JPS6364275B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to nuclear reactor fuel elements, and more particularly to a novel method for manufacturing a composite billet for a composite clad fuel cladding tube in which a zirconium metal barrier is provided on the inner surface of the zirconium alloy tube. Currently, nuclear fuel elements for power reactors encapsulate nuclear fuel material within a tubular, corrosion-resistant, non-reactive, and thermally conductive cladding. Such fuel elements are assembled in a lattice shape at regular intervals to form a fuel assembly, and an appropriate number of these fuel assemblies are combined to form a nuclear fission chain reaction type assembly or reactor core capable of self-sustaining nuclear fission reactions. This core is placed in a reactor vessel through which coolant passes. Coatings are used for several purposes, the first of which is to prevent chemical reactions between the nuclear fuel and the coolant or between the nuclear fuel and the moderator. A second purpose is to prevent radioactive fission products, which are partially gaseous, from escaping from the fuel into the coolant or moderator. Commonly used coating materials are stainless steel and zirconium alloys. Various problems arise in the manufacture and operation of nuclear fuel elements using certain metals and alloys as cladding materials due to mechanical or chemical reactions that occur in these cladding materials under certain conditions. Zirconium and its alloys are excellent nuclear fuel cladding materials under steady-state conditions. The reason is that zirconium and its alloys have a small neutron absorption cross section, and at temperatures below about 400°C, they are strong and ductile in the presence of pure water or steam, commonly used as reactor coolants and moderators. This is because it is extremely stable and non-reactive. However, the behavior of the fuel element is problematic in that interactions between the nuclear fuel, the cladding, and the fission products produced during the fission reaction can cause the cladding to become brittle and crack. It has been determined that this undesirable behavior is further exacerbated by local mechanical stresses in the fuel cladding due to differential thermal expansion between the fuel and the cladding. During operation of a nuclear reactor, fission products are released from the nuclear fuel by fission reactions and are present on the surface of the cladding. In the presence of certain fission products such as iodine and cadmium, stress corrosion cracking occurs due to the action of local stress and strain. As a measure to prevent such damage, attempts have been made to provide various metal barriers between the fuel and the cladding. Among these, a composite cladding tube made of a zirconium alloy tube of moderate purity and metallurgically bonded to the inner surface is considered the most promising as a metal barrier. The thickness of the zirconium barrier is about 5-30% of the cladding thickness. Compared to zirconium alloys, zirconium
It remains soft during irradiation, reducing local strains within the nuclear fuel element and protecting the cladding from stress corrosion cracking. Another advantageous feature is that it does not cause significant neutron capture penalties, heat transfer penalties, or material incompatibility problems. Such a composite cladding tube is usually manufactured by the following method. As shown in FIG. 1, a zirconium ingot serving as a metal barrier and a zirconium alloy ingot serving as a covering material are melted, a zirconium hollow billet is inserted into a zirconium alloy hollow billet, and they are integrated to form a composite billet. Approximately 500 pieces of this composite billet are produced using the normal hot extrusion method.
Extrusion at high temperature ~750℃. Next, the extruded composite tube is subjected to a conventional tube reduction process to complete a cladding tube of desired dimensions. For such composite cladding, it is extremely important that the thickness of the zirconium barrier is controlled to the desired dimensions, and that the zirconium barrier and the cladding material have a complete metallic bond over the entire surface. . To do this, the zirconium hollow billet (inner cylinder) and the zirconium alloy hollow billet (outer cylinder) must be integrated in the composite billet state, and deformed as one body during subsequent extrusion and tube reduction processing. It is. Conventional methods are used to achieve such integration. (1) Insert the inner cylinder into the outer cylinder and integrate them by explosive welding. (2) Insert the inner cylinder into the outer cylinder and heat to diffusion bond and integrate. However, the conventional method described above requires extremely high reliability since the object is a fuel cladding tube, and has the following drawbacks. In other words, method (1) cannot be applied to those with a small inner diameter (approximately φ30 mm or less). Furthermore, the connection between the outer cylinder and the inner cylinder is not uniform in the axial direction. There are problems such as the bonding interface becoming ripple-like.
Method (2) involves long-term high-temperature heating (e.g. 750℃ x 8
Moreover, it is difficult to control the conditions as there are multiple steps, such as applying pressure during heating to ensure uniform diffusion and mechanical pre-bonding in advance. Furthermore, if a vacuum or inert atmosphere is not used, the bonding interface will oxidize, which will interfere with subsequent hot extrusion and tube reduction processing. From the above, in order to achieve uniform and good bonding of the zirconium barrier over the entire surface, the method of integrating the composite billet is an extremely important step in the manufacturing process. An object of the present invention is to provide a method for manufacturing a good composite billet for a fuel cladding tube by integrating a zirconium inner cylinder as a metal barrier and a zirconium alloy outer cylinder as a cladding material. The present invention relates to a method for mechanically bonding a zirconium inner billet to a zirconium alloy outer billet. That is, after inserting the zirconium inner billet into the zirconium alloy outer billet, inserting the elastic body inside the inner billet,
Next, by applying a compressive load to the pressure medium in the axial direction from the outside, the inner cylinder is expanded in the radial direction and is pressed against the inner surface of the outer cylinder billet. In the present invention, by using an elastic body such as rubber as the pressure medium, the pressure medium behaves in the same way as a fluid, so that a uniform force can be applied to the inner cylinder over the entire circumferential direction and axial direction. In addition, the force can be easily controlled, and a sufficiently reliable mechanical connection can be achieved without deforming the wall thickness of the zirconium inner cylinder. The above-mentioned tube expansion method using rubber etc. is already known as the double tube manufacturing method for aluminum, copper alloy, etc.
When manufacturing composite billets for fuel cladding, the following points should be considered. The load during pipe expansion is calculated using the following formula. W=PA _r /η...(1) Where, W: Pipe expansion load (Kg) P: Pipe expansion pressure (Kg/mm ² ) A _r : Cross-sectional area of tube expansion rubber (mm ² ) η: Pipe expansion efficiency (0.85 ~0.9) Figure 2 shows the results of measuring the pull-out fixing force using the above formula and varying the expansion pressure of the zirconium inner cylinder. As the tube expansion pressure increases, the pull-out fixing force also increases. In addition, the pulling-out fixing force is the load required to pull out the inner cylinder after pipe expansion. Generally, the pulling force of a double pipe made of aluminum or copper alloy is sufficient within the range of 100 to 1000 kg, as shown by the diagonal line 1 in the figure. However, in order to manufacture fuel cladding tubes, the resulting composite billet is processed at a processing rate of 80% or more through hot extrusion and tube reduction processing, so it is necessary to have a higher pulling force than in the general case. There is. As a result of various experiments regarding the lower limit value of the pull-out fixing force, the present inventors found that the fixing force should be at least twice that of the conventional one, as shown in the shaded area 2 in the figure. Next, we found that the wall thickness ratio between the zirconium inner tube and the zirconium alloy outer tube during tube expansion is extremely important in order to control the zirconium barrier thickness to the target value. The wall thickness ratio is expressed by the following formula. t = Z _r / Z _ry + Z _r × 100 (%) ...(2) where t: Wall thickness ratio Z _r : Wall thickness of zirconium inner cylinder Z _ry : Wall thickness of zirconium alloy outer cylinder Zirconium inner cylinder and zirconium The alloy outer cylinder was expanded with various wall thickness ratios, a composite cladding tube was finally manufactured, and the wall thickness ratio at the final stage was determined. As a result, the wall thickness ratio at the final stage was 5
It was found that the size was reduced by ~10%. This is because zirconium is easier to process than zirconium alloys. Therefore, in order to obtain the desired thickness of the zirconium barrier, it is better to increase the wall thickness ratio in advance by 5 to 10% during tube expansion. Due to the above characteristics, it is possible to manufacture a composite cladding type fuel cladding tube which has smaller variations in zirconium barrier thickness and has a uniform crimping condition over the entire surface than the conventional method. Examples will be described below to specifically illustrate the present invention. Outer billet is ASTMB353, grade RA-
A standard Zircaloy-2 alloy conforming to No. 1 was melted by arc melting. The material for the inner cylinder billet, which acts as a metal barrier, is arc-melted zirconium.
Pure zirconium was produced. Both ingots were hot forged and each was cut into a hollow billet in order to perform the rubber tube expansion of the present invention. The outer surface of the hollow inner billet and the inner surface of the outer billet have an average surface roughness of 10μm or less, especially
Polish and smooth to a thickness of 6 μm or less. The reason for this is to ensure that the longitudinal thickness of the metal barrier is as uniform as possible in the final composite cladding. Table 1 shows the dimensions of the hollow billet for rubber tube expansion and their wall thickness ratios. Examples 1 and 2 aim for a zirconium barrier thickness of 75±5 μm, and Examples 3 and 4 aim for a zirconium barrier thickness of 80±5 μm.
Note that the protrusion amount of the inner cylinder billet is the same as in Examples 1 and 2.
In Examples 3 and 4, it was 0.5 mm, and in Examples 3 and 4, it was 3 mm.

