Test Method for Biaxial Loading of Fuel Cladding under Loss-of-Coolant Accident Conditions

Abstract

The present disclosure provides a method for testing the biaxial load of a fuel cladding under a loss-of-coolant accident condition. The method includes: assembling and configuring a test section; starting a water loop system and performing temperature and pressure increase treatment on the test section based on preset test conditions; applying biaxial loads in the radial and axial directions to a simulated fuel rod based on a loading frame and a 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 configured in the water loop system; performing a rapid temperature increase 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 the core cooling process; removing the simulated fuel rod from the test section and performing analysis and testing on the simulated fuel rod. By implementing the method of the present disclosure, 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.

Description

Technical Field

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

[0002] The main coolant loss accident (LOCA) is a design basis accident for a reactor. Under the LOCA condition, 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 due to effects such as bulging deformation, re-flooding shrinkage, and transient thermal stress in the axial direction. Therefore, the biaxial or multiaxial mechanics of the cladding tube is a stress state that must be considered. According to the latest international nuclear safety criteria, it is necessary to conduct a LOCA test with loads.

[0003] In the related technologies, the performance verification and evaluation requirements of the fuel cladding under the loss-of-coolant accident in a pressurized water reactor nuclear power plant cannot be met. Summary of the Invention

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

[0005] To this end, the purpose of the present disclosure is to provide a method for testing the biaxial load of a fuel cladding under a loss-of-coolant accident condition, whereby the performance verification and evaluation requirements of the fuel cladding under the loss-of-coolant accident in a pressurized water reactor nuclear power plant can be met.

[0006] To achieve the above object, the method for testing the biaxial load of a fuel cladding under a loss-of-coolant accident condition proposed in the first aspect embodiment of the present disclosure includes:

[0007] Assemble and configure the test section;

[0008] Start the water loop system and perform heating and pressurization on the test section based on preset test conditions;

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

[0010] 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 loop system;

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

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

[0013] Take out the simulated fuel rod from the test section and conduct analysis and testing on the simulated fuel rod.

[0014] The method for testing the biaxial load of a fuel cladding under a loss-of-coolant accident condition provided by the present disclosure includes assembling and configuring a test section; starting a water circuit system, and performing heating and pressurization treatment on the test section based on preset test conditions; applying biaxial loads in the radial and axial directions to a simulated fuel rod based on a loading frame and a 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; performing 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 the core cooling process; taking out the simulated fuel rod from the test section, and analyzing and testing the simulated fuel rod. Thereby, 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, become apparent in part from the following description, or be learned through the 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 be readily understood from the following description of the embodiments in conjunction with the drawings, wherein:

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

[0018] Figure 2 is a schematic diagram of a method for testing the biaxial load of a fuel cladding under a loss-of-coolant accident condition proposed according to the present disclosure. Detailed Embodiments

[0019] The embodiments of the present disclosure will be described in detail below. The examples of the embodiments are shown in the drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the drawings are exemplary only for explaining the present disclosure and should not be construed as limiting the present disclosure. On the contrary, the embodiments of the present disclosure include all variations, modifications, and equivalents falling within the spirit and scope of the appended claims.

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

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

[0022] S101: Assemble and configure a test section.

[0023] In the embodiments of the present disclosure, the test section can be flexibly configured according to test requirements during assembly, and no limitation is imposed thereon.

[0024] Optionally, in some embodiments, when assembling and configuring the test section, a heating rod can be fabricated based on a preset size and a preset shape. 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 from the heating rod and other components, where the other components at least include 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 part is insulated. A connection is established between the test section and a gas pressurization pipeline. Thereby, the configured test section can meet the test requirements of each test link.

[0025] The preset size and preset shape can be the size and shape set for the heating rod in advance according to test requirements, and their specific values can be flexibly configured based on the application scenario, and no limitation is imposed thereon.

[0026] In the embodiments of the present disclosure, when the simulated heating rod is installed in the test section and sealed, leakage during the test can be effectively prevented.

[0027] In the embodiments of the present disclosure, when an electrical connection is established between the simulated heating rod and a power source and the electrical connection part is insulated, safety problems such as short circuit or electric shock can be effectively prevented.

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

[0029] The preset test conditions refer to the conditions that need to be achieved for the test configured in advance, such as temperature conditions, pressure conditions, etc.

