Method and system for testing internal pressure fatigue of fuel rod cladding tube after irradiation

Abstract

The invention discloses an internal pressure fatigue test method for a fuel rod cladding tube after irradiation. The method comprises the following steps: obtaining circumferential yield strength of the cladding tube at an internal pressure fatigue test temperature; sealing after checking that the outer part of the cladding tube sample is not scratched; mounting the sample on a high-pressure joint in a high-temperature vacuum furnace; closing a furnace door and setting fatigue test parameters according to the circumferential yield strength of the cladding tube; exhausting the inside of the sample, then starting preloading operation, and judging the sealing condition of the sample; if the sealing performance of the sample is good, the high-temperature vacuum furnace is started, the furnace body is vacuumized firstly, and heating is started after the vacuum degree meets the requirement; keeping the temperature for more than 20 minutes after the temperature in the furnace reaches the test temperature, starting pressure loading, and circularly loading the internal pressure or strain amplitude of the cladding tube sample; and when the cladding tube sample is broken or the test cycle reaches the rated fatigue life, the fatigue test is stopped. According to the method, the circumferential fatigue performance of the cladding tube can be accurately obtained, and technical support can be provided for development of new fuel.

Description

Technical Field

[0001] The present invention relates to the technical field of post-irradiation inspection, and in particular to a method and system for internal pressure fatigue testing of fuel rod cladding tubes after irradiation. Background Art

[0002] Nuclear fuel assemblies are core components of nuclear power plant reactor cores. To improve the economic efficiency of nuclear power plants, they are constantly striving to increase fuel consumption and extend refueling cycles, placing higher demands on the performance of the fuel cladding tubes. During nuclear reactor operation, the inner surface of the zirconium alloy cladding is subjected to high temperatures of approximately 400°C, while the outer cladding is surrounded by cooling water. Because the loads outside the cladding are greater than those inside, the cladding tubes are filled with helium at a pressure slightly lower than the pressure of the cooling water outside the cladding. As the nuclear fuel burns, the pressure inside the cladding increases further. Due to power grid load fluctuations and frequency control, the combustion of the pellets inside the cladding must be controlled, resulting in certain fluctuations in the internal cladding pressure and the zirconium alloy cladding is subjected to certain circumferential cyclic loads. When the fuel pellets burn in contact with the cladding, the resulting gases have a more pronounced impact on the cladding, resulting in more significant cyclic loads. Therefore, for the safe operation of active nuclear power reactors and the development of advanced fuel elements, it is necessary to establish an internal pressure fatigue test method for irradiated fuel rod cladding tubes based on a hot chamber internal pressure fatigue test system to accurately obtain the fatigue performance of the cladding tubes under circumferential alternating loads. Summary of the Invention

[0003] The purpose of the present invention is to solve the problem of internal pressure fatigue testing of irradiated fuel cladding and provide a method and system for internal pressure fatigue testing of irradiated fuel rod cladding tubes. This method can effectively obtain the circumferential fatigue performance of irradiated fuel rod cladding tubes and provide technical support for the development of new fuel assemblies.

[0004] The present invention is achieved through the following technical solutions: In a first aspect, the present invention provides a method for internal pressure fatigue testing of irradiated fuel rod cladding tubes, comprising the following steps: Conduct internal pressure burst test on cladding tube at internal pressure fatigue test temperature to obtain the circumferential yield strength of cladding tube; After checking that there are no scratches on the outside of the cladding tube sample, perform mechanical sealing on the cladding tube sample; Install the sealed cladding tube sample on the high-pressure joint in the high-temperature vacuum furnace; Close the high-temperature vacuum furnace door and set the fatigue test parameters according to the circumferential yield strength of the cladding tube; Exhaust the inside of the cladding tube sample, then start the preload operation to determine whether the sealing performance of the sample is good; If the sealing performance of the sample is good, start the high-temperature vacuum furnace, first evacuate the furnace body, and then start the heating operation when the vacuum degree in the furnace body reaches a certain level; After the temperature in the furnace reaches the test temperature, the temperature fluctuation and gradient during the test are adjusted in real time according to the temperature fluctuation range so that the temperature fluctuation and gradient range meet the test requirements; When the holding time reaches more than 20 minutes, start the pressure loading. When the pressure is loaded to half of the sum of the peak and the trough, the internal pressure or strain amplitude of the cladding tube sample is cyclically loaded according to a certain waveform. The fatigue test is stopped when the cladding tube specimen is broken or the test cycle reaches the rated fatigue life.

