U.S. patent application number 10/328974 was filed with the patent office on 2004-10-07 for lips-type multi-purposed nuclear fuel assembly spacer grid.
Invention is credited to Chun, Tae Hyun, In, Wang Kee, Jung, Youn Ho, Kang, Heung Seok, Kim, Dae Ho, Kim, Hyung Kyu, Oh, Dong Seok, Park, Gyung Jin, Shin, Chang-Hwan, Song, Kee Nam, Yoon, Kyung Ho.
Application Number | 20040196953 10/328974 |
Document ID | / |
Family ID | 32213458 |
Filed Date | 2004-10-07 |
United States Patent
Application |
20040196953 |
Kind Code |
A1 |
Kim, Dae Ho ; et
al. |
October 7, 2004 |
LIPS-TYPE MULTI-PURPOSED NUCLEAR FUEL ASSEMBLY SPACER GRID
Abstract
A lips-type multi-purposed spacer grid for supporting fuel rods
within a nuclear fuel assembly is disclosed. In the spacer grid,
the fuel rods are in contact with dimples and water strider-type
springs in an equiangular surface contact manner. The spacer grid
distributes load, applied to the springs, to the entire structure
of its inner strips, thus reducing peak stress at the contact
surfaces between the fuel rods and the springs and diminishing
vibration of the fuel rods, and thereby reducing possible fretting
wear of the fuel rods due to hydraulic vibration of the fuel rods.
The spacer grid also enlarges the allowable elastic range of the
springs, and allows the springs to soundly support the fuel rods by
using residual spring force. The spacer grid has mixing blades
capable of minimizing pressure loss and flow interference, so that
the fuel rod cooling efficiency of the nuclear fuel assembly is
improved.
Inventors: |
Kim, Dae Ho; (Daejeon,
KR) ; Song, Kee Nam; (Daejeon, KR) ; Chun, Tae
Hyun; (Daejeon, KR) ; Yoon, Kyung Ho;
(Daejeon, KR) ; Oh, Dong Seok; (Daejeon, KR)
; Kang, Heung Seok; (Daejeon, KR) ; Jung, Youn
Ho; (Daejeon, KR) ; Kim, Hyung Kyu; (Daejeon,
KR) ; In, Wang Kee; (Daejeon, KR) ; Shin,
Chang-Hwan; (Suwon-si, KR) ; Park, Gyung Jin;
(Seoul, KR) |
Correspondence
Address: |
BACHMAN & LAPOINTE, P.C.
900 CHAPEL STREET
SUITE 1201
NEW HAVEN
CT
06510
US
|
Family ID: |
32213458 |
Appl. No.: |
10/328974 |
Filed: |
December 24, 2002 |
Current U.S.
Class: |
376/438 |
Current CPC
Class: |
G21C 3/3563 20130101;
Y02E 30/30 20130101; G21C 3/352 20130101 |
Class at
Publication: |
376/438 |
International
Class: |
G21C 003/34 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 26, 2001 |
KR |
2001-0085115 |
Claims
1. A multi-purposed spacer grid fabricated by a plurality of two
types of inner strips intersecting each other to form a plurality
of unit cells, and used for supporting a plurality of fuel rods in
a nuclear fuel assembly such that one fuel rod is supported within
one unit cell, each of said two types of inner strips being
fabricated by integrating a plurality of unit strip parts into a
linear strip, and having a plurality of notches at junctions of the
unit strip parts such that each notch vertically extends downward
or upward, wherein each of said unit strip parts comprises: a frame
used as a support frame of the unit strip part, said frame
comprising: two support columns vertically disposed in parallel
while being spaced apart from each other at a predetermined
interval; and two support beams horizontally extending between the
two support columns at upper and lower positions to connect the two
support columns to each other and define a middle opening between
the support beams and the support columns; at least one spring
provided in the middle opening of the frame while being projected
in a direction from a vertical surface formed by the frame, said
spring comprising: a curved part axially formed along the spring
and having a predetermined width while being curved within a
direction of the width; two side extensions extending outward in
opposite directions from both sides of the curved part to a
predetermined width while being bent at a predetermined angle; and
four spring legs diagonally extending from upper and lower corners
of the two side extensions, said four spring legs being connected
to inside edges of the frame at four corners of the middle opening
of the frame and being bent in a direction opposed to the bent
direction of the two side extensions; and upper and lower dimples
provided at positions above and under the spring while being
projected from the vertical surface formed by the frame in a
direction opposed to the projecting direction of the spring, said
upper dimple being curved along a lower edge thereof to form an
arc-shaped lower edge, and the lower dimple being curved along an
upper edge thereof to form an arc-shaped upper edge, each of said
upper and lower dimples comprising: a curved dimple part axially
formed along each dimple, and having a predetermined width while
being curved within a direction of the width at a radius of
curvature corresponding to that of an external surface of each fuel
rod; and two side dimple extensions extending outward in opposite
directions from both sides of the curved dimple part to a
predetermined width while being curved at a predetermined
angle.
2. The multi-purposed spacer grid according to claim 1, wherein
upper and lower edges of the spring are curved to form arc-shaped
edges which are symmetrical with respect to a horizontal axis of
the spring.
3. (cancelled):
4. The multi-purposed spacer grid according to claim 1, wherein the
bent parts of the spring legs are projected from the vertical
surface formed by the frame in a direction opposed to the
projecting direction of the curved part.
