Optimized nuclear fuel assembly

Abstract

The nuclear fuel assembly comprises grids distributed along the length of nuclear fuel rods (4), the grids including spacer grids (14), at least one reinforcing grid (16), a lower end grid (18) and an upper end grid (20). The spacer grids (14) define, between them and together with the lower end grid (18) and the upper end grid (20), a plurality of intervals (15). The grids further include one or more intermediate mixing grids (22) and, optionally, an anti-debris grid (24) in which the lower ends of the nuclear fuel rods (24) engage.

Description

[0001] Optimized nuclear fuel assembly

[0002] The present invention relates to the field of nuclear fuel assemblies, in particular for pressurized water reactors (or PWRs for "Pressurized Water Reactor").

[0003] A nuclear fuel assembly for a pressurized water reactor typically comprises a bundle of nuclear fuel rods extending along a longitudinal axis and a support skeleton configured to support the fuel rods. The support skeleton includes a lower end cap and an upper end cap spaced along the longitudinal axis, a plurality of guide tubes extending along the longitudinal axis and connecting the end caps to each other, and spacer grids distributed along and attached to the guide tubes, each spacer grid being configured to support the nuclear fuel rods.

[0004] Each grid-spacer has pencil cells through which the nuclear fuel rods pass, each pencil cell being traversed by a respective nuclear fuel rod and equipped with springs and / or bosses on internal surfaces of the pencil cell to hold transversely and longitudinally the nuclear fuel rod passing through this pencil cell.

[0005] In operation, the nuclear fuel assembly is arranged vertically in the core of a nuclear reactor, resting on a lower core plate equipped with openings through which a cooling fluid enters and circulates vertically from bottom to top through the nuclear fuel assembly.

[0006] In this position, the nuclear fuel assembly may tend to flex along its longitudinal axis and adopt a C, S, or W shape due to fluid-structure interaction involving lateral hydraulic forces that tend to push the fuel assemblies toward the periphery of the core. Furthermore, a decrease in the mechanical stiffness of the fuel assemblies during irradiation and an irradiation creep phenomenon occur, leading to irreversible deformations of the fuel assemblies.

[0007] Such deformations give rise to heterogeneities in spacing between nuclear fuel assemblies which can disrupt the distribution of thermal power in the core, resulting in power imbalances.

[0008] In addition, such deformations can cause operational problems, for example for the insertion of control rod clusters inside the guide tubes, and / or maintenance problems, for example for the insertion and / or extraction of the nuclear fuel assembly into or out of the nuclear reactor core.

[0009] FR2860334A1 and FR2860335A1 disclose nuclear fuel assemblies equipped with reinforcement devices attached to the guide tubes of the nuclear fuel assembly, to stiffen the support structure and thus limit lateral deformation of the nuclear fuel assembly. These reinforcement devices include substantially flat plates, angle-shaped plates, or sets of interlocking plates, the plates being attached to the guide tubes to connect them. These reinforcement devices are provided in addition to the spacer grids. These reinforcement devices have a limited extent. They extend only between the guide tubes and do not extend to the peripheral nuclear fuel rods.These reinforcement devices delimit cells that can receive one, two or more than two nuclear fuel rods and that have dimensions greater than those of the nuclear fuel rods, to receive the nuclear fuel rods with clearance.

[0010] One of the aims of the invention is to propose a nuclear fuel assembly whose rigidity can be improved while limiting the weight of the nuclear fuel assembly and / or the resistance to the flow of a fluid through the nuclear fuel assembly over its entire life.

[0011] To this end, the invention relates to a nuclear fuel assembly comprising nuclear fuel rods extending along a longitudinal axis and a support skeleton configured to carry the nuclear fuel rods, the support skeleton comprising a lower end and an upper end spaced along the longitudinal axis, a plurality of guide tubes extending along the longitudinal axis and connecting the lower and upper ends to each other, and grids distributed between the lower and upper ends, the grids including:

[0012] - grid-spacers fixed to the guide tubes and through which the nuclear fuel rods pass, each grid-spacer being configured to support the nuclear fuel rods longitudinally and transversely;

[0013] - at least one reinforcement grid fixed to the guide tubes to reinforce the support skeleton, each reinforcement grid being crossed by at least part of the nuclear fuel rods;

[0014] - a lower end grid fixed to the guide tubes and through which the nuclear fuel rods pass, the lower end grid supporting the nuclear fuel rods longitudinally and transversely and being located between the lower end and the spacer grid closest to the lower end; and - an upper end grid fixed to the guide tubes and through which the nuclear fuel rods pass, the upper end grid supporting the nuclear fuel rods longitudinally and transversely and being located between the upper end and the spacer grid closest to the upper end; the spacer grids defining, between themselves and with the lower end grid and the upper end grid, a plurality of gaps along the nuclear fuel rods;the grids further including one or more intermediate mixing grids, each intermediate mixing grid being fixed to the guide tubes and through which all the nuclear fuel rods pass, each intermediate mixing grid being fitted with mixing fins configured to deflect a flow of fluid passing through the intermediate mixing grid.

