Fuel cladding biaxial load test method under working condition of water loss accident

Abstract

The invention provides a fuel cladding biaxial load test method under a water loss accident condition. The method comprises the following steps: assembling and configuring a test section; starting the water loop system, and performing temperature and pressure rising treatment on the test section based on preset test conditions; radial and axial biaxial loads are applied to a simulation fuel rod based on the loading frame and the clamp, and the simulation fuel rod is installed in the test section; a high-temperature release valve is started to simulate a transient water loss accident, and the high-temperature release valve is arranged in the water loop system; carrying out a rapid heating test and an oxidation test on the simulated fuel rod in a high-temperature steam environment; performing a high-temperature quenching test on the simulated fuel rod under specific conditions to simulate a reactor core cooling process; and taking out the simulated fuel rod from the test section, and analyzing and testing the simulated fuel rod. By implementing the method disclosed by the invention, the performance verification and evaluation requirements of the fuel cladding under the water loss accident of the pressurized water reactor nuclear power station can be met.

Description

Technical Field

[0001] The present disclosure relates to the field of nuclear power testing technology, and in particular to a biaxial load test method for a fuel cladding under a loss of coolant accident condition. Background Art

[0002] The loss of coolant accident (LOCA) is a design basis accident for reactors. In the LOCA state, the fuel pellets in the cladding tube expand rapidly, and the fission gas generates a large internal pressure in the tube. At the same time, the cladding tube also generates a large axial stress in the axial direction due to bulging deformation, re-flooding shrinkage, transient thermal stress, etc. Therefore, the biaxial mechanics or multiaxial mechanics of the cladding tube is a stress state that must be considered. According to the latest international nuclear safety guidelines, it is necessary to carry out LOCA tests with loads.

[0003] The related technologies cannot meet the requirements for performance verification and evaluation of fuel cladding in a loss of coolant accident in a pressurized water reactor nuclear power plant. Summary of the invention

[0004] The present disclosure aims to solve one of the technical problems in the related art at least to some extent.

[0005] To this end, the purpose of the present disclosure is to propose a biaxial load test method for fuel cladding under loss of coolant accident conditions, thereby being able to meet the performance verification and evaluation requirements of the fuel cladding under a loss of coolant accident in a pressurized water reactor nuclear power plant.

[0006] To achieve the above-mentioned purpose, the first aspect of the present disclosure provides a biaxial load test method for fuel cladding under a loss of coolant accident condition, comprising:

[0007] Assemble and configure the test section;

[0008] Starting the water circuit system and performing temperature and pressure raising treatment on the test section based on preset test conditions;

[0009] Applying radial and axial biaxial loads to the simulated fuel rod based on the loading frame and the fixture, wherein the simulated fuel rod is installed in the test section;

[0010] Starting a high temperature relief valve to simulate a transient loss of coolant accident, wherein the high temperature relief valve is arranged in the water circuit system;

[0011] Conducting a rapid temperature rise test and an oxidation test on the simulated fuel rod in a high-temperature steam environment;

[0012] Performing a high temperature quenching test on the simulated fuel rod under specific conditions to simulate a core cooling process;

[0013] The simulated fuel rod is taken out from the test section, and the simulated fuel rod is analyzed and tested.

[0014] The present disclosure provides a biaxial load test method for fuel cladding under a loss of coolant accident condition, which comprises assembling and configuring a test section; starting a water circuit system, and subjecting the test section to temperature and pressure treatment based on preset test conditions; applying radial and axial biaxial loads to simulated fuel rods based on a loading frame and a fixture, wherein the simulated fuel rods are installed in the test section; starting a high-temperature relief valve to simulate a transient loss of coolant accident, wherein the high-temperature relief valve is configured in the water circuit system; conducting a rapid temperature rise test and an oxidation test on the simulated fuel rods in a high-temperature steam environment; conducting a high-temperature quenching test on the simulated fuel rods under specific conditions to simulate the core cooling process; taking out the simulated fuel rods from the test section, and analyzing and testing the simulated fuel rods. Thus, the performance verification and evaluation requirements of the fuel cladding under a loss of coolant accident in a pressurized water reactor nuclear power plant can be met.

[0015] Additional aspects and advantages of the present disclosure will be given in part in the following description and in part will be obvious from the following description or learned through practice of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The above and / or additional aspects and advantages of the present disclosure will become apparent and easily understood from the following description of the embodiments in conjunction with the accompanying drawings, in which:

[0017] Figure 1 It is a flow chart of a biaxial load test method for a fuel cladding under a loss of coolant accident condition proposed in one embodiment of the present disclosure;

[0018] Figure 2 It is a schematic diagram of a biaxial load test method for fuel cladding under a loss of coolant accident condition proposed in the present disclosure. DETAILED DESCRIPTION

[0019] Embodiments of the present disclosure are described in detail below, examples of which are shown in the accompanying drawings, wherein the same or similar reference numerals throughout represent the same or similar elements or elements having the same or similar functions. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present disclosure, and are not to be construed as limitations of the present disclosure. On the contrary, the embodiments of the present disclosure include all changes, modifications, and equivalents that fall within the spirit and connotation of the appended claims.

