Method for using a thimble of a pressurized water nuclear reactor and device for adjusting the axial position of the thimble

The thimble (10) is introduced into a guidance duct consisting of a guidance tube (5) joining a measurement room (3) to the vessel (1) of the nuclear reactor, a vertical channel passing through the lower internals (14) of the reactor beneath the core (12) and a tube guide (17) of an assembly of the core (12). The closed end of the thimble penetrates into the tube guide of the fuel assembly as far as a defined level. The position of the thimble (10) is modified along the axis of the guidance duct, between two periods of use of the thimble in the nuclear reactor, so as to modify the position of the heavy wear zones of the wall of the thimble (10) with respect to the guidance duct. Preferably, the axial position of the thimble (10) is adjusted by using an extension of the thimble fixed to a seal (8) of the guidance duct and into which the end of the thimble is screwed.

FIELD OF THE INVENTION 
The invention relates to a method for using a measurement duct or thimble, 
closed at one of its ends for a nuclear reactor cooled by pressurized 
water, so as to extend the service life of the thimble and to a device for 
adjusting the position of the thimble in an instrumentation guidance duct. 
BACKGROUND OF THE INVENTION 
Pressurized water nuclear reactors comprise a core formed from assemblies 
of prismatic shape arranged vertically and resting on a support plate, 
inside the vessel of the nuclear reactor. 
During operation of the nuclear reactor, it is necessary periodically to 
perform flux measurements right inside the core. For this purpose fission 
detectors of very small dimensions are used which are displaced by remote 
control with the aid of teleflex cables inside tubes, called thimbles 
which are closed at one of their ends. The thimbles are introduced 
according to a predetermined distribution into certain assemblies of the 
core, after passing inside an instrumentation guidance duct. The 
instrumentation guidance duct comprises a guidance tube connecting a 
measurement room to the bottom of the vessel of the reactor, at the level 
of a penetration sleeve for the bottom and a vertical channel passing 
through the lower internals of the reactor, in alignment with the vertical 
tube-guide of the fuel assembly into which the thimble is introduced. 
By displacement of the flux detectors inside the thimbles introduced into 
the assemblies, flux measurements may be carried out over the entire 
height of the core. 
The thimbles must be able to be extracted from the assemblies of the core, 
for example in order to facilitate reloading of the core of the reactor or 
even of the set of corresponding instrumentation guidance ducts. To this 
end, traction is exerted on the end of the thimbles, from the measurement 
room arranged laterally with respect to the vessel well of the reactor. 
When the thimbles are installed in their respective guidance ducts and in 
the tube guide of the corresponding assemblies, the closed end part of the 
thimble, generally consisting of a bullet-shaped end plug facilitating the 
guidance and the displacement of the thimble, is placed at a reference 
level in the upper part of the tube guide of the assembly. 
The position of the thimble in the axial direction of the guidance duct is 
therefore fixed once and for all, and the same zones of the wall of the 
thimble are always placed facing the same zones of the guidance duct. 
Due to the circulation of the cooling water of the reactor inside the 
guidance ducts, and in certain zones, for example before the inlet into 
the tube guide of the fuel assembly where the thimble is directly exposed 
to the water filling the vessel of the reactor, vibrations of the thimble 
are produced inside the guidance duct, due to the fact that the inside 
diameter of the guidance duct is substantially greater than the outer 
diameter of the thimble so as to allow the thimble to be extracted and 
installed in the guidance duct. 
In certain zones of the guidance duct and in particular in the upper part 
of the vertical channel inside the internals, friction is produced 
generating wear between the wall of the thimble and the inner surface of 
the guidance duct. 
The wear of the wall of the thimble in the zones subjected to friction may 
lead to deterioration and even to breakage of the thimble, due to the fact 
that the wear is always produced in the same zones of the wall of the 
thimble and during long periods of time corresponding to the duration of 
use of the nuclear reactor. 
In the event of a deterioration of the thimble showing up as a loss in 
sealing, contaminated cooling liquid is likely to enter the measurement 
room via the inside of the thimble. 
There have been proposed devices for holding the thimbles transversely 
inside certain parts of the guidance duct, allowing vibration, friction 
and wear to be limited. 
