Nuclear fuel rod cladding and method for producing such cladding

Abstract

The invention relates to cladding comprising a tube (14) that extends along a longitudinal axis and a tubular inner lining (18) housed inside the tube (14), wherein the tube (14) comprises at least one reinforced layer (16) made of a composite material that comprises a ceramic matrix reinforced with ceramic reinforcing fibres, and wherein the thickness of the tube (14) is between 280 µm and 600 µm.

Description

[0001] Nuclear fuel rod cladding and method of manufacturing such cladding

[0002] The present invention relates to the field of nuclear fuel claddings (hereinafter also referred to as "claddings") intended to contain nuclear fuel, in particular nuclear fuel rod claddings, and their manufacturing method.

[0003] Nuclear fuel including fissile material is generally contained in a sealed sheath which prevents the dispersion of the nuclear fuel.

[0004] Nuclear fuel assemblies used in light water or heavy water reactors generally comprise a bundle of nuclear fuel rods, each nuclear fuel rod comprising a tubular cladding containing nuclear fuel, the cladding being closed at each of its two ends by a respective cap.

[0005] The cladding of nuclear fuel assemblies is made, for example, of zirconium-based alloys. Such zirconium-based alloys exhibit high performance under normal operating conditions in nuclear reactors.

[0006] However, they can reach their limits, particularly in terms of temperature, during severe accident conditions, such as a loss of coolant accident (LOCA).

[0007] During such an event, the temperature in the core of the nuclear reactor can reach more than 800°C and the coolant is mainly in the form of water vapor.

[0008] This can cause rapid degradation of the cladding of a nuclear fuel rod, including the release of hydrogen and rapid oxidation of the cladding leading to its embrittlement or even its bursting, and therefore to the release of nuclear fuel from the cladding.

[0009] One of the aims of the invention is to propose a sheath which exhibits improved behavior under normal conditions and accident conditions, while exhibiting improved wear resistance.

[0010] To this end, the invention provides a nuclear fuel rod cladding, the cladding comprising a tube extending along a longitudinal axis and a tubular internal lining received inside the tube, the tube comprising at least one reinforced layer made of a composite material comprising a ceramic matrix reinforced with ceramic reinforcing fibers, the thickness of the tube being between 280 μm and 600 μm. The tube made of a composite material with a ceramic matrix reinforced with ceramic fibers makes it possible to obtain a cladding which is very strong, in particular under accident conditions. The risk of rupture of the cladding and, in so doing, of release of nuclear fuel from the cladding, is therefore reduced. For example, a silicon carbide matrix reinforced with silicon carbide and / or carbon reinforcing fibers is particularly strong.

[0011] The internal lining provided in the tube improves the seal and in particular prevents the release of fission gases outside the cladding.

[0012] According to particular embodiments, the sheath comprises one or more of the following optional features, taken individually or in all technically possible combinations:

[0013] - the tube comprises a single reinforced layer, the thickness of the tube being between 280 pm and 340 pm;

[0014] - the tube comprises exactly two reinforced layers, the thickness of the tube being between 450 pm and 600 pm;

[0015] - the ceramic matrix is ​​made of silicon carbide and / or the ceramic reinforcing fibers are made of silicon carbide and / or carbon;

[0016] - one or each reinforced layer is formed with filament winding of the ceramic reinforcing fibers;

[0017] - one or each reinforced layer is formed with filament winding of the ceramic reinforcing fibers without interlacing of the ceramic reinforcing fibers;

[0018] - one or each reinforced layer is formed with filament winding of the ceramic reinforcing fibers with interlacing of the ceramic reinforcing fibers, for example by braiding the ceramic reinforcing fibers;

[0019] - the tube comprises at least one unreinforced layer, preferably made of the same material as the ceramic matrix;

[0020] - the sheath comprises an internal unreinforced layer covering the reinforced layer(s) on the internal face side of the tube and / or an external unreinforced layer covering the reinforced layer(s) on the external face side of the tube and / or an intermediate unreinforced layer interposed between two reinforced layers of the tube;

[0021] - the tube is formed exclusively from the ceramic matrix and ceramic reinforcing fibers;

[0022] - the internal lining has a thickness of between 50 pm and 200 pm, in particular between 50 pm and 150 pm;

[0023] - the internal lining is metallic; - the internal lining is made of pure zirconium or a zirconium-based alloy containing at least 95% by weight of zirconium;

[0024] - the internal lining is pre-oxidized;

[0025] - the sheath comprises an external protective layer covering an external surface of the tube;

[0026] - the outer protective layer is metallic;

[0027] - the outer protective layer is made of pure chromium or a chromium-based alloy containing at least 85% by weight of chromium or of pure zirconium or a zirconium-based alloy containing at least 85% by weight of zirconium or of pure titanium or a titanium-based alloy containing at least 80% by weight of titanium;

[0028] - the outer protective layer has a thickness between 5 pm and 40 pm.

