SHORT FUEL ELEMENT

ELEMENT COMBUSTIBLE COURT

English Abstract

SHORT FUEL ELEMENT
ABSTRACT OF THE DISCLOSURE
Fuel bundles for use in a pressure tube
reactor each comprise a cylindrical assembly of fuel
rods extending between a pair of end plates to which
the rods are attached. The individual rods have a
sheath construction of predetermined length to permit
the insertion of a plurality of bundles within
existing pressure tube fuel channels, previously
containing a lesser number of longer bundles. The
adoption of shorter rods militates against rod
distortion under thermal and coolant flow forces, to
obviate the former necessity of providing spacing
appendage means attached to the fuel rod sheaths
intermediate the bundle and plates, thereby
simplifying manufacture and enhancing the
metallurgical dependability of the rod sheath. There
is further provided a bundle assembly comprising two
or more of the improved fuel bundles secured in
end-to-end relation, in predetermined angular relation
about the polar axis of the bundle assembly.

Claims

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The embodiments of the invention in which an
exclusive property or privilege is claimed are defined
as follows:-
1. A fuel bundle for use in a liquid cooled
nuclear reactor, having a plurality of plain fuel rods
extending in mutual parallel relation between a pair
of opposed end plates, the length to diameter ratio of
the fuel rods being not greater than about thirty to
one (30:1).
2. The fuel bundle as set forth in claim 1,
said end plates including spacer means extending
outwardly in radial projecting relation, relative to
the radially outer envelope of said fuel rods, to
afford lateral support to the bundle.
3. The fuel bundle as set forth in claim 1, in
combination with at least one other said bundle and
means securing adjacent ones of said end plates in
mutually attached relation, to provide a unitary fuel
assembly.
4. The fuel bundle assembly as set forth in
claim 3, said end plates being mutually orientated
about the polar axis of the bundle assembly in
predetermined angular relation.
5. The fuel bundle assembly as set forth in
claim 4 wherein in use said predetermined angular
relation promotes a low drop in pressure of liquid
coolant flowing past the bundle.
6. The fuel bundle as set forth in claim 2,
said spacer means including rib portions extending

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Claim 6 continued:
from said end plate axially in overlapping relation
with said fuel rods.

Description

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CASE 2964
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S~IORT FUEL ELEMENT
This invention is directed to nuclear fuel
and in particular to a fuel bundle for use in a
pressure tube nuclear reactor.
Pressure tube reactors, particularly as
exemplified by the Canadian deuterium cooled "natural"
uranium fuelled reactor have evolved a sophisticated
fuel bundle arrangement requiring particular
manufacturing processes, leading to high costs, and
possessing certain disadvantages.
In particular, the individual rods o-f
existing fuel bundles are provided with appendages
securing as spacers, attached to the outer surfaces of
the rod sheaths, and serving to maintain certain
prescribed clearances between the sheaths of adjacent
rods, and between the sheaths and the adjacent inner
surface of the pressure tube in which the fuel bundles
repose in axially aligned relation. Some of the
problems associated with this prior art fuel include:
the cost and involvement in locating and
attaching the appendages to the exterior of the
element sheaths, by brazing or welding;
the metallurgical changes produced locally
in the sheath material at the zone affected by heat,
as a consequence of the attachment process;
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CASE No~964
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the increased likelihood of mechanical
failure due to the appendages acting as stress
raisers;
the increase in cost of fuel production due
to the number of rejects arising in the manufacturiny
process.
In order to overcome the existing problems
of the present fuel there is provided a much stiffer
fuel rod construction, to obviate the need of
providing spacer appendages intermediate the rod ends.
In accordance wi.th the present invention
there is provided a fuel bundle having a pluxality of
plain fuel rods extending in mutual parallel relation
between a pair of opposed end plates, the length to
diameter ratio of the fuel rods being not greater than
about 30 to 1.
It is contemplated that multiple bundles can
be assembled, having their end plates in secured
relation to provide a single entity for fuel handling
~0 purposes. In particular, such an assembly permits the
assembling of the respective fuel bundles in mutual
angular orientation about their polar axis, for
puxposes of controlling the pressure drop along the
bundle, and also influencing the local turbulence
effect on the liquid coolant, under operating
conditions.
In order to position the fuel bundles in
radial spaced relation from the inner support surface
of the fuel channel there is provided spacer means
extending from the bundle end plates outwardly in
radial projecting relation, relative to the radially
outer envelope of the fuel rods, to provide lateral
support to the bundle for handling and coolant flow
purposes.

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CASE ~o.2964
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The spacer means may take a number of forms,
generally comprising outwardly extending ribs
connecting with the end plate and extending in
mis-aligned relation with the individual fuel rods.
The radially outer ends of these ribs incorporate
bearing means for contacting the surface of the fuel
channel, and may comprise an axially elongated bearing
pad of streamline form extending in axial overlapping
relation with the ends of the fuel rods. Other
embodiments comprise bearing bands e~tending arcuately
about the bundle. ~ne such embodiment is spaced in
radial clearing relation from the fuel rods. Another
embodiment provides contact of the bearing band inner
surface with the respective rod sheaths.
Certain embodiments of the invention are
described, reference being made to tlle accompanying
drawings, wherein:
Figure 1 is an end view of a portion of a
first embodiment fuel bundle
Figure 2 is a side view of the bundle
portion of Figure l;
Figure 3 is an end view of a portion of a
second embodiment fuel bundle;
Figure 4 is a side view of the bundle
portion of Figure 3;
Figure 5 is an end view of a portion of a
third embodiment fuel bundle;
Figure 6 is a side view of the bundle
portion of Figure 5; and
Figure 7 shows two bundles in coupled
relation.
In Figure 1 the components of the fuel
bundle 10 comprise fuel rod elements 12, the outer rim
portion 15 of end plate 14, rod sheath portion 16 and
fuel pellet 18, and spacer means 20 comprising spacer
rib 22 and spacer pad portion 24.

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CASE No.2~64
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In the Figure 3 embodiment the spacer
ribs 32 connect with spacer band 36 which contacts the
sheaths 16 of the outer row of fuel rods 12.
In the Figure 5 embodiment the spacer ribs 52
are attached to spacer band 56 which enshrouds the
ends of fuel rods 12, to form a flow passage
thereabout.
Figure 7 shows a fuel assembly 70 comprising
a pair of fuel bundles 10 welded together at the end
plates 14 in back-to-back relation. By welding the
end plates 14 so that the spoke portions and other
parts thereof are in axial registry, the hydraulic
pressure drop thus created will be minimised. By
welding the end plates in co-axial but angularly
off-set relation, the hydraulic pressure drop can be
increased. ~enerally, welding is ~ffected at or
adjacent the rim portion 15.