FR2458876A1 - METHOD OF MANUFACTURING SOLDER TUBE FOR NUCLEAR FUEL ELEMENT AND TUBE THUS OBTAINED - Google Patents

Abstract

PROCESS AVOIDING COMPLETE RECRISTALLIZATION OF THE ZIRCONIUM ALLOY. IT INCLUDES: 1 REDUCTION OF THE DIAMETER OF THE BLANKET OF THE CLADDING TUBE BY COLD SCREENING IN A SERIES OF STEPS OF REDUCTION TO THE INSIDE DIAMETER AND TO THE REQUIRED WALL THICKNESS; 2 THE THERMAL TREATMENT OF THIS BLANKET OF THE CLADDING TUBE BETWEEN EACH OF THESE REDUCTION STEPS TO A TEMPERATURE AND FOR A PERIOD SUFFICIENT TO PRACTICALLY TOTALLY RECRISTALLIZE THE ZIRCONIUM ALLOY; 3 THE THERMAL TREATMENT OF THE CLADDING TUBE AFTER THE FINAL STAGE OF THE REDUCTION STAGES TO A LOWER TEMPERATURE AND FOR A TIME THAT ALLOWS A PRACTICALLY COMPLETE RECRISTALLIZATION OF THE ZIRCONIUM LAYER AND A METAL MICROSSTRUCTURE IN THE COATING RELAX THE STRESSES BUT DO NOT COMPLETELY RECRISTALLIZE THE ZIRCONIUM ALLOY. APPLICATION TO NUCLEAR FUEL ELEMENTS.

Description

Cold nuclear reactors are well known and

  moderated by water. We will find a description for example

  in the article by M.M. El-Wakil in "Nuclear Power Engineering"

  McGraw-Hill Book Company, Inc., 1962.

  The fuel elements for these reactors are generally in the form of uranium oxide pellets and / or plutonium oxide encapsulated in an elongated, protective, protective tube made of a suitable metal.

  commonly a zirconium alloy such as Zircaloy-2.

  U.S. Patent No. 3,365,371 discloses an element of

fuel of this type.

  To avoid premature failure of the fuel element sheath and to extend its useful mechanical life, it has been proposed to use various protective shields between the fuel pellet column and the inner surface.

  of the sheath. Among these screens are layers of zirconium

  metal conium related to the inner surface of the tube

zirconium alloy.

  Belgian Patent 835,481 describes a screen layer formed of substantially pure metal zirconium bonded to the surface

inside the tube.

  Belgian Patent 870,342 describes a screen layer formed of metallic zirconium of moderate purity, such as spongy zirconium. In the conventional method of producing cladding tube

  with a layer-screen bonded to the inner surface of this

  Here, a zirconium alloy hollow billet is nested in a hollow billet of zirconium metal for the screen layer, and the composite is coextruded. The composite is then reduced to the final diameter by multi-pass cold work-hardening.

  in a reduction device such as a reduction machine

Pilger tubes.

  After each reduction pass, it is classic to

  annealing the composite by heat treatment at a temperature of

  for a time sufficient for practically recreating

  completely scale the zirconium alloy.

  However, it has been found that the annealing temperatures and the times necessary for complete recrystallization of the zirconium alloy cause grain growth.

  undesirable in the metal zirconium screen layer.

  Therefore, in one aspect of the present invention, the heat treatment of the composite is carried out after the final size reduction step. at a temperature and for a period of time which allow a virtually complete recrystallization of the metal zirconium layer, provide a fine-grained microstructure, which relaxes the stresses

  but do not completely recrystallize the zirconium alloy.

  According to another aspect of the present invention, it is possible, optionally, to improve the crystallographic texture of

  the metal zirconium layer by compressive deformation

  the surface of the layer, for example, by shot peening,

  without deformation of the zirconium alloy of the cladding tube cateosite.

  The remainder of the description refers to the appended figures

which represent respectively;

  Figure 1, a partial sectional view of a communication element

  nuclear fuel; Figure 2, a cross-sectional view of the element of

fuel of Figure 1.

  FIGS. 1 and 2 show a nuclear fuel element 11, which consists of a cladding tube

  composite of elongate form 12 enclosing a column of

  fuel rod 13 and which is sealed at its ends by

  lower and upper end caps 14 and 16.

  A vacuum 17 is provided to allow long-term expansion.

  tudinal fuel and to provide a volume for the gases released by the fuel during operation in the reactor. A spring 18 between the top of the fuel column and the upper end plug 16 holds the fuel column in position. As best shown in FIG. 2, the composite cladding tube 11 is sized relative to the diameter of the fuel pellets to provide an annular clearance or gap 19 between the pellets.

  of fuel and the inner surface of the cladding tube.