[0029] The invention also relates to a nuclear fuel rod comprising a cladding according to any one of the preceding claims, nuclear fuel received inside the inner lining of the cladding, and two plugs, each plug closing a respective axial end of the cladding.

[0030] According to particular embodiments, the nuclear fuel rod comprises one or more of the following optional features, taken individually or in all technically possible combinations:

[0031] - each plug is made of composite material comprising a ceramic matrix reinforced with ceramic reinforcing fibers, in particular a silicon carbide matrix reinforced with silicon carbide and / or carbon reinforcing fibers;

[0032] - the nuclear fuel rod comprises two internal sealing elements, each closing a respective end of the internal liner;

[0033] - the gas contained in the inner lining is helium and / or the pressure of the gas inside the inner lining is equal to or less than 200 bars, preferably equal to or less than 100 bars, more preferably equal to or less than 20 bars.

[0034] The invention and its advantages will be better understood upon reading the following description, given solely as a non-limiting example, and made with reference to the appended drawings, in which:

[0035] - Figure 1 is a schematic longitudinal sectional view of a nuclear fuel rod illustrating a cladding of the nuclear fuel rod;

[0036] - Figure 2 is a schematic sectional view of the nuclear fuel rod of Figure 1, along II - II in Figure 1;

[0037] - Figure 3 is a graph showing a curve illustrating the evolution of the maximum tensile circumferential stress at the internal surface of a tube of the nuclear fuel rod cladding, as a function of the thickness of the tube, under operating conditions; and

[0038] - Figure 4 is a graph showing a curve illustrating the evolution of the maximum tensile circumferential stress at the internal surface of a nuclear fuel rod cladding tube, as a function of the tube thickness, during an inter-cycle cold shutdown.

[0039] Figure 1 illustrates an example of a nuclear fuel rod 2 intended for use in a light water reactor, in particular a pressurized water reactor (or PWR for "Pressurized Water Reactor") or a boiling water reactor (or BWR for "Boiling Water Reactor"), a "VVER" type reactor, a "RBMK" type reactor, or a heavy water reactor, for example of the "CANDll" type.

[0040] Nuclear fuel rod 2 has the shape of an elongated rod along a longitudinal axis A.

[0041] The nuclear fuel rod 2 comprises a cladding 4 containing nuclear fuel. The cladding 4 is tubular and extends along the longitudinal axis A.

[0042] The sheath 4 is for example closed at its axial ends by two plugs 6, each plug 6 closing a respective axial end of the sheath 4, preferably in a sealed manner.

[0043] One of the two caps 6 or, preferably, each cap 6 has a gripping finger 6A

[0044] Each gripping finger 6A allows the nuclear fuel rod 2 to be gripped, for example during the manufacture of the nuclear fuel rod 2, during the insertion of the nuclear fuel rod 2 into a nuclear fuel assembly, or for the extraction of the nuclear fuel rod 2 from a nuclear fuel assembly.

[0045] The nuclear fuel is for example in the form of a stack of pellets 8 stacked axially inside the cladding 4, each pellet 8 containing fissile material. The stack of pellets 8 is also called a “fissile column”.

[0046] The nuclear fuel rod 2 comprises a spring 10 arranged inside the cladding 4, between the stack of pellets 8 and one of the plugs 6, to push the stack of pellets 8 towards the other plug 6. A vacuum or plenum 12 is present between the stack of pellets 8 and the plug 6 on which the spring 10 bears.

[0047] The sheath 4 has for example an external diameter of between 8 mm and 15 mm, in particular between 9 mm and 13 mm, and / or a length of between 1 m and 5 m, in particular between 2 m and 5 m. As visible in Figures 1 and 2, the sheath 4 comprises a tube 14 having an internal surface 14A and an external surface 14B. The internal surface 14A faces the inside of the tube 14 and the external surface faces the outside of the tube 14. The tube 14 extends along the longitudinal axis A.

[0048] The tube 14 is for example closed at its axial ends by the two plugs 6, each plug 6 closing a respective axial end of the tube 14, preferably in a sealed manner.

