Fastening components for nuclear reactor fuel assemblies

Abstract

The attachment unit of a nuclear reactor fuel assembly comprises a fuel assembly tail piece (1), which is made into a top form and fixed in a bushing (7) of a mounting hole (5) of a support plate (2) under the core of a nuclear reactor. The tail piece (1) comprises an outer sleeve (3) with a circular end (4), the sleeve (7) is a split sleeve, and the contact surface between the lower end (13) of the sleeve (7) and the outer sleeve (4) of the tail piece (1) is made into a chamfered shape, forming an angle of 32 to 40 degrees relative to the plane of the support plate (2). The nuclear reactor fuel assembly attachment unit ensures that the fuel assembly is reliably fixed in the support plate (2) under the condition of long-term exposure to liquid metal coolant.

Description

Technical Field

[0001] The present invention relates to a nuclear power plant, which mainly uses liquid metal coolant as a main circuit coolant. Background Art

[0002] A fuel assembly for a pressure tube nuclear reactor is known (see application RU2006137687 published on May 10, 2008, applicant: OJSC Machine-Building Plant, IPC classification: G21C3 / 00), comprising a tail piece, the body of which is made in the form of a shell with a cylindrical portion, the outer surface of which comprises an elastic ring, and a conical lead-in portion, in which holes for coolant channels are made. The elastic ring is made in the form of a liner, the middle portion of which is barrel-shaped, wherein the liner has recesses oriented along the liner grid.

[0003] Due to the small contact area of ​​the barrel liner, reliable fixation of the fuel assembly cannot be guaranteed.

[0004] A nuclear reactor fuel assembly is known (see patent RU2129738 published on April 27, 1999, IPC classification: G21C 3 / 30, G21C 3 / 32), which comprises a fuel element connected to a lower support grid by means of an attachment unit. The attachment unit is made in the form of a cylindrical clamp. The clamp is mounted in a mounting hole of the support grid with a radial gap. The lower part of the clamp comprises a collar, which contacts a thrust surface of an annular groove coaxial with the mounting hole below the upper end of the support grid.

[0005] A disadvantage of the known attachment units in the lower support grid of the fuel assembly is the presence of radial play between cylinders and mounting holes, which leads to increased vibration wear of unit components.

[0006] A fuel assembly for a nuclear reactor with a liquid metal coolant is known (see patent No. RU2594357 published on August 20, 2016, IPC classification: G21C 3 / 32, G21C 3 / 12), comprising a fuel element mounted in a frame, the frame comprising a guide tube, a separation grid and a tail piece. The fuel assembly attachment unit comprises a chuck lock and a locking control mechanism mounted inside the tail piece. The control mechanism comprises a head and a spacer bushing, which are kinematically connected to a rod. The head is made in the form of a bushing with an internal groove. The spacer bushing is located inside the chuck lock so that the spacer bushing can interact with the chuck lock.

[0007] The fuel assembly attachment unit ensures an increase in the reliability of the fixing of the fuel assembly in the supporting structure, but this is achieved by a significant complication of its design.

[0008] A fuel assembly for a nuclear reactor is known (see patent US4344915 published on August 17, 1982, IPC classification: G21C 3 / 32), which comprises fuel elements connected to a lower support grid using attachment units, each attachment unit being made in the form of a cylinder whose cross-section is elastic over at least part of its length starting from the lower end.

[0009] The attachment units of known fuel assemblies do not ensure sufficiently reliable fastening under long-term operating conditions.

[0010] A fuel assembly for a nuclear reactor with a liquid metal coolant is known (see patent RU2594357 published on August 20, 2016, IPC classification: G21C 3 / 32, G21C 3 / 12), comprising a fuel element mounted in a frame, the frame comprising a guide tube, a separation grid and a tail piece. The fuel assembly attachment unit comprises a chuck lock and a locking control mechanism mounted inside the tail piece. The control mechanism comprises a head and a spacer bushing, which are kinematically connected to a rod. The head is made in the form of a bushing with an internal groove. The spacer bushing is located inside the chuck lock so that the spacer bushing can interact with the chuck lock.

[0011] The fuel assembly attachment unit ensures an increase in the reliability of the fixing of the fuel assembly in the supporting structure, but this is achieved by a significant complication of its design.

[0012] A nuclear reactor fuel assembly attachment unit is known (see application JP2019056615 published on April 11, 2019, IPC classification: G21C 5 / 00, G21C 1 / 02, G21C 3 / 33), in which the fuel assembly is fixed in a support structure by a spring-loaded pusher, which includes a release mechanism and is mounted in a groove on the side surface of the support structure.

