Multilayer material resistant to oxidation in a nuclear environment

Abstract

Multilayer material comprising a zirconium-based substrate covered with a multilayer coating, the multilayer coating comprising metallic layers composed of identical or different substances chosen from chromium, a chromium alloy or a ternary alloy of the NbCrTi system. Such a material has improved resistance to oxidation in accident conditions of a nuclear reactor. The invention also relates to a multilayer coating, a part composed wholly or partly of the multilayer material or of the multilayer coating, as well as the method for manufacturing the multilayer material such as for example a magnetron cathodic sputtering process.

Claims

1. Multilayer material comprising a zirconium-based substrate covered with a multilayer coating, said multilayer coating comprising metallic layers composed of substances selected from the group consisting of chromium, a chromium alloy, and a ternary alloy of the NbCrTi system, wherein said metallic layers are not all of identical composition.

2. Multilayer material according to claim 1, said multilayer coating consisting of said metallic layers.

3. Multilayer material according to claim 1, wherein said multilayer coating comprises from 2 to 50 metallic layers.

4. Multilayer material according to claim 1, wherein each of said metallic layers has a thickness from 3 nm to 1 m.

5. Multilayer material according to claim 1, wherein the cumulative thickness of said metallic layers is from 6 nm to 10 m.

6. Multilayer material according to claim 5, wherein said multilayer coating comprises at least ten metallic layers each of which has a thickness of at least 100 nm, the cumulative thickness of said metallic layers being from 1 m to 6 m.

7. Multilayer material according to claim 1, wherein said metallic layers composed of chromium or of a chromium alloy contain at least one chemical element chosen from silicon or yttrium.

8. Multilayer material according to claim 7, wherein silicon or yttrium is present at a content of from 0.1 to 20 at %.

9. Multilayer material according to claim 1, wherein the ternary alloy of the NbCrTi system comprises in atomic percentage from 50% to 75% of niobium, from 5% to 15% of chromium and from 20% to 35% of titanium.

10. Multilayer material according to claim 1, wherein said metallic layer or layers composed of a ternary alloy of the NbCrTi system have a thickness from 5 nm to 500 nm.

11. Multilayer material according to claim 1, wherein said metallic layers are i) one or more layers composed of chromium and/or a chromium alloy and ii) one or more layers composed of the ternary alloy of the NbCrTi system.

12. Multilayer material according to claim 11, wherein a metallic intermediate bonding layer composed of chromium or of chromium alloy is in contact with the zirconium-based substrate.

13. Multilayer material according to claim 1, wherein said metallic layers are independently selected from the group consisting of chromium and a chromium alloy.

14. Multilayer material according to claim 1, wherein said metallic layers are all composed of a ternary alloy of the NbCrTi system.

15. Multilayer coating comprising metallic layers, at least one of which is a ternary alloy of the NbCrTi system, and wherein said metallic layers are not all of identical composition.

16. Multilayer coating according to claim 15, wherein said metallic layers are i) one or more layers composed of chromium and/or a chromium alloy and ii) one or more layers composed of the ternary alloy of the NbCrTi system.

17. Multilayer coating according to claim 15, further comprising an outer bonding layer composed of chromium or of a chromium alloy.

18. Part composed wholly or partly of the multilayer material or of the multilayer coating as defined according to claim 1, said part being a component of a nuclear reactor.

19. Part according to claim 18, said part being a nuclear fuel cladding, a guide tube, a spacer grid or a plate fuel.

20. Method for manufacturing a multilayer material as defined according to claim 1, comprising a plurality of metal deposition steps, as a result of which a zirconium-based substrate is covered with a multilayer coating comprising metallic layers composed of substances independently selected from the group consisting of chromium, a chromium alloy and a ternary alloy of the NbCrTi system, wherein one such metallic layer is deposited in each metal deposition step, and wherein said metallic layers are not all of identical composition.

21. Method of manufacture according to claim 20, wherein the substrate is covered by performing sequential deposition.

22. Method of manufacture according to claim 21, wherein the sequential deposition is carried out at a temperature of at most 580 C.

23. Method of manufacture according to claim 21, wherein the substrate is covered by means of an operation of chemical vapor deposition or of pulsed electrolysis.

24. Method of manufacture according to claim 21, wherein the substrate is covered by means of an operation of physical vapor deposition.

25. Method of manufacture according to claim 24, wherein the operation of physical vapor deposition is cathodic sputtering.

26. Method of manufacture according to claim 25, wherein the cathodic sputtering is of the magnetron type.

27. Method of manufacture according to claim 24, wherein physical vapor deposition is carried out at a temperature of from 50 C. to 700 C.

28. Multilayer material obtained or obtainable by the method of manufacture according to claim 20.

29. A multilayer material comprising a zirconium-based substrate covered with a multilayer coating, said multilayer coating comprising a plurality of layers of identical composition, said composition selected from the group consisting of chromium, a chromium alloy, and a ternary alloy of the NbCrTi system; said multilayer coating being differentiable from a monolayer coating of the same total thickness and composition, by virtue of a difference in the property of resistance to oxidation or corrosion of the zirconium-based substrate of the multilayer coating relative to those of the monolayer coating, wherein said monolayer coating and each layer of the multilayer coating are deposited by the same method, except that the duration of the deposition for each such layer of the multilayer is shorter than that for the deposition of the monolayer.