[0020] Figure 1 It is a flow chart of a biaxial load test method for a fuel cladding under a loss of coolant accident condition proposed in one embodiment of the present disclosure.

[0021] like Figure 1 As shown, the biaxial load test method for the fuel cladding under the loss of coolant accident condition includes:

[0022] S101: Assembling and configuring the test section.

[0023] In the embodiment of the present disclosure, when assembling and configuring the test section, it can be flexibly configured according to the test requirements, and there is no limitation on this.

[0024] Optionally, in some embodiments, when assembling and configuring the test section, a heating rod may be made based on a preset size and a preset shape, wherein the heating rod is made of pure molybdenum material and includes a central heating element, a positive conductor, and a negative conductor; a simulated heating rod is assembled based on the heating rod and other components, wherein the other components include at least: a high-temperature alloy tube and ceramic powder; the simulated heating rod is installed in the test section and sealed; an electrical connection is established between the simulated heating rod and a power source, and the electrical connection portion is insulated; and a connection is established between the test section and a gas charging pipeline. In this way, the configured test section can meet the test requirements of each test link.

[0025] Among them, the preset size and preset shape can be the size and shape set in advance for the heating rod according to the test requirements, and the specific values ​​can be flexibly configured based on the application scenario, and there is no restriction on this.

[0026] In the disclosed embodiment, when the simulated heating rod is installed in the test section and sealed, leakage can be effectively prevented from occurring during the test.

[0027] In the embodiment of the present disclosure, when an electrical connection is established between the simulated heating rod and the power supply and the electrical connection portion is insulated, safety issues such as short circuit or electric shock can be effectively prevented.

[0028] S102: Start the water circuit system and increase the temperature and pressure of the test section based on preset test conditions.

[0029] Among them, the preset test conditions refer to the pre-configured conditions that need to be achieved in the test, such as temperature conditions, pressure conditions, etc.

[0030] Optionally, in some embodiments, the water circuit system is equipped with a measuring device, which is used to measure at least one of the following parameters of the water circuit system: temperature; pressure; dissolved oxygen content; pH value. Thus, real-time detection of various parameters in the water circuit system can be achieved to ensure the reliability of the test environment.

[0031] Optionally, in some embodiments, the water chemical conditions of the primary loop of the PWR can be determined; the measurement results of the measuring device can be obtained; and the water loop system can be adjusted according to the water chemical conditions and the measurement results. In this way, it can be ensured that the water loop conditions during the test are consistent with the water chemical conditions of the primary loop of the PWR, thereby improving the reliability of the test process.

[0032] S103: Applying radial and axial biaxial loads to the simulated fuel rods based on the loading frame and the fixture, wherein the simulated fuel rods are installed in the test section.

[0033] In the embodiment of the present disclosure, when radial and axial biaxial loads are applied to the simulated fuel rod based on the loading frame and the clamp, the stress conditions under actual working conditions can be simulated.

[0034] S104: Start the high temperature relief valve to simulate a transient loss of coolant accident, wherein the high temperature relief valve is configured in the water circuit system.

[0035] Optionally, in some embodiments, a high temperature needle valve is provided before and after the high temperature relief valve, and the opening and closing of the high temperature relief valve and the high temperature needle valve are controlled by a preset automatic program. Thus, the coordinated use of the high temperature relief valve and the high temperature needle valve effectively protects the test equipment and system from being damaged by high temperature and high pressure water, and the automatic control program improves the safety and efficiency of the test process and reduces the risk of human operation errors.

[0036] That is, in the embodiment of the present disclosure, a high-temperature relief valve may be configured in the water circuit system, so that during the test, the high-temperature relief valve may be activated to simulate a transient dehydration accident, and the performance of the simulated fuel rod during the dehydration process may be observed and recorded.

[0037] S105: Conduct rapid temperature rise test and oxidation test on simulated fuel rods in high temperature steam environment.

[0038] In the disclosed embodiment, a high-temperature steam environment can be established in the steam generator to prepare for the subsequent rapid temperature rise test and high-temperature steam oxidation test. Then, the simulated fuel rod is placed in the high-temperature steam environment for a rapid temperature rise test to simulate the rapid temperature rise process of the core. A long-term oxidation test is carried out on the simulated fuel rod in the high-temperature steam environment to study its anti-oxidation performance.

[0039] S106: Conduct high temperature quenching test on simulated fuel rods under specific conditions to simulate the core cooling process.

[0040] S107: Remove the simulated fuel rods from the test section, and analyze and test the simulated fuel rods.

