JPS6055036B2 - Composite cladding for nuclear fuel and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to thermal and mechanical treatment of composite cladding for fuel elements.

Nuclear reactors using water as a coolant and moderator are known, for example from M.M. Elwakil (M.
．． M. E■-Waki Rin's "Nuclear Power Engineering"
Eering) (McGraw-Hill, 1962).

The fuel element for such reactors typically consists of uranium oxide and/or plutonium oxide pellets contained within an elongated guardian casing made of a suitable metal such as a zirconium alloy (e.g. Zircaloy 2). It is customary.

Such fuel elements are described, for example, in U.S. Pat. No. 3,365,37.
It is shown in the specification of No. 1.

In order to prevent premature failure of the fuel element cladding tube and extend its mechanical useful life, it has been proposed to provide various protective partitions between the columnarly arranged fuel pellets and the inner surface of the cladding tube. There is. Some of these partition walls have a layer of metallic zirconium bonded to the inner surface of a zirconium alloy cladding tube. Belgian Patent No. 835,481 describes the case where the partition, which is joined to the inner surface of the cladding tube, consists of a layer of almost pure metallic zirconium. Belgian Patent No. 87034 also describes the case where such a partition consists of a layer of metallic zirconium of low purity, such as sponge zirconium. According to a conventional method of manufacturing a cladding tube having a septum bonded to its inner surface, a hollow billet of zirconium alloy is fitted into a metallic zirconium sleeve for the septum, and then the composite is coextruded.

The diameter of the composite is then reduced to the desired final value by cold working in a device such as a Pilger tube squeezer in multiple passes. After each bath, the composite is typically annealed by heat treatment using a temperature and time sufficient to substantially completely recrystallize the zirconium alloy. However, the annealing temperatures and times required for complete recrystallization of the zirconium alloy have been found to be undesirable due to grain growth in the metallic zirconium partition walls.

According to one feature of the present invention, the heat treatment of the composite material after the final drawing step results in substantially complete recrystallization of the metallic zirconium layer and imparts a fine microscopic structure therein. At the same time, temperatures and times are used that allow stress relief of the zirconium alloy but do not result in its complete recrystallization. According to another feature of the invention, the crystal structure of the metallic zirconium layer is changed without deforming the zirconium alloy of the composite tube by applying compressive deformation to the surface of the zirconium layer of the composite material, for example using shot peening techniques. It can also be improved at will.

The invention will be explained in more detail below with reference to the accompanying drawings. A fuel element 11 for a nuclear reactor such as that shown in FIGS. 1 and 2 consists of a columnar arrangement of fuel pellets 13 and an elongated composite cladding tube 12 for housing them.
Its ends are sealed by a lower end plug 14 and an upper end plug 16.

A plenum space 17 is provided to allow longitudinal expansion of the fuel and to provide room for gases released from the fuel during operation within the reactor.

A spring 18 is disposed between the top fuel pellet and the upper end plug 16 to hold the fuel pellet in place. As best seen in FIG. 2, the dimensions of the composite cladding tube 12 relative to the diameter of the fuel pellets are determined to provide an annular gap 19 between the fuel pellets and the inner surface of the cladding tube. According to one preferred embodiment of the present invention, composite cladding 12 includes a zirconium alloy cladding 21 and a metallic zirconium partition 22 metallurgically bonded to its inner surface.

Zirconium alloys suitable for the cladding tube 21 include Zircaloy 2 and Zircaloy 4.

Zircaloy 2 is approximately 1.5% (by weight) tin, 0.12
(wt)% iron, 0.09 (wt)% chromium, 0.0
0.5% (by weight) nickel and the balance zirconium. Zircaloy 4 has a lower nickel content than Zircaloy 2, but a slightly higher iron content. In each case, such alloys contain more than 5000 ppm of components other than zirconium. The septum 22, which constitutes about 1 to about 30% of the thickness of the composite cladding 12, is comprised of metallic zirconium with a limited impurity content.

That is, such materials can range from substantially pure (i.e., high purity) metallic zirconium having an impurity content of less than 500 ppm up to 5000 ppm, preferably about 420 ppm.
It can even extend to metallic zirconium with an impurity content of less than 0 ppm. Among the impurities, oxygen should be as low as possible and its content should be kept within the range from less than 200 ppm up to about 1200 ppm.

