Multilayer tube in ceramic matrix composite material, resulting nuclear fuel cladding and associated manufacturing processes

Abstract

The invention relates to a multilayer tubular part (1) comprising a metal layer forming a metal tubular body (3) and two layers in ceramic matrix composite material covering the metal tubular body, wherein one of the two layers in ceramic matrix composite material covers the inner surface of the metal tubular body to form an inner tubular body (4), whilst the other of the two layers in ceramic matrix composite material covers the outer surface of the metal tubular body to form an outer tubular body (2), the metal tubular body therefore being sandwiched between the inner and outer tubular bodies. The metal tubular body is in metal or metal alloy. Finally, the metal tubular body has a mean thickness smaller than the mean thicknesses of the inner and outer tubular bodies. A said part is useful in particular for producing nuclear fuel claddings.

Claims

1. A nuclear fuel cladding for a nuclear reactor, wherein the nuclear fuel cladding is a multilayer tubular part which has two ends, at least of said ends being open, the nuclear fuel cladding comprising: a full metal layer forming a metal tubular body; a first layer of ceramic matrix composite material which covers an inner surface of the metal tubular body, thereby forming an inner tubular body, a second layer of ceramic matrix composite material which covers an outer surface of the metal tubular body, thereby forming an outer tubular body; the metal tubular body therefore being sandwiched between the inner and outer tubular bodies and improving hermeticity of the nuclear fuel cladding, and the metal tubular body having a smaller mean thickness than the mean thicknesses of the inner and outer tubular bodies, wherein The inner tubular body is made of a ceramic matrix composite chosen among a C.sub.f/C composite, a C.sub.f/SiC composite or a SiC.sub.f/SiC composite and the outer tubular body is made of a ceramic matrix composite chosen among a C.sub.f/C composite, a C.sub.f/SiC composite or a SiC.sub.f/SiC composite.

2. The nuclear fuel cladding of claim 1, wherein the metal tubular body has a mean thickness of between 5% and 20% of a mean thickness of the multilayer tubular part.

3. The nuclear fuel cladding of claim 1, wherein the metal tubular body is in a material chosen from among niobium and its alloys, tantalum and its alloys, tungsten and its alloys, and titanium and its alloys.

4. A tubular structure having a closed cavity and comprising a nuclear fuel cladding as defined in claim 1 and a cover for each open end of the nuclear fuel cladding, the cover being positioned at an open end thereby sealing fully the open end, the cover comprising an inner layer in metal or metal alloy to be secured to the metal tubular body of the nuclear fuel cladding, the closed cavity of the tubular structure being delimited by an inner wall of the nuclear fuel cladding and by an inner wall of the cover.

5. The tubular structure of claim 4, wherein the nuclear fuel cladding, at the at least one open end, comprises an annular region in which the metal tubular body is not covered by the outer tubular body and wherein the cover is formed of a bottom connected to a side edge, the side edge being adapted to cover the annular region.

6. The tubular structure of claim 4, wherein the closed cavity is configured to contain a nuclear fuel and the fission gases released by the nuclear fuel when irradiated.

7. The tubular structure of claim 6, which has a mean thickness of between 50 and 200 micrometers.

8. A nuclear fuel element comprising nuclear fuel housed the a closed cavity of the tubular structure of claim 4.

9. A process for manufacturing the nuclear fuel cladding of claim 1, comprising: a) providing a tubular body in ceramic matrix composite material to form the inner tubular body, the inner tubular body being made of a ceramic matrix composite chosen among a C.sub.f/C composite, a C.sub.f/SiC composite or a SiC.sub.f/SiC composite, by: preparing a first fibrous preform of continuous fibres on a cylindrical supporting element; applying treatment to cause consolidation of the first fibrous preform by forming a matrix in the first fibrous preform, the treatment being conducted at a temperature lower than the degradation temperature of the first fibrous preform and lower than a degradation temperature of the supporting element, thereby obtaining a first consolidated preform; removing the supporting element from the first consolidated preform by chemical attack of a contact surface of a material of the supporting element with the first consolidated preform; densifying the first consolidated preform at a temperature lower than a degradation temperature of the first consolidated preform; b) forming the metal tubular body on the inner tubular body; c) forming the outer tubular body on the metal tubular body, the outer tubular body being made of a ceramic matrix composite chosen among a C.sub.f/C composite, a C.sub.f/SiC composite or a SiC.sub.f/SiC composite, by: preparing a second fibrous preform of continuous fibres on the outer surface of the metal tubular body; applying treatment to cause densification of the second fibrous preform by forming a matrix in the second fibrous preform, the treatment being conducted at a temperature which is lower than a degradation temperature of the second fibrous preform, lower than a degradation temperature of the metal tubular body and lower than a degradation temperature of the inner tubular body.