5. The multi-purposed spacer grid according to claim 4, wherein one
spring comprising a curved part and two side extensions is provided
in the middle opening of the frame.
6. The multi-purposed spacer grid according to claim 5, wherein an
axial opening is formed along the curved part of the spring.
7. The multi-purposed spacer grid according to claim 6, wherein the
axial opening is enlarged in a direction of a width thereof until
the axial opening reduces the width of the two side extensions.
8. The multi-purposed spacer grid according to claim 4, wherein two
springs, each comprising a curved part and two side extensions, are
provided at upper and lower portions inside the middle opening of
the frame.
9. The multi-purposed spacer grid according to claim 1, further
comprising a mixing blade extending upward to a predetermined
length from a side of an upper edge of the upper dimple.
10. The multi-purposed spacer grid according to claim 9, wherein
the mixing blade is curved in the same direction as the projecting
direction of the spring so as to have a spoon-shaped
configuration.
11. (cancelled):
12. (cancelled):
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates, in general, to spacer grids
used for supporting fuel rods in a nuclear fuel assembly of a
nuclear reactor and, more particularly, to a lips-type
multi-purposed spacer grid used in such nuclear fuel assemblies and
designed to support fuel rods by springs which are in contact with
the fuel rods at equiangular curved contact surfaces having areas
larger than those of springs of conventional spacer grids, thus
enhancing the fuel rod support performance of spacer grids and
accomplishing desired soundness of the spacer grids when the spacer
grids support the fuel rods in a nuclear fuel assembly. The spacer
grid according to the present invention also has a structure
designed such that load applied to the springs is distributed to
the entire structure of the spacer grid, and the coolant deflecting
area of the spacer grid is increased by mixing blades provided at
the upper part of the spacer grid, and the spacer grid has a
structure capable of effectively filtering the debris moving along
with the coolant.
[0003] 2. Description of the Prior Art
[0004] FIG. 1 is a perspective view, showing the construction of a
conventional nuclear fuel assembly.
[0005] In each spacer grid 7 of the nuclear fuel assembly 1,
springs and dimples support a plurality of elongated nuclear fuel
rods 8 and a plurality of guide tubes 5 so as to maintain the
arrangement of the nuclear fuel rods 8 placed at regular intervals
in the nuclear fuel assembly 1. That is, the springs and dimples
maintain the regular intervals between the nuclear fuel rods 8
while preventing impact-induced deformation of the nuclear fuel
rods 8 and the guide tubes 5, thus reliably defining passages for
coolant in the fuel assembly 1 and allowing the coolant to
effectively cool the fuel rods 8 in a reactor core. A plurality of
mixing blades are attached to the upper edges of intersecting
strips of each spacer grid 7 so as to mix the thermally imbalanced
coolant within the nuclear fuel assembly 1. In addition, the spacer
grids 7 may be designed to have a structure capable of effectively
filtering debris from the coolant.
[0006] The coolant mixing function, the wear and the debris
filtering function of the spacer grids 7 are recognized as
important factors in development of nuclear fuel assemblies of high
burn-up and zero defects. In order to develop the nuclear fuel
assemblies of high burn-up and zero defects, it is necessary to
enhance the thermal efficiency of the nuclear fuel assemblies. The
thermal efficiencies of the nuclear fuel assemblies may be enhanced
by improving the flow characteristics of coolant around the fuel
rods.
[0007] The improvement in the flow characteristics of coolant
around the fuel rods may be effectively accomplished by a change of
the structure of a spacer grid. That is, the thermal mixing of
coolant may be improved by attachment of mixing blades to the
spacer grid, a change of the shape of the mixing blades and/or
defining appropriately designed coolant channels in the spacer
grid. However, the above-mentioned methods of enhancing the thermal
efficiencies of nuclear fuel assemblies also generate turbulences
in coolant flowing around the fuel rods, and the turbulences of the
coolant undesirably cause to vibrate the elongated, parallel,
closely spaced fuel rods within the nuclear fuel assembly. When the
fuel rods are so vibrated over a lengthy period of time, the
claddings of the fuel rods are repeatedly and frictionally abraded
at their contact parts at which the fuel rods are brought into
contact with the springs and dimples of the spacer grids. The
claddings are thus reduced in their thickness so as to be finally
perforated at the contact parts. Such an abrasion of the fuel rods
is so-called "fretting wear of fuel rods" in the art.
[0008] When the spacer grids 7 are exposed to neutron irradiation
in the reactor core for a lengthy period of time, the material
characteristics of the springs of the spacer grids 7 are changed,
and the springs gradually lose their elasticity. The springs in
such a case fail to stably and steadily support the fuel rods 8,
and so allow the fuel rods 8 to vibrate. The claddings of the fuel
rods 8 are thus abraded at the contact parts at which the fuel rods
are brought into contact with the springs and dimples of each
spacer grid 7, so that the fretting wear of the fuel rods 8 occurs.
There have been a lot of reports of leakage of radioactive
materials from fuel rods due to perforation of the claddings of the
fuel rods 8 caused by the fretting wear. In the art, it has been
recognized that the fretting wear of the fuel rods is greatly
affected by the design of the spacer grid, including the shapes of
contact parts between the fuel rods and the spacer grids.
