U.S. patent number RE30,065 [Application Number 05/865,000] was granted by the patent office on 1979-08-07 for nuclear reactor.
This patent grant is currently assigned to Combustion Engineering, Inc.. Invention is credited to Andrew J. Anthony, Edward A. Gruber.
United States Patent |
RE30,065 |
Anthony , et al. |
August 7, 1979 |
**Please see images for:
( Certificate of Correction ) ** |
Nuclear reactor
Abstract
.Iadd.A nuclear reactor with control rods in channels between
fuel assemblies wherein the fuel assemblies incorporate guide rods
which protrude outwardly into the control rod channels to prevent
the control rods from engaging the fuel elements. The guide rods
also extend back into the fuel assembly such that they are
relatively rigid members. The guide rods are tied to the fuel
assembly end or support plates and serve as structural members
which are supported independently of the fuel element. Fuel element
spacing and support means may be attached to the guide
rods..Iaddend.
Inventors: |
Anthony; Andrew J.
(Tariffville, CT), Gruber; Edward A. (Windsor Locks,
CT) |
Assignee: |
Combustion Engineering, Inc.
(Windsor, CT)
|
Family
ID: |
24033608 |
Appl.
No.: |
05/865,000 |
Filed: |
December 27, 1977 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
511138 |
Dec 2, 1965 |
03366546 |
Jan 30, 1968 |
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Current U.S.
Class: |
376/353; 376/172;
376/447; 976/DIG.65; 976/DIG.78; 376/435; 976/DIG.19 |
Current CPC
Class: |
G21C
1/08 (20130101); G21C 3/33 (20130101); G21C
3/3563 (20130101); Y02E 30/40 (20130101); Y02E
30/30 (20130101) |
Current International
Class: |
G21C
1/00 (20060101); G21C 3/34 (20060101); G21C
1/08 (20060101); G21C 3/356 (20060101); G21C
3/33 (20060101); G21C 003/30 () |
Field of
Search: |
;176/68,74,76,78 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Nelson; Peter A.
Attorney, Agent or Firm: Berneike; Richard H.
Claims
We claim:
1. A core for a nuclear reactor comprising a plurality of
longitudinally extending generally parallel fuel elements, said
fuel elements being grouped into a plurality of fuel element
assemblies and control rods interposed between certain of said fuel
element assemblies, said fuel element assemblies adjacent said
control rods .Iadd.each .Iaddend.including .Iadd.a pair of spaced
laterally extending support plates with said fuel elements being
supported between said support plates and within the confines of a
laterally extending polygon and further including longitudinally
extending .Iaddend.guide rods .[., said guide rods extending.].
generally parallel .Iadd.and of a length coextensive .Iaddend.with
said fuel elements .Iadd., said guide rods supported between said
spaced support plates independently of said fuel elements and
arranged in said assembly .Iaddend.with a .Iadd.first
.Iaddend.portion .[.of said guide rods.]. .Iadd.thereof
.Iaddend.extending .[.outwardly from the fuel elements grouped in
said assemblies.]. .Iadd.outside of said polygon throughout the
length of said guide rods .Iaddend.towards said control rods
whereby said guide rods will prevent said control rods from
engaging said fuel elements .Iadd.and a second portion thereof
extending inside of said polygon throughout the length of said
guide rods, said guide rods thereby having a lateral dimension
extending from outside said polygon to inside said polygon such
that said guide rods are relatively rigid structural members.
.Iaddend.
2. A core for a nuclear reactor as recited in claim 1 .[.wherein
the lateral cross section of said fuel element assemblies comprises
a polygon.]. and further including fuel element spacing and support
means intermediate the ends of said fuel elements, said spacing and
support means being affixed to said guide rods whereby said spacing
and support means will be maintained in position.
