Fuel element manufacturing method

FIELD: mechanical engineering.

SUBSTANCE: proposed method is intended for manufacturing fuel elements using resistance-butt welding primarily for their joining to zirconium alloy cans. Welding is effected at equipment electrical resistance not over 30-fold resistance of can section forming welded joint having or not point discontinuities in the form of separate spots or chain of such spots forming no continuous line and measuring maximum ten thicknesses of weld in cross-sectional area of welded joint under check disposed at distance equal to two or three thicknesses of can wall from butt-end of plug located inside fuel element, welded joint being checked by uniformity of external fin. Length of poor fin formation section does not exceed 10% of joint perimeter.

EFFECT: enhanced quality of welds and reliability of fuel element sealing.

2 cl

 

The invention relates to mechanical engineering, in particular to the manufacture of fuel elements with the use of contact-butt welding mainly with shells made of zirconium alloys.

Known method of sealing fuel elements representing a tubular sheath made of an alloy of zirconium, contact welding (see RF patent №2127457, 6 G 21 3/10, 21/00, 21/02, 23/11/02, 15/00)consists in preparing the workpiece for welding, contact welding, at least one end of the shell with a cover on the given modes with the fixation end of the shell in a special device with a specified value of electric resistance.

The disadvantage of this method is the lack of guaranteed quality of welds, narrow range of use due to the small allowable change in the electrical resistance of a special device

The closest in technical essence and the achieved result is a method of manufacture of fuel elements for RF patent No. 2140674 (6 G 21 21/02 prototype)consists in preparing the workpiece for welding, contact welding of one of the ends of the shell with the cap detent end of the shell in a special fixture and subsequent ultrasonic testing of this seam.

The disadvantage with the person is the lack of guaranteed quality of welds, since the presence of ultrasonic testing allows a specific error to measure the length of the weld along the axis of the fuel element, and the presence of rough defects such as incomplete penetration. Operational and periodic monitoring, which determines the appearance of such defects, missing. Low technological opportunities due to restrictions on permissible electric resistance of a special device.

An object of the invention is to improve the quality of welds and reliability of the sealing of the fuel Rods with the expansion of technological capabilities of the method.

The solution of the technical problem is achieved by the fact that when the sealing fuel cells by a known method consists in preparing the shell for welding, Assembly fuel element welding, at least one of the contact plugs-butt welding device with a specified value of electric resistance and subsequent inspection of welds according to the invention the welding is carried out at a value of electric resistance device, not exceeding tridtsatipjatiletnego resistance values section of the shell, forming a welded connection with the absence or presence of controlled cross-section of the weld is located on the distance is two or three wall thickness of the sheath from the end of the stub inside of a fuel rod in a point of discontinuity as individual points or chain of such points, not forming a continuous line and having a maximum size of not more than 10 thickness of the weld, and the weld inspection are also on the uniformity of the outer Burr. The length of the section with impaired formation of the Burr does not exceed 10% of the perimeter of the connection.

Presents a collection of characteristics is new, not known from the prior art, and provides the desired effect.

The presence of a range of variation of the resistance welding device to a value not exceeding tridtsatipjatiletnego resistance values section of the shell, forming a welded connection, expanding the technological capabilities of the method. Exceeding of this value leads to overheating of the shell and its surface oxidation.

The welding on the modes, providing control of the presence or absence of controlled in the transverse section of the weld point inhomogeneities in the form of individual points, or chain of such points, not forming a continuous linou and having a maximum size of not more than 10 thickness of the weld, provides the necessary strength and corrosion properties of welded joints, as well as his integrity.

Detected by metallographic inspection of cross sections of point discontinuities are structural defects of the welded connection, the local and the zones with a high level of stress at the boundary of the individual grains, having high etching capacity. The presence of such heterogeneity has no effect on the performance characteristics of the fuel element and does not limit the duration of its operation. As shown by pre - and post-irradiation studies over time under the influence of temperature and, apparently, neutron irradiation, these inhomogeneities disappear completely.

Limiting the magnitude of a point inhomogeneity in the radial direction up to 10 thickness of the weld eliminates the possibility of obtaining in a weld heat-generating element discontinuities channel type, characteristic of poor preparation of the welded parts for welding.

Control the uniformity of the outer Burr along the perimeter of the welded connection also ensures the formation of welded joints to the required length.

The presence of areas disturbed by forming a length of more than 10% of the perimeter of the connection does not guarantee the weld of the required quality.

Implementing periodic monitoring in cross section, spaced from the end face of the stub at a distance of 2-3 wall thickness of the shell (controlled section), makes it possible to more objectively evaluate the stability of the process. The size of the zone that is controlled section (2-3 shell thickness)selected from the usl is via exceptions influence on the evaluation of the quality of the welding edge effects at the ends of the weld, associated with the formation of inner and outer Burr.

The method is implemented as follows.