  In a preferred embodiment of the invention, the composite cladding tube comprises a tube 21 of a zirconium alloy and a screen layer 22 of metallic zirconium.

  metallurgically bonded to the inner surface of the tube 21.

  Among the zirconium alloys suitable for the tube 21, there may be mentioned Zircaloy-2 or Zircaloy-4. Zircaloy-2 contains, on a weight basis, about 1.5% tin, 0.12% iron, 0.09% chromium, 0.05% nickel, the remainder being zirconium. Zircaloy-4 contains less nickel than Zircaloy-2, but contains slightly more iron. In either case, the alloy contains components other than zirconium in an amount greater than 5,000 parts per million. The screen layer 22, which may represent from about 1 to about 30% of the thickness of the composite sheath, is made of zirconium metal, of limited impurity content ranging from

  a zirconium of high or almost pure purity contains

  less than 500 ppm of impurities has an impurity content of up to 5,000 ppm but preferably

  in impurities less than about 4,200 ppm.

  Among the impurities, the maximum oxygen content must be reduced and maintained in a range of 200 ppm or less to a maximum of about 1200 ppm. Other impurities may be in the normal range for commercial grade sponge zirconium and are shown below: aluminum 75 ppm or less; boron 0.4 ppm or less; cadmium 0.4 ppm or less; carbon 270 ppm or less; chromium ppm or less; cobalt 20 ppm or less; copper 50 ppm or less; hafnium 100 ppm or less; hydrogen 25 ppm or less; iron 1500 ppm or less; magnesium 20 ppm or less; manganese 50 ppm or less; molybdenum 50 ppm or less; nickel 70 ppm or less; niobium 100 ppm or less; nitrogen 80 ppm or less; silicon 120 ppm or less; tin 50 ppm or less, tunsgene ppm or less, titanium 50 ppm or less; and uranium 3.5 ppm

or less.

  The metal zirconium screen layer 22 is metal-bonded

  lurgically to the tube of zirconium alloy 21 by a different

  cross fusion between them sufficient to form a strong bond but insufficient for the diffusion contaminates the screen layer 22 to a depth greater than about

  0.00127 to 0.00254 mm from the interface of the link.

  It has been found that a metal zirconium layer thickness of about 5 to 15% of the composite cladding and more particularly recommended thickness of about 10%, prevents the exposure of the zirconium alloy. of the tube

  cladding 21 to corrosive fission products.

  The shielding layer also separates the zirconium alloy tube from a direct mechanical interaction with the fuel pellets and reduces stress

  that can result. It has been found that the screen layer

  Its desirable structural properties, such as yield strength and hardness, can be used at levels much lower than those of conventional zirconium alloys. Indeed, the metal screen does not harden as much as conventional zirconium alloys when subjected to irradiation

  and this, together with the initial elasticity limit-

  low, allows the metal screen to deform

  to relax the constraints induced by

  cells in the fuel element during transients

  power. The constraints induced by the pellets in the

  fuel can occur, for example, during

  swelling of the nuclear fuel pellets at

  erations of reactor operation so that these

  they come in contact with the sheathing.

  The composite cladding of the present invention can be

  make by any of the following methods.

  In one of the processes, a hollow tube of zinc is

  metal conium to make the screen layer and inserted into a hollow billet of zirconium alloy selected to form the tube of the sheath. The assembly is then subjected to a connection by explosion of the tube to the billet. We extrude the

  composite at elevated temperatures of approximately 538 to 7600C using

  reading standard tube extrusion techniques. The extruded composite is then subjected to a process comprising the conventional reduction of the tube to the desired size of

the composite sheath.

  In another process, a hollow tube of zinc is inserted

  metal conium chosen to form the screen layer in a hollow billet of zirconium alloy chosen to form the tube of the sheath. We submit the set to a stage of

  heating, such as 760 ° C for about 8 hours, for

  to create a diffusion bond between the metal zirconium tube and the billet. The composite is then extruded using conventional tube extrusion techniques, and the extruded composite is subjected to a process involving the conventional reduction of the tube to the desired size of the sheath. In yet another method, a hollow metal zirconium tube selected to form the screen layer is inserted into a hollow billet of zirconium alloy selected to form the tube of the sheath. The whole is extruded using conventional tube extrusion techniques. We then submit the

  extruded composite to a process comprising the conventional reduction

  the tube to the desired size.

  The dimensions of the starting materials are determined by cross sectional ratios of the screen layer and

  zirconium alloy parts of the desired composite sheath.

