Filtration plate associated with a lower connector of a fuel assembly of a nuclear reactor

The filtration plate (10), arranged in the lower part of the fuel assembly and ensuring the retention of particles contained in the cooling fluid of the reactor, has at least one active zone where it is pierced with holes (12) of a size determined according to the size of the particles to be retained. The active zones of the filtration plate come into alignment with a water-passage hole through the lower core plate of the reactor when the assembly is in operation. The water-passage orifices of the filtration plate (10) can consist of cutouts of square or triangular form or of stamped parts of the plate or be delimited by parallel lamellae constituting a grating.

FIELD OF THE INVENTION 
The invention relates to a filtration plate of a lower connector of a fuel 
assembly for a nuclear reactor. 
BACKGROUND OF THE INVENTION 
Water-cooled nuclear reactors, especially reactors cooled by pressurized 
water, have a core consisting of prism-shaped fuel assemblies which are 
placed next to one another and arranged vertically and which rest on the 
lower core plate of the nuclear reactor by means of a lower connector. The 
cooling water of the reactor passes through the lower connector commonly 
called "bottom nozzle" and circulates from the bottom upwards in the 
vertical direction within the assembly. 
This cooling water is liable to contain particles which it carries into the 
assembly through orifices passing through the adaptor plate of the lower 
connector. The particles come to rest in gaps located between the fuel 
rods and the walls of the cells of the first spacer grid of the assembly, 
i.e. the spacer grid located nearest to the lower connector of the 
assembly. 
The circulating fluid subjects the particles to axial and transverse 
stresses, to the extent that this can result in wear of the jacket of the 
fuel rod. It is therefore desirable to retain the particles contained in 
the cooling fluid, in order to prevent them from penetrating into the 
assembly. 
U.S. Pat. No. 4,664,880 and U.S. Pat. No. 4,678,627, for example, disclosed 
retention devices in the form of structures made of metal wires or of 
pierced sheet metal and arranged in the lower connector of the assembly. 
It has also been proposed, in U.S. Pat. No. 4,684,496, to use as a 
retention device a filtration plate produced in the form of a grating 
consisting of wafers and associated with the lower connector of the 
assembly. 
These known devices generally introduce a considerable head loss into the 
circulation of the cooling fluid of the reactor, inasmuch as the fluid 
passage cross-sections projected in a plane perpendicular to the flow 
cross-section may only be small. 
EP-A-0,289,829 and EP-A-0,311,037 have also proposed producing the 
filtration device by using the very adaptor plate of the lower connector 
as a filter element, either by providing a special duct in this plate or 
by inserting filtration means locally in orifices machined specially in 
the adaptor plate. However, special machining of this essential structural 
element of the lower connector has disadvantages. 
SUMMARY OF THE INVENTION 
The object of the invention is, therefore, to provide a filtration plate of 
a lower connector of a fuel assembly of a nuclear reactor for the 
retention of particles contained in the cooling fluid of the reactor, the 
lower connector resting on the lower core plate of the reactor when the 
assembly is in operating position, the filtration plate introducing only a 
slight head loss into the circulation of the cooling fluid of the reactor, 
while at the same time ensuring effective filtration. 
To this end, the filtration plate has at least one active zone where the 
plate is pierced with holes of determinate size intended for alignment 
with a water-passage hole through the lower core plate of the reactor.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 shows part of the lower core plate 1 of a nuclear reactor, having 
water-passage holes 2 in the region of the lower connector 3 of a fuel 
assembly. 
The lower connector 3 forms part of the framework of the assembly, to which 
the guide tubes 4 are connected in their lower part. The framework also 
includes a set of spacer grids connected to the guide tubes 4, only the 
lower spacer grid 9 being shown in FIG. 1. 
The connector 3 comprises an adaptor plate 6 with feet 7 which come to rest 
on the upper surface of the lower coreplate 1. 
The lower connector and the spacer grids have a substantially square form 
corresponding to the cross-section of the fuel assembly. 
Two of the feet 7 of the lower connector 3 have orifices which engage onto 
centering and positioning studs 8 for the fuel assembly which are fastened 
to the upper face of the lower core plate 1. 
A filtration plate 10 is inserted between the lower surface of the feet 7 
and the upper surface of the lower core plate 1, so as to ensure the 
retention of particles contained in the cooling fluid of the reactor, 
before these particles can penetrate into the fuel assembly. 
The cooling fluid of the reactor passes through the lower core plate 1 in 
the region of the passage holes 2 and subsequently penetrates into the 
fuel assembly via the orifices of the adaptor plate 6. 
When a filtration plate 10 is interposed in the circulation of the cooling 
fluid at a level below the adaptor plate 6, this filtration plate 10 is 
capable of retaining the particles in circulation, inasmuch as this 
filtration plate has holes of determinate size making it possible to stop 
particles of a size greater than a particular limit. This limit is 
generally determined by the dimension of the gaps present between the fuel 
rods of the assembly and the cells of the first spacer grid 9 and is 
within a range of between two and four millimeters. 