[0009] In the conventional spacer grid 7, the fuel rods 8 are in
contact with the springs and dimples in a manner of point contact
or linear contact mostly. Of the two contact manners, the linear
contact manner confers higher resistance to flow-induced vibration
and abrasion of fuel rods, in comparison with the point contact
manner, so that the linear contact manner more effectively protects
the fuel rods 8 from fretting wear. That is, when the contact
surface areas between the fuel rods 8 and the springs and dimples
of the spacer grid 7 are increased under the condition that the
spring force is not changed, the peak value of contact pressure at
the contact parts is reduced such that the fretting wear of the
fuel rods 8 is diminished.
[0010] In addition, during a process of designing the springs of
spacer grids, it is necessary to preset the spring force of each
spacer grid while considering expected irradiation-induced changes
in the material characteristics of the springs. When the spring
force is too low, the springs may fail to accomplish desired
soundness of a spacer grid when the spacer grid supports fuel rods.
When the spring force is too high, the springs may impose excessive
frictional force on the claddings of fuel rods, thus scratching the
claddings to damage the claddings during a process of installing
the fuel rods in the spacer grid while producing a nuclear fuel
assembly. In addition, the springs having such excessively high
spring force may cause the fuel rods to be undesirably bent when
the fuel rods are elongated due to irradiation-induced growth
during an operation of a nuclear reactor. A desired fuel rod
support soundness of a spacer grid 7 is accomplished when the
springs and dimples of the spacer grid 7 have appropriate spring
force.
[0011] The power distribution of the fuel rods 8 in the reactor
core are unevenly distributed, so that the temperature of coolant
flowing around higher power fuel rods 8 is relatively high. When
the coolant temperature reaches close to the saturation
temperature, an excessive amount of bubbles may be generated at a
certain area of the cladding of a fuel rod 8 as much as to cover
the cladding, so that the bubble crowding remarkably reduce heat
transfer from fuel rod to coolant. In such a case, the temperature
of the local area of the cladding covered with the bubbles is
quickly increased, so that the cladding or the uranium pellets in
the fuel rod 8 may be over-heated up to a melting temperature.
[0012] In an effort to suppress the possibility of the
above-mentioned problem, the spacer grid 7 is designed to forcibly
mix the coolant flowing around the fuel rods 8 of the nuclear fuel
assembly 1, thus creating a uniform temperature distribution of the
coolant in the nuclear fuel assembly 1 as for as possible and thus
enhancing the heat transfer rate of the claddings of the fuel rods
8. The spacer grid 7 thus restricts critical nucleate boiling of
the coolant and allows the nuclear reactor to be safely operated
for a desired lengthy period of time. In order to accomplish the
above-mentioned function, mixing blades as a means for increasing
the heat transfer efficiency between the fuel rods 8 and the
coolant are attached to the spacer grid 7, as a fuel rod support
means.
[0013] In the art, coolant mixing effect in a nuclear fuel assembly
has been accomplished as follows. In a first method, the coolant
mixing may be accomplished by strong vortexes generated in coolants
flowing through the coolant passages defined among fuel rods. Due
to the strong vortexes, the coolants flowing through the coolant
passages of the spacer grid are effectively mixed with each other
to accomplish a uniform temperature distribution of coolant in the
fuel assembly. Second, swirls flow may be generated in the coolants
flowing through the spacer grid, so that the coolant having higher
density is centrifugally moved toward the surfaces of the
claddings, while the bubbles having lower density are centripetally
moved toward the centers of the swirls. The second method using the
swirls thus prevents a reduction in the heat transfer caused by the
bubbles covering the claddings of the fuel rods, so that the fuel
rod cooling performance of the nuclear fuel assembly is
enhanced.
[0014] Of the two methods, the second method using the swirls is
more preferable in that the swirls are more slowly fade out after
passing through a spacer grid 7, in comparison with the turbulent
vortexes used in the first method, and, therefore, the spacer grids
proposed in recent years have been designed on the basis of the
second method using the swirls. When the mixing blades are attached
to the spacer grid, an increase of pressure loss is caused in the
nuclear fuel assembly. Such additional pressure loss in the nuclear
fuel assembly increases the hydraulic load imposed on a pump, thus
reducing the coolant flowing in the nuclear fuel assembly.
Therefore, when attaching the mixing blades to each spacer grid of
a nuclear fuel assembly, it is necessary to design the mixing
blades so as to minimize the pressure loss in the nuclear fuel
assembly under the same projected area of the mixing blades.
[0015] In recent years, a debris filtering and/or capturing device
is also required at a nuclear fuel assembly in an effort to remove
debris from coolant and thereby protect the fuel assembly from
damage caused by the debris. A variety of debris, such as
small-sized bolts or wire pieces, are moved quickly along with
coolant in a reactor core, thus sometimes seriously damaging the
fuel rods. In order to solve the problem by filtering the coolant,
a filtering device may be mounted to a coolant passage in bottom
nozzle 3, or a debris filtering spacer grid specifically designed
to filter the debris. In such a case, the filtering spacer grid is
designed to minimize its interference with the flow of coolant and
capture the debris removed from the coolant in the spacer grid, in
place of having a fuel rod support function accomplished by the
springs and dimples.