[0015] Providing a lower end grid, an upper end grid, at least one reinforcing grid and in addition one or more intermediate mixing grids and, optionally, a debris-free grid in which the lower ends of the nuclear fuel rods are engaged, makes it possible to improve the rigidity of the nuclear fuel assembly without excessively impacting the resistance to the flow of a fluid through the nuclear fuel assembly, while possibly allowing to promote fluid mixing and / or filtering of debris that may be present in the fluid.

[0016] In embodiment examples, the nuclear fuel assembly includes one or more of the following optional features, taken individually or in all technically possible combinations:

[0017] - the nuclear fuel assembly includes at least six grid-spacers, in particular eight grid-spacers;

[0018] - the nuclear fuel assembly includes one reinforcement grid, two reinforcement grids, three reinforcement grids or four reinforcement grids;

[0019] - the nuclear fuel assembly includes a reinforcement grid located in a first interval defined by the grid-spacers and / or a reinforcement grid located in a third interval defined by the grid-spacers and / or a reinforcement grid located in a sixth interval defined by the grid-spacers and / or a reinforcement grid located in a seventh interval defined by the grid-spacers and / or a reinforcement grid located in a ninth interval defined by the grid-spacers.

[0020] - the nuclear fuel assembly includes at least two intermediate mixing grids, in particular four intermediate mixing grids; - the nuclear fuel assembly includes an intermediate mixing grid located in a fifth interval defined by the grid-spacers, an intermediate mixing grid located in a sixth interval defined by the grid-spacers, an intermediate mixing grid located in a seventh interval defined by the grid-spacers and an intermediate mixing grid located in an eighth interval defined by the grid-spacers;

[0021] - the nuclear fuel assembly comprises eight grid-spacers, a reinforcement grid located in a first interval defined by the grid-spacers, the lower end grid, the upper end grid and four intermediate mixing grids located in the fifth, sixth, seventh and eighth intervals defined by the grid-spacers;

[0022] - the nuclear fuel assembly comprises eight grid-spacers, two reinforcement grids located in the first and ninth intervals defined by the grid-spacers, the lower end grid, the upper end grid and four intermediate mixing grids located in the fifth, sixth, seventh and eighth intervals defined by the grid-spacers;

[0023] - the nuclear fuel assembly comprises eight grid-spacers, four reinforcement grids located in the first, third, sixth and ninth intervals by the grid-spacers, the lower end grid and the upper end grid;

[0024] - the nuclear fuel assembly comprises eight grid-spacers, a reinforcement grid located in the first or ninth gap defined by the grid-spacers, the lower end grid and the upper end grid;

[0025] - the nuclear fuel assembly comprises eight grid-spacers, a reinforcement grid located in the ninth interval defined by the grid-spacers, the lower end grid, the upper end grid and four intermediate mixing grids located in the fifth, sixth, seventh and eighth intervals defined by the grid-spacers;

[0026] - the grids further include a debris grid in which the lower ends of the nuclear fuel rods are engaged, the debris grid being configured to support the lower ends of the nuclear fuel rods transversely and to retain debris.

[0027] The invention and its advantages will become apparent from the following description, given solely by way of non-limiting example, and made with reference to the drawings in which:

[0028] - Figure 1 is a side view of an example nuclear fuel assembly; - Figure 2 is a partial top view of a spacer grid of a nuclear fuel assembly; and

[0029] - Figures 3 to 9 are side views of nuclear fuel assemblies according to other examples.

[0030] As illustrated in Figure 1, a nuclear fuel assembly 2 comprises a nuclear fuel rod bundle 4 and a support skeleton 6 configured to support the nuclear fuel rods 4.

[0031] The nuclear fuel assembly 2 is elongated along a longitudinal axis L. The longitudinal axis L extends vertically when the nuclear fuel assembly 2 is placed in a nuclear reactor core in a nuclear reactor vessel.

[0032] In the following description, the terms "vertical", "horizontal", "top", "bottom", "longitudinal", "transverse", "upper" and "lower" are understood by reference to the position of the nuclear fuel assembly 2 in the core of the nuclear reactor, the longitudinal axis L being substantially vertical.

[0033] During operation, a cooling fluid flows vertically from bottom to top through the nuclear fuel assembly 2 as shown by arrow F in Figure 1.

[0034] The support skeleton 6 includes a lower end cap 8, an upper end cap 10, a plurality of guide tubes 12, a plurality of spacer grids 14, at least one reinforcing grid 16, a lower end grid 18 and an upper end grid 20.

[0035] The lower tip 8 and the upper tip 10 are spaced along the longitudinal axis L.

