U.S. patent application number 10/358605 was filed with the patent office on 2003-11-20 for grid for a nuclear fuel assembly, and a strap for such a grid.
Invention is credited to Bonnamour, Michel, Mayet, Roland.
Application Number | 20030215049 10/358605 |
Document ID | / |
Family ID | 9509159 |
Filed Date | 2003-11-20 |
United States Patent
Application |
20030215049 |
Kind Code |
A1 |
Mayet, Roland ; et
al. |
November 20, 2003 |
Grid for a nuclear fuel assembly, and a strap for such a grid
Abstract
At regular intervals, the grid strap has formed therein notches
for assembly with transverse straps. Between at least some pairs of
notches, it has a rod-holding spring comprising two resilient
strips cut out from the plane wall of the strap and integral
therewith. Each strip extends transversely relative to its strap
and is connected to the common portion of the strap at its two
ends. In the free state, the strips project towards the inside of
the cell concerned and they are interconnected in the middle by a
transverse bridge which projects relative to the two strips.
Inventors: |
Mayet, Roland;
(Villeurbanne, FR) ; Bonnamour, Michel; (Lyon,
FR) |
Correspondence
Address: |
KENYON & KENYON
ONE BROADWAY
NEW YORK
NY
10004
US
|
Family ID: |
9509159 |
Appl. No.: |
10/358605 |
Filed: |
February 5, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10358605 |
Feb 5, 2003 |
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09242694 |
Feb 23, 1999 |
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6542567 |
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Current U.S.
Class: |
376/442 |
Current CPC
Class: |
G21C 3/3563 20130101;
Y02E 30/30 20130101 |
Class at
Publication: |
376/442 |
International
Class: |
G21C 003/34 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 11, 1997 |
FR |
97 08873 |
Claims
1/ A grid strap for holding fuel rods, in which there are formed
notches at regular intervals for assembly with transverse straps,
and between at least some pairs of notches, rod-holding springs
(22, 30), each comprising two resilient strips (24) cut out from
the plane wall of the strap and integral therewith, each of which
extends mainly in the flow direction and is connected to the common
portion of the strap via its two ends, the strips projecting, when
in the free state, towards the inside of the cell concerned and
being interconnected in their middle by a transverse bridge (26)
projecting relative to both strips, the two resilient strips (24)
of at least some of the springs (22) touching each other in the
vicinity of their roots and being spaced apart from each other in
their central portions which are interconnected by the bridge
(26).
2/ A grid strap for holding fuel rods, in which there are formed
notches at regular intervals for assembly with transverse straps,
and between at least some pairs of notches, rod-holding springs
(22, 30), each comprising two resilient strips (24) cut out from
the plane wall of the strap and integral therewith, each of which
extends mainly in the flow direction and is connected to the common
portion of the strap via its two ends, the strips projecting, when
in the free state, towards the inside of the cell concerned and
being interconnected in their middle by a transverse bridge (26)
projecting relative to both strips, the shape of the strips and of
the bridge of each spring (22, 30) being such that the strips are
pushed back elastically into the same plane as the common portion
of the strap provided with the spring by the presence of a rod.
3/ A strap according to claim 1 or 2, characterized in that the
springs are of different structures depending on whether the wall
in which the spring is formed lies between two cells that are both
designed to be occupied by respective rods, or between two cells,
one of which is designed to be occupied by a guide tube (20).
4/ A strap according to claim 1, 2, or 3, characterized in that the
two strips of some only of the springs (30) are substantially
parallel to each other and to the flow direction.
5/ A strap according to claim 2, characterized in that the springs
(30) having parallel strips are backed by a cell which has a guide
tube (20) passing therethrough.
6/ A strap according to claim 4 or 5, characterized in that each of
the springs. (30) having parallel strips lies, in the direction
extending transversely relative to the strap, between weld zones
that are substantially plane or slightly curved.
7/ A grid according to any one of claims 1 to 4, characterized in
that each spring designed to be backed by a cell containing a fuel
rod lies, in the direction extending transversely to the long
dimension of the strap, between two bosses (28) projecting in a
direction opposite to that of the spring (22).
8/ A strap according to claim 7, characterized in that the bridge
(26) of a spring (22) and the bosses (28) on either side thereof
and constituting abutment means, are of the same shape.
9/ A grid for holding fuel rods for a nuclear fuel assembly, the
grid being constituted by two crossed sets of straps (12, 14)
according to any preceding claim, and being characterized in that
the upstream portion of each end of each strap includes cutouts or
reinforcements of curved shape serving as bearing points for an
inwardly curved belt (38), at least on the upstream side of the
grid.
