Reconstitutable control assembly having removable control rods with detachable split upper end plugs

A reconstitutable control assembly has a spider structure which supports control rods by detachable attachment joints. Each attachment joint includes a hollow connecting finger on the spider structure and an elongated detachable split upper end plug on each control rod. The split upper end plug includes a pair of separate upper and lower plug portions. The upper plug portion has upper, middle and lower sections, whereas the lower plug portion has upper, middle and lower segments. The upper section of the upper plug portion is rigidly attached to the connecting finger on the spider structure, whereas the lower segment of the lower plug portion is rigidly connected to the control rod. Also, the lower section of the upper plug portion and the middle segment of the lower plug portion have complementary threads defined thereon for rigidly threadably attaching the upper and lower plug portions to one another. Two embodiments of locking features can be used in conjunction with the upper and lower plug portions for locking them together so as to resist their unthreading from one another.

CROSS REFERENCE TO RELATED APPLICATIONS 
Reference is hereby made to the following copending applications dealing 
with related subject matter and assigned to the assignee of the present 
invention: 
1. "Burnable Absorber Rod Push Out Attachment Joint" by Joseph B. Mayers et 
al, assigned U.S. Ser. No. 089,384 and filed Aug. 25, 1987, a division of 
U.S. Ser. No. 774,850, filed Sept. 12, 1985, now abandoned. 
2. "Burnable Absorber Rod Push Out Attachment Joint" by Joseph B. Mayers et 
al, assigned U.S. Ser. No. 089,203 and filed Aug. 25, 1987, a division of 
U.S. Ser. No. 774,850, filed Sept. 12, 1985, now abandoned. 
3. "Reconstitutable Control Rod Spider Assembly" by John M. Shallenberger 
et al, assigned U.S. Ser. No. 162,841 and filed Mar. 2, 1988. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to nuclear reactors and, more 
particularly, is concerned with a reconstitutable control rod spider 
assembly having removable control rods with detachable split upper end 
plugs. 
2. Description of the Prior Art 
In a typical nuclear reactor, such as a pressurized water type, the reactor 
core includes a multiplicity of fuel assemblies. Each fuel assembly is 
composed of top and bottom nozzles with a plurality of elongated 
transversely spaced guide thimbles extending longitudinally between and 
attached at opposite ends to the nozzles. Also, a plurality of transverse 
support grids are axially spaced along and attached to the guide thimbles. 
Further, a plurality of elongated fuel elements or rods transversely 
spaced apart from one another and from the guide thimbles are supported by 
the transverse grids between the top and bottom nozzles. The fuel rods 
each contain fissile material and are grouped together in an array which 
is organized so as to provide a neutron flux in the core sufficient to 
support a high rate of nuclear fission and thus the release of a large 
amount of energy in the form of heat. A liquid coolant is pumped upwardly 
through the core in order to extract some of the heat generated in the 
core for the production of useful work. 
Since the rate of heat generation in the reactor core is proportional to 
the nuclear fission rate, and this, in turn, is determined by the neutron 
flux in the core, control of heat generation at reactor start-up, during 
its operation and at shutdown is achieved by varying the neutron flux. 
Generally, this is done by absorbing excess neutrons using control rods 
which contain neutron absorbing material. The guide thimbles, in addition 
to being structural elements of the fuel assembly, also provide channels 
for insertion of the neutron absorber control rods within the reactor 
core. The level of neutron flux and thus the heat output of the core is 
normally regulated by the movement of the control rods into and from the 
guide thimbles. 
One common arrangement utilizing control rods in association with a fuel 
assembly can be seen in U.S. Pat. No. 4,326,919 to Hill. This patent shows 
a control rod spider assembly which includes a plurality of control rods 
and a spider structure supporting the control rods at their upper ends. 
The spider structure, in turn, is connected to a control drive mechanism 
that vertically raises and lowers (referred to as a stepping action) the 
control rods into and out of the hollow guide thimbles of the fuel 
assembly. The typical construction of the control rod used in such an 
arrangement is in the form of an elongated metallic cladding tube having a 
neutron absorbing material disposed within the tube and with end plugs at 
opposite ends thereof for sealing the absorber material within the tube. 
