FR2526213A1 - COMPOSITE SHEATH FOR NUCLEAR FUEL ELEMENT - Google Patents

Abstract

SHEATH PRESENTING BETTER RESISTANCE TO CORROSION UNDER STRAIN. IT INCLUDES A SUBSTRATE 21 IN ZIRCONIUM ALLOY ON THE INTERIOR SURFACE TO WHICH IS METALLURGICALLY BONDED A SHIRT 22 IN DILUTED ZIRCONIUM ALLOY CONSTITUTED BY ZIRCONIUM AND A METAL CHOSEN FROM IRON, CHROME, IRON PLUS CHROME AND COPPER SHIRT REPRESENTING 5 TO 15 OF THE THICKNESS OF THE COMPOSITE SHEATH. APPLICATION TO NUCLEAR FUEL ELEMENTS.

Description

This invention relates, in general, to a

  refinement of the nuclear fuel elements

  designed for use in the core of fission reactors

  nuclear power and, more particularly, a fuel element

  improved nuclear bond having a composite sheath comprising a diluted zirconium alloy metal jacket consisting of zirconium and a metal selected from the group consisting of iron, chromium, iron plus chromium, and

  copper, bonded to the inner surface of the substrate in alloy

zirconium geometry of the sheath.

  We design, build and operate today

  nuclear reactors in which the nuclear fuel is contained in fuel elements which may be in various geometrical shapes,

  for example in the form of plates, tubes or bars.

  The fuel is usually enclosed in a resistant sheath

  both to corrosion, non-reactive and conducting heat on

  assembles the fuel elements into a network at

  in a channel or region where the refrigerant circulates to form a fuel assembly, and sufficient fuel assemblies are combined to form the assembly where the nuclear fission chain reactions or core of the reactor capable of A self-sustaining fission reaction in turn closes the core in a reactor vessel through which a refrigerant is passed The sheath fills several offices, the two main ones of which are in the first place to prevent contact and reactions between the nuclear fuel and the

  refrigerant or the moderator in case one uses a

  derator or between the nuclear fuel, the refrigerant and the moderator in the case where both are used; and second

  prevent radioactive fission products, some of which

  some are gases, to pass fuel in the refrigeration

  In the case where both a refrigerant and a moderator are used, there may be mentioned among the materials which can conventionally constitute the sheath, stainless steel, aluminum and its alloys,

  zirconium and its alloys, niobium (columbium),

  Some magnesium alloys, etc. Sheath failure, ie leakage of the sheath, may lead to refrigerant or moderator contamination.

  and systems associated with them by products that are

  long-lived dioactives to the point of disrupting the functioning

installation.

  Difficulties were encountered in the manufacture and use of nuclear fuel elements for which certain metals and alloys had been

  material constituting the sheath, difficulties

  mechanical or chemical properties of these materials in certain

  Under normal conditions, zirconium and its alloys

  These are excellent nuclear fuel claddings since they have low neutron absorption cross sections and, at temperatures below about 3980 C, are resistant, ductile, extremely stable and relatively non-reactive. presence of steam

  water or demineralized water that is commonly used

  refrigerants and moderators in the reactors.

  However, difficulties were encountered in the use of fuel elements due to brittle sheath failure due to the combined interactions of nuclear fuel, sheath and fission products.

  formed during the course of nuclear fission reactions.

  this misbehavior was caused by localized mechanical stresses due to differences in expansion between the fuel and the sheath (the constraints

  inside the sheath are concentrated at the level of

  nuclear fuel) The nuclear fuel releases

  corrosive fission products that are found in the

  These fission products are formed in the nuclear fuel during the chain fission reaction during the cracking of the fuel and the cladding surface.

  nuclear reactor operation The important friction

  between the fuel and the sheath exaggerates the

located.