Other impurities may be within the standard range for commercially available nuclear reactor sponge zirconium. To list them, aluminum is 75 ppm or less, boron is 0.4 ppm or less, cadmium is 0.5 ppm or less, and carbon is 270 ppm.
m or less, chromium is less than 200ppm, cobalt is less than 20p
pm or less, copper is 50ppm or less, hafnium is 100p
pm or less, hydrogen is 25 ppm or less, iron is 1500 ppm or less
Below, magnesium is 20ppm or less, manganese is 50ppm or less
ppm or less, molybdenum is less than 50ppm, nickel is less than 70ppm, niobium is less than 100ppm, nitrogen is less than 8
0ppm or less, silicon 120ppm or less, tin 50ppm or less
ppm or less, and uranium is less than 3.5 ppm.
The metallic zirconium partition wall 22 is metallurgically joined to the zirconium alloy cladding tube 22.

Specifically, there is sufficient interdiffusion between the two to create a strong bond, but the diffusion does not contaminate the septum 22 outside about 112 to 1 mil from the interface. shall be. 5 to 15% of the thickness of the composite cladding tube 12
It has been found that exposure of the zirconium alloy cladding 21 to corrosive fission products is prevented if the partition wall 22 is present to a certain extent (particularly preferably 10%).

The bulkhead 22 also prevents the Zirunium alloy cladding 21 from having direct mechanical interaction with the fuel pellets, thus reducing the stresses that may result therefrom.

It has been found that the desired structural properties (eg, yield strength and hardness) of the septum 22 are maintained at significantly lower levels than with conventional zirconium alloys. In fact, upon irradiation, the metallic zirconium partition walls 22 do not harden as much as normal zirconium alloys. Therefore, in view of the low initial yield strength, the bulkhead 22 exhibits plastic deformation during power transients, thereby making it possible to remove pellet-induced stresses in the fuel element. Pellet-induced stresses in the fuel element include those caused by the expansion of the fuel pellets into contact with the cladding under the operating temperature of the nuclear reactor. The composite cladding tube of the present invention can be manufactured by any of the following methods.

According to the first method, a hollow billet made of metallic zirconium selected for the partition wall is inserted into a hollow billet made of zirconium alloy selected for the cladding tube.

The hollow tubes are joined to the hollow billet by subjecting the assembly to an explosive joining process. The composite material thus obtained is processed according to conventional tube extrusion techniques to
It is extruded at an elevated temperature of about 1400 F (about ? to about 760 C). Next, by subjecting the extruded composite material to processing operations including a normal tube drawing step, a composite cladding tube of desired dimensions is obtained. According to the second method, a hollow billet made of metallic zirconium selected for the partition wall is inserted into a hollow billet made of zirconium alloy selected for the cladding tube.

For example, 1400'F (760°C
Diffusion bonding between the hollow tube and the hollow billet is achieved by carrying out a heating step of about 8 hours at ). The composite thus obtained is extruded according to conventional tube extrusion techniques. Next, by subjecting the extruded composite material to processing operations including a normal tube drawing step, a composite cladding tube of desired dimensions is obtained. According to the third method, a hollow billet made of metallic zirconium selected for the partition wall is inserted into a hollow billet made of zirconium alloy selected for the cladding tube.

Such an assembly is extruded according to conventional tube extrusion techniques. Next, by subjecting the extruded composite material to processing operations including a normal tube drawing step, a composite cladding tube of desired dimensions is obtained. The dimensions of the starting material are determined by the ratio of the cross-sectional areas of the septum and cladding body in the desired composite cladding product.

For example, the total cross-sectional area of the final product is given by: A,p=! (0DTp2-1D, p2) TP4FF where ATP is the total cross-sectional area of the final product, 0DTP is the outer diameter of the final product, and IDTp is the inner diameter of the final product.

The desired septum cross-sectional area is given by: However, A8F is the cross-sectional area of the partition wall, 0DBP is the outer diameter of the partition wall,
And IDBp is the inner diameter of the partition wall.