[0005] In the above technical solution, the test parameter selection first determines the yield strength at the fatigue test temperature to basically determine the boundaries of high and low cycles. Then, the Donell-Langer model can be used as a reference to formulate 4-6 groups of stress amplitudes, which can roughly draw the SN curve, effectively reducing the difficulty of the test. The formal start of the test mainly uses a manipulator to pre-install the cladding tube sample with good sealing performance on the high-pressure joint in the high-temperature vacuum furnace, and then uses a long-handled wrench to lock and seal it; close the furnace door to vent the inside of the cladding tube sample and preload it to determine whether the cladding tube sample has leakage; after the cladding tube sample is vented and the sealing inspection is completed, the furnace is evacuated and the temperature is increased. After the temperature reaches the test temperature and is kept warm for more than 20 minutes, the test waveform is loaded, and the cycle number is recorded after the waveform stabilizes. When the test cycle reaches the rated fatigue life or the sample ruptures, the test ends and the final fatigue life of the sample is recorded.

[0006] As a preferred embodiment of the present invention, the fatigue test parameters set according to the circumferential yield strength of the cladding tube include initial pressure, test temperature, frequency, rated fatigue life, rated strain amplitude, and allowable temperature fluctuation range.

[0007] As a preferred solution of the present invention, when sealing the cladding tube sample, end plugs slightly smaller than the inner diameter of the cladding tube sample are used at both ends of the cladding tube sample. When a ferrule is used for extrusion sealing, the sample is clamped inside and outside to avoid deterioration of the sealing performance of the sample.

[0008] As a preferred embodiment of the present invention, the Donell-Langer model is used to confirm 4-6 groups of test parameters when selecting stress amplitude or strain amplitude. After the SN curve is roughly determined, detailed tests are performed again using the group experiment method and the lifting method.

[0009] As a preferred embodiment of the present invention, a group experiment method is used in a high stress level region, and an up-down method is used in a low stress level region to obtain an SN curve or an eN curve.

[0010] As a preferred solution of the present invention, the strain amplitude is adjusted in real time when starting the loading. When the strain amplitude reaches the rated strain amplitude, the waveform stabilizes for a certain number of cycles and then maintains constant pressure peaks and troughs, and subsequent tests are performed with pressure control.

[0011] As a preferred embodiment of the present invention, the criterion for determining fatigue failure in the test is that the peak pressure drops by 20% after reaching the maximum value or the sample fails.

[0012] As a preferred embodiment of the present invention, when the cladding tube sample is pressure-loaded, the pressurized medium is injected into the cladding tube sample through a hydraulic pump, and the loading of the test waveform is started. After the waveform is stabilized, the cycle number is recorded.

[0013] As a preferred solution of the present invention, strain control adjusts the peak pressure in real time during the unstable stage of the test. When the test stress amplitude reaches the set value and stabilizes for a certain period of time, the test control is converted to pressure control.

[0014] In a second aspect, the present invention provides an internal pressure fatigue test system for irradiated fuel rod cladding tubes, which is used to implement the internal pressure fatigue test method for irradiated fuel rod cladding tubes described in the first aspect, and includes a vacuum system, a heating system, a deformation measurement system, a pressure loading system and a data acquisition system. The vacuum system is used to evacuate a high-temperature vacuum furnace, the heating system is used to maintain the test temperature in the high-temperature vacuum furnace, the deformation measurement system is used to monitor the change in the diameter of the cladding tube sample in real time, the pressure loading system is used to cyclically pressurize the cladding tube sample, and the data acquisition system is used to collect relevant test data during the test process.

[0015] Compared with the prior art, the present invention has the following advantages and beneficial effects: The present invention solves the problem of internal pressure fatigue testing of fuel cladding tubes after irradiation, can effectively obtain the fatigue properties of materials, and provide technical support for the service of materials. At the same time, the internal pressure fatigue testing technology can accurately obtain the circumferential fatigue properties of fuel rod cladding tubes after irradiation, which can provide technical support for the development of new fuel assemblies. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following briefly introduces the drawings required for use in the examples. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be considered as limiting the scope. A person of ordinary skill in the art can also derive other relevant drawings based on these drawings without inventive effort. In the drawings: Figure 1 This is a flow chart of the internal pressure fatigue test method for irradiated fuel rod cladding tubes in the present invention; Figure 2It is the pressure peak, trough-cycle curve of the pressure control process in the present invention; Figure 3 The strain amplitude peak, trough and cycle curve of the pressure control process in the present invention; Figure 4 Schematic diagram of the S-N curve of the cladding tube sample in the present invention. DETAILED DESCRIPTION

[0017] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with examples and drawings. The exemplary embodiments of the present invention and their descriptions are only used to explain the present invention and are not intended to limit the present invention.

