Hold-down device of fuel assembly top nozzle employing leaf springs

An improved top nozzle includes leaf spring assemblies interposed between its movable upper hold-down plate and stationary lower adapter plate and arranged along respective peripheries thereof. The leaf spring assemblies bias the upper hold-down plate in an upward direction into contact with the upper core plate of the reactor and thereby impose a hold-down force on the fuel assembly via the lower adapter plate. In the preferred embodiment, each leaf spring assembly includes opposite lower and upper ends, with the lower end being attached to the lower adapter plate adjacent one of the corners on its periphery and the upper end being movably coupled in a groove on the upper hold-down plate adjacent a next one of the corners on its periphery. The leaf spring assemblies are arranged in a single file about the peripheries of the lower adapter plate and upper hold-down plate, with one assembly extending between each pair of succeeding corners on the respective peripheries of the lower and uper plates. In an alternative embodiment, a pair of leaf spring assemblies are interposed between the peripheries of the upper and lower plates. Each leaf spring assembly is generally L-shaped and includes opposite spaced apart upper ends and a lower end located between upper ends, with the lower end being attached to the lower adapter plate periphery adjacent one corner of an opposite pair of the corners thereof and the upper ends being movably coupled in grooves defined on the upper hold-down plate periphery adjacent both corners thereon being located on either side of the one corner on the adapter plate periphery.

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 an improved top nozzle 
for a fuel assembly in which the coil springs of the hold-down device are 
advantageously replaced by a unique arrangement of leaf springs. 
2. Description of the Prior Art 
Conventional designs of fuel assemblies include a multiplicity of fuel rods 
held in an organized array by grids spaced along the fuel assembly length. 
The grids are attached to a plurality of control rod guide thimbles. Top 
and bottom nozzles on opposite ends of the fuel assembly are secured to 
the control rod guide thimbles which extend above and below the opposite 
ends of the fuel rods. At the top end of the fuel assembly, the guide 
thimbles are attached in openings provided in the top nozzle. Conventional 
fuel assemblies also have employed a fuel assembly hold-down device to 
prevent the force of the upward coolant flow from lifting a fuel assembly 
into damaging contact with the upper core support plate of the reactor, 
while allowing for changes in fuel assembly length due to core induced 
thermal expansion and the like. Such hold-down devices have included the 
use of helical coil springs surrounding the guide thimbles, such as seen 
in U.S. Pat. Nos. 3,770,583 (Re. 31,583) and 3,814,667 to Klumb et al. 
Specifically, the fuel assembly of Klumb et al includes a top nozzle having 
an upper hold-down plate, a lower adapter plate (called an upper end plate 
in the patent) and a plurality of coil springs disposed between the upper 
and lower plates and coaxially about alignment posts which serve as upper 
extensions of the guide thimbles (called guide tubes in the patent). The 
alignment posts extend through the upper hold-down plate and are joined to 
the lower adapter plate and to the upper ends of the guide thimbles with 
fastener nuts located on the underside of the lower adapter plate. The 
upper hold-down plate is slidably mounted on the alignment posts and, as 
mentioned above, the coil springs are interposed, in compression, between 
the upper hold-down and lower adapter plates. A radially enlarged shoulder 
on the upper end of each of the alignment posts retains the hold-down 
plate on the posts. 
The use of coil springs in the Klumb et al type top nozzle to supply the 
necessary hold-down force to resist upward lifting of the fuel assembly 
and accommodate thermal growth of the assembly presents several inherent 
problems. 
First, coil springs inherently require considerable axial height even in 
applications where the required hold-down force is small. The extra amount 
of height required for the hold-down springs reduces the length of the 
fuel rods which can be used in the fuel assembly. The failure to maximize 
the length of the active core, as determined by fuel rod length, results 
in increased fuel cycle cost, increased KW per foot and increased peaking 
factors. 
Second, coil springs are subject to coolant flow induced vibration. 
