FR3140703A1 - Debris filter for nuclear fuel assembly bottom tip with variable thickness - Google Patents

Abstract

Anti-debris filter for lower end of nuclear fuel assembly with variable thickness The anti-debris filter is formed of a grid (32) having a first face (32A) and a second face (32B) opposed, the grid (36 ) having passages (36) extending between an inlet (36A) on the first face (32A) and an outlet (36B) on the second face (32B) for the flow of a cooling fluid therethrough the grid (32), the grid (32) having a first zone (Z1) and at least a second zone (Z2) in which the grid (32) has a thickness greater than that of the grid (32) in the first zone (Z1), passages (36) being present in the first zone (Z1) and in each second zone (Z2). Figure for abstract: Figure 7

Description

Debris filter for nuclear fuel assembly bottom tip with variable thickness

The present invention relates to the field of nuclear fuel assemblies, in particular for pressurized water nuclear reactors (or PWR for “Pressurized Water Reactor”).

A nuclear fuel assembly for a pressurized water nuclear reactor generally includes a bundle of nuclear fuel rods extending along a longitudinal axis and a support skeleton configured to support the nuclear fuel rods. The support skeleton comprises a lower end and an upper end spaced apart along the longitudinal axis, a plurality of guide tubes extending along the longitudinal axis by connecting the ends to each other, and distributed spacer grids along the guide tubes and fixed to the guide tubes, each spacer grid being configured to support the nuclear fuel rods.

In operation, the nuclear fuel assembly is arranged vertically in a vessel of a nuclear reactor, being placed on a lower core plate provided with openings through which a cooling fluid enters which circulates vertically from bottom to top through nuclear fuel assembly.

Debris present in the coolant could damage components of the nuclear fuel assembly.

It is possible to equip the lower end of the nuclear fuel assembly with an anti-debris filter allowing the flow of the cooling fluid while retaining any debris that may be present in the cooling fluid. Such an anti-debris filter is disclosed in WO2005/059923A2.

One of the aims of the invention is to provide an anti-debris filter which has limited hydraulic resistance while having a satisfactory debris retention capacity.

For this purpose, the invention proposes an anti-debris filter for the lower end of a nuclear fuel assembly, the anti-debris filter being formed of a grid having a first face and a second opposite face, the grid having passages s 'extending between an inlet located on the first face and an outlet located on the second face for the flow of a cooling fluid through the grid, the grid having a first zone and at least a second zone in which the grid has a thickness greater than that of the grid in the first zone, passages being present in the first zone and in each second zone.

The zones of different thicknesses allow the debris retention capacity and flow resistance of the grid to be adjusted. An area of greater thickness has a higher retention capacity and higher resistance to flow and an area of lower thickness has a lower retention capacity and lower resistance to flow.

In particular embodiments, the anti-debris filter comprises one or more of the following optional characteristics, taken individually or in all possible technical combinations:

- each second zone has passages having an entrance and an exit offset transversely relative to each other;

- each second zone has passages having an entrance and an exit inclined relative to each other with a non-zero angle of inclination;

- each second zone has passages extending in a non-rectilinear manner;

- the grid has in each second zone an increasing thickness from the periphery of the second zone towards the center of the second zone;

- each second zone has a circular outline;

- side walls of the passages located in each second zone are higher than the side walls of the passages located in the first zone;

- at least one passage located in a second zone is delimited between two opposite curved side walls;

- the grid is formed of intersecting grid elements between which the passages are delimited, including at least one grid element of variable height extending in the first zone and at least a second zone, the height of each grid element of variable height being larger in each second zone crossed by the grid element of variable height and smaller in the first zone;

- the grid has a quadrangular contour, in particular a square contour;

- the grid has four second zones;

- the second zones are distributed on the grid in a matrix manner.

The invention also relates to a lower end piece of a nuclear fuel assembly equipped with an anti-debris filter as defined above.

The invention also relates to a nuclear fuel assembly, in particular for a pressurized water nuclear reactor, comprising an anti-debris filter as defined above.