[0049] The tube 14 is at least partly made of a ceramic matrix composite material reinforced with ceramic reinforcing fibers.

[0050] Ceramic reinforcing fibers are, for example, made of carbon or silicon carbide (SiC), preferably of nuclear quality. Nuclear-grade silicon carbide reinforcing fibers are silicon carbide fibers with high crystallinity and / or stoichiometric or quasi-stoichiometric, i.e. with a C / Si ratio in atomic percentage (at%) of between 1.00 and 1.10 (i.e. 1.00 < C%at / Si%at < 1.10) and an oxygen content of less than 1 mass percent (mass%), in particular so-called third-generation silicon carbide fibers. Such silicon carbide reinforcing fibers have a high tolerance to irradiation.

[0051] The ceramic matrix is ​​for example made of silicon carbide. The tube 14 is in this case made at least in part of a composite material with a ceramic matrix of silicon carbide reinforced with silicon carbide fibers (SiC / SiCf) or carbon fibers (SiC / Cf).

[0052] The sheath 4 comprises a tubular inner liner 18 received inside the tube 14. The inner liner 18 is preferably in contact with the inner surface 14A of the tube 14.

[0053] The tube 14 is formed of one or more superimposed layers. The superimposed layers are concentric.

[0054] The tube 14 comprises one or more reinforced layers 16.

[0055] Each reinforced layer 16 is made from the composite material comprising the ceramic matrix reinforced with the ceramic reinforcing fibers.

[0056] Each reinforced layer 16 is made of the ceramic matrix composite material, for example silicon carbide (SiC), reinforced with ceramic reinforcing fibers, preferably silicon carbide (SiC) and / or carbon (C).

[0057] The tube 14 comprises, for example, a single reinforced layer 16 or exactly two reinforced layers 16.

[0058] The thickness of the tube 14, taken between its internal surface 14A and its external surface 14B, is between 280 μm and 600 μm. The provision of a single reinforced layer 16 makes it possible to obtain a sheath 4 having sufficient mechanical strength, while limiting the thickness of the sheath 4. The provision of two superimposed reinforced layers 16 makes it possible to improve the mechanical strength of the sheath 4, while having a sheath 4 whose thickness is contained.

[0059] In each reinforced layer 16, the reinforcing fibers are, for example, arranged in a two- or three-dimensional arrangement.

[0060] The reinforcing fibers of one or each reinforced layer 16 are for example deposited by filament winding (winding a wire under tension around a rotating mandrel).

[0061] This makes it possible to achieve a good surface finish and a high volume fraction of fibers to obtain a tube with the mechanical characteristics required for a nuclear fuel rod cladding.

[0062] In each reinforced layer 16, the reinforcing fibers are for example wound helically around the longitudinal axis A, preferably making a winding angle ±0 of between 30° and 60° with the direction of the longitudinal axis A, in particular a winding angle ±0 of approximately 45° with the direction of the longitudinal axis A.

[0063] Filament winding is carried out without interlacing of the reinforcing fibers, in particular without braiding of the reinforcing fibers, and / or with interlacing of the reinforcing fibers, for example with braiding of the reinforcing fibers.

[0064] Advantageously, the reinforcing fibers are pre-coated with a fiber / matrix interphase (intermediate thin layer with a “mechanical fuse” role) before being incorporated into a ceramic matrix, for example by chemical vapor infiltration (or CVI). This improves the damage tolerance of the composite material of the tube 14.

[0065] Optionally, the tube 14 comprises at least one unreinforced layer 20.

[0066] Each non-reinforced layer 20 is preferably made in the ceramic matrix not reinforced by ceramic reinforcing fibers, or in other words in the same material as the ceramic matrix of the reinforced layer(s) 16 not reinforced by ceramic reinforcing fibers.

[0067] The production of an unreinforced layer 20 in the same material as that of the matrix of the reinforced layers 16 allows good cohesion between the layers of the tube 14, and makes it possible to limit the stresses of thermal origin and linked to the differences in behavior under irradiation.

[0068] The tube 14 comprises for example an internal unreinforced layer 20 covering the reinforced layer(s) 16 on the side of the internal surface 14A of the tube 14, and preferably defining the internal surface 14A of the tube 14, and / or an external unreinforced layer 20 covering the reinforced layer(s) 16 on the side of the external face 14B of the tube 14, and preferably defining the external surface 14B of the tube 14.