Cross-flow from adjacent fuel assemblies occurs because of the radial flow 
maldistribution across pressurized water reactor cores which is caused by 
core inlet flow maldistribution and by temperature differences across the 
core. Thus, there is a radial pressure gradient at the fuel assembly 
outlet which induces cross-flow above the fuel rods of the assembly. The 
coil hold-down springs in the Klumb et al type top nozzle are exposed to 
the cross-flow which has led to spring failure due to fatigue caused by 
flow induced vibration. 
Consequently, a need exists for a different approach to the provision of 
adequate hold-down force in a fuel assembly top nozzle of the type 
disclosed in the Klumb et al patents, one with the objective of 
eliminating the aforementioned problems of suboptimal active core height 
and hold-down spring fatigue but requiring minimal modification of the 
overall top nozzle structure. 
SUMMARY OF THE INVENTION 
The present invention provides an improved top nozzle arrangement designed 
to satisfy the aforementioned needs. Specifically, the improved top nozzle 
of the present invention incorporates leaf springs instead of coil springs 
to provide the necessary hold-down force to prevent lifting of the fuel 
assembly and accommodate thermal expansion of the assembly. Contrary to 
the teachings of the Klumb et al patent (U.S. Pat. No. Re. 31,583), 
wherein leaf springs are characterized as inherently low deflection 
devices that are generally incapable of providing the necessary hold-down 
forces over the entire range of gap distances whigh might be encountered 
between fuel assemblies and the upper core plate given the spring size 
limitations dictated by the reactor core environment, the discovery 
underlying the present invention is that leaf springs applied in unique 
arrangements thereof can avoid the problems inherent in the use of coil 
springs. 
Particularly, coil springs inherently require more axial height than leaf 
springs and coil springs suffer from flow induced vibration whereas leaf 
springs do not. The amount of vertical height saved by the use of a leaf 
spring compared to a coil spring depends upon the fuel assembly hold-down 
force requirement. For a coil spring with a given deflection range, a low 
force requirement can be achieved by a small diameter wire with a 
relatively short spring length, whereas a high force requirement leads to 
a larger diameter wire which requires a longer spring length to maintain 
the same deflection range and stresses. Unlike the coil spring, the height 
of the leaf spring is not porportional to the hold-down force it provides. 
The leaf spring height is much less sensitive to force requirements; 
instead merely more leafs are required for larger forces. To illustrate 
this, the vertical height savings by substituting a leaf spring for a coil 
spring in a small hold-down force design fuel assembly is about 2 inches, 
whereas in a high hold-down force fuel assembly the savings is about 41/2 
inches. To quantify the advantages of a shorter top nozzle, which results 
from use of leaf springs instead of coil springs and permits a longer 
active core, an increase of 3 inches in core length results in a fuel 
cycle cost savings of about 0.5 percent, which is a reduction in KW per 
foot of 2 percent. 
Accordingly, the present invention is directed to improvements in a fuel 
assembly which includes a top nozzle and a plurality of guide thimbles 
having upper end portions mounting the top nozzle. The top nozzle has an 
upper hold-down plate and a lower adapter plate, and the upper end 
portions of the guide thimbles extend through and stationarily mount the 
lower adapter plate and extend through and slidably mount the upper 
hold-down plate for relative movement along the guide thimble upper end 
portions toward and away from the lower adapter plate. The top nozzle 
further has means which defines an upper limit of relative movement of the 
hold-down plate along the guide thimble upper end portions away from the 
lower adapter plate. 
The improvements incorporated by the fuel assembly reside in the top nozzle 
and comprise a plurality of leaf spring assemblies interposed between the 
upper hold-down plate and the lower adapter plate so as to yieldably 
support the movably upper hold-down plate in a spaced relation above the 
stationary lower adapter plate. The leaf spring assemblies are arranged 
along opposing peripheries of the upper hold-down plate and the lower 
adapter plate and engaged with the upper and lower plates at predetermined 
locations on the respective peripheries thereof so as to bias the upper 
hold-down plate in an upward direction and thereby impose a hold-down 
force on the fuel assembly via the lower adapter plate which tends to 
displace the upper hold-down plate to its upper limit along the guide 
thimble upper end portions away from the lower adapter plate. 
