A double-layer straight heat exchange tube for liquid metal cooling reactor

Abstract

The present invention discloses a double-layer straight tube heat exchange tube for liquid metal cooling reactor, which is composed of an inner tube of a multi-layer graphene / 316L composite material doped with graphene and an outer tube of a multi-layer graphene / 316L composite material doped with graphene tightly combined therewith, wherein the inner wall of the inner tube and the outer wall of the outer tube are coated with a graphene nano anti-corrosion coating, and a gallium-based graphene nanofluid is injected into the gap between the two tubes. The present invention adopts a double-layer straight tube, a multi-layer graphene / 316L composite material and a graphene nano anti-corrosion coating that are tightly fitted together, and the probability of heat exchange tube rupture accidents is greatly reduced from the design, thereby effectively improving thermal safety. At the same time, the present invention greatly reduces the heat transfer resistance of the gap at the tightly fitted place by injecting a graphene nanofluid into the gap between the two tubes, and strengthens the convective heat transfer, thereby efficiently deriving the reactor core fission heat, further bringing greater economic benefits.

Description

Technical Field

[0001] The invention belongs to the technical field of heat exchange tubes, and in particular relates to a double-layer straight heat exchange tube for a liquid metal cooling reactor. Background Art

[0002] Sodium-cooled fast reactors use liquid sodium metal as a coolant, and the heat released by the nuclear reaction is taken out of the reactor by the liquid sodium flowing through the core. Sodium has a small neutron absorption cross-section, good thermal conductivity, and is not easy to slow down the neutron speed, which will not hinder the chain reaction in the fast reactor, so it is an ideal cooling liquid. However, the disadvantage of sodium is that it is chemically active and easily reacts with oxygen and water. When using sodium, strict precautions must be taken. In order to ensure the safety of the core when a sodium-water reaction occurs due to a steam generator leak, the sodium-cooled fast reactor adds an intermediate loop with liquid sodium as the working medium and a sodium-sodium intermediate heat exchanger between the sodium coolant loop (primary loop) and the steam-water loop (secondary loop), but the thermal efficiency is bound to be greatly reduced. Therefore, if the intermediate loop can be eliminated and the heat exchange tubes of the main heat exchanger can be improved instead, not only will the core safety be guaranteed, but the thermal efficiency can also be greatly improved.

[0003] An SGTR accident refers to an accident caused by the rupture of one or more heat transfer tubes in the steam generator (including cracks that cause minor continuous leakage). It causes the second barrier of the nuclear power plant (primary circuit pressure boundary) to lose its integrity, and causes the primary and secondary circuits to be connected, causing the secondary circuit to be contaminated by the radioactive primary circuit water. If the condenser is unavailable, the radioactive material will be discharged directly into the atmosphere through the secondary side atmospheric release valve or safety valve. The operating experience of pressurized water reactor nuclear power plants shows that SGTR accidents occupy a primary position in nuclear power plant accidents. Therefore, if the probability of such accidents can be reduced, it will be of great significance to the safety, economy and reliability of nuclear power plant operations.

[0004] Heat exchange tubes are key heat transfer equipment in liquid metal cooled reactors. They are important equipment for transferring core fission heat to the secondary coolant system. They are also a safety barrier to prevent the escape of radioactive liquid metal coolant in the primary circuit. At present, there are relatively few heat exchange tube structural designs for liquid metal cooled reactors in the world. Whether the fission heat of the reactor core can be efficiently exported and whether the heat exchange tubes can be guaranteed not to rupture under harsh conditions of high temperature and high pressure are issues that must be solved and answered in the heat exchange tube design process. The use of double-layer heat exchange tubes can effectively solve the above problems, but at the same time, it also places high demands on the performance of double-layer heat exchange tubes.

