NUCLEAR FUEL CLADDINGS, PRODUCTION METHOD THEREOF AND USES OF SAME AGAINST OXIDATION/HYDRIDING

Abstract

The invention relates to a nuclear fuel cladding comprising: i) a substrate containing a zirconium-based inner layer, optionally coated with at least one intermediate layer formed by at least one intermediate material selected from among tantalum, molybdenum, tungsten, niobium, vanadium, hafnium or the alloys thereof; and ii) at least one protective outer layer placed on the substrate and formed by a protective material selected from either chromium or an alloy of chromium. The nuclear fuel cladding produced using the method of the invention has improved resistance to oxidation/hydriding. The invention also relates to the method for the production of the nuclear fuel cladding by ion etching of the surface of the substrate and deposition of the outer layer on the substrate with a high power impulse magnetron sputtering method (HiPIMS), as well as to the use thereof to protect against oxidation and/or hydriding.

Claims

1. A process for the manufacture of a nuclear fuel cladding comprising i) a substrate containing a zirconium-based internal layer coated or not coated with at least one interposed layer placed over said internal layer and ii) at least one external layer placed over the substrate and composed of a protective material chosen from chromium or a chromium-based alloy; the process comprising the following successive steps: a) ion etching of the surface of the substrate; b) deposition of said at least one external layer over the substrate with a high power impulse magnetron sputtering (HiPIMS) process in which the magnetron cathode is composed of the protective material.

2. A process for the manufacture of a nuclear fuel cladding according to claim 1, wherein the cladding comprises an internal coating placed under said internal layer.

3. A process for the manufacture of a nuclear fuel cladding according to claim 1, having at least one interposed layer placed over said internal layer.

4. A process for the manufacture of a nuclear fuel cladding according to claim 3, wherein said at least one interposed layer is placed over said internal layer by carrying out the following successive steps before the etching step a): a′) ion etching of the surface of said internal layer; and b′) production of a substrate by deposition of said at least one interposed layer over said internal layer with a high power impulse magnetron sputtering (HiPIMS) process in which the magnetron cathode is composed of the at least one interposed material.

5. A process for the manufacture of a nuclear fuel cladding according to claim 1, wherein, after the deposition over the substrate of the first external layer with said HiPIMS sputtering process according to step b), at least a part of the additional external layer(s) are deposited during step b) with a magnetron cathode sputtering process of a different type from the HiPIMS which is carried out simultaneously with said HiPIMS sputtering process according to step b).

6. A process for the manufacture of a nuclear fuel cladding according to claim 3, wherein said at least one interposed layer is composed of at least one interposed material chosen from tantalum, molybdenum, tungsten, niobium, vanadium, hafnium or their alloys.

7. A nuclear fuel cladding obtained or obtainable by the manufacturing process according to claim 1.

8. A nuclear fuel cladding with an interface layer, comprising: i) a substrate containing an internal layer composed of a zirconium alloy comprising, by weight, from 100 ppm to 3000 ppm of iron; ii) at least one external layer placed over the substrate and composed of a protective material chosen from chromium or a chromium-based alloy; and iii) an interface layer positioned between said internal layer and the first external layer and composed of an interfacial material comprising at least one intermetallic compound chosen from ZrCr.sub.2 of cubic crystal structure, Zr(Fe,Cr).sub.2 of hexagonal crystal structure or ZrFe.sub.2 of cubic crystal structure.

9. A nuclear fuel cladding according to claim 8, wherein the interface layer has a mean thickness of 10 nm to 1 μm.

10. A nuclear fuel cladding according to claim 8, wherein the cladding comprises an internal coating placed under said internal layer.

11. A nuclear fuel cladding according to claim 8, wherein each of said at least one external layer has a columnar structure.

12. A nuclear fuel cladding according to claim 8, wherein each of said at least one external layer has a thickness of 1 μm to 50 μm.

13. In a method for protecting nuclear fuel in a humid atmosphere that includes water, comprising cladding the nuclear fuel with a nuclear fuel cladding, the improvement whereby oxidation and/or hybriding of the cladding is combatted, wherein the nuclear fuel cladding is a nuclear fuel cladding as defined by claim 8.