  For example, the total cross-section of the final cladding is given by

A TF = / 4 (ODTF DTF2)

  o ATF is the final product section, ODTF is the outer diameter of the final product, and IDTF is the bottom diameter of the final product. The right section of the desired screen is given by: A = n (OD - _ID2

BF / 4 BF BF

  o ABF is the cross section of the metal screen, ODBF is the outside diameter of the metal screen and ID BF is the inside diameter of the metal screen. The total cross-section of the initial billet of the cladding tube is given by ATI = IR (OD-ID) 2

IT (4DTI IT

2458876-

  o AT is the total straight section of the initial billet including the metal screen, ODTI is the outside diameter of the initial billet and IDTI is the inside diameter of the initial billet. The required cross-section of the initial screen is determined by

ABT = ATI ABF

BIATF

EXAMPLE

  An embodiment of a composite cladding tube 11 according to the present invention is as follows: A billet of alloy cladding tube is machined.

  zirconium and the add-on element for the Zir-

  metal conium, they are cleaned and assembled by

  conventional processes, the dimensions being chosen for the ex-

  trusion of the composite assembly in a hot extrusion press. The billet for the cladding tube consists of normal Zircaloy-2 alloy conforming to ASTM B353 RA-1 grade and the add-on element for the shielding layer consists of metal zirconium having an impurity content within the limits indicated above. . The bores of the

  billet and the insert are formed with a cone

  cited 0.2 mm by 25.4 cm and pressed together to ensure

  good contact between adjacent surfaces.

  By way of example of the dimensions of the machined parts, it can be indicated: for the billet of the sheath tube, 228.6 mm long, 145.8 mm outside diameter, 61.9 mm inside diameter; for the insert of the outer layer 61.9 mm outer diameter, 42.2 mm inner diameter. Before assembly, the surfaces are attacked chemically

  adjacent to the billet and insert to remove

  to remove traces of impurities. A suitable pickling product

  nable is a solution of 70 ml of H20, 30 ml HNO3 (70% aqueous)

and 5 ml HF (48% aqueous).

  To ensure a satisfactory connection during extrusion, the assembly can be preloaded by pressing the conical insert into the conical hole of the billet under a vacuum.

  S 20 pm of mercury while maintaining a temperature of

  viron 7600C for 8 hours with pressure forces

  initials from 13,000 to 20,500 kg. It has been found that one obtains

  20 to 25% of the surface of the

terface.

  To reduce losses at the ends during extrusion

  At each end, Zircaloy-2 pieces 50, 8 mm long can be welded to the preloaded and machined level assembly. a cladding tube is made using the following parameters: extrusion speed 152, 4 mm / min, reduction ratio 6: 1, temperature 5930 C and extrusion force

3.175 tonnes.

  All the surfaces of the billet except the central bore and the floating mandrel can be lubricated with a water soluble lubricant) cooked at a temperature of

  7040C for one hour. After extrusion, remove by

  page, both ends of the tube, the added end pieces and polish the inner surface to eliminate

  all defects and improve the final appearance.

  The final reduction of the composite tube to a suitable size

  for the sheathing of fuel elements, is carried out by cold working in three passes by means of a Pilger tube reduction machine, well known with treatment

  thermal and cleaning between passes. The stages of a

  yield reduction are shown in the table

1 annexed.

  The reduction method is conventional except for the modifications made by the present invention. The basis for these modifications and the beneficial results obtained

will now be discussed.

  The severe cold working that takes place during the reduction passes of the tube leads to a distortion of the shapes of the

  crystallites of the metal and produces many crys-

  tallins in crystallites. Thus, cold-worked metals are in a relatively high energy state that is not thermally stable. The annealing process

  metallurgy uses heat to impart

  to the metal atoms and allow them to rearrange into a lower energy configuration, this annealing being a function of both temperature and time, temperature being the most sensitive parameter. In general, the annealing temperature and time are chosen to be sufficient to provide substantially complete recrystallization but insufficient to allow for growth of

excessive grain.

  Therefore, for the annealing steps (5) and (8) of the

  Reduced yield of Table I, temperatures and times are chosen to provide substantially recrystallization

  complete of the zirconium alloy of the tube 21.

  Cepéndant, the relatively purer metal of the

  screen 22, recrystallizes at a lower temperature and

  found that typical annealing temperatures and times

  Suitable for the zirconium alloy, as in steps (5) and (8), causes grain growth in the metal of the screen layer to an undesirable point.

in the finished product.

