An experimental detection device for high temperature reactor control rod power structure

Abstract

The application relates to an experimental detection device for a high-temperature reactor control rod power structure applied to the field of nuclear reactors, which comprises a container shell for detection through a base, a top disc, a main surrounding ring and a secondary surrounding ring, simulates an actual reactor structure through a reactor core, heats a cooling fluid through a heating rod, simulates the high-temperature condition of the coolant in the actual situation, and adopts multiple transmission pipes as input and output ports of the cooling fluid to simulate the transverse vibration condition of the control rod caused by the coolant flow in the actual situation, gives the power structure a simulation environment close to the real situation, improves the accuracy of the detection result, effectively judges the performance of the power structure, and sets multiple types of secondary surrounding rings according to the number of the transmission pipes, detects the use performance of the power structure under different states and different degrees of flow conditions of the cooling fluid by replacing different types of secondary surrounding rings and adjusting the positions of the main surrounding ring and the secondary surrounding ring, and improves the detection comprehensiveness.

Description

TECHNICAL FIELD

[0001] The application relates to an experimental detection device, in particular to an experimental detection device for a high-temperature reactor control rod power structure. BACKGROUND

[0002] The control rod is made of boron and cadmium and other materials that can easily absorb neutrons. There is a set of power mechanical devices outside the nuclear reactor pressure vessel to control the control rod. When the control rod is fully inserted into the reactor, it can absorb a large number of neutrons to prevent the fission chain reaction from proceeding. When the control rod is pulled out to a certain distance, the reactor starts to operate, and the speed of the chain reaction reaches a certain stable value. The greater the degree of pulling out of the control rod, the fewer the neutrons absorbed by it, and the more neutrons participate in the fission reaction, and the speed of the chain reaction increases.

[0003] The control rod power structure is a key driving device for driving the control rod to insert into the reactor. When the reactor is normally operated, the control rod power structure drags the control rod to move in the reactor at a certain speed to compensate and adjust the backup reactivity of the reactor core, so as to realize the normal start, operation and shutdown of the reactor. Therefore, the performance of the control rod power structure needs to be effectively evaluated during the process of leaving the factory and maintenance.

[0004] For example, Chinese patent CN114373558B discloses a high-temperature reactor control rod driving mechanism test bench, which sets up a test bench with sufficient height to realize effective test of the control rod driving mechanism. By setting an effective damping base on the test bench, the impact force generated by the control rod drop test can be effectively buffered to avoid the impact on the driving mechanism. For example, Chinese patent CN102682859B discloses a control rod driving mechanism power supply switching module test bench, which realizes full-automatic measurement of the switching control module, and the measurement process does not need human participation and can complete data analysis to give measurement results. It also provides a single measurement function of module parameters.

[0005] However, the control rod power structure faces high-temperature environment of the reactor and transverse vibration of the control rod caused by coolant flow in actual use. The above conditions will affect the use performance and positioning accuracy of the control rod power structure. The existing experimental detection means for the control rod power structure do not consider the above actual working conditions, resulting in the defect of insufficient accuracy of the detection of the power structure. SUMMARY

[0006] In view of the above prior art, the technical problem to be solved by the present application is that the existing experimental detection means for the control rod power structure do not consider the high-temperature environment of the reactor and the transverse vibration of the control rod caused by the coolant flow, resulting in the defect of serious lack of accuracy of the experimental detection results.

[0007] To solve the above problems, the application provides an experimental detection device for a high-temperature reactor control rod power structure, which comprises a base, a top disc and a reactor core fixedly connected to the inner bottom surface of the base, a plurality of vertically distributed vertical rods and load-bearing connecting plates fixedly connected between the base and the top disc, wherein the vertical rods are located outside the circular area formed by the load-bearing connecting plates, a plurality of main surrounding rings and a secondary surrounding ring are arranged between the base and the top disc, the main surrounding rings and the secondary surrounding ring are sequentially distributed in the vertical direction, a plurality of cylindrical holes corresponding to the vertical rods are formed in the main surrounding rings, the vertical rods are slidably connected to the inside of the cylindrical holes, the secondary surrounding ring comprises a pair of secondary half rings, a pair of secondary perforated plates are fixedly connected to the side ends of the secondary half rings, a first fastener is threadedly connected between the pair of secondary perforated plates, a transmission pipe is fixedly connected to the outer end of the secondary half ring, the sum of the heights of the secondary surrounding ring and the plurality of main surrounding rings is equal to the distance between the base and the top disc, and the space formed by the base, the top disc, the main surrounding rings and the secondary surrounding ring is filled with a cooling fluid.