3. A nuclear fuel element assembly comprising .Iadd.a pair of
spaced laterally extending support plates, .Iaddend.a plurality of
longitudinally extending, generally parallel fuel elements
.Iadd.supported between said support plates .Iaddend. and at least
one longitudinally extending guide rod generally parallel and
.Iadd.of a length .Iaddend.coextensive with said fuel elements,
said fuel elements arranged in said assembly within the confines of
a laterally extending polygon, said guide rod .Iadd.extending
between said spaced support plates and supported independently of
said fuel elements and .Iaddend.arranged in said assembly with a
.Iadd.first .Iaddend.portion thereof extending outside of said
polygon throughout the length of said guide rod .Iadd.and a second
portion thereof extending inside of said polygon throughout the
length of said guide rod, said guide rod thereby having a lateral
dimension extending from outside said polygon to inside said
polygon such that said guide rod is a relatively rigid structural
member .Iaddend.and further including a fuel element spacing and
support means intermediate the ends of said fuel elements, said
means including a band extending laterally around the outside of
said fuel elements and defining said polygon, said band having a
portion removed, a portion of said guide rod extending through said
removed portion so as to extend beyond the outside of said
band.
4. A core for a nuclear reactor comprising a plurality of
longitudinally extending fuel element assemblies, said fuel element
assemblies spaced apart to provide control rod channels
therebetween, said fuel element assemblies each comprising .Iadd.a
pair of spaced laterally extending support plates, .Iaddend.a
plurality of longitudinally extending, generally parallel fuel
elements .Iadd.supported between said support plates .Iaddend.and
at least one longitudinally extending guide rod generally parallel
to and coextensive with said fuel elements .Iadd.and supported
independently of said fuel elements, .Iaddend.said fuel elements
arranged in said assemblies within the confines of laterally
extending polygons, and said guide rods arranged in said assemblies
with a portion of each of said guide rods extending outside of said
polygons and into said control rod channels substantially
throughout the length of said guide rods thereby providing control
rod guide surfaces extending outwardly from said fuel element
assemblies .Iadd.and with another portion of each of said guide
rods extending inside of said polygons whereby said guide rods have
a lateral dimension extending from outside said polygons to inside
said polygons such that said guide rods are relatively rigid
structural members. .Iaddend.
5. A core for a nuclear reactor as recited in claim 4 wherein each
of said fuel element assemblies includes a plurality of said guide
rods with at least one guide rod on each side of said polygon.
6. A nuclear fuel element assembly as recited in claim .[.5.].
.Iadd.9 .Iaddend.wherein said band is attached to said guide rod
whereby said fuel element spacing and support means will be
maintained in position.
7. A core for a nuclear reactor as recited in claim 4 wherein said
fuel elements and said guide rods have generally circular cross
sections of approximately equal size and wherein said portions
extending outside said polygons comprise longitudinally extending
bars affixed to said guide rods.
8. A core for a nuclear reactor as recited in claim 4 wherein said
fuel elements and said guide rods have generally circular cross
sections and wherein the cross sections of said guide rods are
larger than the cross sections of said fuel elements and wherein
said portions extending outside said polygons comprise
longitudinally extending segments of said guide rods.
9. A core for a nuclear reactor as recited in claim 8 and further
including fuel element spacing and support means intermediate the
ends of said fuel elements, said spacing and support means
including bands extending laterally around the outside of said fuel
elements and defining said polygons, said bands having portions
removed, and said longitudinally extending segments of said guide
rods extending through said removed portions so as to extend beyond
the outside of said bands.
Description
The present invention relates to nuclear reactors and more
particularly to a reactor core design incorporating novel fuel
element assemblies. The invention more specifically relates to
means for both supporting the fuel element assemblies and providing
guide means for control rods whereby control rod followers may be
eliminated.
It is well known that the fuel or fusionable material for
heterogeneous nuclear reactors is frequently contained in a number
of thin tubes comprising fuel elements or rods which are grouped
and joined together in the reactor in fuel element assemblies or
bundles. Each reactor has a number of such fuel assemblies therein
comprising the reactor core. It is also well known that reactors
normally have one or more control rods which serve to regulate the
reactivity of the reactor. These rods, which are composed of a
neutron absorbing material, extend down into the reactor core among
the fuel assemblies to absorb neutrons and reduce the reactivity.