  Within the limits of a sealed fuel element, hydrogen gas may be formed during the

  slow reaction between the sheath and the residual water inside the

  This gaseous hydrogen can accumulate

  under certain conditions, to cause hydrolysis

  Localized sheathing with concurrent concurrent deterioration of sheath mechanical properties Gases such as oxygen, nitrogen, carbon monoxide, and carbon dioxide also exert a deleterious action on the sheath over a wide range of temperatures. zirconium sheath of a nuclear fuel element is exposed to one or

  many of the gases listed above and

  during the irradiation in a nuclear reactor, despite the fact that these gases may not be present in the reactor coolant or moderator and may have been excluded from the atmosphere as far as possible. during the manufacture of the sheath and the combustible element Ceramic and refractory compositions

  sintered, such as the uranium dioxide and

  compositions used as free nuclear fuel

  Measurable quantities of the above-mentioned gases are used when heated, for example during the manufacture of the fuel elements and still release fission products during irradiation. It is known that particulate ceramic and refractory compositions such as powders

  uranium dioxide and other powders used as

  nuclear fuel, release even larger amounts of

  gases previously indicated during irradiation.

  These gases thus released are able to react with the sheath

  of zirconium containing the nuclear fuel.

  Thus, in the light of the foregoing, it has been thought that it would be desirable to minimize the attack of

  sheath by water, water vapor and other gases, in particular

  bind the hydrogen, which are reactive towards the sheath since

  fuel element during the entire period of use of the fuel element for the operation of nuclear power plants.

  to find materials-that react chemically fast-

  with water, water vapor and other gases to remove them from the inside of the sheath.

traps.

  Another attempt was to put on the fuel

  nuclear material with a material selected from various materials

  to prevent moisture from coming into contact with the nuclear fuel The coating of the nuclear fuel causes difficulties in reliability as far as the realization of uniform coatings lacking

  In addition, the deterioration of the coating

  can create difficulties in the behavior of long-term

  the duration of the nuclear fuel.

  In GEAP-4555, dated February 1964, a composite sheath of zirconium alloy having

  a metallurgical-bonded stainless steel inner liner-

  to the zirconium alloy, this composite sheath being

  extruded brick of a hollow billet of zinc alloy

  This sheath has the disadvantage that stainless steel gives rise to brittle phases, and that the stainless steel layer absorbs approximately ten to fifteen times the

  neutrons than a zirconium alloy of the same thickness.

  US Patent No. 3,502,549 discloses a method of protecting zirconium and its alloys by electrolytic deposition of chromium to produce a composite material for use in nuclear reactors. Zircaloy-2 surfaces followed by heat treatment in order to obtain a surface diffusion of the electrolytically deposited metal, in Energia Nucleare, Volume 11, N 9,

  (September 1964), pages 505-508 In Stability and Compatibility

  lity of Hydrogen Barriers Applied to Zirconium Alloys, (Sta-

  and the compatibility of hydrogen barriers applied to zirconium alloys), by F. Brossa et al (European Atomic Energy Community, Joint Nuclear Research Center, EUR 4098 e, 1969), methods of depositing different

  coatings and their effectiveness as barriers to

  fusion of hydrogen as well as a coating of Al-Si considered

  re as the most promising barrier to the diffusion of hydrogen Nickel electroplating processes are described

  on zirconium and zirconium-tin alloys and

  These alloys are thermally bonded to produce diffusion bonds within the alloy in Electroplating on Zirconium and ZirconiumTin (electroplating on zirconium and zirconium-tin alloys), by W C Schickner et al.

  (BMI-757, Technical Information Service, 1952).

  U.S. Patent 3,625,821 discloses a fuel element for a nuclear reactor, comprising a

  lubricating the fuel whose inner surface is coated

  kills a metal having an effective cross section for capturing

  neutrons, such as nickel, and containing

  Finely Scattered Consumable Poisons Reactor Development Program Progress Report (Progress Report on the Reactor Program)

  August 1973 (ANL-RDP-19) describes the use of a chemical trap

  a layer of chromium sacrificed on the surface

  inner face of a stainless steel sheath.