The invention and its advantages will be better understood on reading the description which follows, given solely by way of non-limiting example, and made with reference to the appended drawings, in which:

- there is a side view of a nuclear fuel assembly;

- there is a sectional view of a lower end of the nuclear fuel assembly of the , equipped with an anti-debris filter;

- there is a bottom view of the debris filter;

- there is a bottom view of one quadrant of the debris filter;

- there is a perspective view of the debris filter quadrant shown on the ;

- there is a cut perspective view of the debris filter quadrant shown on the ;

- there is a cutaway view of the debris filter quadrant shown on the ;

- there is a sectional view similar to that of the , illustrating an anti-debris filter according to another exemplary embodiment;

- there is a partial sectional view of an anti-debris filter according to another embodiment, illustrating two adjacent passages of the grid.

Nuclear fuel assembly 2 of the comprises a bundle of nuclear fuel rods 4 and a support skeleton 6 configured to support the nuclear fuel rods 4.

The nuclear fuel rods 4 extend parallel to each other and to a longitudinal axis L.

The longitudinal axis L extends vertically when the nuclear fuel assembly 2 is placed in a core of a nuclear reactor. In operation, a cooling fluid circulates vertically from bottom to top through the nuclear fuel assembly 2 as shown by the arrows F on the .

In the remainder of the description, the terms “vertical”, “horizontal”, “top”, “bottom”, “longitudinal”, “transverse”, “upper” and “lower” are understood with reference to the position of the nuclear fuel assembly 2 in the core of the nuclear reactor, the longitudinal axis L being substantially vertical.

The support skeleton 6 comprises a lower end piece 8, an upper end piece 10, a plurality of guide tubes 12 and a plurality of spacer grids 14.

The lower end 8 and the upper end 10 are spaced along the longitudinal axis L.

The guide tubes 12 extend along the longitudinal axis L and connect the lower end 8 and the upper end 10 between them, maintaining the spacing between the lower end 8 and the upper end 10. The pencils of nuclear fuel 4 are received between the lower end 8 and the upper end 10.

Each guide tube 12 is open at its upper end to allow the insertion of a control bar (not shown) inside the guide tube 12, through the upper end piece 10. Such a control bar allows to control the reactivity of the nuclear reactor core into which the nuclear fuel assembly 2 is inserted.

The spacer grids 14 are distributed along the guide tubes 12 being spaced from each other along the longitudinal axis L. Each spacer grid 14 is rigidly fixed to the guide tubes 12, the guide tubes 12 s 'extending through each spacer grid 14.

Each spacer grid 14 is configured to support the nuclear fuel rods 4 by maintaining them in a configuration in which they are spaced transversely from each other. The nuclear fuel rods 4 are preferably maintained at the nodes of a substantially regular imaginary network.

As illustrated on the , the nuclear fuel assembly 2 is placed on a lower core plate 16 via its lower end piece 8, facing at least one opening 18 for the flow of a cooling fluid.

In operation, the cooling fluid passes through each opening 18, enters the nuclear fuel assembly 2 via the lower nozzle 8, flows along the nuclear fuel rods 4 and exits the nuclear fuel assembly 2 via the upper end 10.

As illustrated on the , the lower end piece 8 comprises for example an end plate 20 and feet 22 extending downwards from the end plate 20 to bear on the lower heart plate 16. The end plate 20 has a lower face 20A and an upper face 20B.

The guide tubes 12 (not shown in Figures 2) are for example fixed to the end plate 20 via fixing screws 24 passing through the end plate 20.

The lower tip 8 is provided with an anti-debris filter 30 configured to filter the cooling fluid.

The anti-debris filter 30 comprises a grid 32. The grid 32 preferably has the shape of a plate.

The grid 32 has a first face 32A and a second face 32B opposite each other.

The grid 32 has a thickness taken between the first face 32A of the grid and the second face 32B of the grid 32.

The first face 32A is intended to be turned downwards, i.e. upwards, considering the direction of flow of the cooling fluid through the grid 32.

