Classifications

• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21C—NUCLEAR REACTORS
  • G21C3/00—Reactor fuel elements and their assemblies; Selection of substances for use as reactor fuel elements
  • G21C3/02—Fuel elements
  • G21C3/04—Constructional details
  • G21C3/06—Casings; Jackets
  • G21C3/07—Casings; Jackets characterised by their material, e.g. alloys
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/0021—Reactive sputtering or evaporation
  • C23C14/0036—Reactive sputtering
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/04—Coating on selected surface areas, e.g. using masks
  • C23C14/046—Coating cavities or hollow spaces, e.g. interior of tubes; Infiltration of porous substrates
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/06—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the coating material
  • C23C14/14—Metallic material, boron or silicon
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/06—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the coating material
  • C23C14/14—Metallic material, boron or silicon
  • C23C14/16—Metallic material, boron or silicon on metallic substrates or on substrates of boron or silicon
  • C23C14/165—Metallic material, boron or silicon on metallic substrates or on substrates of boron or silicon by cathodic sputtering
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/22—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the process of coating
  • C23C14/34—Sputtering
  • C23C14/3435—Applying energy to the substrate during sputtering
  • C23C14/345—Applying energy to the substrate during sputtering using substrate bias
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/22—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the process of coating
  • C23C14/34—Sputtering
  • C23C14/3485—Sputtering using pulsed power to the target
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C14/00—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material
  • C23C14/22—Coating by vacuum evaporation, by sputtering or by ion implantation of the coating forming material characterised by the process of coating
  • C23C14/34—Sputtering
  • C23C14/35—Sputtering by application of a magnetic field, e.g. magnetron sputtering
  • C23C14/351—Sputtering by application of a magnetic field, e.g. magnetron sputtering using a magnetic field in close vicinity to the substrate
• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21C—NUCLEAR REACTORS
  • G21C21/00—Apparatus or processes specially adapted to the manufacture of reactors or parts thereof
  • G21C21/02—Manufacture of fuel elements or breeder elements contained in non-active casings
• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21C—NUCLEAR REACTORS
  • G21C3/00—Reactor fuel elements and their assemblies; Selection of substances for use as reactor fuel elements
  • G21C3/02—Fuel elements
  • G21C3/04—Constructional details
  • G21C3/045—Pellets
• • G—PHYSICS
  • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
  • G21C—NUCLEAR REACTORS
  • G21C3/00—Reactor fuel elements and their assemblies; Selection of substances for use as reactor fuel elements
  • G21C3/02—Fuel elements
  • G21C3/04—Constructional details
  • G21C3/16—Details of the construction within the casing
  • G21C3/20—Details of the construction within the casing with coating on fuel or on inside of casing; with non-active interlayer between casing and active material with multiple casings or multiple active layers
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E30/00—Energy generation of nuclear origin
  • Y02E30/30—Nuclear fission reactors

Definitions

• Nuclear fuel cladding element and method of manufacturing such a cladding element
• the present invention relates to the field of nuclear fuel cladding, in particular nuclear fuel rod cladding, and their method of manufacture.
• the nuclear fuel including the fissile material is generally contained in a sealed sheath which prevents the dispersion of the nuclear fuel.
• the nuclear fuel assemblies used in light water reactors generally comprise a bundle of nuclear fuel rods, each nuclear fuel rod comprising a sheath containing nuclear fuel, the sheath being formed of a sheath tube closed at each of its both ends by a plug.
• the cladding tubes of the nuclear fuel assemblies are made, for example, of zirconium or of a zirconium-based alloy. Such alloys exhibit high performance under normal conditions of use in nuclear reactors. However, they can reach their limits, particularly in terms of temperature during severe accident conditions, such as during a loss of cooling fluid accident (or LOCA for "Loss Of Coolant Accident").
• the temperature can reach more than 800°C and the cooling fluid is essentially in the form of water vapour.
• This can cause rapid degradation of the cladding tube, with in particular the release of hydrogen and rapid oxidation of the cladding tube leading to its embrittlement or even to its bursting, and therefore to the release of nuclear fuel from the cladding.
• part of the hydrogen produced is absorbed (hydriding) by the sheath, leading to its embrittlement.
• WO2016/042262A1 proposes a nuclear fuel cladding comprising a substrate made of zirconium or zirconium alloy and covered with a protective coating made of chromium or chromium alloy, the protective coating having a columnar microstructure.
