Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates ~o nuclear reactor fuel
assemblies each constituted by a cluster of parallel fuel pins
of substantial length and small cross-sectional area each
containing a stack of nuclear fuel pellets within a metal can.
This fuel cluster is associated along the length of the fuel
pins with spacer grids placed at uniform intervals so as to
permit suitable bracing of the fuel pins with respect to each
other while preventing vibrations of these latter and deforma-
tion of the initial geometry of the cluster under the action
of the stresses to which the fuel assembly is subjected
during operation within the reactor core.
In a fuel assembly of this type which is usually
intended to be employed in a light water reactor, the inven-
tion is more particularly applicable to an improvement in thç
construction of the spacer grids of the fuel-pin cluster.
It is known that a spacer grid of the type mentioned
above is usually constituted by an open structure formed by two
sets of sheet metal members which are parallel to each other
but oriented in two perpendicular directions in each set.
Said sheet metal members are intended to interengage in slits
formed lengthwise in these latter and thus constitute a
series of compartments having a generally square cross-section
through which the fuel pins are intended to pass in a
direction parallel to the plane of said sheet metal members.
A number of different solutions have already been proposed in
order to ensure correct positioning of said fuel pins within
each compartment of the spacer grid and especially in order to
ensure suitable stress distribution on the fuel-pin cans at
the level of each spacer grid. In particular, the sheet metal
members of the grid are provided with wlde openings separated
by narrow corrugated strips forming springs and projecting
in tlle canadian Patent N~92~725 o~ the June 2B, 1969
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either inwardly or outwardly from the compartments so as to
provide, in conjunction with other corrugated strips or with
bosses formed in the solid portions of the sheet metal
members, a series of bearing points for each fuel pin at the
location corresponding to the passage of said pin through
each compartment. Each fuel pin is thus maintained in two
different directions at four points spaced at uniform inter-
vals at its periphery and also at a number of points in the
longitudinal direction by means of as many spacer grids as
the total number provided for the fuel assembly. By making
use of narrow corrugated strips for separating open portions
having appreciable dimensions, the coolant fluid which cir-
culates in contact with the fuel assembly ensures efficacious
cooling of the fuel pins ; furthermore, the shape and arrange-
ment of the corrugated strips and of the bearing bosses makes
it possible to ensure efficacious absorption of vibrations and
expansions during operation.
In a practical design of this type, the corrugated
strips which orm springs must maintain the fuel pins with a
suficient bearing force against the fixed bosses of the
sheet metal members, especially in order to prevent wear as a
result of impact of the fuel-pin cans against said bosses
under the action of vibrations produced by the flow of the
coolant fluid. On the other hand, the clamping action thus
produced on the fuel pins must not be too powerful in or~er
to prevent collapse of the fuel cans. However, the springs
constituted by the corrugated strips must provide adequate
compensation for manufacturing tolerances allowed on the
diameters of the fuel pins and on the dimensions of the
spacer grid compartments. In consequence, the curve represent-
ing the force applied by the spring formed by each corrugated
,
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strip as a function of its deflectlon must be as flat as
possible within the operating range corresponding to a bearing
pressure which varies only to a slight extent in respect of a
range of deflection which can on the contrary vary to an
appreciable extent ; it is therefore found necessary to
increase the flexibility of the spring without thereby
permitting the possibility of modifying the spacing of the
sheet metal members of the grid.
The present invention lS directed to an improvement
which is made in spacer grids of the type referred-to in the
foregoing and which meets this requirement.
To this end, the inventlon is characterized in that
the sheet metal members are joined to the corrugated strips by
means of a zone which has lower mechan~cal strength and
increases the deflection of each strip in respect of the same
applied force.
Further properties of a spacer grid which is
improved in accordance with the arrangements of this invention
will become apparent from the following description in which a
number of exemplified embodiments are given by way of indica-
tion and not in any limiting sense, reference being made to
the accompanying drawings, wherein :
- Fig. 1 is a fragmentary diagrammatic view in
perspective showing a grid of a type which is already known in
the technique ;
- Fig. 2 is a sectional view in elevation showing a
corrugated strip of the grid in accordance with Fig. 1 ;
- Figs. 3, 4, 5 and 6 are views in perspective showing
different alternative forms of construction of ths sheet metal
members and corrugated strips in accordance with the arrange-
ments provided by the present inventlon ;
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- Fig. 7 illustrates a serles of curves which give
the variation in contact pressure as a function of the
deflect~on of the springs employed ln the design solutions
shown in Figs. 3 to 5 ;
- Figs. 8 and 9 illustrate two partlcular alternative
embodiments of a grid in accordance with the lnvention.
