A natural circulation simulation test device and method for a megawatt small-sized liquid metal cooled reactor

Abstract

This invention discloses a megawatt-class small liquid metal-cooled reactor natural circulation simulation test device and method. The test device simulates fuel rods with a total power output in the MW range, realistically simulating the high-power operation of a small natural circulation liquid metal reactor. A vacuum insulation layer covering the heating section of the simulated fuel rods is installed between the rising channel of the hot pool and the descending channel of the cold pool to block lateral heat conduction and eliminate the "thermal short circuit" problem. A three-loop coupled cooling design of liquid metal-heat transfer oil-water is adopted, ensuring high safety. Furthermore, an air lift auxiliary device and a heat tracing device are integrated. The former effectively breaks the fluid stagnation during the initial startup phase through gas-liquid two-phase disturbance to assist in establishing natural circulation, while the latter ensures that the working fluid remains in a molten state during non-operational or low-power conditions, preventing equipment damage due to working fluid solidification. This invention can accurately simulate the real natural circulation characteristics of a small liquid metal-cooled reactor, combining high precision, safety, and operational reliability.

Description

Technical Field

[0001] This invention relates to a nuclear reactor thermal-hydraulic test apparatus and method, specifically to a simulation test apparatus and method for studying the natural circulation characteristics of a liquid metal cooled fast reactor (LMFR) core. Background Technology

[0002] With the continuous innovation of nuclear energy technology, small modular reactors (SMRs) with inherent safety have become an important trend in nuclear energy development. Among them, liquid metal-cooled fast reactors use liquid lead, lead-bismuth alloys, or sodium as coolants. Due to their excellent neutronics properties, chemical stability, and extremely high boiling point, they are considered one of the preferred reactor types for fourth-generation nuclear energy systems.

[0003] Thanks to the significant coefficient of thermal expansion and excellent thermal conductivity of liquid metal coolants, small lead-cooled fast reactors are typically designed with a fully natural circulation cooling mode. This design eliminates active equipment such as mechanical pumps in the main loop, avoiding not only the corrosion and wear of mechanical pumps at high temperatures and the manufacturing challenges, but also endowing the reactor with excellent passive safety characteristics. However, the driving force of the fully natural circulation depends entirely on the density difference between the hot and cold fluids, and its flow and heat transfer processes are extremely sensitive to system boundary conditions. Compared with the more mature forced circulation technology, the thermal-hydraulic phenomena involved in liquid metal natural circulation are more complex, urgently requiring mechanistic research and design verification through high-precision, high-power test benches.

[0004] However, existing testing techniques have the following four technical problems:

[0005] (1) Insufficient simulated power leads to distortion of thermal parameters

[0006] Existing liquid metal test rigs are mostly small-scale proof-of-concept devices with typically low heating power (only in the kW range), far below the core power density of real small modular reactors (MW range). This insufficient power directly prevents the test rig from replicating the high heat flux density conditions of a real reactor, resulting in weak driving head and low flow velocity during natural circulation. The natural circulation flow within the vessel remains in the laminar or transitional flow region, failing to simulate the fully developed turbulent heat transfer characteristics of a real reactor core, thus rendering the test data lacking in engineering reference value.

[0007] (2) The test body structure caused the "thermal short circuit" phenomenon.

[0008] In existing monolithic liquid metal test rigs, the rising channel of the hot pool (heat source) and the descending channel of the cold pool (cold source) are typically physically separated only by a simple single-layer metal wall. Due to the extremely high thermal conductivity of liquid metal, heat is easily transferred laterally from the rising channel to the descending channel through the metal wall. This unintended "thermal short-circuit" phenomenon leads to a decrease in fluid temperature in the rising section and an increase in fluid temperature in the descending section, significantly weakening the effective driving head of natural circulation. This results in distorted measured flow rates and heat transfer coefficients, severely impacting the reliability and accuracy of the test data.

[0009] (3) It is difficult and unstable to establish a simulated natural cycle.

[0010] In a fully natural circulation mode, the initial circulation phase presents a significant challenge. Because the temperature difference within the system has not yet been established in the early stages of circulation, the density-driven force is extremely weak, and the fluid often remains stagnant or randomly pulsating. Existing experimental setups lack effective auxiliary start-up methods; simply relying on slow heating with simulated fuel rods easily leads to localized overheating and overall flow stagnation, making it difficult to establish a stable, unidirectional, density-difference-driven natural circulation flow. This results in low experimental success rates and an uncontrollable circulation process.

[0011] (4) Safety risks of direct water cooling

[0012] Traditional cooling systems often use water to directly cool high-temperature liquid metal. To prevent the cooling water from boiling at high temperatures and deteriorating heat transfer, the water circuit usually needs to be maintained at a high pressure. If the heat exchange tubes rupture or leak, the high-pressure water instantly injected into the high-temperature liquid metal can trigger not only violent thermal interactions (such as lead-water reactions) but also severe steam explosions due to the sudden pressure drop and volume expansion, posing a significant safety hazard. Furthermore, the introduction of a high-pressure water system increases the pressure resistance requirements for the main vessel and heat exchanger walls, increasing manufacturing costs.

