Nuclear reactor fuel assembly

Water-cooled nuclear reactor fuel assembly of the type comprising two end pieces, respectively an upper end piece and a lower end piece, said end pieces having openings for the circulation of the light cooling water, the spacing grids made from a single metal have a relaxation effect under irradiation and are constituted by two groups of perpendicular plates, said grids defining cavities, each of which is traversed either by a fuel rod, or by a connecting rod, said spacing grids being distributed along the connecting rods, the walls of each cavity having bearing members for holding in place the fuel rods, wherein the fuel rods are jointed to an end piece and wherein means are provided for maintaining the group of grids against one of the upper or lower end plates, both during the operation of the reactor and when it is shut down.

BACKGROUND OF THE INVENTION 
The present invention relates to a nuclear reactor fuel assembly, whereof 
the rods are fixed to an end piece and in which the system of grids is 
kept supported against one of the end pieces. The invention relates to 
nuclear reactor fuel assemblies and particularly those used for 
water-cooled reactors. 
Each assembly of this type extends over a length of several metres between 
a lower inner plate and an upper inner plate belonging to the equipment 
within the reactor vessel. It rests on the lower inner plate and is 
positioned by centering pins forming an integral part of the two 
aforementioned plates connected to the reactor vessel. It comprises a 
rigid mechanical frame formed by two end pieces, namely the upper and 
lower end pieces, provided with openings permitting the passage of the 
cooling water circulating from bottom to top, as well as a certain number 
of connecting rods ensuring the stability of the system by being fixed to 
the said end pieces. The supporting and spacing of the fuel rods is 
brought about along each assembly, by a certain number of supporting grids 
connected to the connecting rods defining cavities having a square 
cross-section. 
Furthermore, in order to permit the expansion in operation of the fuel 
rods, it is necessary to leave a clearance between the rods and at least 
one of the end pieces. In addition, as the assembly is exposed to a flow 
of cooling water, which circulates from bottom to top in the reactor core, 
it is necessary to avoid, as a function of the type of assembly, either 
the flying off of said assembly, or the flying off of the grids and the 
fuel rods under the effect of the cooling water circulation. 
Numerous embodiments of nuclear reactor fuel assemblies are known providing 
a solution to the aforementioned problems, for example, those described in 
French Pat. Nos. 1,536,257 and 2,049,108 granted to the Westinghouse 
Electric Corporation. French Pat. No. 1,536,257 relates to the design of a 
framework for a fuel assembly equipped with a group of fuel rods sliding 
in the connecting rods. The latter are made from steel and the supporting 
grids are welded thereto and are made from Inconel. The fuel rods are 
supported by springs and bosses stamped into the plates of the grids. 
French Pat. No. 2,049,108 relates to the design of an assembly, whose 
connecting rods are made from Zircaloy, whilst the grids are made from 
Inconel. Due to the metallurgical incompatibility of these two metals, the 
connection between the grid and the connecting rod takes place by the 
interposing of a stainless steel sleeve hard soldered to the grid plates, 
which is then mechanically deformed together with the connecting rod. 
According to another known construction, the spacing grids slide freely on 
the connecting rods. Only the lower grid is held on the lower end plate by 
a fly-off-prevention sleeve. In this case, the spacing grids are made from 
a single material, preferably Inconel, which ensures an adequate fastening 
to the fuel rods for the different grids to be supported and retained by 
means of the said fuel rods. 
The fuel assembly according to the invention has homogeneous grids without 
joined springs and made from zircaloy, whose neutron absorption power is 
particularly low. 
Attempts were firstly made to produce spacing grids entirely from Zircaloy 
by cutting in the walls, springs able to exert an adequate fixing force of 
the fuel rods to ensure the secure fixing between grids and fuel rods 
throughout the life of the assembly. However, tests have shown that this 
result could not be achieved due to the relaxation of Zircaloy under 
irradiation. At the end of a certain time, the fixing effect of the 
springs to the fuel rods slackens and it is no longer possible to ensure 
the fastening between the grids and the rods. 