14. In a method for protecting nuclear fuel in a hydrogenated atmosphere that includes hydrogen, comprising cladding the nuclear fuel with a nuclear fuel cladding, the improvement whereby hydriding of the cladding is combatted, wherein the nuclear fuel cladding is a nuclear fuel cladding as defined by claim 8.

15. A method for combating hydriding according to claim 13, wherein the hydrogenated atmosphere additionally comprises water.

16. A method according to claim 8, wherein the humid atmosphere is at a temperature of between 1200° C. and 1300° C.

17. A composite nuclear fuel cladding comprising i) a substrate containing a zirconium-based internal layer and at least one interposed layer placed over said internal layer and composed of at least one interposed material chosen from tantalum, molybdenum, tungsten, vanadium, hafnium or their alloys and ii) at least one external layer placed over the substrate and composed of a protective material chosen from chromium or a chromium-based alloy.

18. A composite nuclear fuel cladding according to claim 17, wherein the cladding comprises an internal coating placed under said internal layer.

19. A composite nuclear fuel cladding according to claim 17, wherein each of said at least one external layer has a thickness of 1 μm to 50 μm.

20. A composite nuclear fuel cladding according to claim 17, wherein each of said at least one external layer has a columnar structure.

21. A composite nuclear fuel cladding according to claim 20, wherein the constituent columnar grains of the columnar structure have a mean diameter of 100 nm to 10 μm.

22. A process of manufacture of a composite nuclear fuel cladding as defined by claim 17, comprising the following successive steps: A) production of a substrate by deposition, on a zirconium-based internal layer, of at least one interposed layer composed of at least one interposed material chosen from tantalum, molybdenum, tungsten, vanadium, hafnium or their alloys; B) deposition, on the substrate, of at least one external layer composed of a protective material chosen from chromium-based or a chromium alloy.

23. In a method for protecting nuclear fuel in a humid atmosphere that includes water comprising cladding the nuclear fuel with a nuclear fuel cladding, the improvement whereby oxidation and/or hydriding of the cladding is combatted, wherein the nuclear fuel cladding is a composite nuclear fuel cladding as defined by claim 17.

24. In a method for protecting nuclear fuel in a hydrogenated atmosphere that includes hydrogen comprising cladding the nuclear fuel with a nuclear fuel cladding, the improvement whereby hydriding of the cladding is combatted, wherein the nuclear fuel cladding is a composite nuclear fuel cladding as defined by claim 17.

25. A method according to claim 24, wherein the hydrogenated atmosphere additionally comprises water.

26. A method according to claim 24, wherein the hydrogenated atmosphere comprises more than 50 mole % of hydrogen.

27. A method according to claim 25, wherein the humid atmosphere is at a temperature of between 1200° C. and 1300° C.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0140] FIGS. 1A and 1B represent Scanning Electron Microscopy of Field Emission Gun type (SEM-FEG) photographs illustrating the surface condition of a Zircaloy-4 plate respectively provided with a chromium coating deposited with a conventional magnetron PVD process and with an external chromium layer deposited with an HiPIMS sputtering process according to the invention.

[0141] FIG. 1C represents the photograph obtained by scanning electron microscopy with a field emission gun of a section of the plate of FIG. 1B.

[0142] FIG. 1D represent the photographs obtained by Transmission Electron Microscopy (TEM) of the interface region between the substrate and the external layer of the plate of FIG. 1B.

[0143] FIGS. 1E, 1F and 1G represent the photographs obtained by high resolution TEM and the associated electron diffraction photographs of the interface region.

[0144] FIGS. 2A and 2B represent Scanning Electron Microscopy (SEM) photographs of a cross section produced in the thickness of the plates respectively illustrated in FIGS. 1A and 1B, after they have been subjected to an oxidation at 1200° C. for 300 s.

