U.S. patent application number 14/243954 was filed with the patent office on 2017-12-07 for low pressure drop nuclear fuel assembly bottom nozzle.
This patent application is currently assigned to Westinghouse Electric Company LLC. The applicant listed for this patent is Westinghouse Electric Company LLC. Invention is credited to Kirkland D. Broach, Peyton M. Frick, David S. Huegel, Michael L. Lewis, Mark W. Peterson.
Application Number | 20170352438 14/243954 |
Document ID | / |
Family ID | 54480893 |
Filed Date | 2017-12-07 |
United States Patent
Application |
20170352438 |
Kind Code |
A1 |
Broach; Kirkland D. ; et
al. |
December 7, 2017 |
LOW PRESSURE DROP NUCLEAR FUEL ASSEMBLY BOTTOM NOZZLE
Abstract
A nuclear fuel assembly having a bottom nozzle with protrusions
that extend from the upstream (lower or fluid entry) and downstream
(upper or fluid exit) side of a horizontally supported perforated
flow plate. The protrusions have a funnel-like shape that gradually
decreases the lateral flow area on the upstream side of the
perforated flow plate and gradually increases the lateral flow area
on the downstream side of the perforated plate. The protrusions on
the downstream side are preferably recessed to accommodate the ends
of the fuel rods.
Inventors: |
Broach; Kirkland D.;
(Lexington, SC) ; Lewis; Michael L.; (Lexington,
SC) ; Peterson; Mark W.; (Chapin, SC) ;
Huegel; David S.; (Blythewood, SC) ; Frick; Peyton
M.; (Chapin, SC) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Westinghouse Electric Company LLC |
Cranberry Township |
PA |
US |
|
|
Assignee: |
Westinghouse Electric Company
LLC
Cranberry Township
PA
|
Family ID: |
54480893 |
Appl. No.: |
14/243954 |
Filed: |
April 3, 2014 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G21C 3/3305 20130101;
Y02E 30/30 20130101; Y02E 30/38 20130101; G21C 3/322 20130101 |
International
Class: |
G21C 3/33 20060101
G21C003/33; G21C 3/322 20060101 G21C003/322 |
Claims
1. A nuclear fuel assembly including: a plurality of elongated
nuclear fuel rods having an extended axial length; at least a
lowermost grid supporting said fuel rods in an organized array and
having unoccupied spaces defined therein adapted to allow flow of
fluid coolant therethrough and past said fuel rods when said fuel
assembly is installed in a nuclear reactor; a plurality of guide
thimbles extending along said fuel rods through and supporting said
grid; a bottom nozzle disposed below said grid, below lower ends of
said fuel rods, supporting said guide thimbles and having openings
therethrough to allow the flow of fluid coolant into said fuel
assembly, the bottom nozzle comprising: a substantially horizontal
plate, orthogonal to the axis of the fuel rods, having an upper
face directed substantially toward the lowermost grid and a lower
face on an underside of said horizontal plate with said openings
extending therethrough for the flow of coolant, at least some of
said openings in at least one of either the upper face or the lower
face having a funnel-like first appendage respectively extending
above the upper face or below the lower face, as the case may be,
around at least some of the openings in the one of the upper face
or the lower face with an opening at the first appendage's
substantially highest extent, in the case of the upper face or
lowest extent, in the case of the lower face, having a larger
diameter than a diameter of the opening in the one of the upper
face or the lower face, an internal wall of the first appendage
substantially gradually decreasing in diameter from the opening at
the first appendage's substantially highest extent in the case of
the upper face or substantially lowest extent in the case of the
lower face, until the wall of the first appendage transitions to
the opening in the upper face or lower face, as the case may be and
wherein a lip of at least some of the openings at the first
appendage's substantially highest extent in the case of the upper
face or lowest extent in the case of the lower face, has a
scalloped contour.
2. (canceled)
3. The nuclear fuel assembly of claim 2 wherein the scalloped lip
has four substantially equally spaced depressions, resembling the
contour of an egg receptacle in an egg carton.