However, such devices may be difficult to install in the guidance duct and 
may necessitate the use attached components which are likely to become 
detached by breaking and to become loose parts in the primary circuit of 
the reactor. 
To date no method was known enabling the wear of the thimbles to be limited 
without using a transverse holding device. 
The object of the invention is therefore to propose a method for using a 
measurement duct or thimble, closed at one of its ends, for a nuclear 
reactor cooled by pressurized water, so as to extend the service life of 
the thimble ensuring guidance of a probe measuring neutron flux right up 
as far as the inside a fuel assembly of the core of the reactor, the 
thimble being introduced into a guidance duct consisting of a guidance 
tube Joining a measurement room to the vessel the nuclear reactor in which 
the core is placed, a vertical channel passing through the lower internals 
of the reactor, beneath the core in line with a vertical tube guide of the 
assembly and the tube guide of the fuel assembly, so that the closed end 
of the thimble penetrates inside the tube guide of the assembly, as far as 
a predetermined level, this method making it possible to avoid excessive 
wear of the wall of the thimble in exposed zones, during extended use of 
the nuclear reactor. 
With this objective, the position of the thimble is modified along the axis 
of the guidance duct, between two successive periods of continuous use of 
the thimble so as to ensure the guidance of the measurement probe in the 
operating nuclear reactor, so as to modify the position of at least one 
heavy wear zone of the wall of the thimble with respect to at least one 
zone of the guidance duct. 
The invention also relates to a device for adjusting the position of a 
thimble in the axial direction of a guidance duct allowing the 
implementation of the method according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENT 
FIG. 1 shows the vessel 1 of a pressurized water nuclear reactor arranged 
inside a vessel well 2 constituting part of the concrete structure of the 
nuclear reactor. 
A measurement room 3 surrounded by concrete walls is arranged laterally 
with respect to the vessel well. One of the side walls 4 of the room 3 
separates this room from the vessel well 2. Instrumentation guidance ducts 
5 are connected at one of their ends to a vertical penetration sleeve 6 
for passing through the bottom of the vessel 1 and comprise, starting from 
the vessel, a vertical part, a bent part and a horizontal part passing 
through the wall 4 of the measurement room 3 in a sealed manner. 
On the horizontal extension of the guidance duct 5 inside the room 3 there 
are arranged, in particular, a manual safety valve 7, a seal 8 and an 
automatic valve 9. 
The end of the thimble 10 placed inside the guidance duct 5 is accessible 
at the end of the guidance duct 5, inside the room 3, so that it is 
possible to introduce and to displace within the thimble 10 a probe for 
measuring neutron flux fastened to the end of a teleflex cable, from the 
room 3. 
Further more, it is possible to carry out the partial extraction and the 
installation of the thimbles in the guidance ducts from the measurement 
room 3. 
The seal 8 makes it possible to ensure the sealed passage of the thimble to 
which an extension is connected at the exit of the seal 8. 
The vessel 1 of the reactor encloses the core 12 consisting of Juxtaposed 
fuel assemblies of prismatic shape resting on a core support plate 13 
forming part of the internals 14 arranged in the lower part of the vessel 
1, closed by a domed bottom 1a. 
FIG. 2 represents part of the vessel bottom 1a of domed shape, at the level 
of a penetration consisting of sleeve 6 fastened by welding in a hole for 
passing through the vessel bottom 1a. 
The end of instrumentation guidance duct 5 is fixed to the end of the 
penetration sleeve 6, outside the bottom 1a. 
The internals 14 of the vessel comprise, in the axial extension of the 
penetration sleeve 6 of vertical direction, an instrumentation column 15 
the open lower part of which has an inside diameter substantially greater 
than the diameter of the sleeve 6, and which is engaged over the upper end 
of the sleeve with a large radial clearance. 
The upper part of the column 15 is fastened under the core support plate 
13, at the level of a penetration opening of this support plate opening 
out under the lower end piece of a fuel assembly 20 of the core of the 
reactor, by means of a guidance conduit 16 opening out at a small distance 
beneath the lower end of the instrumentation tube guide 17 of the assembly 
20. 