10. The process of claim 9, wherein step b) comprises the vapour phase depositing of a metal or metal alloy layer on the outer surface of the inner tubular body.

11. The process of claim 9, wherein step b) comprises: inserting the inner tubular body in a metal tube made of metal or metal alloy; plating this metal tube onto an outer surface of the inner tubular body, thereby forming a part; optional annealing of the part thus formed.

12. The process of claim 9, further comprising, between steps a) and b), a surface treatment of a surface of the inner tubular body to reduce a roughness thereof.

13. A process for manufacturing a tubular structure for a nuclear reactor, the tubular structure having a closed cavity and comprising a nuclear fuel cladding, wherein the nuclear fuel cladding comprises: a full metal layer which forms a metal tubular body; a first layer of ceramic matrix composite material, which covers an inner surface of the metal tubular body, thereby forming an inner tubular body; a second layer of ceramic matrix composite material which covers an outer surface of the metal tubular body, thereby forming an outer tubular body; the metal tubular body therefore being sandwiched between the inner and outer tubular bodies and improving hermeticity of the nuclear fuel cladding, and the metal tubular body having a smaller mean thickness than the mean thicknesses of the inner and outer tubular bodies; and the inner tubular body being made of a ceramic matrix composite chosen among a C.sub.f/C composite, a C.sub.f/SiC composite or a SiC.sub.f/SiC composite and the outer tubular body being made of a ceramic matrix composite chosen among a C.sub.f/C composite, a C.sub.f/SiC composite or a SiCf/SiC composite; wherein the nuclear fuel cladding has two open ends, at least one of the open ends being open, and a cover for each open end, the cover being positioned at an open end, thereby fully sealing the open end, and the cover comprising an inner layer made of metal or metal alloy to be secured to the metal tubular body of the nuclear fuel cladding, the closed cavity of the tubular structure being delimited by an inner wall of the nuclear fuel cladding and by an inner wall of the cover; wherein the process comprises manufacture of a nuclear fuel cladding according to the process of claim 9 and sealing of each open end of the nuclear fuel cladding by placing a cover on each open end and securing the cover onto the metal tubular body, the cover comprising an inner layer made of metal or metal alloy.

14. The tubular structure of claim 4, wherein the cover further comprises an additional layer in ceramic matrix composite material to be secured to the outer tubular body of the nuclear fuel cladding.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1, already previously mentioned, illustrates the behaviour of a 2D braided tubular SiC.sub.f/SiC composite subjected to uniaxial tensile stress, the strain being shown along the X-axis and stress along the Y -axis. This traction curve evidences an elastic region (proportionality between stress and strain) extending up to a stress of about 80 MPa and strain corresponding to 0.04% elongation, which delimits the usual operating range over which the hermetic seal of the composite can be envisaged: beyond these values multiple cracking of the matrix occurs which causes loss of hermetic sealing. Also FIG. 1 evidences the yield characteristics of the composite, for stress of the order of 300 MPa and elongation of the order of 0.9%. Finally, FIG. 1 illustrates a usual range of mechanical use of the composites as nuclear fuel cladding, with stresses possibly reaching the order of 200 MPa and strain of the order of 0.5%. Since a cladding of a nuclear fuel element must maintain its hermetic seal and mechanical integrity in any sizing situation, the material whose behaviour is illustrated in FIG. 1 meets the requirement of mechanical integrity (in that there is no rupture or excessive deformation of the cladding) but only meets the requirement of hermetic sealing over its elastic range which is more restricted than the range of use. The object of the patent is to extend the range of hermetic sealing of the nuclear fuel cladding in ceramic matrix composite (SiC.sub.f/SiC, in particular), beyond the range of mechanical use of this type of object: the three regions (accessible/use/targeted) are illustrated in the Figure using a hatched system.

(2) FIG. 2 schematically illustrates an axial section view of the multilayer tubular part according to the invention.

