Thimble guide extender for a nuclear power plant

A sleeve-like sealing unit, particularly a thimble guide extender is adapted to be installed on and coaxially with a thimble guide mounted on a lower core plate of a nuclear reaction vessel and projects toward and terminates at a distance from an underface of a bottom nozzle forming part of a fuel assembly and supported on a top face of the lower core plate. The extender has a lower fitting adapted to surround the thimble guide at a bottom length portion thereof adjacent the top face of the lower core plate; and an upper fitting being axially displaceable relative to the lower fitting and being adapted to surround the thimble guide at a top length portion thereof. The upper fitting has an axial passage and a terminal opening defined by an outer annular radial face adapted to be in a face-to-face engagement with the underside of the bottom nozzle. The extender further has an axially compressible, generally cylindrical resilient bellows arranged between the lower and upper fittings generally in axial alignment therewith and being sealingly secured thereto. In an installed state of the extender the belows is compressed and urges the annular radial face of the upper fitting into a sealing contact with underside of the bottom nozzle. A circumferential sealing arrangement is provided at the lower fitting and is adapted to sealingly surround the thimble guide in the installed state of the extender.

BACKGROUND OF THE INVENTION 
This invention relates to a sleeve-like sealing unit, particularly an 
extender for a thimble guide in a nuclear power plant. 
In a conventional pressurized water reactor vessel, in the lower portion 
thereof, a horizontal lower core plate extends, whose top face supports a 
large number of fuel assemblies each having a bottom nozzle. The bottom 
nozzle has a horizontal platform and spaced legs which extend downwardly 
therefrom and which stand on the lower core plate. 
A number of measuring instruments are employed to promote safety and to 
permit proper control of the nuclear reaction. Among other measurements, a 
neutron flux map is generated periodically, for example, every 28 days, 
using data gathered by neutron flux detectors which are moved through a 
number of selected fuel assemblies. To guide the flux detectors during 
their periodic vertical travel parallel to the length dimension of the 
fuel rods of the fuel assemblies, stainless steel tubes known as flux 
thimbles extend through the bottom of the reactor vessel and into the 
selected fuel assemblies. Aligned vertical channels are provided in the 
reactor vessel wall, the lower core plate as well as the platform of the 
bottom nozzle of each selected fuel assembly to accommodate the thimble 
which thus vertically extends from the outside of the reactor vessel 
through the aligned passages to the top of the fuel assembly. The flux 
detector is inserted into the thimble and moved therein along the fuel 
assembly while the detected flux is recorded. During refuelling the 
thimbles are withdrawn from the reactor vessel and subsequently driven 
back into the fresh fuel assemblies. 
For accurately guiding, supporting and stabilizing the thimble, into the 
thimble-receiving vertical passage of the lower core plate there is 
installed, at the top of the lower core plate, a thimble guide which is 
essentially a rigid sleeve member projecting from the top face of the 
lower core plate vertically towards the platform of the associated fuel 
assembly bottom nozzle and in which the thimble may slide. The upper 
terminus of the thimble guide is situated at an appreciable distance from 
the bottom face of the nozzle platform to prevent any abutting engagement 
between the thimble guide and the nozzle. Such an engagement, due to the 
rigid construction of both the thimble guide and the nozzle platform would 
adversely affect the stable footing of the bottom nozzle, sought to be 
ensured by an engagement between the bottom face of the nozzle legs and 
the top face of the lower core plate. 
Due to the above-discussed spacing between the thimble guide and the 
underside of the nozzle platform, the thimble portion which bridges that 
distance is, in the absence of additional shroud arrangements, directly 
exposed to the coolant flow in the reactor vessel. Since considerable 
coolant turbulence exists during operation of the reactor in the region 
between the upper surface of the core plate and the lower surface of the 
lower nozzle platforms of the fuel assemblies, such turbulence may cause 
undesired vibration and wear of the thimbles. These phenomena are enhanced 
by the fact that, in the absence of a shielding arrangement, the coolant 
flowing upwardly through the thimble channel in the core plate and the 
thimble guide does not have a well-defined flow path leading into the 
thimble passage in the nozzle platform. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a sleeve-like sealing unit, 
particularly a thimble guide extender which shieldingly surrounds the 
thimble particularly in the zone between the upper end of the rigidity 
mounted thimble guide and the underside of the fuel assembly nozzle 
platform; further, which effectively shields the thimble at all sides from 
exposure to the coolant flow and which sealingly engages the underside of 
the nozzle platform and sealingly surrounds a bottom part of the thimble 
guide at the top of the core plate without requiring close sliding fits 
between parts and allowing large manufacturing tolerances. 