SUMMARY OF THE INVENTION 
The present invention relates to a nuclear reactor fuel assembly using 
transverse spacing grids having no supporting function during operation 
and which comprises means able to prevent the flying off of fuel rods and 
grids under the effect of the circulation of the cooling water flow. 
More specifically, the invention relates to a water-cooled nuclear reactor 
fuel assembly of the type comprising two end pieces, specifically an upper 
and a lower end piece, said end pieces having openings for the circulation 
of the cooling water, rigid connecting rods fixed by their ends 
respectively to the upper and lower end pieces, spacing grids distributed 
along the connecting rods and which define cavities traversed in each case 
by a fuel rod, the walls of each cavity having supporting members, which 
position the fuel rods, whereby the assembly is characterized in that the 
fuel rods are rendered integral with an end piece and the means maintain 
the system of grids in engagement with one of the upper or lower end 
pieces. 
The said means can consist of the thrust exerted by the cooling water and 
in this case they are preferably supplemented by elastic means which, if 
appropriate, can prevent the drop by gravity of the assemblies, during the 
stoppage of the cooling fluid flow. 
The operation of a fuel assembly according to the invention takes place in 
two successive phases. 
At the start of the life of the assembly, the spacing grids are fixed to 
the fuel rods. Thus, the support points on these rods make it possible to 
ensure an adequate fixing force. Thus, the grids follow the expansion of 
the fuel rods, which moves them upwards. 
During a second phase, there is a relaxation of the support points of the 
grids on the fuel rods, so that the grids slide along the rods. It is then 
that the means, particularly elastic means provided for maintaining the 
grids in place with respect to the one of the upper or lower end pieces 
come into action. These elastic means compensate for the slackening of the 
securing action of the grids on the rods. Nevertheless, as from this time, 
the spacing grids continue to provide the spacing of the fuel rods. Within 
each cavity, the fuel rods are braced without any clearance by the support 
points made in the walls of the partitions of the grid cavities. It is 
therefore necessary to ensure that the fuel rods do not fly off under the 
effect of the upward cooling fluid flow. According to the invention, this 
is brought about by making the fuel rods integral with the lower end 
piece. 
Generally, the fuel rods are constituted by a sheath in which are 
introduced fissile and/or fertile fuel pellets. Each of the ends of the 
sheath is sealed by a plug. 
Preferably, and according to one of the secondary features of the 
invention, the plugs of the fuel elements located at the end joined to an 
end plate are elongated and have at their end a cylindrical fitting 
provided with an annular groove, the corresponding end piece having a hole 
serving to receive the end fitting of the plug. This hole has a 
semi-toroidal annular groove in which is located a floating locking ring. 
Preferably, this hole is extended by a smaller diameter orifice permitting 
the circulation of the water. 
According to a first embodiment of the invention, an elastic stress is 
exerted on the grids, which maintain the latter downwards, i.e. towards 
the lower end piece. According to this embodiment, the elastic means are 
constituted e.g. by a plurality of springs mounted on the connecting rods, 
said springs being secured at one of their ends and whose other end forces 
the grids downwards. More specifically, the fuel assembly according to the 
invention is characterized in that the means for engaging the spacing 
grids with one of the end pieces are constituted by rigid spacers mounted 
on the connecting rods and disposed between the grids, as well as their 
springs forcing each of the grids downwards. 
Preferably, the springs are introduced onto a central rod and are fastened 
at one of their ends and to the said rod. 
According to a second embodiment, there is an upward engagement of the 
spacing grids, i.e. against the upper end piece. Such an embodiment has 
the advantage that the elastic means no longer act, as in the first 
embodiment, against the upward flow of the cooling fluid and instead act 
in the same direction. Consequently, the force required is less, because 
the force exerted by the elastic means and that exerted by the cooling 
fluid act in the same direction. In this case, the springs merely have the 
function of maintaining the system of grids in place when the reactor is 
shut down, i.e. when the cooling fluid flow has stopped. It is then 
sufficient to have a single spring for each connecting rod, or even part 
of the connecting rod. 
