the respective 
collar 68 of the one locating lug 66 through which the stud 70 is mounted. 
Each stud 70 also has an externally-threaded segment 90 defined at its 
lower end which adapts the stud for threaded engagement with one of the 
internally-threaded anchors 62 in the lower adapter plate 30. 
When the spider 64 is installed on the top nozzle 24 with its locating lugs 
66 inserted into respective large coolant flow openings 46 in the upper 
hold-down plate 32, rotation of each of the studs 70 in one predetermined 
direction will thread its threaded end segment 90 into one anchor 62 and 
when the socket element 88 bears on the upper surface 86 of the hold-down 
plate 32 continued threading of the stud into the adapter plate will cause 
movement of the adapter plate and upper hold-down plate toward one 
another. Such movement of the plates 30,32 increases the compression of 
the hold-down springs 56 disposed therebetween and concurrently relieves 
the tension imposed on the upper retainers 50 via the ledges 44 within the 
hold-down plate passageways 42. With tension on the upper retainers 50 so 
relieved, severing of the guide thimble upper end portions at locations 
immediately below the upper retainers 50 can now take place, for example, 
via the method illustrated and described in the third application 
cross-referenced above. 
Once severing of the guide thimbles 16 is completed, rotation of the studs 
70 in the predetermined opposite direction results in unthreading of each 
stud from its anchor 62 and thereby decreases the compression of the 
hold-down springs 56 which then cause the lower adapter plate 30 and upper 
hold-down plate 32 to move away from one another. Upon such movement, the 
ledges 44 of the hold-down plate 32 will engage the severed upper 
retainers 50 and ordinarily cause them to release from the remaining 
unsevered guide thimble upper end portions 28. 
However, if the interference fit between the upper retainers 50 and the 
upper tips of the guide thimbles just below the location of severing is 
tight enough, the upper retainers may not release and a more positive way 
is needed to secure their removal. To satisfy this need, the device 12 
additionally includes an impact-transmitting rod 92 mounted in the central 
spider hub 72 for movement toward and away from the upper hold-down plate 
32. The rod 92 is externally-threaded at its lower end 94 for securement 
in an internally-threaded blind hole 96 in the upper surface 86 of the 
hold-down plate. Means carried on the rod 92 in the form of an 
impact-generating slug 98 and an impact-receiving anvil 100 are actuatable 
together to deliver an upwardly directed driving force on the upper 
hold-down plate 32 via the rod 92 to ensure a positive release of the 
upper retainers 50 from the unsevered guide thimble upper end portions 28. 
Specifically, the impact-generating slug 98 has a central bore 102 through 
which the rod 92 extends for mounting the slug for slidable movement along 
the rod. The impact-receiving anvil 100 is fixed on the 
impact-transmitting rod 92 across the path of the slidable movement of the 
slug 98 along the rod 92. Thus, when the slug 98 is struck against the 
stationary anvil 100, the force of the impact is transmitted from the 
anvil to the hold-down plate 32 via the interconnecting rod 92. 
To summarize its operation, the device 12 is first lowered down onto the 
top nozzle 24 to the position seen in FIG. 5 where the locating lugs 66 
are inserted into the large coolant flow openings 46 in the upper 
hold-down plate 32. Then, the four spring compression studs 70 are 
threaded into the internally-threaded anchors 62 in the lower adapter 
plate 30 by using an extension socket wrench engaged with the socket 
elements 88 on the studs. The studs 70 are then tightened down enough to 
relieve the tension load on the guide thimble upper end portions 28 and 
the upper retainers 50 attached thereto. The guide thimbles 16 are then 
severed just below the bulges 104 which should free the top nozzle 
subassembly from the fuel assembly skelton. The spring compression studs 
70 are loosened to allow the hold-down springs 56 to raise the hold-down 
plate 32 which should pull the upper retainers 50 un the unsevered upper 
end portions 28 of the guide thimbles 16. If the upper retainers 50 do not 
become free, the impact rod 92 is then threaded into the hole 96 in the 
upper hold-down plate 32 and the impact slug 98 is used like a hammer to 
impact the plate 32 in an upward direction via the rod 92 and anvil 100. 
This will then free the upper retainers from the guide thimble upper end 
portions 28 and permit the top nozzle subassembly 24 to be lifted off the 
fuel assembly skelton, as seen in FIG. 6, using the handle 80 or a crane 
attached to the handle. 
It is thought that the invention and many of its attendant advantages will 
be understood from the foregoing description and it will be apparent 
various changes may be made in the form, construction and arrangement 
thereof without departing from the spirit and scope of the invention or 
sacrificing all of its material advantages, the form hereinbefore 
described being merely a preferred or exemplary embodiment thereof.