Note: Descriptions are shown in the official language in which they were submitted.
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CASE-277~
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FUEL BUNDLE WITX S:KEWED ELEM~NTS
This invention is directed to nuclear reactors, and
to fuel for use in nuclear reactors, and methods of
manufacturing such nuclear fuel.
In the Canadian Deuterium Uranium (acronym CANDU)
type of reactor, use is made of fuel channels arranged within
a calandria, somewhat in the manner of a boiler. The
fuel channels each comprises a tube containing pressuriæed
liquid and a so-called string of fuel bundles arranged
in serial, end-to-end relation extending substantially
the full length of the individual channel. Each fuel
bundle has a plurality of clad fuel rods or elements, as
many as but not limited to 37 rods per bundle each rod
being about one half inch in diameter by about twenty
inches long, being arranged in concentric rings and
supported by welding of the rod end caps to a pair of
spaced-apart end plates, with the polar axis of each
rod being parallel with the bundle main axis and the end
plates perpendicular to the bundle main axis.
In the CANDU type reactor the coolant is maintained
usuall~ in liquid phase with a small proportion of
vapour at the exit end of some channels, being pressurized
in order to raise the maximum coolant temperature, the
coolant liquid being deuterium-oxide i.e./ heavy water.
` One of the problems encountered is the matter of
controlling coolant flow in a suitable condition to
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CASE-2774
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effect required heat transfer from the individual fuel
rods to the liquid coolant. In accordance with prior
art and practice, the ~uel rods extend parallel with
the tube main axis, thereby ensuring minimal coolant
pressure drop along the length of the fuel channel,
to thereby minimize pumping requirement for the reactor.
In accordance with the present invention a nuclear
reactor is provided having rodded fuel bundles wherein
each bundle comprises a pluralit:y of fuel rods having
the ends thereof secured to opposed end plates, the rods
being skewed at angles in the typical range oE 0.3 to
0.5, but not restricted to this range, from an axis
parallel with the principal axis of the bundle. A
centre rod lying on the bundle axis remains parallel to
the bundle axis and the end plates perpendicular to the
bundle axis.
One method by which the present invention may be
practiced is to assemble the fuel rods and end plates,
prior to final securement~ in a rectilinear axially
aligned condition, and then rotate one of the end plates
about its polar axis by a desired angle of displacement,
in the typical range of 10 to ~0, but not restricted
to this range from the other plate as to generate symmetrical
skewing of all of the fuel rods. It will be understood
that this method will produce differential axial skewing
between respective rings of rods, with the radially outer
ring of fuel rods being skewed at the greatest angle,
and radially inner rings being progressively less skewed.
The effect of skewing is to shorten the projected
length or distance between the ends of the fuel rods
measured parallel to the bundle axis, but because of the
differentiated skew angles, the outer ring of rods is
shortened more than inner rings, which are progressively
shortened by reduced amounts. A centre rod on the bundle
a~is being not skewed maintains a projected length
equal to its actual length.`
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CASE-2774
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A tabulated analysis given below in Table 1, shows
the decreases in rod projected length resulting from
varying degrees of skew, arrived at by respective
relative rotations of the end plates. This length
variation, according to the ring, can be compensated
by appropriately increasing the fuel rod lengths, or
by providing compensating annular steps or lands on the
end plate.
One effect steming from the skewed construction is
that the fuel bundle takes on a slightly waisted appearance,
i.e., the bundle diameter diminishes progressively from
a maximum value at the ends to a minimum value at the
longitudinal mid point.
This has the effect of causing the interconnected
coolant flow channels which exist in the in-terior of the
fuel bundle, and those exterior to the bundle, to change
in their relative cross sectional areas, thereby tending
to cause a coolant transfer during coolant flow along
the bundle from within the bundle to the outside of the
bundle, and vice versa. This is complementary to the
rotatory effect on coolant flow due to the handling
effect of rod skewing (i.e~, twist).
