Note: Descriptions are shown in the official language in which they were submitted.
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TECHN _AL FIELD
This invention is directed to improvements
in nuclear reactor fuel assemblies, and, more
particularly, to methods and apparatus for securing
and removing burnable poison rods from the spider
in a fuel assembly, and the like.
BACKGROUND ART
_
To produce useful power from nuclear processes
i~ is necessary to assemble a sufficient concentration
of fissionable uranium, or other suitable material,
in a physical configuration that will sustain a con-
tinuous sequence of energy-producing reactions. This
assembly, or reactor core, transfers the heat tha~
is generated in the fission reactions to a working
fluid. Frequently, pressurized water flowing through
the core at high velocities is used for this purpose.
Because the heat, vibration and radiation tha~
is generated within a power reactor core creates a
generally hostile environment, the structural integrity
of the core components is an important consideration.
Consequently, pressurized water power reactor cores
frequently are comprised of groups of fuel assemblies
that are arranged in a generally right circular cylindrical
configuration. Each fuel assembly, moreover usually
comprises an array of about two hundred long slender
fuel rods that are parallel ~o and spaced from each other.
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Each of these fuel rods contains a stack of generally
cylindrical uranium dioxide pellets in which the
uranium provides the fissionable fuel for the power
reactor.
These fuel assemblies are no~ limited to fuel
rods but also support a number of other components.
For example, instrumentation tubes for observing temp-
erature and neutron flux conditions within the core;
end fittings and fuel element grids for stabilizing
the fuel assembly components; and control rods and
control rod guide tubes for regulating the power out-
put from the reactor through the selective absorption
of fission inducing neu~rons within the reactor core
often are made a part of the fuel assembly structure.
Clearly, the neutron distribution will vary
from place-to-place within the reactor core. Illustrative
ly, near the core perimeter it can be expected that the
neutron population will be small relative to the center
of the reactor core because the concentration of neutron
producing uranium is lower at the core perimeter than
it is in the center of the core. Neutrons at the core
perimeter can "escape" from the core more readily through
the core surface than they can from the center of the
core, further tending to reduce the neutron concentration
near the reactor core surface. Because heat generation
within any specific portion of the reactor core is
related to the neutron population within that portion)
there is a definite tendency to produce higher tempera-
tures at the center of the reactor core than at itsmargin. This înclination ~oward producing local tempera-
ture maxima in different regions within the reactor
core is generally undesirable for a number of reasons.
Primarily, the reactor is designed for core operation
that will not exceed a predetermined temperature. If
this core temperature is reached in just one or in a
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few local points within the reactor core, the over-
all heat generating potential of the core can not
be realized. This effect results because the
temperatures elsewhere in the core must be kept
to lower values in order not to exceed the design
temperature at those limited points or "hot spots"
in which the maximum design temperature has been
reached.
Accordingly, in the simplified example under
consideration, o~er-all reactor power can be increased
if the neutron population (and hence, hea~) in ~he
central portion o~ the core is depressed and the neutron
population in the larger volume that characterizes
the peripheral reactor core annulus is allowed to
increase. In this way, by "flattening" the power distri-
bution in the reactor core, the core actually is able
to generate more power than it would be able to generate
if the neutron concentration, temperature and power
was allowed to reach a peak in the center of the core,
or in some other location, as the case may be. In order
to achieve this "flat" power distribution, it has been
the practice to in~ert "burnable poison" rods in the
fuel assemblies. Typically, a burnable poison rod is
a tube filled with a material that has a very high
probability for absorbing neutrons. For example, a
sintered dispersion of boron carbide in an alumina
matrix is suitable for this purpose.
Neutrons, absorbed in this manner by means of
the material within the rod are, in effect, withdrawn
~rom the fission and power generation process. And
so, to "flatten" the power distribution with a reactor
core, burnable poison rods are concentrated in those
fuel assemblies that are located in the central portion
o~ the reactor core.
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Depending on a number of subtle effects it
also might be advisable to provide burnable poison
rod concentrations in other portions of the reactor
core in which specific design or operational features
produce large local neutron populations.
Not only must the fuel assembly support all
of these diverse structural components in spite of
the generally hostile environment within a reactor
core, but the fuel assembly also must be capable of
the somewhat conflicting need for swift and easy
disassembly. For instance, it should be noted that
fuel assemblies become radioactive after exposure in
an operating reactor core. This radiation is so intense
1~ that inspection and repair can be accomplished only
with remote handling equipment behind adequate radiation
shielding.