[0041] Optionally, in some embodiments, when removing the simulated fuel rod from the test section, the heating may be stopped after the test section is finished; when the simulated fuel rod is naturally cooled to room temperature, the simulated fuel rod is removed from the test section. In this way, the safety of the simulated fuel rod removal process can be effectively improved.

[0042] Optionally, in some embodiments, when analyzing and testing the simulated fuel rod, the simulated fuel rod may be cut into a plurality of small sections along the axial direction, and the small sections are analyzed and tested, thereby facilitating microstructure analysis.

[0043] Optionally, in some embodiments, when analyzing and testing a small sample segment, the measurement results of the small sample segment can be obtained based on a scanning electron microscope and an energy spectrometer, wherein the measurement results include: measurement data of the surface morphology, cross-sectional structure and chemical composition of the small sample segment; and a test report is generated based on the measurement results. In this way, the practicability of the obtained measurement results for generating a test report can be effectively improved.

[0044] Optionally, in some embodiments, the test report includes at least one of the following: a performance change analysis report of a simulated fuel rod under high temperature conditions; a performance change analysis report of a simulated fuel rod under high pressure conditions; a performance change analysis report of a simulated fuel rod under oxidizing conditions. Thus, the indication effect of the obtained test report can be effectively improved, and the practicality of the test results can be improved.

[0045] In this embodiment, the test section is assembled and configured; the water circuit system is started, and the test section is heated and pressurized based on the preset test conditions; radial and axial biaxial loads are applied to the simulated fuel rods based on the loading frame and the fixture, wherein the simulated fuel rods are installed in the test section; the high-temperature relief valve is started to simulate a transient loss of coolant accident, wherein the high-temperature relief valve is configured in the water circuit system; the simulated fuel rods are subjected to rapid temperature rise tests and oxidation tests under a high-temperature steam environment; the simulated fuel rods are subjected to high-temperature quenching tests under specific conditions to simulate the core cooling process; the simulated fuel rods are taken out of the test section, and the simulated fuel rods are analyzed and tested. In this way, the performance verification and evaluation requirements of the fuel cladding under a loss of coolant accident in a pressurized water reactor nuclear power plant can be met.

[0046] Optionally, the present disclosure also proposes a biaxial load test method for fuel cladding under a loss of coolant accident condition, wherein the calibration result of the test equipment can be determined before assembling and configuring the test section, and the test equipment can be adjusted according to the calibration result, thereby effectively improving the accuracy and reliability of the test process.

[0047] Based on the above embodiments, Figure 2 As shown, Figure 2The present invention is a schematic diagram of a fuel cladding biaxial load test method under a dehydration accident condition proposed by the present invention, wherein the figure numbers are explained as follows: 1—equipment measurement instrument calibration; 2—heating rod processing and manufacturing; 3—simulated fuel rod assembly; 4—test section assembly and sealing; 5—electrical connection and insulation; 6—gas charging pipeline connection; 7—water chemical parameter measurement; 8—water circuit temperature and pressure increase; 9—simulated fuel rod biaxial loading; 10—transient dehydration test; 11—establishment of high-temperature steam environment; 12—simulated fuel rod rapid temperature increase test; 13—high-temperature steam oxidation test; 14—online high-temperature quenching test; 15—fuel rod heating stop; 16—simulated fuel rod removal; 17—test piece cutting; 18—SEM and EDS measurement and characterization; 19—surface morphology, cross-sectional structure and chemical composition analysis; 20—issuance of research report.

[0048] This flowchart describes a detailed process for biaxial load testing of fuel cladding under simulated PWR nuclear power plant loss of coolant accident (LOCA) conditions. The following is a brief description and explanation of the process:

[0049] 1. First, calibrate all equipment, instruments and measuring instruments used in the test to ensure the accuracy and reliability of the data.

[0050] 2. Processing and manufacturing of heating rods: According to design requirements, the heating rods made of pure molybdenum material are processed and manufactured, including the central heating element, positive conductor and negative conductor, and their size and shape are strictly controlled.

[0051] 3. Simulated fuel rod assembly: Assemble the processed heating rods and other components (such as high-temperature alloy tubes, ceramic powders, etc.) into simulated fuel rods to ensure close connection and insulation between the components.

[0052] 4. Test section assembly and sealing: Install the simulated fuel rods into the test section and seal it to prevent leakage during the test.

[0053] 5. Electrical connection and insulation: Complete the electrical connection between the simulated fuel rods and the power supply, and ensure that all electrical connections are well insulated to prevent safety issues such as short circuits or electric shocks.

[0054] 6. Gas pressurization pipeline connection: Connect the pipeline used for gas pressurization in the cladding tube and check its sealing and reliability.

[0055] 7. Measurement of water chemical parameters: Measuring points are set in the water loop to monitor and record water chemical parameters (such as temperature, pressure, dissolved oxygen content, pH value, etc.) in real time to simulate the water chemical conditions of the primary loop of a pressurized water reactor.