(3) FIGS. 3a and 3b give an axial section and longitudinal section view respectively of a nuclear fuel cladding according to the invention, obtained by placing a nuclear fuel inside a multilayer tubular structure according to the invention.

(4) It is to be noted that the thicknesses of the different tubular bodies are not drawn to scale in FIGS. 2, 3a and 3b. For better visualizing of the metal tubular body its thickness is shown to be comparable to that of the inner and outer tubular bodies which in reality are much thicker.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

(5) With reference to FIG. 2, the tubular part 1 of the invention has an outer layer in CMC material forming an outer tubular body 2, overlying a metal tubular body 3, which itself overlies an inner layer in CMC material forming an inner tubular body 4.

(6) Preferably this tubular part may be used to form a nuclear fuel cladding 10 intended to contain nuclear fuel to form a fuel element 100.

(7) In this case, the nuclear fuel is placed in the tubular part 1, as illustrated in FIG. 3a. The space shown between the inner tubular body 4 and the nuclear fuel 5 may correspond to a gaseous medium or to a porous solid interface joint whose function in particular is: to accommodate, by collapsing, the volume expansion of the fuel without any mechanical load being applied to the cladding; to promote heat transfer from the fuel towards the cladding; to carry released fission gases towards the gas plenum located at the axial end of the fuel element.

(8) The multilayer tubular part of the invention is formed of tubular bodies or tubes which may therefore have two open ends or one open end (the other end being a blind end).

(9) To obtain a nuclear fuel cladding 10, the nuclear fuel 5 must be contained within a sealed enclosure and therefore the open ends of the tubular part 1 must be sealed.

(10) As illustrated in FIG. 3b, one manner proposed by the invention to close the open ends of the tubular part 1 is to provide an annular region in the vicinity of the open ends in which the metal tubular body is not covered by the outer tubular body: the metal layer is therefore accessible on the surround of the open ends of the tubular part. This region can be obtained by removing a portion of the outer layer forming the outer tubular body 2. All that is then required is to position a first metal cover 6 on the two open ends of the tubular part 1, so as to cover the metal layer of the metal tubular body in the annular region and to close the metal tubular body. This closing can be obtained by securing this first cover 6 onto the annular region by forming a weld 8 between the metal walls of the first cover 6 and the annular region. A second cover 7 in CMC material can then be positioned on the first cover to close the outer tubular body 2 in CMC material. It is also possible to use a single cover composed both of an inner metal layer on which a layer in CMC material is deposited.

(11) The second cover is optional if the first cover (metal cover) meets all the constraints for sealing the tubular part, in particular the withstand of the bottom effect well known to those skilled in the art.

(12) Preferably, the cover or covers are mounted and secured to the tubular part so that there is a continuous joining first between the metal layer of the metal tubular body 3 and the first cover 6 (or with the inner metal part of the cover if there is only one cover) and secondly between the outer tubular body 2 and the second cover 7 (or with the outer part in CMC material of the cover if there is only one cover). The said continuous joining can be obtained for example via welding if joining is between two metals or via brazing if joining is between CMC materials.

(13) It is to be noted that the nuclear fuel cladding 10 schematized in FIGS. 2a and 2b is illustrated in the cold state and at the start of irradiation within the reactor, hence the presence of the space between the inner body 4 and the nuclear fuel 5.

(14) The nuclear fuel 5 is in the form of fuel pellets which are stacked inside the fuel cladding 10 the whole forming a fuel element 100.

(15) It is to be noted that the axial space between the nuclear fuel 5 and the first cover 6 is intended to be occupied by an axial positioning device (not illustrated) for the column of fuel pellets (device typically comprising a spring, a spacer and/or wedges).

(16) The CMC materials of the outer and inner tubular bodies of the tubular part may be ceramic matrix composites, for example of SiC.sub.f/SiC, SiC.sub.f/TiC, SiC.sub.f/ZrC or SiC.sub.f/Ti SiC.sub.2 type such as mentioned in document [6].

(17) The hermetic sealing layer is a metal or metal alloy: niobium and its alloys, tantalum and its alloys, tungsten and its alloys, titanium and its alloys; e.g. Nb-1Zr, Nb-1Zr-0,1C, Ta, W-5Re. It is important to note that to guarantee the integrity and properties of the item, the chemical compatibility of the metal or alloy with the CMC material used must be verified over the entire range of temperature of use of the future tubular part, and over the entire temperature range for manufacture of the CMC layers.