[0069] Internal and external unreinforced layers 20 make it possible to control the dimensions of the tube 14, in particular the internal diameter of the tube 14 and the external diameter of the tube 14, and the surface conditions of the internal 14A and external 14B surfaces of the tube 14, for example by machining and / or lapping the unreinforced layers 20 to form the internal 14A and external 14B surfaces of the tube 14.

[0070] When the tube 14 comprises two reinforced layers 16, optionally, the tube 14 comprises an intermediate unreinforced layer 20 interposed between the two reinforced layers 16.

[0071] Preferably, the tube 14 is formed exclusively from the ceramic matrix and the ceramic reinforcing fibers. The tube 14 comprises at least one reinforced layer 16 in which the ceramic matrix is ​​reinforced by the ceramic reinforcing fibers, thereby forming a composite material, and, optionally, at least one unreinforced layer 20 made with the ceramic matrix not reinforced by the ceramic reinforcing fibers.

[0072] A method of manufacturing the tube 14 comprises, for example, producing the reinforced layer(s) 16 by filament winding the reinforcing fibers and then infiltrating the reinforcing fibers with the matrix, for example by chemical vapor infiltration, then depositing an internal non-reinforced layer 20 and / or an external non-reinforced layer 20, then, preferably, reworking the internal surface 14A and / or the external surface 14B of the tube 14, for example by machining and / or lapping.

[0073] To provide an intermediate unreinforced layer 20, the manufacturing method comprises, for example, producing a reinforced layer 16, then depositing the intermediate unreinforced layer 20 over said reinforced layer 16, then producing the other reinforced layer 16 over the intermediate unreinforced layer 20.

[0074] In the following, the thicknesses are taken radially relative to the longitudinal axis A.

[0075] In particular, the tube 14 has a thickness (E1 in Figure 2) taken between the internal surface 14A and the external surface 14B of the tube 14, including the reinforced layer(s) 16, and the possible non-reinforced layer(s) 20.

[0076] In exemplary embodiments, the tube 14 comprises a single reinforced layer 16, the thickness E1 of the tube 14 being between 280 μm and 340 μm.

[0077] In exemplary embodiments, the tube 14 comprises exactly two reinforced layers 16, the thickness E1 of the tube 14 being between 450 μm and 600 μm.

[0078] The inner lining 18 is preferably in contact with the inner surface 14A of the tube 14. The sheath 4 is free of radial play between the tube 14 and the inner lining 18.

[0079] The inner lining 18 is preferably solid. The inner lining 18 is fluid-tight, particularly gas-tight.

[0080] The internal lining 18 is for example metallic, i.e. made of metal.

[0081] The internal lining 18 is for example made of pure zirconium or of a zirconium-based alloy containing at least 95% by weight of zirconium.

[0082] The term "pure zirconium" refers to a material containing at least 99% zirconium by weight. The remainder of the material consists of unavoidable impurities.

[0083] The zirconium-based alloy is for example chosen from one of the known alloys such as M5, ZIRLO, E110, HANA, N36, Zr-2.5Nb, Zircaloy-2 and Zircaloy 4.

[0084] The internal lining 18 has a thickness (E2 in Figure 2) of between 50 pm and 200 pm, in particular between 50 pm and 150 pm.

[0085] The internal lining 18 is for example pre-oxidized in a controlled environment to optimize its sealing characteristics and / or permeability to gas such as tritium.

[0086] The pre-oxidized inner liner 18 is intentionally manufactured with an oxide layer. The oxide layer is present on the inner liner 18 before the nuclear fuel rod is used, particularly in a nuclear reactor.

[0087] The sheath 4 and / or its caps 6 optionally comprise an external protective layer 22. The protective layer 22 covers the external surface 14B of the tube 14.

[0088] The outer protective layer 22 is, for example, metallic.

[0089] The outer protective layer 22 is made of pure chromium or a chromium-based alloy containing at least 85% by weight of chromium or of pure zirconium or a zirconium-based alloy containing at least 85% by weight of zirconium or of pure titanium or a titanium-based alloy containing at least 80% by weight of titanium.

[0090] The term "pure chromium" refers to a material comprising at least 99% chromium by weight. The remainder of the material consists of unavoidable impurities.

[0091] The term "pure titanium" refers to a material comprising at least 99% titanium by weight. The remainder of the material consists of unavoidable impurities.

[0092] Preferably, the outer protective layer 22 has a thickness (E3 in Figure 2) of between 5 μm and 40 μm.