[0013] In summary, problem (1) insufficient power and problem (2) "thermal short circuit" directly lead to the distortion of natural circulation driving force and the deviation of measurement data, which are the core technical barriers that hinder the accurate simulation of the real thermal-hydraulic characteristics of small liquid metal stacks; while problem (3) start-up delay and problem (4) high-pressure water cooling risk seriously restrict the operational reliability and inherent safety of the test device, which are engineering problems that urgently need to be optimized to achieve large-scale and long-term stable testing.

[0014] Therefore, there is an urgent need to develop a new type of small-scale liquid metal cooled reactor natural circulation simulation test device, which can first overcome power limitations and provide accurate adiabatic boundaries to achieve high-fidelity simulation; on this basis, it can further provide a comprehensive solution with high safety redundancy and controllable start-up. Summary of the Invention

[0015] Purpose of the invention: The purpose of this invention is to address the core problems of existing liquid metal natural circulation test benches, such as insufficient heating power leading to simulation distortion and severe "thermal short circuit" interference causing inaccurate data. It provides a small-scale liquid metal cooled reactor natural circulation simulation test device and method with MW-level high-power heating capability and clear adiabatic boundaries, so as to achieve high-precision and high-fidelity simulation of the natural circulation characteristics of real reactor core under high heat flux density.

[0016] Technical Solution: This invention provides a megawatt-class small liquid metal cooled reactor natural circulation simulation test device, comprising a vertically arranged main container shell containing liquid metal and a main heat exchanger located at the top of the main container shell. A flow-blocking wall is coaxially arranged inside the main container shell, forming a hot pool rising channel located inside the flow-blocking wall and a cold pool descending channel located outside the flow-blocking wall. The upper ends of the hot pool rising channel and the cold pool descending channel are connected through the primary side of the main heat exchanger, and their lower ends are connected through flow holes circumferentially opened at the bottom of the flow-blocking wall. The secondary side of the main heat exchanger is connected to the cooling working fluid; the inner side of the flow-blocking wall is located... A sleeve and a vacuum insulation layer are installed above the flow passage, with the vacuum insulation layer located between the sleeve and the flow barrier wall. A core simulation test section is located at the bottom of the main vessel shell, which includes several simulated fuel rods with a total power of MW. Each simulated fuel rod passes upward through the sleeve, and the axial height of the vacuum insulation layer covers the heating section of the simulated fuel rod to block the lateral heat exchange between the high-temperature liquid metal in the rising channel of the hot pool and the low-temperature liquid metal in the descending channel of the cold pool in the heating section region. The power leads at the bottom of each simulated fuel rod are integrated and connected to an external conductive nickel plate for current collection. A thermal insulation and heat tracing device is installed on the outside of the main vessel shell.

[0017] Furthermore, a positioning grid for positioning each simulated fuel rod is fixed on the inner wall of the flow barrier. The positioning grid is located above the sleeve. The annular cavity formed by the positioning grid, the sleeve, and the flow barrier is sealed at the bottom and evacuated to form a vacuum insulation layer.

[0018] Furthermore, the heat tracing device includes a high-temperature resistant heat tracing cable and an insulation layer wound around the outer wall of the main container.

[0019] Furthermore, several simulated fuel rods are sealed at the bottom of the main vessel body by a bottom flange.

[0020] Furthermore, the megawatt-class small liquid metal cooled reactor natural circulation simulation test device also includes a gas lift auxiliary device, which is used to inject preheated pressurized inert gas into the main container to provide auxiliary driving force for the establishment of natural circulation.

[0021] Furthermore, the gas lift auxiliary device includes an argon cylinder group, an argon booster pump, and a gas heating tank connected in sequence. The gas heating tank is wrapped with a heat tracing cable. The gas heating tank is connected to the gas lift auxiliary device inlet pipe on the side wall of the main container, and the gas lift auxiliary device inlet pipe is connected to the cold pool descent channel.

[0022] To address the difficulties or delays in establishing natural circulation during the initial cold start-up of liquid metal reactors due to small temperature differences and insufficient density driving force, this invention integrates a gas lift auxiliary device. This device injects preheated high-pressure inert gas (e.g., argon) into the main container, utilizing the buoyancy effect of the gas in the liquid metal to artificially create a density difference between the gas-liquid two-phase mixture and the single-phase fluid, thereby providing additional auxiliary driving head. This design not only shortens the natural circulation establishment time but also maintains the stability of the system flow rate under low-power conditions, greatly improving the operational flexibility and controllability of the experimental setup. Furthermore, the preheated inert gas avoids the risk of localized solidification of the liquid metal caused by direct injection of cold gas.

[0023] Furthermore, the megawatt-class small liquid metal cooled reactor natural circulation simulation test device also includes a heat transfer oil intermediate cooling circuit and a water cooling circuit. The heat transfer oil intermediate cooling circuit uses heat transfer oil as the cooling medium to remove heat from the liquid metal in the main heat exchanger. The water cooling circuit has an oil-water intermediate heat exchanger and a cooling tower assembly. Water exchanges heat with heat transfer oil in the oil-water intermediate heat exchanger to absorb heat and releases heat in the cooling tower assembly.