The second face 32B is intended to be turned upwards, i.e. downstream, considering the direction of flow of the cooling fluid through the grid 32.

The grid 32 is arranged under the end plate 20 in such a way that the cooling fluid passes through the grid 32 before passing through the end plate 20.

Preferably, the grid 32 extends over the entire extent of the end plate 20, more particularly the entire extent of the lower face 20A of the end plate 20.

The grid 32 has fixing holes 34 for the passage of the fixing screws 24.

As illustrated on the , the grid 32 has passages 36 extending through the grid 32 for the flow of cooling fluid through the grid 32.

The grid 32 is for example formed by elongated grid elements 38, 40 which are intersected.

Each grid element 38, 40 has for example the shape of a bar or an elongated plate.

Each passage 36 is defined between adjacent intersecting grid elements 38, 40.

In particular, each passage 36 has, for example, side walls defined by two adjacent first grid elements 38 intersecting with two adjacent second grid elements 40.

The grid 32 is for example formed of first grid elements 38 extending parallel in a first direction of extension T1 and of second grid elements 40 extending parallel in a second direction of extension T2 perpendicular to the first direction d extension T1.

Each passage 36 is delimited between two first adjacent grid elements 38 and two second adjacent grid elements 40.

The grid 32 has for example an outline of generally quadrangular shape, in particular an outline of generally square shape.

The grid 32 has for example four quadrants which are analogous, only one quadrant being illustrated in Figures 4 to 7 for reasons of clarity.

Furthermore, in Figures 4 to 7, the second face 32B of the grid 32 is facing upwards, this second face 32B being in practice facing downwards when the grid 32 is mounted on the lower end piece 8 and the The nuclear fuel assembly 2 is installed in the core of the nuclear reactor.

As illustrated in particular on the , each passage 36 extends between an inlet 36A, located on the first face 32A, and an outlet 36B, located on the second face 32B.

The inlet 36A of each passage 36 has an input axis A1 and the outlet 36B of each passage 36 has an output axis A2.

Each passage 36 extends from its inlet 36A to its outlet 36B along a central line C of this passage 36.

The central line C of each passage 36 is tangent to the inlet axis A1 at the inlet 36A of the passage 36 and tangent to the outlet axis A2 at the outlet of the passage 36.

The input axis A1 and the output axis A2 of each passage 36 define between them an angle of inclination θ. The angle of inclination θ is also called the angle of deflection.

The angle of inclination θ between the input axis A1 and the output axis A2 of each passage 36 of the grid 32 is for example between 0° and 60°.

Each passage 36 having a zero inclination angle θ (i.e. 0°) corresponds to a rectilinear passage 36. The input axis A1 and the output axis A2 coincide. Passage 36 has zero deflection between its inlet 36A and its outlet 36B. Input 36A and output 36B are aligned

Each passage 36 having a non-zero angle of inclination θ between its inlet axis A1 and its outlet axis A2 presents a non-zero deflection between its inlet 36A and its outlet 36B.

A non-zero angle of inclination θ corresponds to a curved passage 36, i.e. extending along a curved central line C. The central line C of each curved passage 36 preferably has an inclination varying monotonically between the input axis A1 and the output axis A2.

Each passage 36 having a non-zero angle of inclination θ between its inlet axis A1 and its outlet axis A2 preferably has its inlet 36A and its outlet 36B offset transversely relative to each other considering the direction fluid flow through the grid 32.

Advantageously, the grid 32 has passages 36 having angles of inclination θ between their input axis A1 and their output axis A2 different. In other words, the grid 32 has passages 36 having different respective deflections.

On the , three different inclination angles θ are represented, including one which is zero (on the right on the ) and which corresponds to a rectilinear passage 36.

In an exemplary embodiment, the output axes A2 of all passages 36 are parallel to each other, the input axes A1 of passages 36 having different angles of inclination θ being inclined relative to each other.

Preferably, the outlet axes A2 of the passages 36 are parallel to the direction of flow of the fluid. This makes it possible to limit disturbances to the flow of fluid at the outlet of grid 32.