[0036] The guide tubes 12 extend along the longitudinal axis L and connect the lower end 8 and the upper end 10 together, maintaining the spacing between the lower end 8 and the upper end 10. The nuclear fuel rods 4 are received between the lower end 8 and the upper end 10.

[0037] Each guide tube 12 is open at its upper end to allow the insertion of a control rod (not shown) inside the guide tube 12, through the upper end 10. Such a control rod allows the reactivity of the nuclear reactor core into which the nuclear fuel assembly 2 is inserted to be controlled.

[0038] The nuclear fuel rods 4 extend parallel to each other and to the longitudinal axis L. The nuclear fuel rods 4 together form a bundle.

[0039] The spacer grids 14 are distributed along the guide tubes 12, spaced apart from each other along the longitudinal axis L. Each spacer grid 14 is rigidly fixed to the guide tubes 12, the guide tubes 12 extending through each spacer grid 14.

[0040] Each grid-spacer 14 is traversed by the nuclear fuel rods 4, in particular all the nuclear fuel rods 4 of the nuclear fuel assembly 2. Each grid-spacer 14 is configured to support the nuclear fuel rods 4 longitudinally and transversely in a configuration in which the nuclear fuel rods 4 are spaced transversely apart. The nuclear fuel rods 4 are preferably held at the nodes of a substantially regular imaginary lattice. To this end, each grid-spacer 14 includes fuel rod receiving cells, each cell being provided with support elements (e.g., spring(s) and / or boss(es)) to support the nuclear fuel rod 4 received in that cell.Preferably, each nuclear fuel rod 4 is received in a cell and / or each cell receives a single nuclear fuel rod 4. Thus, each spacer grid 14 has a cell associated with each of the nuclear fuel rods 4 of the nuclear fuel assembly 2, in which the associated nuclear fuel rod 4 is received.

[0041] Each grid-spacer 14 is located at a distance from the ends of the nuclear fuel rod bundle 4. The nuclear fuel rod bundle 4 extends downwards from the lowest grid-spacer 14, i.e. the grid-spacer 14 adjacent to the lower end 8, and upwards from the highest grid-spacer 14, i.e. the grid-spacer 14 adjacent to the upper end 10.

[0042] The lower end grid 18 is fixed to the guide tubes 12 and through which the nuclear fuel rods 4 pass, in particular all the nuclear fuel rods 4. The lower end grid 18 supports the nuclear fuel rods 4 longitudinally and transversely and is located between the lower end 8 and the spacer grid 14 closest to the lower end 8. The lower end grid 18 is located between the lower end of the nuclear fuel rod bundle 4 and the first spacer grid 14.

[0043] The lower end grid 18 reinforces the support frame 6. It also provides additional longitudinal and transverse support for the nuclear fuel rods 4. To this end, the lower end grid 18 includes fuel rod reception cells 4, each cell being equipped with support elements (e.g., spring(s) and / or boss(es)) to support the fuel rod received in that cell. Preferably, each fuel rod 4 is received in one cell and / or each cell receives a single fuel rod 4.

[0044] The upper end grid 20 is fixed to the guide tubes 12 and through which the nuclear fuel rods 4 pass. The upper end grid 20 supports the nuclear fuel rods 4 longitudinally and transversely and is located between the upper tip 10 and the spacer grid 14 closest to the upper tip 10. The upper end grid 20 is located between the last spacer grid 14 and the upper end of the nuclear fuel rod bundle 4.

[0045] The upper end grid 20 reinforces the support frame 6. It also provides additional longitudinal and transverse support for the nuclear fuel rods 4. To this end, the upper end grid 20 includes fuel rod reception cells, each cell being equipped with support elements (e.g., spring(s) and / or boss(es)) to support the fuel rod received in that cell. Preferably, each fuel rod 4 is received in one cell and / or each cell receives a single fuel rod 4.

[0046] Sections or intervals 15 are defined by the grid-spacers 14, along the nuclear fuel rod bundle 4. The intervals 15 are defined by the grid-spacers 14 between each other and with the lower end grid 18 and the upper end grid 20. Each interval 15 is defined along the nuclear fuel rod bundle 4 between two adjacent grid-spacers 14 or between an end grid (lower end grid 18 or upper end grid 20) and the grid-spacer 14 adjacent to that end grid.

[0047] In the following, the intervals 15 defined by the grid spacers 14 are ordered from the bottom to the top of the nuclear fuel assembly 2, i.e. from the lower end 8 to the upper end 10. The first interval 15 is that located between the lower end grid 18 and the first grid spacer 14, the second interval 15 is that defined between the first and second grid spacers 14 from the bottom of the nuclear fuel assembly 2, and so on.

[0048] The number of intervals 15 is a function of the number of grid-spacers 14. More precisely, the number of intervals 15 is equal to the number of grid-spacers 14 plus one.