[0093] As visible in Figure 1, the sheath 4 preferably comprises two sealing elements 24 each closing a respective axial end of the internal lining 18, preferably in a fluid-tight manner, in particular gas-tight.

[0094] Each closure element 24 is for example fixed to the inner lining 18 by being welded and / or brazed and / or glued and / or inserted by force at the corresponding axial end of the inner lining 18. Preferably, when closing the inner lining 18, the nature and pressure of the gas contained in the inner lining 18 are controlled, for example by using a valve. The gas is for example helium. The pressure is advantageously equal to or less than 200 bars, preferably equal to or less than 100 bars, more preferably equal to or less than 20 bars. The pressure is for example 15 bars.

[0095] In exemplary embodiments, each closure element 24 is metallic. In particular, each closure element 24 is made of the same material as the inner liner 18, and is preferably welded thereto, and also, optionally, welded, brazed, glued and / or force-fitted.

[0096] The sealing elements 24 closing the internal lining 18 are for example distinct from the plugs 6 closing the axial ends of the tube 14.

[0097] In exemplary embodiments, each plug 6 is made of composite material comprising a matrix reinforced with reinforcing fibers, in particular a ceramic matrix reinforced with reinforcing fibers, even more in particular a silicon carbide matrix reinforced with silicon carbide fibers.

[0098] Each plug 6 is for example fixed to the tube 14 by being brazed and / or glued and / or inserted by force and / or pinned to the corresponding axial end of the tube 14. In advantageous embodiments, each plug 6 is brazed or pinned and brazed.

[0099] When an external protective layer 22 is provided, this covers the tube 14 over its entire length, and preferably partially or completely covers each plug 6, in particular at the junction between the plug 6 and the tube 14.

[0100] According to a method of manufacturing the nuclear fuel rod 2, the tube 14 and the internal lining 18 are for example manufactured separately, then the internal lining 18 is closed, its gas tightness preferably being checked, then the previously closed internal lining 18 is inserted into the tube 14.

[0101] In particular, the manufacturing method comprises manufacturing the tube 14, obtaining the inner lining 18, filling the inner lining 18 with nuclear fuel, for example by inserting the pellets 8 into the inner lining 18, closing the inner lining 18 at its axial ends using the sealing elements 24, preferably checking the gas tightness of the closed inner lining 18, inserting the filled and closed inner lining 18 into the tube 14, then closing the tube 14 at its axial ends by the plugs 6.

[0102] The manufacture of the tube 14 comprises the production of the reinforced layer(s) 16, in particular by filament winding of reinforcing fibers, for example on a cylindrical shape (or mandrel). The filament winding is carried out without interlacing of the reinforcing fibers and / or with interlacing of the reinforcing fibers, for example by braiding.

[0103] The reinforcing fibers are, for example, impregnated before deposition by filament winding, for example by passing through an impregnation bath, and / or after their filament winding, for example by infiltration, in particular by chemical vapor infiltration.

[0104] The method of manufacturing the tube 14 comprises the production of a single reinforced layer 16 or exactly two reinforced layers 16.

[0105] Optionally, the method of manufacturing the tube 14 comprises the deposition of an intermediate non-reinforced layer 20 between the two reinforced layers 16. In this case, the manufacturing method comprises, for example, the production of a reinforced layer 16, then the deposition of an non-reinforced layer 20 on the reinforced layer 16, then the production of another reinforced layer over the non-reinforced layer 20 which is interposed between the two reinforced layers 16.

[0106] Optionally, the production of the reinforced layer(s) 16 is followed by deposition of an internal non-reinforced layer 20 and / or an external non-reinforced layer 20. Optionally, the manufacturing method comprises the deposition of an external protective layer 22 on the external surface 14B of the tube, for example by very low pressure plasma spraying (or VLPPS for “Vacuum Low Pressure Plasma Spraying”) or by physical vapor deposition (or PVD for “Physical Vapor Deposition”).

[0107] The deposition of the external protective layer 22 is for example carried out when the internal lining 18 containing the fuel is present inside the tube 14, and preferably after the tube 14 has been closed using the plugs 6.

[0108] In embodiments, as described above, the tube 14 is formed to a cylindrical shape before inserting the filled and sealed inner liner 18 inside the tube 14.

[0109] In other embodiments, the inner liner 18 is filled with nuclear fuel and closed at its axial ends by the sealing elements 24, then the tube 14 is formed around the inner liner 18. The inner liner 18 serves as a form for the production of the tube 14.

[0110] The proposed sheath 4 exhibits improved behavior under normal conditions of use and in accident conditions.

