Dismountable fuel assembly for nuclear reactor

An easily dismountable fuel assembly for a nuclear reactor comprising a bundle of parallel fuel slugs held spaced apart by cross-pieces transverse to the slugs and by assembly bearing tubes which take the places of some of the slugs, the tubes being longer than the slugs and being secured at each end to a respective plate for maintaining the fuel assembly rigid. Each tube is secured to at least one end plate by a cylindrical sleeve comprising a connection complementary with a corresponding connection borne by the respective end of the tube and formed with a profile for engagement by a tool for rotating the sleeve around its axis in one direction for securing it to the tube and clamping the tube to the end plate in the axial direction, and in the other direction for releasing the tube from the plate. Each sleeve is disposed in an aperture in the plate which aperture is coaxial with the tube and the sleeve extending through the end plate abutting the end plate by a shoulder and comprising a deformable part deformable into a recess in the end plate for preventing the tube from rotating the end of the bearing tube comprising an expanded portion having a prismatic outer surface for engaging in a correspondingly shaped recess formed in the end plate.

FIELD OF THE INVENTION 
The invention relates to a dismountable fuel assembly for a nuclear 
reactor. 
PRIOR ART 
The fuel slugs in nuclear reactors are usually formed into bundles, in 
which the slugs are disposed parallel to one another, the spacing between 
them being maintained by cross-pieces extending transversely to the slugs. 
Some fuel slugs are replaced by bearing tubes, somewhat longer than the 
slugs, and secured at each end to end plates, the tubes being adapted to 
strengthen the fuel assembly. 
When a nuclear reactor core is recharged, it is extremely important to be 
able to change defective slugs in the irradiated fuel elements and replace 
them so that the fuel assembly can continue in use without the reactor 
circuit being contaminated by fission products coming from defective 
slugs. 
This method has the advantage of saving fuel, since the repaired assembly 
can be recharged and re-used after defective slugs have been eliminated 
and replaced. 
In addition, the replacement of defective slugs results in safer reactor 
operation, as far as the installations and staff are concerned, by 
reducing the risk of contamination. 
However, great precautions are necessary when changing defective slugs. The 
irradiated element can be repaired only under a radiological shield. 
Usually the operation is performed under a layer of water in a pool 
adjacent the reactor. 
For certain more specific reasons, e.g. for examining certain slugs or 
making mechanical or physical tests thereon, the assemblies may need to be 
dismantled in order to recover the slugs which have been tested or 
examined. 
In order to reach the slugs and separate them from the rest of the 
assembly, it is necessary to dismantle the end plates secured to the 
bearing tubes. The dismantling operation, which is performed in a pool, 
presents certain difficulties since the element is accessible only at a 
distance and the dismantling operation is performed when the assembly is 
immersed. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an easily dismountable fuel 
assembly for a nuclear reactor, comprising a bundle of parallel fuel slugs 
held spaced apart by cross-pieces transverse to said slugs and by 
assembly-bearing tubes which take the place of some of the slugs, said 
tubes being longer than said slugs and being secured at each end to a 
respective plate for maintaining said fuel assembly rigid, each said tube 
being secured to at least one said end plate by a cylindrical sleeve 
comprising connection means complementary with corresponding means borne 
by the respective end of said tube and formed with a profile for 
engagement by a tool for rotating said sleeve around its axis in one 
direction for securing it to said tube and clamping said tube to said end 
plate in the axial direction, and in the other direction for releasing 
said tube from said plate, wherein each said sleeve is disposed in an 
aperture in said plate which aperture is coaxial with said tube, said 
sleeve extending through said end plate and abutting said end plate by a 
shoulder and comprising a deformable part deformable into a recess in said 
end plate for preventing said tube from rotating, said end of said bearing 
tube comprising an expanded portion having a prismatic outer surface for 
engaging in a correspondingly-shaped recess formed in said end plate. 
The invention will be more fully understood from the following description 
of an embodiment of a fuel assembly according to the invention, given by 
way of example only with reference to the accompanying drawings.