[0049] In some examples, the nuclear fuel assembly 2 includes at least six grid-spacers 14, in particular eight grid-spacers 14.

[0050] Each grid-spacer 14 is configured to deflect the flow through the nuclear fuel assembly 2 along a main flow direction parallel to the longitudinal axis L. For this purpose, each grid-spacer 14 is for example provided with shapes or appendages to deflect the flow through the nuclear fuel assembly 2, i.e. to impart a transverse component to it at the outlet of the grid-spacer 14.

[0051] Preferably, the lower end grid 18 is configured to support the nuclear fuel rods 4 without ensuring mixing of the flow through the nuclear fuel assembly 2, i.e. without deflecting this flow.

[0052] Preferably, the upper end grid 20 is configured to support the nuclear fuel rods 4 without ensuring mixing of the flow through the nuclear fuel assembly 2, i.e. without deflecting this flow.

[0053] Each reinforcing grid 16 is fixed to the guide tubes 12 to reinforce the support skeleton 6, each reinforcing grid 16 being traversed by at least a part of the nuclear fuel rods 4, in particular by only a part of the nuclear fuel rods 4.

[0054] Each reinforcing grid 16 is fixed on at least two guide tubes 12 among the guide tubes 12 present to mechanically link these guide tubes 12 together in an interval 15 defined by the grid spacers 14, i.e. between two grid spacers 14 or between the lower end 8 and the first grid spacer 14 or between the last grid spacer 14 and the upper end 10. This reinforces the support skeleton 6 and limits the deformations of the support skeleton 6.

[0055] Each reinforcement grid 16 differs from a spacer grid 14 in that it does not provide longitudinal and transverse support for the nuclear fuel rods 4.

[0056] Each reinforcing grid 16 provides the support skeleton 6 with additional reinforcement and rigidity beyond that provided by each grid-spacer 14.

[0057] For example, at least one reinforcement grid 16 is located inside the nuclear fuel rod bundle 4 without extending to the periphery of the nuclear fuel rod bundle 4.

[0058] Each reinforcement grid 16 located inside the nuclear fuel rod bundle 4 extends into internal layers of nuclear fuel rods 4 without crossing one or more surface layers of nuclear fuel rods 4 of the nuclear fuel assembly 2.

[0059] Each reinforcement grid 16 located inside the nuclear fuel rod bundle 4 is crossed by only a part of the nuclear fuel rods 4.

[0060] In examples, each reinforcement grid 16 is located inside the nuclear fuel rod bundle 4. In Figure 1, the reinforcement grids 16 are shown in dashed lines to illustrate that they are located inside the nuclear fuel rod bundle 4.

[0061] In some examples, at least one reinforcement grid 16, and in particular each reinforcement grid 16, extends to the periphery of the nuclear fuel rod bundle 4, each of these reinforcement grids 16 comprising, for example, a peripheral belt surrounding the nuclear fuel rod bundle 4.

[0062] In some examples, the nuclear fuel assembly 2 includes one reinforcement grid 16, two reinforcement grids 16, three reinforcement grids 16, four reinforcement grids 16, or more than four reinforcement grids 16.

[0063] Each reinforcement grid 16 is located in one of the intervals 15 defined by the spacer grids 14 along the nuclear fuel rods 4.

[0064] In examples, each reinforcement grid 16 is located in a respective interval 15 among the intervals 15 defined by the grid-spacers 14 along the nuclear fuel rods 4.

[0065] Alternatively, the nuclear fuel assembly 2 includes at least one gap 15 in which two reinforcement grids 16 are arranged. In particular, the nuclear fuel assembly 2 includes one or two reinforcement grids 16 arranged in the first gap 15 from the bottom.

[0066] In some examples, the nuclear fuel assembly 2 includes at least one reinforcement grid 16 located in a first interval 15 defined by the grid-spacers 14 and / or a reinforcement grid 16 located in a third interval 15 defined by the grid-spacers 14 and / or a reinforcement grid 16 located in a sixth interval 15 defined by the grid-spacers 14 and / or a reinforcement grid 16 located in a seventh interval 15 defined by the grid-spacers 14 and / or a reinforcement grid 16 located in a ninth interval 15 defined by the grid-spacers 14.

[0067] As illustrated in Figure 1, the grids of fuel assembly 2 further include:

[0068] - one or more intermediate mixing grids 22, each intermediate mixing grid 22 being fixed to the guide tubes 12 and traversed by the nuclear fuel rods 4, each intermediate mixing grid 22 being provided with mixing fins configured to deflect a flow of fluid passing through the intermediate mixing grid 22; and optionally

[0069] - a debris grid 24 in which the lower ends of the nuclear fuel rods are engaged, the debris grid 24 being configured to support the lower ends of the nuclear fuel rods 4 transversely and to retain debris. Preferably, each intermediate mixing grid 22 is traversed by all the nuclear fuel rods 4 of the nuclear fuel assembly 2.