30. Method for manufacturing a multilayer material according to claim 29, said method comprising a plurality of metal deposition steps, as a result of which a zirconium-based substrate is covered with a multilayer coating comprising metallic layers, wherein one such metallic layer is deposited in each metal deposition step, and the layers are of identical composition, said composition being selected from the group consisting of chromium, a chromium alloy and a ternary alloy of the NbCrTi system.

31. Multilayer material obtained by the method of manufacture according to claim 30.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIGS. 1 to 4 show scanning electron microscopy (SEM) images of materials consisting of a substrate of Zircaloy-4 provided with a Cr monolayer coating (FIG. 1), Cr/Cr multilayer coating (FIG. 2), NbCrTi monolayer coating (FIG. 3) and Cr/NbCrTi multilayer coating (FIG. 4).

(2) FIG. 5 is a graph of the weight increase in nominal conditions, as a function of time, of materials consisting of a substrate of Zircaloy-4 with or without a monolayer or multilayer coating.

(3) The diagram in FIG. 6 illustrates the weight increase in accident conditions, after 850 seconds, of materials consisting of a substrate of Zircaloy-4 with or without a monolayer or multilayer coating. The micrographs in optical microscopy on polished sections in FIGS. 7 to 12 illustrate this weight increase, for materials consisting of a substrate of Zircaloy-4: without coating (FIG. 7), with a NbCrTi monolayer coating (FIG. 8), with a Cr/NbCrTi multilayer coating of reference M600 (FIG. 9) or of reference M1000 (FIG. 10), with a Cr monolayer coating (FIG. 11), with a Cr/Cr multilayer coating (FIG. 12).

(4) FIGS. 13 to 18 are micrographs obtained by light microscopy on polished sections of materials consisting of a substrate of Zircaloy-4: without coating, after undergoing oxidation in nominal conditions (FIG. 13) and then oxidation in accident conditions (FIG. 14), with Cr monolayer coating, after undergoing oxidation in nominal conditions and then in accident conditions (FIG. 15), with Cr/Cr multilayer coating, after undergoing oxidation in nominal conditions (FIG. 16) and then oxidation in accident conditions (FIG. 17). FIG. 18 is a micrograph of the material in FIG. 17 of a zone not shown in this figure and in which the coating has a crack.

(5) FIG. 19 shows the bending strain at room temperature as a function of a stress applied to test specimens of Zircaloy-4 without coating and with Cr/NbCrTi and Cr/Cr multilayer coatings.

(6) FIGS. 20 and 21 are SEM images that illustrate the oxidation after 15000 seconds under steam at 1000 C., for materials consisting of a substrate of Zircaloy-4 without coating (FIG. 20) and provided with a Cr/Cr multilayer coating (FIG. 21). The effects of this oxidation are illustrated by the graph in FIG. 22, which shows the evolution of the weight increase over time for the materials consisting of a substrate of Zircaloy-4 without coating (straight line 1) and provided with a Cr/Cr multilayer coating (straight line 2).

DESCRIPTION OF PARTICULAR EMBODIMENTS

(7) In the following examples, various materials are produced by deposition on a substrate of Zircaloy-4:

(8) i) control monolayer coatings composed of chromium or a ternary alloy of the NbCrTi system, and

(9) ii) multilayer coatings according to the invention alternating with layers of chromium (Cr/Cr), or layers of chromium and layers of the ternary alloy of the NbCrTi system (Cr/NbCrTi).

(10) All the coatings have similar thicknesses.

(11) The ternary alloy of the NbCrTi system chosen is the alloy Nb.sub.67% Cr.sub.10% Ti.sub.23%, the formula of which is expressed in atomic percentage.

(12) The resistance to oxidation and to hydriding, the structural characteristics and the mechanical properties of the materials are tested in nominal conditions (360 C., water at 190 bar) and in conditions representative of an accident of the LOCA type (1100 C., steam) with or without prior oxidation in nominal conditions, according to the conditions representative of those encountered for a nuclear reactor of the PWR type.

(13) The structural analyses are in particular performed by optical microscopy on polished sections. For this purpose, the plates of the materials analyzed are prepared by covering them with a platinum coating (flash) and a gold coating (electrolytic) before embedding in a resin for polishing. These platinum and gold protective coatings prevent the monolayer or multilayer PVD coating, which has become brittle through oxidation, becoming detached during polishing. They also make it possible to improve the image quality in the microscope by electron conduction. These protective coatings are indicated on the micrographs when they appear sufficiently clearly.

1. MANUFACTURE OF MULTILAYER MATERIALS ACCORDING TO THE INVENTION AND OF MATERIALS PRODUCED FOR COMPARISON

(14) The technique of magnetron cathodic sputtering is employed for manufacturing the aforementioned materials.

(15) Plates of Zircaloy-4 with the dimensions 45 mm14 mm1.2 mm are degreased in a strong alkaline solution, rinsed with water, cleaned ultrasonically for 30 minutes in a bath of acetone, and then rinsed with ethanol and stoved.