These control rods fit into channels between the spaced fuel
element assemblies.
Prior art control rods are relatively large in cross sectional area
so as to be of sufficient rigidity. This is necessary to prevent
the control rods from flexing within the control rod channels and
either jamming or damaging the adjacent fuel elements. The large
cross sectional area of the control rods requires that the control
rod channels be of a comparable large size to accommodate the rods.
When the reactor is operating and the control rods are withdrawn
from the core, the control rod channels become filled with the
reactor coolant, which would be water in the case of a water
cooler-water moderated reactor. This concentration of water which
is large compared to the amount of water between the fuel elements
within the fuel assemblies causes a neutron flux peak in the region
of the control rod channels. This is due to the large concentration
of moderator in this region together with the low neutron capture
cross section of the water. This flux peaking causes undesirably
high temperatures in the adjacent fuel elements. One common prior
art method of eliminating this problem is to employ control rod
followers which are attached to the free ends of the control rods
and which occupy the control rod channels when the control rods are
withdrawn. These followers, which are of a relatively non-absorbing
material such as Zircaloy, therefore displace the water from the
channels and eliminate the flux peaking. The followers may also be
of such a length so as to be constantly in engagement with a guide
means so that there will be support for the control rods and
followers at all times from both ends.
The use of control rod followers, however, presents many problems.
The follower itself, of course, involves some considerable expense.
The attachment of the follower to the control rod means that the
control rod-control rod follower assembly is on the order of twice
as long as the control rod without the follower. The means that the
reactor pressure vessel must be considerably longer or higher in
order to accommodate the control rod followers in a position
exterior of the core. This large vessel, of course, involves
considerable expense. The larger pressure vessel also normally
means that the vessel will contain more water which, in turn, means
that the containment system for the reactor must be larger in order
to confine the pressure resulting from the energy release of the
increased amount of water in the event of an incident. The longer
control rod and follower combination also means that there is
additional weight and, therefore, additional problems and expense
involved in the mechanisms for controlling the rods. One of the
major problems in the use of control rod followers is the
connection between the follower and the control rod. These two
sections must, of course, be of different materials since they must
have vastly different neutron absorbing properties. Since the
connection between the rod and the follower travels through the
control rod channel it cannot be a bulky type of connection which
would require that the control rod channel be increased in area.
With prior art attaching means the dimensional accuracy which can
be readily maintained on the connection between the rod and the
follower is less than the accuracy attainable on the control rod
itself thus requiring that the channel be enlarged slightly to
allow for the maximum possible size of the joint. The enlarged
channel, of course, means increased flux peaking.
The present invention therefore proposes the elimination of the
control rod followers with a simultaneous reduction in the cross
sectional area of the control rods and the control rod channels. It
would, of course, be desirable to reduce the size of the control
rod channels such that the fuel elements on either side of the
channel are not substantially farther apart than the fuel elements
within each of the fuel element assemblies. Such a spacing would
virtually eliminate local flux peaking. The provision of control
rod channels of such a size or of a size even approaching this
presents several problems the solution of which the invention is
directed. Reactor control must be available for starting the
reactor, for bringing the power output to the desired level, to
maintain it at that level, and then to shut down the reactor when
necessary. Safety control must also be available to prevent damage
in case of an accident. It is obviously quite essential that the
control rods be constantly free to move in and out of the core
without any possibility of sticking or jamming in the control rod
channels. It is also quite essential that contact between the
control rods and the fuel elements be prevented since such contact
might eventually cause the rupture of the fuel elements and the
resultant contamination with radioactive material. Since the cross
sectional area of the control rods of the present invention has
been reduced as discussed above, the rods no longer have the
rigidity necessary to avoid such contact in control rod channels of
the desired small size. Therefore, to maintain the channels small,
there must be provision to avoid both jamming of the control rods
and damage to the fuel elements.
The solution to the above problem also brought about the solution
to another problem involved in the construction of fuel element
assemblies. The fuel elements in these assemblies are
conventionally rather long, thin tubes arranged in a spaced array.