[0030] Optionally, in some embodiments, the water circuit system is equipped with a measuring device for measuring at least one of the following parameters of the water circuit system: temperature; pressure; dissolved oxygen content; pH value. Thereby, 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 chemistry conditions of the primary loop of a pressurized water reactor can be determined; the measurement results of the measuring device can be obtained; and the water circuit system can be adjusted according to the water chemistry conditions and the measurement results. Thereby, it can be ensured that the water circuit conditions during the test are consistent with the water chemistry conditions of the primary loop of the pressurized water reactor, and further the reliability of the test process can be improved.

[0032] S103: Apply biaxial loads in the radial and axial directions to the simulated fuel rod based on the loading frame and fixture, where the simulated fuel rod is installed in the test section.

[0033] In the embodiments of the present disclosure, when applying biaxial loads in the radial and axial directions to the simulated fuel rod based on the loading frame and fixture, 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, where the high-temperature relief valve is configured in the water loop system.

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

[0036] That is to say, in the embodiments of the present disclosure, a high-temperature relief valve can be configured in the water loop system, so that during the test process, by starting the high-temperature relief valve, a transient loss-of-coolant accident can be simulated, and the performance of the simulated fuel rod during the loss-of-coolant process can be observed and recorded.

[0037] S105: Conduct a rapid heating test and an oxidation test on the simulated fuel rod in a high-temperature steam environment.

[0038] In the embodiments of the present disclosure, a high-temperature steam environment can be established in the steam generator to prepare for the subsequent rapid heating test and high-temperature steam oxidation test. Then, the simulated fuel rod is placed in the high-temperature steam environment for a rapid heating test to simulate the rapid heating process of the reactor core. A long-term oxidation test is conducted on the simulated fuel rod in the high-temperature steam environment to study its oxidation resistance.

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

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

[0041] Optionally, in some embodiments, when removing the simulated fuel rod from the test section, heating can be stopped after the test in the test section ends; when the simulated fuel rod naturally cools to room temperature, the simulated fuel rod is removed from the test section. Thus, the safety of the process of removing the simulated fuel rod can be effectively improved.

[0042] Optionally, in some embodiments, when analyzing and testing the simulated fuel rod, the simulated fuel rod can be axially cut into multiple small cross-section specimen segments; and the small cross-section specimen segments are analyzed and tested. Thereby, it is convenient to perform microstructure analysis.

[0043] Optionally, in some embodiments, when analyzing and testing the small cross-section specimen segments, the measurement results of the small cross-section specimen segments can be obtained based on a scanning electron microscope and an energy spectrometer. Among them, the measurement results include: measurement data of the surface morphology, cross-section structure, and chemical composition of the small cross-section specimen segments; based on the measurement results, a test report is generated. Thereby, the practicability of the obtained measurement results for generating the test report can be effectively improved.

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

[0045] In this embodiment, by assembling and configuring the test section; starting the water loop system, and performing heating and pressure boosting treatment on the test section based on preset test conditions; applying a biaxial load in the radial and axial directions to the simulated fuel rod based on the loading frame and fixture, where the simulated fuel rod is installed in the test section; starting the high-temperature relief valve to simulate a transient loss-of-coolant accident, where the high-temperature relief valve is configured in the water loop system; performing 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 the core cooling process; taking out the simulated fuel rod from the test section, and analyzing and testing the simulated fuel rod. Thereby, 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 method for a biaxial load test of a fuel cladding under a loss-of-coolant accident condition. Before assembling and configuring the test section, the calibration result of the test equipment can be determined; and the test equipment is adjusted according to the calibration result. Thereby, the accuracy and reliability of the test process can be effectively improved.

[0047] Combining the above embodiments, as Figure 2 shown, Figure 2It is a schematic diagram of a biaxial load test method for fuel cladding under loss-of-coolant accident conditions proposed according to the present disclosure. The reference numerals are explained as follows: 1—Calibration of equipment measuring instruments; 2—Manufacture of heating rods; 3—Assembly of simulated fuel rods; 4—Assembly and sealing of the test section; 5—Electrical connection and insulation; 6—Connection of gas pressurization pipelines; 7—Measurement of water chemical parameters; 8—Heating and pressurization of the water loop; 9—Biaxial loading of simulated fuel rods; 10—Transient loss-of-coolant test; 11—Establishment of a high-temperature steam environment; 12—Rapid heating test of simulated fuel rods; 13—High-temperature steam oxidation test; 14—On-line high-temperature quenching test; 15—Stopping heating of the fuel rod; 16—Removal of simulated fuel rods; 17—Cutting of test pieces; 18—SEM and EDS measurement and characterization; 19—Analysis of surface morphology, cross-sectional structure and chemical composition; 20—Issuance of a research report.

