JPS6151626B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing a zirconium-based alloy cladding tube for a nuclear reactor fuel rod.

Clad tubes for nuclear reactor fuel rods are commonly made of thin-walled zirconium-based alloy tubes known as Zircaloy. These alloys include alloying materials such as tin, iron, nickel, and the like. In the case of Zircaloy, the α phase is stable at temperatures below 790°C and the β phase is stable at temperatures above 950°C, whereas the two-phase region, ie, the α plus β phase region, occurs between 790°C and 950°C. In the α phase, zirconium atoms are arranged in a hexagonal close-packed lattice, and in the β phase, they are arranged in a body-centered cubic lattice. During so-called β-quenching of Zircaloy to obtain desired properties, such as improved corrosion properties, the material is heated to a temperature in the β-phase region and then quenched to a temperature in the α-phase region.

Conventionally, in manufacturing Zircaloy clad pipes, an ingot is forged into a rod and then subjected to β-quenching. After manufacturing the rod into an extruded billet,
The billet is extruded to the alpha phase state at a temperature of 680°C and the extrudate is then subjected to several stages of cold rolling, with a 625°- Annealing or intermediate annealing is performed at 700°C. Cooling of the extrudate after each intermediate annealing takes place relatively slowly at a rate of up to 3° C. per minute in a temperature range just below the annealing temperature and without the use of any coolant. After the final cold rolling step, a final anneal is applied to impart the desired properties to the material. This final annealing is carried out at a temperature of 400° to 700°C.

Tubes made from Zircaloy under conditions conventionally employed have generally proven to have sufficient resistance to corrosion at the operating conditions applicable to nuclear reactors. However, the development of the situation is that the operating time of fuel assemblies is becoming longer and longer. Therefore, the cladding material is exposed to corrosive water for a longer period of time than previously, thereby increasing the risk of corrosion damage. It is therefore desirable to obtain better corrosion properties from the alloy used without undesirable changes in its mechanical properties.

By β-quenching finished tubes of Zircaloy, as is already known from US Pat. No. 4,238,251, it is possible to improve the resistance of the tubes to so-called accelerated nodular corrosion in water and high-pressure steam. US Patent Specification No.
No. 3,865,635, Zircaloy tubes with good mechanical properties can be obtained by β-quenching the extrudate before subjecting it to the final cold rolling step.

The exact reason why beta quenching achieves improved resistance to accelerated nodular corrosion has not yet been completely determined. However, improvements are believed to be related to the dimensions of the intermetallic compounds in the material as well as their distribution. The intermetallic compound, the so-called second phase, is composed of a compound containing mainly iron, chromium, and nickel in addition to zirconium, and has a granular form. The dissolution and precipitation achieved by β-quenching results in a reduction in grain size and a redistribution of the evenly distributed grains to those that constitute the arrangement at the grain boundaries of the α grains formed during the β phase transformation.

β-quenching of the finished cladding tube reduces the ductility of the tube, which is a drawback of the conventional manufacturing method described above. Beta quenching the extrudate before cold rolling it to its final dimensions reduces the degradation of the mechanical properties of the finished tube. However, β-quenching, whether performed on the finished tube or before the final cold rolling process, increases the amount of scrap and also forms an oxide layer on the tube surface that must be removed. This will result in a decrease in production.

In accordance with the present invention, it is possible to produce cladding tubes for fuel rods for nuclear reactors that have at least as good a resistance to nodular corrosion as the best cladding tubes known to date, as well as better ductility than such cladding tubes. has been proven. Compared to the conventionally known manufacturing method for clad pipes which uses β-quenching after extrusion, the present invention also uses β-quenching (i.e., β-quenching is performed after extrusion), in which β-quenching is carried out at an early stage of the manufacturing process. This allows the removal of formed oxides over smaller surfaces, resulting in increased throughput due to less scrap and less material loss.

The present invention provides a method for manufacturing zirconium-based alloy cladding tubes for nuclear reactor fuel rods, in which the zirconium-based alloy is extruded and the extrudate is subjected to cold rolling and at least one annealing, i.e., two successive cold rolling steps. In a method of undergoing intermediate annealing during inter-rolling and β-quenching at a stage before final cold rolling,
The preferred temperature for intermediate annealing is
500-610°C, particularly preferably 550-600°C. In other words, β quenching is performed before at least two cold rollings including the final cold rolling.

Extrusion is carried out at any temperature in the alpha phase region.

After the final cold rolling, the extrudate is subjected to a final annealing at a temperature of 400-675°C, preferably 400-610°C, most preferably 550-600°C.

β-quenching of extrudates is carried out at temperatures in the β-phase region, i.e. suitably
This is carried out by heating to a temperature of 950-1250°C, preferably 1000-1150°C, followed by rapid cooling to a temperature in the alpha phase region. Next, from the temperature used in the β phase region,
Cooling is suitably carried out to a temperature of 790°C at a rate of 20-400°C per second, and from 790°C to a temperature of 500°C or less at a rate of 5°C or more per minute.

In manufacturing the clad tube of the present invention, it is important to note that the size of the second phase particles in the finished tube is considerably smaller than the size in the conventional clad tube manufacturing without β quenching after extrusion, such as when β quenching is used. It's clear. However, the second phase particles are evenly distributed in the material, contrary to the case after beta quenching in conventional methods. It may be the small size of the second phase grains achieved by the present invention as well as their uniform distribution that provides a favorable combination of good resistance to nodular corrosion and good mechanical properties.