(16) They are then placed in a cathodic sputtering reactor, and cleaned in situ, operating with an argon partial pressure of 4 Pa and a polarization voltage of 600 V.

(17) On both faces of each plate of Zircaloy-4, the monolayer coatings of pure chromium or of Nb.sub.67% Cr.sub.10% Ti.sub.23%, as well as the corresponding multilayer coatings (Cr/Cr or Cr/Nb.sub.67%Cr.sub.10%Ti.sub.23%) are deposited at 200 C. by cathodic sputtering of chromium targets and of composite targets having inserts of niobium, of chromium and of titanium in suitable proportions.

(18) The argon partial pressure is 0.5 Pa, it is generally between 0.05 Pa and 2 Pa.

(19) The polarization voltage is 100 V. It is typically between 10 V and 400 V.

(20) To facilitate its adherence, the NbCrTi monolayer coating is made on a 500-nm bonding layer of chromium covering the Zircaloy-4. The thickness of the bonding layer may be decreased in order to limit its impact on the overall composition of the coating, especially when the coating comprises few layers.

(21) The Cr/Cr multilayer coatings are produced by interrupting the magnetron discharge several times during deposition, each discharge being separated by a pause time.

(22) The Cr/NbCrTi multilayer coatings are produced by alternately passing the samples opposite each target of Cr and then of NbCrTi, with a discharge time opposite each target fixed as a function of the desired period A. The kinematics of the metal precursors in the enclosure allows precise control of the thickness of each elementary layer forming the multilayer coating. This control is possible starting from a layer thickness of 3 nm.

(23) The operating conditions of magnetron cathodic sputtering and the characteristics of the coatings obtained are shown in Table 1. A period corresponds to the production of a layer of chromium for the Cr/Cr multilayer coatings, or to the motif resulting from the addition of a layer of Cr and a layer of NbCrTi deposited successively for the Cr/NbCrTi multilayer coatings.

(24) TABLE-US-00001 TABLE 1 Cumulative thickness of the Nature of the Coating coating Temperature Power coating Name architecture (m) ( C.) (W) Cr Cr Monolayer 1 to 5 200 400 (Cr) Cr Cr/Cr Multilayer 7 200 400 (Cr) = 500 nm 14 periods Nb.sub.67%Cr.sub.10%Ti.sub.23% NbCrTi Monolayer 4 400 450 (NbCrTi) (at %) (with 500-nm bonding layer of Cr) Cr and Cr/NbCrTi Multilayer 5 to 6 200 400 (Cr) Nb.sub.67%Cr.sub.10%Ti.sub.23% N10 = 2 5 nm 450 450 (NbCrTi) (at %) >500 periods Cr and Cr/NbCrTi Multilayer 5.5 200 400 (Cr) Nb.sub.67%Cr.sub.10%Ti.sub.23% N100 = 2 (50 to 450 (NbCrTi) (at %) 80) nm 40 periods Cr and Cr/NbCrTi Multilayer 6 200 400 (Cr) Nb.sub.67%Cr.sub.10%Ti.sub.23% M600 = 2 300 nm 450 (NbCrTi) (at %) 10 periods Cr and Cr/NbCrTi Multilayer 4 200 400 (Cr) Nb.sub.67%Cr.sub.10%Ti.sub.23% M1000 = 2 400 nm 450 (NbCrTi) (at %) 5 periods

(25) The microstructure of the coatings is observed by SEM on polished section. It is presented in FIGS. 1 to 4, which show the substrate of Zircaloy-4 and the layers of chromium and of NbCrTi.

(26) The interfaces between the 14 layers of the Cr/Cr multilayer coating are not visible in the image in FIG. 4. They may, however, be visualized using a high-resolution technique such as transmission electron microscopy (TEM).

(27) The Cr/NbCrTi multilayer coating shown in FIG. 4 is that which has a period =2300 nm (reference M600 in Table 1), the FIG. 2 indicating the presence of two layers in one and the same period. The light gray and dark gray layers correspond respectively to the layers of the NbCrTi alloy and to the layers of chromium.

(28) These images reveal that the coatings are dense, of uniform thickness and have good adherence to the substrate of Zircaloy-4, without significant defects at the interface.

2. MEASUREMENT OF CORROSION IN NOMINAL CONDITIONS (T=360 C.) AND MICROSTRUCTURE

(29) To evaluate their resistance to oxidation, the zirconium-based plates provided with coating produced in example 1 (with the exception of the plate with Cr/NbCrTi multilayer coating of reference M1000) remain for 60 days in an autoclave whose environment is representative of the conditions in nominal operation of a nuclear reactor of the PWR type.

(30) For purposes of comparison, a control plate of Zircaloy-4 without coating, similar in thickness to the plates with coating, is added.

(31) The environment in the autoclave is water containing 650 ppm of boron and 10 ppm of lithium, heated to 360 C. and pressurized to 190 bar.

(32) Intermediate stops at 10 days and 30 days make it possible to measure the weight increase, reflecting the oxygen uptake, for the various plates.

(33) The results for the weight increases up to 60 days presented in FIG. 5 show that all the monolayer and multilayer coatings containing chromium lead to a significant improvement in oxidation resistance in nominal conditions relative to the control plate of Zircaloy-4 without coating.

