FR2470430A1 - SHEATH FOR NUCLEAR FUEL ELEMENT AND METHOD OF MANUFACTURE - Google Patents

Abstract

SHEATH WITH BETTER RESISTANCE TO HYDRURATION. IT CONSISTS OF A ZIRCONIUM OR ZIRCONIUM ALLOY 12 ENVELOPE, A POROUS COPPER 16 INNER SHIRT AND A LAYER OF ZIRCONIUM OR ITS ALLOY OXIDE, BETWEEN THE INSIDE SURFACE OF THE ENVELOPE AND THE LAYER OF POROUS COPPER. APPLICATION TO NUCLEAR FUEL ELEMENTS.

Description

The invention relates generally to

  made to. the manufacture of fuel ducts

  nuclear fuel used in nuclear fission reactors

  re, of nuclear fuel elements composed of a sheath inside which there is a combustible material such as a compound of uranium, plutonium and thorium, of

  same as the ducts and elements thus manufactured.

  Reactors are being built in which the

  nuclear fuel is contained in fuel elements

  variety of geometric shapes, either in the form of

  plates, tubes or rods, for example. The material

  bustible is usually enclosed in an envelope or sheath

  heat-conducting, non-reactive and resistant to corrosion

  erosion. The elements are assembled in the form of a network, at a fixed distance from each other, in a channel or circulation zone of a refrigerant, to form an assembly of

  fuel, and a sufficient number of these assemblies are assembled

  blages to form a reactor core that can be the seat

  a self-sustaining nuclear reaction. The heart is itself

  even locked in the reactor vessel in which one makes

circulate a refrigerant.

  The nuclear fuel cladding has several functions, including two essential functions: first, it

  avoid contact and chemical reaction between the fuel

  and the refrigerant, or the moderator if it exists

  a moderator, or between the nuclear fuel and,

  refrigerant, on the other hand, the moderator, if it exists

  a moderator and a refrigerant; secondly, it prevents radioactive fission products, some of which are in the form of gas, from contaminating the refrigerant and / or

  the moderator. In general, the material used is gai-

  stainless steel, aluminum and its alloys, zinc

  conium and its alloys, niobium, certain alloys of

  gnesium, etc ... A sheath failure, ie a loss of

  may result in refrigerant or moderator contamination and associated systems,

  long-lived fission, to an extent that compromises

  puts the operation of the installation.

  Problems have arisen in the manufacture and operation of nuclear fuel elements using certain metals or alloys as sheath material, because they are subjected to mechanical stress or are the site of chemical reactions under certain conditions. Under normal conditions, zirconium and its alloys are excellent sheath materials because of their low neutron capture cross-section, and because at temperatures below about 3980C they are robust, ductile, extremely stable, and unresponsive. not with water

  demineralized steam commonly used as refrigeration

rant or moderator.

  It arises, however, during the operation of the

  A problem is that the sheath becomes brittle under the combined effect of the reactions occurring between the fuel, the sheath and the fission products emitted during the nuclear reaction. It was discovered that this bad

  operation is caused by mechanical constraints

  shims due to differences in expansion between the fuel and its sheath (the stresses in the sheath are located at

  level of cracks in the nuclear fuel). Some

  corrosive fission products are emitted by the nu-

  and find themselves at the intersection of the cracks and the surface of the sheath. Fission products are formed

  in the fuel during the chain reaction,

  the operation of the reactor. Local stresses are reinforced by the high friction between the fuel

and the sheath.

  Inside a sealed fuel element, hydrogen gas can be formed by slow reaction between the sheath and the residual water inside this sheath, and result, under certain conditions, by hydriding

  this sheath and the local degradation of its

  mechanical properties. The sheath is also deteriorated by

  gases such as oxygen, nitrogen, carbon monoxide and bio-

  carbon dioxide, over a wide range of temperatures.