[0145] FIGS. 3A and 3B illustrate the corresponding concentrations by weight, measured with an electron microprobe, for the elements zirconium, chromium, iron and oxygen as a function of the distance in micrometers with respect to the metal interface. This interface is that separating the external chromium layer (or if appropriate the Zircaloy-4 substrate, if all the external chromium layer has been oxidized to give Cr.sub.2O.sub.3) from the external chromium oxide Cr.sub.2O.sub.3 layer.

[0146] FIG. 4 represents an optical microscopy photograph of a cross section produced in the thickness of a plate similar to that of FIG. 1B after an oxidation at 1300° C.

[0147] FIG. 5 illustrates the profile of concentration by weight of the chromium as a function of the distance in micrometers with respect to the Zircaloy-4/external chromium layer interface for plates similar to those obtained in FIG. 1B and provided (dotted curve denoted “Cr+Ta”) or not provided (continuous curves denoted “Cr”) with an interposed tantalum layer.

[0148] FIGS. 6A and 6B represent SEM photographs of a cross section produced in the thickness of the plates respectively illustrated in FIGS. 1A and 1B, after they have been subjected to an oxidation under steam at 1000° C. for 15 000 s.

[0149] FIGS. 7A and 7B represent diagrammatic views in cross section of a nuclear fuel cladding of tubular shape, respectively deprived of and provided with an interposed layer.

DESCRIPTION OF PARTICULAR EMBODIMENTS

[0150] 1. Manufacture of a Plate using the Process of the Invention

[0151] This example of implementation of the manufacturing process of the invention is carried out in a cathode sputtering reactor sold by Balzers (BAK 640 model) and equipped with a Hüttinger generator. The experimental conditions applied may nevertheless vary as a function of the reactor used or of its magnetic configuration, of the shape and of the size of the target, . . . .

[0152] In accordance with his general knowledge, a person skilled in the art may, however, easily adapt himself to these variations by modifying at least one of the parameters, such as, for example, the polarization voltage of the substrate which is applied during the step a) of ion etching or the step b) of deposition of the internal layer, the duration, the frequency, the intensity or the polarization voltage of the polarization impulses, the distance between the chromium target and the substrate, or the pressure of the carrier gas.

[0153] More particularly, these parameters influence the mean energy of the Cr.sup.+ Pions which are produced during steps a) or b). This energy can condition the density, the homogeneity, the texture, the microstructure or the state of stress of the external layer.

[0154] 1.1. Step of Ion Etching

[0155] A Zircaloy-4 plate with dimensions of 45 mm×14 mm×1.2 mm is degreased in an alkaline solution, rinsed with water and cleaned ultrasonically in ethanol.

[0156] It is subsequently placed in an HiPIMS cathode sputtering reactor containing a chromium magnetron cathode placed at a distance generally of between 6 cm and 8 cm and in this instance of 8 cm. The chamber of the reactor is placed under a vacuum of less than 2.10.sup.−5 mbar and then filled with a carrier gas composed of argon at a pressure of 0.5 Pa.

[0157] The plate constituting the internal layer and thus the substrate to be coated is polarized negatively with a polarization voltage of −800 V.

[0158] The chromium target is subsequently supplied using the HiPIMS generator according to a polarization voltage of −800 V in order to generate a strongly ionized discharge. The chromium is then sputtered in the form of ions which are accelerated by the electric field of the substrate. The adsorbed carbon-based entities and the nanometric layer of native zirconium oxide or hydroxide are then removed from the surface of the substrate in order to improve the adhesion of the external layer. This ion etching of the substrate lasts 3 minutes in order to limit the heating up of the plate.

[0159] 1.2. Step of Deposition of the External Layer by HiPIMS Sputtering

[0160] The polarization voltage applied to the etched plate is, for example, decreased between −50 V and 0 V, in the present case to −50 V for 8 hours. As the rate of deposition is generally between 0.5 μm/h and 1 μm/h, these conditions lead to the deposition of an external chromium layer with a thickness of 6 μm.