This embodiment has two variants. In both variants, springs are arranged 
around the connecting rods, between the lower end piece and the lower 
grid. According to the first variant, the force exerted by the springs is 
transmitted from the lower grid to the other grids by rigid spacers 
mounted on the connecting rods. Preferably, compensating springs are added 
between the upper grids and the upper end piece. 
According to a second variant, the force exerted by the springs arranged on 
the connecting rods is transmitted from the lower grid to the other grids 
by elastic spacers.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows a first embodiment of the invention and consists of three 
views a, b and c representing a fuel assembly at different stages of its 
life. 
As has been stated, assembly 1 is arranged between a lower core supporting 
plate 2 and an upper core supporting plate 4, represented in the drawings 
by a dotted mixed line, said plates belonging to the internal equipment of 
the reactor vessel. The assembly comprises a rigid mechanical frame formed 
from two end pieces, namely a lower end piece 6 and an upper end piece 8, 
provided with openings for the passage of the cooling water, which 
circulates from bottom to top. The connecting rods 10 ensure the stability 
of the system by their fixing to end pieces 6 and 8. Grids 12, distributed 
along the connecting rods 10 ensure the spacing of the fuel rods 14. The 
spacing of the different grids is ensured by rigid spacers 13. 
The fuel rods 14, whereof only one is shown in FIG. 1 in order not to 
overload the drawing, generally comprises a metal sheath 18 FIG. 4 within 
which are stacked fissile and/or fertile pellets 16. Each of the ends of 
sheath 18 is terminated by a sealing plug 20. According to the invention, 
fuel rods 14 are joined to the lower end piece 6. 
FIG. 4 shows the lower end of a fuel rod 14 and its joining to plate 15 of 
lower end piece 6. Plug 20 is elongated and profiled in such a way that it 
bears on plate 15 of lower end piece 16 between four water passage holes 
22, but does not seal the latter. Its end has a cylindrical end fitting 24 
with a semi-toroidal groove 26. Bore 28 in which is engaged end 24 has in 
the same way a semi-toroidal groove 31 facing groove 26, when the fuel rod 
is in place. A small diameter hole 33 extends bore 28 and permits the 
circulation of water. A retaining ring 30 is placed between the space 
defined by grooves 26 and 31. The bearing of the fuel rod on plate 14 is 
localized on a contact ring 32. There is a clearance between end fitting 
24 and bore 28. Retaining ring 30 thus ensures that fuel rod 14 does not 
fly off by joining the latter to the end piece. 
As can be gathered from FIG. 1, the upper plug has an end fitting 34 which 
can be fitted to a tension clamp for fitting rod 14 in spacing grids 12. 
Rod 14 is fitted by freely introducing end 24 into bore 28 up to locking 
ring 30, a clearance existing between end fitting 24 and bore 28. An axial 
thrust of 0.5 to 1 daN must be exerted in order to introduce locking ring 
30 into grooves 31 and make shoulder 32 bear against plate 15. In this 
position, end fitting 24 can only escape under an axial stress of the same 
magnitude as that necessary for fitting it. However, it has a certain 
freedom of displacement in the radial direction, which means that the grid 
still exerts a fuel rod spacing function. 
As has been stated hereinbefore, the spacing grids 12 are made entirely 
from Zircaloy, for example being grids of the type described in French 
Patent Application No. 81 19483. Due to the particularly significant 
relaxation of this material, no spring ensuring an adequate securing of 
the fuel rods throughout the life of the assembly has been installed. The 
grid only has semi-rigid bearing points on the rods, which ensure the 
spacing of the latter and the cohesion of the system. 
According to the invention, assembly 1 has elastic means making it possible 
to force grids 12 towards one of the end pieces, in the present case the 
lower end piece 6. These means are constituted by springs 36, joined by 
one of their ends to the central instrumentation rod 38 and acting by 
their second end on the corresponding sleeve of each grid. FIG. 5 is a 
perspective view showing the detail of such a spring, made e.g from 
Inconel 718. Two rectangular notches 40 diametrically facing one another 
in the wall of the central instrumentation rod 38 has been provided. 