The zirconium-based alloy is preferably a zirconium-tin alloy, such as the alloys known under the trade names Zircaloy 2 and Zircaloy 4, with an alloy component content of 1.2-1.7% tin and 0.07-0.24% iron.
%, 0.05-0.15% for chrome, 0- for nickel
It is in the range of 0.080%, the remainder being zirconium as well as normal existing impurities, the percentages of which are stated in weight percentages like the other percentages in the text.
Zircaloy 2 contains 1.2-1.7% tin, 0.07-0.20% iron, 0.05-0.15% chromium and 0.03-0.08% nickel. Zircaloy 4 is 1.2−
Contains 1.7% tin, 0.18-0.24% iron, 0.07-0.13% chromium, and no nickel.

The zirconium-based alloy is preferably β-treated before extrusion.
It undergoes a rapid cooling treatment, that is, it is heated to a temperature in the β phase region and rapidly cooled to a temperature in the α phase region. However, it is possible to use zirconium-based alloys without β-quenching. β quenching before extrusion is performed to suitably cool the alloy to 950-1250°C, preferably 1000-1000°C.
This is done by heating to a temperature of 1150°C and rapidly cooling to a temperature in the α phase region. 750 from the operating temperature in the β phase region
This cooling to a temperature of 0.degree. C. is carried out at a rate of 1-50.degree. C. per second, and cooling from 790.degree. C. to a temperature of 500.degree. C. or less is preferably carried out at a rate of greater than 5.degree. C. per minute.

Next, the present invention will be described in detail by giving examples thereof.

Forge Zircaloy 2 ingots into rods with a diameter of 150-200 mm. The rod piece is subjected to a β-quenching treatment by heating it at a temperature of 1050°C for 15 minutes and cooling it to room temperature at a rate of 5-10°C per second. Make an extruded billet from a rod. These billets are 700-740
It is extruded at a temperature in the α phase region. The extrusion was then subjected to a three-stage cold rolling process, which resulted in a final outside diameter of the tube of 12.3 mm. Between the first and second rolling, a high frequency coil placed around the extrudate was used to heat the extrudate to 1050°C for a few seconds.
The sample was heated until then and subjected to β-quenching. After that, the extruded product
It was cooled to room temperature by water jet at a rate of 200°C/sec. Between the second and last rolling the extrudate is annealed to a temperature of 575° C. for 1 hour. After each intermediate annealing, the extrudates are cooled in a helium-filled furnace at a cooling rate of 10°C per minute at the annealing temperature, i.e., in the temperature range from 700°C to 650°C. After the final cold rolling, the tube is finally annealed at a temperature of 565℃.
Both intermediate and final annealing are performed in a vacuum furnace. In the finished tube, the second phase particles are approximately 0.01
In clad tubes produced by conventional methods and not subjected to β-quenching in the finished or early extruded state, with sizes in the range -0.2 μm and average particle size of about 0.1 μm, the second phase particles are approximately
It has a size in the range 0.1-0.6 μm and an average particle size of about 0.3 μm.

During corrosion tests, which have been demonstrated to simulate nuclear reactor operating conditions, the clad tubes made according to the present invention are only a fraction of those obtained with conventional manufacturing without post-extrusion β-quenching treatment. The weight gain is approximately the same as that obtained during production by β-quenching after extrusion, and is 50-100 mg/dm ² in the case of the present invention and 350-4000 mg in conventional production without β-quenching. /dm ² . The ductility of clad tubes made in accordance with the present invention is better than tubes that have undergone beta quenching in the finished state and better than tubes that have undergone beta quenching immediately prior to final cold rolling.

The above corrosion tests were carried out in a steam pressure vessel at a pressure of 9.8 MPa and a temperature of 500°C. Weight gain indicates the degree of corrosion that the tube has undergone.

Claims (1)

[Claims] 1. A method for manufacturing a zirconium-based alloy cladding tube for a nuclear reactor fuel rod, wherein the zirconium-based alloy is extruded and the extrudate is subjected to cold rolling and two consecutive In the above manufacturing method, the extrudate is subjected to at least one intermediate annealing during cold rolling, and the extrudate is further subjected to β quenching at a stage prior to the final cold rolling. At least 2 including
The above manufacturing method, characterized in that the intermediate annealing is carried out before two cold rollings, and the intermediate annealing is carried out at a temperature of 500-675°C between the two cold rollings. 2. A manufacturing method according to claim 1, wherein the intermediate annealing is carried out at a temperature of 500-610°C, preferably 550-600°C. 3 In the manufacturing method according to either claim 1 or 2, the zirconium-based alloy contains 1.2-1.7% tin and 0.07-0.24% iron by weight;
The above manufacturing method containing 0.05-0.15% chromium and 0-0.08% nickel, the remainder being zirconium and any existing impurities of the common type. 4. The manufacturing method according to any one of claims 1 to 3, wherein the zirconium-based alloy used during extrusion is β-quenched. 5. A method according to any one of claims 1 to 4, wherein the extrudate is subjected to a final annealing at a temperature of 400-675° C. after the final cold rolling.