[0018] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application; the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned figure descriptions are intended to cover non-exclusive inclusions.

[0019] In the description of the embodiments of the present application, technical terms such as "first" and "second" are only used to distinguish different objects and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features.

[0020] References herein to "embodiments" mean that a particular feature, structure, or characteristic described in connection with the embodiments may be included in at least one embodiment of the present application. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor does it constitute an independent or alternative embodiment that is mutually exclusive of other embodiments. It is understood, both explicitly and implicitly, by those skilled in the art that the embodiments described herein may be combined with other embodiments.

[0021] In the description of the embodiments of this application, the term "and / or" is simply a description of the association relationship between associated objects, indicating that three relationships can exist. For example, A and / or B can represent the following three situations: A exists, A and B exist at the same time, and B exists. In addition, the character " / " in this document generally indicates that the associated objects are in an "or" relationship.

[0022] In the embodiments of this application, the same reference numerals represent the same components, and for the sake of brevity, detailed descriptions of the same components in different embodiments are omitted. It should be understood that the thickness, length, width, and other dimensions of the various components in the embodiments of this application, as well as the overall thickness, length, width, and other dimensions of the integrated device shown in the drawings are merely illustrative and should not constitute any limitation on this application.

[0023] In the description of the embodiments of the present application, the term "multiple" refers to more than two (including two). Similarly, "multiple groups" refers to more than two groups (including two groups), and "multiple pieces" refers to more than two pieces (including two pieces), unless otherwise clearly and specifically defined.

[0024] In the description of the embodiments of the present application, the technical terms "center", "longitudinal", "lateral", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", "axial", "radial", "circumferential", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the embodiments of the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the embodiments of the present application.

[0025] In the description of the embodiments of the present application, unless otherwise expressly specified or limited, technical terms such as "installed," "connected," "connected," and "fixed" should be understood in a broad sense. For example, they can refer to fixed connections, detachable connections, or integration; mechanical connections or electrical connections; direct connections or indirect connections through an intermediate medium; internal connections between two components or interactions between two components. Those skilled in the art can understand the specific meanings of the above terms in the embodiments of the present application based on specific circumstances.

[0026] Nuclear fuel cladding tubes are designed to support the fissile fuel in the fuel assembly. They are made of elongated zirconium alloy material with a ZrO2 surface coating. Zirconium alloy, as a material for cladding tubes, has a series of advantages such as good neutron economy, high-temperature corrosion resistance, and good mechanical properties. It has become the first choice for pressurized water reactor fuel element cladding materials. However, due to the complexity of its research and development, only a few countries have mastered the research and development technology of cladding tubes.

[0027] As the component directly housing the nuclear fuel, the cladding tube is the reactor's first safety barrier, shouldering the heavy responsibility of preventing nuclear fuel leakage. As the most direct barrier between the nuclear fuel and the coolant, it faces a series of severe challenges, including high temperatures, high pressures, corrosion, and strong radiation. Therefore, the selection of cladding materials is extremely important. The safe and stable operation of the nuclear reactor core is closely related to the performance of the cladding tube.

[0028] During nuclear reactor operation, the inner surface of the zirconium alloy cladding is subjected to temperatures of approximately 400°C, while the outer cladding is surrounded by cooling water. Because the loads outside the cladding are greater than those inside, the cladding tubes are filled with helium at a pressure slightly lower than that of the cooling water outside the cladding.

[0029] As the nuclear fuel burns, the pressure inside the cladding tube increases. Due to power grid load fluctuations and frequency control, the combustion of the pellets inside the cladding tube must be controlled. This causes the internal pressure of the cladding tube to fluctuate, subjecting the cladding tube to a certain degree of circumferential cyclic load. When the fuel pellets burn in contact with the cladding, the resulting gas impacts the cladding more significantly, generating more significant cyclic loads. This circumferential cyclic load can cause transient fatigue fracture failure of the material before yielding, seriously affecting the service safety of the nuclear power plant.

