Lower locking device of reactor fuel assembly

Abstract

The invention discloses a lower locking device of a reactor fuel assembly. The lower locking device is characterized in that a lower reactor core plate is provided with a mounting hole and a locking groove; a lower pipe leg connected with the fuel assembly is inserted into the mounting hole and is provided with a through hole; the locking cam is rotationally arranged in the lower pipe leg and provided with a locking position and an unlocking position in the circumferential direction at intervals. The limiting structure is arranged at a preset position in the lower pipe leg to axially limit the locking cam; each locking assembly can radially slide to protrude out of or retract into the through hole; the guide rod is connected with the locking cam and extends upwards in the axial direction of the fuel assembly. The guide rod drives the locking cam to rotate to the locking position to abut against the locking assembly, the locking assembly partially extends out of the through hole and is connected with the locking groove in an inserted mode, and the locking state is achieved. The guide rod drives the locking cam to rotate to the unlocking position to abut against the locking assembly, the extending part of the locking assembly retracts into the through hole and is separated from the locking groove, and the unlocking state is achieved. According to the device, external force is manually applied to the guide rod to achieve locking and unlocking, and the structure is simple and reliable.

Description

Technical Field

[0001] The invention relates to the technical field of reactor fuel assemblies, and in particular to a lower locking device of a reactor fuel assembly. Background Art

[0002] The fuel assembly is located between the upper and lower core plates in the core. The fuel assembly is fixed by the upper and lower core plates when in service. The fuel assembly is unlocked from the core plates when the fuel assembly is unloaded or replaced. Therefore, the fuel assembly needs to have locking and unlocking functions with the lower end plate of the core.

[0003] Existing lead-based reactor fuel assembly buoyancy locking mechanism, such as Figure 1 As shown, the buoyancy of the floating slide is used to push the ball anchor outward into the ball hole in the core grid plate where the ball hole has been opened, and the fuel assembly is locked by the cooperation of the ball anchor and the ball hole. This locking method relies on the buoyancy of the floating slide to push the ball anchor outward. The designed floating slide is a round tube with limited volume. The density difference between stainless steel and lead-bismuth is small, so the buoyancy provided by the floating slide is limited, and the radial force component on the ball anchor is even more limited. Therefore, this locking method is unreliable.

[0004] The unlocking of this locking method relies on external force to push down the floating slide, providing space for the ball anchor to retract. However, the power for the retraction of the ball anchor comes from the relative movement of the fuel assembly and the core grid plate. When the depth of the ball hole in the core grid plate is close to the radius of the ball anchor, the ball anchor and the ball hole may be stuck, making it impossible to pull out the fuel assembly.

[0005] In addition, lead-based coolants are highly corrosive. Under high temperature conditions, the structural materials of the fuel assembly are strongly eroded and corroded by the lead-based coolant, causing the surface quality of the fuel assembly structural materials to deteriorate and increasing mutual friction. In this case, the friction between the ball anchor and the ball hole and the floating slide increases, making it more likely for the ball anchor to get stuck. Summary of the invention

[0006] The technical problem to be solved by the present invention is to provide a lower locking device for a reactor fuel assembly.

[0007] The technical solution adopted by the present invention to solve the technical problem is: a lower locking device for a reactor fuel assembly, comprising:

[0008] The lower core plate is provided with a longitudinal mounting hole and a plurality of transverse locking grooves, wherein the locking grooves are connected to the mounting hole;

[0009] A lower tube leg connected to the fuel assembly is inserted into the mounting hole and is provided with a plurality of through holes connected to the locking grooves;

[0010] A locking cam is rotatably arranged inside the lower tube leg and is provided with a locking position and an unlocking position at intervals along the circumferential direction;

[0011] A limiting structure, arranged at a preset position inside the lower tube leg to axially limit the locking cam;

[0012] A plurality of locking components, each of which can radially slide and protrude or be received in the through hole;

[0013] A guide rod connected to the locking cam and extending upward along the axial direction of the fuel assembly;

[0014] The lower locking device of the reactor fuel assembly includes a locking state and an unlocking state; the guide rod drives the locking cam to rotate to the locking position and abut against the locking assembly, and the locking assembly partially extends out of the through hole and is plugged into the locking groove, which is the locking state; the guide rod drives the locking cam to rotate to the unlocking position and abuts against the locking assembly, and the protruding part of the locking assembly is retracted into the through hole and disengaged from the locking groove, which is the unlocking state.