The provision of passages 36 with different inclination angles θ makes it possible to differentiate the retention capacity and the resistance to flow of different zones of the grid 32.

A zone of the grid with passages 36 having larger angles of inclination θ (i.e. larger deflections) has a greater debris retention capacity and a greater resistance to flow, while a zone of the grid with passages 36 having smaller inclination angles θ (i.e. smaller deflections) has a smaller debris retention capacity and a smaller flow resistance.

Preferably, the grid 32 has a first zone Z1, the first zone Z1 having a plurality of passages 36, and at least one second zone Z2, each second zone Z2 having a plurality of passages 36, the angles of inclination θ of the passages 36 located in each second zone Z2 being non-zero and greater than or equal to the maximum angle of inclination θ of the passages 36 located in the first zone Z1.

In particular, the output axes A2 of the passages 36 of the first zone Z1 and of each second zone Z2 are for example parallel to each other.

The input axes A1 of passages 36 of each second zone Z2 are for example inclined by a non-zero angle relative to the input axes A1 of the passages 36 of the first zone Z1. The input axes A1 of these passages 36 of each second zone Z2 make a non-zero angle with the input axes A1 of the passages 36 of the first zone Z1.

The angle of inclination θ between the input axis A1 and the output axis A2 of each passage 36 of the first zone Z1 is for example substantially zero. The passages 36 of the first zone Z1 are for example rectilinear.

Preferably, the grid 32 has a second respective zone Z2 associated with each opening 18 of the lower core plate 16 located under the lower end piece 8 when the nuclear fuel assembly 2 is placed on the lower core plate 16.

The grid 32 has for example four second zones Z2 distributed on the grid 32 of quadrangular and in particular square shape, the second zones Z2 being for example distributed according to a 2X2 matrix distribution.

Grid 32 here has four second zones Z2, each second zone Z2 being located in a respective quadrant of grid 32.

The grid 32 has for example a variable thickness, the thickness of the grid 32 in each second zone Z2 being strictly greater than the thickness of the grid in the first zone Z1.

As illustrated in Figures 4 to 7, and in particular on the , in each second zone Z2, the grid 32 has a thickness strictly greater than the thickness of the grid 32 in the first zone Z1.

Preferably, in the first zone Z1, the grid 32 has a substantially constant thickness, called first thickness E1.

The height of each grid element 38, 40 is taken according to the thickness of the grid 32.

In the first zone Z1, each grid element 38, 40 has a substantially constant height or first height H1. The first height H1 corresponds to the first thickness E1.

In each second zone Z2, the grid 32 has a second thickness E2 strictly greater than the first thickness E1 of the grid 32.

The grid 32 has for example a variable thickness, varying between the first thickness E1 at the periphery of each second zone Z2 up to a second thickness E2, for example at the center of the second zone Z2.

In each second zone Z2, at least part of the grid elements 38, 40 has a height strictly greater than the first height H1.

At least part of the grid elements 38, 40 has for example a high variable, varying for example between the first height H1 and a second height H2.

The first height H1 corresponds to the first thickness E1 and the second height H2 corresponds to the second thickness E2.

For example, in each second zone Z2, each grid element 38, 40 has a height greater than the first height H1 or only part of the grid elements 38, 40 has a height greater than the first height H1.

In particular, in each second zone Z2, each first grid element 38 or only part of the grid elements 38 has a height greater than the first height H1 and/or each second grid element 40 or only part of the second elements of grid 40 has a height greater than the first height H1.

In Figures 4 to 7, in each second zone Z2, each first grid element 38 has a height greater than the first height H1 and a second grid element 40 out of three has a height greater than the first height H1.

The side walls of each passage 36 are defined by the portions of the grid elements 38, 40 delimiting the passage 36.

Thus, side walls of the passages located in each second zone Z2 are higher than the side walls of the passages located in the first zone Z1.