[0048] This flow chart describes a detailed process for performing a biaxial load test on fuel cladding under loss-of-coolant accident (LOCA) conditions in a simulated pressurized water reactor nuclear power plant. The following is a simple description and explanation of this process:

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

[0050] 2. Manufacture of heating rods: According to the design requirements, manufacture heating rods made of pure molybdenum material, including a central heating element, a positive conductor and a negative conductor, and strictly control their dimensions and shapes.

[0051] 3. Assembly of simulated fuel rods: Assemble the manufactured heating rods with other components (such as high-temperature alloy tubes, ceramic powders, etc.) into simulated fuel rods to ensure tight connection and insulation between the components.

[0052] 4. Assembly and sealing of the test section: Install the simulated fuel rods into the test section and perform sealing treatment 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 connection parts are well insulated to prevent safety problems such as short circuits or electric shocks.

[0054] 6. Connection of gas pressurization pipelines: Connect the pipelines for gas pressurization inside the cladding tube and check their tightness and reliability.

[0055] 7. Measurement of water chemical parameters: Set measurement points 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. Heating and pressure boosting of the water circuit: Start the water circuit system, perform heating and pressure boosting operations on the test section to reach the predetermined test conditions.

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

[0058] 10. Transient loss-of-coolant test: Start devices such as the high-temperature relief valve to simulate a transient loss-of-coolant accident, and observe and record the performance of the simulated fuel rod during the loss-of-coolant process.

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

[0060] 12. Rapid heating test of the simulated fuel rod: Place the simulated fuel rod in the high-temperature steam environment for a rapid heating test to simulate the rapid heating process of the reactor core.

[0061] 13. High-temperature steam oxidation test: Conduct a long-term oxidation test on the simulated fuel rod in the high-temperature steam environment to study its oxidation resistance.

[0062] 14. Online high-temperature quenching test: Conduct a high-temperature quenching test on the simulated fuel rod under specific conditions to simulate the reactor core cooling process and study the influence of quenching on the performance of the cladding material.

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

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

[0065] 19. Analysis of surface morphology, cross-sectional structure, and chemical composition: Combine the measurement results of SEM and EDS to deeply analyze the performance changes of the simulated fuel rod under conditions such as high temperature, high pressure, and oxidation.

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

[0067] In the biaxial loading test method of fuel cladding under loss-of-coolant accident conditions, key aspects include online measurability and controllability of water chemistry parameters, biaxial simultaneous loading, and high-temperature and high-pressure water transient loss-of-coolant tests. The following is a detailed analysis of the implementation methods, effects, and functions of these key steps:

[0068] Regarding the online measurability and controllability of water chemistry parameters, the loop system integrates equipment such as high-pressure pumps, pipelines, dissolved oxygen meters, pH meters, and conductivity meters. These devices can monitor and control in real time key parameters in the water environment, such as temperature, pressure, dissolved oxygen content, boric acid, and lithium hydroxide concentration. Through the temperature control box and the automated control system, it is ensured that the water chemistry parameters are stable within the range simulating the primary loop water chemistry conditions of a pressurized water reactor. It accurately simulates the water chemistry environment of the primary loop of a pressurized water reactor, providing a true and reliable background condition for the test. Online monitoring and regulation ensure the stability and consistency of water chemistry parameters during the test, improving the accuracy and repeatability of test results. It avoids interference with test results caused by fluctuations in water chemistry parameters, enhancing the reliability and scientific nature of the test.

[0069] Regarding biaxial simultaneous loading, the heating rod is fixed to the test kettle body through a flange and bolts, and the other end is connected to the loading frame. The fixture of the loading frame is connected to the concave adapter of the negative electrode seal. A dynamic seal O-ring is set inside the negative electrode seal to ensure that the loading force is accurately transmitted to the cladding tube, achieving biaxial loading in the radial and axial directions. Biaxial loading simulates the service environment of the fuel cladding under complex stress conditions, improving the simulation authenticity of the test. Precise control and transmission of the loading force ensure the accuracy and reliability of test data. The design of the dynamic seal O-ring reduces the influence of friction on the loading force, improving the stability and precision of the loading system.