[0015] In some embodiments, the locking cam is a variable diameter structure, the diameter of the locking position is larger than the diameter of the unlocking position, and the diameter gradually decreases from the locking position to the unlocking position.

[0016] In some embodiments, the locking assembly includes an elastic member and a locking member, wherein the elastic member is sleeved around the locking member, and two ends of the elastic member are respectively abutted between the end of the locking member and the inner wall of the lower tube leg to push the locking member into the through hole.

[0017] In some embodiments, the locking member includes a locking section and an abutment section connected to each other, the diameter of the locking section is smaller than the diameter of the abutment section, the locking section is slidably inserted into the through hole, and the elastic member is sleeved on the locking section and abuts against the abutment section and the inner wall of the lower tube leg respectively.

[0018] In some embodiments, the outer surface of the abutment section facing the locking cam is a curved surface.

[0019] In some embodiments, the limiting structure includes an upper limit structure and a lower limit structure, the upper limit structure is arranged at the upper part of the locking cam and is fixedly connected to the lower tube leg, and the lower limit structure is arranged at the lower part of the locking cam and is detachably connected to the lower tube leg.

[0020] In some embodiments, the upper limit structure is a plurality of convex ribs disposed inside the lower tube leg, and the convex ribs are in a point-shaped, block-shaped or ring-shaped shape.

[0021] In some embodiments, the lower limit structure is a slat that is radially inserted into the lower tube leg and carries the locking cam.

[0022] In some embodiments, in the locked state, the locking cam is prevented from rotating by locking the outer component with the guide rod.

[0023] In some embodiments, the lower locking device further comprises a plurality of external limiting ribs, wherein the external limiting ribs are connected and fixed to the outside of the lower tube leg and abut against the upper portion of the lower core plate;

[0024] The external limiting convex rib is in a point shape, a block shape or a ring shape.

[0025] By implementing the present invention, the following beneficial effects are achieved:

[0026] The lower locking device of the reactor fuel assembly of the present invention drives the locking cam to rotate to the locking position and abut against the locking assembly through the guide rod, and the locking assembly partially extends out of the through hole and plugs into the locking groove to achieve the locking of the fuel assembly. The guide rod drives the locking cam to rotate to the unlocking position and abuts against the locking assembly, and the protruding part of the locking assembly retracts into the through hole and disengages from the locking groove to achieve the unlocking of the fuel assembly. The lower locking device of the present invention has a simple structure, and uses artificial external force to adjust the abutment position of the locking cam and the locking assembly, thereby locking the fuel assembly. When unlocking, a torque opposite to the locking is applied to complete the unlocking. Both locking and unlocking the fuel assembly are achieved by artificially applying external force, and the controllability is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0028] Figure 1 It is a structural schematic diagram of an existing buoyancy locking mechanism for a lead-based reactor fuel assembly;

[0029] Figure 2 It is a structural schematic diagram of a lower locking device of a reactor fuel assembly according to an embodiment of the present invention;

[0030] Figure 3 yes Figure 2 A cross-sectional view of the lower locking device in a locked state;

[0031] Figure 4 yes Figure 3 A schematic diagram of the relative positions of the middle and lower tube legs, the locking cam and the locking assembly in the locked state;

[0032] Figure 5 yes Figure 2 A cross-sectional view of the lower locking device in an unlocked state;

[0033] Figure 6 yes Figure 5 A schematic diagram of the relative positions of the middle and lower tube legs, the locking cam and the locking assembly in the unlocked state;

[0034] Figure 7 yes Figure 2 A schematic diagram of the structure of the locking cam and the guide rod;

[0035] Figure 8 yes Figure 7 A top view of the locking cam in FIG.

[0036] Fig. 9 yes Figure 2 A schematic diagram of the structure of the locking assembly in FIG.

[0037] Fig.10 yes Figure 2 A schematic diagram of the structure of the lower tube leg in FIG.