[0161] The polarization voltage of the chromium target is maintained at −800 V. Several polarization impulses are applied to the magnetron cathode according to the following characteristics: [0162] duration of an impulse=40 μs; [0163] frequency of the impulses=500 Hz; [0164] overall mean intensity=approximately 2 A; [0165] instantaneous mean intensity=approximately 100 A; [0166] overall mean power=approximately 1 kW; and [0167] instantaneous mean power for an impulse=60 kW.

[0168] The surface condition of the plate coated with the external chromium layer is illustrated by FIG. 1B. By way of comparison, FIG. 1A illustrates a very different surface morphology obtained for a control Zircaloy-4 plate on which a chromium coating of the same thickness was deposited using a conventional magnetron PVD process similar to that described in example 1 of “WO2013/160587”.

[0169] FIG. 1C illustrates the structure in columnar grains of the chromium-based external layer.

[0170] FIG. 1D again illustrates the columnar structure of the external layer, and also the presence of an interface layer.

[0171] FIG. 1E shows the zones 1, 2 and 3 of the interface region which are respectively positioned in the middle of the interface region, between the zirconium-based substrate and the interface region, and between the chromium-based external layer and the interface region. It also illustrates the zone 1 and the structural parameters which identify the intermetallic compound Zr(Fe,Cr).sub.2 of hexagonal crystal structure of which this zone is composed.

[0172] FIG. 1F illustrates the zone 2 and the structural parameters which identify the intermetallic compound ZrFe2 of cubic crystal structure of which this zone is composed.

[0173] FIG. 1G illustrates the zone 3 and the structural parameters which identify the intermetallic compound ZrCr2 of cubic crystal structure of which this zone is composed.

[0174] The combined FIGS. 1E, 1F and 1G thus show that the composition of the interface layer gradually changes from the interface with the Zircaloy-4 substrate toward the interface with the external chromium layer according to the following order of the intermetallic compounds: ZrFe2, Zr(Fe,Cr)2 and ZrCr2. Contrary to all expectations, the iron-containing nanometric phases ZrFe2 and Zr(Fe,Cr).sub.2 were formed by interaction of the zirconium alloy of the substrate with the chromium.

2. Properties with Regard to the Oxidation/Hydriding

[0175] 2.1. Evaluation of the Resistance to Oxidation in Accident Conditions at 1200° C.

[0176] In order to evaluate its resistance to oxidation, a plate based on Zircaloy-4 provided with a single external chromium layer of 6 μm in accordance with example 1 stays for 300 seconds in a furnace in which steam brought to 1200° C. circulates.

[0177] By way of comparison, the same experiment is carried out with a control Zircaloy-4 plate which has been covered with a chromium coating of the same thickness using a conventional cathode sputtering process in accordance with example 1 of “WO2013/160587”.

[0178] The condition of the plates obtained on completion of this oxidation is illustrated by FIGS. 2A and 2B.

[0179] FIG. 2A shows that a layer of chromium oxide Cr.sub.2O.sub.3 of limited thickness is formed. The chromium coating deposited according to the process of the state of the art thus has a partially protective nature with regard to the oxidation at 1200° C. Nevertheless, porosities and cracks are present in the residual metallic chromium layer lying under the layer of chromium oxide Cr.sub.2O.sub.3. They result among others from an exfoliation at the interface between the Zircaloy-4 substrate and the chromium coating, which reflects an embrittling of the residual metallic chromium layer of the control plate and a deterioration in its properties of resistance to oxidation, among others by loss of leaktightness with regard to the diffusion of oxygen.

[0180] On the other hand, even if FIG. 2B illustrates the formation of a peripheral layer of chromium oxide Cr.sub.2O.sub.3 of greater thickness, the underlying layer of residual metallic chromium initially deposited with the HiPIMS sputtering process is for its part not damaged. This is confirmed by the absence of exfoliation at the interface with the internal Zircaloy-4 layer, and also by a very low amount of porosities. This preserved microstructure confirms the protective nature of the chromium coating with regard to the oxidation/hydriding of the zirconium alloy up to at least 1200° C., in particular its ability to limit the diffusion of the oxygen into the internal Zircaloy-4 layer.