The inside bore of the penetration sleeve 6, the inner passage of the 
instrumentation column 15, the penetration opening of the core support 
plate 13 and the bore of the guidance conduit 16 have one and the same 
vertical axis in the extension of the axis of the tube guide 17 of the 
assembly 20. 
These elements constitute the part of the instrumentation guidance duct 
situated inside the vessel and to which the guidance tube 5 is connected, 
at the level of the lower end of the sleeve 6. 
The continuous passage of the thimble 10 can thus be ensured between the 
instrumentation room 3 and the tube guide 17 of the fuel assembly inside 
the vessel 1. 
In FIG. 3, the upper part of the instrumentation tube guide 17 of the 
assembly 20 can be seen fastened to the upper end piece 21 of the 
assembly. 
The thimble 10, which is closed at its end by a bullet-shaped end plug 22, 
is introduced inside the tube guide 17 as far as the vicinity of its upper 
part. 
When the thimble 10 is in its service position inside the tube guide 17, 
the upper end of the bullet-shaped end plug 22 is at a reference level 24 
defined in a very precise manner. 
The probe 25 for measuring neutron flux fastened to the end of a teleflex 
cable 26 may be displaced inside the thimble, so as to be able to carry 
out measurements at any level over the height of the assembly, i.e., at 
any level over the height of the core. 
When the nuclear reactor is operational, cooling water circulates very fast 
inside the vessel, so as to pass through the core in a vertical direction 
from bottom to top. 
The cooling water which is sent at high speed and high flow-rate under the 
core support plate penetrates the instrumentation guidance ducts and comes 
into contact with the thimbles. 
Furthermore, a free space remains between the guidance conduit 16 and the 
lower end of the instrumentation tube guide 17, so that in this free space 
the thimble is not protected and is subjected to the circulating flow of 
cooling water. 
As a result, the thimbles vibrate inside the part of the guidance duct 
situated inside the vessel. 
The thimble is set in vibration inside the guidance duct and comes into 
frictional contact with the inner part of the guidance duct, in certain 
zones situated, in particular in the vicinity of the core support plate. 
Due to the fact that the thimble is always placed in an identical position 
along the axial direction of the guidance duct, this position 
corresponding to the position of the upper part of the bullet-shaped end 
plug 22 at the reference level 24, the thimble wears disproportionately in 
certain zones placed in perfectly defined positions along its length. 
This progressive wear during the use of the thimble may necessitate 
replacement of the thimble due to the deterioration brought about by the 
wear. 
FIG. 4 shows the part of the instrumentation guidance duct penetrating 
inside the measurement room 3. 
The guidance duct passes through the wall 4 of the room, in a sealed 
manner, inside a penetration sleeve 28 fixed to the inside of the room on 
a penetration plate 29. 
The guidance tube 5 constituting the inner part of the guidance duct is 
connected to the manual valve 4 ensuring opening and closing of the 
guidance duct, which valve is itself connected to the seal assembly 8 by 
means of a duct element on which there is arranged a pressurizing 
connector 30. The seal 8 is fastened by means of a seal support 31 and 
comprises a leak detector 32. 
On the outlet end of the seal 8 there is fastened a thimble extension 33. 
The thimble extension 33 is connected to the motorized valve 9 by means of 
a duct element 34. 
The assembly represented in FIG. 4 and situated in the measurement room 3 
makes it possible to ensure sealed outlet of the probe support introduced 
into the thimble, to monitor sealing and to prevent any outlet of primary 
fluid into the measurement room. 
As can be seen in FIG. 5, the seal 8, which is conventional in the art, 
comprises inner sealing elements around the thimble 10 and screwed 
connection rings 8a and 8b making it possible to connect the seal 8 to the 
part of the guidance duct connected to the manual valve 7 upstream and to 
the thimble extension 33 downstream, respectively. 
At its connection end with the extension 33, the thimble 10 comprises an 
externally threaded fastening component 35 extended by a smooth end piece 
35a of small diameter, chamfered at its ends, to which may be screwed the 
thimble extension 33, which comprises a tapped inner bore for this 
purpose. 
The thimble extension 33 also comprises an outer threaded part 33a allowing 
the connection of the end part of the guidance duct to the extension 33 by 
means of a nut 37. 
The threaded part 33a of the extension 33 comprises two flats 33c allowing 
a wrench to be engaged over the body of the extension. 
