RU2513036C1 - Method to manufacture gas-filled fuel element - Google Patents

Abstract

FIELD: power engineering.

SUBSTANCE: method of fuel element manufacturing includes making a "prefabricated pipe" by tight connection of a shell with one of end plugs and formation of a fuel core by installation of fuel pellets into the "prefabricated pipe". Prior to installation of fuel pellets the "prefabricated pipe" is first vacuumised and filled with heavy inert gas, for instance, argon, and subsequent installation of pellets is carried out with the help of a jet of the specified heavy inert gas sent under excessive pressure. Then repeated vacuumisation is carried out, filling of the internal volume with helium and final sealing of the fuel element with the help of the second end plug. When the "prefabricated pipe" is filled with heavy inert gas and during installation of fuel pellets, it is installed vertically, with an open end upwards.

EFFECT: removal of dusty particles of fuel in the process of fuel element manufacturing in a gap between pellets and a shell and provision of contactless action at fuel pellets, improved composition of atmosphere under a shell, increased resource and reliability of fuel elements operation.

10 cl, 10 dwg

Description

The invention relates to nuclear energy, in particular to methods for the manufacture of gas-filled fuel elements (fuel elements) with fuel cores made of uranium nitride or carbonitride.

A known method of manufacturing gas-filled fuel rods, in accordance with which the first assembled unit "pipe assembly" - a shell with a welded first plug, then form the fuel core by equipping the "pipe assembly" with fuel pellets and carry out the final sealing. At the final sealing of the fuel rod assembly with fuel pellets, vacuum, fill with helium and weld the second plug (Reshetnikov F.G. et al. Development, production and operation of fuel elements of power reactors. - M .: Energoatomizdat, 1995, book 2 , p. 159-160, p. 192-193).

Due to the presence of deaf cavities under the cladding of the fuel rod (at the junction points, between the fuel pellets, between the pellets and the cladding), it is not possible to carry out deep evacuation of the fuel rod, therefore, air partially remains in these places. In addition, it is known that carbide, nitride and carbonitride fuels have chemical activity with respect to oxygen and water vapor. This leads to the oxidation of the tablets and to a decrease in the resource of the fuel rod. The butt-lock connection with the molten bead used in the sealing of fuel rods is often accompanied by the formation of a burr beyond the diameter of the cladding, which requires additional machining. In the manufacture (sealing) of such a fuel rod to obtain a weld with a greater or lesser gain and a sufficient penetration depth, the height of the shoulders is selected, however, this does not exclude the appearance of the weld dimensions for the diameter of the sheath. (Reshetnikov F.G. et al. Development, production and operation of fuel elements of power reactors. - M .; Energoatomizdat, 1995, book 2, pp. 58, 91, 92, 176, 177, 185, 187, 189, s .191-194).

The closest known technical solution adopted for the prototype is a method of manufacturing gas-filled fuel rods, according to which a “pipe assembly” is made by hermetically connecting the shell to one of the end caps, form a fuel core by stacking fuel pellets in it, vacuum, fill its internal volume helium and finally seal the fuel rod by welding the second end plug (patent RU No. 2 127 457, IPC G21C 3/10, publ. 03/10/1999).

In the known method for manufacturing a fuel element, there are also blind cavities under its cladding, in which air partially remains after evacuation, which when using uranium nitride or uranium carbonitride tablets leads to oxidation and subsequent swelling of the fuel core and even violation of the integrity of the cladding and fuel tablets. When forming a weld, dusty fuel particles cannot be excluded from entering the weld zone, which impairs its quality. These shortcomings lead to a decrease in the performance of fuel rods.

The objective of the present invention is to increase the reliability and service life of gas-filled fuel rods with a fuel core of uranium nitride or carbonitride.

The problem is achieved in that in the method of manufacturing a gas-filled fuel rod, including the manufacture of a “pipe assembly” by hermetically connecting the shell to one of the end caps, forming a fuel core by laying fuel pellets in a “pipe assembly”, evacuating, filling its internal volume with helium and the final sealing of the fuel rod with the second end plug, according to the invention, before laying the fuel pipe assembly, the vacuum tube is pre-evacuated and filled with heavy inert gas, and the stacking of the tablets is carried out using a jet of said heavy inert gas directed under excessive pressure, after which the fuel rod is again evacuated before filling with helium.

When filling the “pipe assembly” with heavy inert gas and when stacking fuel pellets, it is placed vertically, with the open end up.