[0024] This invention employs a three-loop coupled heat exchange design of "liquid metal-heat transfer oil-water". The secondary loop utilizes chemically stable heat transfer oil as an intermediate medium, physically isolating the high-temperature liquid metal in the main loop from the cooling water in the tertiary loop. This fundamentally eliminates the risk of steam explosion or violent chemical reactions caused by contact between the high-temperature liquid metal and water due to heat exchange tube rupture. Compared to water-cooled loops that require maintaining high pressure to prevent boiling, heat transfer oil has a high boiling point, reaching over 300°C at normal pressure, allowing the secondary loop to operate at low pressure or even normal pressure (e.g., below 0.3 MPa). This significantly reduces the leakage risk of the secondary loop system and the manufacturing cost of pressure-bearing equipment, while also avoiding the additional safety hazards caused by high-pressure liquid jetting. Furthermore, thanks to the wide operating temperature range of the heat transfer oil, this invention can easily achieve a wide range of adjustment of test conditions by adjusting the inlet temperature and flow rate of the secondary loop, making it suitable for testing high-melting-point pure lead as well as low-melting-point working fluids such as lead-bismuth alloys.

[0025] The test method of the megawatt-class small liquid metal cooled reactor natural circulation simulation test device of the present invention includes:

[0026] S1: Turn on the heat preservation and heat tracing device to preheat the main container cylinder, so that the temperature of the main container cylinder and its interior is higher than the melting point of liquid metal, and then inject liquid metal into the main container cylinder to the predetermined liquid level.

[0027] S2: Start the intermediate cooling circuit of the heat transfer oil and the water cooling circuit, adjust the circulation flow of the heat transfer oil and water, and establish a stable secondary side heat dissipation boundary; turn on the simulated fuel rod and increase the heating power according to the preset power curve, so that the liquid metal is heated to generate a density difference, so as to establish natural circulation;

[0028] S3: After the natural circulation is successfully established, various thermal and hydraulic parameters are collected using pre-set sensors.

[0029] Furthermore, in step S2, if the natural circulation is delayed, the gas lift auxiliary device is activated to inject preheated pressurized inert gas into the main container to assist in establishing the natural circulation; the inert gas is preheated to a temperature higher than the melting point of the liquid metal to prevent local solidification of the liquid metal near the injection point; after the liquid metal forms a stable unidirectional flow, the gas lift auxiliary device is gradually shut off, and the natural circulation is maintained by pure thermal driving force.

[0030] Furthermore, in step S2, when adjusting the power of the simulated fuel rod, a stepped power increase method is adopted to reduce the impact of thermal stress on the simulated fuel rod.

[0031] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0032] (1) It has the ability to simulate high power at the MW level and reproduce the thermal characteristics of real liquid metal stacks.

[0033] To address the issue of distorted thermal parameter simulations caused by insufficient heating power in existing liquid metal test devices, this invention achieves MW-level high-power heating output in the reactor core simulation test section by employing specially designed high-heat-load simulated fuel rods and a conductive manifold structure. This design enables the test device to generate a high heat flux density comparable to that of a real small modular liquid metal reactor, thereby generating a sufficiently strong buoyancy driving force to propel the liquid metal into a fully developed turbulent state in the natural circulation loop (rather than laminar or transitional flow at low power). This not only realistically replicates the flow and heat transfer characteristics in a real reactor but also obtains turbulent heat transfer coefficients and flow resistance characteristics that are more consistent with engineering realities, significantly improving the reference value and verification confidence of the test data for reactor design.

[0034] (2) Eliminate "thermal short circuit" and significantly improve test accuracy.

[0035] A vacuum insulation layer is installed between the rising channel of the hot pool and the descending channel of the cold pool. This layer covers the critical area from the hot pool inlet to the top of the simulated fuel rod heating section. The high thermal resistance barrier formed by the vacuum effectively blocks lateral heat exchange between the rising fluid in the hot pool and the descending fluid in the cold pool. This design ensures that the driving head of the natural circulation is entirely derived from the axial temperature difference of the fluid, thereby greatly improving the measurement accuracy of the natural circulation driving force and heat transfer data, and providing high-quality benchmark data for the verification and calibration of relevant theoretical models.

[0036] (3) A complete heat tracing and insulation system ensures the integrity of system operation.

[0037] To address the tendency of liquid metal to solidify, this invention incorporates a comprehensive heat tracing and insulation system outside the main container shell. This system maintains the main container shell temperature above the melting point of the liquid metal working fluid, even during low-power simulated fuel rod tests or non-operational conditions. This effectively prevents mechanical damage to the main container structure, welds, and system equipment caused by accidental solidification of the liquid metal, ensuring the long-term reliable operation of the testing apparatus. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the main container structure in an embodiment of the present invention;

[0039] Figure 2 This is a schematic diagram of the arrangement of the heat tracing and insulation device on the outside of the main container body in an embodiment of the present invention;

[0040] Figure 3 This is a schematic diagram of the air lift auxiliary device in an embodiment of the present invention;

[0041] Figure 4 This is a schematic diagram of the three thermally coupled circuit structures in an embodiment of the present invention. Detailed Implementation

[0042] The invention will now be further described with reference to the accompanying drawings.

