Indirect electric heating rod with axial non-uniform power distribution

Abstract

An indirect electric heating rod with axial non-uniform power distribution comprises a cladding, a ceramic sleeve and a heating section, the ceramic sleeve and the heating section are sequentially arranged in the cladding, electrode sections are arranged at two ends of the heating section, and the electrode sections are respectively connected with a positive electrode and a negative electrode of a direct-current power supply. According to the invention, a central heating and axial heating power stepped distribution structure is adopted, so that the axial non-uniform power distribution of the higher middle part and the lower two ends of the nuclear fuel rod of the reactor can be simulated, and the heating characteristic of the actual nuclear fuel rod can be reflected more accurately.

Description

Technical Field

[0001] The invention relates to a technology in the field of reactors, in particular to an indirect electric heating rod with axial non-uniform power distribution. Background Art

[0002] In order to study the flow and heat transfer mechanism of the reactor, thermal hydraulic tests usually use electric heating rods to simulate the thermal characteristics of nuclear fuel rods, and temperature sensors are set on their surfaces to monitor the wall temperature. The test results are extremely valuable for nuclear reactor design, fluid mechanics model development, and safety analysis program verification. Due to the structural characteristics, the existing non-uniform electric heating technology cannot achieve a radial temperature distribution state with higher center and lower surroundings. Due to the consistency of the material, the thermal expansion is different, which easily causes the temperature measurement point to shift. Summary of the invention

[0003] In view of the shortcomings of the prior art that the thermal characteristics of nuclear fuel rods cannot be accurately simulated, the temperature measurement points are easily offset, and the temperature measurement results have large errors, the present invention proposes an indirect electric heating rod with axial non-uniform power distribution, which adopts a central heating and axial heating power stepped distribution structure, and can simulate the axial non-uniform power distribution of the reactor nuclear fuel rod, which is higher in the middle and lower at both ends, and the radial temperature distribution state of higher center and lower surroundings, and more accurately reflects the heating characteristics of the actual nuclear fuel rod. The positioning gasket and metal spring are used to fix the thermocouple to ensure the positioning of the temperature measurement point and the accuracy of the temperature measurement results.

[0004] The present invention is achieved through the following technical solutions:

[0005] The invention relates to an indirect electric heating rod with axial non-uniform power distribution, comprising: a cladding shell, a ceramic sleeve and a heating section arranged therein in sequence, wherein: electrode sections are arranged at both ends of the heating section, and the electrode sections are respectively connected to the positive and negative electrodes of a DC power supply.

[0006] The heating section is formed by welding a number of alloy round bars of different diameters and lengths end to end. The diameter of each round bar decreases first and then increases from the bottom. The two ends of the heating section are welded to the electrode section. Since the resistance per unit length of each round bar is different, the thermal power per unit length is also different after power is turned on, thereby achieving axial non-uniform power distribution with higher middle part and lower two ends.

[0007] The inner diameter of the ceramic sleeve matches the outer diameter of each section of the alloy round rod, and the inner wall surface thereof is in close contact with the outer wall surface of the alloy round rod. Positioning gaskets are arranged between the ceramic sleeves for fixing the thermocouple.

[0008] A ceramic gasket is preferably provided between the positioning gasket and the heating section, and the positioning gasket is clamped by upper and lower ceramic sleeves.

[0009] A plurality of thermocouples parallel to the central axis of the cladding are arranged between the cladding and the ceramic sleeve. One end of the thermocouple is preferably fixed to the positioning gasket, and the other end is led out through the end of the heating section. The thermocouple leads are installed in the groove on the outer wall of the ceramic sleeve.

[0010] The number of the thermocouples is preferably four and each thermocouple probe is located on a different positioning pad.