[0038] Fig.11 yes Figure 2 Schematic diagram of the structure of the lower core plate. DETAILED DESCRIPTION

[0039] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, specific embodiments of the present invention are now described in detail with reference to the accompanying drawings.

[0040] In the description of the invention, it should be understood that the terms "center", "longitudinal", "lateral", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the invention. In addition, the terms "first", "second", etc. are only used for descriptive purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, features defined as "first", "second", etc. may explicitly or implicitly include one or more of the features. In the description of the present invention, unless otherwise specified, "multiple" means two or more.

[0041] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or a chemical connection; it can be a direct connection or an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood by specific circumstances.

[0042] See also Figures 2 to 11 One embodiment of the present invention discloses a lower locking device for a reactor fuel assembly, which is particularly suitable for locking and unlocking the lower part of a fuel assembly in a lead-based reactor, and is not affected by the corrosiveness of a lead-based coolant. In addition, the device can also be applied to water reactors, sodium metal reactors, and molten salt reactors.

[0043] like Figure 2 As shown, the lower locking device includes a lower core plate 1, a lower tube leg 2 connected to the fuel assembly, a locking cam 3, a limiting structure 4, a plurality of locking assemblies 5 and a guide rod 6. Fig.11 As shown, the lower core plate 1 is provided with a longitudinal mounting hole 11 and a plurality of transverse locking grooves 12, and the locking grooves 12 are connected to the mounting hole 11. The lower tube leg 2 is inserted into the mounting hole 11, and is provided with a plurality of through holes 21 connected to the locking grooves 12, and each locking assembly 5 can be radially slidably protruded or received in the through hole 21. The locking cam 3 is rotatably arranged inside the lower tube leg 2, and a locking position 31 and an unlocking position 32 are arranged at intervals along the circumferential direction. The limiting structure 4 is arranged at a preset position inside the lower tube leg 2 to axially limit the locking cam 3. The guide rod 6 is connected to the locking cam 3 and extends upward along the axial direction of the fuel assembly. Among them, the number of locking assemblies 5 can be one, two, or more. In this embodiment, two locking assemblies 5 are taken as an example for explanation, and multiple locking assemblies 5 are implemented in the same way. Similarly, the setting of the locking position 31 and the unlocking position 32 corresponds to the locking assembly 5 one by one.

[0044] The lower locking device of the reactor fuel assembly includes a locked state and an unlocked state. Figure 3 and Figure 4 As shown, the guide rod 6 drives the locking cam 3 to rotate to the locking position 31 and abuts against the locking assembly 5. The locking assembly 5 partially extends out of the through hole 21 and is plugged into the locking groove 12, which is in a locked state. Figure 5 and Figure 6 As shown, the guide rod 6 drives the locking cam 3 to rotate to the unlocking position 32 and abuts against the locking assembly 5, and the protruding portion of the locking assembly 5 retracts into the through hole 21 and disengages from the locking groove 12, which is an unlocked state.

[0045] By manually applying external force to the guide rod 6 to manipulate the movement of the guide rod 6, the movement of the guide rod 6 drives the locking cam 3 to rotate. In the locked state, the outer component can be locked with the guide rod 6 to prevent the locking cam 3 from rotating. When unlocking is required, first unlock the outer component and the guide rod 6, and then operate the guide rod 6 to unlock the fuel assembly. Among them, the guide rod 6 can be directly rigidly connected and fixed with the locking cam 3, and the rotation of the guide rod 6 synchronously drives the locking cam 3 to rotate. The axial movement of the guide rod 6 can also be converted into the rotational movement of the locking cam 3 through a switching component. This solution is relatively complicated and is not the focus of the present invention, so it will not be described here.

[0046] The lower locking device of the present invention has a simple structure and does not rely on the buoyancy of the parts of the fuel assembly. The fuel assembly is locked by applying an external force manually, and a torque opposite to the locking is applied during unlocking to complete the unlocking.