[0043] Appendix Figures 1 to 4 The accompanying figure labels are as follows:

[0044] 1. Main vessel shell; 2. Insulation layer; 3. Main vessel inlet pipe; 4. Main vessel outlet pipe; 5. Gas lift auxiliary device inlet pipe; 6. Flow barrier wall; 7. Hot pool riser channel; 8. Cold pool descender channel; 9. Main heat exchanger; 10. Core simulation test section; 11. Simulated fuel rods; 12. Conductive nickel plate; 13. Positioning grid; 14. Bottom flange; 15. Metal sealing ferrule; 16. Sleeve; 17. Vacuum insulation layer; 18. Heat exchanger tube bundle; 19. 20. Heat exchanger secondary side inlet pipe; 21. Heat exchanger secondary side outlet pipe; 22. Argon cylinder group; 23. Argon booster pump; 24. Electric valve; 25. Gas heating tank; 26. Heat tracing cable; 27. Gas lift auxiliary device outlet pipe; 28. Secondary loop flow meter; 29. ​​High-level oil expansion tank; 30. Secondary loop regulating valve; 31. Heat transfer oil circulation pump; 32. Oil-water intermediate heat exchanger; 33. Tertiary loop flow meter; 34. Tertiary loop regulating valve; 35. Cooling tower assembly.

[0045] Example 1: As Figures 1 to 4 As shown, Example 1 provides a megawatt-class small liquid metal cooled reactor natural circulation simulation test device, including three thermally coupled loops: liquid metal natural circulation main loop (first loop), heat transfer oil intermediate cooling loop (second loop), and water cooling loop (third loop).

[0046] I. Liquid Metal Natural Circulation Main Circuit

[0047] The liquid metal natural circulation main loop is the core component of this invention, used to simulate the thermal-hydraulic phenomena within a reactor. Its main design parameters are: design pressure 1.0 MPa, design temperature 600°C; normal operating conditions are atmospheric pressure to 1.0 MPa, and temperature range 350~600°C. In this embodiment, liquid lead is used as the simulated working fluid; in other embodiments, other heavy metal fluids such as lead-bismuth alloys can also be used.

[0048] The main structure of the liquid metal natural circulation main loop is a vertically positioned main container, with the main container body 1 made of high-temperature resistant stainless steel. The bottom side of the main container body 1 is equipped with a main container inlet pipe 3 and a main container outlet pipe 4. These two pipes are connected to an external lead melting tank and storage / discharge tank, respectively, and are used only for filling the liquid lead working medium before the test and emptying the liquid lead after the test. During the natural circulation test operation, these two pipes are kept in the closed valve state.

[0049] The main container cylinder 1 is coaxially equipped with a flow-blocking wall 6, which divides the internal space of the main container cylinder 1 into a hot pool region and a cold pool region from the inside to the outside. Specifically, the central flow channel inside the flow-blocking wall 6 constitutes the hot pool region, where the fluid is heated and flows upward along the hot pool rising channel 7; the annular space formed between the outer wall of the flow-blocking wall 6 and the inner wall of the main container cylinder 1 constitutes the cold pool region, where the fluid is cooled and flows downward along the cold pool descending channel 8.

[0050] The lower end of the hot pool rising channel 7 is connected to the lower end of the cold pool falling channel 8 through a flow passage circumferentially opened at the bottom of the flow-blocking wall 6. The upper end of the hot pool rising channel 7 is connected to the upper end of the cold pool falling channel 8 through the primary side of the main heat exchanger 9, thus forming a natural circulation loop inside the main container.

[0051] A core simulation test section 10 is located at the center of the bottom of the main vessel shell 1. This core simulation test section 10 is situated at the entrance of the hot pool riser channel 7 and is used to simulate the reactor core. The core simulation test section 10 consists of several simulated fuel rods 11 arranged in a hexagonal pattern; in this embodiment, 37 rods are preferred. To achieve realistic core power simulation, this invention employs a high-power-density electric heating design. With the support of specially designed high-heat-load simulated fuel rods 11 and an externally matched MW-level high-power DC power supply system, the total power of the core simulation test section 10 can reach the megawatt (MW) level; in this embodiment, the design value is 1.0 MW.

[0052] In terms of structural connection, to withstand high current and reduce contact resistance heating, the high-temperature resistant resistance wire and power lead at the bottom of each simulated fuel rod 11 extend downwards and are integrated and fixed to a high-conductivity conductive nickel plate 12 for current collection. The conductive nickel plate 12 is then connected to an external high-power adjustable DC power supply cabinet via large-section copper busbar leads. This integrated busbar structure effectively solves the heat dissipation and connection problems of high-current power supply for multiple rod bundles.