[0011] A metal spring is arranged between the positioning gasket and the matching thermocouple probe to press the thermocouple probe tightly against the inner wall of the cladding. Technical Effects

[0012] The heating section of the present invention is located at the radial center for indirect heating, and is welded end to end by a plurality of alloy round rods of different diameters and lengths. The diameter of each round rod decreases first and then increases from the bottom, so that the resistance per unit length of each section of the round rod is different. After power is turned on, an axial non-uniform power distribution with a higher middle portion and lower ends can be achieved; the ceramic sleeves with different inner diameters are fitted to the outer wall of the corresponding outer diameter alloy round rods, which not only plays an insulating role to avoid cladding leakage, but also can effectively conduct the heat generated by the heating core. Compared with the prior art, the present invention can simultaneously achieve an axial non-uniform power distribution with a higher middle portion and lower ends and a radial temperature distribution state with a higher center and lower surroundings, and can accurately simulate the thermal characteristics of nuclear fuel rods. The thermal hydraulic test results obtained using the electric heating rod are closer to the actual reactor conditions, while solving the power deviation problem caused by leakage in the prior art, and the inaccuracy of temperature measurement caused by air gaps. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 This is an axial cross-sectional view of the heating rod of the present invention;

[0014] Figure 2 A radial cross-sectional view of the heating rod of the present invention;

[0015] Figure 3 This is the structural diagram of the ceramic casing;

[0016] Figure 4 This is a schematic diagram of the thermocouple probe fixing method;

[0017] Figure 5 The axial temperature distribution of the outer wall of the cladding simulated by COMSOL;

[0018] Figure 6 This is the radial temperature distribution diagram of the highest power section simulated by COMSOL;

[0019] In the figure: 1 electrode segment, 2 heating segment, 3 ceramic sleeve, 4 positioning gasket, 5 ceramic gasket, 6 cladding, 7 thermocouple, 8 ceramic end plug, 9 metal end plug, 10 metal spring. DETAILED DESCRIPTION

[0020] like Figure 1 As shown, this embodiment relates to an indirect electric heating rod with axial non-uniform power distribution, including: a shell 6 and a ceramic sleeve 3 and a heating section 2 arranged therein in sequence, wherein: electrode sections 1 are provided at both ends of the heating section 2, and the electrode sections are respectively connected to the positive and negative electrodes of a DC power supply.

[0021] The heating section 2 is formed by welding a number of alloy round bars of different diameters and lengths end to end. The diameter of each round bar decreases first and then increases from the bottom. The two ends of the heating section are welded to the electrode section. Since the resistance per unit length of each round bar is different, the thermal power per unit length is also different after power is turned on, thereby achieving an axial non-uniform power distribution with a higher middle part and a lower two ends.

[0022] The inner diameter of the ceramic sleeve 3 matches the outer diameter of each section of the alloy round rod, and the inner wall surface thereof is in close contact with the outer wall surface of the alloy round rod. A positioning gasket 4 is arranged between the ceramic sleeves 3 to fix the thermocouple 7 .

[0023] like Figure 2 As shown, a ceramic gasket 5 is preferably provided between the positioning gasket 4 and the heating section 2 , and the positioning gasket is clamped by the upper and lower ceramic sleeves 3 .

[0024] A plurality of thermocouples 7 parallel to the central axis of the cladding 6 are provided between the cladding 6 and the ceramic sleeve 3 . One end of the thermocouple 7 is preferably fixed to the positioning gasket 4 , and the other end is led out through the end of the heating section 2 .

[0025] The number of the thermocouples 7 is preferably four and each thermocouple 7 probe is located on a different positioning pad 4 .

[0026] like Figure 4 As shown, a metal spring 10 is arranged between the positioning gasket 4 and the matching thermocouple 7 probe to press the thermocouple probe against the inner wall of the cladding 6 .

[0027] The outer wall surface of the ceramic sleeve 3 is circumferentially provided with a plurality of grooves for arranging the leads of the thermocouple 7 , and the leads of the thermocouple 7 are installed in the grooves on the outer wall surface of the ceramic sleeve 3 .

[0028] A ceramic end plug 8 and a metal end plug 9 are provided between the electrode segment 1 and the cladding 6. The metal end plugs are welded to the inner wall surfaces at both ends of the cladding to encapsulate and fix the relative axial position of the insulating layer and the cladding.

[0029] The cladding 6 is a seamless tube structure with constant outer diameter and wall thickness.