4. The nuclear fuel assembly of claim 2 wherein the substantially
horizontal plate has a plurality of such openings extending
therethrough and substantially all of the lips of the openings at
the first appendages' substantially highest extents in the case of
the upper face or lowest extents in the case of the lower face,
have a scalloped contour.
5. The nuclear fuel assembly of claim 1 including a funnel-like
second appendage extending outwardly from at least some of the
openings in another of the upper face or the lower face with an
opening at the second appendages' substantially highest extent in
the case of the upper face or lowest extent in the case of the
lower face, having a larger diameter than a diameter of the opening
in the another of the upper face or the lower face, an internal
wall of the second appendage substantially gradually decreases in
diameter from the opening at the second appendages' substantially
highest extent in the case of the upper face or lowest extent in
the case of the lower face until the wall of the second appendage
transitions to the opening in the another of the upper face or the
lower face.
6. The nuclear fuel assembly of claim 5 wherein a lip of at least
some of the openings at the second appendages' substantially
highest extent in the case of the upper face or lowest extent in
the case of the lower face, has a scalloped contour.
7. The nuclear fuel assembly of claim 6 wherein the scalloped lip
has four substantially equally spaced depressions resembling the
contour of an egg receptacle in an egg carton.
8. The nuclear fuel assembly of claim 7 wherein the appendage to
the upper face is at least partially recessed within a
corresponding one of the openings in the upper face.
9. The nuclear fuel assembly of claim 5 wherein substantially all
of the lips of the appendages' substantially highest extent, on the
upper face have a scalloped contour
10. The nuclear fuel assembly of claim 5 wherein the appendages to
the upper face terminate below the lower ends of the fuel rods.
11. The nuclear fuel assembly of claim 10 wherein the highest
extent of the appendages to the upper face is smaller than the
lowest extent of the appendages to the lower face.
12. The nuclear fuel assembly of claim 1 wherein at least some of
the openings in the bottom nozzle substantially align with the
unoccupied spaces in the lowermost grid.
13. A nuclear fuel assembly including: a plurality of elongated
nuclear fuel rods having an extended axial length; at least a
lowermost grid supporting said fuel rods in an organized array and
having unoccupied spaces defined therein adapted to allow flow of
fluid coolant therethrough and past said fuel rods when said fuel
assembly is installed in a nuclear reactor; a plurality of guide
thimbles extending along said fuel rods through and supporting said
grid; a bottom nozzle disposed below said grid, below lower ends of
said fuel rods, supporting said guide thimbles and having openings
therethrough to allow the flow of fluid coolant into said fuel
assembly, the bottom nozzle comprising: a substantially horizontal
plate, orthogonal to the axis of the fuel rods, having an upper
face directed substantially toward the lower most grid and a lower
face on an underside of said horizontal plate with said openings
extending therethrough for the flow of coolant, at least some of
said openings in at least one of either the upper face or the lower
face having a funnel-like first appendage respectively extending
above the upper face or below the lower face, as the case may be,
around at least some of the openings in the at least one of the
upper face or the lower face with an opening at the first
appendage's substantially highest extent, in the case of the upper
face or lowest extent, in the case of the lower face, having a
larger diameter than a diameter of the opening in the at least one
of the upper face the lower face, an internal wall of the first
appendage substantially gradually decreasing the lateral flow area
axially through the first appendage as the first appendage
transitions from the opening at the first appendage's substantially
highest extent, in the case of the upper face or lowest extent, in
the case of the lower face, to the opening in upper face or the
lower face, as the case may be, wherein a lip of at least some of
the openings at the first appendage's substantially highest extent
in the case of the upper face or lowest extent in the case of the
lower face, has a scalloped contour.
14. The nuclear fuel assembly of claim 13 including a funnel-like
second appendage extending up from at least some of the openings in
another of the upper face or the lower face with an opening at the
second appendage's substantially highest extent, in the case of the
upper face, or lowest extent, in the case of the lower face, having
a larger diameter than a diameter of the opening in the another of
the upper face or the lower face, an internal wall of the second
appendage substantially gradually decreases the lateral flow area
axially through the second appendage as the second appendage
transitions from the opening at the second appendage's
substantially highest extent in the case of the upper face or
lowest extent in the case of the lower face to the opening in the
another of the upper face or the lower face, as the case may
be.