[0056] 8. Water circuit temperature and pressure increase: Start the water circuit system and increase the temperature and pressure of the test section to meet the predetermined test conditions.

[0057] 9. Simulated fuel rod biaxial loading: Apply radial and axial biaxial loads to the simulated fuel rods through the loading frame and fixture to simulate the stress conditions under actual working conditions.

[0058] 10. Transient dehydration test: Start the high-temperature relief valve and other devices to simulate a transient dehydration accident, and observe and record the performance of the simulated fuel rods during the dehydration process.

[0059] 11. Establish a high-temperature steam environment: Establish a high-temperature steam environment in the steam generator to prepare for subsequent rapid temperature rise tests and high-temperature steam oxidation tests.

[0060] 12. Rapid temperature rise test of simulated fuel rods: The simulated fuel rods are placed in a high-temperature steam environment for a rapid temperature rise test to simulate the rapid temperature rise process of the core.

[0061] 13. High-temperature steam oxidation test: A long-term oxidation test is conducted on simulated fuel rods in a high-temperature steam environment to study their anti-oxidation properties.

[0062] 14. Online high-temperature quenching test: Conduct high-temperature quenching tests on simulated fuel rods under specific conditions to simulate the core cooling process and study the effect of quenching on the performance of cladding materials.

[0063] 15. Fuel rod heating stop: After completing all tests, stop heating and let the simulated fuel rod cool naturally to room temperature. 16. Simulated fuel rod removal: Remove the simulated fuel rod from the test section and prepare for subsequent analysis and testing. 17. Test piece cutting: Cut the simulated fuel rod axially into a certain number of small sections for microstructure analysis.

[0064] 18. SEM and EDS measurement and characterization: Use scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) to measure and characterize the surface morphology, cross-sectional structure and chemical composition of the sample segment.

[0065] 19. Analysis of surface morphology, cross-sectional structure and chemical composition: Combined with the measurement results of SEM and EDS, an in-depth analysis is conducted on the performance changes of simulated fuel rods under conditions of high temperature, high pressure, oxidation, etc.

[0066] 20. Issue a research report: Based on the test results and analysis data, write a detailed research report, summarize the test results and put forward improvement suggestions.

[0067] In the biaxial load test method of fuel cladding under the condition of loss of coolant accident, such as online measurable and controllable water chemical parameters, biaxial simultaneous loading, and high temperature and high pressure water transient loss of coolant test, it is a key link. The following is a detailed analysis of the implementation methods, effects and functions of these key steps:

[0068] For the online measurable and controllable water chemical parameters, the loop system integrates high-pressure pumps, pipelines, dissolved oxygen meters, pH meters, conductivity meters and other equipment, which can monitor and control key parameters such as temperature, pressure, dissolved oxygen content, boric acid, and lithium hydroxide concentration in the water environment in real time. Through the temperature control box and the automatic control system, the water chemical parameters are ensured to be stable within the range of the simulated pressurized water reactor primary loop water chemical conditions. The water chemical environment of the pressurized water reactor primary loop is accurately simulated, providing real and reliable background conditions for the test. Online monitoring and regulation ensure the stability and consistency of water chemical parameters during the test, and improve the accuracy and repeatability of the test results. It avoids the interference of water chemical parameter fluctuations on the test results, and enhances the reliability and scientificity of the test.

[0069] For biaxial simultaneous loading, the heating rod is fixed to the test kettle body by flanges and bolts, and the other end is connected to the loading frame. The clamp of the loading frame is connected to the concave adapter of the negative electrode seal. A dynamic sealing O-ring is set inside the negative electrode seal to ensure that the loading force is accurately transmitted to the cladding tube to achieve radial and axial biaxial loading. Biaxial loading simulates the service environment of the fuel cladding under complex stress conditions and improves the simulation authenticity of the test. The precise control and transmission of the loading force ensures the accuracy and reliability of the test data. The design of the dynamic sealing O-ring reduces the influence of friction on the loading force and improves the stability and accuracy of the loading system.

[0070] For the transient loss of water test of high temperature and high pressure water, a special high temperature relief valve is set up, which can withstand the transient impact of high temperature and high pressure water and quickly discharge the high temperature water in the autoclave. High temperature needle valves are set before and after the high temperature relief valve to reduce the impact on the high temperature relief valve through buffering. The automatic program controls the opening and closing of the high temperature needle valve and the high temperature relief valve to ensure the safety and controllability of the test process. The transient loss of water test simulates the extreme working conditions of the core loss of water in the primary loop of the pressurized water reactor, and provides an important means for evaluating the high temperature and high pressure resistance of the fuel cladding. The coordinated use of the high temperature relief valve and the high temperature needle valve effectively protects the test equipment and system from damage by high temperature and high pressure water. The automated control program improves the safety and efficiency of the test process and reduces the risk of human operation errors. In summary, the realization of these key technical links not only improves the simulation authenticity and accuracy of the test, but also enhances the safety and controllability of the test process, providing strong support for evaluating the service performance of the fuel cladding under the condition of loss of water accident.