[0030] In order to ensure the safe operation of existing nuclear power reactors and the development of advanced fuel elements, the inventors have conducted in-depth research and established a test method for internal pressure fatigue testing of fuel rod cladding tubes after irradiation. This test method can accurately obtain the fatigue performance of the cladding tubes under the action of circumferential alternating loads.

[0031] Example 1 Please refer to Figures 1 to 4 The embodiment of the present application provides a method for internal pressure fatigue testing of irradiated fuel rod cladding tubes, comprising the following steps: Conduct internal pressure burst test on cladding tube at internal pressure fatigue test temperature to obtain the circumferential yield strength of cladding tube; After checking that there are no scratches on the outside of the cladding tube sample, perform mechanical sealing on the cladding tube sample; Install the sealed cladding tube sample on the high-pressure joint in the high-temperature vacuum furnace; Close the high-temperature vacuum furnace door and set the fatigue test parameters according to the circumferential yield strength of the cladding tube; Exhaust the inside of the cladding tube sample, then start the preload operation to determine whether the sealing performance of the sample is good; If the sealing performance of the sample is good, start the high-temperature vacuum furnace, first evacuate the furnace body, and then start the heating operation when the vacuum degree in the furnace body reaches a certain level; After the temperature in the furnace reaches the test temperature, the temperature fluctuation and gradient during the test are adjusted in real time according to the temperature fluctuation range so that the temperature fluctuation and gradient range meet the test requirements; When the holding time reaches more than 20 minutes, start the pressure loading. When the pressure is loaded to half of the sum of the peak and the trough, the internal pressure or strain amplitude of the cladding tube sample is cyclically loaded according to a certain waveform. The fatigue test is stopped when the cladding tube specimen is broken or the test cycle reaches the rated fatigue life.

[0032] In this embodiment, a video microscope is used to observe whether there are scratches on the outside of the cladding tube. If there are no scratches, an automatic sealing device is used to mechanically seal the cladding tube sample. The sealed cladding tube sample is installed on the high-voltage connector using a manipulator, and the sample is locked with a long-handled wrench. Since there is air inside the cladding tube sample, it is necessary to use an exhaust operation to expel the internal air. When evacuating the furnace body, start the mechanical pump first and then the molecular pump. By evacuating, the high-temperature vacuum furnace can be kept in a vacuum state, so that the degree of oxidation of the cladding tube sample at high temperature is as low as possible; at the same time, since the high-temperature vacuum furnace is in a vacuum state, the amount of gas in the furnace body is greatly reduced, and the heat flow caused by the high temperature is also greatly reduced, which ultimately minimizes the influence of the heat flow on the deformation measurement accuracy.

[0033] According to some embodiments of the present application, fatigue test parameters set based on the hoop yield strength of the cladding tube include parameters such as initial pressure, test temperature, frequency, rated fatigue life, rated strain amplitude, and allowable temperature fluctuation range. It should be noted that the relevant test parameters must be set in advance before starting the fatigue test.

[0034] According to some embodiments of the present application, when sealing the cladding tube sample, end plugs slightly smaller than the inner diameter of the cladding tube sample are used at both ends of the cladding tube sample. When a ferrule is used for extrusion sealing, the sample is clamped inside and outside to avoid deterioration of the sealing performance of the sample.

[0035] According to some embodiments of the present application, when selecting the stress amplitude or strain amplitude, the Donell-Langer model is used to confirm 4-6 groups of test parameters. After the SN curve is roughly determined, detailed tests are performed again using the group experiment method and the lifting and lowering method. In the above technical solution, the fatigue test parameter selection uses the Donnell-Langer model to predict the SN curve and formulate an appropriate stress amplitude or strain amplitude. Among them, the difference between the maximum hoop stress (positive value) and the minimum hoop stress in each stress cycle is called the stress range, and the stress amplitude is half of the stress range. The strain amplitude refers to the difference between the maximum strain and the average strain in the alternating strain.