15. The nuclear fuel assembly of claim 14 wherein the appendage to
the upper face is at least partially recessed within a
corresponding one of the openings in the upper face.
16. The nuclear fuel assembly of claim 1 wherein the openings for
the flow of coolant include both first openings that are aligned
with the unoccupied spaces in the lowermost grid and additional
openings that are aligned with the fuel rods.
17. The nuclear fuel assembly of claim 16 wherein at least some of
the additional openings have appendages substantially the same in
general design as the first appendage.
18. The nuclear fuel assembly of claim 17 wherein the additional
openings respectively have a standoff at a coolant flow exit, with
the standoff configured to prevent the fuel rod from closing off
the coolant flow exit.
19. The nuclear fuel assembly of claim 18 wherein the highest
extend of the appendages to additional openings on the upper face
have a scalloped lip forming peaks and valleys wherein the peaks
form the standoff.
20. The nuclear fuel assembly of claim 16 wherein the additional
openings are smaller in diameter than the openings aligned with the
unoccupied spaces.
21. A nuclear fuel assembly including: a plurality of elongated
nuclear fuel rods having an extended axial length; at least a
lowermost grid supporting said fuel rods in an organized array and
having unoccupied spaces defined therein adapted to allow flow of
fluid coolant therethrough and past said fuel rods when said fuel
assembly is installed in a nuclear reactor; a plurality of guide
thimbles extending along said fuel rods through and supporting said
grid; a bottom nozzle disposed below said grid, below lower ends of
said fuel rods, supporting said guide thimbles and having openings
therethrough to allow the flow of fluid coolant into said fuel
assembly, the bottom nozzle comprising: a substantially horizontal
plate, orthogonal to the axis of the fuel rods, having an upper
face directed substantially toward the lowermost grid and a lower
face on an underside of said horizontal plate with said openings
extending therethrough for the flow of coolant, at least some of
said openings positioned in line with a corresponding one of the
fuel rods and wherein a lip of at least some of the openings'
substantially highest extent in the case of an upper face of the
substantially horizontal plate or lowest extent in the case of the
lower face of the horizontal plate, has a scalloped contour.
22. The nuclear fuel assembly of claim 21 wherein at least some of
the openings are in line with the unoccupied spaces.
23. The nuclear fuel assembly of claim 22 wherein the openings in
line with the unoccupied spaces are larger than the openings in
line with the fuel rods.
Description
BACKGROUND
1. Field
[0001] The present invention relates generally to nuclear reactors
and, more particularly, is concerned with reducing the pressure
drop across the bottom nozzle of a nuclear fuel assembly.
2. Related Art
[0002] The primary side of nuclear reactor power generating systems
which are cooled with water under pressure comprises a closed
circuit which is isolated and in heat exchange relationship with a
secondary circuit for the production of useful energy. The primary
side comprises the reactor vessel enclosing a core internal
structure that supports a plurality of fuel assemblies containing
fissile material, the primary circuit within heat exchange steam
generators, the inner volume of a pressurizer, pumps and pipes for
circulating pressurized water; the pipes connecting each of the
steam generators and pumps to the reactor vessel independently.
Each of the parts of the primary side comprising a steam generator,
a pump, and a system of pipes which are connected to the vessel
form a loop of the primary side.
[0003] For the purpose of illustration, FIG. 1 shows a simplified
nuclear primary system, including a generally cylindrical reactor
pressure vessel 10 having a closure head 12 enclosing a nuclear
core 14. A liquid reactor coolant, such as water, is pumped into
the vessel 10 by pump 16 through the core 14 where heat energy is
absorbed and is discharged to a heat exchanger 18 typically
referred to as a steam generator, in which heat is transferred to a
utilization circuit (not shown), such as a steam driven turbine
generator. The reactor coolant is then returned to the pump 16,
completing the primary loop. Typically, a plurality of the
above-described loops are connected to a single reactor vessel 10
by reactor coolant piping 20.