[0070] Preferably, each intermediate mixing grid 22 extends to the periphery of the nuclear fuel rod bundle 4, the intermediate mixing grid 22 comprising for example a peripheral belt surrounding the nuclear fuel rod bundle 4.

[0071] Preferably, each intermediate mixing grid 22 includes nuclear fuel rod reception cells 4. Preferably, each nuclear fuel rod 4 of the nuclear fuel assembly 2 is received in a cell of the intermediate mixing grid 22 and each cell receives a single nuclear fuel rod 4 from the nuclear fuel assembly 2. Thus, each intermediate mixing grid 22 has a cell associated with each of the nuclear fuel rods 4 of the assembly, in which the associated nuclear fuel rod 4 is received.

[0072] Each intermediate mixing grid 22 is located in one of the intervals 15 defined by the spacer grids 14. Preferably, if several intermediate mixing grids 22 are provided, each intermediate mixing grid 22 is located in a respective interval 15.

[0073] Each interval 15 is not necessarily equipped with an intermediate mixing grid 22.

[0074] The nuclear fuel assembly 2 includes, for example, at least one interval 15 in which an intermediate mixing grid 22 is disposed and at least one interval 15 without an intermediate mixing grid 22.

[0075] The nuclear fuel assembly 2 includes, for example, one intermediate mixing grid 22, two intermediate mixing grids 22, three intermediate mixing grids 22, four intermediate mixing grids 22 or more than four intermediate mixing grids 22.

[0076] The nuclear fuel assembly 2 includes, for example, at least two intermediate mixing grids 22.

[0077] The nuclear fuel assembly 2 includes, for example, an intermediate mixing grid 22 located in a fifth interval 15 defined by the grid-spacers 14, an intermediate mixing grid 22 located in a sixth interval 15 defined by the grid-spacers 14, an intermediate mixing grid 22 located in a seventh interval 15 defined by the grid-spacers 14 and / or an intermediate mixing grid 22 located in an eighth interval 15 defined by the grid-spacers 14. The nuclear fuel assembly 2 is not necessarily equipped with one or more mixing grids 22.

[0078] The debris grid 24 is configured to support the nuclear fuel rods 4 transversely and to retain debris present in the fluid circulating through the nuclear fuel assembly 2.

[0079] The debris grid 24 is preferably arranged below the lower end grid 18, i.e. upstream of the lower end grid 18 considering the direction of fluid flow along the nuclear fuel assembly 2.

[0080] Positioning the debris grid 24 close to the lower end of the nuclear fuel rods 4 allows debris to be filtered upstream, preventing it from entering the nuclear fuel rod bundle 4.

[0081] The debris grid 24 is optionally attached to one or more guide tubes 12. In this case, it reinforces the support skeleton 6. Alternatively, the debris grid 24 is threaded onto the lower ends of the nuclear fuel rods 4 without being attached to the guide tubes 12.

[0082] Preferably, the debris grid 24 is traversed by all the nuclear fuel rods 4 of the nuclear fuel assembly 2.

[0083] Preferably, the debris grid 24 extends to the periphery of the nuclear fuel rod bundle 4, intermediate mixing grid 22 comprising for example a peripheral belt surrounding the nuclear fuel rod bundle 4.

[0084] Preferably the debris grid 24 includes nuclear fuel rod reception cells 4. Preferably, each nuclear fuel rod 4 of the nuclear fuel assembly 2 is received in a cell of the debris grid 24 and / or each cell receives a single nuclear fuel rod 4 from the nuclear fuel assembly 2. Thus, each debris grid 24 has a cell associated with each of the nuclear fuel rods 4 of the assembly, in which the associated nuclear fuel rod 4 is received.

[0085] As illustrated in Figure 2, each grid-spacer 14 comprises a plurality of cells, each cell being intended to receive a nuclear fuel rod 4 or a guide tube 12. Each cell extends along a respective cell axis A, parallel to the longitudinal axis L of the nuclear fuel assembly 2.

[0086] Each grid-spacer 14 is, for example, formed of interlocking metal plates 26 to define the cells. Alternatively, each grid-spacer 14 is formed of a plurality of tubular cell elements fixed together to form a grid, each cell element defining a respective cell. Alternatively, the grid-spacer 14 is obtained by additive manufacturing (or 3D printing).

[0087] Each grid-spacer 14 comprises a plurality of pencil cells 28, each pencil cell 28 being intended to receive a respective nuclear fuel rod 4, the walls 30 of the pencil cell 28 being provided with support elements coming into contact with the external surface of the nuclear fuel rod 4 to hold it longitudinally and transversely.

[0088] The support elements of each cell for pencil 28 include, for example, at least one elastic spring 32 and at least one rigid boss 34, each spring 32 being, for example, configured to push the nuclear fuel pencil 4 against one or more bosses 34.