More particularly, the upper hold-down plate and lower adapter plate of the 
top nozzle each has a plurality of corners on the respective peripheries 
thereof which are opposite to and vertically aligned with one another. 
Also, there are two embodiments of leaf spring assembly arrangements in 
the improved top nozzle. 
In a preferred embodiment, each leaf spring assembly includes opposite 
lower and upper ends. Each leaf spring assembly is attached at its lower 
end to the lower adapter plate adjacent one of the corners on the adapter 
plate periphery and is movably coupled at its upper end to the upper 
hold-down plate adjacent a next one of the corners on the hold-down plate 
periphery. The upper hold-down plate has a plurality of guide means 
defined therein adjacent the corners on its periphery within which are 
movably coupled the respective upper ends of the leaf spring assemblies. 
The leaf spring assemblies are arranged in a single file about the 
peripheries of the lower adapter plate and upper hold-down plate such that 
the upper end of each leaf spring assembly being movably coupled to the 
guide means adjacent one of the corners of the upper hold-down plate 
overlies the lower end of the next succeeding leaf spring assembly in the 
single file thereof which, in turn, is attached adjacent the one corner of 
the lower adapter plate which is aligned below the one corner of the upper 
hold-down plate. The single file of leaf spring assemblies includes one 
leaf spring assembly extending between each pair of succeeding corners in 
the aligned pluralities thereof on the respective peripheries of the lower 
adapter plate and upper hold-down plate. 
In an alternative embodiment, the improved top nozzle includes a pair of 
leaf spring assemblies with each leaf spring assembly including opposite 
spaced apart upper ends and a lower end located between the upper ends. 
Each leaf spring assembly is attached at its lower end to the lower 
adapter plate adjacent one corner of an opposite pair of the corners on 
the adapter plate periphery and is movably coupled at its upper ends to 
the upper hold-down plate adjacent both corners on the hold-down plate 
periphery being located on either side of the one corner on the adapter 
plate. 
While each leaf spring assembly in the alternative embodiment has a 
generally L-shaped configuration compared to the generally linear 
configuration of the leaf spring assembly in the preferred embodiment, 
both embodiments of the leaf spring assemblies may include one or more 
individual leaf springs therein depending upon the particular application. 
These and other advantages and attainments 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 embodiments of the 
invention.

DETAILED DESCRIPTION OF THE INVENTION 
In the following description, like reference characters designate like or 
corresponding parts throughout the several views of the drawings. 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 
Refering now to the drawings, and particularly to FIG. 1, there is shown an 
elevational view of a nuclear reactor fuel assembly, represented in 
vertically foreshortened form and being generally designated by the 
numeral 20. Basically, the fuel assembly 20 includes a lower end structure 
or bottom nozzle 22 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 guide tubes or thimbles 24 which project upwardly 
from the bottom nozzle 22. The assembly 20 further includes a plurality of 
transverse grids 26 axially spaced along the guide thimbles 24 and an 
organized array of elongated fuel rods 28 transversely spaced and 
supported by the grids 26. Also, the assembly 20 has an instrumentation 
tube 30 located in the center thereof and an upper end structure or top 
nozzle 32 attached to the upper ends of the guide thimbles 24 which 
incorporates certain improvements in accordance with the present invention 
which will be fully described below. With such an arrangement of parts, 
the fuel assembly 20 forms an integral unit capable of being 
conventionally handled without damaging the assembly parts. 
As mentioned above, the fuel rods 28 in the array thereof in the assembly 
20 are held in spaced relationship with one another by the grids 26 spaced 
along the fuel assembly length. Each fuel rod 28 includes nuclear fuel 
pellets (not shown) and is closed at its opposite ends by upper and lower 
end plugs 34,36. The fuel pellets composed of fissile material are 
responsible for creating the reactive power of the reactor. A liquid 
moderator/coolant such as water, or water containing boron, is pumped 
upwardly through the guide thimbles 24 and along the fuel rods 28 of the 
fuel assembly 20 in order to extract heat generated therein for the 
production of useful work. 