(34) Relative to the Cr monolayer coating, the resistance to oxidation in nominal conditions is similar for the Cr/Cr multilayer coating, or even higher for two Cr/NbCrTi multilayer coatings (references N10-200 C. and N100-200 C.) when exposure to oxidation is less than 60 days.

(35) These results are confirmed by measuring the thickness of the layer of oxide on the Zircaloy-4 plates, which is from 1.8 m of zirconia (ZrO.sub.2) in the absence of coating, to less than 0.4 m of chromium oxide (Cr.sub.2O.sub.3) with the multilayer coatings that prevent formation of zirconium oxide in the underlying substrate, with the possible exception of oxidation opposite cracks present in the coating.

(36) Moreover, micrographs in optical microscopy on polished section (not reproduced here) are taken on the section of a plate of Zircaloy-4 with Cr/NbCrTi multilayer coating (reference M600 in Table 1). They confirm that this coating prevents the formation of ZrO.sub.2 that was found for a control plate without coating. This property is obtained owing to the formation of a protective layer of chromium oxide (Cr.sub.2O.sub.3) with thickness of 100 nm on the surface of the coating, constituting a diffusion barrier to oxygen, limiting or even preventing the formation of ZrO.sub.2 beneath the multilayer coating.

(37) Relative to a monolayer material with Cr coating, it therefore appears that the multilayer materials with Cr/Cr or Cr/NbCrTi coating preserve good resistance to oxidation in nominal conditions, or even improve it for a Cr/NbCrTi coating.

3. MEASUREMENT OF CORROSION AFTER 850 SECONDS IN ACCIDENT CONDITIONS UNDER STEAM (T=1100 C.) AND MICROSTRUCTURE

(38) Tests are conducted in order to evaluate the oxidation resistance, in accident conditions, of the zirconium-based plates produced in example 1.

(39) The conditions are those of an accident of the LOCA type during which the temperature of the nuclear fuel cladding increases rapidly to above 800 C. or even above 1050 C. and may even reach 1200 C., and then decreases sharply following quenching by the water from the security sprinklers with the aim of flooding the core again. The conditions of the tests correspond to the envelope conditions of an LOCA accident taken into account in the safety calculations.

(40) The plates are held at the end of an alumina rod, and are then placed for 850 seconds in an enclosure in which there is circulation of steam heated to 1100 C. by a furnace permitting oxidation in steam.

(41) The plates are then dropped into a quenching bath filled with water at room temperature. The bottom of the bath is provided with a cushion breaking the fall of the plates and a white cloth for recovering the particles that become detached from the plates in the event of exfoliation of the phases that underwent embrittlement following the thermal shock of quenching.

(42) 3.1. Measurement of the Weight Increase

(43) The oxidized plates and any exfoliated fragments are weighed to determine the weight increase due to the amount of oxygen that has diffused into the plates.

(44) Measurement is repeated once for the Zircaloy-4 plate without coating, and twice for the plates with Cr/NbCrTi multilayer coating (reference M600) and with Cr/Cr multilayer coating. The difference obtained in the values for one and the same plate is due to experimental scatter.

(45) The weight increases measured are presented in Table 2 and are illustrated by the diagram in FIG. 6.

(46) TABLE-US-00002 TABLE 2 Weight increase at 1100 C. Plate (mg/cm.sup.2) Zircaloy-4 without coating 10.82 Zircaloy-4 without coating 12.32 Zircaloy-4 with Cr monolayer coating 6.55 Zircaloy-4 with NbCrTi monolayer coating 8.75 Zircaloy-4 with Cr/NbCrTi coating 7.22 (N10-200 C.) Zircaloy-4 with Cr/NbCrTi coating 3.92 (N10-450 C.) Zircaloy-4 with Cr/NbCrTi coating (N100) 7.22 Zircaloy-4 with Cr/NbCrTi coating (M600) 2.37 Zircaloy-4 with Cr/NbCrTi coating (M600) 2.04 Zircaloy-4 with Cr/NbCrTi coating (M600) 3.16 Zircaloy-4 with Cr/NbCrTi coating (M1000) 4.5 Zircaloy-4 with Cr/Cr multilayer coating 1.01 Zircaloy-4 with Cr/Cr multilayer coating 1.88 Zircaloy-4 with Cr/Cr multilayer coating 1.1

(47) These data suggest that the Cr/NbCrTi and Cr/Cr multilayer coatings improve oxidation resistance in accident conditions, not only relative to absence of a coating, but also significantly relative to the corresponding Cr or NbCrTi monolayer coatings.

(48) This resistance is particularly improved for the multilayer coatings containing at least 10 layers (and therefore with a minimum layer thickness of 100 nm, preferably between 100 nm and 500 nm), more particularly for the Cr/NbCrTi multilayer coatings of reference M600 (10 periods) and Cr/Cr multilayer coatings.

(49) It should nevertheless be noted that, although to evaluate the level of oxidation of Zircaloy-4, the weight increase can be directly correlated with the oxygen uptake in the case of Zircaloy-4 without coating (formation of ZrO.sub.2 and of Zr-(O) in known proportions), this is not possible a priori in accident conditions for coated specimens as the oxidation of the coatings also contributes significantly to this weight increase.