[0036] According to some embodiments of the present application, a group experiment method is used in the high stress level region, and an up-down method is used in the low stress level region to obtain an SN curve or an eN curve. The SN curve is a curve that uses the fatigue strength of a standard material specimen as the ordinate and the logarithm of the fatigue life, log N, as the abscissa, to represent the relationship between the fatigue strength and fatigue life of the standard specimen under certain cyclic characteristics. It is also called a stress-life curve.

[0037] According to some embodiments of the present application, strain-controlled internal pressure fatigue testing is technically challenging. Studies using unirradiated specimens have shown that once the pressure peaks and valleys are determined, the strain amplitude of the specimen does not change significantly. Therefore, the strain amplitude is adjusted in real time during initial loading. When the strain amplitude reaches the rated strain amplitude, the pressure peaks and valleys are stabilized for a certain number of cycles, and subsequent tests are conducted using pressure control.

[0038] According to some embodiments of the present application, fatigue failure is determined by a 20% drop in peak pressure after reaching its maximum value, or by specimen failure. Fatigue failure is the tendency of a material to develop gradually expanding brittle cracks, ultimately leading to fracture, under repeated, alternating, and cyclic stresses far below its normal strength. By employing these fatigue failure criteria, fatigue failure criteria for cladding tube specimens can be clearly defined during testing.

[0039] According to some embodiments of the present application, when the cladding tube sample is pressure-loaded, the pressurized medium is injected into the cladding tube sample through a hydraulic pump, the loading of the test waveform is started, and the recording of the cycle number is started after the waveform stabilizes. After the temperature in the high-temperature vacuum furnace reaches the test requirements for more than 20 minutes, the pressure loading is started, and the pressure or strain amplitude inside the cladding tube sample shows cyclic loading. Specifically, since the cladding tube is connected to the high-pressure joint in the high-temperature vacuum furnace, the pressurized medium is injected into the cladding tube sample from the high-pressure joint through a hydraulic pump during loading to control the cyclic waveform of the pressure, that is, the waveform of the pressure changing from peak to trough and then from trough to peak.

[0040] According to some embodiments of the present application, strain control adjusts the peak pressure in real time during the unstable stage of the test. When the test stress amplitude reaches the set value and stabilizes for a certain period of time, the test control is converted to pressure control.

[0041] The cladding tube internal pressure fatigue test method in this application mainly includes the processes of sample sealing, test parameter selection, test and test data analysis and processing, and is mainly used for the internal pressure fatigue test of fuel rod cladding tubes after irradiation.

[0042] The sealing of the cladding tube sample mainly includes mechanical seals and welding seals. In this application, only mechanical seals are used. The internal filling of the cladding tube is divided into two parts. End plugs with central holes are used at both ends of the cladding tube (the outer diameter of the end plug is required to be as close as possible to the inner diameter of the cladding tube sample), and the middle core shaft should have a guide groove.

[0043] The test parameters are selected by first determining the yield strength at the fatigue test temperature using methods such as internal pressure explosion or hoop tension, which basically determines the limit between high and low cycles. Then the Donnell-Langer model is used to predict the SN curve of the cladding tube ( ), evaluate 4-6 sets of experimental stress amplitudes or strain amplitudes.

[0044] The test officially begins by using a manipulator to pre-install the cladding tube sample with good sealing performance on the high-pressure joint in the high-temperature vacuum furnace. It is then locked and sealed with a long-handled wrench. The high-temperature vacuum furnace door is closed to vent the inside of the cladding tube sample and preload it to determine whether the sample is leaking. After the cladding tube sample is vented and the sealing is checked, the high-temperature vacuum furnace is evacuated and the temperature is increased. After the temperature reaches the test temperature and is kept at this temperature for more than 20 minutes, the test waveform is loaded and the cycle number is recorded after the waveform stabilizes. The test ends when the test cycle number reaches the rated fatigue life or the sample ruptures, and the final fatigue life of the sample is recorded.

[0045] At the same stress level, fatigue life can vary by several times, or even dozens of times. Normal or Weibull distributions are often used to describe the logarithmic fatigue life, lgN. The probability of surviving the fatigue life at various stress levels is calculated based on confidence intervals, and the PSN curve is calculated based on this probability.

[0046] The PSN curve refers to the SN curve corresponding to different survival probabilities P, which is drawn to account for the dispersion of fatigue life. In engineering, it is customary to plot the functional relationship between P, S, and N in the two-dimensional coordinate system of SN. When the survival probability P is constant, an S-N curve is formed with S as the independent variable. When the survival probability P changes, each P value corresponds to an SN curve, forming a family of SN curves, also known as PSN curves.