[0181] Such results are confirmed by FIGS. 3A and 3B, which illustrate the profiles of diffusion of the zirconium, chromium and oxygen measured with the electron microprobe in the thickness of the coating and in the vicinity of the Zircaloy-4/chromium interface.

[0182] The absence of measurable diffusion of oxygen within the residual metal layer of the HiPIMS coating and a fortiori in the zirconium-based internal layer may be observed.

[0183] For the control plate, the measurements of FIG. 3A show a significant diffusion of oxygen through the chromium coating, which exhibits a mean oxygen content of approximately 1% by weight. This diffusion continues actually within the Zircaloy-4, in which the oxygen content is of the order of 0.3% to 0.4% by weight in the first 100 μm from the chromium coating/Zircaloy-4 substrate interface.

[0184] For the plate produced in accordance with the manufacturing process of the invention, the measurements of FIG. 3B show that the mean oxygen content in the Zircaloy-4 is virtually identical to the initial value of 0.14% by weight. The absence of diffusion of oxygen within the residual chromium layer and a fortiori the Zircaloy-4 substrate makes it possible to preserve the mechanical properties of the substrate, among others the residual ductility and the residual toughness. This thus provides a much better margin of protection with regard to the harmful consequences of the oxidation at 1200° C.

[0185] Furthermore, similar experiments of an oxidation at 1200° C. for 300 seconds, followed by a quenching with water at ambient temperature, have confirmed such a behavior when the plate geometries are replaced by tubular cladding geometries nevertheless involving a different crystal texture: the gain in weight representative of the take up of oxygen is from 10 to 30 times less for the tube produced by the process of the invention in comparison with that measured for the tube covered with a chromium coating with a conventional cathode sputtering process.

[0186] 2.2. Evaluation of the Resistance to Oxidation in Accident Conditions at 1300° C.

[0187] Another sample of the plate produced in accordance with the manufacturing process of the invention stays for 5600 seconds in an equimolar oxygen/helium atmosphere brought to 1300° C.

[0188] In this specific oxidation temperature domain, such an atmosphere composition is reasonably representative of the oxidation conditions under steam as, except for in a particular case (confined steam, alloy of mediocre quality, degraded surface condition, . . . ), no significant hydriding of the substrate occurs during the oxidation at 1300° C.

[0189] Although these temperature conditions lie more than 100° C. above the “LOCA” regulatory limits, the photograph of FIG. 4 shows that the plate with a thickness of approximately 1 mm is not destroyed and that only a portion of the external chromium layer with an initial thickness of 15 μm to 20 μm has been oxidized to give chromic oxide Cr.sub.2O.sub.3.

[0190] The Zircaloy-4 substrate predominantly exhibits a structure of Zr-ex-β type which provides most of the residual ductility of the plate.

[0191] In comparison, the metallic residual internal layer of Zircaloy-4 of a control plate not coated with an external chromium layer and subjected to the same oxidation conditions for its part exhibits a wholly α-Zr(O) structure which is brittle at low temperature and which is responsible for a loss of integrity by transverse splitting.

[0192] Even in oxidizing conditions at 1300° C., far above regulatory safety limits, a nuclear fuel cladding obtained by the manufacturing process of the invention may retain its mechanical integrity and exhibit a comfortable residual margin of resistance to oxidation/hydriding.

[0193] 2.3. Evaluation of the Resistance to Oxidation with an Interposed Tantalum Layer

[0194] A plate is produced under conditions similar to those of example 1, apart from the fact that an interposed layer with a thickness of 2 μm to 3 μm approximately composed of tantalum is deposited on the internal layer using an HiPIMS sputtering process. The deposition of the interposed tantalum layer is carried out according to conditions similar to those of the deposition of the external chromium layer for example 1, apart from the fact that the tantalum target is polarized at −800 V for an impulse duration of 25 μs. After carrying out the ion etching (according to step a) of the manufacturing process of the invention) of the interposed tantalum layer, an external chromium layer with a thickness of 4 μm is subsequently deposited on this interposed layer in accordance with step b) of the manufacturing process of the invention.