[0008] As a further supplement to the application, inner grooves matching the vertical rods are formed in the inner walls of the secondary half rings, and the horizontal length direction of the plurality of inner grooves is perpendicular to the side end plane of the secondary half ring.

[0009] As a further supplement to the application, a heating rod is fixedly connected to the side end of the load-bearing connecting plate close to the center line of the base, and a temperature sensor is fixedly connected to the side end of the reactor core.

[0010] As a further supplement to the application, a sealing layer is fixedly connected to the upper end surface of the base, the lower end surface of the top disc, the upper and lower end surfaces of the main surrounding ring and the upper and lower end surfaces of the secondary surrounding ring, and the sealing layer is made of high-temperature-resistant sealing material.

[0011] As a further supplement to the application, an extension ring is fixedly connected to the upper end of the base and the upper end of the main surrounding ring, a ring groove is formed in the lower end of the top disc, the lower end of the secondary half ring and the lower end of the main surrounding ring, the extension ring matches the ring groove, the extension ring is located between the annular area formed by the plurality of vertical rods and the plurality of load-bearing connecting plates, and the cylindrical hole is located outside the extension ring.

[0012] As a further supplement to the application, the main surrounding ring comprises a pair of main half rings, a pair of main perforated plates are fixedly connected to the side ends of the main half rings, and a second fastener is threadedly connected between the pair of main perforated plates.

[0013] As a further supplement to the application, the vertical rod comprises a fixed rod and a telescopic rod, the upper end of the fixed rod is fixedly connected to the lower end of the top disc, and the lower end of the fixed rod is fixedly connected to the upper end of the telescopic rod.

[0014] As a further supplement to the application, the telescopic rod adopts a multi-section telescopic structure, the telescopic rod is in an elongated state in the initial state, the lower end of the telescopic rod is in contact with the upper end of the base, and when the telescopic rod is contracted, the distance between the lower end of the telescopic rod and the base is greater than the sum of the heights of the main half ring and the extension ring.

[0015] In summary, the application comprises a base, a top disc, a main surrounding ring and a secondary surrounding ring to form a container shell for detecting the control rod power structure. The core simulates the actual reactor structure, the heating rod heats the cooling fluid, simulates the high temperature of the coolant in the actual situation, and uses multiple transmission pipes as the input and output ports of the cooling fluid, simulates the lateral vibration of the control rod caused by the flow of the coolant in the actual situation, gives the control rod power structure a simulation environment close to reality, improves the accuracy of the detection result, effectively evaluates the performance of the power structure, and sets multiple types of secondary surrounding rings according to the number of transmission pipes. By replacing different types of secondary surrounding rings and adjusting the positions of the main surrounding ring and the secondary surrounding ring, the use performance of the power structure under different states and different degrees of flow of the cooling fluid can be detected, and the comprehensiveness of the detection is improved. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 It is a perspective view of the first embodiment of the application;

[0017] Figure 2 It is an exploded view of the first embodiment of the application;

[0018] Figure 3 It is a partial perspective view of the first embodiment of the application;

[0019] Figure 4 It is a partial front structure schematic view of the first embodiment of the application;

[0020] Figure 5 It is a perspective view of the first embodiment of the application when the position of the secondary surrounding ring is replaced;

[0021] Figure 6 It is a top structure schematic of the first embodiment of the application Figure 1 ;

[0022] Figure 7 It is a top structure schematic of the first embodiment of the application Figure 2 ;

[0023] Figure 8 It is a perspective view of the second embodiment of the application;

[0024] Figure 9 It is an exploded view of the second embodiment of the application;

[0025] Figure 10 It is a partial front structure schematic view of the second embodiment of the application;

[0026] Figure 11 It is a partial front structure schematic view of the vertical rod of the second embodiment of the application;

[0027] Figure 12Schematic diagram of partial front structure of the second embodiment of the present application when replacing the main surrounding ring Figure 1 ;

[0028] Figure 13 Schematic diagram of partial front structure of the second embodiment of the present application when replacing the main surrounding ring Figure 2 ;

[0029] Figure 14 Schematic diagram of partial front structure of the second embodiment of the present application when replacing the main surrounding ring Figure 3 ;

[0030] Figure 15 Schematic diagram of partial front structure of the second embodiment of the present application when replacing the main surrounding ring Figure 4 .