• One of the aims of the invention is to provide a nuclear fuel cladding element which has satisfactory resistance to hydriding and/or oxidation.
• the invention proposes a nuclear fuel cladding element, the cladding element comprising a substrate made of a material based on zirconium and a protective coating externally covering the substrate, the protective coating being made of a chromium-based material, in which the protective coating has a columnar microstructure composed of columnar grains and has on its outer surface a density of microdroplets lower than 100 per mm 2 .
• the columnar microstructure provides a ductile protective coating that can resist deformation, which limits the risk of cracks appearing in the event of deformation of the cladding element.
• the appearance of a crack would be likely to expose the substrate to the external environment, which could cause its degradation and weakening, and ultimately lead to an opening of the cladding element.
• microdroplets Limiting the density of the microdroplets present on the surface of the protective coating makes it possible to further improve the resistance of the sheathing element. Indeed, the presence of microdroplets limits the protection conferred by the protective coating by allowing the infiltration of cooling fluid along the borders of the microdroplets, reducing the resistance to corrosion and oxidation of the cladding element, especially at high temperature.
• the microdroplets constitute discontinuities in the microstructure of the protective coating, which form points of weakness and which are likely to initiate cracks in the protective coating.
• the presence of microdroplets affects at least locally the microstructure of the protective coating, the columnar grains generally having a larger average diameter under the microdroplets.
• the sheathing element comprises one or more of the following optional characteristics, taken individually or according to all technically possible combinations:
• the columnar grains have an average diameter equal to or less than 1 mhi, preferably equal to or less than 0.5 mhi;
• the columnar grains have an average diameter between 0.05 mhi and 5 mhi, preferably between 0.1 mhi and 2 mhi;
• the microdroplets have a diameter equal to or less than 20 mhi;
• the protective coating has a thickness of between 5 mhi and 25 mhi.
• the protective coating is made of a chromium-based material, for example pure chromium or a chromium-based alloy, for example a binary chromium alloy, in particular a binary chromium-aluminum alloy, a binary chromium alloy - nitrogen or a chromium-titanium binary alloy;
• the sheathing element is a sheathing tube, in particular a nuclear fuel rod sheathing tube.
• the invention also relates to a nuclear fuel element comprising nuclear fuel placed inside a cladding formed by at least one cladding element as defined above.
• the invention also relates to a nuclear fuel rod comprising nuclear fuel disposed inside a sheath formed of a tubular sheathing element as defined above, closed at its ends by plugs.
• the invention also relates to a method of manufacturing a sheathing element as defined above, comprising obtaining the substrate and then depositing the protective coating on the substrate by physical vapor deposition by cathodic sputtering of a target or by physical deposition by cold spraying.
• the deposition of the protective coating by physical vapor deposition or by physical deposition by cold spraying, in particular by magnetron sputtering, makes it possible to obtain the columnar microstructure while limiting the density of microdroplets on the surface of the protective coating. .
• the manufacturing method comprises one or more of the following optional characteristics, taken individually or according to all technically possible combinations:
• the deposition is carried out by physical vapor deposition by magnetron sputtering
• the substrate is in the form of a plate and the deposition step is carried out in such a way that the rate of deposition of the protective coating on the substrate is between 1 pm/h and 30 pm/h;
• the substrate is a tube having a central axis
• the deposition step is carried out by driving the substrate in rotation around its central axis and in such a way that the rate of deposition of the protective coating on the substrate is between 1 /p pm/h and 30/p pm/h;
• the deposition is carried out by physical vapor deposition by supplying the target with direct current so as to obtain a current density of between 0.0005 A/cm 2 and 0.1 A/cm 2 on the target or with pulsed current with current peaks so as to obtain a current density of between 0.01 A/cm 2 and 5 A/cm 2 on the target during current peaks;
• the deposition is carried out by physical vapor deposition with an electrical bias voltage of the substrate relative to the target during the physical vapor deposition, which is negative and between - 10 V and - 200 V.
• FIG. 1 is a schematic view in longitudinal section of a nuclear fuel rod
• FIG. 2 is a schematic cross-sectional view of a cladding tube of a nuclear fuel rod
• FIG. 3 is a micrograph of a section of a protective coating deposited on a substrate
• FIG. 4 is a schematic view of a region of the surface of a protective coating, illustrating the presence of microdroplets
• FIG. 5 is a schematic view of an assembly for depositing a coating on a substrate by physical vapor deposition
• FIG. 8 is a photograph taken under a microscope of the surface of a protective coating deposited on a substrate by physical vapor deposition by cathodic evaporation by arc with heating of the substrate.