10/ A grid according to claim 9, characterized in that the cutouts
or reinforcements of curved shape include a stud (44) engaged in a
hole in the curved portion of the belt.
11/ A grid according to claim 9 or 10, characterized in that the
belt carries reinforced springs, integral therewith.
12/ A grid according to claim 11, characterized in that each spring
of the belt is cut out from a zone of the belt which is inwardly
deformed to constitute a swelling, protecting the springs against
direct external stresses and against excessive outwards
displacement of the rod.
Description
[0001] The invention relates to a grid for holding fuel rods in a
nuclear fuel assembly, and in particular to those used in
pressurized water reactors to hold the rods at the nodes of a
regular array, generally a square array.
[0002] The invention relates in particular to the structure of the
straps which are assembled together to make up the grids, generally
by means of half-depth slots.
[0003] The grids of an assembly define common or regular cells
designed to receive rods, and other cells which have guide tubes
passing through them, the guide tubes often being welded to the
grids. Conventionally, in the grids used for holding rods axially
and laterally, rigid bearing dimples are formed for the rods in two
of the four faces of each common cell for the purpose of receiving
a rod, and two holding springs are cut out from or added to the
other two faces so as to press the rod onto the dimples.
[0004] The invention relates in particular to the straps of a grid
in which the springs are integrally formed with the flat portion of
each strap. At least in the portion of the assembly which is
subjected to a high level of neutron flux, these straps are made of
a zirconium based alloy such as Zircaloy 4. Under neutron
radiation, the mechanical characteristics of such alloys are
degraded. Proposals have already been made to limit the
consequences of such degradation by making the springs in special
manner. Document FR-A-2 338 549 describes a grid strap in which
each spring comprises a flexible corrugated strip extending in the
flow direction of the coolant, i.e. transversely relative to the
strap, or two parallel strips that are interconnected in their
middle portions by a portion which projects into the flow and which
is extended, between the strips, by two tabs. This projecting
portion constitutes an obstacle which disturbs the flow of coolant.
Document WO-A-94/25935 describes a spring constituted by a
resilient strip whose ends are connected to the current portion of
the strap via two stamped bridges. Given their orientation, the
bridges do not hinder the flow of coolant to any significant
extent, but the bend taken up by the blade when pressed against a
rod still constitutes an obstacle to the flow.
[0005] The present invention seeks in particular to provide a strap
having holding springs of a structure such that they provide a
satisfactory compromise between mechanical strength, conserving a
satisfactory level of resilient holding force, and high
transparency to the flow of coolant.
[0006] To this end, the invention provides, in particular, a grid
strap for holding fuel rods, in which there are formed notches at
regular intervals for assembly with transverse straps, and between
at least some pairs of notches, rod-holding springs each comprising
two resilient strips cut out from the-plane wall of the strap and
integral therewith, each strip extending in the axial flow
direction in a grid including the strap and is connected to the
strap at its two ends, the strips projecting, at least when in
released condition, towards the inside of the cell concerned and
being interconnected in their middles by a transverse bridge
projecting relative to both strips.
[0007] The springs of an internal strap may all have the same
structure, or they may be of different structures, for example
depending on whether the wall in which the spring is formed
separates two cells that are both occupied by a rod, or separates
two cells, one of which is occupied by a guide tube.
[0008] In the first case, the two resilient strips advantageously
join together in the vicinity of their roots, i.e. their zones
close to where they connect to the current portion of the strap,
and are on the contrary remote from each other in their central
portions which are interconnected by the bridge. In particular, the
portions close to the roots and the central portions may be
rectilinear in the flow direction; each root is then connected to
the corresponding central portion via a zone that is curved or
oblique. Such a structure avoids having large openings in the strap
which could disturb the flow, in particular by allowing
uncontrolled interchanges between cells.
[0009] In the second case, the strap constitutes a cell wall
backing a guide tube; the spring can be constituted by two mutually
parallel strips in the flow direction, having their central
portions interconnected by the bridge. There is no drawback in
having an opening in the strap at this location, given that the
opening is closed by the guide tube.
[0010] On either side of such a spring, in the flow direction,
there remains a substantially flat zone in which the strap can be
fixed to the guide tube by welding.
[0011] The rod is provided with support on its side opposite from
its side against which the spring presses. For this reason, the
spring, situated in a middle portion of the strap, is generally
located between two bearing areas constituted by stamped bridges
extending transversely to the flow so as to avoid significantly
altering the flow section for the coolant. The bridges of the
springs and those of the bearing areas may be of the same
shape.
[0012] The initial shape given to the spring strips by stamping is
advantageously such that the strips are pushed back substantially
into the same plane as the current portion of the strap when a rod
is inserted and is to be held by the spring. The bridges and the
dimples advantageously project equally into the cell when the
springs are in the active position.