[0053] The upper part of the simulated fuel rod bundle 11 passes through the positioning grid 13 to maintain a hexagonal geometric spacing. The outer edge of the positioning grid 13 is fixedly connected to the lower inner side of the baffle wall 6. The bottom of the entire rod bundle is fixed to the bottom plate of the main vessel shell 1 via a bottom flange 14. A metal sealing sleeve 15 is provided at the flange connection to ensure no leakage of liquid lead under high temperature and pressure. The overall assembly design of the rod bundle facilitates disassembly and maintenance. In addition, in order to accurately measure the wall temperature under high heat flux density, multiple sets of armored thermocouples are pre-embedded in the outer shell wall of the simulated fuel rod 11, and their signal leads are sealed and exit from the bottom of the rod bundle.

[0054] The sleeve 16 is coaxially fitted onto the outside of the core simulation test section 10. The sleeve 16 is higher than the flow passage on the baffle wall 6, and its axial length covers the heating section of the simulated fuel rod 11. The sleeve 16, the positioning grid 13, and the baffle wall 6 form an annular cavity. After the bottom of this annular cavity is sealed and evacuated, a vacuum insulation layer 17 is formed. This vacuum insulation layer 17 effectively blocks the lateral heat conduction between the high-temperature liquid lead in the rising channel 7 of the hot pool and the low-temperature liquid lead in the descending channel 8 of the cold pool in the heating section region, eliminating the "thermal short circuit" phenomenon and ensuring the authenticity of the natural circulation driving force.

[0055] Four main heat exchangers 9 are configured and symmetrically suspended and fixed to the top of the main vessel shell 1. Each main heat exchanger 9 is a vertical shell-and-tube heat exchanger, comprising a shell and internal heat exchange tube bundles 18. The lower opening of the primary side (outer shell side of the heat exchange tube bundle 18) of the main heat exchanger 9 connects to the upper end of the cold pool descending channel 8, and its side opening connects to the upper end of the hot pool ascending channel 7. The working medium flowing on the primary side is liquid lead from the hot pool inside the main vessel. The secondary side (inner tube side of the heat exchange tube bundle 18) of the main heat exchanger 9 is equipped with a secondary side inlet pipe 19 and a secondary side outlet pipe 20. The working medium flowing on the secondary side is a cooling medium (heat transfer oil) from the external secondary loop.

[0056] During the natural circulation test run, the liquid lead working fluid is sealed within the main container cylinder 1. The working principle is as follows:

[0057] Liquid lead is heated by simulated fuel rods 11 at the core simulation test section 10 at the bottom of the main container 1, causing its temperature to rise and its density to decrease. Under the action of buoyancy, it flows upward along the hot pool rising channel 7. Subsequently, the high-temperature liquid lead enters the primary side of the main heat exchanger 9 and fully exchanges heat with the secondary loop heat transfer oil flowing in the heat exchange tube bundle 18. After being cooled, the liquid lead's temperature decreases and its density increases. After flowing out of the main heat exchanger 9, it enters the annular cold pool descending channel 8 and sinks downward under the action of gravity. Finally, the low-temperature liquid lead flows back to the bottom of the main container and re-enters the core simulation test section 10 for heating. Due to the presence of the vacuum insulation layer 17, there is no heat exchange between the rising and descending fluids. The stable and continuous natural circulation of single-phase liquid metal within the main container is formed entirely by the huge fluid density difference generated by the core heating and heat exchanger cooling.

[0058] like Figure 2As shown, to prevent liquid lead from solidifying during low-power testing or non-operational conditions, the outer surface of the main container cylinder 1 is covered with a heat tracing and insulation device. The heat tracing and insulation device includes a high-temperature resistant heat tracing cable tightly wound around the outer wall of the main container cylinder 1 and an insulation layer 2. In this embodiment, a total of nine cables, each 40 meters long, are arranged, with a line power density of 120W / m, using a 380V three-phase power supply (star connection). The insulation layer 2 uses aluminum silicate fiber insulation cotton of appropriate thickness (e.g., 300mm) to reduce the loss of high-temperature heat to the environment.

[0059] like Figure 3 As shown, in order to solve the problem of insufficient system driving force in the initial stage of natural circulation startup, and in order to study the effect of air lift effect on the flow characteristics of liquid lead, the experimental device of the present invention also integrates a backup air lift auxiliary device.

[0060] In the gas supply section, the outlet of argon cylinder group 21 is connected to argon booster pump 22 to pressurize the argon gas to a hydrostatic pressure higher than the corresponding depth of the main container. An electric valve 23 is installed on the pressurized argon gas pipeline to control the on / off state of the gas path and regulate the flow rate. The pipeline is then connected to a gas heating tank 24, which is externally wrapped with a heat tracing cable 25 to preheat the argon gas to a temperature higher than the liquid metal in the main container.

[0061] On the main container side, such as Figure 1 As shown, two gas lift auxiliary device inlet pipes 5 are also provided on the side of the main vessel shell 1, which are connected to the cold pool descent channel 8. These two pipes are distributed vertically in the axial direction: one is located in the lower part of the main vessel (near the core simulation test section 10), and the other is located in the upper part of the main vessel (near the primary side outlet area of ​​the main heat exchanger 9). The two gas lift auxiliary device outlet pipes 26 on the gas heating tank 24 are respectively connected to the two gas lift auxiliary device inlet pipes 5.

