Note: Descriptions are shown in the official language in which they were submitted.
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RECONSTITUTING A NUCLEAR REACTOR EUEL ASSEMBLY
BACKGROUND OF THE INVENTION
The present invention relates generally to fuel
assemblies for nuclear reactors and, more particularly, to
a method for reconstituting a fuel assembly by removal and
attachment of top nozzles and to a reconstituted fuel
a~sembly with an improved top nozzle attachment system.
Con~entional designs of fuel assemblies include
a multiplicity of fuel rods and control rod guide thimbles
held in an organized array by grids spaced along the fuel
assembly length. The grids are attached to the control
rod guide thimbles. Top and bottom nozzles on opposite
ends thereo are secured to the control rod guide thimbles
which extend slightly above and below the ends of the fuel
rods. At the top end of the assembly, the guide thimbl~s
are attached in openings provided in the top nozzle. The
guide thimbles may each be surrounded by a sleeve for
attachment to the top nozzle and to the uppermost grid.
During operation of such assembly in a nuclear
reactor, the fuel rods may occasionally develop cracks
along their length resulting primarily from internal
stresses thus establishing the possibility that fission
products having radioactive characteristics may seep or
otherwise pass into the primary coolant of the reactor.
Such products may also be released into a flooded reactor
cavity during refueling operations or into the coolant
circulated through pools where the spent fuel assEmblies
are stored.
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Under these circumstances, it is difficult to
detect and remove failed fuel rods because they are part
of an integral assembly of guide tubes welded to the top
and bottom nozzles. Consequently, to gain top access to
individual fuel rods, it is necessary to remove the
affected assembly from the nuclear reactor core and then
break the welds which secure the top nozzle to the control
rod guide thimbles. In so doing, the destructive action
often renders the fuel assembly unfit for further use in a
reactor because of the damage done to both the guide
thimbles and the top nozzle which prohibits rewelding.
In view of the high costs associated with replac-
ing fuel assemblies, both domestic and foreign utilities
have indicated an interest in reconstitutable fuel assem-
blies in order to minimize both their operating and maint-
enance expenses.
Conventional reconstitutable fuel assemblies
incorporate design features (at the time of original
manufacture) arranged to permit the rapid removal of
individual failed fuel rods, the option to replace rods,
followed by the additional use in the reactor and/or
normal handling and storage of the affected fuel assembly.
Reconstitution has been made possible by providing a fuel
assembly with a removable top nozzle. The top nozzle is
mechanically fastened usually by a threaded arrangement to
the upper end of each control rod guide thimble assembly,
and the top nozzle can be removed remotely from an irrad-
iated fuel assembly while it is still submerged in neutron-
absorbing liquid. With rod removal/replacement and after
3~ the top nozzle has been remounted on the control rod guide
thimble tubes, the reconstituted assembly can then be
reinserted into the reactor and used until the end of its
useful life, and/or stored in spent fuel pools or other
places in a safe, normal manner.
The conventional design philosophy for existing
reconstitutable fuel assemblies has been to implement
their costly reconstruction features on every fuel assem-
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bly at the time of original manufacture. The various
threaded or other arrangements to removably attach each
control rod guide thimble to the top nozzle represent a
high cost in time and money for the added manufacturing
operations, when only a very small percentage of the fuel
assemblies will ever experience a leaky fuel rod and hence
the need for reconstitution.
SUMMARY OF THE INVENTION
Briefly stated, the invention is directed towards
a method for reconstituting a nuclear reactor fuel assembly
which has its top nozzle adapter plate attached (such as
by welding3 to its control rod guide thimbles (or control
rod guide thimble sleeves). The guide thimbles (or
sleeves) are severed from the adapter plate by circumferen-
tial cutting below the attachment area. The top nozzle isremoved from the cut guide thimbles (or sleeves). A
second top nozzle is obtained with a modified adapter
plate having some of its control rod passageways refash-
ioned with a groove. The cut sleeves are inserted in the
second top nozzle adapter plate control rod passageways.
