Extended burnup top nozzle for a nuclear fuel assembly

A fuel assembly includes an array of fuel rods and guide thimbles disposed in laterally spaced relation to one another. The guide thimbles have upper ends extending above the upper ends of the fuel rods. The fuel assembly also includes a top nozzle defined solely by a flat rectangular adapter plate having a main central portion and a peripheral portion surrounding and merging with the main central portion. The main central portion of the adapter plate has a plurality of attachment holes receiving the upper ends of guide thimbles for attachment of the adapter plate upon the guide thimble upper ends in spaced relation above the fuel rod upper ends. A first sest of holes are defined through one pair of diagonal corners of the adapter plate for use in attaching sets of spring assemblies directly to and upon the adapter plate for alignment along the peripheral portions thereof. Abutments formed at the other pair of diagonal corners of the adapter plate define a second set holes through the adapter plate for use in latching handling equipment directly to the adapter plate for lifting of the top nozzle.

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 top nozzle allowing 
improved utilization of space for accommodating greater thermal growth and 
burnup of fuel rods of the 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. The upper and 
lower core support plates are directly or indirectly attached to a support 
barrel which surrounds the entire core and extends between the ends 
thereof. 
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. The guide thimbles extend slightly above 
and below the ends of the fuel rods. 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 fuel assemblies are arranged 
vertically resting on the lower core support plate. To facilitate handling 
and installation, the fuel assemblies are generally not secured to the 
lower core support plate. 
Temperatures at various times within the core may vary greatly, such as, 
from cold shutdown to normal operating conditions. Also, different 
materials exhibit different thermal growth characteristics. Since the 
materials of fuel assembly components are generally different than those 
used in the core support barrel and undergo greater thermal expansion, the 
resulting increase in length of the fuel assemblies in the axial or 
vertical direction must be accommodated. For this reason, the fuel 
assemblies are not usually attached to the upper and lower core plates but 
rather are supported in a manner which permits some relative motion 
therebetween. 
The axial thermal expansion differential between the fuel assemblies and 
the core support barrel has been accommodated by insuring that the axial 
spacing between the upper and lower core support plates is somewhat 
greater than the axial length of the fuel assemblies. Normally, this is 
accomplished by providing an axial gap between the top of the fuel 
assemblies and the upper core support plate. However, the presence of the 
gap can result in upward lifting of the fuel assemblies due to the 
hydraulic forces produced on the fuel assemblies in the upward direction 
by coolant flow. Thus, fuel assemblies have also employed hold-down 
devices with the top nozzles to prevent the force of upward coolant flow 
from lifting the fuel assemblies into damaging contact with the upper core 
support plate, while at the same time allowing for changes in fuel 
assembly length due to core-induced thermal expansion and the like. 
Representative of prior art fuel assemblies with hold-down devices are 
those disclosed in Hatfield (U.S. Pat. No. 4,792,429) and Wilson et al 
(U.S. Pat. No. 4,684,502) and Japanese Pat. Nos. 62-91891 and 62-102186. 
As mentioned previously, the guide thimbles of fuel assemblies extend 
slightly above and below the ends of the fuel rods. Thus, the top and 
bottom nozzles of fuel assemblies secured at opposite ends of the guide 
thimbles likewise are spaced above and below the fuel rod ends. This space 
between the opposite ends of the fuel rods and adjacent portions of the 
top and bottom nozzles accommodates increase in length of the fuel rods 
due to thermal growth as fuel rod burnup occurs during normal reactor 
operation. 
With improvements in various aspects of fuel assembly design, it has become 
feasible to increase the allowable burnup of the fuel rods. This increase 
in burnup is desirable because it decreases the frequency of plant 
shutdowns and the buildup of spent fuel. However, to permit the fuel rods 
to operate to a higher burnup, an increase of approximately 0.5 inch 
minimum in fuel rod length is necessary due to extra thermal growth. This 
necessitates an increase in the space between the adapter plates of the 
top and bottom nozzles to accommodate this additional fuel rod growth. At 
the same time, there still must be enough space between the upper core 
plate and adapter plate of the top nozzle to allow inserting and latching 
of the handling equipment to the top nozzle. 