• Figure 1 illustrates a nuclear fuel rod 2 intended for example to be used 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 reactor of the “VVER” type, a reactor of the “RBMK” type, or a heavy water reactor, for example of the “CANDU” type.
• a pressurized water reactor or PWR for "Pressurized Water Reactor”
• BWR Boiling Water Reactor
• the nuclear fuel rod 2 has the shape of an elongated rod along a rod axis A.
• the nuclear fuel rod 2 comprises a sheath 4 containing nuclear fuel.
• the sheath 4 comprises a tubular sheathing element 6 (or “sheathing tube”), extending along the pencil axis A and closed at each of its ends by a plug 8.
• the nuclear fuel is in the form of a stack of pellets 10 stacked axially inside the cladding element 6, each pellet 10 containing fissile material.
• the stack of pellets 10 is also called "fissile column”.
• the nuclear fuel rod 2 comprises a spring 12 arranged inside the sheathing element 6, between the stack of pellets 10 and one of the plugs 8, to push the stack of pellets 10 towards the other cap 8.
• a void or plenum 14 and present between the stack of pellets 10 is the plug on which the spring 12 bears.
• FIG. 2 which shows a cross-sectional view of the sheathing element 6, the latter comprises a substrate 16 covered on the outside with a protective coating 18.
• Substrate 16 has an inner surface 16A facing the inside of sheath 4, and an outer surface 16B facing outside of sheath 4.
• Protective coating 18 covers outer surface 16B of substrate 16 to protect it of the external environment.
• the sheathing element 6 is here a tube, and, correspondingly, the substrate 16 here has a tubular shape.
• the substrate 16 is for example made of a material based on zirconium.
• a zirconium-based material means a pure zirconium or zirconium-based alloy material.
• a pure zirconium material is a material comprising at least 99% by weight of zirconium.
• a zirconium-based alloy is an alloy comprising at least 95% by weight of zirconium.
• the zirconium-based alloy is for example chosen from one of the known alloys such as M5, ZIRLO, E110, HANA and N36.
• the substrate 16 has for example a thickness of between 0.4 mm and 1 mm.
• the thickness of the substrate 16 is the distance between the internal surface 16A and the external surface 16B of the substrate 16.
• the protective coating 18 is a thin layer having for example a thickness strictly less than that of the substrate 16.
• the protective coating 18 has for example a thickness comprised between 5 ⁇ m and 25 ⁇ m, in particular a thickness comprised between 10 ⁇ m and 20 ⁇ m.
• the thickness of the protective coating 18 is taken perpendicular to the surface on which the protective coating 18 is deposited, here the outer surface 16B of the substrate 16.
• the protective coating 18 is the outermost layer of the sheath element 6.
• the protective coating 18 is in contact with the external environment.
• the protective coating 18 is made of a chromium-based material.
• a chromium-based material means a pure chromium material or a chromium-based alloy.
• a pure chromium material is a material comprising at least 99% by weight of chromium.
• a chromium-based alloy is an alloy comprising at least 85% by weight of chromium.
• the chromium-based material is a chromium-based alloy chosen from: a binary chromium-aluminum alloy (CrAI), a binary chromium-nitrogen alloy (CrN) and a binary chromium-titanium alloy (CrTi ).
• the protective coating 18 comprises a single layer made of chromium-based material or several superimposed layers made of chromium-based material, preferably with the same chromium-based material.
• the structure of the protective coating 18 in several superimposed layers results for example from the deposition process used to deposit the protective coating 18 on the substrate 16.
• the protective coating 18 has a columnar microstructure.
• the microstructure of the protective coating 18 has columnar grains 20, ie grains which have the general shape of an elongated cylinder in a direction of extension DE perpendicular to the surface on which the protective coating 18 is deposited. , here the outer surface 16B of the substrate 16.
• Each columnar grain 20 has a height, taken along the direction of extension DE of the columnar grain 20.
• Each columnar grain 20 has a diameter.
• the diameter of a columnar grain 20 is for example measured on a micrograph by measuring the width of the columnar grain 20, i.e. its dimension perpendicular to its direction of extension.