[0070] Regarding the high-temperature and high-pressure water transient loss-of-coolant test, a special high-temperature relief valve is set, 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 a buffering effect. The automated 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-coolant test simulates the extreme condition of core loss-of-coolant in the primary loop of a pressurized water reactor, providing an important means for evaluating the high-temperature and high-pressure resistance performance 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 being damaged 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 implementation of these key technical aspects 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 loss-of-coolant accident conditions.

[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, in the test system for simulating the loss-of-coolant accident (LOCA) conditions of a pressurized water reactor nuclear power plant, an efficient and high-temperature-resistant electric heating element can be designed, and its core part is made of pure molybdenum material. The heating element includes a thin molybdenum rod heating element with a diameter of 2 mm (strictly controlled within the range of 2 ± 0.2 mm) and a length of 500 mm, as well as pure molybdenum positive and negative conductors at both ends, which have diameters of 4 mm and lengths of 340 mm and 360 mm respectively. This design ensures that the heat generation mainly concentrates on the central molybdenum rod, while the electrodes at both ends have relatively lower temperatures due to their larger diameters, which is beneficial for electrical connection and the sealing and stability of the heating element.

[0077] To improve the high-temperature resistance and heat conduction performance of the heating element, an 800 nickel-based alloy tube is sleeved outside, with an inner diameter of 6.5 mm, an outer diameter of 8.2 mm, a wall thickness of 0.85 mm, and a total length of 1200 mm. Magnesium oxide ceramic powder is tightly filled between the alloy tube and the molybdenum rod to ensure good heat conduction and electrical insulation, and at the same time prevent the expansion and deformation of materials at high temperatures. In addition, a cladding tube is wrapped outside the alloy tube, and the gas in the tube is pressurized through a small gap (0.08 mm), further enhancing 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 16 mm and a length of 60 mm, which is directly connected to the positive conductor through nesting technology, avoiding potential damage caused by welding. At the same time, special insulating O-rings are equipped on both the positive and negative seals, which not only achieve electrical insulation but also ensure the gas tightness inside the cladding tube.

[0079] To address the potential threat of high temperature to the seals during 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, through the venting small holes and venting ducts, on-line pressurization and pressure monitoring of the gas inside the cladding tube are achieved.

[0080] On the high-temperature and high-pressure water spray discharge loop system, a high-temperature relief valve capable of withstanding high-temperature and high-pressure impacts and a high-temperature needle valve for buffering are installed to ensure the safety and stability during the test process. At the same time, the water chemistry loop system simulates the water chemistry conditions of the primary loop of the pressurized water reactor, providing a precisely controlled high-temperature and high-pressure water environment for the test section.

[0081] During the flooding stage of the LOCA accident, cooling water is injected into the test section through the accumulator tank to simulate the process of core cooling. A pressurizing device is used to adjust the injection rate to study the influence of different cooling rates on the high-temperature quenching process of the fuel cladding.

[0082] After the test, by cutting the simulated fuel rod and conducting microstructure analysis (such as SEM, EDS, XRD, TEM, etc.), the effects of high-temperature oxidation during the loss-of-coolant accident on the surface and cross-sectional microstructure and chemical composition distribution of the fuel cladding tube were studied in detail, providing important data support for evaluating the performance characteristics of the fuel cladding tube under the loss-of-coolant accident conditions of a pressurized water reactor nuclear power plant.