The active zones of the filtration plate 10, in which this filtration plate 
is pierced with orifices of determinate size, correspond substantially to 
the zones of the plate coming opposite the water-passage holes of the 
lower core plate when the assembly is in the operating position, as shown 
in FIG. 1. 
These active zones have a generally circular form corresponding to the 
cross-section of the water-passage holes in the lower core plate. 
FIG. 2 shows part of an active zone of a filtration plate according to the 
invention, of which the water-passage and particle-retention orifices have 
a square form. 
The orifices 12 cut out from the filtration plate have a side of a 
dimension corresponding to the minimum dimension of the particles to be 
retained. 
FIG. 2 shows approximately one eighth of an active part of a filtration 
plate according to the invention, the radius of this active part 
corresponding to the radius of a water-passage hole 2 in the lower core 
plate. 
FIG. 3 shows an alternative embodiment, in which the square cutouts 13 
passing through the filtration plate have a dimension substantially larger 
than the dimension of the cutouts 12 of the filtration plate shown in FIG. 
2. 
These cutouts of square form therefore have a dimension substantially 
larger than the minimum dimension of the particles to be retained. 
As shown in FIG. 3A, it can be seen that each of the cells 13 of square 
form cut out from the filtration plate has four bosses 15 directed towards 
the inside of the cell. The bosses 15 delimit, within the cell 13, 
water-passage holes with a dimension smaller than the minimum dimension of 
the particles to be retained. 
FIG. 4 shows a third embodiment of the active zone of a filtration plate 
according to the invention, this active zone comprising cutouts in the 
form of rightangled triangles 14 placed next to one another to form a 
water-passage and particle-retention network of a dimension greater than a 
particular limit dependent on the size of the triangular orifices 14. 
FIGS. 5 and 6 illustrate a fourth embodiment of the filtration plate 
according to the invention. 
The plate 20 has an active zone which is located opposite each of the 
water-passage holes 22 of the lower coreplate 21 and in which the 
filtration plate 20 is stamped to form orifices 23, the form of which can 
be seen in FIG. 6. Each of the orifices 23 is delimited by four fins 24 
pushed out of the plane of the plate 20 at the time of stamping. 
The central passage hole 23 delimited by the fins 24 makes it possible to 
obtain the passage of water and retention of particles of a size greater 
than a particular limit. 
If the filtration plate is produced by stamping, the stamped zone must 
correspond perfectly to the water-passage cross-section 22 in which it is 
received. 
FIGS. 7 and 8 illustrate a filtration plate 25, the active zone 26 of which 
is produced in the form of a grating comprising parallel lamellae 27, the 
spacing of which corresponds to the minimum size of the particles to be 
retained in the region of the filtration plate 25. 
The lamellae 27 are placed next to one another within circular orifices 
which are made in the plate 25 and the dimension of which corresponds to 
the dimension of the water-passage holes 29 of the lower coreplate 30. 
These lamellae 27 can be welded at their ends to the edges of the orifice 
of the filtration plate. 
To limit the loss of head of the cooling fluid during the passage through 
the filtration plate 25, the lamellae 27 have a profiled form, as can be 
seen in FIG. 8A. This profiled form matched to the flow of the fluid in a 
direction perpendicular to the filtration plate allows an appreciable 
reduction of head loss. 
The lamellae 27 of the active parts of the filtration plate 25 are fastened 
only at their ends and are all arranged parallel to one another. These 
lamellae are liable to vibrate under the effect of the hydraulic stresses 
of the cooling fluid passing through the active zones of the filtration 
plate 25. 
To prevent these vibrations, it is possible to arrange stiffeners 31 in 
directions perpendicular to those of the lamellae 27', as can be seen in 
FIG. 9. The stiffeners 31 can be welded or otherwise fastened to the 
lamellae 27' and to the edges of the orifice passing through the 
filtration plate 25. 
The filtration plate according to the invention ensures effective retention 
of the particles which are contained in the cooling fluid of the reactor 
and which are of a size greater than a particular limit. 
This filtration plate introduces only a moderate head loss into the circuit 
of the cooling fluid, inasmuch as the orifices of the filtration plate are 
located in the extension of the passage orifices of the lower core plate. 
If these orifices are made by the stamping of the plate, the deflector 
elements 24 of the plate are pointed in the direction of flow, thus making 
it possible to reduce the loss of head. 
The perforations or orifices may be cut out or machined in the filtration 
plate in a manner and form different from those described. 
The filtration plate can be produced from any material resistant to the 
cooling fluid and to the operating conditions of the reactor, for example 
from a nickel-based alloy or from martensitic steel. 
The filtration plate according to the invention can be arranged not only in 
contact with the upper surface of the lower core plate, as described, but 
can also be arranged in the lower connector of the assembly in an 
intermediate position between its lower end and its adaptor plate, or be 
fastened in contact with the lower face of the adaptor plate. 
At all events, the orifices of the filtration plate must be located 
opposite the cooling-fluid passage orifices passing through the lower core 
plate, in order to limit head losses. 
The invention is used for any fuel assembly of a water-cooled nuclear 
reactor.