Since the elements are of relatively great length as compared not
only to their individual diameters but also as to the lateral
dimension of the fuel assembly it has become the practice to
provide lateral spacing and support means along the length of the
fuel assembly at selected locations. These spacing and support
means prevent any bowing of the fuel elements and maintain the
elements in parallel alignment as is necessary for proper reactor
operation. The fuel elements within a single fuel assembly
frequently undergo unequal thermal expansion which must be freely
permitted to avoid excessive stresses in the fuel elements. It is
therefore necessary that the spacing and support means permit the
fuel elements to slide relative thereto while at the same time
maintaining the elements in lateral alignment. For this reason the
spacing and support means cannot be affixed directly to any of the
fuel elements. It is necessary, however, that the spacing and
support means be maintained in the proper position along the length
of the fuel assemblies. Means other than the fuel elements
themselves must be provided for this purpose.
The present invention accordingly has for an object the provision
of a reactor core arrangement employing thin control rods and
control rod channels thereby eliminating the necessity for rod
followers.
The invention also has for an object the provision of novel and
effective guide means for reactor control rods.
Another object of the invention is the provision of supporting
means for the various components of a nuclear reactor fuel element
assembly.
A further object of the invention is the provision of novel support
means for fuel element assemblies which also serve as guide means
for control rods.
Another object of the invention is the provision of a novel fuel
element assembly construction which will provide guide means for
control rods and which can be readily fabricated with a minimum of
components.
A further object of the invention is the provision of control rod
guide means which will not substantially increase the size of the
control rod channel.
These and other features, advantages and objects of the present
invention will be explained more fully in, and will be apparent
from, the following description of the invention when read in
conjunction with the accompanying drawings wherein:
FIG. 1 is a vertical section view of a nuclear reactor suitable for
use with the present invention;
FIG. 2 is a horizontal cross section view of the reactor taken
along line 2--2 of FIG. 1;
FIG. 3 is an elevation view of a fuel assembly;
FIG. 4 is a plan view of the fuel assembly of FIG. 3;
FIG. 5 is a partial cross section view of a fuel assembly taken
along line 5--5 of FIG. 3;
FIG. 6 is an isometric view of a portion of a fuel element and the
associated spacing and supporting means;
FIG. 7 is a schematic illustration of an arrangement of fuel
assemblies and control rods embodying the present invention;
FIG. 8 is a schematic illustration similar to FIG. 7 incorporating
a modified fuel assembly;
FIG. 9 is an isometric illustration of the guide rod of the present
invention together with the adjacent fuel rods and a portion of the
spacing and support means;
FIGS. 10, 11, and 12 illustrate various stages of a procedure for
connecting the spacing and support means to the guide rod;
FIGS. 13 and 14 illustrate alternative control rod guide means;
and
FIG. 15 illustrates the invention when applied to hexagonal fuel
assemblies .
The reactor 10 shown in FIGS. 1 and 2 comprises a pressure vessel
12, a thermal shield 14 and a core support barrel 16 surrounding
and containing a reactor core 18. This core is composed of a
plurality of fuel assemblies 20 disposed in mutually perpendicular
spaced rows. Interposed in the spaces between certain of the fuel
assemblies are control rods 22 which are of conventional cruciform
configuration. The general arrangement of the reactor components is
depicted in FIG. 1 wherein only two of the control rods have been
illustrated for purposes of clarity. The construction of the
overall reactor and the manner in which the fuel assemblies and
control rods are mounted in the reactor form no part of the present
invention, however, and may be of any one of the many conventional
designs which are well-known in the art requiring no detailed
description.
The fuel assemblies 20 each comprise a plurality of fuel elements
24 as shown in FIG. 3. These fuel elements may be formed of tubes
of stainless steel or other materials such as Zircaloy partially
filled with pellets, discs, compacts, or powders of fissionable
material. The tubes are capped at the bottom and top ends,
respectively, by caps 26 and 28. The caps 26 at the lower end are
rigidly affixed to a bottom support plate 30 such as by a force fit
between pins 32 on the end caps 26 and blind poles 34 in the
support plate 30. The upper ends of the fuel elements are held in
position by a top support plate 36, the plan view of which is shown
in FIG. 4. Pins 38 on the upper end caps 28 slidably fit into the
holes 40 in the top suport plate. This slidable engagement between
the fuel elements and the top support plate permits differential
thermal expansion between the various fuel elements due to unequal
heating.