[0083] During the high-temperature test of the loss-of-coolant accident, the surface temperature of the cladding tube reached over 1200 °C, and the high temperature would damage the seals and make them unable to seal. Two cooling water jackets were installed in front of the positive and negative seals respectively, and circulating cooling water was introduced for long-term cooling to ensure the reliability of the seals during operation and prevent high-temperature failure accidents, greatly improving the safety of the test. A ventilation hole with a diameter of 3 mm was located at the center of the concave adapter, and an external ventilation duct was connected to it, which was connected to the external gas pressurization circuit to achieve gas pressurization loading inside the cladding tube, and the pressure inside the cladding tube was measurable and controllable online. One end of the high-temperature electric heating element was fixed to the test kettle body through a flange and bolts, and the other end was connected to a loading frame. The clamp of the loading frame was connected to the concave adapter of the negative seal, and the loading force was directly applied to the cladding tube. Since an O-ring was installed inside the negative seal and this O-ring was a dynamic seal, the negative copper rod only received a small dynamic friction force. Therefore, the magnitude of the axial force applied to the cladding tube was determined. This high-temperature electric heating element achieved biaxial loading in the radial and axial directions. The maximum temperature of the cladding tube could reach 1263.8 °C, and the holding time at a temperature above 1200 °C was greater than 30 minutes, meeting the requirements of simulating the loss-of-coolant accident conditions of a pressurized water reactor nuclear power plant. The important characteristic process of the LOCA accident was the instantaneous discharge of high-temperature and high-pressure water in the test section. On the high-temperature and high-pressure water discharge loop system, a special high-temperature relief valve was set. It could withstand the transient impact of high-temperature and high-pressure water (temperature 350 °C, pressure 17.2 MPa) without leakage and quickly discharge the high-temperature water in the high-pressure kettle to simulate the loss-of-coolant state of the primary loop core of a pressurized water reactor. The valve core of the high-temperature relief valve was customized with a special alloy and could withstand rapid cooling and heating as well as transient impact pressure without safety accidents. Due to the large thermal shock force caused by the transient discharge of high-temperature and high-pressure water, a break accident was likely to occur. High-temperature needle valves needed to be set in front of and behind the high-temperature relief valve respectively. The high-temperature needle valves were made of special materials. Through the buffering effect of the high-temperature needle valves, the instantaneous impact of high-temperature and high-pressure water on the high-temperature relief valve was reduced, protecting the high-temperature relief valve from leakage. At the same time, the high-temperature needle valves also alleviated the impact of high-temperature and high-pressure water on the pipe valves and the pressure relief box, protecting the loop system from damage. The opening and closing of the high-temperature needle valves were also fully automated, and the operators were far away from the valves, improving the safety guarantee of the operating system.

[0084] Before the occurrence of LOCA, the fuel element is in a high-temperature and high-pressure water environment. The water environment temperature is 350 °C and the water environment pressure is 17.2 MPa. The water chemical loop system provides an on-line circulating high-temperature and high-pressure water environment for the test section, which is composed of a high-pressure pump, pipelines, a dissolved oxygen meter, a pH meter, a conductivity meter, a loop frame, a temperature control box, etc. Parameters such as temperature, pressure, dissolved oxygen content, boric acid, and lithium hydroxide concentration simulate the primary loop water chemical conditions of a 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 from special alloy and can withstand rapid cooling and heating as well as transient impact pressure. The opening and closing actions of the high-temperature relief valve are all completed by an automatic control program, avoiding human operation errors and enhancing the safety protection of the test process. A high-temperature needle valve is used to reduce the instantaneous impact of high-temperature and high-pressure water on the high-temperature relief valve, protect the high-temperature relief valve from leakage, and at the same time relieve the impact of high-temperature and high-pressure water on pipe fittings and the pressure relief box. An electric ball valve is installed on the steam pipeline. By opening and closing this ball valve, the high-temperature steam is controlled to enter and exit the high-temperature test section. This electric ball valve has the advantages of high temperature resistance, high pressure resistance, and good sealing performance. Through a digital system, automatic control is realized to ensure safety. The steam generator is divided into upper and lower parts. The lower part is the main generator, and the upper part is the steam drum cavity. The two are connected by 2 connecting pipes with a size of 168 mm × 30 mm. The main generator is made of seamless pipe with a material of 316L stainless steel, a total volume of 120 L, a design temperature of 400 °C, and a pressure of 25 MPa. The medium in the main generator is ultrapure water, and internal heating is carried out by an electric heating rod group to generate high-temperature steam. The liquid level in the main generator is measured on-line by a liquid level gauge to avoid dry burning due to too low liquid level. If the liquid level is too low, water can be replenished through an automatic water replenishing device. The steam enters the upper steam drum cavity through the connecting pipe. There is also an electric heating rod group in the steam drum cavity for heating 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 air outlet.

[0085] During the flooding stage of the LOCA accident, water is injected into the test section to cool down the high-temperature fuel cladding, avoiding the occurrence of core melting events. An accumulator tank is installed in the loop system, made of 316L stainless steel, with a length of 1480 mm, a width of 320 mm, and an internal volume of 60 L. Its maximum operating temperature is 150 °C and the maximum operating pressure is 25 MPa. After a LOCA accident occurs, the cooling water in the accumulator tank 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 change significantly. After cooling for a period of time, the temperature of the fuel cladding reaches an equilibrium state, and the LOCA process ends. The water in the accumulator tank is provided by the storage tank through a charging pump. An electromagnetic valve on the flooding loop is used to control the release of the cooling water in the accumulator tank into the high-temperature test section, achieving water flooding and simulating the core cooling process to achieve the effect of emergency intervention in the LOCA accident. A pressurization device is used to change the pressure inside the accumulator tank and adjust the water injection rate from the accumulator tank to the test section to meet the requirements of different flooding speeds, which has an important impact on the high-temperature quenching of the fuel cladding. By changing the power of the pressurization device, the LOCA quenching process of the fuel cladding under different water injection rates is obtained. The whole process simulation test system for the loss-of-coolant accident realizes the whole process of instantaneous discharge of high-temperature and high-pressure water, rapid heating and temperature rise inside the fuel element, and flooding, meeting the test requirements for studying the service performance of the fuel cladding under the loss-of-coolant accident conditions of a pressurized water reactor nuclear power plant.