[Table] Figure 3 shows the assembled composite billet. The method of the present invention for mechanically joining and integrating this composite billet is shown in FIG. In FIG. 4, the left half shows the state before processing, and the right half shows the state after processing. The device fixes base plates 14 and 15 via a support member 13 with a nut 16, and includes a restraining mold 31 of a composite billet, a pressure medium 41 inserted into the billet, and a pressure medium 41 disposed on both end surfaces of the pressure medium 41. a pressurizing rod 61 that loads the pressure medium 41 and an axial compressive load, and the pressurizing rod 6.
Hydraulic cylinder 7 and lower end plate 11 connected to 1
The extrusion member 8 is fixed to the extrusion member 8.
By tightening the composite billet to the restraining mold 31, the hydraulic member 8 is pressed against the inner cylinder billet 2. In this state, the pressure medium 41 is compressed in the axial direction by the axial load generated by the hydraulic cylinder 7, and the inner billet 2 is expanded in the radial direction and mechanically connected to the outer billet. During the expansion process, the pressing member 8 is moved in the axial direction by the restoring force of the spring 10 attached to the end plate 11 by the tightening bolt 12 and washer 9 by the amount that the inner cylinder billet has shrunk in the axial direction, and the pressure medium 41 is prevented from plastically flowing through the gap. In this embodiment, in order to uniformly connect the inner cylinder billet to the end face, the axial length of the pressure medium 41 is made longer than the billet length. The above-mentioned apparatus was used to reduce pressure in Examples 1 to 4.
Bonding was carried out at 3000Kg/cm ² . Non-uniform deformation of the wall thickness of the inner cylinder billet was not clearly measured. The measurements of the final finished tube dimensions and zirconium barrier thickness are shown in Table 2. The zirconium barrier thickness satisfied the target value over the entire length. Furthermore, ultrasonic tests and cross-sectional observations revealed that the zirconium barrier and the Zircaloy cladding tube had no defects over the entire length, and a good metallurgical bond had been achieved.