The spider structure typically includes a plurality of radially extending 
vanes supported on and circumferentially spaced about a central hub. The 
vanes are flat metal plates positioned on edge and being connected at 
their inner ends to the central hub. Cylindrical shaped control rod 
connecting fingers are mounted to and supported by the vanes, with some of 
the vanes having only a single connecting finger and other vanes having a 
spaced pair of connecting fingers associated therewith. 
Typically, the upper end plug of each control rod has a threaded outer end 
which is receivable into a bore in the lower portion of one finger of the 
spider structure and threadable into a tapped hole formed in the finger at 
the inner end of the bore. The end plug is then secured or locked therein 
by a key or pin inserted into the side of the finger and the end plug and 
then welded therein. Generally speaking, the above-described control rod 
spider assembly is not reconstitutable, that is, the assembly cannot 
readily be taken apart and worn or damaged components replaced. Instead, 
the whole assembly must be discarded. 
In many older nuclear reactor plants, the control rods are approaching the 
end of their design life. Furthermore, in a number of newer plants with 
modified reactor internals, the control rods are experiencing accelerated 
wear. In most cases wear occurs over local areas on the cladding of some, 
but not all, control rods of a given control rod spider assembly. Wear is 
believed to be caused by coolant flow-induced vibration. The severity of 
the clad wear is dependent upon the type of nuclear plant and internals 
design and typically occurs only on a limited number of control rods of a 
given assembly. 
Consequently, there is a growing interest in and need for reconstitutable 
control rod spider assemblies wherein selected individual control rods 
that are prematurely worn or damaged can be removed and replaced. A spider 
assembly which allows removal of a control rod from a spider structure by 
severing or cutting off the threadably fastened top end of the control rod 
and the fastener is disclosed in French patent application No. 86/08381. A 
spider assembly which seems to allow removal of a control rod from a 
spider by removing a set screw and unthreading the upper end plug of the 
control rod from a support rod axially movably mounted to a spider vane 
finger is disclosed in Nakazato U.S. Pat. No. 4,711,756. Spider assemblies 
which allow removal of a control rod from a spider structure by deforming 
a locking element on the top end of the control rod are disclosed in U.S. 
patents to Edwards et al U.S. Pat. No. 4,314,885 and Walton U.S. Pat. No. 
4,381,283. However, none of these disclosures would appear to be the 
optimum approach to providing a reconstitutable control rod spider 
assembly. 
SUMMARY OF THE INVENTION 
The present invention provides a reconstitutable control rod spider 
assembly designed to satisfy the aforementioned needs. The reconstitutable 
control assembly of the present invention incorporates a removable control 
rod with a split upper end plug for detachably attaching the control rod 
to a vane finger of the spider. Specifically, the split upper end plug is 
composed of two separate upper and lower portions. 
The upper and lower portions of each upper end plug are attached together 
by screw threading and are torqued to incorporate joint preload. Also, a 
locking feature is defined by the portions in order to prevent their 
loosening during reactor operation. Locking is accomplished by 
swaging/crimping a thin-walled upper segment of the lower portion onto a 
flats-bearing middle section of the lower portion of the upper end plug. 
The flats are thus provided on the upper portion to prevent subsequent 
joint loosening. Removal of a control rod is accomplished by unscrewing 
the rod which will rotate the lower portion of the upper end plug relative 
to the upper portion thereof and overcome the crimp retainer locking 
feature. 
A replacement control rod can then be reinstalled and crimped for rod 
retention. The thin-walled upper section to be crimped is virgin material 
on the lower portion of the upper end plug on the replacement control rod. 
The detachable attachment features incorporated by the separate upper and 
lower portions of the upper end plug do not adversely impact an existing 
desired feature such as the flexure joint which is retained by the control 
rod upper end plug. 