(50) Such a correlation is, however, possible in nominal conditions in view of the very limited oxidation of the coatings.

(51) 3.2. Structure and Measurement of the Thickness of the Oxidized Layers

(52) In addition, the thickness of the ZrO.sub.2 and Zr-(O) phases in the plates with coating is evaluated using microstructural examination by optical microscopy on polished section.

(53) The micrographs obtained show the microstructures after oxidation in accident conditions of the plates without coating (FIG. 7), and of the plates with NbCrTi monolayer coating (FIG. 8), Cr/NbCrTi multilayer coatings (reference M600: FIG. 9; reference M1000: FIG. 10), Cr monolayer coating (FIG. 11) and Cr/Cr multilayer coating (FIG. 12) that led to the smallest weight increases at 1100 C.

(54) Each figure reveals a microstructure that has the following successive layers: FIG. 7: outer layer of ZrO.sub.2, layer of Zr-(O), substrate of Zircaloy-4; FIG. 8: outer layer of platinum and of gold, Cr/NbCrTi multilayer coating partly oxidized, dark zone corresponding to detachment during polishing due to the presence of brittle phase, layer of Zr-(O), substrate of Zircaloy-4; FIGS. 9 and 10: Cr/NbCrTi multilayer coating partly oxidized (with clear outer layer consisting of gold in FIG. 10), layer of Zr-(O) extending in places in the substrate of Zircaloy-4 in the form of needles of Zr-(O) with less than 100 m of projected length, whose presence is delimited in FIG. 9 as an example, substrate of Zircaloy-4; FIG. 11: coating of chromium partly oxidized, layer of Zr-(O) extending in places in the substrate of Zircaloy-4 in the form of needles of Zr-(O) with less than 100 m of projected length, substrate of Zircaloy-4; FIG. 12: outer layer of Cr.sub.2O.sub.3 resulting from oxidation of the Cr/Cr multilayer coating, sound Cr/Cr multilayer coating, substrate of Zircaloy-4.

(55) The Cr/NbCrTi and Cr/Cr multilayer coatings (FIGS. 9, 10 and 12) therefore prevent formation of the brittle oxide ZrO.sub.2, whereas this oxide covers the uncoated plate to a thickness of 60 m (FIG. 7).

(56) The improvement in oxidation resistance of Zircaloy-4 in accident conditions through the use of the Cr/Cr or Cr/NbCrTi multilayer coatings is also confirmed by measuring the thickness of the layers of ZrO.sub.2 and the equivalent thickness of the layers of Zr-(O) and by determining the equivalent thickness of oxidized Zircaloy-4, measured on six plates. These measurements are presented in Table 3. They are corroborated by complementary analyses by Castaing microprobe (WDS assays) of the concentration profiles on the section of the oxidized plates.

(57) In this table, the equivalent thickness of Zr-(O) corresponds to the thickness of the layer of Zr-(O), plus the thickness of a layer whose area is equivalent to the area of the Zr- (O) needles.

(58) The equivalent thickness of oxidized Zircaloy-4 (i.e. of Zircaloy-4 made brittle by penetration of oxygen) is calculated from the following formula:

(59) Equivalent thickness of oxidized Zircaloy-4=Equivalent thickness of Zr-(O)+Thickness of ZrO.sub.2/1.56

(60) The Pilling-Bedworth coefficient, which has a value of 1.56, reflects the density change on oxidation of zirconium to ZrO.sub.2.

(61) It can be seen from Table 3 that the Cr/Cr multilayer coating displays good hermeticity, since although there is some penetration of oxygen into Zircaloy-4, this penetration is not significant enough for Zr-(O) to appear.

(62) TABLE-US-00003 TABLE 3 Thickness of the phases (m) Zr-(O) (equivalent Oxidized thickness)/ Zircaloy-4 etch on (equivalent ZrO.sub.2 needles thickness) Zircaloy-4 without coating 60 62/NO 101 Zircaloy-4 with NbCrTi 25 80/NO 96 monolayer coating Zircaloy-4 with Cr/NbCrTi 0 8/YES 8 multilayer coating (M600) Zircaloy-4 with Cr/NbCrTi 4.90 51/YES 54.70 multilayer coating (M1000) Zircaloy-4 with Cr 53 57/YES 92 monolayer coating Zircaloy-4 with Cr/Cr 0 0/NO 0 multilayer coating

(63) Even though finer observation of the microstructure shows that the Cr/NbCrTi (M600) and Cr/Cr coatings respectively have, for about 2 m, a partly oxidized layer (a layer of mixed oxide of chromium and niobium, and a layer of chromium oxide, respectively), these oxidized layers have a protective and sacrificial role with respect to oxidation of the underlying Zircaloy-4.

(64) 3.3. Measurement of the Oxygen Content

(65) Using WDS assay on polished section, the content by weight of oxygen in the Zr-ex- layer obtained after quenching is also measured at the core of the plates over a distance of 400 m.

(66) The measurements presented in Table 4 show that the oxygen content of the zirconium alloy is lowered significantly owing to the presence of the multilayer coatings.