[0005] Due to the influence of the surface roughness of the double-layer tube wall, no matter how closely the inner and outer layers of the heat exchange tube are in contact, in reality they are only in contact at some discrete points, and the uncontacted areas are filled with air. Since the thermal conductivity of the air in the gap is much smaller than the thermal conductivity of the solid material used in the heat exchange tube, it has a larger heat transfer resistance. Injecting graphene nanofluid into the gap can not only reduce the heat transfer resistance, but also greatly enhance convective heat transfer and improve the reliability of the heat exchanger.

[0006] 316L stainless steel is an austenitic stainless steel with good gloss, good corrosion resistance, mechanical properties and welding performance. It is often used in pipelines, food, chemicals, watches, construction, medicine and other fields, and has extremely wide applications. At present, most of the heat exchange tubes of liquid metal cooled reactors in the world use 316L and T91 stainless steel materials. Graphene has many excellent properties in optics, electricity, mechanics and heat, and can be called a super material. The thermal conductivity of a single layer of graphene is as high as 5300Wm -1 K -1 It can be seen that graphene has very good thermal conductivity. The multilayer graphene / 316L composite material prepared by laser deposition additive manufacturing, combined with their respective excellent properties, not only improves the heat conduction rate of the entire material, but also greatly improves other performances. Therefore, if a double-layer heat exchange tube made of multilayer graphene / 316L composite material is used, the economy and safety of the reactor will inevitably be improved.

[0007] Single-layer graphene is a two-dimensional mesh material with a theoretical single-layer thickness of only 0.34nm. It not only has very good thermal conductivity, but also has considerable research value in anti-corrosion coatings. Graphene nano anti-corrosion coatings composed of special resins, graphene, a small amount of nano-composite ceramics and other materials have many excellent properties such as corrosion resistance, high thermal conductivity, high hardness, and impact resistance. It can be seen that covering the wall of the heat exchange tube with graphene nano anti-corrosion coatings has a considerable value. Summary of the invention

[0008] The purpose of the present invention is to meet the needs of liquid metal cooled reactor heat exchange tubes, overcome the shortcomings of the current existing technology that the system structure is complex and affects the economy and safety of the reactor, and provide a double-layer straight tube heat exchange tube suitable for liquid metal cooled reactors, which has a simple structure, small shell side coolant pressure loss, can reduce the probability of heat exchange tube rupture accidents from the design, and can efficiently extract the fission heat of the reactor core.

[0009] The technical solution adopted by the present invention is as follows:

[0010] A double-layer straight heat exchange tube for a liquid metal cooled reactor, the double-layer straight heat exchange tube comprising an inner tube of a multi-layer graphene / 316L composite material doped with graphene for entry and exit of a secondary circuit coolant, an outer tube of a multi-layer graphene / 316L composite material doped with graphene tightly combined with the inner tube, and a gap between the inner tube and the outer tube; wherein the inner wall of the inner tube is coated with a first graphene nano anti-corrosion coating and the outer wall of the outer tube is coated with a second graphene nano anti-corrosion coating, and a gallium-based graphene nanofluid is injected into the gap.

[0011] Furthermore, the inner tube and the outer tube are tightly fitted together by mechanical force, and both are straight tubes.

[0012] Compared with the prior art, the advantages of the present invention are:

[0013] (1) The present invention adopts double-layer straight tubes that are tightly fitted together. Compared with the Italian MYRRHA single-tube straight heat exchange tube, any crack in any tube will stop expanding at the place where the two layers of tubes contact without interfering with each other. Therefore, the probability of heat exchange tube rupture accidents is greatly reduced from the design point of view, thereby effectively improving the thermal safety of the reactor; compared with the EU ELSY spiral heat exchange tube, it can effectively reduce the pressure loss on the primary side and improve the natural circulation capacity of a circuit. At the same time, the power consumption of the pump is small, the fluid vibration amplitude is small, the tube bundle is not easy to wear, the structural design is simple, and the service life is longer.