[Table] As is clear from the above examples, the present invention has the following effects. (1) Since the dimensions of the composite billet are easy to control, the thickness of the metal barrier can be controlled with high precision. (2) Applicable even if the inner diameter of the composite billet is small. (3) Processing time is shorter than conventional methods, and work conditions can be easily managed. In order to improve the soundness between the inner cylinder and outer cylinder billet of the composite billet according to the present invention, the joining operation between the two is carried out in a vacuum or in an inert gas, and air, moisture, or Contaminants may be removed or replaced with inert gas. The present invention is not limited to the above embodiments,
Various modifications and additions of known techniques can be made. For example, when manufacturing a composite billet, if the billet is heated to cause diffusion, the bonding strength between the billets will increase, and therefore the pressing force can be reduced.
The diffusion conditions may be those conventionally known.

[Brief explanation of drawings]

Figure 1 is a manufacturing process diagram of a conventional composite fuel cladding tube, Figure 2 is a diagram explaining the pressurization conditions in the present invention, and Figure 3 is a vertical cross section showing the state in which the inner billet is inserted into the outer billet. 4 are longitudinal sectional views of the manufacturing apparatus used in the present invention.

Claims (1)

[Claims] 1 Insert a hollow inner billet made of zirconium that is slightly longer than the outer billet into a hollow outer billet made of nuclear fuel cladding material, insert an elastic body into the inner billet, and then insert the elastic body in the axial direction of the billet. A method for manufacturing a composite billet for a fuel cladding tube, characterized in that the outer surface of the inner billet is brought into close contact with the inner surface of the outer billet by applying pressure.