[0004] An exemplary conventional reactor design is shown in more
detail in FIG. 2. In addition to the core 14 comprised of a
plurality of parallel, vertical, co-extending fuel assemblies 22,
for the purposes of this description, the other vessel internal
structures can be divided into the lower internals 24 and the upper
internals 26. In conventional designs, the lower internals'
function is to support, align and guide core components and
instrumentation as well as direct flow within the vessel. The upper
internals restrain or provide a secondary restraint for the fuel
assemblies 22 (only two of which are shown for simplicity in FIG.
2), and support and guide instrumentation and components, such as
control rods 28. In the exemplary reactor shown in FIG. 2, coolant
enters the reactor vessel 10 through one or more inlet nozzles 30,
flows down through an annulus between the reactor vessel and the
core barrel 32, is turned 180.degree. in a lower plenum 34, passes
upwardly through lower support plate 37 and lower core plate 36
upon which the fuel assemblies are seated and through and about the
fuel assemblies. In some designs, the lower support plate 37 and
the lower core plate 36 are replaced by a single structure, a lower
core support plate having the same elevation as 37. The coolant
flow through the core and surrounding area 38 is typically large on
the order of 400,000 gallons per minute at a velocity of
approximately 20 feet per second. The resulting pressure drop and
frictional forces cause an upward force on the fuel assembly whose
movement is restrained by the upper internals including a circular
upper core plate 40. Coolant exiting the core 14 flows along the
underside of the upper core plate 40 and upwardly through a
plurality of perforations 42. The coolant then flows upwardly and
radially to one or more outlet nozzles 44.
[0005] The upper internals 26 are supported from the vessel or the
vessel head and include an upper support assembly 46. Loads are
transmitted between the upper support assembly 46 and the upper
core plate 40, primarily by a plurality of support columns 48. A
support column is aligned above a selected fuel assembly 22 and
perforations 42 in the upper core plate 40.
[0006] Rectilinearly moveable control rods 28, which typically
include a drive shaft 50 and a spider assembly 52 of neutron poison
rods (shown and described more fully with regard to FIG. 3), are
guided through the upper internals 26 and into aligned fuel
assemblies 22 by control rod guide tubes 54. The guide tubes are
fixedly joined to the upper support assembly 46 and the top of the
upper core plate 40. The support column 48 arrangement assists in
retarding guide tube deformation under accident conditions which
could detrimentally effect control rod insertion capability.
[0007] FIG. 3 is an elevational view, represented in vertically
shortened form, of a fuel assembly being generally designated by
reference character 22. The fuel assembly 22 is the type used in a
pressurized water reactor and has a structural skeleton which, at
its lower end includes a bottom nozzle 58. The bottom nozzle 58
supports the fuel assembly 22 on lower core plate 36 in the core
region of the nuclear reactor. In addition to the bottom nozzle 58,
the structural skeleton of the fuel assembly 22 also includes a top
nozzle 62 at its upper end and a number of guide tubes or thimbles
84 which align with guide tubes 54 in the upper internals. The
guide tubes or thimbles 84 extend longitudinally between the bottom
and top nozzles 58 and 62 and at opposite ends are rigidly attached
thereto.
[0008] The fuel assembly 22 further includes a plurality of
transverse grids 64 axially spaced along and mounted to the guide
thimbles 84 and an organized array of elongated fuel rods 66
transversely spaced and supported by the grids 64. Also, the
assembly 22, as shown in FIG. 3, has an instrumentation tube 68
located in the center thereof that extends between and is captured
by the bottom and top nozzles 58 and 62. With such an arrangement
of parts, the fuel assembly 22 forms an integral unit capable of
being conveniently handled without damaging the assembly of
parts.
[0009] As mentioned above, the fuel rods 66 in the array thereof in
the assembly 22 are held in spaced relationship with one another by
the grids 64 spaced along the fuel assembly length. Each fuel rod
66 includes a plurality of nuclear fuel pellets 70 and is closed at
its opposite ends by upper and lower end plugs 72 and 74. The
pellets 70 are maintained in a stack by a plenum spring 76 disposed
between the upper end plug 72 and the top of the pellet stack. The
fuel pellets 70, composed of fissile material, are responsible for
creating the reactive power of the reactor. The cladding which
surrounds the pellets functions as a barrier to prevent fission
byproducts from entering the coolant and contaminating the reactor
system.