[0016] In accordance with an extension of fuel life cycle due to
provision of nuclear fuel assemblies of high burn-up and zero
defects, the quantity of neutrons irradiated to the spacer grids is
increased, so that the problem of irradiation-induced changes in
the material characteristics of springs, resulting in a reduction
in the spring force, becomes worse. Therefore, in order to provide
nuclear fuel assemblies with zero defects in terms of fretting
wear, it is necessary to provide a spacer grid having a mechanism
capable of compensating for the reduction in the spring force and
effectively supporting the fuel rods without allowing flow-induced
vibration of the fuel rods.
[0017] In some high burn-up nuclear fuels, the level of nuclear
fuel enrichment may be increased. The increased nuclear fuel
enrichment results in much higher power peak than an average fuel
rods. Under this situation, the enhanced heat transfer rate from
the claddings may become more important considerably. Therefore, it
is necessary to provide an advanced spacer grid having a fuel rod
cooling performance higher than that of the conventional spacer
grid. In addition, it is also necessary to provide a spacer grid
capable of solving the problem of excessive pressure loss caused by
mixing blades and preventing damage to fuel rods due to debris.
SUMMARY OF THE INVENTION
[0018] Accordingly, the present invention has been made keeping in
mind the above problems occurring in the prior art, and an object
of the present invention is to provide a lips-type multi-purposed
spacer grid for nuclear fuel assemblies, which allows fuel rods to
be in contact with springs and dimples of the spacer grid in an
equiangular surface contact manner, different from conventional
spacer grids, so that contact force generated at equiangular
contact surfaces between the fuel rods and the springs and dimples
is evenly distributed to the contact surfaces, thus reducing
possible fretting wear of the fuel rods due to flow-induced
vibration of the fuel rods, and which enlarges the allowable
elastic range of the springs, thus allowing the springs to soundly
support the fuel rods by using residual spring force.
[0019] Another object of the present invention is to provide a
lips-type multi-purposed spacer grid for nuclear fuel assemblies,
which has a structure designed such that load applied to the
springs in longitudinal and latitudinal directions is distributed
to the entire structure of the spacer grid, thus reducing peak
stress at the contact surfaces between fuel rods and the springs,
and which soundly supports the fuel rods regardless of any
directional external force applied to the fuel rods or a variation
in the pressure field generated in a nuclear fuel assembly, and
which has mixing blades provided at the upper edge of dimples, thus
minimizing pressure loss and flow interference caused by
conventional mixing blades that have been provided on typical
spacer grids to generate vortexes or swirls in coolant, and the
mixing blades of which each have a spoon-shaped curve and are sized
twice as large as conventional mixing blades, so that the coolant
deflecting surfaces and the coolant mixing effect of the mixing
blades are maximized, and which minimizes pressure loss of the
nuclear fuel assembly by controlling the angle of the mixing blades
and reducing the number of the mixing blades by a half compared to
the conventional spacer grid, so that the fuel rod cooling
efficiency is improved, and which has arc-shaped edges at the
springs and dimples to define gaps between the springs and dimples,
so that lateral passages for the coolant are formed by the gaps
between the arc-shaped edges of the springs and dimples and debris
from the coolant are captured at the gaps between the springs and
dimples, and which thus minimizes damage to the fuel rods due to
such debris.
[0020] In order to accomplish the above objects, the present
invention provides a lips-type multi-purposed spacer grid
fabricated by a plurality of two types of inner strips intersecting
each other to form a plurality of unit cells, and used for
supporting a plurality of fuel rods in a nuclear fuel assembly such
that one fuel rod is supported within one unit cell, each of the
two types of inner strips being fabricated by integrating a
plurality of unit strip parts into a linear strip, and having a
plurality of notches at junctions of the unit strip parts such that
each notch vertically extends downward or upward, wherein each of
the unit strip parts comprises a frame used as a support frame of
the unit strip part, at least one water strider-type spring, and
upper and lower dimples. The frame comprises two support columns
vertically disposed in parallel while being spaced apart from each
other at a predetermined interval, and two support beams
horizontally extending between the two support columns at upper and
lower positions to connect the two support columns to each other
and define a middle opening between the support beams and the
support columns. The water strider-type spring is provided in the
middle opening of the frame while being projected in a direction
from a vertical surface formed by the frame, and comprises an
equiangular curved part axially formed along the spring and having
a predetermined width while being curved within a direction of the
width at a radius of curvature corresponding to that of an external
surface of each fuel rod, two side extensions extending outward in
opposite directions from both sides of the equiangular curved part
to a predetermined width while being bent at a predetermined angle,
and four spring legs diagonally extending from upper and lower
corners of the two side extensions such that the four spring legs
are connected to inside edges of the two support columns at four
corners of the middle opening of the frame. The upper and lower
dimples are provided at positions above and under the water
strider-type spring while being projected from the vertical surface
formed by the frame in a direction opposed to the projecting
direction of the water strider-type spring. The upper dimple is
curved along a lower edge thereof to form an arc-shaped lower edge,
and the lower dimple is curved along an upper edge thereof to form
an arc-shaped upper edge. Each of the upper and lower dimples
comprises a curved dimple part axially formed along each dimple and
having a predetermined width while being curved within a direction
of the width at a radius of curvature corresponding to that of an
external surface of each fuel rod, two side dimple extensions
extending outward in opposite directions from both sides of the
curved dimple part to a predetermined width while being curved at a
predetermined angle.
[0021] In the lips-type multi-purposed spacer grid, upper and lower
edges of the water strider-type spring are curved to form
arc-shaped edges which are symmetrical with respect to a horizontal
axis of the water strider-type spring.