In Fig. 1, -the spacer grid under consideration is
of a type known ~ and generally designated by the
reference numeral 1. This grid comprises two series of
parallel sheet metal members 2 and 3 located at right angles
to each other so as to define compartments A of approximately
square cross-section. Said compartments are provided for the
passage of canned fuel pins (not shown) of a fuel assembly for
a nuclear reactor and especially a light-water reactor. The
two series of perpendlcular sheet metal members 2 and 3 are
interengaged, for example by means of narrow slits formed-at
intervals along the sheet metal members, said members being
also fixed on an outer frame. The two sides o:E said frame
which are shown in the drawing are designated respectively by
the references 2a and 3a.
~ In order to ensure that the fuel pins are maintained
within the compartments A of the spacer grid, the sheet metal
members 2 and 3 are provided with central open portions 4 of
substantial width separated by corrugated strips 5 each having
at least one projecting portion 6 which is directed towards
the interior of the opposite compartment. These projecting
portions 6 of the corrugated strips 5 are intended to apply
the fuel pins against bosses 7 and 8 respectively, only a few
of which are illustrated in order not to complicate the
drawing unduly. Said bosses are formed in the sheet metal
members 2 and 3 at the top and bottom portions of these latter.
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The sectional view of Fig. 2 shows in greater detail the
profile and re]ative arrangement of the bosses 7 and 8 and of
the corrugated strip 5 in any particular sheet metal member 2
of the spacer grid.
The top and bottom edges of the sheet metal members
2a and 3a of the spacer grid frame are advantageously provided
with teeth 9 having a slightly rounded shape and inclined
towards the center of the spacer grid, said teeth being
intended to facilitate the positLoning of said grid and as far
as possible to prevent interengagement of two adjacent grids
in two adjacent fuel assemblies at the time of operations
which entail the need for relative handling of said assemblies.
In accordance with the invention and as shown in
greater detail in Fig. 3, the flexibility of each corrugated
strip 5 is improved by making provision in the sheet metal
members 2 and 3 for zones of reduced mechanical strength and
especially slits 10 near the points of junction between said
members and each corrugated strip. In the example of con-
struction under consideration, said slits 10 extend in a
direction which is parallel to the opposite edges of the sheet
metal members and which is substantially perpendicular to the
direction of the corresponding corrugated strip. By means of
said strips 10 which are formed in the solid portions of the
sheet metal members between the strips 5 and the bosses 7 and
8, the twisting of that portion of the sheet metal member
which is located between said slit and the strip itself is
thus made to take part in the bending stress applied to each
corresponding strip, thus considerably improving the flexibi-
lity of the complete assembly. In all other respects, the
spacer grids thus provided are similar to the grid shown in
Fig. 1, especially in regard to the shape of the compartments
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and of the outer frame.
In another alternative embodiment shown in Fig. 4,
the corrugated strips 5 are formed through the sheet metal
members 2 and 3 of the spacer grid, thus directly incorporating
- the bosses 7 and 8. By means of this solution, said bosses on
which the fuel pins are applied within qach compartment are no
longer in stationarily ~ixed positions. This results in inter-
dependence of the compartments since a force of greater
magnitude applied to any one corrugated strip which forms a
spring is transmitted to the bosses of the adjacent compartment
and so on in sequence within the spacer grid, thus achieving
equalization of the forces applied to the fuel pins throughout
the entire spacer grid.
In another alternative embodiment, the corrugated
strips of the grid are still joined to the flat sheet metal
members at their extremities so as to form light-weight
junction zones in accordance with either of the arrangements
contemplated in the foregoing but are in turn provided in this
instance with open portions so arranged as to form in said
2Q strips bearing-arms which have relative elasticity with respect
to the strips~ Said strips therefore provide an association of
two spring assemblies which produce combined actions on the
fuel pencils which pass through the spacer grid, with a
resultant deflection of considerably increased value in
respect of a given bearing pressure.