[0010] To control the fission process, a number of control rods 78
are reciprocally moveable in the guide thimbles 84 located at
predetermined positions in the fuel assembly 22. Specifically, a
rod cluster control mechanism 80, positioned above the top nozzles
62 of selected fuel assemblies, supports a plurality of the control
rods 78. The control mechanism has an internally threaded
cylindrical hub member 82 with a plurality of radially extending
flukes or arms 52 that form the spider previously noted with regard
to FIG. 2. Each arm 52 is interconnected to a control rod 78 such
that the control rod mechanism 80 is operable to move the control
rods vertically in the guide thimbles 84 to thereby control the
fission process in the corresponding fuel assembly 22, under the
motive power of a control rod drive shaft 50 which is coupled to
the control rod hub 80, all in a well-known manner.
[0011] It is desirable to have a balanced flow across the reactor
core, i.e., substantially the same pressure drop across each of the
fuel assemblies, so that some of the fuel assemblies do not operate
at higher temperatures than other fuel assemblies. Power output is
limited by the hottest operating fuel element. Reducing pressure
drop without comprising other beneficial features provides the fuel
assembly designer the opportunity to add other features to make up
for the reduced pressure drop, that can, for example, promote
mixing which will enhance heat transfer that can translate into
increased power output of the core. The bottom nozzles of the fuel
assemblies, which include a horizontal top plate with a large
number of flow through holes, is a significant contributor to that
pressure drop. That is especially true for debris filter bottom
nozzles which require that the holes be small enough to prevent the
passage of debris that could damage the fuel rod cladding such as
is described in U.S. Pat. No. 7,822,165, assigned to the Assignee
of this application. Any modification that reduces the pressure
drop across the fuel assemblies without otherwise adversely
impacting the operation of the reactor core is desirable.
[0012] Therefore, it is an object of this invention to reduce the
pressure drop across the fuel assemblies by modifying the design of
the bottom nozzles to alter the shape of the flow through
holes.
[0013] It is a further object of this invention to achieve that
reduction in pressure drop by gradually changing the lateral flow
area on either or both the upstream and downstream sides of the
perforated flow plate of the bottom nozzle.
SUMMARY
[0014] These and other objects are achieved by a nuclear fuel
assembly having a plurality of elongated nuclear fuel rods with an
extended axial length. At least a lower most grid supports the fuel
rods in an organized array having unoccupied spaces defined therein
adapted to allow flow of fluid coolant therethrough and past the
fuel rods when the fuel assembly is installed in a nuclear reactor.
A plurality of guide thimbles extend along the fuel rods through
and supporting the grid. A bottom nozzle is disposed below the
lower most grid, below lower ends of the fuel rods and supports the
guide thimbles. The bottom nozzle has openings therethrough to
allow the flow of fluid coolant into the fuel assembly. The bottom
nozzle includes a substantially horizontal plate supported
orthogonal to the axis of the fuel rods. The horizontal plate has
an upper face directed substantially toward the lower most grid and
a lower face on an underside of the horizontal plate with the
openings extending therethrough for the flow of coolant. At least
some of the openings in the lower face have a funnel-like first
appendage respectively extending below the lower face, around at
least some of the openings in the lower face with an opening at the
first appendage's substantially lowest extent having a larger
diameter than a diameter of the opening in the lower face. An
internal wall of the first appendage substantially gradually
decreases in diameter from the opening at the first appendages
substantially lowest extent until the wall of the first appendage
transitions to the opening in the lower face. In one preferred
embodiment, a lip in the opening in at least some of the first
appendage's substantially lowest extent has a scalloped contour and
preferably, the scalloped lip has a plurality of spaced
depressions, resembling the contour of an egg receptacle in an egg
carton and more preferably all of the lip of the opening at the
first appendage's substantially lowest extent has such a scalloped
contour.