Certain embodiments of the invention are described,
reference being made to the accompanying drawings,
wherein;
Figure 1 is a side view of a fuel bundle according
to the present invention.
Figure 2 and Figure 3 are end views taken at 2-2
and 3-3 of Figure 1.
Figure 4 is a diametrical sectioned view of an end
plate taken on line 4-4, a bundle having all rods of
uniform length.
For illustratory purposes, a bundle having 37 rods
is shown,arranged as three concentric rings of rods
about a single rod lying on the bundle axis.
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CA5E-2774
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Referring to the drawings, the bundle 20 is the
same as a normal rectilinear bundle, but having a 40
counter clockwise twist applied between the ends
thereof. Thus, in the illustratad embodiment, the
respective rod positions for the rods 22 are displaced
by two intervals in the case of the numbered, radially
outer ring of rods, as shown in Figures 2 and 3.
The Figure 4 sectioned view is somewhat schematic,
in that perforations of the end plate 24, which permit
through-flow to assist coolant circulation~ have not
been illustrated, to better show the provision of
annular lands 26, 28 and 30 for the respective three
annular rows of rods 22 contained in the bundle. The
differences in the heights of the annular lands 26,
28 and 30 exaggerated for purposes of illustration,
compensate for the differential axial shortening
taking place in the equal length rods of the respective
rings of rods, due to the different angles in inclination,
as set forth below in Table I, with corresponding
differences in effective axial length, as measured
between the bundle end plates. Alternatively, a flat
end plate of constant thickness, withou-t the annular
lands 26, 28 and 30, may be used with rods having
lengths that are increased to compensate for the
differential axial shortening taking place due to the
different angles of inclination in the respective rings
as set forth in Table I.
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TABLE 1
SOME RELEVANT DIMENSIONS FOR THE ILLUSTRATED
EMBODIMENT OF A 37-ROD BUNDLE
Mid Length ¦ Mid Length Pitch
Subchannel Area Circle Diam. Decrease
End~ Change - inche.s
Plate
Rotary ___________________________ _ _ ______~_~
Angle Exterior Interior Outer Mid Inner
Channels Channels Ring Ring Ring
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00 O O o O O
10 +3.3 -2.1 .0133 .0086 .0046
20 -~12.7 -8.1 .0518 .0344 .0178
30 +28.3 -18.0 .1162 .0771 .0399
40 +49 -31.5 .2056 .1365 .0707
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End Rod Angle to Flow Porjected Axial RodLen9th
Plate Change - inches
Rotatory _ _ ~ _ _ ___~
Angle outer Mid Inner Outer Mid Inner
_ Ring Ring Ring Ring Ring Ring
O O O O O O
10 0.9 0.6 0.3 .0023 .0010 .0003
20 1.8 1.2 0.6 .0091 .0040 .0011
30 2.6 1.7 0.9 .0201 ~ .0089 .0024
40 3.5 2.3 1.2 .0351 .0155 .0041
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C~SE-277
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Referring to Table I, in the case of the 40
rotatory angle between the end plates, there is a
projected rod length change, as measured axially between
the end plates of the fuel bund:Le, amounting to
approximately 35 thousandths of an inch ("thou."), in the
case of the radially outer ring of rods, 15 thousandths
of an inch in the case of the m:id ring of rods, and the
4 thousand-ths of an inch in the case of the inner ring
of rods. These are the lengths by which the rods in the
respective rings must be increased for the use of end
plates without steps.
In the case where rods of uniform length are used
throughout the bundle, both end plates 24 are approximately
stepped, to compensate ~or axial projected length
differences, the stepped differentials of the annular
rings or lands 26, 28 and 30 would be halved to values
of 17.5 thou., 7.5 thou., and 2 thou., approximately.
In the other three tabulated cases of a fuel bundle
having a lesser angle of skew, the axial distance
differentials are correspondingly reduced, to the extend
that, for a ten degree skew (Table I) the difference to
be made up at each'end would not exceed 1.15 thou., which
could be accommodated in the welding of the rod end cap
to the plate 24.
