Note: Descriptions are shown in the official language in which they were submitted.
1~385~s
This invention relates to long nuclear fue~ bundles
which employ grids to support the ~uel elements at intermediate
points. More partlcularly it relates to a grid assembly feature
to provide a long fuel bundle with variable rigidit~
With the present design of long fuel bundles, there must
be an allowance made ~or the fact that the fuel channel may become
warped and no line can be straight and true. If sufficient allow~
ance is not made for this, a fuel bundle may ~am ln the fuel
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channel~ because, although the fuel bundles are relatively long, ~ -~
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they may be fairly rigid and not bend to an appreciable extenk or ~;
alternatively too flexible for convenient handling. These long ~ -
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fuel bundles are a feature of many o~ the pre~e~t reactors. It
would be advantageous if a sy tem could be provided which would
enable the fuel to be desiened to ~ill the channel to the maximum
possible extent~ without the attendant danger of the ~uel bundle -~
jamming in the fuel channel, as this would raise the burn up or
channel power of the reactor without additional expense. Alter-
natively it would be advantageous lf a system could be provided
to give a predetermined stiffness to a fuel bundle.
It has been found that the above desiderata may be ob~
tained by attaching grids to a cage o~ tie rods so as to enable ~ ;
the rigidity of the whole assembly to be adjusted to give a wLde
variation from very stiff to relatively flexible. The tie rod
cage is attached to the grid by flexible strap means of such a
length and cross-section so as to have a predetermined torsional
rigidity and thereby provide a variable degree of stiffness to the
whole assembly thus enabling the fuel designer to choose the des~
ired amount of flexibility to cope with the deviations and out of
straightness of an actual fuel channel. ;
A preferred embodiment o~ the present invention provides
a nuclear fuel assembly which has a plurality o~ fuel rods, a grid
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me~ns having apertures therein for receiving the fuel rods, the
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fuel rods having c~n axis which is normally p0rpendicular to the
plane of the grid means, tie rod means extending parallel to the
fuel rods, flexible strap means attaching the tie rod means to
the grid means so as to prevent relative axial movement between the
tie rod means and the grid means so as to permi~ relative angular
motion therebetween, the relative angular motion therebetween, the
relative angular motion occurring when the Xul~l bundle is caused
to bend in a longitudinal plane. One end of the ~lexible strap
means ls attached ~o the tie rod means and another end to the grid
means, with the intermediate portion having a predete~ined tor-
sional rigidity tending to oppose the relative angular motion
which occurs when the fuel bundle is caused to bend in the longi-
tudinal plane.
The present inventlon will be more particularly described
with re~erence to the figures in which:
Figure 1 is a view of a preferred embodiment o~ the fuel
blmdle of the present invention;
Figure 2 is a cross-sectional view tc~ken a~ Section A-A
of the fuel bundle of the present invention;
Figure 3 shows a preferred embodiment of the means of
providing variable rigidity~ and
Figure 4 is an enlarged view of a pre~erred embodim~nt
o~ the flexlble members exaggerated ~or clarit~.
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In Figure 1 there is shown a side view of the fuel bun-
dle on which the present inven~ion is practised in which the ~uel ~ ;
bundle has end plates 10 and 12 at either end o~ the fuel bundle
which along with the grids maintains the fuel elements in an accur-
ately spaced configuration as shown in Figure '20 `
The indi~idual fuel elements 14, as shown in Fic~ure 1, ;
are fastened to end plate 10 b~, for example, hot upsetting the
element end 16.
F~gure 2 is a cross-section view of Figure 1 tc~ken along
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Section A-A ~howing the tie rods 18 fastened to the grids 20 by the
flexible strap means 22. The fuel elements 14 are shown disposed
within the grids 20. ~ ~
Figure 3 is a partial view of two tie rods 18, a portion ~ ;
of the grid structure 20 and the flexible strap means 22 for ~asten
ing the tie rods 18 to the grid 20. As shown in the preferred em-
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bodiment o~ Figure 3, the flexible strap means 22 may be strips o~
any suitable length wh~ch will provide the desired sti~fness or
rigidity in the assembly. m e strap means 22 may be fastened to -
the tie rod 18 at a point 24 by any suitable means, such as elec- ;
tron beam, resistance9 or arc welding as well as by brazing with
an alloy compatible with the strip and tie rod. The strap means
22 may also be fastened to the grid 22 at a point 26 in a similar
manner.
The action of the strap means 22 will be described in more
detail with reference to Figure 4. In Figure 4 the strap means 22
is shown connected to a por~ion of the tie rod 18 and a portion of `~
the grid 20. The distance D found in Flgure 4 has been exaggerated
for clarity. Any bending moment applied to the fuel bundle would
result in a twisting moment being applied to the free portion L of ;~
the strap means 22.
Al~hough as shown in the preferred embodiment the strap
mean8 22 i8 shown as having a greater dimension disposed in the
radial direction and the narrow dimension being in a circumferen~
tial direction the strap means may assume other cross-sectional
dimensions, such as round, trapezoidal or square.
Since a single fuel bundle will ha~e a large number of
flexible strap means 22, the torsional rigidity of the indi~idual
flexible strap means 22 will depend upon the stiffness desired for
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the fuel bundle as a whole as well as the number of flexible means
which are effective in contributing to the stiffness of the bundle.
For instance, since the flexible strap means of the preferred em-
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bodiment have a rectangular shape, only those strips which are ~`
oriented to provide maximum ~orsional rigidity, i~e~ in ths present
embodiment those located the furthest distance from the bending
A-A will contribute significantly to the stiffness of the fuel
element since those flexible means 22 c-ose t;o the axis A-A will
be oriented along their axis of minimum torsional rigidity as
shown in Figure 2~
In fact, in the preferred embodiment shown9 the flexible
strap means 22 close to the æis A-A are subjec~ to substantially -~
pure bending about the narrow dimension and therefore ser~e mainly
to prevent relative axial movement o~ the tie rod cage with res~
pect to the grids and contribu~e very little to the rigidity of
the fuel bundle.
From th~ above description it can be seen that the con-
tribution of each of the flexible means 22 depends on its location
with respect to the axis of bending A-A and may be varied by
changing the cross-sectional shape of the flexible strap means 22
to provide a predete~mined torsional rigidity necessary to achieve ;~
the desired bundle stif~ness.
In summary, it has been shown how a fuel bundle having
a con~rolled rigidity or stiffness may be produced using in the
preferred embodiment, a flexible strap means 22 o~iented in such a
manner as to produce maximum stiffness at the point furthest away ~
~rom the axis of rotation. ~ -
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