  The zirconium cladding of a nuclear fuel element

  area is exposed to one or more gases that have just been

  enumerated as well as to fission products during the

  radiation in a nuclear reactor, although these gases or fission products may not be present in the refrigerant or the moderator, and although they may have been - excluded to the extent possible from the ambient atmosphere during the manufacture of the sheath and the element. Sintered ceramic and refractory compositions, such as uranium dioxide, and other compositions used

  nuclear fuel emit the gases mentioned above.

  in measurable quantities when heated during the manufacture of the fuel element, and,

  plus, release fission products under irradiation. cer-

  certain compositions, such as urax dioxide powder,

  nium, are known to release very large quantities of the gases mentioned under irradiation, and these gases are susceptible

  to react with the zirconium sheath that encloses the fuel

nuclear power.

  It has therefore appeared desirable to reduce the

  than sheath by water, water vapor, hydrogen and

  other gases released by the fuel and react

  with this sheath for the duration of use of

  the fuel element in a nuclear power plant.

  One of the proposed routes was to find materials that would react chemically quickly with water, water vapor, hydrogen and other gases to remove them from the inside of the sheath, such materials being known under

the trap name or "getter".

  A number of solutions have been proposed in United States Patents 4,022,662, 4,029,545.

and 4,025,288.

  It has been proposed recently, in response to the problem of

  the rupture of fuel element sheaths due to

  teractions between zirconium or its alloys constituting the

  sheath material and nuclear fuel and / or

  fission products that this fuel releases, to form a

  swimming or lining of copper inside the sheath, or

  screen layer. It is generally considered that a layer of

  vre, formed by cladding or veneer, functions primarily as a chemical screen prohibiting fission products, such as cadmium, cesium, iodine and the like, from coming

  in contact and to attack the zirconium or zirconium alloy

which constitutes the envelope.

  Although a sheath made of zirconium or zirconium alloy,

  coated internally with a layer of copper, proves to be

  in many respects with regard to interac-

  mentioned above, it appears that the whole is not sufficient

  effectively with regard to its resistance to hydro-

  gene and the phenomena of hydriding that can occur

  when the sheath or fuel element is defective.

  For example, steam entering a sheath or

  defective or damaged can react with the bio-

  ranium, generating a moist hydrogen atmosphere in the

  inside the sheath. Studies have shown that zirconium or its alloys hydride rapidly and become brittle in a hydrogen atmosphere when the rate at which the vapor or oxygen falls is below the value

  which allows the formation and the maintenance of an oxide film pro-

  on the exposed surfaces of zirconium or its

  bonding. This value was determined to correspond to a very low partial pressure of water vapor, of the order of 0.140 g / cm 2 (0.1 mm Hg). The copper jacket, which reduces or prevents the arrival of water vapor or oxygen to the underlying zirconium or zirconium alloy, thus renders the zirconium or zirconium alloy sheath very vulnerable to

  hydriding, since hydrogen will diffuse pre-

  ferential through a metal layer such as copper

  fever. The diffusion coefficients of hydrogen and o-

  xygen in copper (DC-1.5 x 10-5 and DCu = 2.95 x 10-10 cm 2 / s,

  at 400% C) make it possible to envisage, as more likely, a

  much higher than the oxidation rate

  tion. The sheath rendered fragile by hydriding zirconium or

  of the zirconium alloy which constitutes it leads to an accen-

  degradation of defective fuel elements

  killer and, ultimately, to their total damage.

  The invention relates to a nuclear fuel element composite sheath for a nuclear reactor and to its method of manufacture, this sheath being composed of a sheath

  zirconium or zirconium alloy which has on its surface

  inner side, a gas permeable sleeve formed by

  copper, and a layer of zirconium oxide or alloy

  of zirconium formed on the inner surface of the envelope

  pe, between this envelope and the permeable copper jacket

to gases.

  In what follows, the term "zirconium" will be used for

  designate both zirconium and one of its alloys.