[0013] A disadvantage of known attachment units for nuclear reactor fuel assemblies is that the fixing of the fuel assembly to the support structure is complex and unreliable.

[0014] An attachment unit for a nuclear reactor fuel assembly using liquid metal coolant is known (see application US20200027575 published on January 23, 2020, IPC classification: G21C 1 / 02, G21C7 / 08, G21C 9 / 027, G21C 19 / 20, G21C 5 / 06), which is made in the form of a wedge mechanism, connected to a drive with a transmission device to fix the fuel assembly in the reactor vessel.

[0015] A disadvantage of a nuclear reactor fuel assembly attachment unit is its complex design.

[0016] A nuclear reactor fuel assembly attachment unit is known (see application number WO2006119140 published on November 9, 2006, IPC classification: G21C 15 / 00), which is most similar to the basic features of the present technical solution and is used as a prototype. The attachment unit of the fuel assembly for the prototype nuclear reactor includes a fuel assembly tail piece, which is in the form of a top end, has two petals, and is fixed in a bushing of a mounting hole of a support plate under the core, wherein the diameter of the bushing of the mounting hole is smaller than the diameter of the tail piece. The tail piece petals can be compressed in the radial direction to provide friction when fixed in the bushing of the mounting hole of the support plate under the core.

[0017] The known nuclear reactor fuel assembly attachment unit cannot ensure sufficient reliability in fixing the tail piece in the mounting hole of the lower support grid during long-term operation of the fuel assembly because it is fixed in the mounting hole only by the friction force of the elastic petal of the clamp tail piece against the mounting hole wall. Summary of the invention

[0018] The problem solved by the technical solution is to develop a nuclear reactor fuel assembly attachment unit, which can ensure that the fuel assembly is reliably fastened in the support plate under the condition of long-term exposure to liquid metal coolant.

[0019] The problem is solved in such a way that a nuclear reactor fuel assembly attachment unit comprises a tail piece of a fuel assembly in the form of a top end, which is fixed in a first bushing of a mounting hole of a support plate under a core, wherein the inner diameter of the first bushing is smaller than the diameter of the tail piece. The novelty of the attachment unit is that the tail piece comprises an outer sleeve with rounded ends, the first sleeve is split, and the contact surface between the lower end of the first sleeve and the outer sleeve of the tail piece of the fuel assembly is formed to form a chamfer of 32 to 40 degrees relative to the plane of the support plate.

[0020] The first split bushing may include an annular groove, an upper wall of which forms an angle of 32 to 40 degrees relative to a plane of the support plate.

[0021] The upper end of the first split bushing may include an outwardly facing collar.

[0022] A gasket is installed above the first split bushing.

[0023] A second split bushing coaxial with the first bushing may be installed on the outer side of the first bushing.

[0024] A third split bushing coaxial with the first bushing may be installed on the outer side of the second bushing.

[0025] The upper end of the third split bushing may include an inwardly facing collar.

[0026] The height of the collar of the third split bushing may be made smaller than the height of the collar of the first split bushing.

[0027] A lower end of the second split bushing may protrude beyond lower ends of the first split bushing and the third split bushing.

[0028] The locking bushing can be inserted into the groove of the mounting hole from below and connected to the lower core support plate by welding.

[0029] The upper end surface of the locking bushing may have an annular protrusion, the diameter of which is smaller than the inner diameter of the second split bushing.

[0030] The nuclear reactor fuel assembly attachment unit ensures that the fuel assembly is fixed in the working position in the lower support plate of the core and fixed in the middle position of the additional annular groove to prevent the fuel assembly from being separated from the normal position without permission. When the coolant flows through the core, the force required to extract it exceeds the force generated by the Archimedean force and the fluid dynamics acting on the fuel assembly, while not causing its collapse or damage. At the same time, there are no obstacles in the internal volume of the tail piece of the fuel assembly, so that the jacket of the control and protection system (CPS), the hydraulic resistance device or other necessary elements fixed to the lower support plate can be installed along the center line of the fuel assembly. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The design of the fuel assembly attachment unit is illustrated by the accompanying drawings, in which:

[0032] Figure 1 A portion of a fuel assembly attachment unit in a lower support plate is shown;

[0033] Figure 2 Shows Figure 1 AA section view of the attachment unit shown;