[0030] The alloy round rod is located in the radial center, and the temperature is high after power is turned on, and the temperature of the insulation layer and the cladding decreases in turn. A ceramic sleeve is installed between the cladding and the heating core, and a ceramic gasket and a ceramic end plug are installed on the inner wall of the positioning gasket and the metal end plug, so that the heating core and the cladding are mutually insulated to ensure the accuracy of the actual heating power.

[0031] This embodiment relates to a method for manufacturing the above-mentioned indirect electric heating rod, by processing 6 sections with a resistivity of 1.09×10 -6 The diameter of the Cr20Ni80 round rod with a diameter of Ω·m shows a trend of decreasing first and then increasing. The specific values ​​are 4.5mm, 4mm, 3.5mm, 3mm, 4mm, and 4.5mm, and the corresponding lengths are 1000mm, 1000mm, 700mm, 300mm, 500mm, 160mm and two sections of pure nickel electrode round rods with a diameter of 4.5mm and a length of 350mm and 450mm respectively. Among them, the total length of the Cr20Ni80 round rods after splicing is 3.66m, which is the same as the length of most pressurized water reactor nuclear fuel rods. Then, multiple sections of boron nitride ceramic sleeves with the same outer diameter and inner diameter matching the outer diameter of each section of Cr20Ni80 round rod and pure nickel electrode are made, and 4 grooves parallel to the axis are set on the circumference of the outer surface. The boron nitride ceramic sleeves are respectively sleeved on the outer wall of the round rods with corresponding outer diameters, and the positioning gasket and ceramic gasket are installed at the temperature measurement position and fixed by the upper and lower boron nitride ceramic sleeves. The assembled structure is arranged in the order of decreasing and then increasing the diameter of the Cr20Ni80 round rod from bottom to top, and the pure nickel electrodes are arranged at both ends, and cold welding and straightening are performed in sequence. The grooves of each ceramic tube and the positioning gasket are aligned in sequence along the axial direction, and 4 copper metal springs are spot welded at the positioning gasket grooves. Four K-type armored non-grounded thermocouples with a temperature measurement range of more than 1000℃ and a diameter of 0.5mm are used. The four thermocouple probes are welded to the copper metal springs at the corresponding positions by laser cold welding, and the leads are arranged in the axial grooves on the outer wall of the boron nitride ceramic sleeve and led out from one end of the electric heating rod. The Inconel 600 seamless tube with an outer diameter of 9.5mm and a wall thickness of 0.84mm is cold-rolled by a mold, and is sleeved on the outer wall of the insulation layer. The metal end plugs are welded at both ends and insulated with ceramic end plugs. The thermocouple is pressed against the inner wall of the Inconel 600 seamless tube by the copper metal spring.

[0032] In order to further verify the effectiveness of the indirect electric heating rod with axial non-uniform power distribution in the present invention in simulating the thermal characteristics of nuclear fuel rods, the COMSOL software was used for electrothermal coupling simulation analysis. The specific method is that the electrode segment at one end of the electric heating rod is grounded, and 127.3A DC is added to the other end to meet the minimum diameter alloy round rod linear power density of 2.5kW / m, that is, the maximum linear power density of the electric heating rod is 2.5kW / m, and the heat transfer coefficient of the outer wall of the cladding is set to 10W / (m 2·K) The convection heat transfer boundary condition with a temperature of 30°C is used for transient calculation. When the heating continues until the temperature of the highest power section reaches 1000°C, the temperature distribution state is recorded: Figure 5 The axial temperature distribution of the outer wall of the cladding shows that the temperature in the middle of the heating section is significantly higher than that at both ends and there are 6 temperature steps, which proves the accuracy of the heating rod structure of the present invention in simulating the axial temperature distribution of nuclear fuel rods. Figure 6 This is the radial temperature distribution in the highest power section. The highest temperature at the radial center is 1011°C, and gradually decreases to 1000°C with the radius outward, forming a radial temperature distribution state with higher center and lower surroundings, which is consistent with the actual radial temperature distribution characteristics of nuclear fuel rods.

[0033] The electric heating rod of the present invention has an outer diameter of 9.5 mm, a heating length of 3660 mm, a total length of 4460 mm, and a 6-stage stepped distribution of heating power with a higher middle portion and lower ends. The heating rod is provided with 4 temperature measuring points on the inner wall of the cladding at different axial heights, and the maximum rod surface temperature can reach 1000°C, meeting the requirements of various thermal hydraulic tests such as flow boiling heat transfer test, reflooding test, and critical heat flux density test.