[0047] In some embodiments, Figure 7 and Figure 8 As shown, the locking cam 3 is a variable diameter structure, the diameter of the locking position 31 is larger than the diameter of the unlocking position 32, and the diameter gradually decreases from the locking position 31 to the unlocking position 32. The locking cam 3 is a variable diameter structure, and the outer peripheral wall surface of the locking cam 3 is an arc surface with different diameters, which can reduce the friction between the locking cam 3 and the locking assembly 5, facilitate the operation of the guide rod 6, and reduce the wear of the locking cam 3 and the locking assembly 5, protect the integrity of the parts, and reduce the risk of failure. Among them, Figure 8 The specific profile of the locking cam 3 is shown, the diameter of the locking position 31 refers to the outer diameter of the locking cam 3 at the locking position 31, and the diameter of the unlocking position 32 refers to the outer diameter of the locking cam 3 at the unlocking position 32. The maximum outer diameter of the locking cam 3 is the same as the inner diameter of the lower leg 2 to achieve radial limitation and prevent the locking cam 3 from moving radially.

[0048] Specifically, the outer wall surface of the locking cam 3 at the locking position 31 is closer to the inner wall of the lower leg 2 than the outer wall surface of the locking cam 3 at the unlocking position 32. Generally, the distance between the locking position 31 and the inner wall of the locking groove 12 of the lower core plate 1 is greater than or equal to the length of the locking assembly 5, so as to ensure that when the locking position 31 abuts against the locking assembly 5, the protruding portion of the locking assembly 5 can be plugged into the locking groove 12, thereby achieving the lower locking of the fuel assembly. The distance between the unlocking position 32 and the outer wall of the lower leg 2 is the same as or similar to the length of the locking assembly 5, so as to ensure that when the unlocking position 32 abuts against the locking assembly 5, the locking assembly 5 can just retract the protruding portion into the through hole 21 and disengage from the locking groove 12, thereby achieving the lower unlocking of the fuel assembly.

[0049] In some embodiments, Fig. 9 As shown, the locking assembly 5 includes an elastic member 51 and a locking member 52. The elastic member 51 is sleeved on the periphery of the locking member 52, and the two ends of the elastic member 51 are respectively abutted between the end of the locking member 52 and the inner wall of the lower tube leg 2 to push the locking member 52 into the through hole 21. The locking member 52 is used to be inserted into the locking groove 12 to achieve the locking function and retract into the through hole 21 to achieve the unlocking function. The elastic member 51 provides elastic force in the locked state and the unlocked state and in the conversion process between the two states. The elastic member 51 can be a spring. The locking member 52 can be a pin.

[0050] Specifically, when the locking member 52 abuts against the locking cam 3 at the locking position 31, the elastic member 51 remains in a compressed state. During the rotation of the locking cam 3, from the locking member 52 abutting against the locking position 31 to the abutting against the unlocking position 32, due to the elastic force, the locking member 52 always abuts against the locking cam 3, and the protruding portion of the locking member 52 is gradually retracted into the through hole 21, until the locking member 52 abuts against the locking cam 3 at the unlocking position 32, and the elastic member 51 can remain in a normal state or a compressed state.

[0051] In some embodiments, the locking member 52 includes a locking section 521 and an abutting section 522 connected to each other, the diameter of the locking section 521 is smaller than the diameter of the abutting section 522, the locking section 521 is slidably plugged with the through hole 21, and the elastic member 51 is sleeved on the locking section 521 and abuts between the abutting section 522 and the inner wall of the lower tube leg 2. The locking section 521 is used to be plugged into the locking groove 12 to achieve the locking function and retract into the through hole 21 to achieve the unlocking function. The abutting section 522 is used to receive the elastic force from the elastic member 51 and the thrust from the locking cam 3, and transmit them to the locking section 521 to achieve the corresponding movement. Preferably, the outer surface of the abutting section 522 facing the locking cam 3 is an arc surface. Similar to the arc surface of the outer peripheral wall of the locking cam 3, the friction between the abutment section 522 and the locking cam 3 can be reduced, which facilitates the operation of the guide rod 6 and reduces the wear of the abutment section 522 and the locking cam 3, thereby protecting the integrity of the parts and reducing the risk of failure.