(67) TABLE-US-00004 TABLE 4 Oxygen content in the ex- phase (wt %) Zircaloy-4 without coating 0.40 0.07 Zircaloy-4 with Cr/NbCrTi 0.23 0.03 multilayer coating Zircaloy-4 with Cr/Cr 0.16 0.03 multilayer coating

(68) This is particularly advantageous because when this oxygen content is above 0.4 wt %, the ex- phase has the drawback of adopting brittle behavior at 20 C.

(69) Combined with a dramatic decrease in thickness of the layers of ZrO.sub.2 and Zr-(O), the materials with Cr/NbCrTi and particularly Cr/Cr multilayer coatings are therefore able to ensure ductility at the core of zirconium-based nuclear fuel cladding. Such a property is decisive with respect to behavior on quenching and after quenching of the cladding in order to satisfy the safety criteria connected with LOCA.

(70) 3.4. Influence of the Multilayer Character

(71) The data in Tables 2, 3 and 4 clearly reveal a very significant improvement in the resistance of the substrate of Zircaloy-4 to oxidation in accident conditions through the use of a multilayer coating instead of a monolayer coating of equivalent composition.

(72) This improvement is also illustrated by comparing the microstructure: of the substrates of Zircaloy-4 with NbCrTi monolayer coating (FIG. 8) or Cr/NbCrTi multilayer coating (reference M600: FIG. 9, and to a lesser extent reference M1000: FIG. 10), of the substrates of Zircaloy-4 with Cr monolayer coating (FIG. 11) or Cr/Cr multilayer coating (FIG. 12) for which the oxidation resistance of the substrate appears total.

4. INFLUENCE OF THE COMPOSITION IN NOMINAL CONDITIONS AND IN ACCIDENT CONDITIONS

(73) To determine the influence of the composition of the multilayer coating of the invention, a plate is prepared consisting of a substrate of Zircaloy-4 provided with a TiN/AlTiN multilayer coating based on titanium nitride and mixed nitride of aluminum and titanium. The TiN/AlTiN multilayer coating with a total thickness of 3.4 m consists of a sublayer of TiN with a thickness of 200 nm, which is superposed with more than 400 alternating layers of AlTiN or TiN with a thickness of about 7 nm, having a cumulative thickness of 3 m, then a final layer of AlTiN with a thickness of 200 nm.

(74) This multilayer coating is tested in nominal conditions and in accident conditions according to the protocols of examples 2 and 3. The oxidation resistance of the substrate of zirconium alloy is improved by the TiN/AlTiN coating in nominal conditions, but no improvement is found in accident conditions.

(75) The weight increase of about 10 mg/cm.sup.2 after 800 seconds and 18 mg/cm.sup.2 after 3000 seconds, as well as the thickness of the oxides formed, are in fact comparable to the plate of Zircaloy-4 without coating.

(76) The various measurements in example 3 show that it is indeed the combination of the structure and composition of the multilayer material of the invention that makes it possible to improve the oxidation resistance in accident conditions.

5. MEASUREMENT OF CORROSION IN ACCIDENT CONDITIONS UNDER STEAM (T=1100 C.) AFTER PRIOR OXIDATION IN NOMINAL CONDITIONS (T=360 C.) AND MICROSTRUCTURE

(77) A hypothetical accident scenario of the LOCA type may occur at any stage in the life of the nuclear fuel cladding in service, therefore after some low-temperature oxidation.

(78) The following measurements are intended to evaluate the effect of prior oxidation in nominal conditions on the efficacy of the Cr/NbCrTi multilayer coating, Cr monolayer coating and Cr/Cr multilayer coating with respect to protection against oxidation in accident conditions.

(79) For this purpose, the following plates are submitted to the protocol of oxidation and measurement, successively, according to example 2 (nominal conditions) and then according to example 3 (accident conditions): a plate of Zircaloy-4 without coating; a plate of Zircaloy-4 with Cr monolayer coating; a plate of Zircaloy-4 with Cr/Cr multilayer coating; a plate of Zircaloy-4 with Cr/NbCrTi multilayer coating (Reference M600).

(80) 5.1. Structure and Measurement of the Thickness of the Oxidized Layers

(81) The micrographs obtained by optical microscopy on polished section are reproduced in FIGS. 13 to 18. They show that the presence of a layer of pre-oxide (ZrO.sub.2 or Cr.sub.2O.sub.3) formed in nominal conditions on the surface of the plates only has a slight influence on subsequent oxidation in accident conditions.

(82) As before, a layer of ZrO.sub.2 forms in nominal conditions on the surface of the plate of Zircaloy-4 without coating (FIG. 13). This layer of oxide then becomes thicker notably in accident conditions and is accompanied by formation of an underlying layer of Zr-(O) with a thickness of 62 m (FIG. 14). This behavior is similar to that of oxidation in accident conditions only.

(83) For the plate of Zircaloy-4 with Cr monolayer coating, a layer of Cr.sub.2O.sub.3 forms at the surface in nominal conditions (not shown). In accident conditions (FIG. 15), the layer of Cr.sub.2O.sub.3 becomes thicker (dark gray layer of 1.5 m), the unoxidized layer of Cr is still present (white layer of 2 m) but is no longer protective, which leads to oxidation of the substrate in the form of a layer of ZrO.sub.2 of about twenty microns and an underlying layer of Zr-(O) of about sixty microns.