[0062] The injected high-temperature, high-pressure argon gas forms bubbles in the cold pool descending channel 8 and rises upward, generating strong gas-liquid two-phase disturbances. This dual-site gas injection and disturbance can quickly break the stagnation and thermal stratification of the liquid metal in the cold pool during the initial startup phase, prompting the fluid inside the main vessel to generate initial macroscopic convection and flow field pulsation. Combined with the heating of the core simulation test section 10, under the action of the initial flow field induced by the gas lift disturbance, the heat generated by the core can be carried out more quickly, prompting the rapid establishment of a temperature difference between the hot pool and the cold pool, thereby greatly accelerating the formation of natural circulation driving force. When the natural circulation flow rate of the main loop tends to stabilize and can be completely self-sustaining by thermal driving force, the electric valve 23 and the argon booster pump 22 are gradually closed, the gas lift circulation assistance is withdrawn, and after the argon gas bubbles in the loop are exhausted, the system smoothly transitions to a pure single-phase thermally driven natural circulation mode.

[0063] It should be noted that this air lift auxiliary device serves as a backup and disturbance means for natural circulation startup. It is only activated when the initial resistance cannot be overcome and circulation needs to be established by relying solely on thermal driving force, or when specific two-phase flow disturbance tests need to be conducted, in order to avoid unnecessary gas injection interfering with the pure single-phase flow thermal-hydraulic test data.

[0064] II. Intermediate cooling circuit for heat transfer oil and water cooling circuit

[0065] like Figure 4 As shown, in order to ensure the effective removal of heat from the main circuit's natural circulation and to guarantee experimental safety, the experimental device of this invention also integrates an intermediate heat transfer oil cooling circuit (secondary circuit) and a water cooling circuit (tertiary circuit).

[0066] The secondary loop, serving as an intermediate isolation and heat transfer system, utilizes heat transfer oil as the working fluid. It is responsible for carrying away the heat from the liquid lead in the main loop through the main heat exchanger 9 and transferring it to the tertiary loop. Unlike traditional high-pressure water cooling, the high boiling point of the heat transfer oil enables this system to operate safely at low pressure. Its main design parameters are: design pressure 0.3 MPa, design temperature 320℃; normal operating conditions are atmospheric pressure to 0.3 MPa, temperature range 200~320℃.

[0067] The second loop adopts a mechanically driven forced loop mode, and its loop path and component functions are as follows:

[0068] Powered by the heat transfer oil circulation pump 30, the low-temperature heat transfer oil enters the secondary side of the main heat exchanger 9 to absorb heat. The high-temperature heat transfer oil after heat absorption is then drawn out and flows sequentially through the secondary loop flow meter 27 and the high-level oil expansion tank 28 on the pipeline. The secondary loop flow meter 27 is used to monitor the mass flow rate of the heat transfer oil in real time, which is convenient for heat balance calculation. The high-level oil expansion tank 28 is set at the high point of the loop pipeline to accommodate the volume of the heat transfer oil that has expanded due to heat, maintain system pressure stability, and prevent pump cavitation.

[0069] Subsequently, the high-temperature heat transfer oil enters the hot side of the oil-water intermediate heat exchanger 31 to release heat. After being cooled, the heat transfer oil flows through the secondary loop regulating valve 29 and returns to the inlet of the heat transfer oil circulation pump 30, forming a closed loop. The secondary loop regulating valve 29 is an electrically operated regulating valve used to precisely control the circulation flow rate of the secondary loop, thereby adjusting the overall heat exchange power of the system.

[0070] The third loop serves as the final heat dissipation system, using water as the working fluid to ultimately discharge the heat transferred from the second loop to the atmospheric heat sink. Its main design parameters are: design pressure 0.3 MPa, and inlet water temperature range 5~40℃.

[0071] The third loop also adopts a forced circulation mode. The cooling water is driven by a circulating water pump integrated in the cooling tower assembly 34 and flows through the cold side of the oil-water intermediate heat exchanger 31. The oil-water intermediate heat exchanger 31 is a key thermal coupling device connecting the second and third loops, realizing physical isolation heat exchange between the two working fluids, oil and water.

[0072] After absorbing heat from the heat transfer oil in the oil-water intermediate heat exchanger 31, the cooling water's temperature rises. It then flows through the three-loop flow meter 32 and the three-loop regulating valve 33, finally returning to the outdoor cooling tower assembly 34. There, the heat is dissipated to the atmosphere through air cooling and water evaporation. The cooled water is then pumped back for reuse. The three-loop flow meter 32 and the three-loop regulating valve 33 work together to monitor and regulate the cooling water flow rate to control the system's final heat dissipation capacity.