In order to extract a thimble from the guidance duct, the extension of the 
duct is disconnected by unscrewing the nut 37, then the extension 33 of 
the seal 8 is disconnected by unscrewing the ring 8b. 
Traction can then be exerted on the thimble by means of the extension 33. 
The thimble can be reinstalled by exerting a thrust on the extension 33. 
Sealing around the thimble is ensured by the seal 8 which comprises sealing 
gaskets constituting sealed sliding bearings for the thimble 10. 
Reference will now be made to FIGS. 6 and 7A, 7B, 7C to describe, in more 
detail, the method for joining the thimble 10 and the extension 33 by 
means of the threaded component 35, and the means making it possible to 
implement the method according to the invention by adjusting the axial 
position of the thimble 10 in the guidance duct. 
The extension 33 comprises a cylindrical body pierced with a central bore 
allowing the passage of the probe support into the extension of the 
thimble 10 and comprising a widened tapped part for fastening the thimble 
10, by means of the threaded component 35, extended by a smooth opening 
allowing the end piece 35a to be received. 
The extension 33 comprises a shoulder 33b on which the threaded ring 8b for 
connecting to the seal 8 comes to bear. Sealing gaskets 38 and 39 are 
interposed between the shoulder 33b of the extension 33 and the nut 8b and 
the seal 8, respectively. 
The threaded connection component 35 is welded to the end of the thimble 10 
and comprises two cavities 41 in which two pins of a pin wrench may be 
engaged when the adapter is fastened, or two pins or a spacer 40 may be 
engaged allowing the axial position of the thimble 10 in the guidance duct 
to be adjusted. 
As can be seen in FIGS. 7B and 7C, the axial position of the thimble 10 is 
adjusted by screwing the threaded component 35 right to the bottom of the 
bore and by using one or more spacers such as 40 (FIG. 7B) or 40' and 40" 
(FIG. 7C). 
In order to adjust the thimble 10 in the axial position implementing one or 
more spacers such as 40, 40' and 40", thimble extensions 33' and 33" 
comprising a large-diameter inner bore of a suitable length are used. 
The spacer such as 40 comprises a radial slot 42 allowing the engagement of 
the thimble 10 in the spacer 40 as far as its central part, by simply 
introducing the spacer laterally, and two flats such as 40a for the 
engagement of a holding wrench. 
The spacer such as 40 also comprises, at one of its ends, two projecting 
pins 44 and, at its other end, two orifices 43 enabling two pins such as 
44 of another spacer to be received (in the event of assembling several 
spacers such as 40' and 40" represented in FIG. 7C) or making it possible 
to receive the pins of a pin wrench, so as to tighten the extension on the 
thimble. 
Mounting the extension 33 on the thimble 10 may fasten the thimble in one 
of the three axial positions corresponding to FIGS. 7A, 7B or 7C. 
In order to perform the mounting represented in FIG. 7A, corresponding to a 
first position of the thimble 10, called a top position, inasmuch as the 
end of the thimble is in its uppermost position inside the fuel assembly 
(position 24a in FIG. 3), the extension 33 is simply screwed onto the 
threaded fastening component 35, for example by means of a wrench engaged 
with the flats 33c. An O-ring seal 45 is engaged over the smooth end piece 
35a and is placed in a housing consisting of a diametrically widened part 
of the smooth opening for receiving the end piece 35a. 
Tightening may be carried out by rotationally locking the thimble by 
introducing a pin wrench into orifices 41 in component 35. 
The extension 33 and the thimble 10 may then be fixed to the seal assembly 
8 by the connection ring 8b, as shown in FIG. 7A. The thimble is then 
immobilized inside the guidance duct in a fixed axial position 
corresponding to its top position. 
In order to fasten the thimble inside the guidance duct in an intermediate 
position, as represented in FIG. 7B (position 24 in FIG. 3), a spacer such 
as 40 is introduced laterally onto the tubular body of the thimble 10, by 
virtue of the slot 42. The spacer 40 is then displaced axially along the 
thimble 10, so that the pins 44 of the spacer come to be engaged in the 
orifices 41 of the fastening component 35. 