[0015] In one embodiment, the nuclear fuel assembly includes a
funnel-like second appendage that extends up from at least some of
the openings in the upper face with an opening at the second
appendage's substantially highest extent having a larger diameter
than a diameter of the opening in the upper face. An internal wall
of the second appendage substantially, gradually increases in
diameter from the transition at the opening in the upper face to
the second appendage's substantially highest extent. In this latter
embodiment, a lip of the opening at the second appendage's
substantially highest extent has a scalloped contour. Desirably,
the second appendage is at least partially recessed within the
opening in the upper face. In one embodiment the highest extent of
the second appendage terminates below the lower ends of the fuel
rods and desirably, the highest extent of the second appendage is
smaller than the lowest extent of the first appendage. At least
some of the openings in the bottom nozzle substantially align with
the unoccupied spaces in the lower most grid.
[0016] In general, the internal wall of the first appendage
gradually decreases the lateral flow area axially through the first
appendage as the first appendage transitions to the opening in the
lower face. The internal wall of the second appendage gradually
increases the lateral flow area axially through the second
appendage as the second appendage transitions from the opening in
the upper face to the unoccupied flow spaces defined within the
organized array of fuel rods.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] A further understanding of the invention can be gained from
the following description of the preferred embodiments when read in
conjunction with the accompanying drawings in which:
[0018] FIG. 1 is a simplified schematic of a nuclear reactor system
to which this invention can be applied;
[0019] FIG. 2 is an elevational view, partially in section, of a
nuclear reactor vessel and internal components to which this
invention can be applied;
[0020] FIG. 3 is an elevational view, partially in section, of a
fuel assembly illustrated in vertically shortened form, with parts
broken away for clarity;
[0021] FIG. 4 is an isometric view of a portion of one embodiment
of the bottom nozzle top plate and flow through holes of this
invention showing recesses in the upper face that interface with
the fuel rod end plugs;
[0022] FIG. 5 is an isometric view of the embodiment shown in FIG.
4 with the fuel rod end plugs in position;
[0023] FIG. 6 is a partial side view of the embodiment illustrated
in FIGS. 4 and 5;
[0024] FIG. 7 is a bottom plan view of the embodiment illustrated
in FIGS. 4 and 5;
[0025] FIG. 8 is a top plan view of the embodiment illustrated in
FIGS. 4 and 5 (with portions of the end plugs removed);
[0026] FIG. 9 is a sectioned isometric view of another embodiment
of the bottom nozzle top plate and flow through holes of this
invention;
[0027] FIG. 10 is the sectioned isometric view shown in FIG. 9 with
the lower portion of a number of fuel element end plugs shown
interfacing with the appendages of this invention;
[0028] FIG. 11 is a top view of the horizontal lower nozzle plate
of FIGS. 9 and 10, showing the positioning of the flow through
holes;
[0029] FIG. 12 is a bottom view of the hole pattern shown in FIG.
11; and
[0030] FIG. 13 is a sectioned side view of the flow holes (shown
with the lower end of the fuel rod end plugs).
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0031] The present invention relates to a bottom nozzle 58 for a
fuel assembly which, in addition to supporting the fuel assembly 22
on the lower core plate 36, also contains features which function
to reduce the pressure drop across the nozzle. This can be
appreciated from FIG. 3. The bottom nozzle includes a support
means, for example, the skirt 56 shown in FIG. 3. The support
means, skirt 56 in this embodiment, includes a plurality of corner
legs 60 for supporting the fuel assembly 22 on the lower core plate
36. A generally rectangular, planar plate 86 is suitably attached
to the upper surface of the support skirt 56. In the nozzle plate
86 of this embodiment, a large number of relatively small holes are
provided to accommodate the passage of coolant from below the plate
86 to and through the lower most grid 88. These holes may be small
enough to trap debris to shield the fuel element cladding from
damage as described in U.S. Pat. No. 7,822,165, though it should be
appreciated that this invention can provide a benefit to most any
type of flow through hole in a fuel assembly seeking to minimize
pressure drop.
[0032] This invention recognizes that a significant portion of the
pressure drop associated with the bottom nozzle flow plate 86 is
due to abrupt changes in flow area. This advanced bottom nozzle
concept incorporates "egg-crate" type features on either or both
the upstream and downstream sides of the bottom nozzle flow plate
86 to gradually change the lateral flow area in the flow through
path through the flow plate 86.