[0195] By way of comparison, several corresponding control plates, apart from the fact that they are devoid of interposed tantalum layer, are produced.

[0196] After a residence of 300 seconds in a furnace in which steam at 1200° C. circulates, the profiles of diffusion of the chromium of the external layer toward the internal Zircaloy-4 layer are measured from the interface between these layers.

[0197] These measurements illustrated in FIG. 5 show:

[0198] a very good reproducibility of the results obtained with regard to the control plates devoid of interposed tantalum layer;

[0199] a diffusion of the chromium into the internal Zircaloy-4 layer which is greater for the control plates. This is because, at 1200° C., the external chromium layer is consumed at relatively similar proportion via an internal phenomenon of diffusion of the chromium toward the zirconium alloy and via the external oxidation of the chromium to give chromium oxide;

[0200] the beneficial effect in oxidizing conditions at 1200° C. of the interposed tantalum layer, which acts as a diffusion barrier: in comparison with the control plate, the total amount of chromium which diffuses from the external layer toward the internal Zircaloy-4 layer is thus divided by approximately 4 and the lifetime of the external layer may optionally be multiplied by 2.

[0201] Generally, the interposed layer reduces, indeed even eliminates, the phenomenon of diffusion, which increases the lifetime of the external layer and thus of the corresponding nuclear fuel cladding, amongst others in accident conditions, such as, for example, the dewatering of a spent fuel storage pool or those defined by the criteria of dimensioning accident of LOCA type.

[0202] Furthermore, the impact of the interposed layer on the diffusion of the chromium toward the zirconium alloy also has the advantage of delaying the formation of a eutectic between the zirconium and the chromium above 1330° C. and thus the production of a surface liquid phase, which makes it possible to avoid or limit the potentially negative consequences which might result therefrom in the event of an incursion above ˜1320° C.

[0203] 2.4. Evaluation of the Resistance to Hydriding at 1000° C.

[0204] Hydriding is a phenomenon which occurs within a nuclear fuel cladding in nominal conditions or in certain accident conditions. The hydriding results from the sequence of the following reactions (1) and (2): the zirconium present in the nuclear fuel cladding is oxidized by the pressurized water or the steam according to the reaction,

Zr+2H.sub.2O.fwdarw.ZrO.sub.2+2H.sub.2   (1)

[0205] then a portion of the hydrogen thus released diffuses into the zirconium alloy of the cladding and may form a hydride with the zirconium of the cladding which has not yet oxidized, according to the reaction

Zr+xH.fwdarw.ZrH.sub.x.   (2)

[0206] The index “x” indicates that hydrides of variable stoichiometry may be formed, this index being in particular equal to or less than 2.

[0207] According to the overall hydrogen content and/or the temperature, all or a portion of the hydrogen will precipitate, the remainder remaining in solid solution (in insertion in the α-zirconium crystal lattice).

[0208] For example, at 20° C., virtually all of the hydrogen is precipitated in the form of hydrides, whereas their dissolution may be total at high temperature (typically greater than 600° C.)

[0209] Hydrogen in solid solution, but especially in the form of zirconium hydride precipitate, has the disadvantage of decreasing the ductility of zirconium alloys and thus of causing embrittlement of the cladding, among others at low temperature. This embrittlement is all the more to be feared when it is desired to reach high burn-up rates as, at these rates, an increase in the proportion of zirconium oxidized according to the reaction (1) and thus in the amount of hydrides formed according to reaction (2) is found. It then generally results in the corrosion of the usual industrial alloys at levels which are harmful with regard to the criteria of safety and integrity of the cladding, and may present problems for post-service transportation and storage.

[0210] Observed in normal conditions with regard to the zirconium alloys M5™ or Zirlo™ of a nuclear fuel cladding, hydriding is generally observed in accident conditions only in the vicinity of 1000° C., or toward 800° C. for longer oxidation times. This phenomenon, known as “breakaway”, is associated with an increase in the kinetics of oxidation beyond a certain critical time. It results from the appearance of cracks and/or porosities in the ZrO.sub.2 phase related to the presence of stresses generated at the Zr/ZrO.sub.2 interface probably related to the reversible transformation of tetragonal ZrO.sub.2 into monoclinic ZrO.sub.2. The consequences of this uptake of hydrogen are, in the same way as in normal conditions, an embrittlement of the material in the vicinity of 1000° C. which can result in the fracturing thereof during a quenching or after returning to low temperature.