As an alternative to the above-described locking feature, a radially 
flexible and expandable thread-defining coil can be employed between the 
threaded lower section and middle segment of the respective upper and 
lower end plug portions to provide frictional resistance to rotation once 
attachment of the portions has been completed. The coil makes it possible 
to rotate the control rod against the frictional resistance posed by the 
coil without the need for overriding or overcoming a crimp retainer-type 
locking feature. Where the crimp locking feature was previously employed, 
the middle section and upper segment of the respective upper and lower end 
plug portions are of cylindrical configuration and form a slip fit type 
joint instead. 
Accordingly, the present invention is directed to an end plug for use in 
facilitating replacement of a neutron absorber control rod on a control 
assembly spider structure. The end plug comprises a pair of separate upper 
and lower plug portions with the upper plug portion having integral upper, 
middle and lower sections and the lower plug portion having integral 
upper, middle and lower segments. The upper section of the upper plug 
portion is configured for rigid attachment to the control assembly spider 
structure for supporting the control rod. The lower segment of the lower 
plug portion is configured for rigid connection to the control rod. Also, 
the lower section of the upper plug portion and the middle segment of the 
lower plug portion are complementarily configured for rigid threaded 
attachment to one another. Further, the middle section of the upper plug 
portion and the upper segment of the lower lug portion are complementarily 
configured for interlocking attachment to one another so as to resist 
unthreaded detachment of the upper and lower plug portions from one 
another. 
Also, the present invention sets forth in a reconstitutable control 
assembly including a spider structure and at least one control rod, an 
attachment joint for detachably fastening the control rod to the spider 
structure. The attachment joint comprises: (a) a hollow connecting finger 
on the spider structure; and (b) an elongated detachable split upper end 
plug on the control rod having a pair of separate upper and lower plug 
portions. The upper plug portion has integrally-connected 
tandemly-arranged upper, middle and lower sections. The lower plug portion 
has integrally-connected tandemly-arranged upper, middle and lower 
segments. 
More particularly, the upper section of the upper plug portion is rigidly 
attached to the connecting finger on the spider structure, whereas the 
lower segment of the lower plug portion is rigidly connected to the 
control rod. Also, the lower section of the upper plug portion and the 
middle segment of the lower plug portion have complementarily threaded 
means defined thereon for rigidly threadably attaching the upper and lower 
plug portions to one another. Further, the middle section of the upper 
plug portion and the upper segment of the lower plug portion have 
complementary interlocking means defined thereon for locking the upper and 
lower plug portions together so as to resist unthreading of the upper and 
lower plug portions from one another. 
Further, the present invention is directed to a modified end plug for use 
in facilitating replacement of a neutron absorber control rod on a control 
assembly spider structure. The modified end plug comprises a pair of 
separate upper and lower plug portions with the upper plug portion having 
integral upper, middle and lower sections and the lower plug portion 
having integral upper, middle and lower segments. Also, the end plug 
includes means in the form of a radially flexible and expandable 
thread-defining coil. 
Further, the upper section of the upper plug portion is configured for 
rigid attachment to a control assembly spider structure for supporting a 
control rod, whereas the lower segment of the lower plug portion is 
configured for rigid connection to the control rod. Still further, the 
lower section of the upper plug portion and the middle segment of the 
lower plug portion are threaded complementarily to receive therebetween 
the radially flexible and expandable thread-defining coil for threaded 
engagement therewith in a relationship in which the thread-defining coil 
frictionally resists rotation of the upper and lower plug portions 
relative to one another. Finally, the middle section of the upper plug 
portion and the upper segment of the lower plug portion are configured 
complementarily for defining a slip fit joint between the upper and lower 
plug portions. 
These and other features and advantages of the present invention will 
become apparent to those skilled in the art upon a reading of the 
following detailed description when taken in conjunction with the drawings 
wherein there is shown and described an illustrative embodiment of the 
invention.