As illustrated in particular on the , each passage 36 is delimited between two opposite side walls which extend substantially parallel while being curved

Thus each of these passages 36 extends in a curvilinear manner, its inlet 36A and its outlet 36B being offset transversely with respect to one another and inclined with respect to one another.

In particular, in each second zone Z2, each first grid element 38 of variable height, and in particular each first grid element 38, has an upper portion 42 which extends higher than the rest of the first face 32A of the grid 32 in the first zone Z1.

The upper portion 42 of each first grid element 38 is for example curved to define the curved passages 36.

Each of the second grid elements 40 extends for example substantially along a plane.

In each second zone Z2, second grid elements 40 of variable height have upper portions 44 which intersect the upper portions 42 of the first grid elements 38 of variable height.

The grid 30 is for example produced by a manufacturing process by adding material (additive manufacturing) and/or by a manufacturing process with removal of material (machining).

To obtain the grid 32 with passages 36 having input axes A1 inclined relative to each other and output axes A2 parallel to each other, a grid blank 32 is for example initially manufactured with a constant thickness , each passage 36 extending along a curved central line C with a non-zero angle of inclination between its entry axis A1 and its exit axis A2, the passages 36 extending parallel to each other.

Initially, the input axes A1 of the passages 36 are parallel to each other and the output axes A2 of the passages 36 are parallel to each other.

Then, the grid blank 32 is machined on its first face 32A so as to form the first zone Z1 and each second zone Z2.

Due to the variation in thickness of the machined grid 32, certain passages 36 are reduced in height, and passages 36 finally have input axes A1 inclined relative to each other.

In particular, the passages 36 located in the thinnest regions of the machined grid 32 have the least inclined input axes A1 and the smallest inclination angles θ, and the passages 36 located in the thickest regions of grid 32 have the most inclined input axes A1 and the largest angles of inclination θ.

Thus, passages 36 located in each second zone Z2 have a greater angle of inclination θ than the passages 36 located in the first zone Z1.

The thickness of the grid 32 is for example reduced in the first zone Z1 in such a way that only a rectilinear exit section of passage 36 of the grid 32 remains in each passage 36 of the first zone Z1 before machining, each passage 36 of the first zone Z1 therefore being rectilinear in the machined grid 32.

In a variant illustrated on the , which is analogous to the and on which the numerical references to similar elements are included, a grid 32 has for example a constant thickness, the differences in inclination between the passages 36 resulting from differences in curvature between the central lines C of the passages 36, for example from differences in curvatures between side walls of passages 36.

Such a grid 32 is for example obtained by additive manufacturing.

As illustrated on the , it is possible to provide passages 36 of variable cross section along the passage 36, each passage 36 of variable cross section having a converging inlet section 46 (considering the direction of flow of the fluid in the passage 36 of the 'inlet 36A towards outlet 36B) and/or a diverging outlet section 48.

A converging inlet section 46 or a diverging outlet section 48 is obtained for example by providing a chamfer on the side walls respectively of the inlet section 46 and the outlet section.

Each passage 36 of variable cross section is provided with a converging inlet section 46 and a diverging outlet section 48, with or without intermediate section of constant cross section, a converging inlet section 46, the rest of the passage having a constant cross section or of a diverging outlet section 46, the rest of the passage having a constant cross section.

A passage 36 of variable cross section provided with a converging inlet section 46 and a diverging outlet section 48 without an intermediate section of constant cross section, has for example curved and convex side walls.

The variation in cross section of a passage 36 makes it possible to generate a venturi effect in the fluid circulating in the passage 36.

As also illustrated on the , preferably the lower edge and/or the upper edge of each side wall of each passage 36 is preferably rounded. This makes it possible to promote the flow of the fluid by limiting the resistance to the flow of the grid 32.

The provision of passage 36 with different inclination angles θ between the inlet 36A and the outlet 36B makes it possible to differentiate the retention capacity and the resistance to flow of different zones of the grid 32.