[0211] The “breakaway” phenomenon generally occurs after 5000 seconds at 1000° C. for a zirconium alloy, such as the Zircaloy-4 or M5™.

[0212] In order to evaluate the resistance to the hydriding of a nuclear fuel cladding according to the invention, another sample of the plate produced in accordance with the manufacturing process of the invention stays for 15 000 seconds in an atmosphere of steam brought to 1000° C.

[0213] By way of comparison, the same experiment is carried out with a control plate of Zircaloy-4 which has been covered with a chromium coating of the same thickness using a conventional cathode sputtering process in accordance with example 1 of “WO 2013/160587”.

[0214] The results obtained are illustrated by FIGS. 6A and 6B.

[0215] FIG. 6A shows that the control plate exhibits a local exfoliation of the external chromium layer which has partly oxidized to give Cr.sub.2O.sub.3. The content of hydrogen dissolved in the plate is measured by analysis of the gases after reduction melting in an analyzer provided for this purpose, instead of the indirect and imprecise evaluation according to example 1 of the document “WO 2013/160587”: this content is approximately 1000 ppm by weight. The Zircaloy-4 plate also comprises brittle phases of α-Zr(O) structure at low temperature due to the diffusion of oxygen into the Zircaloy-4. In point of fact, it is known that, after a quenching with water from the β domain (>900-1000° C.), Zircaloy-4 loses its residual ductility at low temperature (20-150° C.), when the hydrogen content increases in weight above 600 ppm approximately.

[0216] On the other hand, FIG. 6B shows that the plate produced in accordance with the manufacturing process of the invention exhibits a layer of oxide Cr.sub.2O.sub.3 with a thickness 5 times smaller than for the control plate. Furthermore, the content of dissolved hydrogen is at most from 60 ppm to 80 ppm by weight and no phase of α-Zr(O) structure appears in the zirconium-based substrate. The plate has a significant residual ductility since its mechanical strength is for its part approximately 900 MPa and the mode of fracture is transgranular ductile dimple with an elongation at break of several %. These results confirm the very good resistance to hydriding of a nuclear fuel cladding according to the invention, for example under “post-breakaway” conditions.

3. Geometry of the Nuclear Fuel Cladding According to the Invention

[0217] The nuclear fuel cladding obtained by the manufacturing process of the invention is described with reference to FIGS. 7A and 7B, in the nonlimiting specific case of a tubular geometry.

[0218] According to a first embodiment of the invention, the cladding illustrated by FIG. 7A is composed of a zirconium-based internal layer (1), the internal surface of which delimits a closed volume capable of receiving the nuclear fuel. The internal layer (1) forms a substrate on which is placed an external layer (2) composed of a chromium-based protective material which makes it possible to improve the resistance to oxidation of the cladding at very high temperature.

[0219] According to a second embodiment illustrated by FIG. 7B, the cladding may be provided with an interposed layer (3) placed between the internal layer (1) and the external layer (2). In this case, the combination of the internal layer (1) and of the interposed layer (3) forms the substrate. The interposed layer (3) is composed of at least one interposed material, such as, for example, tantalum, capable of preventing or limiting the diffusion of the chromium from the external layer (2) toward the internal layer (1).

[0220] According to a third nonillustrated embodiment, an internal coating is placed under the internal layer (1), and thus directly facing the volume capable of receiving the nuclear fuel.

[0221] It emerges from the preceding description that the process of the invention makes it possible to manufacture a nuclear fuel cladding exhibiting an improvement in the resistance to oxidation at very high temperature. The additional safety margins thus obtained make it possible among others to prevent or delay the deterioration in the cladding in the event of worsening or persistence of the accident situation.