  Another attempt was to introduce a bar-

  between the nuclear fuel and the sheath containing the nuclear fuel as described in US Pat. No. 3,230,150 (copper foil), German Patent Publication No. 1,238,115 (titanium layer), US Patent A. No. 3,212,988 (sheet of zirconium, aluminum or beryllium),

  U.S. Patent No. 3,018,238 (Crystalline Carbon Barrier)

  tallin between the UO 2 and the zirconium alloy sheath) and the

  U.S. Patent No. 3,088,893 (stainless steel sheet).

  Although the idea of using a barrier proved to be

  Some of the previous references use materials that are incompatible with the nuclear fuel (for example, carbon may combine with the oxygen of the nuclear fuel) or with the sheath (for example, copper and other metals may react with the sheath and in

  alter the properties), or with the nu-

  (for example, by acting as neutron absorbers

  trons) None of the cited references gives a solution to the problem of localized chemical-mechanical interactions

  between the nuclear fuel and the sheath.

  Other attempts, still using the idea of barriers, are found in US Pat. No. 3,969,186.

  (refractory metal such as molybdenum, tungsten,

  nium, niobium and their alloys, used in the form of a single or multiple layer tube or sheet or coating on the inner surface of the sheath) and in US Patent No. 3,925,151 ( zirconium shirt,

  niobium or their alloys between the nuclear fuel

  re and the sheath with a layer of a lubricious material

  high between the shirt and the sheath).

  US Patent No. 4,045,288 discloses a composite sheath comprising a zirconium alloy substrate, a metal barrier metallurgically bonded to the substrate and an inner layer of zirconium alloy metallurgically bonded to the metal barrier. The barrier is selected from the group consisting of niobium, aluminum, copper, nickel, stainless steel and iron The buried metallic barrier reduces corrosion due to fission products and corrosive gases, but is subject to corrosion cracking due to corrosion. stress and embrittlement by

-Structure of liquid metal.

  U.S. Patent No. 4,200,492 discloses a composite sheath comprising a zirconium alloy substrate and a spongy zirconium jacket. The zirconium jacket

  soft minimizes localized deformation and decreases fissure

  due to stress corrosion and embrittlement by formation of liquid metal, but leads to losses by grinding, etc., during its manufacture and by oxidation In addition, if a gap opens in the sheath, allowing the water

  and / or water vapor to enter the trading bar

  bustible, the zirconium shirt tends to oxidize

  pidement. It therefore remained desirable to produce nuclear fuel elements which minimized the difficulties

indicated above.

  The invention relates to a naked fuel element

  key particularly effective for use in the

  heart of a nuclear reactor that includes a composite sheath

  consisting of a substrate on the inner surface of which a metal jacket of diluted zirconium alloy is metallurgically bonded. The diluted zirconium alloy comprises zirconium and a metal selected from the group consisting of

  iron, chromium, iron plus chromium, and copper, the amount of

  the zirconium alloyed iron content being from about 0.2% to about 0.3% by weight, the amount of chromium being about 0.05%

  and about 3% by weight, the total amount of iron and chromium

  between about 0.15% and about 0.3% by weight, the ratio

  from the weight of iron to the weight of chromium being between about

  1: 1 and about 4: 1 and the amount of copper being

  is between about 0.02% and about 0.2% by weight.

  The substrate of the sheath remains completely unchanged in its design and function compared to the prior practice with regard to nuclear reactors and is made of a material chosen from the materials conventionally used for sheaths such as zirconium alloys. of zirconium alloy sheath has a content

  higher alloy than the zirconium alloy jacket

  diluted nium The diluted zirconium alloy sleeve forms

  a continuous screen between the substrate and the nuclear fuel

  contained in the sheath, and also isolates the alloy of zirconium

  conium or any other substrate of the sheath of

fission and gases.