[0034] Compared with the prior art, the heating core of the device is located at the radial center, and the heating section is formed by welding alloy round rods of different lengths and diameters. After power is turned on, it can simultaneously meet the axial non-uniform power distribution with higher middle part and lower ends and the radial temperature distribution with higher center and lower surroundings, which more realistically simulates the thermal characteristics of the fuel rods in the reactor; the specific power distribution state can be accurately controlled by adjusting the length and diameter of the alloy round rods; there is an insulating layer between the heating core and the cladding for electrical insulation, and the outer wall surface is not electrified when power is turned on, ensuring the accuracy of the actual heating power; the thermocouple probe is pressed against the inner wall surface of the cladding by the metal spring sheet, is not affected by the air gap, the temperature measuring point is accurately positioned, and the temperature measurement accuracy is high; the structural design allows monitoring of the temperature of multiple points on the inner wall surface of the cladding, and the temperature measuring points are accurately positioned; the cladding is a complete metal tube without welds, has good axial straightness, and consistent surface characteristics.

[0035] The above-mentioned specific implementation can be partially adjusted in different ways by those skilled in the art without departing from the principle and purpose of the present invention. The protection scope of the present invention shall be based on the claims and shall not be limited by the above-mentioned specific implementation. Each implementation scheme within its scope shall be subject to the constraints of the present invention.

Claims

1. An indirect electric heating rod with axial non-uniform power distribution, characterized in that: include: The cladding shell and the ceramic sleeve and the heating section are sequentially arranged therein, wherein: the two ends of the heating section are provided with electrode sections, and the electrode sections are respectively connected to the positive electrode and the negative electrode of the DC power supply; The heating section is formed by welding a number of alloy round bars of different diameters and lengths end to end. The diameter of each round bar decreases first and then increases from the bottom. The two ends of the heating section are welded to the electrode section. Since the resistance per unit length of each round bar is different, the thermal power per unit length is also different after power is turned on, thereby achieving an axial non-uniform power distribution with higher power in the middle and lower power at both ends and a radial temperature distribution state with higher power in the center and lower power around.

2. The indirect electric heating rod with axial non-uniform power distribution according to claim 1 is characterized in that: A plurality of thermocouples parallel to the central axis of the cladding are arranged between the cladding and the ceramic sleeve. One end of the thermocouple is fixed to the positioning gasket, and the other end is led out through the end of the heating section. The thermocouple leads are installed in the groove on the outer wall of the ceramic sleeve.

3. The indirect electric heating rod with axial non-uniform power distribution according to claim 2 is characterized in that: The inner diameter of the ceramic sleeve matches the outer diameter of each section of the alloy round rod, and the inner wall surface thereof is in close contact with the outer wall surface of the alloy round rod. Positioning gaskets are arranged between the ceramic sleeves for fixing the thermocouple.

4. The indirect electric heating rod with axial non-uniform power distribution according to claim 2 or 3, characterized in that: A ceramic gasket is arranged between the positioning gasket and the heating section, and the positioning gasket is clamped by the upper and lower ceramic sleeves.

5. The indirect electric heating rod with axial non-uniform power distribution according to claim 2 or 3, characterized in that: There are four thermocouples and each thermocouple probe is located on a different positioning pad.

6. The indirect electric heating rod with axial non-uniform power distribution according to claim 2 or 3, characterized in that: A metal spring is arranged between the positioning gasket and the matching thermocouple probe to press the thermocouple probe tightly against the inner wall of the cladding.

7. The indirect electric heating rod with axial non-uniform power distribution according to claim 1 is characterized in that: A ceramic end plug and a metal end plug are provided between the electrode segment and the cladding. The metal end plugs are welded to the inner wall surfaces at both ends of the cladding to encapsulate and fix the relative axial position of the insulating layer and the cladding.

8. The indirect electric heating rod with axial non-uniform power distribution according to claim 1 is characterized in that: The cladding is a seamless tube structure with constant outer diameter and wall thickness.

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