Filling the "pipe assembly" with a heavy inert gas is carried out using a jet of said heavy inert gas directed under excessive pressure.

A jet of inert gas under excessive pressure is created using a tubular tip in the form of a nozzle equipped with axial and radial holes, and argon is used as a heavy inert gas.

Uranium nitride or carbonitride is used as the fuel pellet material.

The tight connection of the shell with the end caps is made by fusion welding.

In this case, the end caps at the places of tight connection with the shell are provided with collars with a width of (1.2 ÷ 1.4) b and a height (0.8 ÷ 1.1) b of the bore diameter of the end cap, where b is the shell wall thickness, at this collars provide chamfers with a width of (0.6 ÷ 0.9) b, made from the side of the shell end at an angle to it (45 ÷ 60) degrees.

When the fuel rod is finally sealed, the length of the landing part of the second end plug is selected to be greater than the length of the landing part of the first end plug in (1.5 ÷ 2).

After laying the tablets, the fuel core is pressed with a helical spring.

Filling the “tube assembly” with a heavy inert gas, such as argon, allows air to be displaced from the internal volume of the shell, including water vapor contained in it, which subsequently negatively affects the material of the tablets, leading to swelling of the fuel, especially fuel based on uranium nitrides and carbonitrides. The best result is achieved when using an inert gas jet directed under excess pressure, which is created using a tubular tip. Stacking of fuel pellets can be done using the same tip, the end of which can be made in the form of nozzles with axial and radial holes, which allows, thanks to the formation of a gas cushion, to provide a non-contact effect on fuel pellets (both solid and with a central hole). At the same time, during the laying of fuel pellets, the inner surface of the fuel element is cleaned of dusty fuel particles, which further contributes to the formation of a better weld without extraneous inclusions. In addition, a jet of heavy inert gas directed under excessive pressure allows the air to be displaced from the dead cavities of the fuel rod, to improve the atmosphere composition under the cladding in order to maximally eliminate the effect of air and water vapor on the tablet material.

The sealing of a fuel rod can be performed by one type of welding, which simplifies its manufacture. The shape of the seam when performing fusion welding of the proposed bevels with a chamfer does not require additional machining. To ensure heat dissipation when performing a weld, which eliminates the heating of the tablet material and annealing of a fixing coil spring located in the recess of the plug, the length of the mounting portion of the second end plug is selected (1.5-2) times longer than the length of the mounting portion of the first end plug.

The essence of this technical solution is illustrated by figures of graphic images.

Figure 1 shows the connection diagram of the shell with the plug and the shape of the shoulder.

Figure 2 shows the equipment circuit "pipe assembly".

Figure 3 shows a diagram of a chamber for welding the second plug and the shape of the end of the tip.

Figure 4 shows the welding circuit of the second plug.

Figure 5 shows the appearance of the welds.

Figure 6 shows a section of a weld made in accordance with the present invention.

Figure 7 shows the image of the violation of the shape of the cladding of a fuel rod.

Fig. 8 shows an image of a tablet shaped from oxidation.

Figure 9 shows the image of the oxidized ends of the fuel pellets.

Figure 10 shows the image of the fuel pellets from a fuel rod made by the proposed method.