  The diluted zirconium alloy jacket represents

  from about 1 to about 20 percent of the thickness of the sheath.

  The shirt remains soft relative to the substrate during

  irradiation and minimizes local constraints

  embedded within the nuclear fuel element, thereby protecting the sheath from stress corrosion cracking or embrittlement by liquid metal formation The diluted zirconium alloy sheath opposes the reaction between the substrate and fission products or volatile impurities present within

  the nuclear fuel element and protects, from this

  the substrate of the sheath of the attack by

  fission or volatile impurities.

  This invention has the remarkable advantage of protecting the sheath substrate against stress corrosion cracking and embrittlement by liquid metal formation and, in addition, contact with

  fission, corrosive gases, etc. thanks to the alloyed shirt

  diluted zirconium and this shirt does not increase

  appreciable neutron capture, and does not introduce heat transfer or incompatibility

  In addition, the shirt provides a resistance

  higher than the oxidation by steam or hot water compared to non-alloy zirconium in the case

  opening a gap in the sheath.

  The following description refers to the figures

  attached which respectively represents: FIG. 1, a partial sectional view of a fuel assembly containing nuclear fuel elements

  manufactured in accordance with the teachings of this invention.

  Figure 2, an enlarged cross-sectional view

  of a nuclear fuel element of Figure 1, illus-

  the teachings of this invention.

  If we refer now more particularly to

  FIG. 1 shows in partial section an assembly

  This fuel assembly 10 is constituted by

  killed by a tubular flow channel of approximately square cross section provided at its upper end with a lifting stirrup 12 and at its lower end with a nose (not shown since the lower part of the assembly has been omitted) 10) The upper end of the channel it is open at the level of the exit 13 and the lower end

  the nose has openings for the circulation of refrigerant

  A network of elements or fuel rods 14 is enclosed in the channel 11 and is held there by means of an upper endplate 15 and a lower endplate (not shown since omitted the lower part) The refrigerating liquid generally enters through the openings in the lower end of the nose, rises around the fuel elements 14 and discharges through the upper outlet 13 at a high temperature in the partially vaporized state. for boiling water reactors or

  in the non-vaporized state for pressurized reactors.

  Nuclear fuel elements or rods 14

  are sealed at their ends by means of end plugs.

  Means 18 welded to the sheath 17, which may comprise rods 19 to facilitate mounting of the fuel rod in the assembly. A gap 20 was left at one end of the element to allow longitudinal expansion of the element.

  fuel and the accumulation of gases released by

  A device for retaining the fuel has been placed

  24, in the form of a helical element

  inside space 20 to oppose the axial displacement

  the column of pellets, especially during

  pulation and transport of the fuel element.

  The fuel element has been designed to provide excellent thermal contact between the sheath and the fuel, minimum parasitic neutron absorption, and good warp and vibration resistance.

  are occasionally caused by fast-moving traffic.

the refrigerant is rising.

  FIG. 1 is a fragmentary sectional view of a

  14 or nuclear fuel bar, manufactured in accordance with

  In accordance with the teachings of this invention, the fuel element comprises a core or cylindrical central portion of nuclear material 16, here represented as a set of fuel pellets of fissile material and / or

  fertile placed inside a structural sheath 17.

  In some cases, the fuel pellets may be

  of various shapes, for example cylindrical or spherical

  In other cases, it is possible to use forms of

  different fuel, and for example a fuel

  The physical form of the fuel is of no importance to this invention. Various nuclear fuel materials may be used, including compounds

  Uranium, plutonium compounds,

  and mixtures thereof It is recommended to use as a com-

  uranium dioxide or a mixture comprising

  uranium dioxide and plutonium dioxide.