FIG. 4 also illustrates a plurality of holes 42 interspersed
between the holes 40. Similar holes also are formed in the bottom
support plate 30 for the purpose of permitting the cooling fluid to
flow upwardly through the bottom support plate, upwardly between
the fuel elements of the fuel assembly and then out through the
holes 42 in the top support plate. Also affixed to the top support
plate 36 is a cruciform projection 44 which is for the purpose of
handling the fuel assemblies and maintaining them in the correct
position in the reactor core.
Since the fuel elements are not rigidly fastened to the top support
plate it is necessary to provide means for holding this support
plate in position. For this purpose a number of rods 46 are
employed. These rods have threaded end projections 48 which project
through appropriately positioned holes in the top and bottom
support plates. Nuts 50 are then threaded on the projections 48 and
preferably welded in place to prevent loosening during operation.
Rods 46 together with the nuts 50 will then hold the fuel
assemblies together as a unit. These rods 46 also serve other
functions which will be pointed out hereinafter. It can be seen in
FIG. 4 that the fluid flow holes 52 which are adjacent the nuts 50
are smaller than the fluid holes 42 so as not to unduly weaken the
support plates at this point and interfere with the nuts 50.
The fuel elements 24 are of relatively great length as compared not
only to their individual diameters but also as to the lateral
dimensions of the fuel assembly which renders them susceptible to
bending or bowing as previously pointed out. Such misalignment
means that the fuel elements are closer together in certain
portions of the assembly and that the area of the liquid flow path
is altered in the same area. This will cause flux peaking in the
liquid channels adjacent the bowed elements as well as unequal
distribution of coolant flow which will result in overheating of
the fuel elements in that portion of the assembly. The bowing of
peripherally located fuel elements can also jam or obstruct control
rod movement. It is therefore necessary to provide lateral spacing
and support means along the length of the fuel assembly to maintain
the fuel elements in parallel alignment.
A spacing and support means 54 is illustrated in FIGS. 3, 5 and 6.
The spacing and support means comprises a frame 56 around the
periphery of the assembly encircling the array of fuel elements.
Within the boundaries of the frame 56 are a plurality of members 58
which form a gridlike structure. These grid-forming components or
members 58 are formed from thin elongated strips or bands of metal
as is the frame 56. The grid-forming members 58 have slots 60
therein at the intersections of the members which permit the
members to be interlocked together as shown in FIG. 6. This
"egg-crate" like structure provides a plurality of compartments 62
through which the fuel elements 24 extend.
Each of the grid-forming members 58 is bent at points 63 generally
corresponding to the mid-points of the compartments 62. Each bend
will, therefore, extend outwardly from one compartment and into the
adjacent compartment to form fuel element engaging arches therein.
Formed from the grid-forming members 58 are spring tabs 64 which
extend into the compartments 62 from two adjacent sides. These
spring tabs engage the fuel elements and force them against the
opposing arches.
The perimeter frame 56 is punched out as at 66 in FIGS. 3, 9, 13
and 14 at locations corresponding to the engagement of the frame
with the fuel elements. These cut-outs permit local cooling of the
fuel element cladding which touches the frame.
Since the spacing and support means 54 is not affixed directly to
the fuel elements, but is slideable relative thereto, it is
necessary to provide means for maintaining the support means 54 in
position. This can be accomplished by attaching the support means
54 to the rods 46 such as by welding. The specific manner in which
this attachment is preferably accomplished will be discussed
hereinafter. The rods 46 are, therefore, serving not only to tie
the entire fuel assembly together but also to maintain the spacing
and support means in its proper position along the length of the
assembly. It is obvious that there may be a plurality of spacing
and support means 54 along the length of each of the fuel
assemblies to provide the required bracing.