The manufacture of a fuel rod is as follows. First, prepare the shell 1 (figure 1) and the end elements 2 (figure 1) and 3 (figure 2) with beads 4 for fusion welding, for example butt-lock gas-arc welding. A chamfer 5 is made on the flange from the side of the shell end face at an angle of (45 ÷ 60) degrees by an amount (0.6 ÷ 0.9) of the shell wall thickness b, the height of the flanges is equal to (0.8 ÷ 1.1) b, and width - (1.2 ÷ 1.4) b. This shape of the bead, due to the presence of a chamfer and the aforementioned size ratio, provides a sufficient penetration depth of the weld, improves the degassing conditions in the joint zone, eliminates the exit of the diameter of the weld beyond the diameter of the shell. After manufacturing the pipe assembly 7 (FIG. 3) by welding the end cap 2 to the shell 1 (FIG. 1), it is placed vertically, with its open end up in the fuel equipment chamber 8 (FIG. 3). The camera is pre-vacuum to a level of 10 ^-1 mm RT.article, and then filled with argon. To achieve the best result, before placing the fuel pellets into the “tube assembly”, a tubular tip 9 is introduced through which argon is supplied with excessive pressure, which allows air to be displaced from the inner volume of the shell. The fuel pellets are laid by means of the same tip 9, the end part of which can be made in the form of nozzles with axial and radial openings (Fig. 3), which form a directed gas stream, which makes it possible to ensure a contactless effect on the fuel pellets (both solid and center hole) and the inner surface of the shell. In this case, due to the formation of a gas cushion during the laying of fuel pellets, the inner surface of the fuel element is cleaned from dusty fuel particles, which contributes to the formation of a better weld without foreign inclusions. At the end of the pill placement, a temporary sealing plug is installed. In this form, the fuel elements are placed horizontally in the chamber 9 (Fig. 4), where it is repeatedly evacuated with a temporary plug removed to a level of 10 ^-3 mm Hg, filled with helium to 1.1 atm, and a fixing spring 10 is installed (Fig. 2) ), the second end plug 3, and in the helium atmosphere, the second plug is welded using arc welding with a tungsten electrode 11, which is installed in the middle of the bead width. With the length of the seating portion of the first plug ℓ (FIG. 1), it is advisable to fit the seating portion of the second plug with a length of (1.5-2) ℓ (FIG. 2) to reduce the effect of heating during welding on the fixing spring 10 and fuel pellets 12. In FIG. .5 shows the appearance of the welds, and Fig.6 is a section of the weld. It can be seen from the figures that the seam is even, has no burr and does not protrude beyond the diameter of the shell. The microstructure of the weld is quite homogeneous, without cracks and pores, the microhardness of the weld and shell are close in value, which indicates the ductility of the weld. The shape and quality of the seam do not require additional machining.

An example implementation.

For a gas-filled fuel rod with a fuel core made of uranium-zirconium carbonitride, the shell and end caps were made of 06X18H10T stainless steel. The length of the shell was 500 mm, the diameter was 12 mm, and the wall thickness was 0.6 mm. End caps were made with beads with a height equal to the shell wall thickness of 0.6 mm. When the wall thickness of the shell (for example, 0.8 mm), the height of the shoulder for joints that do not extend beyond the diameter of the shell is made less than approximately 0.7 mm, i.e. within (0.6 ÷ 0.9) of the shell wall thickness. The width of the collar was selected within (1.2 ÷ 1.4) the thickness of the shell wall. With a shell wall thickness of 0.6 mm, the bead width is 0.8 mm. The bevels were chamfered at an angle of (45 ÷ 60) degrees from the side of the bore diameter of the plug by (0.6 ÷ 0.9) of the wall thickness of the shell (Fig. 1). When the angle of the bevel is 45 degrees, the depth of the bevel was chosen 0.6 of the width of the bead, i.e. approximately 0.5 mm. At 60 degrees - 0.8 of the width of the shoulder. With this design, the flange of the end caps provides a sufficient penetration depth, as well as the weld absenteeism for the diameter of the fuel rod cladding (Figs. 5, 6). The landing part of the end cap 3 (FIG. 2) was made 1.5 times longer than the length of the landing part of the end cap 2 (FIG. 1) to ensure heat removal during welding and to prevent annealing of the fixing spring 10 located in the recess of the cap 3 (FIG. 2) .2). The connection of the end cap 2 with the shell 1 (Fig. 1) was carried out by argon-arc welding (the apparatus “ORBIMAT 165 SA” was used). Welding occurred during orbital rotation with a non-consumable electrode in a local chamber with argon supply at a welding current of 40A, providing the necessary penetration depth of about 1.5 shell thickness (see Fig.6). Next, the “pipe assembly” 6 was placed vertically open end up in the chamber 7 (figure 3), which was previously evacuated and filled with argon to a pressure of (0.9 ÷ 1.1) atm. Cylindrical fuel pellets made of uranium-zirconium carbonitride were placed in a “tube assembly” 6 using a tubular tip 8 (FIG. 3), through which argon was supplied with an overpressure of (1.1–1.2) atm, which simplifies the stacking process the minimum gap between the inner diameter of the shell and the outer diameter of the fuel pellet, and in addition, at the same time, the internal volume of the fuel element is cleaned from dusty particles of fuel, which often occur when the fuel core is equipped. The use of argon makes it possible to displace air residues from the blind cavities of a fuel rod and thereby exclude its effect on uranium-zirconium carbonitride and, in addition, contactless stacking of fuel pellets is provided. The presence of air leads to the oxidation and swelling of the uranium-zirconium carbonitride and damage to the cladding of the fuel rod during operation of the fuel rod, which is illustrated by the images shown in Figs. 7, 8, 9.