DETAILED DESCRIPTION OF THE INVENTION 
In the following description, like references characters designate like or 
corresponding parts throughout the several views. Also in the following 
description, it is to be understood that such terms as "forward", 
"rearward", "left", "right", "upwardly", "downwardly", and the like, are 
words of convenience and are not to be construed as limiting terms. In 
General 
Referring now to the drawings, and particularly to FIG. 1, there is shown 
an elevational view of a nuclear reactor fuel assembly, represented in 
vertically foreshortened form and being generally designated by the 
numeral 10. Being the type used in PWR, the fuel assembly 10 basically 
includes a lower end structure or bottom nozzle 12 for supporting the 
assembly on the lower core plate (not shown) in the core region of a 
reactor (not shown), and a number of longitudinally extending guide tubes 
or thimbles 14 which project upwardly from the bottom nozzle 12. The 
assembly 10 further includes a plurality of transverse grids 16 axially 
spaced along the guide thimbles 14 and an organized array of elongated 
fuel rods 18 transversely spaced and supported by the grids 16. Also, the 
assembly 10 has an instrumentation tube 20 located in the center thereof 
and an upper end structure or top nozzle 22 removably attached to the 
upper ends of the guide thimbles 14 to form an integral assembly capable 
of being conveniently handled without damaging the assembly parts. 
As mentioned above, the fuel rods 18 in the array thereof in the assembly 
10 are held in spaced relationship with one another by the grids 16 spaced 
along the fuel assembly length. Each fuel rod 18 includes nuclear fuel 
pellets 24 and the opposite ends of the rod are closed by upper and lower 
end plugs 26,28 to hermetically seal the rod. Commonly, a plenum spring 30 
is disposed between the upper end plug 26 and the pellets 24 to maintain 
the pellets in a tight, stacked relationship within the rod 18. The fuel 
pellets 24 composed of fissile material are responsible for creating the 
reactor power of the nuclear reactor. A liquid moderator/coolant such as 
water, or water containing boron, is pumped upwardly through the fuel 
assemblies of the core in order to extract heat generated therein for the 
production of useful work. 
Prior Art Control Rod Spider Assemblies 
Turning now to FIGS. 2 and 3 as well as FIG. 1, there is shown a typical 
embodiment of a conventional control rod spider assembly, generally 
designated 32, designed for use in the conventional fuel assembly 10 of 
FIG. 1. In its basic components, the control assembly 32 includes a 
plurality of control rods 34 and a spider structure 36 which supports the 
control rods at their upper ends. The spider structure 36 holds the 
control rods 34 in a pattern matched to that of the guide thimbles 14 
which adapts them to be inserted through the top nozzle 22 and downward 
through the guide thimbles 14 of the PWR fuel assembly 10. The spider 
structure 36 is connected to a control mechanism (not shown) which is 
operable in a known manner to move the control rods 34 so as to regulate 
core power. 
In a typical construction, each control rod 34 of the control assembly 32 
is composed of an elongated metallic cladding tube 38 having a neutron 
absorbing material disposed therein and upper and lower end plugs 40,42 
attached at opposite ends of the cladding tube 38 for sealing the absorber 
material therewithin. The spider structure 36 of the control assembly 32 
typically includes a plurality of radially extending flutes or vanes 44 
supported on and circumferentially spaced about a central hub 46. 
Cylindrical shaped control rod connecting fingers 48 are mounted to and 
supported by the vanes 44. Some of the vanes 44 have only a single 
connecting finger 48 attached thereon, whereas other vanes 44 have a 
spaced pair of connecting fingers 48 associated therewith. 
Turning now to FIGS. 4-9, there is illustrated one prior art attachment 
joint, generally indicated by the numeral 50, provided between each 
control rod connecting finger 48 on the vane 44 of the control assembly 
spider structure 36 and the upper end plug 40 of each control rod 34. 