The associated guide thimbles (or sleeves) are circumferen-
tially bulged into the grooves. The method can also be
employed to reconstitute a fuel assembly (such as a pre-
viously reconstituted one) having its top nozzle attached
to its guide thimbles (or sleeves) by a circumferential
bulge/groove system.
The invention is also directed towards a nuclear
reactor fuel assembly having a number of its top nozzle
adapter plate control rod passageways with a groove and
; 30 having the associated control rod guide thimbles (or
control rod guide thimble sleeves, or both) circumferen-
tially bulged into the grooves.
BRIEF DESCRIPTION OF THE DRAWINGS
The subject matter of the invention is particu-
larly pointed out and distinctly claimed in the concludingportion of this specification. The invention, however,
both as to organization and method operation, together
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with further advantages thereof, may best be understood by
reference to the following description taken in connection
with the accompanying drawings in which:
Fig. 1 is a view in elevation partially in
section, illustrating the design of a fuel assen~ly which
incorporates an embodiment of the invention's top nozzle
to control rod guide thimble attachment system.
Fig. 2 is a plane view of the fuel assembly
shown in Fig. 1.
Fig. 3 is an enlarged cross-sectional view of a
prior art top nozzle to guide thimble welded attachment
system upon which the invention's method of fuel assembly
reconstitution can be employed.
Fig. 4 is an enlarged cross-sectional view of a
portion of Fig. 1 showing in greater detail the invention's
attachment system embodiment. Fig. 4 also represents the
end result of the invention's method of fuel assembly
reconstitution carried out on the attachment system of
Fig. 3.
Fig. 5 shows an alternate embodiment of the
attachment system of Fig. 4. Fig. 5 also represents the
end result of the invention's method of fuel assembly
reconstitution carried out on the attachment system of
Fig. 4 (such as in a second reconstitution of a fuel
assembly).
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings wherein like
reference characters designate like or corresponding parts
throughout the several views, there is shown in Figs. 1
and 2 a nuclear reactor fuel assembly 10 including an
array of fuel rods 12 held in spaced relationship to each
other by grids 14, 15 and 17 (only three of which are
shown in Figure 1) spaced along the fuel assembly length.
Each fuel rod includes nuclear fuel pellets 16 and a
spring 18 located in the plenum of each fuel rod, and the
ends of the rods are closed by end plugs 20, all in a
conventional manner.
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To control the fission process, a multiplicity
of control rods 21 are reciprocally movable in control rod
guide tubes or guide thimbles located at predetermined
positions in each selected fuel assembly in the reactor.
The guide thimbles are attached to the grids 14, 15 and
17. The reactor includes a top nozzle 24 and a bottom
nozzle 26 to which opposite ends of the control rod guide
thimbles are attached to form an integral assembly capable
of being conventionally handled without damaging the
assembly components.
Typically the guide thimbles have sleeves for
weld compatibility with the upper 14 and lower 17 grids
and with the top 24 and bottom 26 nozzles. In this case,
as shown in Figs. 1 and 4, a sleeve 53 is used to join the
guide thimble 50 to the upper grid 14 and the top nozzle
24. Each guide thimble 50 extends the full length of the
fuel assembly 10 between the top nozzle 24 and the bottom
nozzle 26. The sleeve 53 only extends from the top nozzle
24 to the upper grid 14, and the sleeve 54 only extends
from the bottom nozzle 26 to the lower grid 17. The
guide thimble 50 is attached to the sleeve 53 by a bulge
fit. The bulge fit typically consists of bulges 55
arrayed in three axially spaced sets (two sets between the
top nozzle adaptor plate 32 and the upper grid 14 and one
set below the upper grid 14), with each set consisting of
four symmetrically spaced apart bulges. The guide thimble
50 can be attached to the sleeve 54 in a like manner.
As illustrated in Figs. 1 and 2, the top nozzle
is square in cross section and comprises a housing 28
having an upper plate 30 spaced from a (lower) adapter
plate 32. Assembly hold-down springs 34 attached to
opposite sides of upper plate 30 are held in place by
bolts 36 and are adapted to be compressed when the reactor
upper core plate (not shown) is placed in position. The
top nozzle further includes a rod cluster control assembly
37 comprising an internally threaded cylindrical member 38
having radially extending flukes or arms 40. A connector
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42 interconnects each control rod 21 with the arms, the
arrangement being such that the rod cluster assembly moves
the control rods vertically in the control rod guide
thimbles to thereby control the fission process in the
5 assembly.