(84) Regarding the plate of Zircaloy-4 with Cr/Cr multilayer coating, a layer of Cr.sub.2O.sub.3 forms at the surface in nominal conditions (FIG. 16). Opposite a crack in the coating (rare defect), islands of ZrO.sub.2 may form, with thickness comparable to the material without coating. In accident conditions, the layer of Cr.sub.2O.sub.3 then becomes thicker while continuing advantageously to perform a sacrificial protective role, since the underlying Cr/Cr multilayer coating still keeps a significant thickness (FIG. 17) and prevents oxidation of the inner layer of Zircaloy-4. It should be noted that the behavior of the Cr/NbCrTi multilayer coating (micrograph not reproduced) is similar to that of the Cr/Cr multilayer coating.

(85) The only signs of oxidation are those located opposite the few rare cracks present in the initial Cr/Cr multilayer coating: a small island of ZrO.sub.2, formed in nominal conditions and marked in FIG. 16, gives rise in accident conditions to islands of ZrO.sub.2 and of Zr-(O), which are delimited with dotted lines in FIG. 18. This demonstrates the major role played by the multilayer coating in the oxidation resistance of Zircaloy-4.

(86) Table 5 shows, for the various plates, the equivalent thicknesses of Zircaloy-4 oxidized in accident conditions without (example 3) or with prior oxidation in nominal conditions (example 4).

(87) The microstructural observations and Table 5 confirm that prior oxidation in nominal conditions does not affect the subsequent efficacy of the Cr/NbCrTi and Cr/Cr multilayer coatings with respect to protection against oxidation of the substrate of Zircaloy-4 in accident conditions.

(88) TABLE-US-00005 TABLE 5 Equivalent thickness of oxidized Zircaloy-4 (m) Oxidation Oxidation 60 days at 850 s at 360 C. + 850 s 1100 C. at 1100 C. Zircaloy-4 without coating 101 99 Zircaloy-4 with Cr/Cr 0.2 8 multilayer coating

(89) The oxidation found for the multilayer material with Cr/Cr coating is in particular due to the presence of a crack in the coating, which allows diffusion of oxygen and oxidation of the Zircaloy-4 opposite this crack, as is illustrated in FIGS. 16 and 18.

(90) 5.2. Influence of the Multilayer Character

(91) The data in Table 5, as well as comparison of the microstructure of the substrate of Zircaloy-4 with Cr monolayer coating (FIG. 15) or with Cr/Cr multilayer coating (FIG. 17), confirm that the multilayer coating makes it possible to improve the resistance of the substrate to oxidation in nominal conditions and then in accident conditions, relative to the monolayer coating of equivalent composition.

6. MECHANICAL PROPERTIES AFTER OXIDATION AT 1100 C.

(92) The residual ductility of nuclear fuel cladding submitted to accident conditions, or during and after quenching following the accident, is essentially provided by the thickness of the residual layer of Zr-ex-, provided that the oxygen content of this layer remains below the maximum content of 0.4 wt % at 20 C.

(93) In order to evaluate their residual ductility, test specimens with dimensions of 25 mm to 45 mm3 mm1 mm of Zircaloy-4 without coating and with Cr/NbCrTi multilayer coating (reference M600) and Cr/Cr multilayer coating are taken from plates that have undergone oxidation in accident conditions according to example 3. Their mechanical strength is then tested in three-point bending at room temperature.

(94) The stress/strain curves obtained are reproduced in FIG. 19.

(95) They show that, beyond the elastic region, the test specimen of Zircaloy-4 without coating displays some strain. Instabilities caused by successive cracking of ZrO.sub.2 and Zr-(O) gradually lead to destruction of the material, but make it possible to accommodate the deformation of the underlying residual ductile Zircaloy-4 and avoid complete fracture of the test specimen.

(96) Regarding the test specimens of Zircaloy-4 with multilayer coatings, they display deformation without fracture that is at least as great, while avoiding the phenomena of surface spalling of brittle phase. The test specimen of Zircaloy-4 with Cr/Cr multilayer coating in particular has notably improved mechanical strength, since it has a deflection of 5 to 6 mm for a stress that may reach 42 MPa to 47 MPa.

(97) This improvement in mechanical strength may prove decisive for the good mechanical behavior of nuclear fuel cladding before and after quenching.

7. MEASUREMENT OF CORROSION AND HYDROGEN UPTAKE IN ACCIDENT CONDITIONS AFTER 15 000 SECONDS UNDER STEAM (T=1000 C.)

7.1. Measurement of Corrosion

(98) The plates with substrate of Zircaloy-4 without coating and with Cr/Cr multilayer coating are oxidized at 1000 C. for 15000 seconds.

(99) The temperature of 1000 C. is within a range that leads to instability of the layer of ZrO.sub.2 that may be formed on the surface.

(100) The variation over time of the weight increase of the plates is illustrated in FIGS. 20 and 21, as well as in Table 6, to which the graph in FIG. 22 corresponds.