The total cross-sectional area of the original hollow billet including the bulkhead is given by: However, A-cho! is the total cross-sectional area of the original hollow billet including the bulkhead, 0DT is the outer diameter of the original hollow billet, and ■DT! is the inner diameter of the hollow billet.

In that case, the required cross-sectional area of the original bulkhead AB! is given by the following equation. EXAMPLE An example of a method for manufacturing a composite cladding tube 12 according to the present invention is shown below.

The billet for the zirconium alloy cladding and the insert for the metallic zirconium septum are machined, cleaned and assembled in a conventional manner.

In addition, these dimensions are selected to be compatible with extrusion of the assembly in a hot extrusion press. The build for the cladding is of conventional Zircaloy 2 meeting ASTM B353, grade RA-1, and the insert for the septum is of metallic zirconium with an impurity content within the ranges mentioned above. The bores of the billet and insert are formed with a taper of 8 mils per inch (203p) so that their mating under pressure ensures good contact between the mating surfaces. Examples of dimensions of such machined parts are as follows.

That is, the billet for cladding tube has a length of 9.
0 inch (22.9 cm), outer diameter 5.74 inch (1
4.6 cm) and has an inner diameter of 2.44 inches (6.20 cm)
m). The bulkhead insert has an outer diameter of 2.44 inches and an inner diameter of 1.66 inches (4.2 inches).
2cm). Prior to assembly, trace impurities are removed by lightly etching the billet and insert contact surfaces.

A suitable caustic agent for such purpose is 70 ml of H2O,
30 ml of HNO3 (70% aqueous solution) and 5 ml of 8
(48% aqueous solution). 20 mercury to ensure a satisfactory bond during extrusion.
Pre-bonding of the assemblies may be accomplished by press-fitting a tapered insert into the tapered bore of the billet in a submicron vacuum.

In that case, 30 to 45,000 bonds (13.7 to 20
While applying an initial pressure of 400k9), about 1400. F(
It is sufficient to maintain the temperature of 76(s) for 8 hours. It has been found that this results in a bond over 20-25% of the contact area. To reduce end loss during extrusion, a 2 inch long piece of Zircaloy 2 tubing may be welded to each end of the pre-bonded assembly and machined to a flat surface. .

When extruding such a pre-bonded aggregate into a composite material, the following parameters may be used.

That is, the extrusion speed is 6 inches/min and the processing ratio is 6:1.
, the temperature is 1100°F, and the extrusion load is 3500 tons. All billet surfaces except the bore and floating mandrels may be lubricated with a water-based lubricant that has been heat treated at 1300'F (704C) for 1 hour.

After extrusion, additional tubing is cut from each end of the composite and the interior surface is honed to remove flaws and improve the finish. In order to obtain a cladding tube of suitable dimensions for a fuel element by drawing such a composite material, cold working with three baths in a known Pilger tube drawing machine and heat treatment and cleaning operations between successive baths are required. Just go.

The steps of a typical operating procedure are shown in FIG. The operating procedure is conventional except for the changes made in accordance with the present invention.

The rationale for such changes and the beneficial results obtained therefrom will be discussed below. The intense cold working performed in the tube drawing process causes distortion of the shape of the metal microcrystals and creates a large number of lattice defects inside the microcrystals.

That is, the metal after cold working is in a relatively high energy state, which is thermally unstable. The annealing process uses heat to impart mobility to the atoms of the metal, thereby serving to rearrange such atoms into a lower energy state. The parameters that affect such annealing are temperature and time, with temperature being the more sensitive. The general ratios, annealing temperatures and times are selected to be sufficient to provide substantially complete recrystallization but insufficient to cause excessive grain growth. That is,
Annealing steps (5) and (8) during the operating procedure in Figure 3
As regards temperature and time, the temperature and time are selected to result in substantially complete recrystallization of the zirconium alloy of cladding tube 21.

However, since the relatively pure metal constituting the partition wall 22 recrystallizes at a lower temperature, steps (5) and (8)
Typical annealing temperatures and times suitable for zirconium alloys such as those used in the present invention result in an undesirable degree of grain growth in the partition wall 22 in the finished product.