A preferred embodiment of the invention's method
for reconstituting a nuclear reactor fuel assembly 10 by
removal and replacment of top nozzles is illustrated in
Fig. 3 as the "before" drawing and Fig. 4 as the "after"
10 drawing. The control rod guide thimbles 50 (only one of
which is illustrated in Fig. 3) have their uppermost
sleeves 52 coaxially positioned within the controd rod
passageways 60 of the top nozzle adapter plate 62. The
sleeves 52 are attached to the top nozzle adapter plate 62
15 preferably by welding (or other metallurgical attachment)
at the adapter plate's upper surface 64. This type of
conventional attachment system is less costly in time and
money when compared with existing reconstitutable fuel
assemblie using threaded attachment systems in all assem-
20 blies at the time of original manufacture.
When a decision is made to reconstitute a fuelassembly 10 (which will occur in only a small percentage
of fuel assemblies during their useful life), one would,
using the method of the invention, sever the sleeves 52
25 from the adapter plate 62 by circumferentially cutting the
sleeve walls below the area of attachment. Such cutting
can be accomplished, for example, by using a conventional
in-tube cutter, within the control rod passageway 60, to
cut the sleeve wall (from inside out) below the welded
30 attachment. Alternatively, such cutting can be accom-
plished by using a conventional hole-saw type cutter
proceeding from the upper surface 64 of the adapter plate
62 at a diameter and to a depth sufficient to cut the
weld. The top nozzle with its adapter plate 62 is then
35 removed from the grid sleeves by conventional means. The
cutting and removal operations are performed remotely on
the irradiated fuel assembly 10 while it is submerged in a
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neutron-absorbing liquid, such as water, thus providing
access at the top end of the fuel assembly for the detec-
tion and removal of failed fuel rods.
A modified top nozzle 24 is obtained, preferably
generally identical to the removed top nozzle, with its
adapter plate 32 having control rod passageways 72, but
with a plurality (and preferably all) of such passageways
72 having a channel or groove 74. The groove 74 is located
in an area axially below the corresponding area on the
removed top nozzle adapter plate 62 from which the sleeves
52 are severed. The groove 74 could be ~ny shaped indenta-
tion, of various dimensions, in the *~ of the passageway
72. Preferably, the groove 74 is a coaxial, fully circu-
lar, circumferential groove. Preferably, the modified top
nozzle 24 is an unirradiated one, but the removed top
nozzle could be refashined if desired. The formation of
the grooves can be accomplished by conventional cutting
means known to those skilled in the art.
The cut sleeves 53 with the control rod guide
thimbles 50 are inserted into the control rod passageways
72 of the adapter plate of the modified top nozzle 24, to
an axial distance above the grooves 74. Preferably, tha
cut sleeves 53 are inserted from below in the passageways
72 to an axial position generally equal to that where they
were previously cut. The corresponding plurality of the
inserted sleeves 53 are circumferentially bulged into the
grooves 74. By "corresponding plurality" is meant the
sleeves 53 which are inserted in the plurality of passage-
ways 72 having the grooves 74. The cir~-umferential bulg-
ing is performed by conventional means known to thoseskilled in the art. For example, the bulging could be
accomplished by hydraulic pressure of a fluid introduced
into the control rod passageway through holes in a fluid
supply tube having seals above and below the groove to
contain the fluid in the passageway to bulge the sleeve
into the groove. Another method could employ a piston and
its rod in the passageway, with the piston below the
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groove being drawn by its rod up the passageway closer to
the groove to compress an introduced material (such as
polyurethane) against a seal ~located above the groove)
through which the shaft is drawn to contain the material
in the passageway to bulge the sleeve into the groove.
Both the insertion and bulging operations can be performed
remotely.