[0111] The tube 14 made of a ceramic matrix composite material reinforced with silicon carbide fibers makes it possible to obtain a cladding 6 which is very resistant, particularly in accident conditions. The risk of rupture due to the release of nuclear fuel from the cladding is therefore reduced. A silicon carbide matrix reinforced with silicon carbide reinforcing fibers is particularly resistant.

[0112] The internal lining 18 provided in the tube 14 makes it possible to improve the sealing and in particular to prevent the release of fission gases outside the sheath 6.

[0113] The addition of a protective coating (i.e. the external protective layer 22) covering the external surface of the tube 14 of the cladding 4, and preferably also on the plugs 6, makes it possible to limit the risk of hydrothermal corrosion by the fluid circulating in the nuclear reactor around the nuclear fuel rod 2. In particular, it makes it possible to avoid or at least limit the erosion of the composite material of the tube 14 and / or to limit the release of silicon into the primary fluid (when the tube 14 comprises silicon) and / or to avoid or at least limit deposits, in particular CRUD deposits (from the English “Chalk River Unidentified Deposits”).

[0114] The thickness E1 of the tube 14, between 280 pm and 340 pm if the tube 14 comprises a single reinforced layer 16 or between 450 pm and 600 pm if the tube 14 comprises exactly two reinforced layers 16, makes it possible to form a cladding 4 having dimensions (in particular internal diameter and external diameter) compatible with existing nuclear fuel pellets.

[0115] The thickness E1 of the tube 14 of between 280 pm and 340 pm if the tube 14 comprises a single reinforced layer 16 or between 450 pm and 600 pm if the tube 14 comprises exactly two reinforced layers 16 allows, with the tube 14 made at least partly of composite material, in particular of the SiC / SiCf or SiC / Cf type, a satisfactory compromise between heat transfer and mechanical strength in normal operation and in accidental operation, with good tolerance to damage that may be suffered by the sheath 4.

[0116] The lower limit of the thickness E1 of the tube 14 makes it possible to withstand stress and deformation levels observed during the use of the cladding 4 in a nuclear reactor, in normal or incident operating conditions.

[0117] Preferably, the lower limit of the thickness E1 is configured to hold the tube 14 under mechanical tensile stress with an elongation greater than 0.3% of deformation and / or a stress level before rupture greater than 100 MPa.

[0118] The upper limit of the thickness E1 of the tube 14 makes it possible to ensure satisfactory heat transfer during use, between the nuclear fuel contained in the tube 14 and the heat transfer fluid circulating outside the tube 14 in a nuclear reactor in which a nuclear fuel assembly incorporating the fuel rod 2 is inserted. Indeed, during use, a thermal gradient is established in the thickness of the tube 14 due to the heat exchanges between the nuclear fuel contained in the cladding 4 and the heat transfer fluid circulating outside the cladding 4.

[0119] This thermal gradient leads to direct internal stresses, by thermal expansion, and indirect internal stresses, by establishing a swelling gradient under irradiation, in the thickness of the tube 14.

[0120] Calculations of the circumferential tensile stress (or azimuthal stress) at the internal surface 14A of the tube 14 as a function of the thickness of the tube 14 for normal or incidental operating conditions, show that this maximum circumferential tensile stress increases as a function of the thickness E1 of the tube 14.

[0121] These calculations can be performed by taking into account the pressure difference between the inside and outside of the sheath 4, the thermal expansion of the sheath 4 and the swelling of the sheath 4.

[0122] Figure 3 is a graph illustrating the evolution of the maximum tensile circumferential stress at the internal surface 14A of the tube 14 as a function of the thickness E1 of the tube 14 (curve C1), for normal or incident operating conditions in a nuclear reactor.

[0123] Calculations similar to the previous ones show that during a cold stop in intercycles, the internal pressurization of the rod 2 leads to a tensile stress on the ceramic matrix composite material reinforced with ceramic fibers, the maximum tensile circumferential stress increasing when the thickness of the tube 14 decreases.

[0124] Figure 4 is a graph illustrating the evolution of the maximum tensile circumferential stress at the internal surface 14A of the tube 14 as a function of the thickness E1 of the tube 14 (curve C2) during an inter-cycle cold shutdown in a nuclear reactor.

[0125] Thus, the upper limit of the thickness E1 of the tube 14 makes it possible to limit the circumferential tensile stresses at the internal surface 14A of the tube 14 under normal and incident operating conditions in a reactor, and the lower limit of the thickness E1 of the tube 14 makes it possible to limit the circumferential tensile stresses during cold shutdown phases of the nuclear reactor, for example between cycles.