These objects and others to become apparent as the specification 
progtesses, are accompalished by the invention, acocrding to which, 
briefly stated, the extender has a lower fitting adapted to surround the 
thimble guide at a bottom length portion thereof adjacent the top face of 
the lower core plate; and an upper fitting being axially displaceable 
relative to the lower fitting and being adapted to surround the thimble 
guide at a top length portion thereof. The upper fitting has an axial 
passage and a terminal opening defined by an outer annular radial face 
adapted to be in a face-to-face engagement with the underside of the 
bottom nozzle. The extender furether has an axially compresible, generally 
cylindrical resilient bellows arranged between the lower and upper 
fittings generally in axial alignment therewith and being sealingly 
secured thereto. The bellows is compressed to a greater or lesser degree 
by the lower and upper fittings dependent upon axial distances 
therebetween. In an installed state of the extender the bellows is 
compressed and urges the annular radial face of the upper fitting into a 
sealing contact with the underside of the bottom nozzle. A circumferential 
sealing arrangement is provided at the lower fitting for sealingly 
surrounding the thimble guide in the installed state of the extender.

DESCRIPTIN OF THE PREFERRED EMBODIMENTS 
Turning to FIG. 1, a pressurized water reactor (not shown in detail) has a 
hemispherical bottom end wall 1 and supports a horizontal lower core plate 
2, on the top face 3 of which there are supported fuel assemblies, of 
which one is shown and generally designated at 4. The fuel assembly 4 
comprises a plurality of fuel rods 5 supported at their lower end by a 
bottom nozzle generally designated at 6. The nozzle 6 comprises a 
horizontal platform 7 having a bottom face or underside 8, as well as 
spaced legs 9 which extend downwardly from the platform 7 and stand on the 
top face 3 of the lower core plate 2, thus supporting upright the entire 
fuel assembly 4. 
Through axially aligned vertical passages 10, 11 and 12 provided, 
respectively, in the reactor wall 1, the lower core plate 2 and the 
platform 7 there passes a thimble 13 which is a slender stainless tube 
reaching to the top part of the fuel assembly 4 at its upper end and 
projectin out of the reactor wall 1 in a downward direction to be 
associated with measuring instrumentation, not shown. The thimble 13 
serves the purpose of guiding therein a probe (also not shown) which 
travels vertically along the length of the fuel assembly 4 to sense the 
neutron flux. 
The thimble 13 is axially moved out of the fuel assembly during refueling 
and is subsequently reinserted into the fresh fuel assembly. In the zone 
between the reactor wall 1 and the lower core plate 2 the thimble 13 is 
surrounded by a permanently installed sleeve construction 14 whose 
securing elements and thimble guide components are generally designated at 
14a. 
A thimble guide 15 constituted by an elongated sleeve-like component is 
inserted at the upper end of the passage 11 of the lower core plate 2 and 
extends vertically upwardly, in alignment with the passage 12 provided in 
the nozzle platform 7. The thimble guide 15 is rigidly held--for example, 
by a tightened threaded connection--in the core plate 2. The thimble guide 
15 serves as a sliding guide and for positioning and stabilizing the 
thimble 13. Above the nozzle platform 7 the thimble 13 is positioned 
within an instrumentation tube 16 extending upwardly from the platform 7 
and forming a permanent part of the fuel assembly 4. 
On the thimble guide 15 there is inserted an only symbolically shwon 
thimble guide extender generally designated at 17 for the particular 
purpose of providing a shield about the thimble 13 in the zone situated 
between the top of the thimble guide 15 and the underface 8 of the 
platform 7. 
The thimble guide extender 17 constitutes a first preferred embodiment of 
the invention, now to be described in detail with reference to FIG. 2 in 
which the extender 17 is shown in its installed state. 
The extender 17 has a longitudinal axis A and comprises a lower fitting 18 
having a flange 19 and an upper fitting generally designated at 20, being 
vertically spaced from and generally in alignment with the lower fitting 
18. The upper fitting 20 has a hollow cap 21 having an inner radial 
terminal shoulder 21a and provided with a passage 22 adapted to closely 
surround the thimble 13 (not shown in FIG. 2) and an integral or welded 
sleeve 23 having a radially inwardly extending flange 24 of downwardly 
flaring configuration at 25 and a narrow, annular contact area 25a. 
A cylindrical bellows 26 of circumferentially closed configuration is 
sealingly welded at its top to a radial lower face of the sleeve 23 and 
is, at its bottom, sealingly welded to a radial top face of the flange 19 
of the lower fitting 18. The bellows 26 is resiliently deformable in 
several directions and may be made, for example, of No. 316 stainless 
steel. 
Within the bellows 26, coaxially therewith, there is arranged a retainer 
ring generally designated at 27 having an upper terminal radial face 31a. 
Approximately the lower two-thirds of the length of the retainer ring 27 
is slit to form a plurality of circumferentially arranged, radially 
resilient legs 28 each having an inwardly extending bottom projection 29. 