[0071] Based on the above embodiments, the biaxial load test method for fuel cladding under loss of coolant accident conditions proposed in the present disclosure includes at least the following technical effects:

[0072] The water chemical loop system provides an online circulating high-temperature and high-pressure water environment for the test section. Through high-pressure pumps, pipelines, dissolved oxygen meters, pH meters, conductivity meters and other equipment, it realizes accurate simulation and control of parameters such as temperature, pressure, dissolved oxygen content, boric acid, and lithium hydroxide concentration. These parameters can be measured and adjusted in real time to accurately simulate the water chemical conditions of the primary loop of the pressurized water reactor. It can highly restore the water chemical environment of the primary loop of the pressurized water reactor, ensure the accuracy and reliability of the test results, monitor and adjust the water chemical parameters in real time, improve the automation level and response speed of the test, and control multiple parameters at the same time, which increases the complexity and difficulty of regulation of the system, but also improves the comprehensiveness and accuracy of the test.

[0073] The high-temperature electric heating element realizes radial and axial biaxial loading. One end fixed to the test kettle body by flange and bolts, and the other end connected to the negative electrode seal through the loading frame, jointly apply radial and axial forces. This design enables the cladding tube to withstand complex mechanical environments during the test. Biaxial loading takes into account radial and axial loading at the same time, and more comprehensively simulates the mechanical environment under actual working conditions. Accurate control of axial force is achieved through the design of dynamic sealing O-rings and precise loading frames. The design of biaxial loading increases the complexity of the structure and the difficulty of processing, but improves the simulation accuracy and reliability of the test.

[0074] When simulating the loss of coolant accident (LOCA) condition of a pressurized water reactor nuclear power plant, the transient discharge of high-temperature and high-pressure water (temperature 350°C, pressure 17.2MPa) was achieved through a specially designed high-temperature relief valve and high-temperature needle valve. The high-temperature relief valve can withstand rapid cooling and heating as well as transient impact pressure, while the high-temperature needle valve plays a buffering role and protects the system from damage.

[0075] It can simulate the transient discharge process of high-temperature and high-pressure water in LOCA accidents, which is difficult to achieve with traditional tests. The high-temperature relief valve and high-temperature needle valve are customized with special alloys and can withstand rapid cooling and heating and transient impact under extreme working conditions. The switching actions of the high-temperature relief valve and high-temperature needle valve are all completed by automatic control programs, which improves the safety and reliability of the test. Through buffering and pressure reduction design, other components of the system are protected from the impact and damage of high-temperature and high-pressure water.

[0076] For example, the present disclosure can design an efficient and high temperature resistant electric heating element in a test system simulating the working conditions of a loss of coolant accident (LOCA) in a pressurized water reactor nuclear power plant, the core part of which is made of pure molybdenum material. The heating element includes a thin molybdenum rod heating element with a diameter of 2mm (strictly controlled within the range of 2±0.2mm) and a length of 500mm, and pure molybdenum positive and negative conductors at both ends, which have a diameter of 4mm and a length of 340mm and 360mm respectively. This design ensures that the heat is mainly concentrated on the central molybdenum rod, while the electrodes at both ends have a relatively low temperature due to their larger diameter, which is conducive to the electrical connection and the sealing and stability of the heating element.

[0077] In order to improve the high temperature resistance and thermal conductivity of the heating element, an 800 nickel-based alloy tube is placed on the outside, with an inner diameter of 6.5mm, an outer diameter of 8.2mm, a wall thickness of 0.85mm, and a total length of 1200mm. Magnesium oxide ceramic powder is tightly filled between the alloy tube and the molybdenum rod to ensure good thermal conduction and electrical insulation, while preventing the expansion and deformation of the material at high temperatures. In addition, a layer of cladding tube is wrapped around the outside of the alloy tube, and the gas in the tube is pressurized through a small gap (0.08mm), further improving the thermal stability and pressure-bearing capacity of the overall structure.

[0078] The power input end uses a solid copper rod with a diameter of 16mm and a length of 60mm, which is directly connected to the positive conductor through nesting technology to avoid potential damage caused by welding. At the same time, both the positive and negative seals are equipped with special insulating O-rings, which not only achieve electrical insulation, but also ensure the sealing of the gas in the cladding tube.

[0079] In order to deal with the potential threat of high temperature to the seals in the loss of coolant accident, the system is also equipped with a cooling water jacket to provide long-term cooling protection for the seals. In addition, the online pressurization and pressure monitoring of the gas in the cladding tube are realized through the ventilation holes and ventilation ducts.