[0031] Explanation of reference numerals in the figure:

[0032] 1 base, 2 top disc, 3 main surrounding ring, 301 cylindrical hole, 31 main half ring, 32 main perforated plate, 4 auxiliary surrounding ring, 41 auxiliary half ring, 4101 inner groove, 42 auxiliary perforated plate, 43 transmission pipe, 5 vertical rod, 51 fixed rod, 52 telescopic rod, 6 load-bearing connecting plate, 7 heating rod, 8 core, 9 extension ring, 10 ring groove. DETAILED DESCRIPTION

[0033] The two embodiments of the present application will be described in detail below with reference to the accompanying drawings.

[0034] First embodiment:

[0035] The present application provides an experimental detection device for a high-temperature reactor control rod power structure, please refer to Figures 1-3 , including a base 1, a top disc 2 and a core 8 fixedly connected with the inner bottom surface of the base 1, the core 8 simulates the reactor structure in the prior art for the control rod to be inserted, a plurality of uniformly distributed vertical rods 5 and load-bearing connecting plates 6 are fixedly connected between the base 1 and the top disc 2, and the vertical rods 5 are located outside the circular area formed by the plurality of load-bearing connecting plates 6, the base 1 and the top disc 2 are provided with an auxiliary surrounding ring 4 and a plurality of main surrounding rings 3, and the main surrounding rings 3 and the auxiliary surrounding ring 4 are sequentially distributed along the vertical direction, a cylindrical hole 301 corresponding to each of the plurality of vertical rods 5 is formed in the main surrounding ring 3, the vertical rod 5 is slidingly connected to the inside of the cylindrical hole 301, the load-bearing connecting plate 6 is used to connect and fix the base 1 and the top disc 2, and plays a supporting role for the top disc 2, the base 1, the top disc 2 and the load-bearing connecting plate 6 form a fixed whole, which is convenient for subsequent position adjustment of the main surrounding ring 3 and the auxiliary surrounding ring 4, the vertical rod 5 plays a limiting role for the main surrounding ring 3, so that the main surrounding ring 3 can stably move up and down, and a mounting hole is formed in the top disc 2, during detection, the control rod power structure is conveniently mounted on the upper end of the top disc 2 through the mounting hole, and the power structure drives the control rod to be released from the upper end of the top disc 2 into the core 8 from top to bottom.

[0036] Please refer to Figure 1 and Figure 2 The secondary ring 4 comprises a pair of secondary half rings 41, the side ends of the secondary half rings 41 are fixedly connected with a pair of secondary belt hole plates 42, a first fastener is threadedly connected between the pair of secondary belt hole plates 42, the first fastener can be a bolt and a nut, the inner wall of the secondary half ring 41 is provided with an inner groove 4101 matched with the vertical rod 5, the horizontal length direction of the plurality of inner grooves 4101 is perpendicular to the side end plane of the secondary half ring 41 (as shown in Figure 5 and Figure 6 ), the outer end of the secondary half ring 41 is fixedly connected with a transmission pipe 43, the sum of the heights of the plurality of primary rings 3 and the secondary ring 4 is equal to the distance between the base 1 and the top disc 2, the space formed by the base 1, the top disc 2, the primary ring 3 and the secondary ring 4 is filled with cooling fluid.

[0037] The load connecting plate 6 is fixedly connected with a heating rod 7 at the side end close to the center line of the base 1, the side end of the core 8 is fixedly connected with a temperature sensor (not shown in the figure), the heating rod 7 can adopt a resistance wire, and an indirect resistance heating method is used to heat the cooling fluid, so that the heating temperature can reach above 1000℃, simulating the high temperature condition of the coolant in the actual situation, the temperature sensor adopts a high-temperature-resistant thermocouple type temperature sensor, which can realize continuous temperature measurement from-40℃ to 1600℃, and is convenient for monitoring the heating temperature of the cooling fluid.