Therefore, according to one feature of the invention, the cladding obtained after the final tube drawing step is subjected to a heat treatment at a low temperature as shown in step (12).

In that case, the temperature and time of the heat treatment in step (12) are selected so that the metallic zirconium constituting the partition wall 22 undergoes substantially complete recrystallization but does not cause crystal grain growth.

The partition walls thus obtained have a fine equiaxed crystal structure and therefore exhibit improved strength and ductility, increased resistance to stress corrosion cracking, and a high degree of plastic stabilization. The temperature and time of the heat treatment step (12) are also selected to allow complete stress relief of the zirconium alloy of the cladding tube 21, but not complete recrystallization thereof.

As a result, zirconium alloys retain the elongated grain structure created by the tube drawing operation and have the advantage of exhibiting increased strength under high strain rates along with internal stress relief. Annealing process (2)
, (5) and (8) are conditions suitable for approximately 100
Approximately 1~ at a temperature of 0~1300F (538~704℃)
Conditions include 1 hour, preferably about 1 to 4 hours.

In addition, conditions suitable for the heat treatment step (12) are approximately 82
Examples include conditions for about 1 to 4 hours at 5 to 9501 F (440 to 510 C).

According to another feature of the invention, the crystal structure (i.e. the degree of preferred crystal orientation) of the metallic zirconium partition wall may optionally be improved by applying mechanical compressive deformation to the surface thereof. can.

For example, by subjecting the partition wall to shot peening from the inside of the composite material prior to the heat treatment step (12), compressive deformation of the partition wall can be achieved without substantially deforming the zirconium alloy cladding tube. Such mechanical treatment, shown as step (10) in FIG. 3, results in an improved crystal structure in which the bottom poles (0002) are highly aligned along the radial direction of the composite cladding. You will get it.

[Brief explanation of the drawing]

FIG. 1 is a partially cut-away elevational view of the fuel element, FIG. 2 is a cross-sectional view of the fuel element of FIG. 1, and FIG. This is a chart showing.

Claims (1)

[Scope of Claims] 1. A zirconium alloy tube containing more than about 5000 ppm of components other than zirconium, and a cladding material consisting of a metal zirconium layer metallurgically bonded to the inner surface and containing less than 500 ppm of impurities. , a method of manufacturing an elongated composite cladding tube useful for containing nuclear fuel and constructing a fuel element for a nuclear reactor, comprising: (1) forming the cladding material to desired dimensions by cold working in a series of tube drawing steps; (2) between successive tube drawing steps, using a temperature and time sufficient to effect substantially complete recrystallization of the zirconium alloy; Heat treatment is performed, and (
3) After the final tube drawing step, which results in substantially complete recrystallization of the metallic zirconium layer to provide a fine microscopic structure within it and at the same time allows for stress relief of the zirconium alloy; A method comprising the initial step of heat treating the cladding using low temperatures and times that do not result in complete recrystallization. 2 The temperature and time of the step (2) are each about 53
8 to about 704°C and about 1 to about 15 hours, and the temperature and time of step (3) are about 440 to about 510°C and about 1 to about 4 hours, respectively. Method described. 3. The method of claim 1 or 2, further comprising the step of applying a substantially uniform compressive deformation to the surface of the metallic zirconium layer prior to step (3). 4. The method of claim 3, wherein the compressive deformation is applied by shot peening. 5 Consisting of a zirconium alloy tube containing more than about 5000 ppm of components other than zirconium and a metal zirconium layer with an impurity content of less than 500 ppm metallurgically joined to the inner surface of the tube, it accommodates nuclear fuel and is used as fuel for a nuclear reactor. In the elongated composite cladding tube that serves to construct the element, the metallic zirconium layer undergoes substantially complete recrystallization and exhibits a fine microscopic structure, and the zirconium alloy tube undergoes substantially complete stress relief. A composite cladding tube that exhibits oxidation but has not undergone complete recrystallization. 6 Compressive deformation is applied to the surface of the metal zirconium layer. A composite cladding tube according to claim 5. 7. The composite cladding tube according to claim 6, wherein the compressive deformation is applied by shot peening.