[0089] Each grid-spacer 14 includes at least several guide tube cells 36, each guide tube cell 36 being configured to receive a guide tube 12.

[0090] Each grid-spacer 14 is configured to ensure mixing of the fluid flowing through the nuclear fuel assembly 2, in particular by diverting the fluid passing through the grid-spacer 14.

[0091] Each spacer grid 14 includes, for example, mixing fins 40 arranged to deflect the fluid flowing through the spacer grid 14 mainly along the longitudinal axis L, to impose a flow with a transverse component at the outlet of the spacer grid 14.

[0092] A reinforcement grid 16 differs from a spacer grid 14 in particular in that it does not support the nuclear fuel rods 4 to hold them longitudinally and transversely.

[0093] Each reinforcement grid 16 includes, for example, one or more interlocking plates fixed to several guide tubes 12 to mechanically link them together.

[0094] The lower end grid 18 differs from a grid-spacer 14 in particular in that it is not configured to ensure mixing of the fluid circulating through the nuclear fuel assembly.

[0095] The lower end grid 18 is configured to provide longitudinal and transverse support for the nuclear fuel rods 4 without ensuring mixing of the fluid circulating through the nuclear fuel assembly 2.

[0096] The lower end grid 18 is for example formed from a single elementary grid or from two superimposed elementary grids to form the lower end grid 18.

[0097] Preferably, the lower end grid 18 exhibits greater radiation resistance than a spacer grid 14. The upper end grid 20 differs from a spacer grid 14, notably in that it is not configured to ensure mixing of the fluid circulating through the nuclear fuel assembly. The upper end grid 20 is configured to provide longitudinal and transverse support for the nuclear fuel rods 4 without ensuring mixing of the fluid circulating through the nuclear fuel assembly 2.

[0098] The upper end grid 20 is for example formed from a single elementary grid or from two superimposed elementary grids to form the upper end grid 20.

[0099] Preferably, the upper end grid 20 has a higher resistance to radiation than a grid-spacer 14.

[0100] An intermediate mixing grid 22 differs from a spacer grid 14 in particular in that it does not provide support for the nuclear fuel rods 4 to hold them longitudinally or transversely.

[0101] An intermediate mixing grid 22 is configured to deflect the fluid flowing through the intermediate mixing grid 22 mainly along the longitudinal axis L, to impose a flow with a transverse component at the outlet of the intermediate mixing grid 22.

[0102] An intermediate mixing grid 22 is for example provided with mixing appendages, for example mixing fins similar to those of the spacer grid 14 illustrated in Figure 2.

[0103] An intermediate mixing grid 22 is for example formed of intersecting metal plates 26 provided on their upper edge with mixing fins similar to those of the spacer grid 14 illustrated in Figure 2.

[0104] Other shapes or appendages can be formed to deflect the fluid to impose a flow with a transverse component at the outlet of the intermediate mixing grid 22, such as inclined channels formed on intersecting metal plates 26 forming the intermediate mixing grid 22.

[0105] Each intermediate mixing grid 22 is optionally provided with bosses 34 to limit the transverse movements of the nuclear fuel rods 4, for example to prevent contact of the nuclear fuel rods 4 with mixing appendages such as fins 40.

[0106] Each intermediate mixing grid 22 is traversed by all the nuclear fuel rods 4 of the nuclear fuel assembly 2. In particular, it has a respective receiving cell for each nuclear fuel rod 4 of the nuclear fuel assembly 2. This ensures a mixing effect of the fluid distributed over the entire section of the intermediate mixing grid 22, i.e. on and around all the nuclear fuel rods 4, unlike a grid configured to achieve a mixing of the fluid but which would only receive some of the nuclear fuel rods, which would achieve an incomplete or heterogeneous mixing of the fluid over its entire section.

[0107] The debris grid 24 differs from a spacer grid 14 in particular in that it is configured to retain debris present in the coolant circulating through the nuclear fuel assembly 2.

[0108] As illustrated in Figure 1, the nuclear fuel assembly 2 comprises eight spacer grids 14 defining, between themselves and with the end grids (lower end grid 18 and upper end grid 20) nine intervals 15, a reinforcement grid 16 located in the first interval 15, a lower end grid 18, an upper end grid 20, four intermediate mixing grids 22 located in the fifth, sixth, seventh and eighth intervals 15 and a debris grid 24 located at the lower end of the nuclear fuel rod bundle 4.

[0109] Figure 1 illustrates an example of a nuclear fuel assembly 2 with optional grids. Other examples including only some of the grids shown in Figure 1 are possible.

[0110] In examples, as illustrated in Figure 3, the grids of the nuclear fuel assembly 2 comprise, preferably exclusively, eight grid-spacers 14, a reinforcement grid 16 located in a first interval 15 defined by the grid-spacers 14, the lower end grid 18, the upper end grid 20 and four intermediate mixing grids 22 located preferably in the fifth, sixth, seventh and eighth intervals 15 defined by the grid-spacers 14.