DETAILED DESCRIPTION 
The fuel assembly shown in FIG. 1 comprises a set of parallel slugs 1 held 
in a rigid bundle by transverse cross-pieces 2 disposed at substantially 
regular intervals along the slugs. 
FIG. 2 shows how the slugs 1 are transversely disposed in a cross-piece 2. 
Cross-piece 2 is formed with cells 4 containing the slugs. Some cells 4 
are occupied by bearing tubes 5, which take the place of a certain number 
of fuel slugs. The tubes 5 are somewhat longer than the slugs 1, and are 
joined at their respective ends to an end plate 3. 
The assembly can be dismantled by removing the end plate 3 after 
disconnecting the bearing tubes 5 from plate 3. 
The slugs can then be extracted and replaced by sliding them horizntally 
out of the cross-pieces. 
FIG. 3 shows a tube 5 connected to a plate 3. In FIG. 3, the end 7 of tube 
5 comprises an expanded portion which rests in a recess 8 in plate 3. In 
the configuration shown in FIG. 3, the recess 8 is prismatic and the 
expanded portion 7 has a hexagonal outer surface. The inner surface of 
portion 7 is cylindrical with a circular cross-section. The inner 
cylindrical surface has a thread 9 which extends over a certain length of 
portion 7. Recess 8 opens into a passage 10 extending right through plate 
3. Accordingly, the passage 10 in plate 3 is coaxial with tube 5 when 
positioned in the bundle. In the direction from recess 8 to the top 
surface of the plate 3, passage 10 comprises a cylindrical part 11 having 
a relatively small diameter circular cross-section and a larger diameter 
part 12 opening at the upper surface of end plate 3. A flat shoulder 13 is 
formed between the two parts 11, 12 of the passage. 
At the upper end of part 12 of passage 10 there is a recess 15 which does 
not have symmetry of revolution about the axis of tube 5. 
Inside the passage 10 in plate 3, a sleeve 16 is disposed so that, when 
tube 5 is in the locked position as shown in FIG. 3, a thread 18 on sleeve 
16 is screwed into the thread 9 of tube 5. Thread 18 is formed on the 
lower end part of sleeve 16, which is cylindrical with a circular 
cross-section. The diameter of the last mentioned part of sleeve 16 is 
slightly less than the part 11 of the passage 10 in plate 3. The smaller 
diameter part of sleeve 16 is prolonged by a larger diameter part 19 
comprising a solid part formed with an internal slot 21 for inserting a 
screwdriver blade, the solid part being in turn prolonged by a thin collar 
23, the part of the collar in recess 15 of bore 12 being bent out by 
expansion inside the recess 15 after the sleeve 16 has been completely 
screwed engage shoulder 22 on the solid part of the sleeve with shoulder 
13 of the passage in the plate 3. 
When sleeve 16 has been completely screwed in by using a screwdriver 
engaged in the slot 21 formed on the solid part of the sleeve, the 
expanded end 7 of tube 5 abuts the bottom of recess 8, and as mentioned 
above, the shoulder 22 of sleeve 16 abuts the corresponding shoulder 13 of 
the passage in the plate 3. The bearing tube is then firmly secured to the 
end plate without longitudinal clearance. Finally, part of the collar 23 
is expanded inside recess 15 in order to prevent further rotation of the 
tube. 
When the tubes have thus been secured to the end plate, the fuel assembly 
is rigid. 
In order to change slugs in the assembly, tube 5 can be disconnected from 
plate 3 inside a pool, simply by inserting a screwdriver blade into the 
slot 21 and unscrewing the sleeve. That part of the thin collar 23 which 
has penetrated into recess 15 during the expansion of the collar is 
deformed by the force exerted during unscrewing and finally yields and 
releases sleeve 16, which can be completely unscrewed so as to disconnect 
plate 3 from the tube 5. 
Accordingly, by a simple unscrewing operation, which can easily be 
performed at a distance using a special tool, each bearing tube can be 
dismantled and the end plate 3 can be released, thus giving access to the 
slug. 
When a fuel element is assembled, each collar 23 can be expanded inside 
recess 15 in a very simple manner, by pressing a large diameter tool 
against the inner wall of collar 23. 
The advantage of the above described arrangement is that the bearing tubes 
are locked in a completely mechanical manner and can therefore be 
dismantled without machining operations, e.g. grinding or cutting, which 
might contaminate parts of the fuel assembly. Similarly, the device 
eliminates any welding operation, which has the same risks of 
contamination. 
Another advantage of the above described connection between the tube and 
the plate is that no torsion is exerted on tube 5, which is prevented from 
rotating in recess 8. 
Sleeve 16 is usually made of stainless steel of the same quality as the 
other parts of the assembly. 
The invention is not intended to be limited to the embodiment which has 
been described, but includes all variants, and equivalent means may be 
used without thereby departing from the scope of the invention. For 
example, the end of tube 5 can have a cross-section of any external shape, 
the end being disposed in a recess formed for holding it in the end plate 
3. Sleeve 16 can be secured to the end of tube 5 in any manner, involving 
rotation of sleeve 16. For example, a bayonet connection can be used 
between sleeve 16 and the end of tube 5. 
Instead of a slot for a screwdriver, sleeve 16 can have a hexagonal or 
square recess into which a correspondingly shaped key can be inserted. The 
sleeve can be entirely tubular, as shown in FIG. 3, or it can have solid 
parts preventing access to the interior of the bearing tube from the 
exterior of the assembly. 
It may however be advisable to provide a passage from the exterior of the 
assembly through the interior of the tube, e.g. for inserting control 
elements or measuring instruments or for a flowing fluid. 
The deformable collar 23 of sleeve 16 can be disposed in any part of the 
collar, for example, it can form part of a smaller diameter region of the 
sleeve, in which case one or more recess 15 are provided in the smaller 
diameter part 11 of the passage in end plate 3. 
The description in connection with FIG. 3 concerns the securing of a 
bearing tube to a top end plate of a fuel assembly. However, the bearing 
tubes can be secured as described above to either the top end plate or the 
bottom end plate or to both end plates. In some cases it is desirable to 
dismantle the assembly by removing the bottom end plate, in which case the 
bearing tubes are secured to the bottom end plate as described above.