[0014] (2) Gallium-based graphene nanofluid is injected into the gap between the two tubes of the present invention. When cracks appear somewhere in the inner tube or the outer tube, they can be detected in time, so that the occurrence of heat exchange tube rupture accidents can be prevented in time, further improving the thermal safety of the reactor. Since the gap is injected with gallium-based graphene nanofluid, the original air gap heat transfer resistance is greatly reduced. Since the width of the gap is very narrow, the tiny channel increases the convective heat transfer coefficient, effectively strengthening the convective heat transfer. At the same time, the presence of a large number of nanoparticles not only increases the heat capacity and surface area of ​​the fluid, but also greatly increases the effective thermal conductivity of the fluid, which is beneficial to the extraction of heat released by reactor fission, further improving the economy of the reactor.

[0015] (3) The graphene nano anti-corrosion coating used in the present invention has excellent properties of acid and alkali resistance and corrosion resistance. It covers the inner wall of the inner tube and the outer wall of the outer tube, effectively avoiding the corrosion of the heat exchange tube material caused by the contact between the inner wall of the inner tube and the second-circuit coolant and the outer wall of the outer tube and the first-circuit coolant, and plays a good shielding role for the coolant. At the same time, due to the impact resistance of the graphene nano anti-corrosion coating, the magnitude of the tensile stress inside the heat exchange tube and the vibration of the heat exchange tube are effectively reduced, thereby greatly reducing the stress corrosion of the material and further improving the thermal safety of the reactor. At the same time, due to the very high thermal conductivity of the graphene nano anti-corrosion coating, it is conducive to the transfer of heat and further optimizes the heat transfer performance of the heat exchange tube. At the same time, due to the hardness advantage of the graphene nano anti-corrosion coating, the heat exchange tube has a greater pressure bearing capacity and is more resistant to temperature differences, further reducing the probability of heat exchange tube rupture accidents.

[0016] (4) Compared with 316L stainless steel, the multilayer graphene / 316L composite material used in the present invention has a higher hardness, which further improves the pressure bearing capacity of the heat exchange tube and is more resistant to temperature differences, effectively improving the thermal safety of the reactor. At the same time, due to the faster heat conduction rate and higher thermal conductivity of the multilayer graphene / 316L composite material, it is beneficial to the extraction of heat released by reactor fission, further improving the economic efficiency of the reactor. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of the partial structure of the double-layer straight heat exchange tube of the present invention;

[0018] Figure 2 It is a schematic diagram of an enlarged local gap of a double-layer straight heat exchange tube of the present invention;

[0019] Figure 3 It is a top view of the double-layer straight tube heat exchange tube of the present invention.

[0020] In the figure: 1-secondary circuit coolant, 2-inner tube, 3-gap, 4-outer tube, 5-primary circuit coolant, 6-first graphene nano anti-corrosion coating, 7-second graphene nano anti-corrosion coating. DETAILED DESCRIPTION

[0021] The present invention provides a double-layer straight tube heat exchange tube for a liquid metal cooling reactor. The specific implementation of the present invention is described in detail below in conjunction with the accompanying drawings:

[0022] Figure 1 , Figure 2It is a partial structural schematic diagram and a partial gap magnified schematic diagram of a double-layer straight tube heat exchange tube for a liquid metal cooled reactor of the present invention. The double-layer straight tube heat exchange tube of the present invention comprises an inner tube 2 formed of a multi-layer graphene / 316L composite material doped with graphene for the entry and exit of a secondary circuit coolant 1, and an outer tube 4 of a multi-layer graphene / 316L composite material doped with graphene tightly combined with the inner tube 2. The inner wall of the inner tube 2 is coated with a first graphene nano anti-corrosion coating 6 and the outer wall of the outer tube 4 is coated with a second graphene nano anti-corrosion coating 7, and the gap 3 between the inner tube 2 and the outer tube 4 is injected with a gallium-based graphene nanofluid.