[0047] Since the inner and outer walls near the cladding tube mouth need to remove the oxide layer and polish the weld after welding, a nuclear fuel cladding tube grinding device is used for processing. The device consists of a cladding tube clamping mechanism and a grinding mechanism. Specifically, the relevant grinding operations can be performed by using a manipulator.

[0048] The internal pressure fatigue test method for irradiated fuel rod cladding tubes in this embodiment can accurately obtain the circumferential fatigue performance of the cladding tube specimens, thereby providing technical support for the development of new fuel assemblies. At the same time, this test method uses the Donell-Langer model as a reference to formulate 4-6 groups of stress amplitudes, which can roughly draw the SN curve, effectively reducing the test difficulty, accelerating the project implementation progress, and shortening the research and development cycle of new nuclear fuels.

[0049] Example 2 A post-irradiation fuel rod cladding tube internal pressure fatigue test system provided in the embodiments of the present application is used to implement the post-irradiation fuel rod cladding tube internal pressure fatigue test method described in Example 1, and includes a vacuum system, a heating system, a deformation measurement system, a pressure loading system and a data acquisition system. The vacuum system is used to evacuate a high-temperature vacuum furnace, the heating system is used to maintain the test temperature in the high-temperature vacuum furnace, the deformation measurement system is used to monitor the change in the diameter of the cladding tube sample in real time, the pressure loading system is used to cyclically pressurize the cladding tube sample, and the data acquisition system is used to collect relevant test data during the test process.

[0050] The vacuum system consists of two stages of vacuum, namely a mechanical pump and a molecular pump. Their main functions are: (1) to maintain a vacuum state in the high-temperature vacuum furnace, so that the degree of oxidation of the cladding tube sample at high temperature is as low as possible; (2) when the high-temperature vacuum furnace is in a vacuum state, the amount of gas in the furnace body is greatly reduced, and the heat flow caused by the high temperature is also greatly reduced, ultimately minimizing the impact of heat flow on the deformation measurement accuracy. When the high-temperature vacuum furnace is vacuumed, the mechanical pump is started first to pump air, and then the molecular pump is started to pump air. A mechanical pump is a device that uses mechanical principles to extract gas from the evacuated space through the movement of mechanical parts. It is mainly used for low vacuum applications. Compared with mechanical pumps, molecular pumps can achieve higher vacuum levels. The function of molecular pumps is to reduce the pressure of the vacuum chamber to a very low range.

[0051] Due to their respective advantages and disadvantages, mechanical pumps and molecular pumps are often used together to achieve higher vacuum levels. Mechanical pumps can extract gas molecules within a low vacuum range, while molecular pumps can remove gas molecules from the system within a high vacuum range. This combined use can achieve high vacuum levels and is widely used in fields such as semiconductors, electronics, chemistry, and biology.

[0052] The heating system utilizes resistance wire heating and is removably mounted on the high-temperature vacuum furnace. The entire heating element is portable and replaceable. The heating system is primarily used to heat the high-temperature vacuum furnace and regulate the temperature to maintain it within the internal pressure fatigue test temperature range. Once the furnace reaches the test temperature, the temperature fluctuation and gradient are adjusted in real time based on the temperature fluctuation range to ensure that the temperature fluctuation and gradient meet the test requirements.

[0053] The deformation measurement system uses a laser caliper to monitor changes in the diameter of the cladding tube specimen in real time. Since the cladding tube specimen undergoes slight changes in diameter after internal pressure is applied, the laser caliper can monitor this in real time. Laser calipers can be broadly categorized as laser scanning calipers, CCD projection calipers, and laser diffraction calipers. The first two operate on optical geometry, while the latter utilizes the optical wave principle.

[0054] The pressure loading system is based on a hydraulic station and uses high-temperature silicone oil as a pressurized medium. This oil is pumped into the cladding tube specimen via an oil pump, enabling cyclic loading of the cladding tube's internal pressure. Using high-temperature silicone oil as the pressurized medium enables high-frequency, high-cycle fatigue testing. Due to load fluctuations and frequency control within the power grid, the combustion of the fuel pellets within the cladding tube requires certain control. As a result, the air pressure within the cladding tube fluctuates, subjecting the cladding tube to a certain degree of circumferential cyclic loading. By cyclically loading the cladding tube's interior with a high-pressure joint through the pressure loading system, the circumferential cyclic loading experienced by the cladding tube during operation can be simulated.