  Referring now to Figure 2, the fuel

  the nuclear core 16 forming the central core of the fuel element 14 is surrounded by a sheath 17, that in this

  The invention is also referred to as the composite sheath.

  posite encloses the fissile nucleus so as to leave

  valle 23 between the core and the cladding when used in a nuclear reactor The composite cladding comprises a substrate

  external 21 selected from materials that are used classically

  for claddings such as stainless steel and zirconium alloys and, in a preferred embodiment of this invention, the substrate is a zirconium alloy.

like Zircalloy-2.

  A dilute zirconium alloy jacket 22 is metallurgically bonded to the inner circumference of the substrate 21, so that the diluted zirconium alloy jacket forms a shield protecting the substrate, between the substrate and the nuclear fuel 16 therein The diluted zirconium alloy jacket is preferably from about 1 to about 20% of the thickness of the sheath. It would be difficult in a commercial production.

  to make a diluted zirconium alloy jacket representing

  less than about 1% o of the thickness of the sheath, and the use of a diluted zirconium alloy jacket

  representing more than 20% of the thickness of the sheath

  You do not have any extra advantage.

  having more than about 20% of the thickness of the sheath

  signifies a concomitant decrease in the thickness of the

  trat and a possible weakening of the sheath.

  The diluted zirconium alloy is composed of zirconium and an alloying member selected from the group consisting of iron, chromium, iron plus chromium and copper. As used herein, the expression diluted zirconium alloy means a zirconium alloy having an alloy content sufficiently low to cause a higher ductility and a higher rate of deformation than would be present in the material constituting the substrate in

  equivalent conditions of stress.

  The amount of iron alloyed with zirconium is included

  between about 0.2% and about 0.3% by weight, and preferably

  from about 0.2% to about 0.25% by weight.

  The chromium content is from about 0.05% to about 0.3% by weight and preferably about 0.15%

and about 0.25% by weight.

  Iron plus chromium can be incorporated so that the total amount of both components is included

  between about 0.15% and about 0.3% by weight and preferably

  between about 0.2% and about 0.25% by weight, the ratio of

  weighted port of chromium iron being between about

  1: 1 and about 4: 1, and preferably about 2: 1.

  The copper is used in an amount of from about 0.02% to about 0.2% by weight and preferably

  between about 0.05 and about 0.15% by weight.

  The diluted zirconium alloy jacket isolates the substrate from gaseous impurities and fission products, protects this portion of the sheath from being in contact with and in reaction with these impurities and these products.

  fission and opposes the creation of localized constraints.

  The addition to zirconium of small amounts of a metal selected from the group consisting of iron, chromium, iron plus chromium and copper improves the corrosion resistance, and in particular the resistance to oxidation by hot water or water vapor if the element content

  of addition is part of the range indicated for this element.

  The lower limit of the amount of metal alloyed with zircon

  specified for each of the alloying elements is sufficient

  to significantly improve the corrosion resistance

  compared to that of unalloyed zirconium.

  The upper limit of the amount of zirconium-alloy metal indicated for each of the alloying elements generally corresponds to the maximum amount of metal that significantly improves corrosion resistance with respect to

  that of spongy zirconium. Additions of metals in

  above the upper limit indicated does not improve

  do not significantly increase corrosion resistance

  zirconium and can be harmful by reducing the

  Ability and ductility of the shirt.

  It has been indicated as defining the recommended ranges

  the quantities of zirconium-alloy metal

  the most significant improvement in resistance to corrosion

  sion, for each of the alloying elements.

  Iron, chromium and copper are weakly

  Zirconium-free Zirconium alloys can be heat-treated to contain one or more of these metals to produce a material having a fine dispersion of intermetallic particles which are noble with respect to the zirconium matrix.

  of alloy are poorly soluble, a hardening

  zirconium by the solid solution.

  The curing effect is sufficiently weak to preserve

  the necessary malleability of the diluted zirconium alloy jacket to prevent or

  fuel due to the interaction pellets-sheath.