  Therefore, according to one aspect of the present invention, after the final reduction pass, the composite tube is heat treated at a lower temperature as indicated in step (12). Thus, the temperature and duration of the heat treatment of the step (12) are chosen such that the metal zirconium of the screen layer 22 is substantially completely recrystallized without grain growth. This provides a screen layer with an equiaxed microstructure to

  fine grains with excellent resistance and ductility

  improved resistance to corrosion cracking

  under stress and high plastic stability.

  The temperature and the duration of the heat treatment of step (12) are also chosen in view of obtaining a total relaxation of the stresses, but not of a complete recrystallization of the zirconium alloy of the tube 21. It provides the added benefit that the zirconium alloy retains the elongated grain structure imparted by the reduction process and has higher strength at high strain rates while being

  still relaxed internal constraints.

  Suitable temperatures and times for the annealing steps (2), (5) and (8) are between 5380C and 7040C for about 1 to 15 hours and preferably for about 1 to 4 hours. Suitable temperatures and times for the heat treatment step (12) range from about 440 to 510 ° C for

a duration of about 1 to 4 hours.

  According to another aspect of the present invention, the texture

  crystallographic (that is, the degree of crystallographic

  Preferably, the metallographic zirconium screen layer may be optionally enhanced by mechanical deformation of the surface. For example, before the final heat treatment step 12, the screen layer can be subjected to shot blasting. from within the assembly, to provide compressive deformation of this layer without significant deformation of the zirconium alloy tube.

  Such a mechanical treatment before the heat treatment

  The final value given in step (10) of Table 1 provides an improved crystallographic structure with base plates {00021 strongly aligned in the radial direction of the

composite cladding tube.

T A B L E A U I

REDUCTION AND TREATMENT OF THE TUBE

  Step STEP Thickness Diameter Diameter N of the composite Exterior Interior (cm) (cm) (cm) (1) Starting with tube blank 1.09 6.35 4.16 (1) Cleaning before annealing (degreasing-caustic-based product soap) 1.09 6.35 4.16 (2) Annealing 677 C - 1 hour (3) First reduction pass 0.56 3.7 2.56 (4) Cleaning before annealing (5) Annealing 621 C - 1 hour o (6) Second reduction pass 0.24 2.03 1.55 (7) Cleaning before annealing (8) Annealing 621 C - 1 hour (9) Third pass reduction 0.084 1.23 1.06 (10) Shot-blasting of the inner surface (11) Cleaning before heat treatment (12) Heat treatment 482 C - 4 hours (13) Attack up to 0.081 1.23 1.07 FINAL THICKNESS OF THE SCREEN LAYER 0, 0863 0.0076 mm Cal Co CD

Claims (7)

R E V E N D I C A T IO N S   A method of manufacturing an elongate composite cladding tube for containing nuclear fuel   and forming a fuel element for a nuclear reactor   area, this cladding tube being formed from a blank   of a sheath tube made of a zirconium alloy tube   Nium containing constituents other than zirconium in an amount greater than about 5000 ppm and having a zirconium metal layer having amounts less than 500 ppm metallurgically bonded to its inner surface, characterized in that it comprises: (1) reduction of the diameter of the blank of the tube   cold-working sheathing in a series of stages of   ducting to the desired inside diameter and wall thickness; (2) heat treatment of this tube blank   sheathing between each of these reduction steps at a temperature of   for a period of time sufficient to   to recrystallize the zirconium alloy; (3) the heat treatment of the cladding tube after   the final stage of the reduction steps at a lower temperature   and provide a fine-grained microstructure therein which relaxes the stresses but does not recrystallize   completely the zirconium alloy.   The method of claim 1, characterized in that the temperature and duration of step (2) is from about 5380C to about 704C and from about 1 hour to about 15 hours respectively, and that temperature and the duration of step (3) range from about 4400C to about 5100C and about   1 hour to about 4 hours, respectively.   3. Method according to any one of claims 1   at 3, characterized in that it further comprises a step of deformation by substantially uniform compression of the surface of the metal zirconium layer before the step heat treatment (3).   4. Method according to claim 3, characterized in   this deformation is done by shot blasting.   5. An elongated composite cladding tube for containing nuclear fuel as a fuel element for a nuclear reactor, said composite cladding tube containing a zirconium alloy tube containing constituents other than zirconium. in an amount greater than about 5000 ppm and having a metal zirconium layer containing impurities in an amount of less than 500 ppm metallurgically bonded to its inner surface, said metal zirconium layer being substantially completely recrystallized to provide a fine grain microstructure,   and the zirconium alloy tube being substantially   relaxed but not completely recrystallised   Lisa. Composite cladding tube according to Claim 5, characterized in that the surface of the zirconium layer   metal is deformed by compression.   Composite cladding tube according to claim 6,   characterized in that this deformation is effected by naillage.