• each columnar grain 20 is not perfectly cylindrical and has a diameter that can vary along the columnar grain 20.
• columnar grains 20 do not all have the same diameter.
• an average diameter of the columnar grains 20 of the protective coating 18 near the outer surface 18B is also possible to determine an average diameter of the columnar grains 20 of the protective coating 18 near the outer surface 18B, as the sum of the diameters of the columnar grains 20 visible on a micrograph of the coating of protection 18 near the outer surface 18B, divided by the number of columnar grains 20 considered.
• the columnar grains 20 have an average diameter equal to or less than 1 ⁇ m, in particular an average diameter equal to or less than 0.5 ⁇ m.
• the very fine columnar grains 2 at the interface between the substrate 16 and the protective coating 18 allow good cohesion of the protective coating 18 on the substrate 16.
• the diameter of the columnar grains 20 tends to increase.
• the columnar grains 20 Preferably, near the outer surface 18B of the protective coating 18, the columnar grains 20 have an average diameter of between 0.05 ⁇ m and 5 ⁇ m, preferably between 0.1 ⁇ m and 2 ⁇ m.
• the relatively fine columnar grains 20 on the outer surface 18B of the protective coating 18 limit the fragility and the risk of flaking of the protective coating 18.
• microdroplets 22 may appear on the outer surface 18B of the protective coating 18.
• the density of microdroplets 22 on the outer surface 18B of the coating 18 is equal to or less than 100 per mm 2 , in particular equal to or less than 10 per mm 2 .
• the density of microdroplets 22 on the outer surface 18B of the protective coating 18 is for example determined by observation under an optical or electron microscope of a given reference region of the outer surface 18B, preferably on which represents a sample representative of the homogeneity of the outer surface 18B.
• the density of microdroplets 22 on the outer surface 18B of the protective coating 18 is for example determined as the number of microdroplets 22 present in the reference region of the outer surface 18B of the protective coating 18, divided by the area of the region reference.
• the reference region represents for example a fraction of the outer surface 18B of the protective coating 18.
• the area of the reference region is large enough for the measurement carried out to be representative.
• the area of the reference region is equal to or greater than 10 mm 2 .
• the microdroplets 22 present on the external surface 18B have a diameter equal to or less than 20 ⁇ m.
• the outer surface 18B is devoid of microdroplets having a diameter greater than 20 ⁇ m.
• Each microdroplet 22 present on the outer surface 18B of the protective coating 18 promotes in operation the infiltration of cooling fluid (typically water) along the boundaries between the microdroplet 22 with the columnar grains 22, which reduces the resistance corrosion of the cladding element and the resistance to oxidation of the cladding element, in particular at high temperature (typically 280°C to 350°C in normal operation and between 800°C and 1200°C in accident conditions, in a pressurized water reactor).
• cooling fluid typically water
• each microdroplet 22 defines a discontinuity in the microstructure of the protective coating 18, which weakens the protective coating 18 by constituting a point of weakness and by being liable to initiate cracks in the protective coating 18.
• each microdroplet 22 locally affects the formation of the microstructure of the protective coating 18, generally by causing the growth of grains of larger diameter under the microdroplet 22.
• microdroplets can be the source of crack creation.
• the absence of microdroplets with a diameter greater than 20 mhi limits this risk.
• the deposition of the protective coating 18 is carried out in such a way as to avoid the formation of microdroplets with a diameter greater than 20 mhi.
• the sheathing element 6 described above is a sheathing tube for the production of a nuclear fuel rod 2 comprising nuclear fuel disposed inside the sheathing element 6 closed in leaktight manner at each of its ends. by a cap.
• the cladding element 6 is in the form of a plate, for example to form a nuclear fuel element 2 in the form of a plate comprising a layer of nuclear fuel interposed (ie sandwiched) between two elements of sheathing 6 in the form of a plate.
• Such a sheathing element 6 in the form of a plate is produced analogously to what has been described above, in particular as regards the material of the substrate 16, the material of the protective coating 18, the thickness of the substrate 16 , the thickness of the protective coating 18, the microstructure of the protective coating 18 and the droplet limitation on the outer surface 18B of the protective coating 18.
• the manufacturing method includes a step for obtaining the substrate 16.
• the substrate 16 is a tube, which has for example an external diameter 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.
• Such a tube is for example obtained in a known manner by crawl step rolling, from a tubular blank of larger diameter and shorter length than the tube.