To control the fission process, a number of control rods (not shown) are 
reciprocally movable in the guide thimbles 24 located at predetermined 
positions in the fuel assembly 20. Since the control rods are inserted 
into the guide thimbles 24 from the top of the fuel assembly 20, the 
placement of the components forming the improved top nozzle 32 must 
accommodate the movement of the control rods into the guide thimbles 24 
from above the improved top nozzle 32. 
Improved Top Nozzle Employing Leaf Springs 
Turning now to FIGS. 1 to 6, there is shown the preferred arrangement of a 
plurality of leaf spring assemblies 38 employed by the improved top nozzle 
32 in accordance with the present invention. The leaf spring assembly 
arrangement eliminates the problems associated with the prior art coil 
springs and results in the capability of incorporating longer fuel rods 28 
in the fuel assembly 20, as represented in FIG. 12. 
The top nozzle 32 incorporating the improvements of the present invention 
is conventionally mounted on the upper end portions 40 of the guide 
thimbles 24 and includes an upper hold-down plate 42 and a lower adapter 
plate 44. The upper end portions 40 of the guide thimbles 24, being in the 
form of alignment posts, extend through a plurality of passageways 46 in 
the upper hold-down plate 42 and fit through a plurality of openings 47 in 
the lower adapter plate 44 and are connected thereto with fastener nuts 48 
located on the underside 50 of the lower adapter plate. The upper 
hold-down plate 42 is slidably mounted on the guide thimble upper end 
portions 40 and radially enlarged nuts 52 are threaded on the upper ends 
thereof which define an upper limit of relative movement of the hold-down 
plate along the guide thimble upper end portions away from the lower 
adapter plate 44. The upper hold-down plate 42 is composed of an array of 
hubs 54 and ligaments 56 which extend between and interconnect the hubs. 
Each hub 54 has one of the passageways 46 defined therethrough. In 
addition, the upper hold-down plate 42 and the lower adapter plate 44 each 
have a rectangular configuration with a plurality of corners (preferably 
four in number) 58 and 60 defined on respective peripheries thereof being 
opposite to and vertically aligned with one another. Therefore, in 
summary, the upper end portions 40 of the guide thimbles 24 extend through 
and stationarily mount the lower adapter plate 44, whereas they extend 
through and slidably mount the upper hold-down plate 42 for relative 
movement toward and away from the lower adapter plate. 
The preferred embodiment of the improvements provided by the present 
invention relates to the plurality of leaf spring assemblies 38 interposed 
between the upper hold-down plate 42 and the lower adapter plate 44 in the 
single file arrangement seen in FIGS. 1 to 3, by which arrangement the 
movable upper hold-down plate is yieldably supported in a spaced relation 
above the stationary lower adapter plate. The leaf spring assemblies 38 
are arranged along the respective peripheries of the upper hold-down plate 
42 and the lower adapter plate 44 and engaged with the upper and lower 
plates adjacent predetermined ones of their corners 58,60. In such 
arrangement, the leaf spring assemblies 38 bias the upper hold-down plate 
in an upward direction toward and into contact with the upper core plate 
(not shown) and thereby impose a hold-down force on the fuel assembly 20 
via the lower adapter plate 44. 
As seen in FIG. 3, each leaf spring assembly 38 has a generally linear, 
inclined configuration and includes opposite lower and upper ends 62,64. 
The pair of leaf springs composing the assembly 38 illustrated in FIG. 3 
merely serve as an example. Some applications may require more than two 
leaf springs and others only one leaf spring. The lower end 62 of the 
assembly 38 is bent slightly relative to the remainder thereof and a hole 
66 is formed in the lower end 62 to facilitate its attachment by a bolt 
68, as seen in FIGS. 2 and 3, to the lower adapter plate 44 adjacent one 
of the corners 60 on the adapter plate periphery. 