[0022] In an embodiment, one water strider-type spring comprised of
a long equiangular curved part and two long side extensions is
provided in the middle opening of the frame.
[0023] In another embodiment, two water strider-type springs, each
comprising a short equiangular curved part and two short side
extensions, are provided at upper and lower portions inside the
middle opening of the frame.
[0024] In the spacer grid, a mixing blade extends upward to a
predetermined length from a side of an upper edge of the upper
dimple.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The above and other objects, features and other advantages
of the present invention will be more clearly understood from the
following detailed description taken in conjunction with the
accompanying drawings, in which:
[0026] FIG. 1 is an exploded perspective view, showing the
construction of a conventional nuclear fuel assembly;
[0027] FIG. 2 is a perspective view of a lips-type multi-purposed
spacer grid for nuclear fuel assemblies according to a primary
embodiment of the present invention;
[0028] FIGS. 3a and 3b are front views of two types of inner strips
constituting the spacer grid according to the present
invention;
[0029] FIGS. 4a and 4b are a plan view and a bottom view of the
spacer grid according to the present invention, respectively;
[0030] FIG. 5 is a front view of a unit strip part constituting an
inner strip used in the spacer grid according to the present
invention;
[0031] FIGS. 6a, 6b and 6c are a front view, a perspective view and
a plan sectional view of a water strider-type spring included in
the inner strips constituting the spacer grid according to the
present invention, respectively;
[0032] FIGS. 7a, 7b, 7c and 7d are a front view, a plan view, a
left side view and a right side view of an upper dimple having a
mixing blade included in the inner strips constituting the spacer
grid according to the present invention, respectively;
[0033] FIGS. 8a and 8b are a front view and a bottom view of a
lower dimple included in the inner strips constituting the spacer
grid according to the present invention, respectively;
[0034] FIGS. 9a and 9b are a perspective view and a plan view of a
four-walled unit cell of the spacer grid according to the present
invention, with a fuel rod supported within the cell,
respectively;
[0035] FIGS. 9c and 9d are a perspective view and a plan view of
intersecting inner strips of the spacer grid according to the
present invention, respectively;
[0036] FIG. 10 is a front view of an inner strip constituting a
lips-type multi-purposed spacer grid for nuclear fuel assemblies
according to a second embodiment of the present invention; and
[0037] FIG. 11a is a front view of an inner strip constituting a
lips-type multi-purposed spacer grid for nuclear fuel assemblies
according to a third embodiment of the present invention, and FIG.
11b is a perspective view of the lips-type multi-purposed spacer
grid fabricated by intersecting the inner strips of FIG. 11a.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] Reference should now be made to the drawings, in which the
same reference numerals are used throughout the different drawings
to designate the same or similar components.
[0039] As shown in the accompanying drawings, a lips-type
multi-purposed spacer grid 50 according to the present invention
receives and supports a plurality of elongated nuclear fuel rods 8
at positions spaced at regular intervals in a nuclear fuel
assembly, and comprises a plurality of two types of inner strips 12
and 13. The two types of inner strips 12 and 13 intersect each
other at right angles in accordance with a designed array, thus
forming the spacer grid 50 with a plurality of four-walled unit
cells for receiving and supporting the elongated nuclear fuel rods
8, as shown in FIGS. 2, 4a and 4b. Each of the inner strips 12 and
13 is fabricated, as shown in FIGS. 3a and 3b, by integrating a
plurality of unit strip parts 45 into a linear strip, and each of
the unit strip parts 45 is comprised of a water strider-type spring
16, an upper dimple 15 and a lower dimple 17, as will be described
later herein. The inner strips 12 and 13 each have a plurality of
notches at the junctions of the unit strip parts 45 such that each
notch having a predetermined length vertically extends downward or
upward. In the nuclear fuel assembly, one fuel rod 8 is received
and supported within one cell.
[0040] As shown in FIGS. 5 to 7d, each of the unit strip parts 45
constituting the inner strips 12 and 13 is comprised of a frame
used as a support frame of the unit strip part 45, the water
strider-type spring 16 formed in an opening defined at the middle
part of the frame, and the upper and lower dimples 15 and 17
provided at the upper and lower parts of the frame, respectively.
That is, the upper and lower dimples 15 and 17 are formed at
positions above and under the water strider-type spring 16 in each
unit strip part 45. The unit strip part 45 also has a mixing blade
14. The mixing blade 14 extends upward to a predetermined length
from a side of the upper edge of the upper dimple 15, as best seen
in FIG. 5.
[0041] In the spacer grid 50 according to the present invention,
the spring 16 has a shape similar to the profile of a water strider
that is an aquatic insect of the family Gerridae, having six
slender legs fringed with hairs, enabling the insect to dart about
on the surface of water. Of the six legs of the water strider, four
long legs except for two relatively short forelegs extend from the
body in diagonal directions in a manner similar to that of the
spring 16, so the spring 16 is a so-called "water strider-type
spring".
[0042] The frame of each unit strip part 45 is comprised of two
vertical support columns 40 and two horizontal support beams 25.
The two support columns 40 are vertically disposed in parallel
while being spaced apart from each other at a predetermined
interval. The two horizontal support beams 25 horizontally extend
between the two columns 40 at vertically symmetrical upper and
lower positions, thus connecting the two columns 40 to each other
and defining a rectangular middle opening between the beams 25 and
the columns 40. An upper opening which is open upward is defined
between the two columns 40 and the upper beam 25, and a lower
opening which is open downward is defined between the two columns
40 and the lower beam 25.