The retainer ring 27 further has at its bottom, approximately at the axial 
location of the leg projections 29, radially outwardly extending ring 
segments 30. At the top of the retainer ring 27 there is formed a radially 
outwardly extending flange 31. 
The retainer ring 27 is, at its lower part, in a stabilizing engagment with 
the inner face of the lower fitting 18 by means of the ring segments 30. 
At its upper portion, the retainer ring 27 is in circumferential 
engagement with the surrounding sleeve 23 of the upper fitting 20. The 
circumferential contact between the sleeve 23 and the retainer ring 27 is 
along the narrow, annular contact area 25a. Such small area contact 
permits a rocking motion of the upper fitting 20 relative to the retainer 
ring 27 about an axis transverse to the longitudinal extender axis A for 
purposes to become apparent as the specification progresses. 
A cooperation between a lower radial face of the flange 31 of the retainer 
ring 27 and an upper radial face of the flange 24 of the sleeve 23 forming 
part of the upper fitting 20 limits an outwardly directed axial motion of 
the upper fitting 20 on the retainer ring 27 as urged by the bellows 26. 
The cylindrical bellows 26 is spacedly and fully surrounded by a 
cylindrical shield 32 which is secured at its bottom part to the lower 
fitting 18 and which has an open top through which projects the sleeve 23 
of the upper fitting 20. 
The thimble guide extender 17 is installed on the thimble guide 15 
previously secured to the lower core plate 2 from above, in the absence of 
the fuel assembly 4. For this purpose, the extender 17 is guided 
downwardly such that the tapered top part of the thimble guide 15 is 
introduced into the opening defined by the bottom part of the retainer 
ring 27 and is slid down thereon until the projections 29 of the 
respective legs 28 snap into a bottom circumferential groove 33 of the 
thimble guide 15. In this manner the retainer ring 27 is axially 
immobilized on the thimble guide 15 which closely fits into the retainer 
ring 27. In the absence of the fuel assembly 4 the biased bellows 26 ruges 
the sleeve 23 of the upper fitting 20 into abutting engagement with the 
flange 31 of the retainer ring 27 and, at the same time, the bellows 26 
ruges the lower fitting 18 downwardly such that its smooth annular radial 
bottom face 18a surrounding the thimble guide 15 is in an annular contact 
with the top face 3 of the core plate 2. 
The fuel assembly 4 is lowered onto the core plate 2 such that the passage 
12 of the nozzle platform 7 is in alignment with the passage 22 of the 
upper fitting 20 of the extender 17. The length of the extender 17 is 
designed such that before the legs 9 of the fuel assembly nozzle 6 contact 
the upper face 3 of the core plate 2, the upper annular radial surface 
area 34 of the cap 21 engages the bottom face 8 of the nozzle platform 7 
and, as the fuel assembly 4 continues to move downwardly, the bellows 26 
is compressed by virture of the downwardly moving upper fitting 20 causing 
an increasing, axially oriented resilient force of the bellows 26 to be 
exerted downwardly on the lower fitting 18 and upwardly on the upper 
fitting 20. Since the lower fitting 18 is free to slide axially along the 
ring segments 30 on the lower part of the retainer ring 27, no spring 
forces are taken up by the retainer ring 27; these forces are applied 
axially downwardly on the lower fitting 18 further pressing it into 
engagement with the upper face 3 of the core plate 2. 
The extender 17 is ready for use as the fuel assembly nozzle 7 assumes its 
supported position on the core plate 2 as shown in FIG. 1. 
The upper fitting 20, particularly by virtue of the only narrow annular 
contact area 25a between the sleeve 23 and the outer face of the retainer 
ring 27, is capable of executing a slight rocking motion about an axis 
transverse to the longitudinal extender axis A, readily permitted by the 
resilient bellows 26. By virtue of such rocking motion any deviation from 
a strict parallelism between the underside 8 of the nozzle platform 7 and 
the top face 3 of the core plate 3 and/or between the upper radial annular 
face 34 of the upper fitting 20 and the bottom annular face 18a of the 
lower fitting 18 are compensated for and thus face-to-face, sealing 
engagements between the upper fitting 20 and the platform 7 as well as 
between the lower fitting 18 and the core plate 2 are ensured. 
By virtue of the bellows construction and the above-described sealing 
engagement of the extender 17 with the bottom nozzle 7 and the core plate 
2, the inside of the extender 17 is sealed from the surrounding 
environment, that is, the region between the core plate 2 and the platform 
7 of the lower nozzle 6, without the need of accurately fitting, 
relatively sliding extender parts. 
Further, by virtue of its sealing contact with the underside 8 of the 
nozzle platform 7, the cap 21 of the upper fitting 20 seals the passage 12 
in the nozzle platform from the surrounding environment and, with its 
upwardly tapering outer conical face allows the coolant water into 
platform passages 35 situated adjacent the thimble passage 12. 