Consequently, because disassembly procedures
are expensive and time consuming, the need for a sturdy,
yet readily dismountable structure is of considerable
commercial importance.
The burnable poison rods that are used in many
fuel assemblies are a part of this s~ructural picture.
Generally, the burnable poison rods that have characterized
~5 the prior art are mounted for movement in a direction
that is parallel to the longitudinal orientation of
the fuel rods. A "spider", in the form of a centrally
disposed hub from which a number of arms radiate often
is used to couple the burnable poison rods together ~or
longitudinal movement relative to the balance of the
fuel assembly. This motion is required to permit
the power reactor operator to insert or withdraw the
burnable poison rods from the reactor core in response
to power flattening needs.
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Eventually, lumped burnable poison rods must be
removed from operation, packayed i,n a cask that provides
adequate radi,ation shieldiny and then shipped for dispo~al
at a suitable site. It is desirable to remove each of the
rods from the spider in order to gconomize on the volume of
the shipping cask. Unfortunately, removing the burnable
poison rods from the spider is complicated and potentially
hazardous for a number of reasons. The irradiated poison
rods have developed, after sufficient irradiation in the re-
actor core, an internal gas pressure. The cladding ortubing in which the burnable poison has been loaded also
becomes qui~e brittle as a result of a period of irradiation.
The burnable poison rods usually are joined to their res-
pective spider arms by means of threaded fasteners. In
these circumstances, the most frequently suggested techni-
ques for removing the rods from the spider are by means of
shearing or sawing. Sawing the rods permits these rods to
be handled more gently - an important consideration in view
of the gas pressure within the rods - but the sawing process
generates radioactive chips. Shearing overcomes this prob-
lem to a large extent,' but does nevertheless result in un-
desirably rough handling.
There is, then a need for some suitable means -Eor
joining burnable poison rods to the spider arms in a manner
that is proof against the reactor core environment but per-
mits these rods to be removed from the spider simply, swiftly
and delicately.
According to the invention, there is pr~vided a
control component structure comprising a spider having a
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plurality of arms, at least onc spider bore forrned in said
plurality of arms, sa:id spider bore including an enlarged
recess and a small recess with an upright truncated conical
section forming a transition from the enlaryed recess to the
small recess, and a burnable poison rod includiny a tube
terminating in transverse end, a plug with a chamfered end
that leads into a cylindrical portion, said cylindrical por-
tion of the plug snugly fitting ~J.ithin the tube and termina-
ting in a radially protruding shoulder which engages the
transverse end of the tube to which it is welded, and a stem
protruding in the longitudinal direction from the central
portion of the shoulder having a longitudinal stem bore ex-
tending through about half of the length of the stem, a~
least part, of the stem which defines the stem bore being
fixed within the truncated conical section, the enlarged re-
cess and the small recess of the spider by outward deforma-
tion of that part of the stem in order to releasably attach
the rod to the spider, said stem being adapted to substant.i-
ally reshape itself by movement of the rod in a longitudinal
direction with respect to the spider bore, while maintaining
the structural integrity of the poison rod and maintaining
the structural integrit~ of the spider.
The invention also extends to a method oE assemb-
ling and disassembling a control component structure for a
nuclear reactor comprising the steps of providing a spider
having a plurality of arms, at least one spider bore formed
in said plurality of arms for receiving a burnable poison
rod having a deformable stem protruding from one end thereof,
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insertiny the stem into the spider bore, fasten:iny the stem
within the spider bore by deforminy a port,ion of the stem
into engagement with a taper in said bore t and separating the
burnable pOiSOTI rod from the spider by applyiny a longitudi-
nal force to the stem of the burnable poison rod sufficientto extrude the deformed portion through the taper while
maintaining structural unityof the burnable poison rod and
maintaining structural unity of the spider. Preferably the
stem has an axial bore, and a mandrelo~'smaller diameterthan
the spider bore is inserted within the stem bore to apply
the longitudinal force.
In these circumstances, although considerable force
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may be applied to press the stem through the spider a~m bore~
it will be noted ~hat the force i8 applied in a longitudin~l
direction relative to ~h~ stem ThiS feature of ~he in~ention
not only protects the brittle, pressurized burnable poison rod
from risk of fracture or other damage due to the rough hand-
ling that characterizes shearing but also avoids producing a
mass of radioactive chips if the rod is to be sawed free from
the spider.