  The diluted zirconium alloy jacket of the composite sheath is resistant to radiation hardening as compared to Zircaloy or other conventional alloys

* zirconium, and this allows the ziron alloy jacket

  diluted conium to maintain, after prolonged irradiation, recommended structural properties such as yield strength and hardness, at considerably

  lower than those of conventional zirconium alloys.

  In fact, the diluted zirconium alloy jacket does not harden as much as conventional zirconium alloys when

  irradiation and this, added to its initial elastic limit of

  low, allows the zirconium alloy jacket

  diluted plastically deform and relax the contraindications

  These are induced by the pellets in the fuel element, which can be caused, for example, by the swelling of the nuclear fuel pellets at the operating temperatures of the reactor (3000 C to 3500 C), the pellets in contact with the sheath.

  The use of a dilute zirconium alloy jacket comprising zirconium and a metal selected from the group consisting of iron, chromium, iron plus chromium and copper-and preferably from about 5 to about 15 percent of the thickness of the sheath, in which it is bonded to a conventional zirconium alloy substrate, provides sufficient stress reduction

  to avoid or reduce breaks in the composite sheath.

  The purity of metallic zirconium alloyed with iron, chromium, iron plus chromium or copper is important and makes it possible to obtain particular properties for the shirt.

  diluted zirconium alloy Metallic zirconium

  generally closes less than 5000 ppm of impurities Among these impurities oxygen should be at a concentration as low as possible in practice but up to

about 1000 ppm.

  In the composite sheath of the fuel element

  of this invention, a zirconium alloy jacket

  diluted nium is metallurgically bound to the substrate

  metallographic evidence shows that there is a trans-

  sufficient soil between the substrate and the zirconium alloy jacket diluted to form metallurgical bonds,

  but insufficient to form an important alliance

  ge with the diluted zirconium alloy liner itself.

  Of the conventional zirconium alloys suitable

  as substrates, mention may be made of Zircaloy-2 and Zircaloy-4.

  Zircaloy-2 comprises, by weight, about 1.5 percent tin, 0.12 percent iron, 0.09 percent chromium, and 0.005 percent nickel, and is used extensively in jet reactors. water cooling Zircaloy-4 contains

  less nickel than Zircaloy-2 but slightly more iron.

  It is possible to manufacture the composite sheath used in

  of this invention by

  which of the following methods.

  In one of these methods, a tube of material is inserted

  material to form the zirconium alloy jacket diluted in a hollow billet of the material chosen for the

  substrate, then the whole is subjected to a connection by explo-

  The tube is extruded from the substrate.

  conventional tube extrusion process at temperatures of

  The extruded composite is subjected to a process using a conventional reduction of tube to obtain the size.

  desired for the sheath. The wall thicknesses

  hollow billet and tube of material in front of

  constitute the zirconium alloy liner diluted

  to obtain in the finished tube constituting the sheath, the

desired thickness ratio.

  In another method, a tube of material to form the zirconium alloy jacket diluted in a hollow billet of the material chosen for the substrate is inserted, and the assembly is then subjected to a heating step (for example at 750 ° C. for 8 minutes). hours) by exerting compression to ensure good metal-to-metal contact and bonding by

  diffusion between the tube and the billet.

  bound by diffusion using a conventional method of

  tubes as described above in para-

  preceding graph Then the extruded composite is subjected to a process using a conventional reduction of tube

  to obtain the desired size for the sheath.

  In yet another method, a tube of material to form the zirconium alloy jacket diluted in a hollow billet of the material selected for the substrate is inserted and the assembly is extruded using a process.

  conventional extrusion tube as previously described

  Then the extruded composite is subjected to a process

  implementing a conventional tube reduction to obtain

nize the desired size for the sheath.

  The previous methods of manufacturing the composite sheath of this invention are economical compared to other methods used for the manufacture of sheaths,

  as electroplating or vapor deposition processes.