[0033] FIGS. 4 through 8 show a portion of one embodiment of a flow
plate 86 incorporating the features claimed hereafter. FIG. 4 shows
an isometric view of a portion of the flow plate 86 with parts of
the flow through holes 90 broken away to observe the interior of
the flow through holes. FIG. 5 is the isometric view illustrated in
FIG. 4 with the fuel rod end plugs 74 shown in position above the
flow plate 86. FIG. 6 is a side view of a portion of the flow plate
shown in FIG. 5. FIG. 7 is a bottom plan view of the flow plate
shown in FIG. 5; and FIG. 8 is a top plan view of the portion of
the flow plate shown in FIG. 4. On the upstream side (i.e., from
the underside of the flow plate 86), which can best be appreciated
from FIG. 7, streamlined "egg-crate" protrusions 92 gradually
reduce the lateral flow area to minimize form losses associated
with the rapid contraction that the coolant flow must undergo as it
enters the perforated flow plate at the entrance to the flow holes
90. These "egg-crate" protrusions 92 also eliminate high pressure
pockets of recirculating flow below each fuel rod location. The
protrusions 92 are funnel-like extensions of the openings of the
flow through holes 90 with a lip 98 that surrounds an opening on
the lower most extent of the protrusions 92 having depressions 94
that in one embodiment are approximately equally spaced around its
circumference; though it should be appreciated that the depressions
need not be equally spaced to obtain some reduction in pressure
drop. The depressions in the lip 94 form a scalloped contour.
Additionally, though the protrusions 92 extending on either side of
the flow plate 86 are shown to be approximately the same height,
the height may vary over the surface of the plate and still obtain
a reduction in pressure drop.
[0034] On the downstream side (i.e., between the flow plate 86 and
the fuel rods), streamlined "egg-crate" protrusions 96 gradually
increase the lateral flow area to minimize form losses associated
with the rapid expansion and contraction that the coolant
experiences in the transition from the flow plate 86 to the fuel
rod bundle. Due to the close proximity of the fuel rod bottom end
plugs 74, the downstream "egg-crate" protrusions are recessed in
the upper face of the plate 86 to interface with the fuel rods 66.
There are no changes to the axial elevations of the fuel rods.
[0035] The development of advanced fabrication techniques such as
additive manufacturing makes the manufacture of this design more
convenient, though it should be appreciated that traditional
manufacturing techniques can also be employed. Though the egg-crate
protrusion design has been applied to the upper and lower surfaces
of the flow through plate 86 it should be appreciated that either
of these designs may be employed alone to obtain some reduction in
pressure drop or together to minimize the pressure drop for maximum
benefit.
[0036] Furthermore, an additional reduction in pressure drop can be
achieved employing the embodiment illustrated in FIGS. 9-13. This
embodiment retains the streamlined flow passages unique to the
foregoing embodiment, which has the flow through holes
substantially aligned with the unoccupied spaces between the
lowermost grid and the fuel rod, but adds an additional flow path
substantially in-line with the fuel rods. The additional flow holes
100 are of a similar design to the other flow holes 90, but are
positioned directly under the fuel rods, are preferably smaller in
diameter and have a set of standoffs 102 supporting the fuel rods
and allowing the coolant flow to exit the bottom nozzle. The
standoff may be the peaks of the scalloped lips of the appendages
and ensure that the fuel rods don't block the flow holes during
operation. Because the additional flow holes 100 are directly under
the fuel rods they provide a "no-line-of-sight" path for the flow
which helps minimize debris from passing thru the bottom nozzle yet
help reduce the overall loss coefficient of the bottom nozzle by
providing an additional flow path. Testing of this added feature
showed a significant improvement over the embodiment employing the
appendages without the additional flow holes in-line with the fuel
rods.
[0037] While specific embodiments of the invention have been
described in detail, it will be appreciated by those skilled in the
art that various modifications and alternatives to those details
could be developed in light of the overall teachings of the
disclosure. Accordingly, the particular embodiments disclosed are
meant to be illustrative only and not limiting as to the scope of
the invention which is to be given the full breadth of the appended
claims and any and all equivalents thereof.
* * * * *