Spring 36 has been split or slit and deforms so as to introduce two tabs 
42 into each of the windows 40. Thus, spring 36 is stopped in translation 
with respect to connecting rod 38. At its other end, it bears on sleeve 44 
of grid 12, both being diagrammatically shown by dotted lines. 
In FIG. 1a, assembly 1 is shown at the start of its life. In this stage, 
the semi-rigid bearing points on the rods ensure an adequate securing 
force to immobilize each of the grids 12 relative to the fuel rods 14. The 
spacing grids are consequently secured relative to the fuel rods by this 
force. 
During the operation of the reactor shown in FIG. 1b, the fuel rods 14 
expand under the effect of irradiation. The grids 12 follow the expansion 
of the fuel rods, which displaces them upwards, because the lower part of 
the fuel rods is fixed to the lower end piece 6. The upward displacement 
of grids 12 leads to a compression of each of the springs 36. 
Thus, there is a clearance .DELTA. e between each grid and the spacer 13 
separating it from the grid placed in the immediately lower position, 
because sleeve 13 slides freely on connecting rod 10 and consequently 
rests on the lower grid. 
During the life of the assembly, the Zircaloy relaxation phenomenon occurs. 
There is a slackening of the securing force of the semirigid bearing 
points on fuel rods 14. Due to the reduction of this force, grids 12 slide 
downwards under the action of the force exerted by springs 36. The 
downward displacement of grids 12 stops when the clearance .DELTA. e which 
appeared at the start of the life of the assembly has been cancelled out, 
i.e. when all the grids bear against a spacer 13. The system of grids is 
then applied without clearance to the lower end piece 6 via spacers 13. 
As has been stated, this makes it possible to prevent the flying off of the 
fuel rods and grids, whilst still using spacing grids made entirely of 
Zircaloy, i.e. without having to use for the springs a material having a 
high relaxation resistance, such as Inconel 718. 
FIGS. 2 and 3 show two variants of a second embodiment of the invention, 
which differs from the first embodiment described relative to FIG. 1 in 
that the elastic means equipping the assembly exerts a force directed from 
bottom to top and not from top to bottom. The advantage of this embodiment 
is that the force exerted by the elastic means is lower, because it is 
exerted in the same direction as the cooling water flow and not in the 
opposite direction. 
It is therefore possible to use one spring per connecting rod and to 
arrange the same at the lower end thereof, which simplifies the 
construction compared with that of FIGS. 1 and 5. 
In the embodiment of FIG. 2, the elastic means are constituted by springs 
50 arranged around the connecting rods 10. In the same way as in the case 
of the assembly of FIG. 1, rigid spacers 13 are arranged between the grids 
12. The force exerted by springs 15 on the lower grid is then transmitted 
between the grids via said spacers 13. 
FIG. 2a shows assembly 1 at the start of its life. As has been explained 
hereinbefore, at this stage the securing force of the semi-rigid supports 
13 is adequate to maintain the grids in place with respect to the fuel 
rods 14. 
FIG. 2b shows the assembly during its life. A certain expansion of rods 14 
has taken place under the action of irradiation. As hereinbefore, grids 12 
follow this expansion of the rods, because the securing force on the rods 
is still high. Thus, as explained with reference to FIG. 1, a clearance 
.DELTA. e appears between the spacers and the grid positioned immediately 
above it. 
FIG. 2c also shows the assembly during its life. As a result of the 
relaxation of the Zircaloy from which the spacing grids 12 are made, the 
securing force of the semi-rigid supports on the fuel rods has slackened. 