There are a number of more subtle advantages that
accrue through the practice of the invention. Manufacturing
costs, for example, are relatively low in comparison to the
threaded fasteners that have characterized the prior art. Joint
performance, moreover, is superior at the temperatures commonly
reachedwithin nuc~earrea~tor cores. For instance, the greater
thermal expansion of the spider arm (frequently made from
stainless steel) relative to a zircalloy pin on the burnable
poison end plug causes undesirable axial stress that must be
compensated for by other means. This is achieved in the inven-
tion by a corresponding radial loosening, thereby reducing the
chance for generating undesirahle thermal stresses in this
region. Without this automatically provided stress relief,
these thermally induced forces otherwise might cause the stem
to shear and thus permit the burnable poison rod to become an
undesirable loose object within the reactor core.
The various features o novelty which characterize
the invention are pointed out with particularity in the claims
annexed to and forming a part of this specification. For a
better understanding of the invention, its operating advantages
and specific objects attained by its use, reference should be
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had to the accompanying drawiny and descriptive matter in which
there is illustrated and descri~ea a preferxed embodiment of
the invention.
BRIEF DESC~IPTION OF THE D~AWING
Fig. 1 shows a typical spidex and burnable poison rod
combination, to which the invention may be applied;
FigO 2 illustrates in full section a typical embodi-
ment of a portion of a stem of a rod in a partially assèmbled
condition in accordance with the invention;
Fig. 3 shows in broken section the stem fully assem-
bled in a portion of a spider of the invention; and
Fig. 4 is a drawing in broken section illustrating a
technique for disassemoling the stem from the ~t~ _ t ~
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DETAILED DESCRIPTION OF T~ PREFERRED EM~ODIMENTS
For a more detailed understanding of the invention,
attention is invited to ~ig. 1. As illustrated, a
spider 10, formed from stainless steel, or the like,
is provided with a centrally disposed,generally cylindrical
hub 11 that has a longitudinal axis 12. The spider 10
has a number of spider arms that extend radially from
the hub 11 in a plane that is perpendicular to the
axis 12. For purposes of simplified description, only
spider arms 13, 14, 15, 16, 17, 20 and 21 which are viewed
in the plane of of Fig. 1 of the drawing are shown.
A group of burnable poison rods 22, 23, 24,
25, 26, 27 and 30 are attached to their respective spider
arms. All of these burnable poison rods are long, slender
tubes in which the longitudinal axis of each of these
tubes is parallel with the longitudinal axis 12 of the
hub 11. As illustrated in more detail with respect
to the burnable poison rod 30, a hollow tube 31 forms the
outer surface of the rod. On longitudinal end of the
tube 31 is sealed by means of a solid plug 32 that is
welded or otherwise secured in the open tube end. Within
the tube 31 and bearing against the plug 32 is a stack
of short cylindrical burnable poison pellets 33. The
individual cylindrical pellets in the stack 33 are
pressed together in a longitudinal direction by means of
a spring 34 which is lodged between the last pellet
in the stack 33 and a plug 35 that is secured in the
other open end of the hollow tube 31.
Turning now to Fig. 3, it can be seen that the
plug 35 is provided with a chamfered end 36 that provides a
lead in for a cylindrical portion 37. The inside diameter
of the hollow tube 31 matches the outside diameter of
the cylindrical portion to an extent that a snug fit is
achieved.
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As shown in the drawing, ~he cylindrical portion,
37 terminates in a radially protruding shoulder 40
that engages the transverse end of the hollow tube 31
to which it is welded. In the illustr~tive erabodiment
of the invention, the diameter of the hollow tube 31
in order to establish a flush rela~ionship between
an exposed cylindrical portion 41 of the plug 35 that
extends beyond ~he transverse end of the tube 31 and
the outer surface of the tube.
A ~ransverse flange 42 that is spaced longi-
tudinally from the shoulder 40 terminates the exposed
cylindrical portion 41 of the plug 35. A stem 43 pro-
trudes in a longitudinal direction from the central
portion of the transverse flange 42. Most of the
shank that forms the stem 43 is formed from solid metal
stock. A longitudinal bore 44 is formed, however, in
the stem 43 for a depth that extends from the free
end of the stem through a distance that is about half
of the stem length.