An adapter 33' comprising a threaded large-diameter inner bore whose axial 
length corresponds to the sum of the axial length of the component 35 and 
of the spacer 40 is screwed onto the fastening component 35. 
The outer diameter of the spacer 40 is less than the minimum diameter of 
the thread of the large-diameter bore, so that the spacer connected to the 
component 35 by the pins 44 may be introduced inside the large-diameter 
bore. 
In order to produce the screwing of the adapter 33' onto the fastening 
component 35 of the thimble, the latter may be rotationally immobilized by 
a wrench comprising pins which are introduced into the orifices 43 of the 
spacer 40 or by a wrench engaged on the flats 40a. 
When the extension 33' is fastened onto the seal 8 via the tapped ring 8b, 
the spacer 40 is held firmly between the conduit 8 and the fastening 
component 35. 
It should be noted that the adapter 33' has an outer shape and dimensions 
identical to those of the extension 33. 
In the same manner, the axial position of the thimble 10 may be modified 
and adjusted without in any way modifying the elements of the guidance 
duct connected to the extension. 
However, in order to be able to distinguish, from the outside, the method 
for mounting and for positioning the thimble, a number of grooves 
corresponding to the type of extension and to the position of the thimble 
are machined on the outside surface of the extensions 33 and 33'. 
FIG. 7C represents a third type of mounting of the thimble 10 allowing it 
to be placed in a third axial position, called a bottom position (position 
24b in FIG. 3), inasmuch as the end of the thimble is then inside the fuel 
assembly in a position situated at a level lower than the level 
corresponding to the mountings of FIGS. 7A and 7B. 
In order to produce this mounting, an extension 33" is used, comprising a 
large-diameter threaded inner bore of a length equal to the sum of the 
axial lengths of the fastening component 35 and of the spacers 40' and 40" 
which are interlocked in one another by means of end pins 44' of the 
spacer 40' engaged in orifices of the spacer 40". 
The fastening of the extension 33" by screwing onto the thimble may be 
carried out by using a wrench comprising pins which are engaged in the 
orifices 43' of the spacer 40' or a wrench engaged with the flats of the 
spacer, so as to immobilize the thimble. 
The extension 33" has a shape and external dimensions identical to those of 
the extension 33 and of the extension 33'. 
Outside grooves in a different number may allow the extension 33" to be 
distinguished from the extensions 33 and 33'. 
The thimble can thus be adjusted into the third or bottom position without 
modifying the elements for mounting the extension on the guidance duct. 
The axial position of a thimble can thus be modified very easily inside the 
guidance duct, after a certain time of use of the thimble inside the 
nuclear reactor, so as to modify the position of the wear zones along the 
length of the thimble and therefore to extend the service life of the 
thimble. 
Generally, thimbles of pressurized water nuclear reactors are mounted as 
represented in FIG. 7B, so that the end of the thimble inside the 
tube-guide of the fuel assembly is at an intermediate reference level. 
It is therefore possible, by using means represented in FIGS. 6 and 7A to 
7C, to modify the position of the thimble twice, once by removing the 
spacer 40 and by producing a mounting according to FIG. 7A and a second 
time by using two spacers such as 40' and 40" represented in FIG. 7C. 
The corresponding extension will be machined internally so as to have a 
large-diameter tapped bore whose length corresponds to the type of 
mounting. 
The method and the devices according to the invention therefore make it 
possible to extend the service life of a thimble in a guidance duct, in a 
simple manner and without modifying the means for connecting the guidance 
duct. 
The thimble may be displaced axially with a variable amplitude, by using 
one or more spacers of suitable length and an extension comprising a 
large-diameter inner bore whose length corresponds to the sum of the axial 
lengths of the connection component of the thimble and of the spacer or 
spacers. 
It is possible to provide for only two positions of the thimble, naively a 
top position and a bottom position, instead of three positions as in the 
example described. 
The invention applies to any pressurized water nuclear reactor regardless 
of the arrangement of the measurement room with respect to the vessel well 
and regardless of the arrangement of the guidance duct in the measurement 
room. 
In particular, the method and the device according to the invention apply 
equally well in the case of a guidance duct having a horizontal 
arrangement in the measuring room as to a guidance duct having a vertical 
end part (U-shaped instrumentation duct).