[0043] The water strider-type spring 16 is formed in the
rectangular middle opening of the frame. As shown in FIGS. 6a to
6c, the water strider-type spring 16 is comprised of an equiangular
curved part 23, two side extensions 36 and 37, and four spring legs
28. The equiangular curved part 23 is axially formed along the
central axis of the spring 16, and has a predetermined width while
being curved within the direction of the width at a radius of
curvature allowing the equiangular curved part 23 of the spring 16
to come into surface contact with the cladding of a fuel rod 8. The
two side extensions 36 and 37 extend outward to a predetermined
width while being curved at a predetermined angle in opposite
directions from both sides of the equiangular curved part 23. The
four spring legs 28 diagonally extend from upper and lower corners
of the two side extensions 36 and 37. The four spring legs 28 are
connected to the inside edges of the frame at the four corners of
the rectangular middle opening of the frame. The four spring legs
28 are thus integrated with the frame of the unit strip part 45 at
four points, so that the water strider-type spring 16 has a four
point support structure in which the spring 16 supports a fuel rod
at the four points. The spring 16 is also projected in a direction
from a vertical surface formed by the frame.
[0044] The upper and lower dimples 15 and 17 are formed at
positions above and under the water strider-type spring 16 in each
unit strip part 45. As shown in FIGS. 7a to 8b, each of the upper
and lower dimples 15 and 17 is comprised of a curved dimple part 20
and two side dimple extensions 21. The curved dimple part 20 is
axially formed along the central axis of each dimple 15, 17, and
has a predetermined width while being curved within the direction
of the width at a radius of curvature allowing the curved dimple
part 20 to come into surface contact with the cladding of the fuel
rod 8. The two side dimple extensions 21 extend outward in opposite
directions from both sides of the curved dimple part 20 to a
predetermined width while being curved at a predetermined
angle.
[0045] The upper dimple 15 is curved along the lower edge thereof
to form an arc-shaped lower edge 32, while the lower dimple 17 is
curved along the upper edge thereof to form an arc-shaped upper
edge 31. The upper and lower dimples 15 and 17 are also projected
from the vertical surface formed by the frame in a direction
opposed to the projecting direction of the water strider-type
spring 16.
[0046] The radius of curvature of the equiangular curved part 23 of
the water strider-type spring 16 is determined to be slightly
larger than that of the cladding of each fuel rod 8, so that the
equiangular curved part 23 comes into close surface contact with
the cladding of the fuel rod 8 and soundly supports the fuel rod 8
even when the fuel rod 8 vibrates or is impacted by external force.
That is, the radius of curvature of the equiangular curved part 23
is designed to be slightly larger than that of the cladding of each
fuel rod 8 before the fuel rod 8 is installed in a unit cell of the
spacer grid. However, after the fuel rod 8 is installed in the unit
cell of the spacer grid, the radius of curvature of the equiangular
curved part 23 becomes equal to that of the cladding because the
cladding pushes the spring 16 in a direction opposed to the
projecting direction of the spring 16.
[0047] An axial opening 29 is formed along the central axis of the
equiangular curved part 23 to have a slender appearance, so that
coolant is completely collected in the gap between the cladding of
the fuel rod 8 and the equiangular curved part 23. Therefore, it is
possible to prevent disturbance of heat transfer at a part of the
cladding due to the coolant remaining at the gap between the
cladding and the equiangular curved part 23, so that the spacer
grid does not cause nucleate boiling at the claddings of fuel rods
8.
[0048] The four spring legs 28 of the water strider-type spring 16
may have bent parts 24 at which the spring legs 28 are bent in a
direction opposed to the bent direction of the two side extensions
36 and 37. In such a case, the bent parts 24 of the spring legs 28
are projected from the vertical surface formed by the frame in a
direction opposed to the projecting direction of the equiangular
curved part 23. In addition, it is possible to control the fuel rod
support force of the water strider-type spring 16 by adjusting the
bent angle of the bent parts 24.
[0049] The upper and lower edges of the water strider-type spring
16 are curved to form arc-shaped edges which are symmetrical with
respect to a horizontal axis of the spring 16. That is, the upper
edge of the spring 16, formed by the upper edges of the equiangular
curved part 23, the two side extensions 36 and 37 and the two upper
spring legs 28, is downwardly curved to form an arc-shaped edge. In
the same manner, the lower edge of the spring 16, formed by the
lower edges of the equiangular curved part 23, the two side
extensions 36 and 37 and the two lower spring legs 28, is upwardly
curved to form an arc-shaped edge. The arc-shaped upper and lower
edges of the water strider-type spring 16 are symmetrical with
respect to the horizontal axis of the spring 16.
[0050] Therefore, when the spacer grid of the present invention
fabricated by the intersecting inner strips 12 and 13 is sectioned
along a horizontal direction as shown in FIG. 4b, each of the water
strider-type springs 16 is viewed in the form of a lower lip, while
each of the upper and lower dimples 15 and 17 is viewed in the form
of an upper lip. In each unit strip part 45 of the spacer grid, the
water strider-type spring 16 is projected in a direction opposed to
that of the upper and lower dimples 15 and 17, so that the spring
16 and the dimples 12 and 13 of each unit strip part 45 support
different fuel rods 8, separately. In addition, when the spring 16
and the dimples 12 and 13 of each unit strip part 45 are viewed
from the top or the bottom of the spacer grid, they form a pair of
lips.