[0086] Remove the simulated fuel rod from the test section and axially cut it into a certain number of small sample segments. Preferably use SEM, EDS, XRD, and TEM to study the chemical composition and microstructure of the cladding tube. In the SEM observation, both observe the change characteristics of the surface state of the cladding tube and conduct microscopic measurements on its cross-sectional structure. Study the influence of high-temperature oxidation of the fuel cladding tube in the loss-of-coolant accident on its surface microstructure and the distribution of its chemical composition. Along the axis of the RIA simulated fuel rod, analyze the surface and cross-sectional microstructures of the cladding tube in different temperature regions, and combine the corresponding EDS composition change curves to clarify the influence of high-temperature oxidation in the loss-of-coolant accident on the microstructure and chemical composition distribution of the fuel cladding tube, and evaluate the performance characteristics of the fuel cladding tube under the loss-of-coolant accident conditions of a pressurized water reactor nuclear power plant.

[0087] Those skilled in the art will readily conceive of other embodiments of the present disclosure after considering the specification and the practice of the invention disclosed herein. The present disclosure is intended to cover any variations, uses, or adaptations of the present disclosure, which follow the general principles of the present disclosure and include the common general knowledge or conventional technical means in the technical field not disclosed in the present disclosure. The specification and the embodiments are only regarded as exemplary, and the true scope and spirit of the present disclosure are pointed out by the following claims.

[0088] It should be understood that the present disclosure is not limited to the exact structures described above and shown in the drawings, and various modifications and changes can be made without departing from its scope. The scope of the present disclosure is only limited 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 "a plurality" is two or more.

[0090] Any process or method description shown in the flowchart or described in other ways herein can be understood as representing a module, segment, or part of code including one or more executable instructions for implementing a specific logical function or process. The scope of the preferred embodiments of the present disclosure includes additional implementations, where the functions can be executed in a substantially simultaneous manner or in the reverse order according to the functions involved, rather than in the order shown or discussed. This should be understood by those skilled in the technical field to which the embodiments of the present disclosure belong.

[0091] It should be understood that various parts of the present disclosure can be implemented by hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented by software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented by hardware, as in another embodiment, any one of the following techniques known in the art or a combination thereof can be used: discrete logic circuits having logic gate circuits for implementing logical functions on data signals, application-specific integrated circuits having appropriate combinational logic gate circuits, programmable gate arrays (PGAs), field-programmable gate arrays (FPGAs), etc.

[0092] Those of ordinary skill in the art of this technology can understand that all or part of the steps carried by the method of implementing the above embodiments can be completed by instructing relevant hardware through a program. The program can be stored in a computer-readable storage medium. When the program is executed, it includes one or a combination of the steps of the method embodiments.

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

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

[0095] In the description of this specification, the descriptions referring to terms such as "one embodiment", "some embodiments", "examples", "specific examples", or "some examples" etc. mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present disclosure. In this specification, the schematic representations of the above terms do 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 can be understood that the above embodiments are exemplary and should not be construed as limiting the present disclosure. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to 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 configured in the water circuit system, and high-temperature needle valves are respectively provided before and after the high-temperature relief valve, and the switches of the high-temperature relief valve and the high-temperature needle valve are controlled by a preset automatic program; 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 cut into a plurality of small sections along the axial direction. Measurement results of the small sections are obtained based on a scanning electron microscope and an energy spectrometer, and a test report is generated based on the measurement results, wherein the test report includes an analysis report on performance changes of the simulated fuel rod under high temperature conditions, an analysis report on performance changes under high pressure conditions, and / or an analysis report on performance changes under oxidizing conditions; wherein the measurement results include: measurement data of surface morphology, cross-sectional structure and chemical composition of the small sections.

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 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.

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