The relationship of the control rods to the fuel elements is shown
in FIGS. 7 and 8 as well as in FIG. 2. The FIG. 2 arrangement
illustrates groups of four fuel assemblies surrounded by portions
of four control rods while the FIGS. 7 and 8 arrangement has each
fuel assembly surrounded by portions of two control rods. Many
different arrangements are possible for which the invention is
equally applicable. It can be seen from these illustrations that if
a control rod were to rotate as a unit only slightly or to shift
laterally in the control rod channel, the control rod would contact
the fuel assemblies. Since the control rods are thin and flexible,
it is also possible for the individual blades of the cruciform
control rods to flex relative to each other and contact the
assemblies. The presence of the spacing and support means 54 and in
particular the presence of the encircling frame 56 which protrudes
outwardly from the exterior fuel elements presents a control rod
jamming problem. The ends of the control rods could engage the top
of the frame 56 upon insertion of the rod and thus cause the rod to
jam. This, of course, could be diastrous.
The prevention of contact between the control rods and the fuel
elements themselves as well as the prevention of jamming is
accomplished by the use of guide surfaces 68. These guide surfaces
are formed by and comprise the exterior surfaces of guide means
which are a part of the fuel element assemblies. These guide means
may take various forms and several modifications will be described.
There are, however, several basic necessary or desirable features
of the guide surfaces and guide means which are common to the
various modifications. First of all, the guide surfaces should
extend the full length of the fuel assemblies or at least
substantially the full length so as to protect the fuel elements
throughout the full excursion of the control rods. It is also
essential that the guide means be relatively rigid so that they
will remain straight and in position under operating conditions.
This requires that the dimension of the guide means in a direction
perpendicular to the sides of the fuel assemblies be substantial so
as to prevent the guide means from flexing in this plane. At the
same time it is essential that the guide means protrude outwardly
from the sides of the fuel assemblies only a very small distance so
as to keep the control rod channels small. These two latter
requirements are counter to each other and the solution requires
that the guide means extend some distance back into the interior of
the fuel assemblies to provide the necessary rigidity. This,
however, presents other design problems since the guide means
cannot interfere with the fuel elements such as by causing hot
spots.
The guide means illustrated in FIGS. 3, 5, 9 and 10-12 comprise the
previously described rods 46 which already serve to tie the fuel
assemblies together and to hold the spacing and support means 54 in
position. The guide surfaces 68 comprise the protruding outer
surfaces of the rods 46 which are larger in diameter than the
associated fuel elements as most clearly shown in FIG. 5.
Longitudinally extending segments of the rods 46 and therefore the
guide surfaces 68 protrude through cut-out portions in the frame
56. These rods 46 provide all the desirable features mentioned
above for the guide means, i.e., (1) they extend the full length of
the fuel assemblies, (2) they extend outwardly from the sides of
the fuel assemblies only a very small distance, (3) they are rigid
since they extend back into the interior of the fuel assemblies.
FIGS. 5 and 7 illustrate the use of eight guide rods per fuel
assembly while the fuel assembly of FIG. 8 employs only four guide
rods. There may, of course, be any number of guide rods in each
fuel assembly so long as the necessary control rod support and fuel
assembly support are provided. The advantage of the symmetrical
FIG. 7 arrangement over the unsymmetrical FIG. 8 arrangement is
that the fuel assemblies can be inserted into the reactor core
without regard for orientation while the assemblies of FIG. 8 must
be inserted in the proper manner so as to have the guide rods
properly positioned.
The guide rods 46 may be attached to the frames 56 in a number of
ways. The frame in FIG. 9 has merely been cut out to provide the
proper size gap 70 in the frame and then fusion welded at 72 to the
guide rod. FIGS. 10-12 illustrate another manner of attaching the
frame to the guide rods. In this procedure a gap 70 is also cut
into the frame member 56 in the proper location adjacent the guide
rod 46. This forms tabs 74 on the frame 56. Recesses 76 are then
cut into the guide rod 46 underlying the tabs 74. These recesses 76
are formed to the depth necessary to provide the proper guide
surface protrusion. Holes 78 are then drilled in the guide rod 46
in the recesses between the tabs 74. The holes are then tapped and
the threaded pin 80 inserted. The protruding portion of the pin 80
is then fusion welded over the tabs 74 of the frame 56 to form an
intimate bond between the pin 80, the frame 56 and the guide rod
46. After fusion welding, the exterior surface of the weld is
blended to the contour of the guide rod diameter as shown in FIG.