An area of the grid with passages 36 having larger angles of inclination θ has a greater debris retention capacity and greater flow resistance, while an area of the grid with passages 36 having Smaller tilt angles θ have smaller debris holding capacity and smaller flow resistance.

The provision of a first zone Z1 with a first thickness E1 and one or more second zones Z2 with a thickness strictly greater than the first thickness E1 makes it possible to easily form passages 36 with angles of inclination θ between the entrance 36A and the outlet 36B smaller in the first zone Z1 and passages 36 with angles of inclination θ between the inlet 36A and the outlet 36B larger in each second zone Z2, from a grid blank of constant thickness whose passages 36 are parallel with angles of inclination θ between the inlet 36A and the outlet 36B which are identical and non-zero.

The second zone(s) Z2 can be placed on the grid 30 at the location(s) where the probability of debris passing is greatest, which is most of the time facing the openings 18 of the lower core plate 16 through which the cooling fluid arrives under the nuclear fuel assemblies 2.

Thus, it is possible to obtain a grid 30 having limited hydraulic resistance while having a satisfactory debris retention capacity.

The grid 30 can be easily manufactured, for example by additive manufacturing and/or by machining.

The invention is not limited to the example and the variants described, other examples and other variants being possible.

The passages 36 of the grid in Figures 4 and 7 are delimited between first curved grid elements 38 and second flat grid elements 40, so that the input axes A1 of the passages 36 are all inclined in the same direction. The input axes A1 of the passages 36 are all parallel to the same reference plane. The input axes A1 can be parallel if the angles of inclination of the passages 36 are the same or not if the passages have different angles of inclination.

Of course, in a variant, input axes A1 of passages 36 can be inclined in different directions. To do this, it is possible to provide only the second grid elements 40 which are also curved.

Furthermore, it is possible to provide output axes A2 which are not parallel between two, with in particular output axes A2 which have a non-zero inclination with the longitudinal axis L of the nuclear fuel assembly 2 when the grid 30 is mounted on the lower end 8.

In particular, it is possible to provide that the input axes A1 are parallel to each other and that the output axes A2 are inclined relative to each other to obtain different inclination angles, the output axes A2 being by example all parallel to the same reference plane or inclined in different directions.

The provision of a first zone Z1 having a first thickness E1 and at least a second zone Z2 having a second thickness E2 is advantageous independently of the fact that the passages 36 have angles of non-zero inclination between an input axis A1 and an output axis A2, in particular differentiated inclination angles between different passages 36.

Thus, according to another aspect, the invention proposes an anti-debris filter for a lower end piece of a nuclear fuel assembly, the anti-debris filter being formed of a grid 32 having a first face 32A and a second face 32B opposite each other. , the grid 32 having passages 36 extending between an inlet 36A located on the first face 32A and an outlet 36B located on the second face 32B for the flow of a cooling fluid through the grid 32, the grid 32 having a first zone Z1 and at least a second zone Z2 in which the grid 32 has a thickness greater than that of the plate in the first zone Z1, passages 36 being present in the first zone Z1 and in each second zone Z2.

Each second zone Z2 thicker than the first zone Z1 has a higher retention capacity than the first zone Z1 and a higher flow resistance than the first zone Z1.

The passages 36 located in each second zone Z2 are a priori longer than the passages 36 located in the second zone Z2, and allow for example longer debris to be filtered.

The provision of a first zone Z1 and at least a second zone Z2 thicker than the first zone Z1 makes it possible to locally adapt the retention capacity, depending on the probability of the presence of debris, while limiting the resistance to the flow of the grid 32 as a whole.

Such a grid with a first zone Z1 and at least a second zone Z2 with different thicknesses can be provided with rectilinear passages 36 and/or curved passages 36, and in particular with passages 36 which are all rectilinear or all curved, the passages 36 curved with identical or differentiated inclination angles.

The characteristics discussed in relation to the embodiment of Figures 4 to 7 can be provided as an option on grid 32.