[0013] The straps may also be provided with deflector fins for
improving mixing of the coolant, which fins are curved in
shape.
[0014] As a general rule, the grid has outer straps constituting a
belt and in abutment against innner straps via tenon-and-mortise
type links. The outer straps may have guide fins for limiting the
risk of catching between the grids of adjacent fuel assemblies
while a fuel assembly is being loaded or unloaded.
[0015] In an advantageous embodiment, the outside plates are curved
at the base of the guide fins so as to follow the shape of the
terminal edges of the common inner straps which have curved shapes
for this purpose.
[0016] The outer straps may also contribute to holding peripheral
rods and may be provided with springs. Given the relatively small
number of grid cells concerned, and in order to make these straps
very strong, the springs which are obtained by cutting out and
stamping, may be disposed in the flow direction.
[0017] In particular, the straps may be made out of one of the
zirconium alloys known as "Zircaloy 4", in the recrystallized
state.
[0018] The above characteristics, advantageously usable in
combination, but capable of being used separately, together with
other characteristics, appear more clearly on reading the following
description of particular embodiments, given as non-limiting
examples. The description refers to the accompanying drawings, in
which:
[0019] FIG. 1 is a perspective view of a fraction of a grid and a
guide tube, the inside straps being provided with springs
constituting a particular embodiment, and shown in the free
state;
[0020] FIG. 2 is similar to FIG. 1, showing the springs in the
"active" position, i.e. in the state where they are forced back by
an abutting rod;
[0021] FIG. 3 is a cross-sectional view along a plane containing
the axis of a cell containing a rod, showing one spring in its
active position and another spring in the free state;
[0022] FIG. 4 is a plan view of FIG. 3;
[0023] FIG. 5 is a view similar to a fragment of FIG. 1, showing a
modified embodiment that makes it possible to increase the strength
of the grids;
[0024] FIG. 6 is a front view of a fragment of an outside strap
belonging to the belt of the grid, showing a spring for holding a
rod, which spring is of reinforced resilience and is less sensitive
to accidental external stresses;
[0025] FIGS. 7 and 8 are section views of FIG. 6 respectively along
lines VII-VII and VIII-VIII;
[0026] FIG. 9 shows a fragment of a strap including springs of the
kind shown in FIG. 1 and its junction with a belt strap; and
[0027] FIG. 10 is a perspective view showing a fragment of a grid
of the kind shown in FIG. 9.
[0028] The grid 10, a fragment of which is shown in FIGS. 1 and 2,
is constituted in particular by two sets of crossed straps 12 and
14 that are mutually engaged via half-depth notches provided for
this purpose. These straps are fixed to one another, e.g. by
welding at the cross-points 16. The straps 12 and 14 define cells
that are to receive fuel rods 18, such as those shown in
chain-dotted lines in FIG. 2. However, some of the cells receive
guide tubes 20 designed to provide a passage for the absorbent rods
of control rod clusters. In general, at least some of the guide
tubes are fixed to the grids. They interconnect top and bottom fuel
assembly nozzles (not shown).
[0029] At least some of the grids of an assembly are designed to
hold the rods at the nodes of a regular array and to support them
vertically. The grid shown by way of example in FIGS. 1 and 2
performs both of these functions. For this purpose, in two
orthogonal directions, the grid includes bearing means formed in
one face of a cell and a spring cut out from the opposite face of
the cell serving to press the rod against the bearing means.
[0030] Two of the faces of each cell containing a rod have
respective springs of resilience urging them to project into the
cell. The structure of such a spring varies depending on whether it
is backed by a cell that is to receive a fuel rod or a cell that
contains a guide tube 20.
[0031] Each of the springs 22 between two cells occupied by rods is
constituted by two resilient strips 24 cut out from the plane wall
of the strap, and integral therewith, which strips are
interconnected in their middles by a transverse bridge 26
projecting relative to the two strips. Each strip extends
transversely to the long dimension of the strap, i.e. in the flow
direction. Each strip 24 is connected to the strap at both ends. At
rest, a strip slopes towards the inside of the cell from each end
(FIG. 1). The bridge projects further relative to the two strips
that it interconnects.
[0032] In the flow direction, each spring 22 lies between bearing
means constituted by two bosses 28. The bosses 28 and the bridges
26 may be of exactly the same shape.
[0033] The springs and the bridges can be fabricated in
conventional manner, by stamping and stretching.
[0034] The resilient strips 24 touch each other close to their
roots, and they are spaced apart from each other in their middle
portions so as to leave the room required for the bridge. In this
way, the gaps between the two strips which puts two adjacent cells
each occupied by a rod into communication is narrow in width; it
hardly disturbs the flow.