[0080] A high-temperature relief valve and a high-temperature needle valve for buffering that can withstand high-temperature and high-pressure impacts were installed on the high-temperature and high-pressure water spraying circuit system to ensure safety and stability during the test. At the same time, the water chemical circuit system simulated the water chemical conditions of the primary circuit of the pressurized water reactor, providing a precisely controlled high-temperature and high-pressure water environment for the test section.

[0081] During the flooding phase of the LOCA accident, cooling water was injected into the test section through the injection tank to simulate the core cooling process. The water injection rate was adjusted using the booster device to study the effect of different cooling rates on the high-temperature quenching process of the fuel cladding.

[0082] After the test, the simulated fuel rods were cut and microstructured (such as SEM, EDS, XRD, TEM, etc.) to study in detail the effects of high-temperature oxidation in a loss of coolant accident on the surface and cross-sectional microstructure and chemical composition distribution of the fuel cladding tubes, providing important data support for evaluating the performance characteristics of the fuel cladding tubes under loss of coolant accident conditions in pressurized water reactor nuclear power plants.

[0083] In the high-temperature test of the loss of water accident, the surface temperature of the cladding tube reaches above 1200℃. The high temperature will damage the seal and make it impossible to seal. Two cooling water jackets are installed, located in front of the positive seal and the negative seal respectively, and circulating cooling water is introduced for long-term cooling to ensure the reliability of the seal during operation and prevent high-temperature damage and failure accidents, which greatly improves the safety of the test. A ventilation hole is located at the center of the concave adapter. The diameter of the hole is 3mm. It is connected to a ventilation duct, which is connected to the external gas pressurization circuit, so as to realize the gas pressure loading in the cladding tube. The pressure inside the cladding tube can be measured and controlled online. One end of the high-temperature electric heating element is fixed to the test kettle body by a flange and bolts, and the other end is connected to a loading frame. The clamp of the loading frame is connected to the concave adapter of the negative electrode seal, and the loading force is directly applied to the cladding tube. Since an O-ring is set inside the negative electrode seal, the O-ring is a dynamic seal, and the negative electrode copper rod is only subjected to a small dynamic friction force. Therefore, the axial force applied to the cladding tube is determined. The high-temperature electric heating element realizes radial and axial biaxial loading. The maximum temperature of the cladding tube can reach 1263.8℃, and the high temperature holding time above 1200℃ is greater than 30 minutes, which meets the requirements of simulating the loss of coolant accident conditions of a pressurized water reactor nuclear power plant. An important characteristic process of the LOCA accident is the instantaneous release of high-temperature and high-pressure water in the test section. A special high-temperature relief valve is installed on the high-temperature and high-pressure water release loop system. It can withstand the transient impact of high-temperature and high-pressure water (temperature 350°C, pressure 17.2MPa) without leakage, and quickly discharge the high-temperature water in the autoclave to simulate the dehydration state of the primary loop core of a pressurized water reactor. The valve core of the high-temperature relief valve is customized with special alloys, which can withstand rapid cooling and heating and transient impact pressure without causing safety accidents. Since the transient discharge of high-temperature and high-pressure water causes a great thermal shock force, rupture accidents are prone to occur. A high-temperature needle valve needs to be installed in front and behind the high-temperature relief valve. The high-temperature needle valve is made of special materials. Through the buffering effect of the high-temperature needle valve, the instantaneous impact of high-temperature and high-pressure water on the high-temperature relief valve is reduced, and the high-temperature relief valve is protected from leakage. At the same time, the high-temperature needle valve also alleviates the impact of high-temperature and high-pressure water on pipes, valves and pressure relief boxes, protecting the circuit system from damage. The opening and closing of the high-temperature needle valve also adopts an automated program. The operator is away from the valve, which improves the safety of the operating system.