Currently, there is not enough room between the adapter plates of the top 
and bottom nozzles to permit the 0.5 inch growth in fuel rod length. 
Consequently, a need exist for a way to accommodate extra fuel rod thermal 
growth without impairing the handling capability of the core equipment 
currently in use. 
SUMMARY OF THE INVENTION 
The present invention provides a top nozzle having an improved construction 
designed to satisfy the aforementioned needs. The top nozzle of the 
present invention is reduced in overall height by about 1.75 inches 
compared to the prior art top nozzle which enables a fuel assembly to 
accommodate increased fuel rod length and growth and thereby allow a 
reactor to operate at a higher burnup rate. The additional space created 
by the shortened height of the top nozzle can be effectively utilized to 
provide high burnup capability for the fuel assembly. The additional space 
may be utilized for additional rod growth and/or making the fuel rod 
longer with a larger plenum if needed. It is estimated that this 
additional space of 1.75 inches will provide about 20,000 MWD/MTU in 
additional burnup capacity. 
Accordingly, the present invention is directed to a top nozzle for a 
nuclear fuel assembly which permits increased fuel rod thermal growth and 
burnup. The top nozzle comprises: (a) a flat adapter plate having a main 
central portion and a peripheral portion surrounding and merging into the 
main central portion; (b) means defining a first set of holes through a 
first pair of diagonally-opposed locations on the peripheral portions of 
the adapter plate for use in attaching sets of spring assemblies directly 
to the adapter plate for alignment along the peripheral portions thereof; 
and (c) means defining a second set holes through a second pair of 
diagonally-opposed locations on the peripheral portions of the adapter 
plate for use in latching handling equipment directly to the adapter plate 
for lifting of the top nozzle. The means defining the second set of holes 
is an abutment formed at each of the second diagonally-opposed locations 
so as to project upwardly from the plane of the upper surface of the 
adapter plate, with each hole being defined through the respective 
abutment. The peripheral portion of the adapter plate extending between 
the first and second pairs of holes is of solid construction. Further, a 
plurality of spring assemblies are attached directly upon the adapter 
plate at the holes through the first diagonally-opposed locations. 
Also, the present invention relates to a fuel assembly which comprises: (a) 
an array of fuel rods disposed in laterally spaced relation to one another 
and having upper ends; (b) an array of guide thimbles disposed in 
laterally spaced relation to one another and to the fuel rods and having 
upper ends extending above the upper ends of the fuel rods; and (c) a top 
nozzle defined solely by the flat rectangular adapter plate described 
above being attached upon the guide thimble upper ends in spaced relation 
above the fuel rod upper ends. 
These and other 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 an illustrative embodiment of the 
invention.

DETAILED DESCRIPTION OF THE INVENTION 
In the following description, like references 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 
an elevational view of a fuel assembly, represented in vertically or 
longitudinally foreshortened form and being generally designated by the 
numeral 10. The fuel assembly 10 basically includes a lower end structure 
or bottom nozzle 12 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 14 which project upwardly 
from the bottom nozzle 12. The assembly 10 further includes a plurality of 
transverse grids 16 axially spaced along the guide thimbles 14 and an 
organized array of elongated fuel rods 18 transversely spaced and 
supported by the grids 16. Also, the assembly 10 has an instrumentation 
tube 20 located in the center thereof and an upper end structure or top 
nozzle 22 attached to the upper ends of the guide thimbles 14. With such 
an arrangement of parts, the fuel assembly 10 forms an integral unit 
capable of being conventionally handled without damaging the assembly 
parts. 