Typically, the upper end plug 40 of each control rod 34 has a threaded 
outer end 52. Each connecting finger 48 is mounted to the vane 44 in a 
bayonet-type of welded connection, and has an axial bore 54 formed in a 
lower portion 56 thereof with a smaller-diameter threaded hole 58 tapped 
therein at the inner end of the bore 54. The threaded outer end 52 of the 
upper end plug 40 is threadably received in the tapped hole 58 when the 
plug 40 is received within the axial bore 54. The end plug 40 is secured 
or locked therein by a key or pin 60 inserted through aligned holes 62,64 
in the sides of the finger 48 and end plug 40 and then welded thereto. 
Parenthetically, it should be pointed out that the axial bore 54 
terminates at the start of an upper portion 66 of each connecting finger 
48 where the finger connects with the vane 44. 
A major disadvantage of this conventional control assembly 32 is that it is 
not reconstitutable; that is, the assembly 32 cannot readily be taken 
apart and have worn or damaged components thereof replaced. Instead, the 
whole assembly has to be discarded. 
However, as mentioned earlier, control rod spider assemblies having 
removable control rods are known in the prior art. One recent control rod 
spider assembly that is reconstitutable is disclosed in the aforecited 
French patent application No. 86/08381. Similar to the above-described 
prior art control assembly, the French control assembly includes a spider 
structure with connecting fingers on vanes and a plurality of control rods 
with upper end plugs having a threaded outer end. However, the attachment 
joint employed to secure each control rod to one connecting finger is 
modified somewhat from that described above. Each connecting finger of the 
French control assembly has an axial bore extending therethrough from end 
to end. When the upper end plug of one control rod is inserted through the 
axial bore, its threaded outer end extends above the top end of the 
finger. A fastener or nut is threaded onto the outer end of the control 
rod upper end plug until it contacts the top end of the finger. Then a 
tubular locking cup formed on the control rod upper end plug above the 
threaded outer end and extending above the threaded nut is deformed 
radially outward to lock within a groove in the nut to retain the nut 
thereon. 
However disadvantageously, to remove the control rod from the spider 
structure, the portion of the end plug which includes the threaded outer 
end with the nut fastened thereon must first be severed or cut off. Thus, 
the fastening nut must be replaced after removal. 
Control Rod Spider Assembly of the Present Invention 
Turning now to FIGS. 10-16, there is shown particularly in FIG. 10, in 
fragmentary form, a reconstitutable control rod spider assembly, generally 
designated by the numeral 68 and constructed in accordance with the 
principles of the present invention, which provides an improved 
arrangement for removing and replacing a control rod. The reconstitutable 
control assembly 68 basically includes a spider structure 70, a plurality 
of control rods 72 (only one fragmentarily shown), and a plurality of 
improved attachment joints 74 (only one shown) for detachable fastening 
the control rods 72 to the spider structure 70. Except for the parts 
thereof to be described hereinafter, the spider structure 70 and control 
rods 72 of the control assembly 68 are identical to that of the prior art 
control assembly 32 of FIGS. 2 and 3. 
Each attachment joint 74 includes an elongated connecting finger 76 on the 
spider structure 70 and an elongated detachable split upper end plug 78 on 
each control rod 72. The split upper end plug 78 includes a pair of 
separate upper and lower plug portions 80,82. The upper plug portion 80 
has integrally-connected tandemly-arranged upper, middle and lower 
sections 80A,80B,80C, whereas the lower plug portion 82 has 
integrally-connected tandemly-arranged upper, middle and lower segments 
82A,82B,82C. 
Being substantially identical to the prior art attachment joint 50 of FIG. 
8, the connecting finger 76 of the attachment joint 74 of the present 
invention has an axial bore 84 formed in a lower portion 76A of the finger 
76 so as to open at a lower end 86 of the finger, and an 
internally-threaded hole 88 at an upper end of the bore 84. Further, the 
upper section 80A of the upper plug portion 80 has external threads 90 
allowing it to be rigidly threadably attached within the threaded hole 88 
of the connecting finger 76. Also, the lower segment 82C of the lower plug 
portion 82 is of a smaller diameter size than an upper open end 92 of an 
elongated hollow tube 94 of the control rod 72 allowing it to be mated and 
rigidly connected within the control rod tube upper end 92. However, the 
remaining middle and lower sections 80B,80C of the upper plug portion 80 
and the upper and middle segments 82A,82B of the lower plug portion 82 are 
different from the upper end plug 40 of the prior art attachment joint 50 
of FIG. 8. 