[0084] Before LOCA occurs, the fuel element is in a high-temperature and high-pressure water environment with a water environment temperature of 350℃ and a water environment pressure of 17.2MPa. The water chemical loop system provides an online circulating high-temperature and high-pressure water environment for the test section. It is composed of a high-pressure pump, pipeline, dissolved oxygen meter, pH meter, conductivity meter, loop frame, temperature control box, etc. Parameters such as temperature, pressure, dissolved oxygen content, boric acid, and lithium hydroxide concentration simulate the water chemical conditions of the first loop of the pressurized water reactor. Water chemical parameters such as dissolved oxygen concentration, pH value, and conductivity can be measured and controlled. The valve core of the high-temperature relief valve is customized with special alloys and can withstand rapid cooling and heating and transient impact pressure. The switching action of the high-temperature relief valve is all completed by automatic control programs, avoiding human operating errors and enhancing the safety protection of the test process. The use of high-temperature needle valves reduces the instantaneous impact of high-temperature and high-pressure water on the high-temperature relief valve, protects the high-temperature relief valve from leakage, and also alleviates the impact of high-temperature and high-pressure water on pipe valves and pressure relief boxes. An electric ball valve is installed on the steam pipeline. By switching the ball valve, the high-temperature steam is controlled to enter and exit the high-temperature test section. The electric ball valve has the advantages of high temperature resistance, high pressure resistance, and good sealing performance. Through the digital system, automatic control is achieved to ensure safety. The steam generator is divided into two parts, the lower part is the main generator, and the upper part is the steam drum cavity. The two are connected by two 168mm×30mm connecting pipes. The main generator is made of seamless pipes, made of 316L stainless steel, with a total volume of 120L, a design temperature of 400℃, and a pressure of 25MPa. The medium in the main generator is ultrapure water, and an electric heating rod group is used for internal heating to generate high-temperature steam. The liquid water level in the main generator is measured online by a liquid level gauge to avoid dry burning due to too low liquid level. If the liquid level is too low, it can be replenished by an automatic water replenishment device. The steam enters the steam drum cavity above through the connecting pipe. The steam drum cavity is also heated by an electric heating rod group to increase the steam temperature and make the steam temperature reach a uniform and stable state. After the temperature reaches the experimental set value, the steam is output through the outlet.

[0085] During the flooding stage of the LOCA accident, the high-temperature fuel cladding is cooled by flooding the test section to avoid the occurrence of core melting events. A safety injection box is set in the loop system. The material is 316L stainless steel, the length is 1480mm, the width is 320mm, the internal volume is 60L, its maximum operating temperature is 150℃, and the maximum working pressure is 25MPa. After the LOCA accident occurs, the cooling water in the safety injection box is injected into the high-temperature test section. When the high-temperature fuel cladding encounters the cooling water, a high-temperature quenching state is formed, and the microstructure and integrity of the cladding material are significantly changed. After cooling for a period of time, the temperature of the fuel cladding reaches a balanced state, and the LOCA process ends. The water in the safety injection box is provided by the water storage tank through the water injection pump. Through a solenoid valve on the flooding loop, the cooling water in the safety injection box is controlled to be released into the high-temperature test section to achieve water flooding, simulate the core cooling process, and achieve the effect of emergency intervention in the LOCA accident. A booster device is used to change the pressure inside the injection box and adjust the water injection rate from the injection box to the test section to meet the requirements of different water injection speeds. This has an important impact on the high-temperature quenching of the fuel cladding. By changing the power of the booster device, the LOCA quenching process of the fuel cladding under different water injection rates is obtained. The full-process simulation test system for the loss of coolant accident realizes the entire process of instantaneous discharge of high-temperature and high-pressure water in the loss of coolant accident, rapid heating of the fuel element, and water injection, meeting the test requirements for the service performance research of the fuel cladding under the simulated loss of coolant accident conditions.

[0086] Take out the simulated fuel rod from the test section and cut it into a certain number of small sections along its axial direction. Select SEM, EDS, XRD and TEM to study the chemical composition and microstructure of the cladding tube. In the SEM observation, both the surface state change characteristics of the cladding tube are observed and the cross-sectional structure is microscopically measured. The influence of high-temperature oxidation of the fuel cladding tube on its surface microstructure and chemical composition distribution in the loss of coolant accident is studied. Along the axial direction of the RIA simulated fuel rod, the surface and cross-sectional microstructure of the cladding tube in different temperature zones are analyzed, and combined with the corresponding EDS component change curve, the influence of high-temperature oxidation in the loss of coolant accident on the microstructure and chemical composition distribution of the fuel cladding tube is clarified, and the performance characteristics of the fuel cladding tube under the loss of coolant accident conditions of the pressurized water reactor nuclear power plant are evaluated.

[0087] Those skilled in the art will readily appreciate other embodiments of the present disclosure after considering the specification and practicing the invention disclosed herein. The present disclosure is intended to cover any variations, uses, or adaptations of the present disclosure that follow the general principles of the present disclosure and include common knowledge or customary techniques in the art that are not disclosed in the present disclosure. The description and examples are to be considered exemplary only, and the true scope and spirit of the present disclosure are indicated by the following claims.

[0088] It should be understood that the present disclosure is not limited to the exact structures that have been described above and shown in the drawings, and that various modifications and changes may be made without departing from the scope thereof. The scope of the present disclosure is limited only by the appended claims.

[0089] It should be noted that, in the description of the present disclosure, the terms "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance. In addition, in the description of the present disclosure, unless otherwise specified, the meaning of "plurality" is two or more.

[0090] Any process or method description in a flowchart or otherwise described herein may be understood to represent a module, segment or portion of code that includes one or more executable instructions for implementing the steps of a specific logical function or process, and the scope of the preferred embodiments of the present disclosure includes alternative implementations in which functions may not be performed in the order shown or discussed, including performing functions in a substantially simultaneous manner or in the reverse order depending on the functions involved, which should be understood by those skilled in the art to which the embodiments of the present disclosure belong.