As mentioned above, the fuel rods 18 in the array thereof in the assembly 
10 are held in spaced relationship with one another by the grids 16 spaced 
along the fuel assembly length. Each fuel rod 18 includes nuclear fuel 
pellets 24 and the opposite ends of the rod are closed by upper and lower 
end plugs 26,28 to hermetically seal the rod. Commonly, a plenum spring 30 
is disposed between the upper end plug 26 and the pellets 24 to maintain 
the pellets in a tight, stacked relationship within the rod 18. The fuel 
pellets 24 composed of fissile material are responsible for creating the 
reactive power of the nuclear reactor. A liquid moderator/coolant such as 
water, or water containing boron, is pumped upwardly through the fuel 
assemblies of the core in order to extract heat generated therein for the 
production of useful work. 
To control the fission process, a number of control rods 32 are 
reciprocally movable in the guide thimbles 14 located at predetermined 
positions in the fuel assembly 10. Specifically, the top nozzle 22 
includes a rod cluster control mechanism 34 having an internally grooved 
cylindrical member 36 with a plurality of radially extending flukes or 
arms 38. Each arm 38 is interconnected to a control rod 32 such that the 
control mechanism 34 is operable to move the control rods 32 vertically in 
the guide thimbles 14 to thereby control the fission process in the fuel 
assembly 10, all in a well-known manner. 
Prior Art Top Nozzle Construction 
Referring now to FIGS. 2 and 3 as well as FIG. 1, it can be seen that the 
prior art top nozzle 22 of the fuel assembly 10 includes an enclosure or 
housing 40 formed by a transversely extending lower adapter plate 42 and 
an upper annular flange 44 with an upstanding sidewall 46 extending 
between and integrally interconnecting the adapter plate 42 and flange 44 
at their respective peripheries. Suitably clamped to the upper annular 
flange 44 are a plurality of spring assemblies 48 which constitute a 
hold-down device for the fuel assembly 10. Each spring assembly 48 is 
composed of a set of leaf springs 48A disposed in a stack relation. The 
spring assemblies 48 cooperate with the upper core plate (not shown) in a 
conventional manner to prevent hydraulic lifting of the fuel assembly 10 
caused by upward coolant flow while allowing for changes in fuel assembly 
length due to core induced thermal expansion and the like. Also, the rod 
cluster control assembly 34 (not shown in FIG. 2) is disposed within a 
central top opening 50 of the top nozzle 22 defined by the annular flange 
44. Flow openings 51 and guide thimble attachment holes 52 are defined in 
spaced apart relation from one another through the main central portion 
42A of the lower adapter plate in alignment with the central top opening 
50 of the upper annular flange 44. 
As seen in FIGS. 1-3, each spring assembly 48 at a base end 48B is fastened 
and held in its operative position on the top nozzle upper flange 44 by 
using a spring clamp 53 which includes a corner block 54 and a spring 
screw 56. There are two spring clamps 53 for holding two pairs of the 
spring assemblies 48. One spring clamp 53 is provided at each of one pair 
of opposite diagonal corners 22A of the top nozzle 22 and each spring 
clamp 53 preferably includes the one clamp block 54 and a pair of spring 
screws 56 which share the same clamp block for clamping a pair of the 
spring assemblies 48 at their respective base ends 48B. Each screw 56 is 
installed through a counterbore 58 defined in the corner block 54 and a 
hole 60 defined in the base end 48B of the spring assembly 48. The screw 
56 is threaded into a threaded hole 62 tapped in the upper annular flange 
44. Together with the corner clamp block 54, when the spring screw 56 is 
tightened down it clamps the spring assembly 48 at its respective base end 
48B to the peripheral upper annular flange 44 of the top nozzle 22. Once 
the screw 56 is tightened down, the corner clamp block 54 is then fixedly 
attached to the top nozzle flange 44 by welds (not shown). Further, the 
spring screw 56 is locked against rotation and is retained in place by a 
lock pin (not shown) which is welded to the inside of the counterbore 58 
in the clamp block 54. 
In their operative positions, the spring assemblies 48 extend in inclined 
upward relationship along and within the outer perimeter of the top nozzle 
housing 40 where they contact the upper core plate. A tang 64 extending 
downwardly from an upper one of the leaf springs 48A in each of the sets 
projects into an elongated slot 65 defined in the flange 44 for 
maintaining the leaf spring sets 48A in alignment with the flange 44 and 
preventing the leaf springs 48A from inadvertently swinging over the 
central top opening 50 defined by the upper annular flange 44 where they 
would interfere with the operation of the rod cluster control mechanism 
34. 