• the manufacturing method then comprises a step of depositing the coating on the outer surface 16B of the substrate 16, for example by physical vapor deposition by cathodic sputtering.
• the substrate 16 and a target 24, which is made of a suitable material to form the protective coating 18, are placed in a rarefied atmosphere, formed for example of an inert gas, such as argon, and an electrical potential difference is generated between target 24 and substrate 16, target 24 defining a cathode and substrate 16 defining an anode (target 24 is brought to an electrical potential higher than that of substrate 16) .
• a rarefied atmosphere formed for example of an inert gas, such as argon
• the target 24 is for example made of pure chromium.
• the target 24 is for example made of a chromium alloy having different proportions, but allowing the deposition of a protective coating with the targeted proportions (for example a target in chromium-aluminum alloy with 15% by weight of aluminum in order to obtain a protective coating in chromium-aluminum alloy with 10% by weight of aluminum).
• the deposition step is carried out using a physical vapor deposition installation 26, comprising a chamber 28, the target 24 disposed inside the chamber 28, and a pump 30 whose inlet is fluidically connected to chamber 28 to generate a rarefied atmosphere in chamber 26, and an electric circuit 32 to generate a potential difference between the target 24 and the substrate 16 introduced inside the chamber 28.
• the substrate 16 is introduced into the chamber 28, a rarefied atmosphere is created in the chamber 28 using the pump 30, and the potential difference between the target 24 and the substrate 14 is generated by the electric circuit 32, which makes it possible to carry out the physical vapor deposition.
• the physical vapor deposition is carried out by magnetron sputtering.
• a magnetic field is generated, preferably at least close to the target 24.
• the magnetic field is generated for example by one or more permanent magnets 34, as illustrated in Figure 4, and/or one or more electromagnets.
• the substrate 16 has the shape of a tube, in particular a tube having a symmetry of revolution around a central axis, as is the case for a substrate 16 for a sheathing tube of a pencil of nuclear fuel, during the deposition step, the substrate 16 is rotated around its central axis.
• the protective coating 18 is preferably deposited on the tube-shaped substrate having a central axis by causing the substrate 16 to rotate around this central axis and in such a way that the deposition speed of the protective coating 18 on the substrate 16 is between 1/p prn/h and 30/p prn/h.
• a sheathing element 6 in the form of a plate, so that the outer surface 16B of the substrate 16 is substantially planar.
• a cladding element 6 makes it possible to form a nuclear fuel element in the general shape of a plate, comprising nuclear fuel interposed (or sandwiched) between two cladding elements 6 in the form of a plate.
• the deposition step is carried out without rotation of the substrate 16.
• the deposition of the protective coating 18 on a plate-shaped substrate is carried out in such a way that the rate of deposition of the protective coating 18 on the substrate 16 is between 1 ⁇ m/h and 30 ⁇ m/h.
• the deposition rate of the protective coating 18 conditions the microstructure of the protective coating 18 which will be obtained.
• the deposition rates proposed above make it possible to obtain a desired microstructure, ie a columnar microstructure with columnar grains having a small diameter at the interface between the substrate 18 and the protective coating 18, and a diameter not too large at the outer surface 18B of the protective coating, as indicated above.
• the deposition rate of the protective coating 18 by physical vapor deposition by cathode sputtering, in particular by magnetron cathode sputtering, is a function in particular of the current density passing through the target 24 and of the polarization of the substrate, ie of the difference le electrical potential of substrate 16 and the electrical potential of target 24 during deposition.
• physical vapor deposition can be performed with continuous current density (i.e. applying a continuous electric current to target 24) or pulsed current density (i.e. applying a pulsed electric current comprising pulses).
• the current density of the target is the intensity of the current passing through the target divided by the area of the active surface of the target, ie the surface of the target which is turned towards the substrate 16 and which receives the charged particles of the plasma projected onto target 24.
• the deposition of the protective coating 18 is carried out by supplying the target 24 with a direct current so as to obtain for the target 24 a current density of between 0.0005 A/cm 2 and 0.1 A /cm 2 , preferably between 0.0005 A/cm 2 and 0.05 A/cm 2 , or by supplying the target 24 with a pulsed current with current peaks so as to obtain for the target 24 a density of current of between 0.01 A/cm 2 and 5 A/cm 2 during current peaks (ie peak current density), preferably of between 0.01 A/cm 2 and 0.5 A/cm 2 .