[0091] It should be understood that the various parts of the present disclosure can be implemented in hardware, software, firmware or a combination thereof. In the above-mentioned embodiments, multiple steps or methods can be implemented in software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, it can be implemented by any one of the following technologies known in the art or their combination: a discrete logic circuit having a logic gate circuit for implementing a logic function for a data signal, a dedicated integrated circuit having a suitable combination of logic gate circuits, a programmable gate array (PGA), a field programmable gate array (FPGA), etc.

[0092] A person skilled in the art may understand that all or part of the steps in the method for implementing the above-mentioned embodiment may be completed by instructing related hardware through a program, and the program may be stored in a computer-readable storage medium, which, when executed, includes one or a combination of the steps of the method embodiment.

[0093] In addition, each functional unit in each embodiment of the present disclosure may be integrated into a processing module, or each unit may exist physically separately, or two or more units may be integrated into one module. The above-mentioned integrated module may be implemented in the form of hardware or in the form of a software functional module. If the integrated module is implemented in the form of a software functional module and sold or used as an independent product, it may also be stored in a computer-readable storage medium.

[0094] The storage medium mentioned above can be a read-only memory, a magnetic disk or an optical disk, etc.

[0095] In the description of this specification, the description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present disclosure. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner.

[0096] Although the embodiments of the present disclosure have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present disclosure. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present disclosure.

Claims

1. A biaxial load test method for fuel cladding under loss of coolant accident conditions, characterized in that: include: Assemble and configure the test section; Starting the water circuit system and performing temperature and pressure raising treatment on the test section based on preset test conditions; Applying radial and axial biaxial loads to the simulated fuel rod based on the loading frame and the fixture, wherein the simulated fuel rod is installed in the test section; Starting a high temperature relief valve to simulate a transient loss of coolant accident, wherein the high temperature relief valve is arranged in the water circuit system; Conducting a rapid temperature rise test and an oxidation test on the simulated fuel rod in a high-temperature steam environment; Performing a high temperature quenching test on the simulated fuel rod under specific conditions to simulate a core cooling process; The simulated fuel rod is taken out from the test section, and the simulated fuel rod is analyzed and tested.

2. The method according to claim 1, characterized in that Before the assembly configuration test section, it also includes: Determine the calibration results of test equipment; The test equipment is adjusted according to the calibration results.

3. The method according to claim 1, characterized in that The assembly configuration test section includes: A heating rod is manufactured based on a preset size and a preset shape, wherein the heating rod is made of pure molybdenum material and comprises a central heating element, a positive conductor and a negative conductor; The simulated heating rod is obtained by assembling the heating rod and other components, wherein the other components at least include: a high-temperature alloy tube and ceramic powder; Installing the simulated heating rod into the test section and performing sealing treatment; Establishing an electrical connection between the simulated heating rod and a power source, and insulating the electrical connection portion; Establish a connection between the test section and the gas charging pipeline.

4. The method according to claim 1, characterized in that The water circuit system is equipped with a measuring device, which is used to measure at least one of the following parameters of the water circuit system: temperature; pressure; Dissolved oxygen content; pH value.

5. The method according to claim 4, characterized in that The method further comprises: Determine the water chemistry of the primary circuit of a pressurized water reactor; obtaining a measurement result of the measuring device; The water circuit system is adjusted according to the water chemical conditions and the measurement results.

6. The method according to claim 1, characterized in that A high-temperature needle valve is respectively arranged before and after the high-temperature relief valve, and the switch of the high-temperature relief valve and the high-temperature needle valve is controlled by a preset automatic program.

7. The method according to claim 1, characterized in that The removing the simulated fuel rod from the test section comprises: After the test section ends the test, the heating is stopped; After the simulated fuel rod is naturally cooled to room temperature, the simulated fuel rod is taken out from the test section.

8. The method according to claim 1, characterized in that The analyzing and testing of the simulated fuel rod includes: Cutting the simulated fuel rod into a plurality of small sections along the axial direction; The small sample sections were analyzed and tested.

9. The method according to claim 8, characterized in that The analyzing and testing of the small sample segment includes: Obtaining measurement results of the small-section sample segment based on a scanning electron microscope and an energy spectrometer, wherein the measurement results include: measurement data of the surface morphology, cross-sectional structure and chemical composition of the small-section sample segment; Based on the measurement results, a test report is generated.

10. The method according to claim 9, characterized in that The test report shall include at least one of the following: An analysis report on the performance changes of the simulated fuel rods under high temperature conditions; An analysis report on the performance changes of the simulated fuel rod under high pressure conditions; An analysis report on the performance changes of the simulated fuel rod under oxidizing conditions.

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