[0038] In the actual working process of the nuclear reactor, in order to transfer the heat of fission, a coolant is needed in the reactor vessel, common coolants include light water, heavy water, helium and liquid metal sodium, etc., such as: when used in a high-temperature gas cooled reactor, helium is usually used as a coolant; after the coolant is heated, it flows to the steam generator, and when it flows through the pipeline in the steam generator, it exchanges heat with the external circulating water, so that the water is heated to become steam, the steam drives the steam engine, and the steam engine drives the engine to generate electricity, and the cooled coolant will return to the reactor vessel again;

[0039] Based on the above actual situation, in the present application, the container shell for detecting the control rod power structure is composed of the base 1, the top disc 2, the main surrounding ring 3 and the auxiliary surrounding ring 4, the reactor structure in the actual situation is simulated by the core 8, so that the power structure driving the control rod is closer to the real state when it is inserted and pulled, the cooling fluid material in the present application is the same as the actual coolant material, such as helium, and the cooling fluid can be heated by the heating rod 7 during the detection process, the high temperature condition of the coolant in the actual situation is simulated, the external delivery pump is communicated with the transmission pipe 43, the heated cooling fluid in the present application is extracted and delivered to the external heat exchange equipment, and after indirect heat exchange with water, it is re-delivered to the container shell through another transmission pipe 43, so as to simulate the lateral vibration of the control rod caused by the flow of the coolant in the actual situation. The above multiple simulations are combined to give the control rod power structure a simulation environment close to the real situation, improve the accuracy of the detection result, and effectively evaluate the performance of the power structure.

[0040] In addition, when the control rod power structure is put into actual use, the reactor vessel structure it faces is not fixed, for example: the number of steam generators can be one, two or even more, the more the number of steam generators, the greater the flow of the coolant in the reactor vessel, and the greater the influence on the control rod. Therefore, it is necessary to detect the performance of the power structure under different flow of the coolant. For example: for different reactor vessels, the height position of the input port and the output port of the coolant may be different, and the input and output of the coolant at different positions will also affect the overall flow of the coolant, and further affect the control rod and the power structure. Therefore, for the above different use environments, the present application gives the control rod power structure different detection environments by replacing the auxiliary surrounding ring 4 and adjusting the positions of the main surrounding ring 3 and the auxiliary surrounding ring 4, as follows:

[0041] First level adjustment of detection condition: if the number of steam generators is one, one coolant input port and one coolant output port are needed on the reactor vessel, if the number of steam generators is two, two coolant input ports and two coolant output ports are needed on the reactor vessel, and so on. For the above situation, the present application can set multiple types of auxiliary surrounding rings 4 according to the number of transmission pipes 43, so that different types of auxiliary surrounding rings 4 have different numbers of transmission pipes 43, and by replacing different types of auxiliary surrounding rings 4 during detection, the performance of the power structure under different flow states of the cooling fluid can be detected.

[0042] Second level adjustment of detection condition: after selecting a certain type of auxiliary surrounding ring 4, such as Figure 5 and Figure 7As shown, taking four transmission pipes 43 as an example (two transmission pipes 43 are arranged on each of the pair of secondary half rings 41, two of which are used as cooling fluid input ends, and the other two are used as cooling fluid output ends), first, the plurality of primary surrounding rings 3 are slid up and down correspondingly, and adjacent primary surrounding rings 3 are attached to each other, leaving a space at the required height position, then a pair of secondary half rings 41 are installed at the space, and during installation, a plurality of inner grooves 4101 are respectively inserted into a plurality of vertical rods 5, and the two secondary half rings 41 are enclosed into a complete ring structure between a pair of primary surrounding rings 3 (supplementary description: if the secondary surrounding ring 4 is installed on the upper side of all the primary surrounding rings 3, the secondary surrounding ring 4 is located between the top disc 2 and the primary surrounding ring 3, and if the secondary surrounding ring 4 is installed on the lower side of all the primary surrounding rings 3, the secondary surrounding ring 4 is located between the base 1 and the primary surrounding ring 3), then a pair of secondary hole plates 42 are connected by using the first fastener, so that the pair of secondary half rings 41 are stably connected.