In Fig. 5, which corresponds to the alternative
embodiment mentioned above, the sh~et metal members such as 2
or 3 of the spacer grid which are joined together by means of
the corrugated strips 5 are provided with lateral slits 12
having a suitable profile in order to improve the deflection
of the strip in its zone of junction with the sheet metal
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members of the grid. In this alternative embodiment, however,
each corrugated strip is in turn provided with open portions
13 so as to form two bearing-arms 1~ extending on each side of
the central portion 6 of each strip and joined elastically to
said strip at one end, the profile and length of which are so
determined that the bearing-arms 14 cannot penetrate through
the slits 13 and then completely withdraw, even in the event
of an appreciable force applied on these latter. At the time
of positioning of a fuel-pin spacer grid within each compart-
ment A, the fuel pin applies a force first to one and then tothe other bearing-arm 14 ; this force increases and first has
the effect of abuttingly applying the extremities 14a against
the solid portion of the strip 5. If the force continues to
increase, the ~uel pin bears on the central portion 6 and
then produces the deflection of the strip 5 itself. The
embodiment considered therefore utilizes two successive
elastic actions which are combined with each other while
permitting more effective transfer of forces applied on the
fuel pins, especially in regard to vibrations or deformations
~0 during use.
Fig~ 6 illustrates another alternative embodiment
which combines the arrangements of the examples shown in Figs.4
and 5 with an increase in length of the corrugated strips, the
ends of which carry the bearing bosses within an adjacent com-
partment of the grid and the central portions of which carry
the resilient bearing-arms.
In the graphic representation of Fig. 7, the set of
curves gives the compared values of deflections in respect of
a predetermined range of contact pressures of the springs on
the fuel-assembly pins corresponding to the practical range of
use of said springs. The cross-hatched zone in the drawing
gives a diagrammatic indication of this range. The curve A
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corresponds to a bearing spring of the prior art as recalled
at the beginning of this description. The curves B, C and D
correspond to variants of Figs. 3, 4 and 5 respectively. It
can thus be seen that the deflection increases substantially
in respect of a given bearing pressure when changing over from
one of these variants to another. A study of these curves
makes it possible to choose the most suitable form of con-
struction while taking other parameters into account. In
particular, the curve D relating to the variant in accordance
with Fig. 5 has two successive portions Dl and D2 correspond-
ing on the one hand to application of the resilient bearing-
arms 14 and on the other hand to deflection of the corrugated
strip 5. It is thus apparent that, at the cost of a slightly
more complex practical construction, the deflection of the
springs can vary to a substantial extent without thereby
causing the bearing pressure to depart from the intended
range of utilization. This carrieCl an advantage in that
adjustment o the pressure of the springs is facilitated when
the spacer grid is init~ally employed.
Finally, Figs. 8 and 9 illustrate an advantageous
improvement in an assembly grid which makes use of the
corrugated strips described in any one of the alternative
embodime~ts given in the foregoing. This improvement
provides more favorable conditions for positioning or with-
drawal of the diffexent fuel assemblies in the reactor core,
especially without entailing any danger of accidental inter-
engagement of adjacent assembly grids. It is in fact known
that, in conventional designs, certain difficulties are
liable to arise at the time of positioning of a fuel assembly
within the reactor core or withdrawal from this latter,
especially after a long period of operation within the core.
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~s a result of deformations caused by irradiation, the edges
of the sheet metal members constituting the spacer grids af
any given fuel assembly are liable to engage accidentally with
the grids of adjacent fuel assemblies, thus preventing inser-
tion or withdrawal of said assembly. It should be noted in
addition that the withdrawal of a fuel assembly can normally
take place only by handling the upper end of said assembly
since there is no possibility of handling this latter either
from the lower end or along the sides.
In order to circumvent the disadvantages noted above,
it has already been proposed to provide rounded teeth such as
those designated by the reference 9 in Fig. 1, both at the top
and bottom of the sheet metal members which constitute the
external sides o~ each spacer grid. Said teeth are inclined
towards the center of the spacer grid in order to ensure that
the grids can be more readily guided with respect to each other
when they form part of two adjacent fuel assemblies. However,
this arran-~ement may not prove sufficient in practice to en~ure
that the spacer grids are placed in position with an appre-
ciable degree of safety and without any attendant danger of
accidental interengagement. In order to overcome this draw-
back, the teeth 9 provided on the sides of the outer frame
of the spacer grid 1, for example on the sides 2a and 3a, are
associated with projecting ribs 20 for guiding the spacer
grids with respect to each other at the time of positioning
of the fuel assemblies. There is thus formed between two
adjacent frames in two adjacent fuel assemblies a lateral
clearance which is at least equal to the thickness of said
ribs 20. In the example of construction shown in Fig. 8, the
ribs 20 aforesaid are oriented 50 as to extend at right angle$
to the longitudinal direction of the sides 2a or 3a which
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constitute the outer frame of the spacer grid. The arrange- -
ment of the ribs in the sheet metal members is such that said
ribs are located at predetermined intervals along the length
of said members.