  A nuclear fuel element can be formed by

  a composite sheath open at one end and composed of a substrate and an inner liner of diluted zirconium alloy consisting of zirconium and a metal selected from the group consisting of iron, chromium, iron plus chromium , and the copper metallurgically bonded to the inner surface of the substrate, by filling the composite sheath with the nuclear fuel, leaving a cavity on the open end side, inserting into the cavity a device for retaining the nuclear fuel, applying a closure system at the open end of the sheath leaving the cavity in communication with the nuclear fuel, and then by binding the end of the sheath to this closure system so as to form between them a

hermetic seal.

  The present invention has several advantages ensuring a prolonged duration of use of the element of

  nuclear fuel, among which may be mentioned the

  the chemical interaction of the sheath, the minimization of the localized stresses on the part of the cladding constituted by the zirconium alloy substrate, the minimization of the stress corrosion of the part of the sheath formed by the zirconium alloy substrate, and decreasing the probability of crack failure occurring in the zirconium alloy substrate. The diluted zirconium alloy jacket not only minimizes stresses and stress corrosion in the substrate, but is also resistant to oxidation by water vapor and hot water in case the sheath would break, while unalloyed zirconium oxidizes rapidly under these conditions The diluted zirconium alloy exhibits plasticity similar to that of unalloyed zirconium and allows its advantages to be benefited while providing increased resistance to corrosion, particularly to oxidation by hot water and steam. The composite sheath of the present invention exhibits

  important property to benefit from improvements

  tions without adversely increasing the absorption of

  This reactor sheath will be readily accepted in nuclear reactors since it will not result in eutectic formation in a refrigerant loss accident or an accident involving the fall of a bar. In addition, the composite sheath only affects heat transfer very little, as there is no thermal barrier to heat transfer, as is the case when inserting a separate sheet into an element. The composite sheath of this invention can also be examined by conventional non-destructive testing methods during various stages of its production.

manufacture and its use.

Claims (7)

  1 Composite sheath for a nuclear reactor characterized   in that it comprises an outer part made of zinc alloy   conium forming a substrate (21) and a jacket (22) in alloy   of zirconium diluted zirconium and a metal selected from the group consisting of iron, chromium, iron plus chromium, and copper, metallurgically bonded to the   inner surface of the substrate, the alloy zir-   conium representing from about 5 to about 15 percent of the thickness of the composite sheath.   Composite sheath according to claim 1, characterized   in that the diluted zirconium alloy jacket   from about 0.2% to about 0.3% by weight iron, the rest being constituted by zirconium.   Composite sheath according to claim 1, characterized in that the diluted zirconium alloy jacket comprises from about 0.2% to about 0.25% by weight of iron, the rest being constituted by zirconium.   Composite sheath according to claim 1, characterized in that the diluted zirconium alloy jacket comprises from about 0.05% to about 0.3% by weight of chromium, the rest being constituted by zirconium.   Composite sheath according to claim 1, characterized in that the jacket of diluted zirconium alloy comprises from about 0.15% to about 0.25% by weight of chromium, the remainder being composed of zirconium 6 Composite sheath according to claim 1, characterized in that the diluted zirconium alloy jacket comprises iron and chromium, the total amount of iron and chromium being from about 0.15% to about 0.3% by weight, the remainder being zirconium and the weight ratio I   chromium iron being from about 1: 1 to about 4: 1.   Composite sheath according to claim 1, characterized in that the diluted zirconium alloy jacket comprises iron and chromium, the total amount of iron and chromium being between about 0.2% and about 0.25% by weight.   weight, the remainder being zirconium and the ratio   weighted port of chromium iron being between about 1: 1 and about 4: 1.   Composite sheath according to claim 1, characterized in that the diluted zirconium alloy liner comprises from about 0.02% to about 0.2% by weight of copper,   the rest being zirconium.   Composite sheath according to claim 1, characterized   in that the diluted zirconium alloy jacket comprises from about 0.05% to about 0.15% by weight copper,   the rest being zirconium.