[0126] The minimum and maximum limits of the thickness E1 of the tube 14 are chosen so that the circumferential tensile stresses remain below a target elastic tensile limit of the ceramic matrix composite material reinforced with ceramic fibers (in particular SiC / SiCf or SiC / Cf).

[0127] The target tensile yield strength is preferably equal to or greater than 80 MPa.

[0128] The provision of an unreinforced layer 20 on the external face, preferably having a micrometric surface condition, makes it possible to ensure protection of the reinforced layer(s) 16 from the outside, in particular from erosion / corrosion in contact with the heat transfer fluid circulating in a nuclear reactor in which a nuclear fuel assembly integrating the fuel rod 2 is inserted.

[0129] Preferably, the total thickness of the reinforced layer(s) 16 is greater than 280 μm.

[0130] The total thickness of the reinforced layer(s) 16 is the sum of the thicknesses of the reinforced layers 16 of the tube 14.

[0131] Preferably, the thickness of the unreinforced layer 20 on the external face is equal to or greater than 20 μm, in particular equal to or greater than 30 μm. This ensures the protection function of the reinforced layer(s) 16 from the heat transfer fluid.

[0132] The tube 14 closed at its axial ends by the plugs 6 and, preferably provided with the external protective layer 22, protects the internal lining 18 from the external environment under normal operating conditions and under accident conditions.

[0133] The thickness range provided for the internal lining 18 allows sufficient mechanical strength before insertion into the tube 14, limited neutron absorption and satisfactory heat transfer.

[0134] The internal lining 18 ensures gas tightness with limited impact on the internal diameter of the sheath 4.

[0135] The liner material 18 must have low interaction with the tube material 14 and with the nuclear fuel for the temperature conditions of severe accident conditions.

[0136] The thickness range provided for the outer protective layer 22 allows limited neutron absorption and satisfactory heat transfer, and ensures a good service life, with a controlled risk of delamination. It makes it possible to produce an outer protective layer 22 which remains protective despite swelling of the tube 14 under irradiation, which is generally less than 2%.

[0137] Preferably, the total thickness of the sheath 4, comprising the tube 14, the internal lining 18, and optionally the external protective layer 22, is equal to or less than 600 μm, in particular equal to or less than 570 μm.

[0138] The total thickness of the sheath 4, comprising the tube 14, the internal lining 18, and optionally the external protective layer 22 corresponds to the sum of the thicknesses E1 (tube 14), E2 (internal lining 18) and E3 (external protective layer 22) in Figure 2.

[0139] A nuclear fuel rod may be provided for use in a nuclear fuel assembly comprising a bundle of parallel nuclear fuel rods arranged in an NxN matrix configuration, N being an integer, in particular a 17x17 or 14x14 matrix configuration.

[0140] The total thickness of the sheath 4 is for example limited according to the number N of the NxN matrix configuration.

[0141] In examples, the cladding 4 is configured for a nuclear fuel rod 2 for a nuclear fuel assembly comprising a bundle of parallel nuclear fuel rods arranged in a 17x17 matrix configuration, and the total thickness of the cladding 4 is equal to or less than 570 pm.

[0142] In examples, the cladding 4 is configured for a nuclear fuel rod 2 for a nuclear fuel assembly comprising a bundle of parallel nuclear fuel rods arranged in a 14x14 matrix configuration, and the total thickness of the cladding 4 is equal to or less than 600 μm.

[0143] Limiting the total thickness of the cladding 4 ensures the compatibility of the cladding 4 with the fuel pellets 8 and existing nuclear fuel assemblies.

[0144] The invention also relates to a nuclear fuel assembly comprising a bundle of parallel nuclear fuel rods arranged in an NxN matrix configuration, N being an integer, in particular a 17x17 or 14x14 matrix configuration, each rod having a cladding, the claddings of the nuclear fuel rods being as defined above.

Claims

CLAIMS 1. Nuclear fuel rod sheath, comprising a tube (14) extending along a longitudinal axis and a tubular internal lining (18) received inside the tube (14), the tube (14) comprising at least one reinforced layer (16) made of a composite material comprising a ceramic matrix reinforced with ceramic reinforcing fibers, the thickness of the tube (14) being between 280 μm and 600 μm.

2. Sheath according to claim 1, in which the tube (14) comprises a single reinforced layer (16), the thickness of the tube (14) being between 280 μm and 340 μm.