(18) The thickness values of the different layers of the multilayer tubular part are preferably within the following ranges: inner CMC layer (inner tubular body): 0.2 to 0.5 metal hermetic sealing layer (metal tubular layer): 50 to 200 m; outer CMC layer (outer tubular body): 0.3 to 1 the thicknesses of the inner and outer CMC layers being chosen however so that they are greater than the thickness of the hermetic sealing layer, preferably 3 times greater or more than the hermetic sealing layer. It is sought to minimize the thickness of the hermetic sealing layer and the overall thickness of the multilayer tubular part, to optimize the neutron.

(19) The multilayer tubular part of the invention is advantageous in that it uses a large majority of ceramic matrix composite phases in lieu and stead of a fully metal part. The purpose of the metal layer here is solely to guarantee the hermetic sealing of the overall part. For many envisaged applications (pressure chamber operating at high temperature for example), the use of metal is not recommended on account of its density and/or weak mechanical strength at high temperature and/or on account of its neutron capture cross-section for nuclear applications. It is therefore necessary to limit the thickness of the metal layer to a strict minimum.

(20) The manufacturing of a multilayer part according to the invention can be broken down into several steps, namely:

(21) 1) the manufacture of the inner tubular body in CMC material;

(22) 2) the preparation of the outer surface of this inner tubular body (this step being optional but preferable);

(23) 3) the manufacture of the metal tubular body (hermetic sealing tube of narrow thickness;

(24) 4) plating, or using any other deposit technique known to those skilled in the art, the metal tubular body onto the inner tubular body ;

(25) 5) preparing the outer surface of the metal tubular body thus obtained (this step being optional but preferable);

(26) 6) producing the fibrous preform of the outer tubular body directly on the metal tubular body obtained at step 5, followed by densification thereof, and finally optional final coating thereof (this coating being optional but preferable) leading to obtaining of the multilayer part.

(27) It is to be noted that it is also possible, instead of manufacturing the inner tubular body and the metal tubular body, to use ready-made tubes.

(28) The first step to manufacture the part is to prepare a tube intended to form the inner tubular body in CMC material. To do so, a fibrous reinforcement is formed around a cylindrical mandrel chosen to suit the type of composite to be prepared.

(29) For composites with reinforcement and matrix of carbon or silicon carbide type, it is preferable to use a mandrel in silica glass so that the mandrel is able to be easily removed at the end of the process by mere chemical dissolution. A good inner surface of the inner tube and heed of dimensions and tolerances (which are most important requirements for the cladding of a nuclear fuel element in particular) are more accessible with this type of mandrel than with a mandrel in graphite that is conventionally used.

(30) For the manufacturing of ceramic matrix composites using oxide phases, the type of mandrel must be adapted to the subsequent densification process.

(31) The shaping of the fibrous architecture of the reinforcement can be achieved using one of the techniques derived from the textile industry suitable for geometric parts having an axis of revolution, such as fibre winding, 2D braiding or 3D interlock.

(32) The thickness of the reinforcement (number of braiding or wind layers) is chosen in accordance with the thickness chosen for this inner tube.

(33) Once the forming of the reinforcement on the mandrel is completed, the reinforcement is densified. For this purpose, a chemical vapour infiltration process or CVI, well known to persons skilled in the art, can be used although other processes such as sintering, Polymer Infiltration Process or PIP, liquid- or mixed-route processes can also be used.

(34) When the densifying of the reinforcement is completed, the mandrel used as support for the reinforcement is removed.

(35) The second step, which is optional but preferable, is to prepare the outer surface of the inner composite tube thus obtained, that is rough and abrasive by nature, so as to obtain a surface having a maximum RMS roughness of 1 to 2 m to allow optimal plating of the future metal tube acting as hermetic sealing layer onto the inner composite tube.

(36) The outer surface of the composite tube can be machined. Experience has shown that diamond grinding of the outer surface of the composite tube using the <<centreless grinding>> technique gives good results: mean surface roughness values of about 1 m can be obtained for a surface subjected to such grinding as compared with 50 to 100 m without any preparation.