[0111] This particular example differs from that in Figure 1 in that the nuclear fuel assembly 2 does not include a reinforcement grid 16 in the ninth gap 15 and does not include a debris grid 24.

[0112] In particular examples, as illustrated in Figure 4, the grids of the nuclear fuel assembly 2 comprise, preferably exclusively, eight spacer grids 14, two reinforcement grids 16 located in the first and ninth intervals 15 defined by the spacer grids 14, the lower end grid 18, the upper end grid 20 and four intermediate mixing grids 22 located preferably in the fifth, sixth, seventh and eighth intervals 15 defined by the spacer grids 14.

[0113] This particular example differs from that of Figure 1 in that the nuclear fuel assembly 2 does not include a debris grid 24. In particular examples, as illustrated in Figure 5, the grids of the nuclear fuel assembly 2 include, preferably exclusively, eight spacer grids 14, a reinforcement grid 16 located in the ninth interval 15 defined by the spacer grids 14, the lower end grid 18, the upper end grid 20 and four intermediate mixing grids 22 located in the fifth, sixth, seventh and eighth intervals 15 defined by the spacer grids 14.

[0114] This particular example differs from that in Figure 1 in that the nuclear fuel assembly 2 includes a single reinforcement grid 16, located in the ninth gap 15, and does not include a debris grid 24.

[0115] In variants of the nuclear fuel assemblies in Figures 3 to 5, the grids of nuclear fuel assembly 2 further include a debris grid 24.

[0116] In other examples, the grids of nuclear fuel assembly 2 do not include an intermediate mixing grid 22.

[0117] In examples, as illustrated in Figure 6, the grids of the nuclear fuel assembly 2 include, for example, spacer grids 14, preferably at least six spacer grids 14 and in particular eight spacer grids 14, one or more reinforcement grids 16, in particular one, two, three or four reinforcement grids 16 each located, for example, in an interval 15 chosen in particular from the first, third, sixth, seventh and ninth intervals 15 defined by the spacer grids 14, the lower end grid 18 and the upper end grid 20, and optionally a debris grid 24, without including an intermediate mixing grid 22.

[0118] In examples, as illustrated in Figures 7 to 9, the grids of the nuclear fuel assembly 2 comprise spacer grids 14, preferably at least six spacer grids 14 and in particular eight spacer grids 14, one or more reinforcement grids 16, in particular one, two, three or four reinforcement grids 16 each located for example in an interval 15 chosen from the first, third, sixth and ninth intervals 15 defined by the spacer grids 14, the lower end grid 18 and the upper end grid 20, without including an intermediate mixing grid 22 or a debris grid 24.

[0119] In one particular example, as illustrated in Figure 7, the grids of the nuclear fuel assembly 2 comprise, preferably exclusively, eight spacer grids 14, four reinforcement grids 16 located in the first, third, sixth and ninth intervals 15 defined by the spacer grids 14, the lower end grid 18 and the upper end grid 20. In particular examples, as illustrated in Figure 8, the grids of the nuclear fuel assembly 2 comprise, preferably exclusively, eight spacer grids 14, one reinforcement grid 16 located in the ninth interval 15 defined by the spacer grids 14, the lower end grid 18 and the upper end grid 20.

[0120] In particular examples, as illustrated in Figure 9, the grids of the nuclear fuel assembly 2 comprise, preferably exclusively, eight grid-spacers 14, a reinforcement grid 16 located in the first gap defined by the grid-spacers 14, the lower end grid 18 and the upper end grid 20.

[0121] Providing one or more intermediate mixing grids 22 and / or a debris grid 24 makes it possible to improve the rigidity of the nuclear fuel assembly 2 without excessively impacting the resistance to the flow of a fluid through the nuclear fuel assembly, while possibly allowing to promote mixing of the fluid and / or filtering of debris that may be present in the fluid.

[0122] The grids can be combined and arranged to obtain a satisfactory compromise between the rigidity of the support skeleton 6, to avoid deformations of the nuclear fuel assembly 2 in normal use or in the event of an incident, for example an earthquake, and the resistance to the flow of the cooling fluid in the nuclear fuel assembly, with the addition of functions such as mixing or filtering debris.

[0123] The specific examples indicated above allow for particularly satisfactory compromises to be obtained.