(101) TABLE-US-00006 TABLE 6 Weight increase (mg/cm.sup.2) Zircaloy-4 with Oxidation time Zircaloy-4 Cr/Cr multilayer (seconds) without coating coating 0 0 0 5000 10.05 1.32 5000 10.68 1.22 7500 20.75 15000 40.3 3.14

(102) Each figure reveals a microstructure that has the following successive layers: FIG. 20: outer layer of ZrO.sub.2, layer of Zr-(O), substrate of Zircaloy-4; FIG. 21: outer layer of gold, layer of Cr.sub.2O.sub.3, Cr/Cr multilayer coating, layer of Zr-(O), substrate of Zircaloy-4.

(103) These data reveal that the weight increase resulting from the formation of oxide is still greatly limited by the Cr/Cr multilayer coating, even after 15 000 seconds of oxidation in accident conditions.

(104) This behavior is confirmed by the microstructural observations, which show a smaller thickness of the layer of Zr-(O) for the substrate of Zircaloy-4 with Cr/Cr multilayer coating (FIG. 21), relative to that without coating (FIG. 20) which moreover shows partial stripping of the layer of Zr-(O) during polishing due to the presence of brittle phase and formation of a layer of ZrO.sub.2.

(105) Moreover, after 15 000 seconds of oxidation, the images of the surface of the plates (not reproduced here) show that only the surface of the plate without coating displays marked exfoliation connected with the low mechanical strength of ZrO.sub.2. The plate with Cr/Cr multilayer coating, for its part, does not display any spalling.

7.2. Measurement of Hydrogen Uptake

(106) In order to determine their content of dissolved hydrogen, the plates are heated to 600 C. in order to dissolve all the hydrides that have formed.

(107) Using calorimetry, the hydrogen content is then measured by integration of the exothermic peak of precipitation of the hydrides after cooling.

(108) The results are presented in Table 7.

(109) TABLE-US-00007 TABLE 7 Weight increase Hydrogen uptake after 15000 s after 15000 s Zircaloy-4 without 40.30 mg/cm.sup.2 2000 ppm coating Zircaloy-4 with 3.14 mg/cm.sup.2 200 ppm Cr/Cr multilayer coating

(110) The measured hydrogen content shows the gain supplied by the material with Cr/Cr multilayer coating for the resistance to hydriding during oxidation in accident conditions.

8. CONCLUSIONS

(111) The foregoing examples demonstrate that coating a zirconium-based substrate with a multilayer coating according to the invention comprising metallic layers composed of identical or different substances chosen from chromium, a chromium alloy or a ternary alloy of the NbCrTi system offers the following advantages: in nominal conditions: limiting or even preventing oxidation and/or hydriding of the zirconium-based substrate (among other things, formation of the ZrO.sub.2 phase causing embrittlement), especially when some or all of the multilayer coating layers are composed of a niobium alloy; in accident conditions, with or without prior oxidation in nominal conditions: limiting or preventing formation of Zr-(O), or completely preventing formation of ZrO.sub.2, as both of these oxides may cause embrittlement of the zirconium-based substrate; lowering the concentration of oxygen in the Zr-ex- layer in order to improve the residual ductility and mechanical strength of the zirconium-based substrate after oxidation; and decreasing the hydrogen uptake, which may also lead to embrittlement of the cladding. It should be noted in particular that the use of a multilayer coating instead of a monolayer coating of identical or similar composition improves the oxidation resistance of the zirconium-based substrate.

(112) It follows from the aforementioned advantages that the use of a multilayer material according to the invention for manufacturing a zirconium-based nuclear fuel cladding has important practical consequences for the behavior of this cladding during and after an accident, for example of the LOCA type: slowing of high-temperature oxidation, in order to prevent or at least delay possible runaway of oxidation, which would lead to rapid degradation of the cladding associated with uptake or considerable production of hydrogen; improvement in resistance to high-temperature oxidation, giving a significant gain in mechanical properties of the cladding, among others an increase in the residual ductility of the cladding through better behavior during quenching and after quenching. Now, most of the nuclear safety authorities throughout the world have defined a critical degree of oxidation that must not be exceeded in order to meet the margins for ensuring more or less long-term cooling of the core of a nuclear reactor after an accident of the LOCA type. The use of the multilayer material of the invention would enable margins to be gained at the level of the grace periods and critical oxidation temperatures to be observed. This makes it possible to envisage potential simplifications of the safety systems for emergency cooling and/or greater flexibility in the management of nuclear reactors; increase in the mechanical strength of nuclear fuel cladding in order to preserve its structure during ballooning and gain safety margins with respect to the problems of the degree of clogging of the inter-rod channels.

(113) Moreover, apart from accident conditions, the multilayer material of the invention also has the advantages that it has little effect on: the general mechanical properties of the cladding in service conditions; the weight of the fuel rods; neutron behavior making it possible to use coatings optionally with little capture; use of proved methods of deposition facilitating transfer to industrial application; the current and/or future geometry of the nuclear fuel claddings and therefore of the nuclear reactor core. This allows the multilayer material of the invention to be considered for use in the core of various types of nuclear reactors (PWR, BWR, FNR, etc.), for nuclear propulsion, and more generally for any reactor core or nuclear steam generator, whether or not compact, requiring increased resistance to oxidation.