[0023] Under normal operation conditions of the reactor, such as the reactor power operation mode (RP). At this time, the first graphene nano anti-corrosion coating 6 coated on the inner wall of the inner tube 2 is in direct contact with the secondary circuit coolant 1, and the second graphene nano anti-corrosion coating 7 coated on the outer wall of the outer tube 4 is in direct contact with the primary circuit coolant 5. Under the action of the pump, the primary circuit coolant 5 flows from top to bottom in the shell side, and the secondary circuit coolant 1 flows from bottom to top in the tube side, forming a parallel countercurrent heat exchange mode. Due to the temperature difference between the secondary circuit coolant 1 in the inner tube 2 and the primary circuit coolant 5 outside the outer tube 4, the primary circuit coolant 5 absorbs the heat released by nuclear fission when passing through the core, and passes through the second graphene nano anti-corrosion coating 7 coated on the outer wall of the outer tube 4, the outer tube 4, the gap 3 injected with gallium-based graphene nanofluid, the inner tube 2, and the first graphene nano anti-corrosion coating 6 coated on the inner wall of the inner tube 2, and finally transfers the heat to the secondary circuit coolant 1.

[0024] The inner tube 2 and the outer tube 4 are tightly fitted together by mechanical force, and both are straight tubes. Compared with other processes, the gap can be minimized by tightly fitting together by mechanical force. If a crack appears in any tube, it will stop expanding at the place where the two layers of tubes contact, and will not interfere with each other. Therefore, the probability of heat exchange tube rupture accidents is greatly reduced from the design. For lead alloy cooled reactors, such a design can reduce the primary side pressure loss, help establish a natural circulation flow mode, and reduce the probability of heat exchange tube rupture accidents from the design.

[0025] The materials used for the inner tube 2 and the outer tube 4 are both multi-layer graphene / 316L composite materials with better performance. The use of such materials makes the heat exchange tube have greater pressure bearing capacity and better resistance to temperature difference, further reducing the probability of heat exchange tube rupture accidents. At the same time, its heat conduction rate is faster and the thermal conductivity is higher, thereby effectively improving the thermal safety of the reactor.

[0026] The inner wall of the inner tube 2 is coated with a first graphene nano anti-corrosion coating 6 and the outer wall of the outer tube 4 is coated with a second graphene nano anti-corrosion coating 7. Such coatings effectively avoid the corrosion of the heat exchange tube material caused by the contact between the inner wall of the inner tube 2 and the secondary circuit coolant 1 and the outer wall of the outer tube 4 and the primary circuit coolant 5, and play a good shielding role for the coolant, while further reducing the probability of heat exchange tube rupture accidents.

[0027] The graphene nanofluid injected into the gap 3 between the inner tube 2 and the outer tube 4 greatly reduces the original air gap heat transfer resistance, effectively strengthens the convective heat transfer, and further improves the economy of the reactor.

[0028] The graphene nanofluid is made of liquid gallium as the base liquid and a new carbon nanotube material. Due to the low melting point, high boiling point and high thermal conductivity of metal gallium, it can be seen that using gallium as the base liquid of graphene nanofluid will not produce boiling phenomenon due to high temperature, and is suitable for liquid metal cooling reactors. In addition to meeting the required operating temperature range, it also ensures compatibility with the heat exchange tube wall material.

[0029] In the design of heat exchange tubes, the amount of graphene added must be strictly controlled. When preparing multi-layer graphene / 316L composite materials by laser deposition additive manufacturing, if too much graphene is added, it will be counterproductive. Compared with 316L steel, its various properties will be greatly reduced, which is further not conducive to improving the safety and economy of the reactor.

[0030] In the design of heat exchange tubes, the proportion of various materials used to prepare graphene nano anti-corrosion coatings must be strictly controlled. Graphene nano anti-corrosion coatings are two-component anti-corrosion coatings made of special resins, graphene, functional pigments, fillers, additives, solvents and other materials. For different applications, the proportion of each material is completely different. For the primary circuit coolant 5 side with strong corrosion ability and high temperature, the proportion of each material should be appropriately increased.