3. Sheath according to claim 1, in which the tube (14) comprises exactly two reinforced layers, the thickness of the tube (14) being between 450 μm and 600 μm.

4. Sheath according to any one of the preceding claims, in which the ceramic matrix is made of silicon carbide and / or the ceramic reinforcing fibers are made of silicon carbide and / or carbon.

5. A sheath according to any preceding claim, wherein one or each reinforced layer (16) is formed with filament winding of the ceramic reinforcing fibers.

6. A sheath according to any preceding claim, wherein one or each reinforced layer (16) is formed with filament winding of the ceramic reinforcing fibers without interlacing of the ceramic reinforcing fibers.

7. A sheath according to any preceding claim, wherein one or each reinforced layer (16) is formed with filament winding of the ceramic reinforcing fibers with interlacing of the ceramic reinforcing fibers, for example by braiding the ceramic reinforcing fibers.

8. Sheath according to any one of the preceding claims, in which the total thickness of the reinforced layer(s) (16) is equal to or greater than 280 μm.

9. Sheath according to any one of the preceding claims, in which the tube (14) comprises at least one unreinforced layer (20), preferably made of the same material as the ceramic matrix.

10. Sheath according to claim 9, comprising an internal non-reinforced layer (20) covering the reinforced layer(s) (16) on the side of the internal face of the tube (14) and / or an external non-reinforced layer (20) covering the reinforced layer(s) (16) on the side of the external face of the tube (14) and / or an intermediate non-reinforced layer (20) interposed between two reinforced layers (16) of the tube (14).

11. Sheath according to claim 9 or 10, in which the thickness of the external unreinforced layer (20) is equal to or greater than 20 μm, in particular equal to or greater than 30 μm.

12. Sheath according to any one of the preceding claims, in which the tube (14) is formed exclusively from the ceramic matrix and the ceramic reinforcing fibers.

13. Sheath according to any one of the preceding claims, in which the internal lining has a thickness of between 50 pm and 200 pm, in particular between 50 pm and 150 pm.

14. A sheath according to any preceding claim, wherein the inner lining (18) is metallic.

15. A sheath according to any preceding claim, wherein the inner lining (18) is made of pure zirconium or a zirconium-based alloy containing at least 95% by weight of zirconium.

16. A sheath according to any preceding claim, wherein the inner lining (18) is pre-oxidized.

17. A sheath according to any preceding claim, comprising an outer protective layer (22) covering an outer surface of the tube (14).

18. Sheath according to claim 17, in which the outer protective layer (22) is metallic.

19. Sheath according to claim 17 or 18, in which the outer protective layer (22) is made of pure chromium or a chromium-based alloy containing at least 85% by weight of chromium or of pure zirconium or a zirconium-based alloy containing at least 85% by weight of zirconium or of pure titanium or a titanium-based alloy containing at least 80% by weight of titanium.

20. Sheath according to any one of claims 17 to 19, in which the outer protective layer (22) has a thickness of between 5 μm and 40 μm.

21. Cladding according to any one of the preceding claims, wherein the total thickness of the cladding (4) is equal to or less than 600 pm, the cladding (4) preferably being configured for a nuclear fuel rod (2) for a nuclear fuel assembly comprising a bundle of parallel nuclear fuel rods arranged in a 14x14 matrix configuration, or wherein the total thickness of the cladding (4) is equal to or less than 570 pm, the cladding (4) preferably being configured for a nuclear fuel rod (2) for a nuclear fuel assembly comprising a bundle of nuclear fuel rods arranged in a 17x17 matrix configuration.

22. Nuclear fuel rod comprising a cladding according to any one of the preceding claims, nuclear fuel received inside the inner lining (18) of the cladding, and two plugs (6), each plug (6) closing a respective axial end of the cladding.

23. Nuclear fuel rod according to claim 22, in which each plug (6) is made of a composite material comprising a ceramic matrix reinforced with ceramic reinforcing fibers, in particular a silicon carbide matrix reinforced with silicon carbide and / or carbon reinforcing fibers.

24. A nuclear fuel rod according to claim 22 or claim 23, comprising two internal sealing elements (24), each closing a respective end of the internal liner (18).

25. Nuclear fuel rod according to any one of claims 22 to 24, wherein the gas contained in the internal lining (18) is for example helium and / or the pressure of the gas inside the internal lining (18) is equal to or less than 200 bars, preferably equal to or less than 100 bars, more preferably equal to or less than 20 bars.

Images