Claims

DEMANDS 1. A nuclear fuel assembly comprising nuclear fuel rods (4) extending along a longitudinal axis (L) and a support skeleton (6) configured to carry the nuclear fuel rods (4), the support skeleton (6) comprising a lower end (8) and an upper end (10) spaced along the longitudinal axis (L), a plurality of guide tubes (12) extending along the longitudinal axis (L) and connecting the lower end (8) and the upper end (10) to each other, and grids distributed between the lower end (8) and the upper end (10), the grids including: - grid-spacers (14) fixed on the guide tubes (12) and through which the nuclear fuel rods (4) pass, each grid-spacer (14) being configured to support the nuclear fuel rods (4) longitudinally and transversely; - at least one reinforcing grid (16) fixed on the guide tubes (12) to reinforce the support skeleton (6), each reinforcing grid (16) being crossed by at least part of the nuclear fuel rods (4); - a lower end grid (18) fixed to the guide tubes (12) and through which the nuclear fuel rods (4) pass, the lower end grid (18) supporting the nuclear fuel rods (4) longitudinally and transversely and being located between the lower end (8) and the spacer grid (14) closest to the lower end; and - an upper end grid (20) fixed to the guide tubes (12) and through which the nuclear fuel rods (4) pass, the upper end grid (20) supporting the nuclear fuel rods (4) longitudinally and transversely and being located between the upper tip (10) and the spacer grid (14) closest to the upper tip; the spacer grids (14) defining, between themselves and with the lower end grid (18) and the upper end grid (20), a plurality of gaps (15) along the nuclear fuel rods (4); the grids further including one or more intermediate mixing grids (22), each intermediate mixing grid (22) being fixed to the guide tubes (12) and through which all the nuclear fuel rods (4) pass, each intermediate mixing grid (22) being provided with mixing fins configured to deflect a flow of fluid passing through the intermediate mixing grid (22).

2. Nuclear fuel assembly according to claim 1, comprising at least six grid-spacers (14), in particular eight grid-spacers (14).

3. Nuclear fuel assembly according to claim 1 or claim 2, comprising one reinforcement grid (16), two reinforcement grids (16), three reinforcement grids (16) or four reinforcement grids (16).

4. Nuclear fuel assembly according to any one of claims 1 to 3, comprising a reinforcement grid (16) located in a first interval defined by the grid-spacers (14) and / or a reinforcement grid (16) located in a third interval defined by the grid-spacers (14) and / or a reinforcement grid (16) located in a sixth interval defined by the grid-spacers (14) and / or a reinforcement grid (16) located in a seventh interval defined by the grid-spacers (14) and / or a reinforcement grid (16) located in a ninth interval defined by the grid-spacers (14).

5. Nuclear fuel assembly according to any one of claims 1 to 4, comprising at least two intermediate mixing grids (22), in particular four intermediate mixing grids (22).

6. Nuclear fuel assembly according to any one of claims 1 to 5, comprising an intermediate mixing grid (22) located in a fifth interval defined by the grid-spacers (14), an intermediate mixing grid (22) located in a sixth interval defined by the grid-spacers (14), an intermediate mixing grid (22) located in a seventh interval defined by the grid-spacers (14), and an intermediate mixing grid (22) located in an eighth interval defined by the grid-spacers (14).

7. Nuclear fuel assembly according to any one of claims 1 to 6, comprising eight grid-spacers (14), a reinforcement grid (16) located in a first interval defined by the grid-spacers (14), the lower end grid (18), the upper end grid (20) and four intermediate mixing grids (22) located in the fifth, sixth, seventh and eighth intervals defined by the grid-spacers (14).

8. Nuclear fuel assembly according to any one of claims 1 to 6, comprising eight grid-spacers (14), two reinforcement grids (16) located in the first and ninth intervals defined by the grid-spacers (14), the lower end grid (18), the upper end grid (20) and four intermediate mixing grids (22) located in the fifth, sixth, seventh and eighth intervals defined by the grid-spacers (14).

9. Nuclear fuel assembly according to any one of claims 1 to 6, comprising eight grid-spacers (14), four reinforcement grids (16) located in the first, third, sixth and ninth intervals defined by the grid-spacers (14), the lower end grid (18) and the upper end grid (20).

10. Nuclear fuel assembly according to any one of claims 1 to 6, comprising eight grid-spacers (14), a reinforcement grid (16) located in the first or ninth gap (15) defined by the grid-spacers (14), the lower end grid (18) and the upper end grid (20).

11. Nuclear fuel assembly according to any one of claims 1 to 6, comprising eight grid-spacers (14), a reinforcement grid (16) located in the ninth interval defined by the grid-spacers (14), the lower end grid (18), the upper end grid (20) and four intermediate mixing grids (22) located in the fifth, sixth, seventh and eighth intervals defined by the grid-spacers (14).

12. Nuclear fuel assembly according to any one of claims 1 to 11, wherein each nuclear fuel rod (4) is received in a cell of the intermediate mixing grid (22) and each cell receives a single nuclear fuel rod (4).

13. Nuclear fuel assembly according to any one of claims 1 to 12, the grids further including a debris grid (24) in which the lower ends of the nuclear fuel rods are engaged, the debris grid (24) being configured to support the lower ends of the nuclear fuel rods (4) transversely and to retain debris.

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