In a final alternative embodiment shown in Fig. 3
in which the same reference numerals have again been employed
to designate similar components, the ribs which serve to
achieve relative spacing of two adjacent grids by projecting
from the apparent contour of the outer frame are constructed
by suitably shaping the rounded teeth 9 formed on the top edges
of the sheet metal members. In particular, said teeth are
slightly bent back in the outward direction before being bent
inwards and consequently have outwardly projecting bulges 21
which, in the example shown in the figure, extend transversely
and parallel to the longitudinal direction of the sides 2a and
3a.
Whatever form of construction may be adopted, the grid
provided in accordance with the invention for supporting and
guiding the pins of a fuel cluster in a fuel assembly offers
outstanding advantages. Especially when use is made of
corrugated strips forming springs as illustrated in the
accompanying drawings, there is accordingly obtained suitable
equalization of the bearing pressures as well as an increased
bearing distance or length of span, particularly in the case of
the alternative embodiments shown in Figs. 4 and 6 in which the
projecting bosses within one compartment form an integral part
of a corrugated strip which in turn penetrates into an
adjacent compartment. This results in a more uniform weight
distribution throughout the spacer grid, in more effective
damping of vibrations and in a lower degree of wear of the
fuel pins.
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It is also worthy of note that, in the alternative
embodiment shown in Figs. 5 and 6 in which a substantial
deflection can be obtained in spite of a short span between
bearing points, a further advantage lies in the possibility of
manufacture without excessively close dimensional tolerances.
This is not the case with conventional springs formed in one
piece in which the permissible deflection of the order of
0.4 mm at the point of passage of a fuel pin entails the
need for highly accurate manufacture and strict standards of
position control which cannot readily be put into practice in
an industrial manufacturing process. When making use of a
corrugated strip in accordance with the invention, the spring
which is thus formed has two separate and distinct portions
which can easily be manufactured by cutting-out and bending
of the bearing-arms in the strip itself. The strip is set at
the center of the span of the bear:Lng-arms, with the result
that the respective deflections are permitted to build-up at
the time of application of a load on the corrugated strip.
There takes place an overall deflection of the strips and
bearing-arms ; these latter accordingly begin to undergo
deflection (curve Dl of Fig. 7) until the fuel pins come into
contact with the central boss ; this is followed by deflection ;
of the strips alone (curve D2).
The constructional dimensions of the essential parts
of a fuel assembly grid in accordance with the invention,
specially in the alter~ative embodiment shown in Fig. 4, are
given hereunder by way of indication :
- width of corrugated strips forming springs : 2.5 mm
- thickness of strips : 0.4 mm
- depth of the end bosses : 1.4 mm
- distance of projection of the strips with
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respect to the plane of the sheet metal
members of the grid : 2.5 mm
- ma~imum distance between the ends of the
slits in the sheet metal members : 33 mm.
In the case of the alternative embodiment shown in
Fig. 5, a particular example of construction leads to the
adoption of the following dimensions :
- total width of corrugated strips : 5.~ mm
- width of the central bearing-arms : 2.4 mm
- distance between the sheet metal members : 23 mm
- thickness of strips : 0.4 mm
- distance of projection of the bearing-arms
with respect to the plane of the sheet metal
members : 3 mm
- maximum def~ection of the bearing-arms : 0.5 mm.
Finally, in the case of a grid in accordance with
either Fig. 8 or Fig. 9 in which any one of the embodiments
shown in Figs. 3 to 6 is employed i-or the corrugated strips,
the preferential dimensions adopted for one particular
example of construction are :
a) Fig. 8
width of projecting spacing ribs : 1.6 mm
- relative spacing of said ribs : 4.8 mm
- length of ribs : 40 mm
- width of sheet metal members of grid frame : 10 mm
- length of said sheet metal members : 212.8 mm
b) Fig. 9
- distance of projection of end teeth : 0.5 mm
- angle of bending-back towards the interior : 30.
As will be readily apparent, the invention is not
limited solely to the examples which have been more especially
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contemplated in the foregoing but extends on the contrary to
all alternative forms. In particular, no special assumption
has been made in regard to the material constituting the grid
and corrugated strips but preference will be given to the use
of the alloy known under the trade mark "Inconel 718" or of
a suitable zirconium alloy.
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