The complete tube filled with fuel pellets was sealed with a temporary plug. The product was placed horizontally in another chamber 9 (Fig. 4), where there was an argon-arc welding head 13. The chamber and the product located in it were evacuated with a temporary plug removed to a pressure of (2 ÷ 3) × 10 ^-3 mm Hg. The chamber with the product was filled with helium to a pressure of (1.1 ÷ 1.2) atm. The second end cap 3 with a clamping coil spring 10 was sent to the shell 2 (figure 2), then welding was performed in helium medium with the electrode 13 in the middle of the flange of the end cap 3 (figure 2). Welding occurred when the fuel rod was rotated at 5 rpm. They made (1.5–2) turns at a welding current of 20A, then the welding current was reduced by (40–50)% and made another (1–1.5) turns, which made it possible to reduce post-welding stresses in the weld. During welding, the bead is completely melted and forms a weld. Further, the product was subjected to a leak test on a helium leak detector, ultrasonic and x-ray inspection of welds.

Figure 7 shows that the oxidation of tablets from uranium-zirconium carbonitride leads to disruption of the shape of the cladding of a fuel rod. Fig. 8 shows an image of a tablet shaped from oxidation. Figure 9 shows the oxidation of the ends of the fuel pellets due to the presence of a blind cavity between them. Figure 10 shows the image of fuel pellets, also extracted from a fuel rod manufactured by the proposed method, after a month of storage under the same conditions as the known fuel rods, from which it is clear that the above disadvantages are absent.

Thus, filling the internal volume of a fuel rod with a heavy inert gas and subsequent stacking of fuel pellets using a jet of heavy inert gas directed under excessive pressure allows dust particles to be removed in the gap between the pellets and the cladding and to provide a non-contact effect on the fuel pellets and to improve the atmosphere composition under shell. This in turn allows you to increase the resource and reliability of the fuel rods. In addition, when using the proposed technical solution, the fuel rod can be sealed with one type of welding, which simplifies its manufacture. Filling the internal volume of the pipe assembly with argon allows the remaining air to be displaced. This embodiment of gas-filled fuel elements is appropriate for experimental fuel elements with a low internal gas pressure in the range from 1 kg / cm ² to 2 kg / cm ² and a length of up to 1000 mm.

Claims (10)

1 A method of manufacturing a gas-filled fuel element, including the manufacture of a “pipe assembly” by hermetically connecting the shell to one of the end caps, forming a fuel core by stacking fuel pellets in a “pipe assembly”, evacuating, filling the internal volume with helium, and finally sealing the fuel rod with the second end cap, characterized in that before laying the fuel pellets, the “tube assembly” is pre-evacuated and filled with heavy inert gas, and the subsequent The tablets are laid by means of a jet of said heavy inert gas directed under excessive pressure. 2. The method according to claim 1, characterized in that when filling the "pipe assembly" with a heavy inert gas and laying the tablets, it is placed vertically, with its open end up. 3. The method according to claim 1, characterized in that the filling of the "pipe assembly" with a heavy inert gas is carried out using a jet of said heavy inert gas directed under excessive pressure. 4. The method according to claim 1, characterized in that a stream of inert gas directed under excessive pressure is created using a tubular tip in the form of a nozzle equipped with axial and radial openings. 5. The method according to any one of paragraphs. 1-4, characterized in that as a heavy inert gas using argon. 6. The method according to claim 1, characterized in that the material of the fuel pellets use uranium nitride or carbonitride. 7. The method according to claim 1, characterized in that the hermetic connection of the shell with the end caps is made by fusion welding. 8. The method according to claim 7, characterized in that the end caps at the places of tight connection with the shell are provided with collars with a width (1.2 ÷ 1.4) b and a height (0.8 ÷ 1.1) b from the bore diameter of the end cap , where b is the wall thickness of the shell, while the shoulders provide chamfers with a width of (0.6 ÷ 0.9) b, made from the side of the end of the shell at an angle to it (45 ÷ 60) degrees. 9. The method according to claim 7, characterized in that during the final sealing of the fuel rod, the length of the landing part of the second end plug is selected (1.5 ÷ 2) times longer than the length of the landing part of the first end plug. 10. The method according to claim 1, characterized in that after laying the tablets, the fuel core is pressed with a helical spring.

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