[0031] In the design of heat exchange tubes, within the range allowed by the working fluid, the width of gap 3 should be processed to the minimum as much as possible, so that the flow molecular effect of the graphene nanofluid at gap 3 is more obvious, further strengthening the convective heat transfer. However, it should also be noted that if gap 3 is too small, it will not be possible to prevent the cracks in any tube from extending to another tube. If gap 3 is too large, it will be unfavorable for heat transfer. Therefore, the average value is generally 10um-35um.

[0032] In the design of heat exchange tubes, due to the influence of the surface roughness of the double-layer tube wall, no matter how tightly the inner and outer layers of the heat exchange tubes are in contact, they are actually only in contact at some discrete points, and the gap 3 where there is no contact is filled with air. The gap 3 between the inner tube 2 and the outer tube 4 should be completely sealed by the upper tube sheet and the lower tube sheet, and the gap 3 should be evacuated before sealing.

[0033] In the design of the heat exchange tube, the horizontal uniformity of heat conduction should be ensured, while avoiding the situation where the temperature in local areas is too high. Therefore, attention should be paid to the uniformity and integrity of the first graphene nano anti-corrosion coating 6 coated on the inner wall of the inner tube 2 and the second graphene nano anti-corrosion coating 7 coated on the outer wall of the outer tube 4.

[0034] Taking the double-layer straight tube heat exchanger of a lead-bismuth cooled reactor as an example, the main technical parameters and related operating conditions of the heat exchanger tube are explained below.

[0035] The thermal power of the entire reactor is 10MW. The designed double-layer straight tube heat exchange tube of the liquid metal cooled reactor consists of an inner tube 2 of a multilayer graphene / 316L composite material doped with graphene for entry and exit of a secondary circuit coolant 1, an outer tube 4 of a multilayer graphene / 316L composite material doped with graphene tightly combined with the inner tube 2, and a gap 3 between the inner tube 2 and the outer tube 4, wherein the inner wall of the inner tube 2 is coated with a first graphene nano anti-corrosion coating 6 and the outer wall of the outer tube 4 is coated with a second graphene nano anti-corrosion coating 7, and the gap 3 between the two tubes is injected with gallium-based graphene nanofluid.

[0036] In order to better perform quantitative analysis, the heat exchange tube model of the present invention is simplified here:

[0037] Assume that there is no axial heat transfer and the heat is only transferred uniformly in the radial direction;

[0038] It is assumed that only heat conduction and heat convection exist in the heat transfer process, and heat radiation is ignored;

[0039] Assume that the lead-bismuth alloy as the primary coolant 5 and the pressurized water as the secondary coolant 1 are kept at a constant temperature;

[0040] Assume that the inner tube 2 and the outer tube 4 are smooth, and the gap 3 is d;

[0041] Since the gap 3 can be ignored relative to the radius of the double-layer straight heat exchange tube, it is assumed that the inner diameter of the inner tube 2 is d3, the outer diameter of the inner tube 2 is d2, the inner diameter of the outer tube 4 is d2, and the outer diameter of the outer tube 4 is d1;

[0042] Assume that the thickness of the graphene nano anti-corrosion coating 6 on the inner wall of the inner tube 2 and the graphene nano anti-corrosion coating 7 on the outer wall of the outer tube 4 are both δ;

[0043] Assume that the length of the heat exchange tube is l;

[0044] The thermal conductivity of the multilayer graphene / 316L composite material can be measured by laser flash-Raman spectroscopy. Here, the thermal conductivity of the multilayer graphene / 316L composite material is replaced by the thermal conductivity of 316L steel and is assumed to be λ1;

[0045] The thermal conductivity of graphene nano anti-corrosion coating is assumed to be λ2;

[0046] Assume that the convective surface heat transfer coefficient of gallium-based graphene nanofluid (graphene mass fraction is 0.05%) is h1;

[0047] Assume that the convective surface heat transfer coefficient of air is h2;

[0048] Assume that the convective surface heat transfer coefficient of pressurized water is h3;

[0049] Assume that the convective surface heat transfer coefficient of the lead-bismuth alloy is h4;