[0034] Figure 3 Shows Figure 1 An enlarged cross-sectional view of area B of the attachment unit is shown. DETAILED DESCRIPTION

[0035] Nuclear reactor fuel assembly attachment unit (see Figure 1) comprises a tail piece 1 of a fuel assembly (FA), which is made in the form of a cylindrical top end and fixed in a support plate 2 of a nuclear reactor core. The tail piece 1 comprises an externally shaped collar 3 with a circular end 4, which ensures minimum contact resistance when the fuel assembly tail piece 1 is inserted into a mounting hole 5 of the support plate 2. The mounting hole 5 comprises a cylindrical groove 6, wherein a first shape bushing 7, a second bushing 8 and a third bushing 9 are coaxially mounted in the cylindrical groove 6. In the bushings 7, 8 and 9, longitudinal cuts 10 are formed to ensure their compression and expansion. By using a pin 11 (see Figure 3 ) to prevent the angular displacement of the bushing 7, the bushing 8 and the bushing 9, the pin 11 enters the cutout 10 and is properly installed in the locking bushing 12 through the lower end, and the upper end surface of the locking bushing 12 has an annular protrusion with a diameter smaller than the inner diameter of the second split bushing 8, which is used to accommodate the ends of the bushing 8 and the bushing 9. After assembling the entire attachment unit, the fixing bushing 12 is welded to the support plate 2, which fixes the bushing 7, the bushing 8 and the bushing 9 to prevent their vertical and radial movements. The contact surface between the lower end 13 of the bushing 7 and the external forming collar 3 (see Figure 2) is made in the form of a chamfer, forming an angle α of 32 to 40 degrees relative to the plane of the support plate 2, providing the required clamping force and extraction force for the fuel assembly. The upper end of the liner 7 includes an outward-facing collar 14, and the upper end of the liner 9 includes an inward-facing collar 15. The collar 14 and the collar 15 ensure the installation and fixation of the liner 7 and the liner 9 on the liner 8, simplifying the pre-assembly and subsequent installation of a set of liners 7, 8 and 9. In this case, the height of the liner 7 and the liner 9 is less than the height of the liner 8, so that the lower end of the liner 8 protrudes beyond the lower ends of the split liner 7 and the split liner 9, and the collar 14 on the liner 7 is higher than the collar 15 on the liner 9. Therefore, after installing a set of bushings 7, 8 and 9, their upper ends are in contact with the thrust washer 16 only through the end face of the collar 14 of the bushing 7, and are in contact with the bushing 12 only through the lower end face of the bushing 8. The bushing 9 has no contact on its end face. This set design of the bushings 7, 8 and 9 ensures that the end faces of the bushings 7, 8 and 9 will not be stuck or clamped during the radial movement caused by the tail piece 1 during installation and removal. The thrust washer 16 is located in the groove 6 of the support plate 2, between the support plate and the set of bushings 7, 8 and 9, relieves the increased local load on the upper end face of the groove 6, and disperses the force of the impact from the upper end face of the collar 14 of the bushing 7 to a large surface, thereby protecting the support plate 2 from potential corrosion and mechanical damage. A set of bushings 7, 8 and 9 with longitudinal grooves 10 acts as a spring element, which expands in the radial direction to ensure the relative freedom of downward movement of the tail piece 1. After the tail piece 1 is placed in the lower position, the combination of the bushings 7, 8 and 9 returns to the initial state due to the spring characteristics of the material, ensuring that the tail piece 1 is engaged with the bushing 7 along the chamfer at an angle α.

[0036] The range of values ​​of the angle α is selected because when the angle α is less than 32 degrees, the tail piece 1 of the fuel assembly may collapse under the influence of forces generated during the operation, installation and extraction of the fuel assembly.

[0037] When the angle α is greater than 40 degrees, reliable retention of the fuel assembly under operating load cannot be guaranteed.

[0038] The first split bushing 7 may include an annular groove 17 (see Figure 2 ), whose upper wall 18 forms an angle of 32 to 40 degrees with respect to the plane of the support plate 2, and is designed to fix the tail piece 1 in an intermediate position in the event that the fuel assembly is unauthorizedly separated from its normal position.