[0073] Example 2: Example 2 provides a test method for the natural circulation simulation test device of the megawatt-class small liquid metal cooled reactor described in Example 1. Before implementation, the main loop environment needs to be established. Specifically, firstly, all external interface valves, such as the main loop inlet pipe 3 and the main container outlet pipe 4, are closed. The main container and pipelines are then evacuated until a predetermined vacuum level (e.g., 10⁻² Pa) is reached to remove impurity gases. Next, the external argon injection system is activated to refill the main loop with high-purity argon for purging. Then, the vacuum is evacuated again, and the replacement operation is repeated to thoroughly remove residual air and moisture, preventing oxidation of the high-temperature liquid lead. Finally, a slightly positive pressure argon gas (e.g., 0.12 MPa) is introduced into the gas phase space of the main container as a covering gas. After the main loop environment is established, the following test process is carried out:

[0074] S1: Working fluid charging and melting

[0075] (1) Turn on the heat preservation and heat tracing device on the outer wall of the main container cylinder 1, set the target temperature to 350℃ (higher than the melting point of lead 327℃), and preheat the main container and its internal components uniformly. The heating rate is controlled at, for example, 1℃ / min to prevent thermal stress damage.

[0076] (2) After the temperature of the main container wall stabilizes, the preheated liquid lead is pumped into the main container through the main container inlet pipe 3 via a mechanical lead pump until the liquid level monitoring device shows that the liquid level has reached the specified test liquid level height.

[0077] (3) Close the filling valve of the main container inlet pipe 3, keep the heat tracing device running, and keep the liquid lead in molten flow dynamic.

[0078] S2: Establishment of Natural Cycle

[0079] (1) Start the heat transfer oil circulation pump 30 of the second loop and the cooling tower assembly 34 of the third loop, adjust the regulating valve 29 of the second loop and the regulating valve 33 of the third loop to establish a stable secondary side heat dissipation boundary.

[0080] (2) Turn on the power control cabinet and supply power to the simulated fuel rods 11 of the core simulation test section 10. Increase the heating power according to the preset power curve. In the initial stage, use low power operation. As the fluid temperature difference is established, gradually increase the heating power in a stepwise manner.

[0081] (3) During the heating process, temperature field data is collected in real time through wall temperature thermocouples embedded in the surface of the simulated fuel rod 11 and fluid thermocouples arranged at different heights in the hot pool rising channel 7 and the cold pool descending channel 8; at the same time, differential pressure transmitters connected to the upper and lower pressure measuring ports of the main vessel 1 are used to acquire real-time data on the circulating drive pressure head and core pressure drop. Based on the real-time feedback from the above sensors, the natural circulation flow state is determined:

[0082] Scenario A (Normal Start-up): If the fluid thermocouple shows a stable upward trend in the temperature difference between the hot and cold pools, and the differential pressure transmitter shows a continuous and stable unidirectional circulation drive head (such as core pressure drop fluctuation within ±5Pa), it indicates that the fluid has formed a stable unidirectional natural circulation flow, and then directly enters the steady-state thermal parameter measurement stage.

[0083] Scenario B (Auxiliary Start-up): If, after heating for a certain period, the thermocouples at each height level show severe thermal stratification, and the differential pressure transmitter data exhibits irregular pulsations or stagnation, it indicates that the system's initial resistance is too high, causing a delay in establishing a pure thermal-driven natural circulation. In this case, actively activate the gas lift auxiliary device: start the argon booster pump 22, open the electric valve 23, and inject preheated high-temperature argon gas into the cold pool descending channel 8 through the gas lift auxiliary device inlet pipe 5. Utilize the strong gas-liquid two-phase disturbance generated by rising bubbles to break fluid stagnation and provide initial mechanical driving force to assist in "draining." After the thermocouple and differential pressure transmitter data show that the circulating flow field has been stably established, gradually shut down the gas lift auxiliary device, allow it to stand for a period of time until the argon bubbles in the main container are completely discharged from the liquid surface, and the system smoothly transitions to the pure thermal-driven natural circulation mode before measuring thermal parameters.

[0084] S3: Data Acquisition and Analysis

[0085] Once it is determined that the natural circulation has been successfully established, and the various thermal parameters of the system (including the inlet and outlet fluid temperatures, absolute pressures, and flow rates of the hot and cold pools) do not change over time within a specified period (e.g., 10 minutes), and the thermal balance error is within the allowable range, steady-state data is recorded and analyzed.

[0086] In summary, this invention addresses the challenges of realism, accuracy, and safety in small liquid metal reactor natural circulation simulation test devices. It constructs a comprehensive test platform comprising a main container structure with a vacuum insulation layer, a MW-level high-power heating unit, and a lead-oil-water three-loop coupling system, supplemented by an air-lift auxiliary device and a heat tracing device. Through the coordinated operation of the above-mentioned system, this invention successfully solves key technical problems existing in current test devices, such as power simulation distortion, delayed natural circulation start-up, "thermal short-circuit" interference, and the risk of lead-water reaction. This test device can realistically reproduce the entire process of a small liquid metal reactor from cold start-up, natural circulation establishment, to full-power operation, while ensuring safety. It fills the technical gap between existing kW-level test benches and real reactors, providing high-precision experimental data support and a technical verification platform for the design verification, system safety analysis validation, and passive safety characteristic research of future advanced small liquid metal cooled reactors.