(37) It is also possible to have recourse to the chemical or physical vapour depositing (CVD or PVD) of a layer having a thickness of a few hundred nanometers on the outer surface of the said inner composite tube, or to deposit a ceramic coating obtained by liquid route.

(38) Depending on the material chosen for the said layer, this layer may also have the purpose of accommodating differences in deformation between the CMC composite tube and the metal layer applied to the outer surface at the following step of the process. For example, for a composite tube in SiC.sub.f/SiC it is possible to use a material of pyrocarbon type to form this layer.

(39) At the third step a metal tube is prepared which will be used as hermetic sealing layer to form the metal tubular body of the multilayer part. The choice of type of metal is highly important for the intended application and will depend on the type of composite used for the inner and outer tubes and conditions of use. In particular, a metal phase must be chosen that is compatible with the ceramic phases of the composite, whether over the range of operating temperatures of the final part or over the range of manufacturing temperatures of the composite as per the chosen densification process.

(40) If the composites are C.sub.f/C, C.sub.f/SiC or SiC.sub.f/SiC for example, the possible densification processes require implementation temperatures close to 1000 C. In this case, it is therefore necessary to choose only those metals which have good chemical compatibility with the carbon and silicon carbide phases at 1000 C. The proposed metals are niobium and its alloys (Nb-1Zr, Nb-1Zr-0,1C), but also tantalum and its alloys, tungsten or titanium to a lesser extent.

(41) Evidently, if different ceramic phases are concerned, other alloys may be more suitable.

(42) To limit the thickness of the metal tube to the strict minimum, it is chosen to use a technique for forming the metal tube to the desired dimensions which allows a minimum thickness of up to 0.1 mm to be obtained, even lower. It is possible for example to use a cold rolling technique, this technique having the advantage of adapting to numerous metals and alloys. Here the inventors used an HPTR rolling bench but other cold or hot rolling benches can be used. It is also possible to use drawing or extrusion technique.

(43) For the dimensions of the metal tube, an inner diameter is targeted that is equal to the outer diameter of the composite inner tube after grinding, to which clearance is added so that inner composite tube is able to be inserted into the metal tube, this clearance however being as narrow as possible to facilitate the plating step described below. For the conducted tests a diametric clearance of the order of 0.1 mm was used. It is to be recalled here that rolling has the effect of hard working the metal or alloy used. The main consequence of this working is to increase the hardness of the material and to limit its ultimate deformation. To restore the properties of the metal or alloy close to the initial values, it is preferable to perform annealing after rolling, particular to each material.

(44) The following step entails plating the metal tube onto the inner composite tube. The purpose of this operation is to guarantee close contact between these two elements. This plating can be obtained using several techniques. It is possible for example to have recourse to controlled drawing of the metal tube onto the inner composite tube. For this purpose, the inner composite tube is inserted in the metal tube made possible by the clearance provided; the metal tube is then mechanically subjected to tensile stress so that it retracts onto the wall of the inner composite tube (Poisson effect).

(45) Other plating techniques can be envisaged such as hot drawing or magnetic pulsing which uses a magnetic field of strong intensity to plate the metal tube onto the non-magnetic inner composite tube.

(46) It is also possible to replace the manufacturing step of the metal tube and plating step by using a chemical vapour or physical vapour depositing technique, (CVD or PVD), to deposit a metal layer directly on the inner composite tube.

(47) At a fifth step that is optional but preferable, the bilayer tube once formed can optionally undergo grinding to reduce roughness on its outer metal surface, or vapour phase depositing of an additional layer as described previously for the inner composite tube.

(48) The last step consists of manufacturing the outer composite tube. The procedure is similar to that used to manufacture the inner composite layer. The fibrous reinforcement is first shaped using the same techniques as those previously mentioned (fibre winding, 2D braiding or 3D interlock) followed by densification. For this manufacture, and contrary to the first step which required the use of a temporary supporting mandrel, the outer composite tube here is directly shaped on the prepared composite/metal bilayer tube.

(49) This then gives a multilayer ceramic matrix composite part that is hermetically sealed up to yield point such as illustrated in FIG. 2.

(50) Optionally, if it is desired that the final part should have minimum roughness on its outer surface, this outer surface (i.e. the outer composite tube) can be ground using centreless grinding or an additional layer can be applied thereto.