The shell is cut to size with a given geometry of the ends, if necessary, degreasing it and plugs. Before welding fuel elements check the amount of resistance welding equipment. Depending on the available welding equipment, special purpose welding fixtures value of its electrical resistance can be within their own resistance material of this device (from units macroom) to tridtsatipjatiletnego resistance values section of the shell in the zone of the welded joint, and then perform the welding special designs, made of longitudinal (along the axis of the fuel element) and transverse (across the axis) sections. The welding modes depend on the available welding equipment, special purpose welding fixtures, designs and geometric dimensions of the stub. Thus, the amplitude of the welding current for shells with a diameter of about 9 mm of zirconium alloys may be 12 to 18 kA, the duration of the flow of welding current from 20 to 150 MS. In appearance samples with the desired amount of movement of the stub, control the availability and uniformity of the external grata around the perimeter of the compound. In case of positive testing results from samples made of longitudinal and transverse sections. On the longitudinal cut metallographic when the control is determined by the length of the weld in the direction along the axis of the weld, in cross section, spaced 2-3 wall thickness of the sheath from the end of the stub, the presence of a penetration and a point of discontinuity. The presence of such inhomogeneities, their number, and the location on the perimeter of the weld characterizes the stability of quality of preparation of parts for welding, the stability of the cooking process and the correctness of the selected modes. Therefore, for samples this parameter is quantitatively limited, for example, the percentage of the perimeter of the weld. In case of positive results of monitoring carried out welding of heat-generating elements on the welding mode samples. Perform periodic monitoring of the appearance of welds on the presence of external grata around the perimeter of the welded connection. No grata in excess of the set value is invalid factor. Prior to Assembly of fuel elements in the cartridge, this control is repeated for all products. After making additional finished goods produced during a certain period, selected products for metallographic weld inspection and finally what about the conclusion on the welding quality of these products.

The proposed method is generally implemented in the manufacture of fuel elements of the assemblies of VVER with shells made of zirconium alloys in the following way. Fuel elements are manufactured technological batch, quantity of 80-120 pieces of Shell fuel elements are cut to size with simultaneous receiving surface side at the contact welding shape close to spherical, with a radius equal to 0,15-0,4 its thickness. If necessary, the parts to be welded is subjected to degreasing in special cleaning solutions. After subsequent technological operations connected with welding electron beam welding of the first stub, weighing the empty and filled with fuel shells, shell processed batch come to the welding operation. Before welding products welding is performed two special samples for metallographic inspection. Of the samples made of longitudinal and transverse thin section, which control the length of the welded joint in the axial direction, which must be not less than three thicknesses of the walls of the shell and to have continuity, and the presence and number of point discontinuities in the testing section, located at a distance of 2-3 wall thickness of the sheath from the end of the stub inside the fuel element

If the length of the welded joint is not less than three thicknesses of the shell and the cross-cut discontinuities are absent or have a point inhomogeneity length of not more than 5% of the perimeter of the weld, and with a uniform distribution of the outer Burr around the perimeter of the connection is permitted welding of standard products. If some parameter is not satisfied with the results, configure, repair welding equipment, accessories or replacement with subsequent fabrication and control of new samples. When getting positive results at the same modes that were welded samples, the welding of standard products. Welding is carried out on the capacitor power supply with pulse duration of the welding current 60-120 MS when its amplitude is 12-14 kA and the resistance of a special device 600-1200 µohm. During this movement of the stub shall not be less than four thicknesses of the shell. The number of products covered by the results of the control samples is determined replaceable output per unit of welding equipment, if during the shift was not made its repair, replacement equipment and changing modes of welding. During welding of fuel elements control the appearance of the welded connection and the availability and distribution of outdoor grata around the perimeter of the weld. When there are areas with no grata, a length of more than 10% of the perimeter of the compound, the product on the Assembly is not allowed, welding ends and is made up of the equipment, followed by repeating the procedure of welding and control samples, also before the Assembly of fuel elements in the cartridge of this control are all fuel elements. In addition, replaceable produce selected products for metallographic weld inspection. However, if the transverse cut of point discontinuities are absent or are in the form of single and strochechnyh formations, not merging in a continuous line with the increase over three hundred times their value is less than 10 thickness of the weld, the products are suitable and do further operations.

1. The method of manufacture of fuel elements, including the preparation of the shell, the Assembly of fuel elements, weld ends, at least one of the contact plugs-butt welding in the welding equipment specified by the value of electrical resistance and subsequent testing of welded joints, characterized in that the welding is carried out at a value of electric resistance welding equipment, not exceeding tridtsatipjatiletnego resistance values section of the shell, forming the weld the connection with the absence or presence of controlled cross-section of the weld, located at a distance of two or three wall thickness of the sheath from the end of the stub, which is located inside the fuel element, a point of discontinuity in the form of individual points, or chain of such points, not forming a continuous line and having a maximum size of not more than ten thickness of the weld, and the weld inspection carried out by the uniformity of the outer Burr.

2. The method according to claim 1, characterized in that the uniformity of the outer Burr is defined as the ratio of the lengths of the sections with no grata to the total length of the weld, not exceeding 10%.



 

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SUBSTANCE: proposed method is intended for manufacturing fuel elements using resistance-butt welding primarily for their joining to zirconium alloy cans. Welding is effected at equipment electrical resistance not over 30-fold resistance of can section forming welded joint having or not point discontinuities in the form of separate spots or chain of such spots forming no continuous line and measuring maximum ten thicknesses of weld in cross-sectional area of welded joint under check disposed at distance equal to two or three thicknesses of can wall from butt-end of plug located inside fuel element, welded joint being checked by uniformity of external fin. Length of poor fin formation section does not exceed 10% of joint perimeter.

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