Thus, the system of grids has moved upwards under the action of the force 
exerted by the group of springs 50, as well as the upward force of the 
cooling water stream, said upward force alone being sufficient to obtain 
this result. Thus, the position shown in FIG. 2c is reached, in which the 
upper gird abuts against the upper end piece 8 and the clearances .DELTA. 
e of FIG. 2b have been compensated. Thus, the system of grids bears 
against the upper end piece 8 and springs 50 are relaxed. One of the 
functions of springs 50 is to maintain the system of grids upwards during 
reactor shutdown. It is obviously possible to use any equivalent means. 
According to a preferred variant, between the upper grid and the upper end 
piece are arranged compensating springs. 
In the position of FIG. 2, these compressed springs only maintain the upper 
grids in place. Subsequently, they compensate the displacement of the 
grids and, in the position of FIG. 2c, their turns are made contiguous and 
form an abutment for the upper grid. As a function of the state of the 
material used and particularly the heat treatment applied thereto, it is 
possible to arrange the system of grids, from the outset, so as to bear 
against the upper end plate 8. In such a solution, the displacement of 
grids described in FIGS. 2a and 2b no longer occurs. 
FIGS. 3a, b and c represent a variant of the second embodiment of the 
invention, in which the grids are forced against the upper end piece 8. 
Like the embodiment of FIG. 2, it has springs 50 wound around the 
connecting rods 10 and positioned between the lower end piece 6 and the 
lower grid of the assembly. The difference between these two variants is 
based on the spacers. In this case, the spacers are elastic and not rigid 
like spacers 13 shown in FIGS. 1 and 2. FIG. 6 is a perspective view 
giving details of such a spacer 52. It is in the form of a sleeve into 
which has been cut a helical slot 54 defining turns identical to those of 
a spring. In the free state, the turns of spacer 52 are contiguous as 
shown in FIG. 3a. When a tensile stress is exerted on ends 52a, the 
position shown in FIG. 6 is obtained. A notch 56 is formed at each end 52a 
for cooperating with an identical notch 58 made in sleeve 44 of each of 
the grids 12 shown in dotted mixed line for in FIG. 6. 
The installation of the spacers 52 is shown. Firstly, end 52a of each 
elastic spacer 52 is introduced into notch 58 of each of the sleeves of 
grids 12. When this has taken place, connecting rod 10 is mounted, by 
sliding it in the spacers or in the sleeves of the grids in accordance 
with the direction of arrow f. 
In FIG. 3a, as in FIGS. 1a and 2a, the bearing force of the semi-rigid 
bosses of rods 14 is adequate to maintain them in place with respect to 
the latter. 
In FIG. 3b, under the action of the expansion of fuel rods 14, grids 12 
follow said rods and are consequently displaced with respect to the 
connecting rods 10. The spacers 52 fixed to grids 12 at each of their ends 
assume the position in which the turns are spaced apart, as shown in FIG. 
3. Thus, the elastic spacers exert a tensile stress on the grids, which 
tends to move them towards one another. However, this force is less than 
the frictional forces existing at the rigid support points on the rods and 
therefore, at this stage, the grids still remain stationary with respect 
to the fuel rods 14. 
The relaxation of the Zircaloy has taken place in FIG. 3c, which represents 
assembly 1 during its life. The fastening of the rods has slackened, in 
such a way that the grids have moved close to one another under the action 
of the tensile action exerted by each of the elastic spacers 52, 
superimposed on the upward force of the cooling fluid. Thus, the group of 
grids has moved upwards and bears against the upper end piece 8. Thus, a 
position identical to that shown in FIG. 2c is reached, in which the 
springs 50 are relaxed. 
The difference between the variants of FIGS. 2 and 3 is that in the latter, 
at no time is there a clearance between the spacers and the grids. This is 
due to the fact that the spacers are elastic and can absorb clearance 
.DELTA. e, to which reference has been made with respect to FIGS. 1 and 2. 
This eliminates any vibration of the spacers during the operation of the 
reactor. 
Moreover, this embodiment retains the aforementioned advantage of engaging 
the group of grids against the upper end piece, which reduces the force 
necessary, because it acts in the same direction as the force due to the 
cooling water flow. Thus, it is merely necessary to dimension springs 50 
in such a way that they are able to maintain the grids and spacers in the 
position occupied by them in FIG. 3c during reactor shutdown. 