A portion of a stem 45 as manufactured, and
before full installation on a spider arm 46, is shown
in ~ig. 2. Thus the stem 45 has a generally cylindrical
exterior configuration. Bore 47, that extends from
the free end of the stem 45 to about half its length,
also is of a generally cylindrical shape with the excep-
tion of the bottom of the bore which is formed into
the shape of a shallow cone.
It will be recalled that the stem 45 is received
in the spider arm 46. To accomplish this end, the spider
arm 46 is provided with a generally cylindrical passage-
way 50 ~hat has a longitudinal axis 51 which is essentially
coincident with the longitudinal axis of the stem 45
and the burnable poison rod tnot shown in Fig. 2). In
accordance with a characteristic feature of the invention,
an enlarged recess 52 is formed within the passageway 50.
Note in this respect, that the recess 52 overlaps only
a portion of the bore 47 in the stem 45.
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A truncated conical surface 53 affords a
transition from the recess 52 to a bore 54 which forms
the balance of the passa~eway 50. As shown in Fig. 2,
the bore 54 has a slightly larger diameter than the
outside diameter of the stem 45 that is lodged in
the passageway 50.
In operation, the stem 45 is inserted into the
appropria~e passageway 50. The stem is mechanically
de~ormed through hydroforming 9 mechanical flaring or
other suitable means, to match the contour of the truncated
conical surface 53 and the enlarged recess 52 and to pro-
vide a designed "weak point" for poison rod removal, as
illustrated in Fig. 3. In this manner, ~he burnable
poison rod is secured to the associated spider arm with-
out making use of costly and difficult to handle weldfittings, and the like.
It will be recalled, moreover, that different
metals often are used for the spider arm and for the
burnable poison rod. The truncated conical surface 55,
however, provides a tapered surface that alleviates
the effect of the different coefficients of thermal ex-
pansion. Typically, as the temperature increases, the
exemplary stainless steel spider arm expands in an
axial dire~tion more than the poison rod end plug 35
(usually Zircalloy). In this manner, ~he axial stresses
~hat otherwise would have been generated are alleviated
because of the radial loosening of the parts. Thus,
because the spider arm also expands radially more than
the plug 35, the conical interface between the adjacent
deformed portion of the stem 43 and the conical surface
53 shifts axially downward. The two parts remain
in intima~e contact radially and axially, but the strain
in the stem 45 has been materially reduced Tela~ive to
the stresses that otherwise would have been generated.
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Thus, although a tight joint is maintained throughout
the entire range of reactor operating temperatures,
the structure nevertheless provides a predetermined
design "weak point" to facilitate removal of the
burnable poison rod as described subsequently in more
complete detail.
A further salient fea~ure of the invention is shown
in Fig. 4. As illustra~ed, a plug 56 seals an end of
a burnable poison rod 57. The plug 56 has a stem 60,
the longitudinal axis OI which is coincident with longi-
tudinal axis 61 of the burnable poison rod 57.
A bore 62 extends longitudinally through about
half the length of the stem 60. As shown in the draw-
ing, this bore has been expanded or upset through about
half of its length to produce an enlarged cavity that
enables the outer diameter of the adjacent portion of
the stem 60 to bear against an enlarged recess 64 in
a spider arm 65.
The bore 62 has a truncated conical taper 63 that
also provides a transition to a smaller diameter bore
portion 66. The smaller diameter bore portion 66, more-
over, terminates in a cone 67.
In accordance with a salient feature of ~he in-
vention, a mandrel 70 is pressed into the bore 62 in
; 25 the direction of arrow 71. The mandrel 70, preferably
formed inthe shape of a slender, cylindrical metal
shaft, has a diameter that is slightly smaller than
the diameter of the smaller diameter bore portion 66.
The longitudinal axis of the mandrel 70 is generally
- 30 coincident with the longitudinal axis 61 of the burnable
poison rod 57.
To ~emove the burnable poison rod 57 from the
spider arm 65 in a manner that will avoid undesirable
shearing or sawing proceduTes it is only necessary to press
the mandrel 70 in the direction of the arrow 71 with
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force sufficient to extrude the stem 60 through small
diame~er recess 72, threby freeing the stem 60 from
the spider arm 65.
Naturally although not shown in the drawing,
a group of mandrels can be ganged together on a plate
These mandrels, aligned with and inserted into corres-
ponding bores on respective burnable poison rods
mounted in a spider, can press all of the rods out
of the spider in one operation. In this way, a potentially
hazardous, expensive and tedious process of shearing or
sawing each burnable poison rod from the spider is
avoided.
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