[0039] The number and total thickness of the group of the bushings 7, 8 and 9 are selected using known calculation methods. In this case, the increase in the total thickness of the group of the bushings 7, 8 and 9 proportionally increases the compressive force and the corresponding force required to extract the fuel assembly. When the bushings 7, 8 and 9 are radially stretched during the insertion of the tail piece 1 of the fuel assembly into the lower operating position, the increase or decrease in the number of bushings 7, 8 and 9 in the group will conversely reduce or increase the maximum tensile (compressive) stress on the inner (outer) surface of the bushings 7, 8 and 9. The stable characteristics of the coefficient of friction between the steel surfaces depend on the required coolant quality, that is, by keeping the controlled impurity level within the required range. In order to eliminate the influence of local deviations in the coolant quality in the contact area of ​​the tail piece 1 of the fuel assembly with the elements of the attachment unit of the fuel assembly in the core support plate 2, a hole 19 is made in the tail piece 1 of the fuel assembly, which provides a bypass flow of coolant between the elements of the group of bushings 7, 8 and 9. The coolant flow for cooling the core is divided into two main flows. The main coolant flow is directed to the core cooling through the channel 20, and the rest enters the cooling channel of the CPS rod, which ends at the fuel assembly tail piece 1. At the inlet of the fuel assembly tail piece 1, part of the coolant is directed to the bypass channel mentioned above and returns to the CPS cooling channel through the hole 19.

[0040] In one example, in order to confirm the operability of the design and determine the optimal joint angle, two tail piece and fuel assembly attachment unit models with joint angles of 30 degrees and 35 degrees were made and tested. The model was tested by repeatedly inserting and withdrawing the tail piece from the operating position. During the plugging and unplugging process, the plugging and unplugging forces at different movement speeds, the force to hold the tail piece in the operating position, and the surface condition of the tail piece and the mounting hole after the test were checked. Based on the research results, the results were statistically processed to obtain the dependence of the plugging and unplugging force on the joint angle, and to estimate the dependence of the stress in the design on the joint angle. After testing and analyzing the results, it was found that the performance was best when the joint angle was in the range of 32 degrees to 40 degrees.

[0041] At an engagement angle greater than 40 degrees, the tail piece cannot be securely fixed. When the engagement angle is less than 32 degrees, the extraction force is too high, posing a risk to the structural integrity of the tail piece.

[0042] The fuel assembly attachment unit model test shows that the chamfer at the lower end of the first liner and the contact surface of the outer ring of the fuel assembly tail piece form an angle of 32 to 40 degrees relative to the support plate, ensuring the reliable fixation of the fuel assembly and the safe withdrawal from the support plate.

Claims

1. An attachment unit for a nuclear reactor fuel assembly, the attachment unit comprising: a tail piece of the fuel assembly, which is made into a top form, fixed in a first split liner, and located in a groove of a mounting hole of a lower support plate of the core, wherein: The inner diameter of the first split bushing is smaller than the diameter of the tail piece, and is characterized in that: the tail piece includes an outer ring with a circular end, the first split bushing is in a split form, the contact surface between the lower end of the first split bushing and the outer ring of the tail piece of the fuel assembly is formed to form a chamfer of 32 to 40 degrees relative to the plane of the support plate, and a second split bushing coaxial with the first split bushing is installed on the outer side of the first split bushing.

2. The attachment unit according to claim 1, characterized in that: The first split bushing includes an annular groove, and an upper wall of the annular groove forms an angle of 32 degrees to 40 degrees relative to the plane of the support plate.

3. The attachment unit according to claim 1, characterized in that :The upper end of the first split bushing includes an outwardly facing ring.

4. The attachment unit according to claim 1, characterized in that : A thrust washer is installed above the first split bushing.

5. The attachment unit according to claim 1, characterized in that : A third split bushing coaxial with the first split bushing is installed on the outer side of the second split bushing.

6. The attachment unit according to claim 5, characterized in that :The upper end of the third split bushing includes an inwardly facing ring.

7. The attachment unit according to claim 6, characterized in that :The height of the collar of the third split bushing is less than the height of the collar of the first split bushing.

8. The attachment unit according to claim 6, characterized in that :The lower end of the second split bushing protrudes from the lower ends of the first split bushing and the third split bushing.

9. The attachment unit according to claim 1, characterized in that : Also includes a fixing bushing, which is inserted into the groove of the mounting hole from below and connected to the lower core support plate by welding.

10. The attachment unit according to claim 9, characterized in that :The upper end surface of the fixed bushing includes an annular protrusion, and the diameter of the annular protrusion is smaller than the inner diameter of the second split bushing.

11. The attachment unit according to claim 1, characterized in that A plurality of holes are provided in the tail piece for allowing coolant to flow into the inner space of the tail piece.

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