[0035] Springs 30 backing onto a cell containing a guide tube 20
also comprise two resilient strips that are cut out from the strap
and interconnected in their middle portions by a bridge 26.
However, the two strips 32 of the spring 30 extend substantially
parallel to each other and to the flow direction. The gap that this
establishes is not a drawback since the gap is closed by the guide
tube 20.
[0036] The grid can be fixed to the guide tubes by spot welds,
either in zones 34 that are substantially plane or slightly curved
situated on either side of the springs 30, or else on tabs formed
at the top and/or bottom portions of the straps in register with
the cell containing the guide tube 20.
[0037] The extent to which the bridges 26 project relative to the
strips, and the shape of the strips at rest are advantageously
selected in such a manner that the strips are forced back
substantially into the same plane as the current portion of each
strap when the rods 18 are in place, as can be seen in FIGS. 2, 3,
and 4.
[0038] By way of example, in FIG. 3, the spring 22 of the strap 12
is straight and projecting because it is not being pushed back by a
rod. In contrast, the spring 22 of the left strap is bent by a rod
18 at a point such that its strips lie in practically the same
plane as the common portion of the strap 20, thereby preventing
them from impeding the flow.
[0039] To increase the strength of such grids, the straps can be
welded together not only at the cross-points 16 in each of the
major faces of the grid, but also at additional locations. By way
of example, FIG. 5 shows straps 12 having pairs of additional
windows 36 in alignment with each line of crossovers with other
straps. This provides access making it possible to weld the straps
together at two additional points at least. Welding is performed by
means of a tool which is inserted into the grid, e.g. a laser beam
tool. The windows 36 are generally situated between the bosses,
level with a spring, in the flow direction.
[0040] The grid may have four outer straps that are interconnected
to form a belt. While a fuel assembly is being loaded or unloaded,
there is a risk of a belt of the assembly catching on the belt of
an adjacent assembly. To reduce this risk, proposals have already
been made to curve the corners of the belt inwards. The risk of
catching is further reduced with a belt of the kind shown in part
in FIGS. 9 and 10. The belt-constituting outside straps have a
special shape. These straps are curved inwards in their portions 40
close to their upstream edges and 42 close to their downstream
edges. The ends of the common straps 12 and 14 are cut out
accordingly. The upstream portion of the end of each strap 12 or 14
is rounded or chamfered and may also have a stud 44 engaged in a
slot of the curved portion of the belt strap.
[0041] The downstream portion at the end of each strap 12 or 14
includes a projection 46 directed in the flow direction, having
rounded or chamfered edges, and against which the belt bears. This
projection may also have a stud 44. The extent to which the
projection extends may be substantially equal to the extent to
which the flow deflection fins 48 extend in conventional
manner.
[0042] The belt shown in FIGS. 9 and 10 may be used with common
straps 12 and 14 having springs of a structure that is different
from that shown in FIGS. 1 to 4; in particular they may be of
conventional structure.
[0043] The springs for holding the peripheral rods situated against
the outer belt can be exposed to accidental stresses which run the
risk of spoiling their support function. In an advantageous
embodiment of the invention, the grid has a belt 38 in which
reinforced springs are provided. For this purpose, the room for
displacement of the resilient blades is restricted, and they are
provided with additional protection by setting them back relative
to the outside face of the grid.
[0044] Reinforcement can be provided in particular by means of the
kind shown in FIGS. 6 to 8. The spring 52 is constituted by cutting
out and bending a zone of the belt which is deformed inwards so as
to constitute a swelling 54. In FIG. 6, it can be seen that this
swelling is of elongate shape in the flow direction, and thus in
the spring direction, with parallel edges interconnected by
semicircular portions. The swelling may also be made by stamping or
by shaping in a press.
[0045] The spring 52 itself is of conventional shape, with a
projecting central portion 56, however this shape is not essential.
At rest it may project by an amount that is different from the
amount by which the springs 22 and 30 project. Because of the
presence of the swelling, the spring is protected against any risk
of direct external attack, thereby making it possible to take full
advantage of the lateral cutouts to give it optimum resilience and
stiffness. Furthermore, the swelling puts a natural limit on the
maximum bending of the spring by limiting the extent to which a rod
can move towards the outside of the grid to a value such that the
rod comes to bear thereagainst.
[0046] When it is necessary to reduce the headlosses imparted to
the coolant to as small a value as possible, the upstream edges of
the straps, of the belt, of the dimples, and/or of the bridges can
be shaped, for example as described in the patent application filed
on the same day as the present application and entitled "A grid
strap for a nuclear fuel assembly, and a grid including such
straps".
* * * * *