In conventional manner, the fuel assembly grids can have sleeves arranged 
in cavities of the said grids traversed by the connecting rods. These 
sleeves are welded to the grids and rigid spacers 12 and bear on the ends 
of the grid sleeves. 
The invention relates to a simpler construction of these grids, guided 
axially on the connecting rods, in which the sleeves are eliminated. 
According to a first variant shown in FIG. 7, plates 12a of grid 12 have no 
extension. To ensure the contact between grid 12 and rigid spacer 13, the 
latter has a central part 13a, whose diameter slightly exceeds the 
external diameter of connecting rod 10 onto which it is mounted. In this 
way, spacer 13 slides on the connecting rod. Moreover, spacer 13 has on 
either side of the central part 13a, two end parts, whereof only one is 
shown in FIG. 7. Parts 13b are intended to bear on the edges of plates 
12a, which constitute the spacing grids 12 of the assembly fuel rods. 
Thus, part 13b has a variable section. It is circular at the point where 
it is connected to the circular part 13a and its section varies until is 
becomes polygonal at the point where it bears on grid 12. This polygonal 
section has at least four opposite sides, which are parallel in pairs, and 
whose extensions intersect at right angles. For example, it can be a 
square section, octogonal section, or a square section with rounded 
angles, in the manner shown in FIG. 7. 
To prevent spacer 13 from rotating about connecting rod 10, which would be 
prejudicial to its bearing on grid 12, the spacer is provided with means 
able to prevent a relative rotation of these two members. In the 
embodiment of FIG. 7, these means are constituted by a tenon 13c, which is 
placed in a mortise cut into one of the plates 12a forming the grid. No 
shaping is required on plates 12a of the grid, because the contact surface 
between spacer 13 and the latter is always adequate. This contact is 
permanently maintained during operation, as a result of means able to 
prevent the aforementioned rotation. 
According to another variant shown in FIGS. 8 and 9, the central parts of 
plates 12a of grid 12, which form the four sides of a cavity traversed by 
a connecting rod 10 are shaped so as to have a substantially circular 
rounded form in their central part. This deformation of the plate can be 
made over the entire height of the latter or only over part thereof, as 
shown in FIG. 8. An oblong hole 12c is made, which limits the rounded 
section portion 12b. The rigid spacer 13, shown in dotted mixed line form 
in FIG. 8, has a central part 13a, whose internal diameter slightly 
exceeds the external diameter of connecting rods 10, as well as two end 
parts 13b, whereof only one is shown in FIG. 8. Part 13b has a final 
diameter corresponding to that of the rounded parts 12b of FIG. 12. Once 
the grid has been assembled, spacer 13 rests on the latter by parts 12b. 
The contact surface is less than the section of spacer 13, but is still 
adequate. It should be noted that there is no need to provide means for 
preventing the relative rotation of the spacer with respect to the grid, 
because the bearing zone has a symmetry of revolution. 
The deformation can be more or less pronounced. Thus, it is possible to 
obtain a contact area forming a complete circle, as shown in FIG. 9. In 
this case, the bearing area of spacer 13 of grid 12 is increased. 
This construction in no way reduces the water passage cross-section. Thus, 
it reduces the pressure drop around connecting rods 10. The engagement 
function of connecting rods 10 in the grid is maintained by slightly off 
centering the deformed parts 12b, which are thus applied to rods 10 with a 
pressure due to the local bending of the plate forming the grid. It should 
also be noted that this construction is simple, easy to realise and leads 
to a material saving. It requires no supplementary welding during the 
assembly of the grid. 
It should be noted that with the constructions of the grid and the guide 
tubes described with reference to FIGS. 7 to 9, the bearing of the spacers 
on the grids provides an adequate surface for preventing the 
interpenetration of the materials as a result of the contact pressure. 
This bearing is maintained, even in the case of a possible separation of 
the contact surfaces.