[0051] As shown in FIGS. 9a to 9d, the mixing blade 14 extends
upward to a predetermined length from the upper edge of one side
dimple extension 21 of the upper dimple 15 while being smoothly
curved in the same direction as the projecting direction of the
water strider-type spring 16. The mixing blade 14 thus has a
spoon-shaped configuration which is concave at a side surface
thereof facing the fuel rod 8. It is preferable to determine the
bent angle of the mixing blade 14 relative to a vertical surface of
the unit strip part to 90.degree. or less. That is, the mixing
blade 14 is curved such that an acute angle is formed between a
normal line at the uppermost end of the mixing blade 14 and an
axial line of the side dimple extension 21 of the upper dimple
15.
[0052] In addition, the upper edge of each mixing blade 14 is
placed along a circle which has a radius larger than that of the
cladding of a fuel rod 8, as shown in FIG. 9b, so that the mixing
blades 14 do not scratch or damage the cladding of the fuel rod 8
during a process of installing fuel rods 8 in the spacer grid while
producing a nuclear fuel assembly. In order to space the mixing
blades 14 apart from the cladding of the fuel rod 8, the upper
edges of the mixing blades 14 are designed such that they are
placed along a circle having a diameter larger than that of the
cladding.
[0053] As shown in FIGS. 4a and 4b, the lips-type multi-purposed
spacer grid 50 according to the present invention is fabricated by
the two types of inner strips 12 and 13 which are each comprised of
a plurality of unit strip parts 45 integrated into a linear strip,
and which intersect each other at right angles to form a plurality
of four-walled unit cells in the spacer grid 50 for receiving and
supporting the elongated nuclear fuel rods 8 such that one fuel rod
8 is received and supported within one cell.
[0054] The water strider-type spring 16 of each unit strip part 45
is projected from a vertical surface formed by the frame of the
unit strip part 45 in a direction opposed to the projecting
direction of the upper and lower dimples 15 and 17. Therefore,
within each four-walled unit cell of the spacer grid 50 defined by
four unit strip parts 45, the two water strider-type springs 16 of
two neighboring unit strip parts 45 meeting each other at a right
angle are projected toward the center of the unit cell, and the
upper and lower dimples 15 and 17 of the remaining two unit strip
parts 45 are projected toward the center of the unit cell. The fuel
rod 8, installed within the four-walled unit cell of the spacer
grid 50, is thus supported at six points by the two springs 16 and
the four dimples 15 and 17.
[0055] In addition, the mixing blade 14 of each unit strip part 45
extends upward from the upper dimple 15 while being smoothly curved
in the same direction as that of the water strider-type spring 16
of the unit strip part 45. Therefore, the mixing blades 14 of unit
strip parts 45 are directed toward fuel rods 8 installed in
neighboring unit cells.
[0056] When viewing the spacer grid 50 of the present invention
from the top as shown in FIG. 4a, the upper dimples 15 and the
water strider-type springs 16 support the fuel rods 8 while being
curved at their fuel rod contact surfaces at the same radius of
curvature as that of the claddings of the fuel rods 8, and the
mixing blades 14 extending from the upper dimples 15 are outwardly
curved to be directed over neighboring unit cells. When viewing the
spacer grid 50 from the bottom as shown in FIG. 4b, the lower
dimples 17 and the water strider-type springs 16 support the fuel
rods 8 while being curved at their fuel rod contact surfaces at the
same radius of curvature as that of the claddings of the fuel rods
8.
[0057] FIGS. 9a and 9b show a four-walled unit cell 18 of the
spacer grid according to the present invention, with a fuel rod 8
supported within the cell 18. Particularly, these drawings show the
surface contact between the fuel rod 8 and the water strider-type
springs 16 and the upper and lower dimples 15 and 17 of unit strip
parts 45 defining the four-walled unit cell 18. FIG. 9d shows
currents of coolant guided by the mixing blades 14 of the
intersecting strips 12 and 13. Due to the mixing blades 14, the
spacer grid 50 forcibly mixes coolants flowing through the coolant
passages of the spacer grid, thus enhancing the fuel rod cooling
efficiency of the nuclear fuel assembly.
[0058] Since the upper dimple 15 has the arc-shaped lower edge 32,
and the lower dimple 17 has the arc-shaped upper edge 31, it is
possible to reduce pressure loss inside the spacer grid 50. In
addition, the spacer grid 50 of the present invention does not have
any horizontal support beam at a position under the lower dimple
17, different from conventional spacer grids, so that the inventive
spacer grid 50 effectively removes impurities from coolant when the
coolant having the impurities flows into the spacer grid 50 through
the lower end of the spacer grid 50.
[0059] The equiangular curved part 23 of each water strider-type
spring 16 is axially formed along the central axis of the spring 16
such that the curved part 23 has a substantial length. Therefore,
the fuel rod support surface of the springs 16 is enlarged, and the
fuel rod support force of the springs 16 is increased. Soundness of
the spacer grid 50 supporting the fuel rods 8 within a nuclear fuel
assembly is thus improved. Another advantage of the spacer grid 50
according to the present invention resides in that load, applied to
the water strider-type spring 16 of each unit strip part 45 from a
fuel rod 8 through the equiangular curved part 23, is effectively
distributed to the entire structure of the unit strip parts 45
through the four spring legs 28.