[0052] In some embodiments, Fig.10 As shown, the limiting structure 4 includes an upper limit structure 41 and a lower limit structure 42. The upper limit structure 41 is arranged on the upper part of the locking cam 3 and is connected and fixed to the lower leg 2. The lower limit structure 42 is arranged on the lower part of the locking cam 3 and is detachably connected to the lower leg 2. The spacing between the upper limit structure 41 and the lower limit structure 42 is slightly greater than or equal to the thickness of the locking cam 3. Among them, the upper limit structure 41 is used to limit the axial upward movement of the locking cam 3, and is a plurality of convex ribs arranged inside the lower leg 2. The convex ribs are point-shaped, block-shaped or ring-shaped. The lower limit structure 42 is used to limit the axial downward movement of the locking cam 3. It is a slat that is radially plugged into the lower leg 2 and carries the locking cam 3. Preferably, at least two plug-in holes 22 are provided on the lower leg 2 for detachable connection of the lower limit structure 42.

[0053] In some embodiments, the lower locking device further includes a plurality of external limiting ribs 43, which are connected and fixed to the outside of the lower tube leg 2 and abut against the upper part of the lower core plate 1. The external limiting ribs 43 are point-shaped, block-shaped or ring-shaped. The external limiting ribs 43 are used to limit the downward movement of the lower tube leg 2 and ensure accurate installation and positioning.

[0054] When the lower locking device is initially assembled, after the locking cam 3 is assembled with the lower tube leg 2, the lower limit structure 42 is assembled to position the locking cam 3. After the locking member 52 is assembled with the elastic member 51, it is assembled with the locking cam 3 inside the lower tube leg 2. The locking member 52 is retracted inward by the elastic force of the elastic member 51, and the lower locking device is in an unlocked state, that is, the fuel assembly is in an unlocked state.

[0055] The method of using the lower locking device of the present invention is as follows:

[0056] Locking method: After the fuel assembly is assembled to the lower core plate 1, the lower tube leg 2 is inserted into the mounting hole 11 of the lower core plate 1, and the external limiting rib 43 abuts against the upper part of the lower core plate 1 to achieve installation positioning. Apply external force to the guide rod 6 to rotate the locking cam 3 counterclockwise, and the locking member 52 is pushed outward to overcome the elastic force of the elastic member 51 and move outward. The locking member 52 cooperates with the lower core plate 1 and is inserted into the locking groove 12 to complete the locking of the fuel assembly.

[0057] Unlocking method: When the fuel assembly needs to be unlocked from the lower core plate, an external force is applied to the guide rod 6 to rotate the locking cam 3 clockwise, and the locking member 52 returns under the elastic force of the elastic member 51, and the protruding part of the locking member 52 is finally completely retracted into the through hole 21, that is, recovered into the lower tube leg 2, and the fuel assembly is unlocked. The fuel assembly can be pulled out from the lower core plate 1.

[0058] By implementing the present invention, the following beneficial effects are achieved:

[0059] The lower locking device of the reactor fuel assembly of the present invention drives the locking cam 3 to rotate to the locking position 31 and abut against the locking assembly 5 through the guide rod 6. The locking assembly 5 partially extends out of the through hole 21 and plugs into the locking groove 12 to achieve the locking of the fuel assembly. The guide rod 6 drives the locking cam 3 to rotate to the unlocking position 32 and abuts against the locking assembly 5. The protruding part of the locking assembly 5 retracts into the through hole 21 and disengages from the locking groove 12 to achieve the unlocking of the fuel assembly. The lower locking device of the present invention has a simple structure and uses artificial external force to adjust the abutment position of the locking cam 3 and the locking assembly 5, thereby locking the fuel assembly. When unlocking, a torque opposite to the locking is applied to complete the unlocking. Both locking and unlocking the fuel assembly are achieved by artificially applying external force, and the controllability is high.

[0060] It can be understood that the above embodiments only express the preferred implementation modes of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the patent scope of the present invention. It should be pointed out that, for ordinary technicians in this field, without departing from the concept of the present invention, the above embodiments or technical features can be freely combined, and several deformations and improvements can be made, which all belong to the protection scope of the present invention, that is, the embodiments described in "some embodiments" can be freely combined with any of the above and below embodiments. Therefore, all equivalent changes and modifications made to the scope of the claims of the present invention should fall within the scope of the claims of the present invention.