  The remainder of the description refers to the attached drawing

  which represents an example of a fuel element sheath

  nuclear device in accordance with the invention, illustrated in cross section.

  versale. The invention is based on the realization of a composite structure or combination of materials, with a porous layer or sleeve of copper permeable to steam and hydrogen and

  formed on the surface of zirconium or its alloy which

  a substrate and the essential component of the cladding of a nuclear fuel element. The possibility given to the

  vapor to penetrate quickly into the layer or shirt

  The copper channel, and passing through it, leads to the in situ formation of zirconium oxide at the underlying surface of the structure, ie to the surface of the zirconium coated with the above-mentioned layer or jacket. The prior deposition of a copper layer on the zirconium surface which constitutes a metal substrate makes it possible to use the most efficient and fastest deposition methods, such as electrolytic deposition.

  The invention thus allows the formation of zirconium oxide

  on the surface underlying the pre-formed copper jacket.

  formed, so that a protective barrier is

  oxide teat between this jacket and the zirconium sheath, which avoids any interdiffusion phenomenon between zirconium

  and copper and the resulting adverse effects. But we

  In addition, the steam can permanently pass through the porous copper jacket and reach the underlying surface

  of the zirconium or zirconium alloy envelope.

  As a result, in the event of a fault or damage to the

  nuclear fuel element according to the invention,

  accidentally the zirconium casing coated with

  porous water or refrigeration vapor generating a moist hydrogen atmosphere, the vapor will penetrate the copper and reach the underlying surface, maintaining

  the intermediate layer of zirconium oxide, therefore the barrier

  protector formed by this oxide, so that the hydrogen will not reach the zirconium, and that the latter will not be

rendered fragile by hydriding.

  The sheath shown in the cross-sectional drawing consists of a casing 12 of zirconium or zirconium alloy, a layer of zirconium oxide or zirconium alloy 14 formed on the inner surface of the casing,

  and a porous copper jacket 16, permeable to steam.

  The nuclear fuel 18, consisting for example of granules, is oxide of uranium, plutonium and / or thorium contained inside the sheath to isolate it from any contact with the cooling medium that circulates in the

reactor.

  The nuclear fuel sheaths 10, which constitutes a recommended embodiment of the invention, may be manufactured by depositing the copper layer 16 to a thickness of less than 10, preferably to a thickness of between 0.5 and About 5 p, on the surface

  interior of the envelope 12, this layer being therefore strong

  permeable to steam. We can use this end

  appropriate procedures, such as those described in the

  of United States of America N0 4.017.368, 4.137.131 and

4093756.

  After depositing the porous layer of copper 16 on the surface

  face of the zirconium envelope 12, the whole is subjected to

  vapor from which the air has been removed at a temperature of

  re of the order of 3000C to 4000C, for example in an autoclave, to

  4000C under a pressure of 0.7 kg / cm 2 for 24 h. The

  Dry fear penetrates through the surface layer of

  porous and reacts with the underlying zirconium in the oxy-

  thus forming a protective oxide layer between

  the zirconium casing and the copper jacket.

  Test sleeves were made from zirconium alloy tubes (Zircaloy 2, see US Pat.

  United States of America No. 4,164,420) internally coated with

  of copper in different thicknesses, ie in thicknesses of the order of 0.5-1 p, 1-2 p, and 10 p. These

  test tubes were autoclaved for 24 hours.

  res, in the presence of dry steam at 4000C. The same sheaths were subjected to a hydrogen test consisting of

  place in conditions of potential hydriding, for example

  position in a moist hydrogen atmosphere for a period of 72 h or 300 h. After exposure for 72 h in a humid hydrogen atmosphere, the test sheath with a 1-2 p thick copper jacket "hung" hydrogen

  approximately 150 ppm, whereas under the same conditions

  the test sheath with a copper jacket of

  p hung hydrogen at 1000 ppm.

  Test ducts exposed to 300 h exposure

  in a hydrogen atmosphere were subjected to a neutral examination

  trographique; all sheaths coated with a 0.5-1 and 1-2 V copper jacket had hung hydrogen

  in an amount less than the detectable amount by

  neutron count, in the order of 500-800 ppm. As for the jackets with copper jacket 10 p thick, they contained hydrogen at the rate of several thousand

- ppm, and more.