For coupling with the upper end 64 of each assembly 38, the upper hold-down 
plate 42 has a plurality of guide means defined therein adjacent its 
peripheral corners 58. Each guide means is in the form of a groove 70 
defined on the underside 72 of a flange 74 formed on the upper hold-down 
plate 42 adjacent the respective corners 58 thereof. The upper end 64 of 
the assembly 38 has a semi-cylindrical cross-sectional configuration and 
fits within the groove 70 so as to movably couple the assembly 38 to the 
upper hold-down plate 42. 
Due to the inclined configuration of the leaf spring assembly 38, the upper 
end 64 of each assembly is coupled to the upper plate 42 adjacent the next 
succeeding one of the corners 58 on the periphery thereof after the one of 
its corners 58 aligned above the one corner 60 of the adapter plate 44 
adjacent to which the lower end 62 of the assembly 38 is attached. Thus, 
in the single file arrangement of the preferred embodiment of the leaf 
spring assemblies 38, each assembly 38 extends along one side of the 
four-sided, generally rectangular upper and lower plates 42,44 between 
each pair of succeeding corners 58,60 in the aligned pluralities thereof 
on the respective peripheries of the upper and lower plates. 
Turning now to FIGS. 7 to 11, there is shown an alternative embodiment of 
the leaf spring assemblies. In the alternative arrangement, a pair of leaf 
spring assemblies 76 are interposed between the upper hold-down plate 42 
and the lower adapter plate 44 so as to yieldably support the movable 
upper hold-down plate above the stationary lower adapter plate. Like 
before, the leaf spring assemblies 76 are arranged along the respective 
peripheries of the upper hold-down plate 42 and the lower adapter plate 44 
and engaged with the upper and lower plates adjacent predetermined ones of 
peripheral corners 58,60 thereon. Due to its L-shaped configuration, each 
leaf spring assembly 76 is coextensive with two adjacent sides of the 
four-sided, generally rectangular upper and lower plates 42,44. 
Here also, the upper hold-down plate 42 has guide means defined therein 
adjacent predetermined ones of its corners 58. This time the guide means 
takes the form of a groove 78 defined in the underside of a flange 80 on 
each of a pair of diagonally opposite pair of the corners 58. In view of 
its generally L-shaped configuration, each leaf spring assembly 76 
includes a pair of opposite spaced apart upper ends 82 and a lower end 84 
located between the upper ends. Each leaf spring assembly 76, which can be 
composed of one or more individual leaf springs, has a hole 86 defined in 
its center lower end 84 by which it is attached by a bolt 88 to the lower 
adapter plate 44 adjacent one corner 60 of an opposite pair of the 
peripheral corners 60 of the adapter plate. Each assembly 76 is movably 
coupled at its upper ends 82 to the guide grooves 78 on the upper 
hold-down plate 42 adjacent both of the peripheral corners 58 thereon 
being located on either side of the one corner 60 on the adapter plate 44 
to which the assembly's lower end 84 is attached. 
This alternative arrangement of the leaf spring assemblies 76 is more 
symmetrical in configuration than the preferred arrangement of assemblies 
38. However, it engages the upper and lower plates 42,44 at only two of 
four diametrically opposite corners, whereas the alternative arrangement 
engages the plates at each of the four corners. 
FIGS. 12A and 12B are diagrammatic representations of two fuel assemblies 
90 and 20 having the same overall axial height. However, the fuel assembly 
20 of FIG. 12B having the improved top nozzle with the preferred 
arrangement of leaf springs 38 of the present invention has fuel rods 28 
with greater height as compared with the height of the fuel rods 92 in the 
assembly 90 of FIG. 12A having the prior art top nozzle with coil springs 
94. The difference is due to the fact that the prior art coil springs 94 
require greater axial height than that of the leaf springs 38 in order to 
deliver the same hold-down force. 
It is thought that the present invention and many of its attendant 
advantages will be understood from the foregoing description and it will 
be apparent that 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 forms 
hereinbefore described being merely exemplary embodiments thereof.