12. A smooth continuous guide surface is thereby formed.
The guide rods need not be cylindrical as have been illustrated
thus far. FIG. 13 shows a guide rod 82 which is somewhat
semi-circular in cross sectional configuration with a protruding
portion 84. This portion 84 extends through a gap in the frame 56
to form the guide surface 68. The frame 56 is then suitably
attached to the rod 82. It is obvious that guide rods of many
configurations may be employed. The guide means discussed thus far
have been solid rods rather than fuel containing tubes. Since there
is normally a neutron flux differential across the width of a fuel
assembly, the fuel elements on one side of the assembly are apt to
be at a different temperature than the elements on the other side.
Since the guide rods which have been illustrated are also serving
as structural members to tie the fuel assemblies together, a
temperature differential between these tying rods on opposite sides
of the assembly would cause a bowing of the fuel assembly as a
unit. The solid rods which contain no fuel are, on the other hand,
all essentially at the temperature of the reactor coolant. There is
no temperature differential which would cause bowing of the fuel
assemblies. It is therefore preferable that fuel containing tubes
not be used as the guide rods. However, it may in certain instances
be permissible or even desirable to use tubes as guide rods. These
tubes could either be poison containing tubes or fuel containing
tubes in which case they might contain a fuel of a lower
enrichment. In the case of the lower fuel enrichment there would
not be such a pronounced tendency for temperature differential and
thus their use might be permissible. Since there is normally a
neutron flux peak adjacent the corners of the fuel assemblies, it
might, in certain instances, be advantageous to employ guide rods
containing a burnable poison in that area to reduce the neutron
flux.
FIG. 14 illustrates the use of a tube 86 as a guide rod. This tube
may contain a fuel or a poison. Rather than employ a portion of the
tube 86 itself as the guide surface, a bar 88 is attached
throughout its length by fusion welding to the exterior surface of
the tube 86. The outside surface 68 of the bar 88 thereby serves as
the guide surface. The bar 88 standing by itself unattached to tube
86 would not be rigid enough to serve as a guide means. By
attaching the bar to the tube 86, however, the guide means is given
the necessary effective depth and thereby the necessary rigidity.
Frame 56 is attached to the guide means by welding it to the bar
88. The opposite side of tube 86 may also contain a bar
corresponding to bar 88 which will serve to balance the temperature
of the cladding.
The invention has been illustrated thus far as applied to
rectangular fuel assemblies and to cruciform control rods. It is
obvious that the principles and advantages of the invention may be
utilized with various forms of fuel assemblies. FIG. 15
schematically illustrates the invention as applied to hexagonal
fuel assemblies 90 and Y-shaped control rods 92. A symmetrical
arrangement of guide rods 94 is illustrated. The invention could as
readily be applied to triangular fuel assemblies.
The guide rods not only serve as guides for the control rods but
they also protect the fuel elements from damage during the handling
of the fuel assemblies. During insertion of the fuel assemblies
into the reactor core, it is virtually impossible to keep the
assemblies from engaging the previously inserted adjacent
assemblies. Absent the guide rods the fuel elements would most
certainly rub against at least one of the top support plates 36
thus risking damage and rupture of an element. With the guide rods
in position, however, it is the guide rods and not the fuel
elements themselves which engage the various portions of the
adjacent fuel assemblies. It is therefore evident that the guide
rods in the forms proposed by the present invention serve a
plurality of functions.
While preferred embodiments of the invention have been shown and
described it will be understood that such showings are illustrative
rather than restrictive and that changes in construction,
combination and arrangement of parts may be made without departing
from the spirit and scope of the invention as claimed.
* * * * *