  These results demonstrate the important role played by the

  porosity of the copper jacket and the possibility of penetrating

  in this steamed shirt, as well as their

  effect on the possibilities of coupling hydrogen left

with zirconium.

R E Y E N D I C A T I 0 N S

  1 - Sheath for nuclear reactor fuel element

  area, characterized in that it consists of:

  (a) a zirconium or zirconium alloy envelope

  nium (12), b) a porous jacket (16) formed by depositing copper on the inner surface of the envelope, and

  (c) a layer of zirconium oxide or alloy oxide

  of zirconium (14) formed on the inner surface of the

  veloppe, between this inner surface and the porous-shirt

of copper of which it is coated.

  2- sheath according to claim 1, characterized in that

  the porous copper sleeve formed on the surface of the

  envelope has a thickness of about 5%. 3 - Sheath according to claim 1, characterized in that

  the porous copper sleeve formed on the surface of the

  the envelope has a thickness of between 0.5 and about p.

  4 - Sheath according to one of claims 1 to 3, characterized

  in that the sheath contains a noncombustible

  selected from uranium compounds, the compounds of

  plutonium, thorium compounds, and mixtures.

  Sheath for nuclear reactor fuel element, characterized in that it consists of:

  (a) a zirconium or zirconium alloy envelope

  (b) a gas-permeable jacket formed by deposition of copper on the inner surface of the shell and forming a layer having a thickness of between 0.5 and 5 per cent, and

  (c) a layer of zirconium oxide or alloy oxide

  of zirconium formed in situ on the inner surface of

  the envelope, between this inner surface and the shirt

  gas that it is coated.

  6 - Sheath according to claim 5, characterized in that the oxide layer has a thickness between 0.5 p

and about 5 p.

  7 - Sheath for nuclear reactor fuel element

  area, characterized in that it consists of

  (a) a zirconium or zirconium alloy envelope

  (b) a vapor permeable jacket formed by depositing copper on the inner surface of the shell and forming

  a layer whose thickness is between 2 p and 5 p

viron, and

  (c) a layer of zirconium oxide or alloy oxide

  of zirconium, with a thickness between 0.5 p and 1 Vi

  viron, formed in situ on the inner surface of the envelope

  pe, between this inner surface and the shirt permeable to

the steam with which it is coated.

  8 - Process for manufacturing a sheath for a nuclear reactor fuel element, characterized in that

consists of: -

  a) to take a zirconium or zirconium alloy envelope, b) to form, by deposition of copper, a porous jacket on the inner surface of the envelope, then c) to oxidize the inner surface of the envelope to form a layer of zirconium oxide or zirconium alloy oxide, between this inner surface and the jacket

porous which it is coated.

  9 - Process according to claim 8, characterized in that it consists in using steam-to oxidize the surface

inside the envelope.

  - Process according to claim 8, characterized in that it consists in using dry steam to oxidize the

inner surface of the envelope.

  11 - Method according to one of claims 8 to 10,

  characterized in that it consists in using steam at a temperature between 3000C and 400'C approximately to oxidize

  the inner surface of the envelope.

  12 - Process according to one of claims 8 to 11,

  characterized by forming the porous layer of copper deposited on the inner surface of the envelope to a thickness of up to 5 percent.

  13, - Method according to one of claims 8 to 11,

  characterized in that it consists in forming the porous layer of copper deposited on the inner surface of the envelope on

  a thickness of between 0.5 and 5 i approximately.

  14 - Method according to one of claims 8 to 11,

  characterized in that it consists in forming the porous layer of copper deposited on the inner surface of the envelope on

  a thickness of between 2 iV and 5 p approximately.

  Method according to one of claims 8 to 14,

  characterized in that it consists in oxidizing the inner surface of the envelope by application of vapor passing through the layer

  porous formed by deposition of copper on this interior surface.

re.

  16 - Method according to one of claims 8 to 15,

  characterized in that it consists in oxidizing the inner surface of the envelope so as to form an oxide layer of which

  the thickness is between 0.5 p and 1 p approximately.