[0050] Assume the overall heat transfer coefficient is k;

[0051] Assume that the above parameters are constant and do not change at any time;

[0052] For the existing double-layer tube, the gap 3 is air and there is no graphene nano anti-corrosion coating on the inner wall of the inner tube 2 and the outer wall of the outer tube 4. This problem is a one-dimensional problem, using cylindrical coordinates, and the total thermal resistance of heat transfer is:

[0053]

[0054] For the present invention, the gap 3 is a gallium-based graphene nanofluid and the inner wall of the inner tube 2 is covered with a graphene nano-anticorrosion coating 6 and the outer wall of the outer tube 4 is covered with a graphene nano-anticorrosion coating 7. The problem is still a one-dimensional problem. Using cylindrical coordinates, the total thermal resistance of heat transfer is:

[0055]

[0056] The specific values ​​of the relevant parameters are shown in the following table:

[0057]

[0058] Substituting the specific values ​​of the above parameters into R1 and R2, we can get:

[0059] R1≈442.1K / W R2≈7.0K / W

[0060] It can be seen that compared with the existing double-layer tube, the barrier effect of the present invention on heat transfer is greatly reduced, which is more conducive to heat transfer.

[0061] In summary, the present invention uses double-layer straight tubes, multi-layer graphene / 316L composite materials and graphene nano anti-corrosion coatings that are tightly fitted together, which greatly reduces the probability of heat exchange tube rupture accidents from the design point of view, thereby effectively improving thermal safety. At the same time, the present invention greatly reduces the heat transfer resistance of the gap at the tightly fitted part by injecting graphene nanofluid into the gap between the two tubes, and strengthens the convective heat transfer, thereby efficiently extracting the fission heat of the reactor core, further bringing greater economic benefits. The parts not elaborated in detail in the present invention belong to the known technology in the field.

[0062] Although the above describes an illustrative specific embodiment of the present invention so that those skilled in the art can understand the present invention, it should be clear that the present invention is not limited to the scope of the specific embodiment. For those of ordinary skill in the art, as long as various changes are within the spirit and scope of the present invention as defined and determined by the attached claims, these changes are obvious, and all inventions and creations using the concept of the present invention are protected.

Claims

1. A double-layer straight heat exchange tube for a liquid metal cooled reactor, characterized in that: The double-layer straight heat exchange tube comprises an inner tube (2) of a multi-layer graphene / 316L composite material doped with graphene for the entry and exit of a secondary circuit coolant (1), an outer tube (4) of a multi-layer graphene / 316L composite material doped with graphene tightly bonded to the inner tube (2), and a gap (3) between the inner tube (2) and the outer tube (4); wherein the inner wall of the inner tube (2) is coated with a first graphene nano-anticorrosion coating (6) and the outer wall of the outer tube (4) is coated with a second graphene nano-anticorrosion coating (7), and the gap (3) is injected with a gallium-based graphene nanofluid; Under normal operating conditions of the reactor, the first graphene nano anti-corrosion coating coated on the inner wall of the inner tube is in direct contact with the secondary circuit coolant, and the second graphene nano anti-corrosion coating coated on the outer wall of the outer tube is in direct contact with the primary circuit coolant; the primary circuit coolant flows from top to bottom in the shell side under the action of the pump, and the secondary circuit coolant flows from bottom to top in the tube side, forming a parallel countercurrent heat exchange mode; due to the temperature difference between the secondary circuit coolant in the inner tube and the primary circuit coolant outside the outer tube, the primary circuit coolant absorbs the heat released by nuclear fission when passing through the core, and passes through the second graphene nano anti-corrosion coating coated on the outer wall of the outer tube, the outer tube, the gap injected with gallium-based graphene nanofluid, the inner tube, and the first graphene nano anti-corrosion coating coated on the inner wall of the inner tube in sequence, and finally transfers the heat to the secondary circuit coolant; The inner tube (2) and the outer tube (4) are tightly fitted together by mechanical force and are both straight tubes.

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