More particularly, in the attachment joint 74 of the present invention, the 
lower section 80C of the upper plug portion 80 and the middle segment 82B 
of the lower plug portion 82 have complementarily threaded means 96,98 
defined thereon for rigidly threadably attaching the upper and lower plug 
portions 80,82 of the split upper end plug 78 to one another. The 
complementarily threaded means 96 defined on the lower section 80C of the 
upper plug portion 80 is preferably in the form of external male threads 
96. The complementarily threaded means 98 on the middle segment 82B of the 
lower plug portion 82 preferably includes internal female threads defined 
in a recess 100 at the inner end of a central opening 102 extending 
through the upper and middle segments 82A,82B of the lower plug portion 
82. The external threads 96 of the upper plug portion 80 are threadably 
received within the internal threads 98 of the lower plug portion 82 when 
the portions are attached together. Further, lower section 80C of the 
upper plug portion 80 has two different diametric sizes. At the smaller 
one of the two diameters of the lower section 80C is defined the external 
threads 96. Bridging the two diameter sizes of the lower section 80C is an 
annular shoulder 104 which faces toward the lower plug portion 82 and is 
located just above the external threads 96. The central opening 102 of the 
middle segment 82B of the lower plug portion 82 is larger in diameter than 
the internally threaded recess 100 thereof so as to define therebetween an 
annular ledge 106 located just above the recess 100. The upper plug 
portion shoulder 104 is capable of being bottomed out in a preloaded 
condition against the lower plug portion ledge 106 when the upper and 
lower plug portions 80,82 are threadably attached together. 
Further, in the attachment joint 74 of the present invention, the middle 
section 80B of the upper plug portion 80 and the upper segment 82A of the 
lower plug portion 82 have complementary interlocking means 108,110 
defined thereon for locking the threadably attached upper and lower plug 
portions 80,82 together so as to resist unthreading thereof from one 
another during normal reactor operation. The complementary interlocking 
means 108 defined on the middle section 80B of the upper plug portion 80 
is a plurality of flat surfaces 108 formed on the exterior thereof at 
locations circumferentially spaced from one another. There are preferably 
four flat surfaces 108 being circumferentially displaced about ninety 
degrees from one another. The complementary interlocking means 110 defined 
on the upper segment 82A of the lower plug portion 82 is a hollow sleeve 
110 fitted over and about the flat surfaces 108 and deformably crimped 
thereagainst so as to resist unthreading of the upper and lower plug 
portions 80,82 from one another. However, the sleeve 110 is thin-walled so 
as to reformable back to a generally cylindrical configuration upon 
application of a predetermined torque to the control rod 72 to rotate the 
same and unthread the lower and upper plug portions 80,82 of the split 
upper end plug 78 from one another. As can be seem in FIGS. 10, 15 and 16, 
the upper segment 82A of the lower plug portion 82 defining the sleeve 110 
is much thinner in wall thickness than the middle segment 82B thereof. 
To summarize, the reconstitutable control assembly 68 of the present 
invention incorporates a plurality of removable control rods 72 each with 
a split upper end plug 78 for detachably attaching the control rod 72 to a 
vane finger 76 of the spider structure 36. Specifically, the split upper 
end plug 78 is composed of two separate upper and lower plug portions 
80,82. The upper and lower plug portions 80,82 of each upper end plug 78 
are attached together by screw threading and are torqued to incorporate 
joint preload. Also, the interlocking flat surfaces 108 and crimped sleeve 
110 prevent loosening of the plug portions 80,82 during reactor operation. 
Locking is accomplished by swaging/crimping the thin-walled sleeve 110 of 
the lower plug portion upper segment B2A onto flat surfaces 108 of the 
upper plug portion middle section 80B. The flat surfaces 108 are thus 
provided on the upper plug portion to prevent subsequent joint loosening. 