[0060] FIG. 10 shows an inner strip constituting a lips-type
multi-purposed spacer grid for nuclear fuel assemblies according to
a second embodiment of the present invention. As shown in the
drawing, the water strider-type spring 16' provided in each unit
strip part of the inner strip constituting the spacer grid
according to the second embodiment is designed such that the width
of the axial opening formed along the central axis of the
equiangular curved part 23 is enlarged to reduce the width of the
two side extensions 36 and 37. The spacer grid according to the
second embodiment of the present invention improves heat transfer
efficiency thereof, thus more effectively transferring heat from
the claddings of fuel rods to coolant. In this embodiment, the size
of the axial opening of the equiangular curved part 23 may be
adjusted in an effort to control the fuel rod support force of the
water strider-type spring 16', thus improving the fuel rod support
force of the springs 16' and soundness of the spacer grid
supporting the fuel rods in a nuclear fuel assembly.
[0061] FIGS. 11a and 11b show an inner strip according to a third
embodiment of the present invention, and a lips-type multi-purposed
spacer grid fabricated by intersecting the inner strips,
respectively.
[0062] As shown in the drawings, each unit strip part of the inner
strip according to the third embodiment is designed such that two
water strider-type springs 16", each having a short equiangular
curved part 23, two short side extensions 36 and 37, and four short
spring legs 28, are formed at upper and lower portions inside the
rectangular middle opening of the unit strip part. Therefore, a
fuel rod 8, installed within a four-walled unit cell of the spacer
grid, is supported at eight points by four water strider-type
springs 16" and four dimples, so that the spacer grid more stably
supports the fuel rods inside a nuclear fuel assembly. In addition,
even though one of the two springs 16" of each unit strip part is
broken by, for example, impurities, the remaining spring 16"
effectively supports the fuel rod, so that soundness of the spacer
grid supporting the fuel rods is improved.
[0063] As described above, the present invention provides a
lips-type multi-purposed spacer grid for nuclear fuel assemblies.
In the spacer grid of the present invention, fuel rods are in
contact with dimples and water strider-type springs in an
equiangular surface contact manner. The spacer grid thus soundly
supports the fuel rods even when the fuel rods are excessively
loaded in any direction due to a variation in operational
conditions of a nuclear reactor. Particularly, the fuel rod support
surface of the springs is enlarged, and the fuel rod support force
of the springs is increased. Soundness of the spacer grid
supporting the fuel rods within a nuclear fuel assembly is thus
improved, and the spacer grid reduces possible fretting wear of the
fuel rods due to hydraulic vibration of the fuel rods.
[0064] The springs of the inventive spacer grid each have four
spring legs designed in the form of four long slender legs of a
water strider. In addition, the four legs of the water strider-type
spring each have a bent part which appropriately controls the fuel
rod support spring force of the water strider-type spring, and
enlarges the allowable elastic range of the spring.
[0065] The upper and lower edges of the water strider-type spring
are curved to form arc-shaped edges which are symmetrical with
respect to a horizontal axis of the spring. Due to the arc-shaped
upper and lower edges of the water strider-type spring, the torsion
applied to the spring from a fuel rod through the equiangular
curved part is effectively distributed to the entire structure of
the unit strip part, so that the spacer grid effectively supports
the fuel rods during an operation of a nuclear reactor.
[0066] An axial opening is formed in the equiangular curved part of
the water strider-type spring, so that it is possible to prevent
disturbance of heat transfer at a part of the cladding due to
coolant remaining at a gap between the cladding and the equiangular
curved part. Therefore, the spacer grid does not cause nucleate
boiling at the claddings of the fuel rods. It is also possible to
control the fuel rod support spring force of the spacer grid to a
desired level by appropriately changing the size of the axial
opening formed at the equiangular curved part of the water
strider-type spring. It is thus not necessary to impose excessive
force on the fuel rods when installing or removing the fuel rods in
or from the spacer grid, so that the claddings of the fuel rods are
less likely to be scratched or damaged by the springs of the spacer
grid. The claddings of the fuel rods are thus prevented from
corrosion caused by such scratched or damaged parts. This results
in an extension of life spans of the fuel rods.
[0067] In addition, a mixing blade extends upward from the upper
edge of an upper dimple while being smoothly curved to have a
spoon-shaped configuration, so that the mixing blade changes the
axial flow of coolant to a lateral flow within each unit
four-walled cell of the spacer grid, thus effectively mixing the
coolant within the spacer grid. Since the upper dimple has an
arc-shaped lower edge, and the lower dimple has an arc-shaped upper
edge, it is possible to reduce pressure loss inside the spacer
grid. In addition, the flow direction of coolant flowed in from at
the lower end of the spacer grid is changed, and the debris of
coolant is guided to the gaps between the dimples and the water
strider-type springs of the spacer grid, so that debris are
effectively captured at the gaps. The spacer grid thus minimizes
damage to the fuel rods due to such debris.
[0068] Although the preferred embodiments of the present invention
have been disclosed for illustrative purposes, those skilled in the
art will appreciate that various modifications, additions and
substitutions are possible, without departing from the scope and
spirit of the invention as disclosed in the accompanying
claims.
* * * * *