The above-described method also'perm ts access
to fuel rods in an assembly to allow for the transfer of
partially spent fuel rods from one damaged skeleton, for
example, to another thus permitting the fuel rods to
achieve their intended burnup levels. The method further
permits access for the removal and/or rearrangement of
fuel rods to attain better uranium utilization in the
reactor.
The method of the invention is equally appli-
cable to reconstitute fuel assemblies having their control
rod guide thimbles directly attached (such as by welding)
to the top nozzle adapter plate without any intervening
sleeves. In this case, the cutting and bulging operations
would be performed on the quide thimbles instead of the
sleeves. It can be appreciated by those skilled in the
art that the method of the invention is also applicable to
reconstitute fuel assemblies having (at the time of orig-
inal manufacture or, preferably, at the time of firstreconstitution) their guide thimbles (or sleeves) circum-
ferentially bulged into grooves on their adapter plate
control rod passageways. This allows fo~ additional
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reconstitution of a previously rc~ons~i ~u~ed fuel assem-
~0 bly. For example, Fig. 4 could represent the `'before"
drawing and Fig. 5 could represent the "after" drawinq of
a twice reconstituted fuel assembly. In Fig. 5, the
twice-cut sleeve 80 of the control rod guide thimble 50 as
well as the control rod guide thimble 50 itself are circum-
ferentially bulged into a lower groove 90 in the controlrod passageway 92 of the adapter plate 94 of another top
nozzle. The groove 90 is located below the corresponding
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area of the previous top nozzle adapter plate 32 (see Fig.
4) from which the sleeve 53 was severed. It should be
noted that due to the lower location of the groove 90, the
circumferential bulging of the sleeve 80 also requires the
circumferential bulging of the overlapping control guide
thimble 50.
From the previous discussion it can be seen that
the invention encompasses a nuclear reactor fuel assembly
top nozzle to control rod guide thimble attachment system.
The attachment system can be used at the time of original
manufacture of the fuel assembly, or preferably at the
time of fuel assembly reconstitution. Employment of the
attachment system of the invention only at reconstitution
results in lower total costs in time and money when the
original fuel assembly uses, for example, a welded top
nozzle to control rod guide thimble attachment. This
results from the fact that only a small percentage of the
fuel assemblies will ever need reconstitution.
A preferred embodiment of the top nozzle to
guide thimble attachment system, as shown in Eig. 4,
employs a top nozzle with its adapter plate 32 having a
number (at least two and preferably all) of its control
rod passageways 72 modified with a groove 74. The attach-
ment system also utilizes control rod guide thimbles 50
having sleeves 53. The sleeves are coaxially positioned
in the control rod passagaway. A corresponding number of
sleeves are positioned axially above the grooves and are
circumferentially bulged into the grooves. By "correspond-
ing number" is meant the sleeves which are associated with
the number of control rod passageways which have grooves.
An alternate attachment system (not shown in the
drawings) has guide thimbles without sleeves with the
guide thimbles circumferentially bulged into the grooves.
Another attachment system (shown in Fig. 5) has both the
sleeves 80 and the guide thimbles 50 circumferentially
bulged into the grooves 90.
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Optionally, means for positioning the sleeves
(or guide thimbles) at a predetermined axial distance in
the control rod passageways are employed. Pr~ferably, (as
shown in Fig. 5) such means includes an upper reduced
diameter section of the passageway joined to the normal
diameter section of the passageway by a shoulder surface
96 preferably axially located to maintain a constant
height of the top nozzle in the fuel assembly by account-
ing for the length of the sleeve previously cut off.
Alternatively, the sleeve 80 could be fitted with a collar
abutting the lower surface of the adapter plate 94.
Tensile strength tests on a control rod guide
thimble sleeve portion circumferentially bulged into a
coaxial, fully circular, circumferential groove in a
control rod passageway of a top nozzle adapter plate
section showed the attachment to be stronger than the
sleeve itself.
It will be apparent that many modifications and
variations are possible in light of the above teachings.
It, therefore, is to be understood that within the scope
of the appended claims, the invention may be practiced
other than as specifically described.