Thus, the thimble extender 17 sufficient shields the thimble 15 from the 
coolant flow turbulences in the zone between the upper end of the thimble 
guide 15 and the underface 8 of the nozzle platform 7 and also in the 
passage 12 and the adjoining instrumentation tube 16. In sealing this 
area, the extender 17 defines a continuous flow path through the sleeve 
construction (instrumentation column) 14, the passage 11 and the 
instrumentation tube 16. By virtue of an orifice (not shown) at the top of 
the instrumentation tube 16 a reduced flow is obtained through this flow 
path, resulting, in turn, in smaller forces thus reducing the 
vibration-causing effect thereof. 
The cylindrical shield 32 surrounding the bellows 26 protects the bellows 
from flow turbulences that may cause vibrations thereof and further may be 
used --by virtue of the cooperation of its upper edge 36 with the 
underface 37 of teh sleeve 23--to limit the downward axial sliding motion 
of the upper fitting 20, thus preventing an excessive compression of the 
bellows 26 particularly during installation of the extender 17 on the 
thimble guide 15. Such a limiting effect may also be obtained by the 
cooperation beetween the radial face 21a of the cap 21 and the radial face 
31a of the retainer ring 27. 
In FIG. 3 there is illustrated a thimble guide extender 40 constituting a 
second preferred embodiment of the invention and shown--similarly to the 
FIG. 2 embodiment--in its installed state, surrounding thimble guide 15 
which is shown in section. 
The extender 40 comprises a lower fitting generaly designated at 41 having 
a radially outwardly oriented flange 42 at its upper portion and a 
cylindrical skirt 43 extending downwardly from the flange 42 and having, 
at its bottom part, a plurality of circumferentially distributed, radially 
inwardly projecting resilient locating fingers 44. 
An upper fitting generally designated at 45 has a cap 46 and a plurality of 
downwardly extending circumferentially distributed radially resilient 
fingers 47. The lower and upper fittinsg 41 and 45 are interconnected by a 
cylindrical, circumferentially closed resilient bellows 48 welded at the 
top to the lower part of the cap 46 and, at the bottom to the flange 42 of 
the lower fitting 41. 
Installation of the thimble guide extender 40 onto the thimble guide 15 is 
effected in a manner similar to that described in connection with the 
first embodiment. Thus, the upper conical terminus of the thimble guide 15 
is, as the extender 40 is lowered, introduced into the bottom opening 
defined by the skirt 43 of the lower fitting 412 and the extender 40 is 
pushed down until the locating fingers 44 snap into the circumferential 
groove 33 of the thimble guide 15. The inner wall 42a of the flange 42 and 
the inner wall 43a of the lower part of the skirt 43 engage the respective 
outer surface portions of the thimble guide 15 with a close, sealing fit 
and by virtue of the locating fingers 44 engaging into the groove 33, the 
lower fitting 41 is axially immobilized on the lower part of the thimble 
guide 15. 
The sealing engagement with the bottom face 8 of the nozzle platform 7 and 
the upper annular face of the cap 46 is effected by the resilient force of 
the bellows 48. In this embodiment, however, the spring force of the 
bellows 48 is taken up at the bottom by the locating fingers 44 in the 
groove 33 of the thimble guide 15 and also, if required, by contact 
between the base of the skirt 43 with the top face 3 of the core plate 2. 
The lower seal is therefore effected by virtue of the close fit, between 
the lower fitting 41 and the thimble guide 15, rather than between the top 
face 3 of the core plate 2 and the lower edge of the skirt 43. 
The resilient finger 47 of the upper fitting 45 guidingly surrounding the 
thimble guide 15 permit a rocking motion of the upper fitting 45 about an 
axis transverse to the longitudinal axis A with the same result and 
advantages as described in connection with the first embodiment. Also 
similarly to the first embodiment, the resilient bellows 48 provides a 
fluidtight closure for the inside of the extender 40 without the need of 
small tolerances or closely interfitting, operationally sliding extender 
components. 
For limiting the extent of compression of the bellows 48, particularly 
during installation, that is, for limigint the relative motion between the 
lower fitting 41 and the upper fitting 45, the lower ends 47a of the 
fingers 47 are in alignment with an upper radial face 49 of the flange 42 
of the lower fitting 41 whereby these components are brought into an 
abutting relationship upon a predetermined motion path which compresses 
the bellows 48. 
While in the two described preferred embodiments the sleeve-like sealing 
unit serves as an extender of and cooperates with a thimble guide mounted 
on a core plate, it is to be understood that the sleeve-like 
bellows-equipped sealing unit of the invention may find application in an 
environment which does not include a thimble guide. 
It will be understood that the above description of the present invention 
is susceptible to various modifications, changes and adaptations, and the 
same are intended to be comprehended within the meaning and range of 
equivalents of the appended claims.