• the target power density is the electrical power passing through the target divided by the active surface area of the target.
• the deposition of the protective coating 18 is carried out by supplying the target 24 with a direct current so as to obtain for the target 24 a power density of between 0.5 W/cm2 and 100 W/cm 2 , preferably a density of power between 0.5 W/cm2 and 50 W/cm 2 , or by supplying the target 24 with a pulsed current with current peaks so as to obtain for the target 24 a power density of between 10 W/cm2 and 50,000 W/cm 2 (ie the peak power density), preferably a power density of between 10 W/cm 2 and 5,000 W/cm 2 .
• the electrical bias voltage of substrate 16 relative to target 24 during physical vapor deposition is negative and between -10 V and -200 V, more preferably between -50 V and -150 V.
• the deposition is carried out with a pulsed current with one or more of the following parameters:
• the average power density (the average over time of the electrical power density passing through the target 24) is between 1 W/cm 2 and 5 W/cm 2 ;
• the peak power density (the electrical power passing through the target 24 at each current pulse per unit area of the target 24) is between 30 W/cm2 and 100 W/cm2;
• the frequency of the current pulses is between 50 Hz and 5000 Hz;
• the duration of the current pulses is between 10 ps and 50 ps;
• the pressure residing inside the chamber in which the physical vapor deposition is carried out is between 0.1 Pa and 0.4 Pa;
• the distance between the substrate 16 and the target 24 is between 50 mm and 200 mm, more preferably between 80 mm and 140 mm.
• the physical vapor deposition is carried out on the substrate 16 without any heat input other than that resulting from the bombardment of the substrate 16 by the particles (atoms, ions, etc.) torn from the target 24 due to the setting in implementation of physical vapor deposition.
• the substrate 16 is not heated using a heating device. This makes it possible to limit the risk of exceeding the phase transition temperature of the material of the substrate 16.
• Physical vapor deposition by magnetron cathode sputtering can be carried out using one of the following techniques or a combination of at least two of the following techniques: DC magnetron cathode sputtering, cathode sputtering magnetron in pulsed direct current (in English "Pulsed Direct Current” or “DC pulsed"), sputtering High Power Impulse Magnetron Sputtering (HiPIMS or HPMS), Bipolar Magnetron Sputtering (MSB), Dual Magnetron Sputtering Magnetron Sputtering” (DMS)), unbalanced magnetron sputtering (in English “Unbalanced Magnetron Sputtering” (UBM)).
• DC magnetron cathode sputtering cathode sputtering magnetron in pulsed direct current (in English "Pulsed Direct Current” or “DC pulsed")
• HiPIMS or HPMS sputtering High Power Impulse Magnetron Sputtering
• MSB Bipolar Magnetr
• Figures 6 to 8 are photographs taken under a microscope of the surface of protective coatings 18 deposited on a substrate 16 by physical vapor deposition by magnetron sputtering with pulsed current, under an argon atmosphere, with different sets of parameters, shown in Table 1 below:
• Example 1 and Example 2 are carried out respecting the physical vapor deposition parameters indicated above, while Example 3 is not carried out respecting all these parameters.
• the protective coating 18 has few droplets G on its outer surface 18B.
• the protective coating 18 has on its outer surface 18B a density of microdroplets of 50 per mm 2 .
• Example 1 gave a result similar to that of example 2, the protective coating 18 having on its outer surface 18B a density of microdroplets of 50 per mm 2 .
• the protective coating 18 of example 3 illustrated by FIG. 7 was also produced by physical vapor deposition by magnetron sputtering but outside the recommended ranges, in particular for the pressure and the peak power density.
• the density of microdroplets is in this case approximately 2500 microdroplets per mm 2 , which is much higher than the desired maximum density of microdroplets which is 100 microdroplets per mm 2 .
• the protective coating 18 of the example in Figure 8 was fabricated by physical vapor deposition by sputtering by arc with heating of the substrate and parameters different from those of magnetron sputtering.
• the protective coating 18 has on its outer surface 18B a density of microdroplets greater than 10,000 per mm 2 .
• the deposition of the protective coating can be carried out according to another technique, for example by physical deposition by cold spraying.
• the invention it is possible to obtain a nuclear fuel cladding element which has good resistance to oxidation and to hydriding, in normal operation of the nuclear reactor and in accident conditions, for example during a loss of coolant accident.

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• 2020
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