[0043] Please refer to Figure 2 and Figure 4 The upper end of the base 1 and the upper end of the primary surrounding ring 3 are fixedly connected with an extension ring 9, the lower end of the top disc 2, the lower end of the secondary half ring 41 and the lower end of the primary surrounding ring 3 are all provided with a ring groove 10, the extension ring 9 is matched with the ring groove 10, the extension ring 9 is located between the annular area formed by the plurality of vertical rods 5 and the plurality of load-bearing connecting plates 6, the cylindrical hole 301 is located outside the extension ring 9, through the matching of the extension ring 9 and the ring groove 10, the contact area of the connection between the adjacent structures of the base 1, the top disc 2, the primary surrounding ring 3 and the secondary surrounding ring 4 is increased, and at the same time, a sealing layer is fixedly connected to the upper end face of the base 1, the lower end face of the top disc 2, the upper and lower end faces of the primary surrounding ring 3 and the upper and lower end faces of the secondary surrounding ring 4, the sealing layer can be made of flexible graphite, which has good corrosion resistance, can withstand high temperature above 1000℃, has good compression resilience and high strength, in this application, the sealing layer not only adapts to the high temperature environment of the cooling fluid, but also effectively seals the contact gap between the adjacent structures of the base 1, the top disc 2, the primary surrounding ring 3 and the secondary surrounding ring 4, so that the cooling fluid is not easy to overflow from the gap, similarly, the side end faces of the pair of secondary half rings 41 are also fixedly connected with the sealing layer, so that the cooling fluid is not easy to overflow.

[0044] The second embodiment:

[0045] Based on the first embodiment, the following content is added: please refer to Figure 8 and Figure 9 The primary surrounding ring 3 comprises a pair of primary half rings 31, and the side end of the primary half ring 31 is fixedly connected with a pair of primary hole plates 32, and a second fastener is threadedly connected between the pair of primary hole plates 32, and the second fastener can also be a bolt and a nut, the vertical rod 5 comprises a fixed rod 51 and an extension rod 52, the upper end of the fixed rod 51 is fixedly connected with the lower end of the top disc 2, and the lower end of the fixed rod 51 is fixedly connected with the upper end of the extension rod 52, please refer to Figure 10And Figure 11 The telescopic rod 52 adopts a telescopic structure, including a plurality of sleeve rods sleeved with each other, so that the length of the telescopic rod 52 after being stretched is more than 2.5 times of the length after being contracted. The telescopic rod 52 is in the stretched state in the initial state, and the lower end of the telescopic rod 52 is in contact with the upper end of the base 1. When the telescopic rod 52 is contracted, the distance between the lower end of the telescopic rod 52 and the base 1 is greater than the sum of the height of the main half ring 31 and the extension ring 9. The inner diameter of the cylindrical hole 301 is slightly greater than the maximum outer diameter of the telescopic rod 52, facilitating the up-and-down sliding of the main half ring 31.

[0046] The embodiment divides the main surrounding ring 3 into two main half rings 31 (supplementary description: the extension ring 9 at the upper end of the main surrounding ring 3 is also divided into two half rings), and cooperates with the telescopic setting of the vertical rod 5, so that the main surrounding ring 3 can be disassembled while being used normally. When the main surrounding ring 3 is damaged (for example, the sealing layer on the surface is damaged), the damaged main surrounding ring 3 can be replaced alone. The specific operation is as follows:

[0047] Step one: as shown in Figure 12 , first, the first fastener on the auxiliary surrounding ring 4 is removed, the pair of auxiliary half rings 41 is disassembled and removed, and then the plurality of main surrounding rings 3 are moved upward to expose the lower section of the telescopic rod 52;

[0048] Step two: as shown in Figure 13 , the lower end of the telescopic rod 52 is manually moved upward to be contracted to the bottom end of the lowermost main half ring 31 (there is a large friction force between the plurality of sleeve rods on the telescopic rod 52, and the telescopic rod 52 is not easy to be stretched and unfolded when it is contracted without being affected by other external forces), so that the telescopic rod 52 can be kept in the contracted state;

[0049] Step three: as shown in Figure 14 , the lowermost main half ring 31 is moved downward until it is in contact with the base 1. At this time, a small part of the lower end of the telescopic rod 52 is located inside the cylindrical hole 301. The side end of the telescopic rod 52 is manually operated to be continuously contracted upward until the lower end of the telescopic rod 52 is located on the upper side of the extension ring 9. At this time, the lowermost main surrounding ring 3 is not limited by the vertical rod 5. After the second fastener is removed, the pair of main half rings 31 can be disassembled and replaced (as shown in Figure 15 ). Similarly, when the new main surrounding ring 3 is installed, the above steps can be performed in reverse.