(51) The multilayer tubular part thus prepared can be used to produce a pressurised fluid duct or a pressure chamber, such as cladding for a nuclear fuel element. A description is given below of the manufacture of cladding for nuclear fuel element, this being the priority targeted application of the invention.

(52) As described above, first an inner CMC tube is prepared, here a tube in SiC.sub.f/SiC of inner diameter 7 mm and thickness of 300 m, optionally completed by the coating of the outer surface of the composite tube with a layer of material of pyrocarbon type.

(53) A metal tube in tantalum is then prepared of inner diameter 7.7 mm and thickness of 100 m, either by rolling followed by plating (using a magnetic forming or drawing technique) over the inner CMC tube or by vapour phase depositing a layer directly onto the inner CMC tube.

(54) The tantalum layer is optionally coated with a layer of material of pyrocarbon type having a thickness of a few hundred nanometres.

(55) Finally an outer composite tube in SiC.sub.f/SiC is produced of thickness 600 m directly on the part obtained at the preceding step, by forming the reinforcement in SiC/SiC and impregnating this reinforcement following usual procedure, optionally completed by coating or grinding the outer surface of this outer composite tube. This latter coating, performed by vapour phase (PVD or CVD)depositing of a material of SiC type to a thickness of a few hundred nanometers, is intended to produce a smooth surface finish if this meets a requirement (this may be necessary to limit pressure drops associated with friction of the coolant along the cladding for example).

(56) To obtain a multilayer structure (multilayer part whose open ends are sealed off), the ends of the multilayer tubular part are cleared by machining the outer CMC layer, for example over a length of 5 mm, without damaging the underlying metal layer.

(57) One of the two ends of the tubular part is then closed by welding a cover or metal lid 6 onto the part of the metal tube previously cleared (cover having a thickness of 100 m for example obtained by stamping) then brazing a cover in CMC material (e.g. SiC.sub.f/SiC) onto the outer CMC tube. In FIG. 3b, the weld bead is identified by reference 8 whilst the braze bead is identified by reference 9.

(58) It is optionally possible to combine the metal cover and CMC cover in a single lid.

(59) The CMC cover may be optional, provided a metal is chosen for the cover having adequate thermomechanical properties in terms of refractoriness and resistance to inner pressure under normal and accidental operation. It is possible for example to use a metal cover of thickness 700 m, obtained by stamping or machining, welded to the metal tube and brazed to the outer CMC tube of the multilayer tubular part.

(60) It will be noted that this sealing step of an end of the multilayer tubular structure can become optional if the preceding steps are carried out so as to manufacture a blind tube (closed at one of its ends).

(61) The pellets of nuclear fuel 5 can then be inserted (which here have a diameter of 6.71 mm) inside the tubular structure together with the inner fittings (spring, spacer, wedges) which are arranged between the column of pellets and the cover (not illustrated in FIG. 3b).

(62) Finally the closing of the other open end of the tubular part can be carried out following the procedure described above.

(63) The innovation of the proposed solution lies in the multilayer nature of the cladding concept, with the positioning of the metal layer acting as hermetic sealing layer between two CMC layers meeting requirements of refractoriness and mechanical strength to obtain an advanced fuel element.

(64) With this particular positioning it is possible to obtain a hermetic sealing layer of narrow thickness (50 to 200 m) without any risk of weakening through excessive deformation up to high temperatures and neutron flows, and without any risk of damage by the nuclear fuel and its fission products up to high combustion levels.

(65) This narrow thickness of the metal layer and the lack of any strong interaction with the fuel and its fission products allow recourse to a wide range of hermetic sealing materials.

(66) This solution proposes a durable containment mode of fission products for a fuel element having a cladding in CMC material. In this respect, it opens up prospects of use for this type of cladding whose refractoriness (accompanied by good neutron properties) should allow increased safety of the fuel element whilst guaranteeing its geometric integrity (guaranteed control over reactivity and coolability of the reactor core) up to the very high temperatures of accidental transients which are to be taken into account in the sizing thereof.

(67) The outer and inner tubes in CMC material not only act as mechanical reinforcement but also as refractory reinforcement intended to consolidate the resistance of a conventional metal cladding to accidental transients (thermal stability and creep resistance), which allows the safety objectives to be reached that are typically targeted for GFRs or for a major improvement in the resistance of cladding to the high temperature conditions of some accidental transients in PWRs, BWRs and SFRs.

BIBLIOGRAPHY

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