Claims

1. A megawatt-class small liquid metal cooled reactor natural circulation simulation test device, characterized in that, The system includes a vertically arranged main container cylinder (1) containing liquid metal and a main heat exchanger (9) located at the top of the main container cylinder (1). A flow-blocking wall (6) is coaxially arranged inside the main container cylinder (1), forming a hot pool rising channel (7) inside the flow-blocking wall (6) and a cold pool descending channel (8) outside the flow-blocking wall (6). The upper ends of the hot pool rising channel (7) and the cold pool descending channel (8) are connected through the primary side of the main heat exchanger (9), and their lower ends are connected through a flow-through hole opened circumferentially at the bottom of the flow-blocking wall (6). The secondary side of the main heat exchanger (9) is connected to a cooling working fluid. A sleeve (16) and a vacuum insulation layer (17) are arranged on the inner side of the flow-blocking wall (6) above the flow-through hole. The heat insulation layer (17) is located between the sleeve (16) and the flow barrier wall (6); the bottom of the main container shell (1) is provided with a core simulation test section (10), which includes several simulated fuel rods (11) with a total power of MW; each simulated fuel rod (11) passes upward through the sleeve (16), and the axial height of the vacuum heat insulation layer (17) covers the heating section of the simulated fuel rod (11) to block the lateral heat exchange between the high temperature liquid metal of the hot pool rising channel (7) and the low temperature liquid metal of the cold pool descending channel (8) in the heating section area; the power lead at the bottom of each simulated fuel rod (11) is integrated and connected to the external conductive nickel plate (12) for current convergence; the outside of the main container shell (1) is provided with a heat preservation and heat tracing device.

2. The megawatt-class small liquid metal cooled reactor natural circulation simulation test device according to claim 1, characterized in that, The inner wall of the flow-blocking wall (6) is fixed with a positioning grid (13) for positioning each simulated fuel rod (11). The positioning grid (13) is located above the sleeve (16). The annular cavity formed by the positioning grid (13), the sleeve (16) and the flow-blocking wall (6) forms a vacuum insulation layer (17) after being sealed at the bottom and evacuated.

3. The megawatt-class small liquid metal cooled reactor natural circulation simulation test device according to claim 1, characterized in that, The heat tracing device includes a high-temperature heat tracing cable and a heat insulation layer (2) wrapped around the outer wall of the main container (1).

4. The megawatt-class small liquid metal cooled reactor natural circulation simulation test device according to claim 1, characterized in that, Several simulated fuel rods (11) are sealed and installed at the bottom of the main container body (1) by a bottom flange (14).

5. The megawatt-class small liquid metal cooled reactor natural circulation simulation test device according to any one of claims 1 to 4, characterized in that, It also includes an air lift auxiliary device for injecting preheated pressurized inert gas into the main container cylinder (1) to provide auxiliary driving force for the establishment of natural circulation.

6. The megawatt-class small liquid metal cooled reactor natural circulation simulation test device according to claim 5, characterized in that, The gas lift auxiliary device includes an argon cylinder group (21), an argon booster pump (22), and a gas heating tank (24) connected in sequence. The gas heating tank (24) is wrapped with a heat tracing cable (25). The gas heating tank (24) is connected to the gas lift auxiliary device inlet pipe (5) on the side wall of the main container (1). The gas lift auxiliary device inlet pipe (5) is connected to the cold pool descent channel (8).

7. The megawatt-class small liquid metal cooled reactor natural circulation simulation test device according to claim 5, characterized in that, It also includes a heat transfer oil intermediate cooling circuit and a water cooling circuit. The heat transfer oil intermediate cooling circuit uses heat transfer oil as the cooling medium and removes the heat of the liquid metal in the main heat exchanger (9). The water cooling circuit has an oil-water intermediate heat exchanger (31) and a cooling tower assembly (34). Water exchanges heat with heat transfer oil in the oil-water intermediate heat exchanger (31) to absorb heat and releases heat in the cooling tower assembly (34).

8. A test method for a natural circulation simulation test device for a megawatt-class small liquid metal cooled reactor as described in claim 7, characterized in that, include: S1: Turn on the heat preservation and heat tracing device to preheat the main container cylinder (1) so that the temperature of the main container cylinder (1) and its interior is higher than the melting point of liquid metal. Then, inject liquid metal into the main container cylinder (1) to the predetermined liquid level. S2: Start the intermediate cooling circuit of heat transfer oil and the water cooling circuit, adjust the circulation flow of heat transfer oil and water, and establish a stable secondary side heat dissipation boundary; turn on the simulated fuel rod (11) and increase the heating power according to the preset power curve, so that the liquid metal is heated to generate a density difference, so as to establish a natural circulation; S3: After the natural circulation is successfully established, various thermal and hydraulic parameters are collected using pre-set sensors.

9. The test method according to claim 8, characterized in that, In step S2, if the natural circulation starts slowly, the gas lift auxiliary device is turned on to inject preheated pressurized inert gas into the main container (1) to help establish the natural circulation; the inert gas is preheated to a temperature higher than the melting point of the liquid metal to prevent local solidification of the liquid metal near the injection point; after the liquid metal forms a stable unidirectional flow, the gas lift auxiliary device is gradually turned off, and the natural circulation is maintained by pure heat driving force.

10. The test method according to claim 8, characterized in that, In step S2, when adjusting the power of the simulated fuel rod (11), a stepped power increase method is adopted to reduce the impact of thermal stress on the simulated fuel rod (11).

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