[0050] The above embodiments adopted by the present application are not limited to the scope of the present application. Various changes made within the knowledge of those skilled in the art without departing from the concept of the present application still fall within the protection scope of the present application.

Claims

1. An experimental testing device for the dynamic structure of control rods in a high-temperature reactor, comprising a base (1), a top plate (2), and a reactor core (8) fixedly connected to the inner bottom surface of the base (1), characterized in that: Multiple evenly distributed vertical rods (5) and load-bearing connecting plates (6) are fixedly connected between the base (1) and the top plate (2), and the vertical rods (5) are located outside the circular area formed by the multiple load-bearing connecting plates (6). A secondary ring (4) and multiple main rings (3) are provided between the base (1) and the top plate (2), and the main rings (3) and secondary rings (4) are distributed sequentially along the vertical direction. The main rings (3) are provided with cylindrical holes (301) corresponding to the multiple vertical rods (5), and the vertical rods (5) are slidably connected to the inner part of the cylindrical holes (301). The sub-ring (4) includes a pair of sub-half-rings (41), the side ends of which are fixedly connected to a pair of sub-perforated plates (42), and the pair of sub-perforated plates (42) are threaded together with a first fastener. The outer end of the sub-half-ring (41) is fixedly connected to a transmission pipe (43). The sum of the heights of the sub-ring (4) and the plurality of main rings (3) is equal to the distance between the base (1) and the top plate (2). The space formed by the base (1), the top plate (2), the main rings (3) and the sub-ring (4) is filled with cooling fluid.

2. The experimental testing device for the dynamic structure of a high-temperature reactor control rod according to claim 1, characterized in that: The inner wall of the sub-half ring (41) is provided with an inner groove (4101) that matches the vertical rod (5), and the horizontal length direction of the multiple inner grooves (4101) is perpendicular to the side end plane of the sub-half ring (41).

3. The experimental testing device for the dynamic structure of a high-temperature reactor control rod according to claim 1, characterized in that: A heating rod (7) is fixedly connected to the side end of the load-bearing connecting plate (6) near the center line of the base (1), and a temperature sensor is fixedly connected to the side end of the core (8).

4. The experimental testing device for the dynamic structure of a high-temperature reactor control rod according to claim 1, characterized in that: The upper end face of the base (1), the lower end face of the top plate (2), the upper and lower end faces of the main ring (3) and the upper and lower end faces of the secondary ring (4) are all fixedly connected with a sealing layer, which is made of high temperature resistant sealing material.

5. The experimental testing device for the dynamic structure of control rods in a high-temperature reactor according to claim 1, characterized in that: An extension ring (9) is fixedly connected to the upper end of the base (1) and the upper end of the main ring (3). The lower end of the top plate (2), the lower end of the secondary half ring (41) and the lower end of the main ring (3) are all provided with ring grooves (10). The extension ring (9) matches the ring groove (10). The extension ring (9) is located between the annular area formed by multiple vertical rods (5) and multiple load-bearing connecting plates (6). The cylindrical hole (301) is located on the outside of the extension ring (9).

6. The experimental testing device for the dynamic structure of a high-temperature reactor control rod according to claim 1, characterized in that: The main circumferential ring (3) includes a pair of main semi-rings (31), and a pair of main belt perforated plates (32) are fixedly connected to the side ends of the main semi-rings (31). A second fastener is threaded between the pair of main belt perforated plates (32).

7. The experimental testing device for the dynamic structure of high-temperature reactor control rods according to claim 1, characterized in that: The vertical rod (5) includes a fixed rod (51) and a telescopic rod (52). The upper end of the fixed rod (51) is fixedly connected to the lower end of the top plate (2), and the lower end of the fixed rod (51) is fixedly connected to the upper end of the telescopic rod (52).

8. The experimental testing device for the dynamic structure of a high-temperature reactor control rod according to claim 7, characterized in that: The telescopic rod (52) adopts a multi-section telescopic structure. In the initial state, the telescopic rod (52) is in an extended state, and the lower end of the telescopic rod (52) is in contact with the upper end of the base (1). When the telescopic rod (52) is retracted, the distance between its lower end and the base (1) is greater than the sum of the heights of the main half ring (31) and the extension ring (9).

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