The other pair of opposite corners 22B of the top nozzle 22 have upwardly 
projecting abutments 66 formed on the flange 44 and defining holes 67 
which receive members of top nozzle handling equipment (not shown). 
Sufficient space is provided in the top nozzle housing 40 between the 
upper annular flange 44 and lower adapter plate 42 to accommodate 
inserting of the handling equipment members through the holes 67 and 
latching thereof under the flange 44 of the top nozzle 22. 
Improved Top Nozzle Construction of the Present Invention 
Referring now to FIGS. 4 and 5, there is illustrated a top nozzle, 
generally designated 68, having an improved construction in accordance 
with the present invention which provides the top nozzle 68 with a reduced 
height. The construction of the reduced height top nozzle 68 has only a 
flat rectangular adapter plate 70 which in and of itself is substantially 
the same as the lower adapter plate 42 of the prior art top nozzle 22, 
except for the features pointed out below. The adapter plate 70 retains 
the main central portion 70A with the array of flow openings 72 and guide 
thimble holes 74. 
However, in the reduced height top nozzle 68, the upper annular flange 44 
and upstanding sidewall 46 of the housing 40 of the prior art top nozzle 
22 have been eliminated. In effect, the reduced height top nozzle 68 and 
the adapter plate 70 are now one and the same component. The main central 
portion 70A of the flat adapter plate 70 is surrounded by a peripheral 
portion 70B with the main and peripheral portions 70A, 70B defining upper 
surface portions which merge into one another and lie in a common plane. 
Unlike the lower adapter plate 42 of the prior art top nozzle 22, the 
adapter plate 70 of the reduced height top nozzle 68 has a pair of leaf 
spring assembly attachment holes 76 defined through each of one pair of 
diagonal corners 70C thereof. Further, the adapter plate 70 has an 
upwardly projecting abutment 77 formed on each of the other pair of 
diagonal corners 70D which defines a handling equipment latch hole 78 
through each diagonal corner 70D. Thus, in the case of the adapter plate 
70 of the reduced height top nozzle 68, the mounting of the sets of leaf 
springs 48A is relocated directly to upon the adapter plate 70 and the 
latching of the handling equipment is directly under the adapter plate 70. 
The sets of leaf springs 48 now extend along and directly overlie the 
peripheral portions 70D of the adapter plate 70. 
Each spring assembly 48 at its base end 48B is fastened and held in its 
operative position on the adapter plate corners 70B by using the spring 
clamp 53, the same as before, which includes the corner block 54 and the 
spring screw 56. There are two spring clamps 53 for holding two pairs of 
the spring assemblies 48. One spring clamp 53 is provided at each of one 
pair of opposite diagonal corners 70B of the adapter plate 70 and each 
spring clamp 53 preferably includes the one clamp block 54 and pair of 
spring screws 56 which share the same clamp block for clamping the pair of 
the spring assemblies 48 at their respective base ends 48B. The spring 
screws 56 thread into the holes 76, as seen in FIG. 5. It will be noted 
that the tangs 64 have been eliminated from the spring assemblies 48. 
By comparing the prior art top nozzle 22 of FIG. 3 with the reduced height 
top nozzle 68 of FIG. 5, it becomes readily apparent the substantial 
increase in the growth gap between the upper end plugs 26 and the bottom 
side of the respective adapter plate 70 of the top nozzle 68 compared to 
that of the prior art top nozzle 22. The reduced or shortened height of 
the top nozzle 68 can be utilized for more fuel rod growth and/or longer 
fuel rods 18 with larger plenums, enabling the fuel assembly to achieve 
higher burnup by as much as 20,000 MWD/MTU. It should be mentioned that 
the adapter plate 70 can be made slightly thicker if needed to minimize 
deflection and stresses. 
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 form 
hereinbefore described being merely a preferred or exemplary embodiment 
thereof.