Removal of a control rod 72 is accomplished by unscrewing the rod which 
will rotate the lower plug portion 82 of the upper end plug 78 being 
fixedly attached to the control rod tube 94 relative to the upper plug 
portion 80 thereof being fixedly attached to the spider vane connecting 
finger 79 and overcome the crimp retainer locking feature provided by the 
deformed sleeve 110. A replacement control rod can then be reinstalled and 
crimped for rod retention. The replacement thin-walled sleeve to be 
crimped is virgin material on the lower plug portion of the upper end plug 
on the replacement control rod. The detachable attachment features 
incorporated by the separate upper and lower plug portions 80,82 of the 
split upper end plug 78 do not adversely impact any existing desirable 
features, such as the flexure joint 112 on the upper plug portion 80 and 
the intended clearance gap 114 between the flexure joint 112 and the lower 
end 86 of the finger 76; these prior art features are retained in the 
split upper end plug 78 of the present invention. 
Turning now to FIGS. 17-21, a modified split upper end plug 116 is 
illustrated having upper and lower plug portions 118,120 and employing a 
different type of locking feature as an alternative to the above-described 
crimp retainer-type locking feature. In all respects but the following 
ones the modified split upper end plug 116 is identical to the 
earlier-described split upper end plug 78. 
First, in place of the thin-walled crimpable hollow sleeve 110 of the upper 
section 82A of the upper end plug lower portion 82 and the flats-bearing 
middle segment 80B of the upper end plug upper portion 80, a 
thicker-walled cylindrical upper segment 120A is provided on the lower 
plug portion 120 and a cylindrical nonflat-bearing middle section 118B is 
provided on the upper plug portion 118. Thus, where the crimp locking 
feature was previously employed, the corresponding middle section 118B and 
upper segment 120A of the respective upper end plug portions 118,120 are 
of cylindrical configuration and merely form a slip fit type joint 
instead. 
Second, the threaded lower section 118C of the upper plug portion 118 and 
threaded middle segment 120B of the lower plug portion 120 of the modified 
split upper end plug 116 are greater in length than the corresponding 
threaded lower section 80C and middle segment 82B of the respective upper 
and lower portions 80,82 of the split upper end plug 78. Finally, the 
modified locking feature is provided by a member 122 in the form of a 
radially flexible and expandable thread-defining coil deployed between the 
threaded lower section and middle segment 118C,120B of the respective 
upper and lower end plug portions 118,120. More particularly, the coil 122 
threadably fits between and at its respective interior and exterior 
peripheries makes threaded frictional engagement with external male 
threads and internal female threads 124,126 on the respective upper plug 
portion lower section 118C and lower plug portion middle segment 120B. 
The coil 122 thus provides frictional resistance to relative rotation of 
the upper and lower plug portions 118,120 once attachment of the plug 
portions has been completed. However, the coil makes it possible to rotate 
the control rod against the frictional resistance posed by the coil 
without the need for overriding or overcoming a crimp retainer-type 
locking feature. 
As best seen in FIGS. 22 and 23, the radially flexible and expandable 
thread-defining coil 122 is a screw thread coil insert which per se is a 
commercial product marketed under the tradename HeliCoil. The locking 
action of the coil is achieved by one or more of the windings 128 of the 
coil 122 having a series of straight segments or "chords" 128A. When the 
threaded lower section 118C of the upper plug portion 118 enters the 
"grip" coil 122, these chordal segments 128A flex outward, creating 
pressure on the threaded section and providing frictional resistance to 
rotation of the upper plug portion 118 relative to the lower plug portion 
120. 
It is thought that the present invention and many of its attendant 
advantages will be understood from the foregoing description and it will 
be apparent that various changes may be made in the form, construction and 
arrangement thereof without departing from the spirit and scope of the 
invention or sacrificing all of its material advantages, the form 
hereinbefore described being merely a preferred or exemplary embodiment 
thereof.