Claims

1. A lower locking device for a reactor fuel assembly, characterized in that: include: The lower core plate (1) is provided with a longitudinal mounting hole (11) and a plurality of transverse locking grooves (12), wherein the locking grooves (12) are connected to the mounting hole (11); A lower tube leg (2) connected to the fuel assembly is inserted into the mounting hole (11) and is provided with a plurality of through holes (21) connected to the locking grooves (12); A locking cam (3) is rotatably disposed inside the lower tube leg (2) and is provided with a locking position (31) and an unlocking position (32) at intervals along the circumferential direction; A limiting structure (4) is arranged at a preset position inside the lower tube leg (2) to axially limit the locking cam (3); A plurality of locking components (5), each of the locking components (5) being capable of radially sliding and protruding from or being received in the through hole (21); A guide rod (6) connected to the locking cam (3) and extending upward along the axial direction of the fuel assembly; The lower locking device of the reactor fuel assembly includes a locking state and an unlocking state; the guide rod (6) drives the locking cam (3) to rotate to the locking position (31) and abuts against the locking component (5), and the locking component (5) partially extends out of the through hole (21) and is plugged into the locking groove (12), which is the locking state; the guide rod (6) drives the locking cam (3) to rotate to the unlocking position (32) and abuts against the locking component (5), and the protruding part of the locking component (5) is retracted into the through hole (21) and disengaged from the locking groove (12), which is the unlocking state.

2. The lower locking device of the reactor fuel assembly according to claim 1, characterized in that: The locking cam (3) is a variable diameter structure; the diameter of the locking position (31) is greater than the diameter of the unlocking position (32), and the diameter gradually decreases from the locking position (31) to the unlocking position (32).

3. The lower locking device of the reactor fuel assembly according to claim 1, characterized in that: The locking assembly (5) comprises an elastic member (51) and a locking member (52); the elastic member (51) is sleeved around the locking member (52), and two ends of the elastic member (51) are respectively abutted between the end of the locking member (52) and the inner wall of the lower tube leg (2) to push the locking member (52) into the through hole (21).

4. The lower locking device of the reactor fuel assembly according to claim 3, characterized in that: The locking member (52) comprises a locking section (521) and an abutting section (522) connected to each other, the diameter of the locking section (521) is smaller than the diameter of the abutting section (522), the locking section (521) is slidably plugged into the through hole (21), and the elastic member (51) is sleeved on the locking section (521) and abuts against the abutting section (522) and the inner wall of the lower tube leg (2), respectively.

5. The lower locking device of the reactor fuel assembly according to claim 4, characterized in that: The outer surface of the abutment section (522) facing the locking cam (3) is a curved surface.

6. The lower locking device of the reactor fuel assembly according to claim 1, characterized in that: The limiting structure (4) comprises an upper limit structure (41) and a lower limit structure (42); the upper limit structure (41) is arranged on the upper part of the locking cam (3) and is connected and fixed to the lower tube leg (2); the lower limit structure (42) is arranged on the lower part of the locking cam (3) and is detachably connected to the lower tube leg (2).

7. The lower locking device of the reactor fuel assembly according to claim 6, characterized in that: The upper limit structure (41) is a plurality of convex ribs arranged inside the lower tube leg (2), and the convex ribs are in the shape of dots, blocks or rings.

8. The lower locking device of the reactor fuel assembly according to claim 6, characterized in that: The lower limit structure (42) is a strip which is radially inserted into the lower tube leg (2) and carries the locking cam (3).

9. The lower locking device of a reactor fuel assembly according to any one of claims 1 to 8, characterized in that: In the locked state, the locking cam (3) is prevented from rotating by locking the outer component with the guide rod (6).

10. The lower locking device of a reactor fuel assembly according to any one of claims 1 to 8, characterized in that: The lower locking device further comprises a plurality of external limiting convex ribs (43), wherein the external limiting convex ribs (43) are connected and fixed to the outside of the lower tube leg (2) and abut against the upper part of the lower core plate (1); The external limiting convex rib (43) is in the shape of a point, a block or a ring.

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