In particular, in embodiments, the grid 32 includes one or more of the following optional characteristics, taken individually or in all technically possible combinations:

- each second zone Z2 has passages 36 having an inlet 36A and an outlet 36B offset transversely relative to each other;

- each second zone Z2 has passages 36 having an inlet 36A and an outlet 36B inclined relative to each other with a non-zero angle of inclination.

- each second zone Z2 has passages 36 extending in a non-rectilinear manner;

- the grid 32 has in each second zone Z2 an increasing thickness from the periphery of the second zone Z2 towards the center of the second zone Z2;

- each second zone Z2 has a circular outline.

- at least one passage 36 located in a second zone Z2 is delimited between two opposite curved side walls;

- the grid 32 is formed of intersecting grid elements 38, 40 between which the passages 36 are delimited, including at least one grid element 38, 40 of variable height extending in the first zone Z1 and at least one second zone Z2, the height of each grid element 38, 40 of variable height being greater in each second zone Z2 crossed by the grid element 38, 40 of variable height and smaller in the first zone Z1;

- the grid 32 has a quadrangular contour, in particular a square contour;

- grid 32 has four second zones Z2;

- the second zones Z2 are distributed on the grid 32 in a matrix manner;

According to said other aspect, the invention also proposes a lower end piece for a nuclear fuel assembly equipped with an anti-debris filter as defined above and/or a nuclear fuel assembly, in particular for a pressurized water nuclear reactor. , comprising an anti-debris filter as defined above.

Claims (14)

Anti-debris filter for a lower tip of a nuclear fuel assembly, the anti-debris filter being formed of a grid (32) having a first face (32A) and a second face (32B) opposite each other, the grid (36) having passages (36) extending between an inlet (36A) on the first face (32A) and an outlet (36B) on the second face (32B) for the flow of a cooling fluid through the grid (32), the grid (32) having a first zone (Z1) and at least a second zone (Z2) in which the grid (32) has a thickness greater than that of the grid (32) in the first zone (Z1 ), passages (36) being present in the first zone (Z1) and in each second zone (Z2). Anti-debris filter according to claim 1, in which each second zone (Z2) has passages (36) having an inlet (36A) and an outlet (36B) offset transversely relative to each other. An anti-debris filter according to claim 1 or claim 2, wherein each second zone (Z2) has passages (36) having an inlet (36A) and an outlet (36B) inclined relative to each other with a non-zero inclination angle. Anti-debris filter according to any one of the preceding claims, in which each second zone (Z2) has passages (36) extending in a non-rectilinear manner. Anti-debris filter according to any one of the preceding claims, in which the grid (32) has in each second zone (Z2) a thickness increasing from the periphery of the second zone (Z2) towards the center of the second zone (Z2 ). Anti-debris filter according to any one of the preceding claims, in which each second zone (Z2) has a circular outline. Anti-debris filter according to any one of the preceding claims, wherein side walls of the passages (36) located in each second zone (Z2) are higher than the side walls of the passages (36) located in the first zone (Z1 ). Anti-debris filter according to any one of the preceding claims, in which at least one passage (36) located in a second zone (Z2) is delimited between two opposite curved side walls. Anti-debris filter according to any one of the preceding claims, in which the grid (32) is formed of intersecting grid elements (38, 40) between which the passages (36) are delimited, including at least one grid element (38, 40) of variable height extending in the first zone (Z1) and at least one second zone (Z2), the height of each grid element (38, 40) of variable height being greater in each second zone (Z2) crossed by the grid element (38, 40) of variable height and smaller in the first zone (Z1). Anti-debris filter according to any one of the preceding claims, in which the grid (32) has a quadrangular contour, in particular a square contour. Anti-debris filter according to any one of the preceding claims, wherein the grid (32) has four second zones (Z2). Anti-debris filter according to any one of the preceding claims, in which the second zones (Z2) are distributed on the grid (32) in a matrix manner. Lower end of a nuclear fuel assembly equipped with an anti-debris filter according to any one of the preceding claims. Nuclear fuel assembly, in particular for a pressurized water nuclear reactor, comprising an anti-debris filter according to any one of claims 1 to 12.