[0055] The data acquisition system is used to collect relevant test data during the test process, such as vacuum level data and temperature data within the high-temperature vacuum furnace, cladding tube specimen diameter data, and cladding tube specimen internal loading data. The data acquisition system is primarily composed of sensors, signal conditioning modules, data acquisition cards, computer systems, and software systems.

[0056] This application has effectively solved the problem of internal pressure fatigue testing of fuel cladding after irradiation. Based on the load fluctuation and frequency control of the power grid, the combustion of the internal pellets needs to be controlled to a certain extent, so that the air pressure inside the cladding will fluctuate to a certain extent. The fuel rod cladding is subjected to a certain circumferential cyclic load, which causes the material to fail due to instantaneous fatigue fracture before yielding, seriously affecting the service safety of the nuclear power plant. The internal pressure fatigue test method of the fuel rod cladding tube after irradiation can effectively obtain the fatigue properties of the material and provide technical support for the service of the material. At the same time, this application masters the internal pressure fatigue test technology of the cladding tube after irradiation. After the internal pressure fatigue test technology can accurately obtain the circumferential fatigue properties of the irradiated fuel rod cladding tube, it can provide technical support for the development of new fuel assemblies.

[0057] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for internal pressure fatigue testing of irradiated fuel rod cladding tubes, characterized in that: The following steps are involved: Conduct internal pressure burst test on cladding tube at internal pressure fatigue test temperature to obtain the circumferential yield strength of cladding tube; After checking that there are no scratches on the outside of the cladding tube sample, perform mechanical sealing on the cladding tube sample; Install the sealed cladding tube sample on the high-pressure joint in the high-temperature vacuum furnace; Close the high-temperature vacuum furnace door and set the fatigue test parameters according to the circumferential yield strength of the cladding tube; Exhaust the inside of the cladding tube sample, then start the preload operation to determine whether the sealing performance of the sample is good; If the sealing performance of the sample is good, start the high-temperature vacuum furnace, first evacuate the furnace body, and then start the heating operation when the vacuum degree in the furnace body reaches a certain level; After the temperature in the furnace reaches the test temperature, the temperature fluctuation and gradient during the test are adjusted in real time according to the temperature fluctuation range so that the temperature fluctuation and gradient range meet the test requirements; When the holding time reaches more than 20 minutes, start the pressure loading. When the pressure is loaded to half of the sum of the peak and the trough, the internal pressure or strain amplitude of the cladding tube sample is cyclically loaded according to a certain waveform. The fatigue test is stopped when the cladding tube specimen is broken or the test cycle reaches the rated fatigue life.

2. The internal pressure fatigue test method for irradiated fuel rod cladding tubes according to claim 1, characterized in that: The fatigue test parameters set according to the circumferential yield strength of the cladding tube include initial pressure, test temperature, frequency, rated fatigue life, rated strain amplitude, and allowable temperature fluctuation range.

3. The internal pressure fatigue test method for irradiated fuel rod cladding tubes according to claim 1, characterized in that: When sealing the cladding tube sample, end plugs slightly smaller than the inner diameter of the cladding tube sample are used at both ends of the cladding tube sample. When a ferrule is used for extrusion sealing, the sample is clamped inside and outside to avoid deterioration of the sealing performance of the sample.

4. The internal pressure fatigue test method for irradiated fuel rod cladding tubes according to claim 1, characterized in that: When selecting the stress amplitude or strain amplitude, the Donell-Langer model is used to confirm 4-6 groups of test parameters. After the SN curve is roughly determined, the group experiment method and the lifting and lowering method are used again to conduct detailed tests.

5. The internal pressure fatigue test method for irradiated fuel rod cladding tubes according to claim 4, characterized in that: The group experiment method is used in the high stress level area, and the lifting and lowering method is used in the low stress level area to obtain the SN curve or eN curve.

6. The internal pressure fatigue test method for irradiated fuel rod cladding tubes according to claim 1, characterized in that: The strain amplitude is adjusted in real time when starting the loading. When the strain amplitude reaches the rated strain amplitude, the waveform stabilizes for a certain number of cycles and the pressure peaks and troughs are constant, and subsequent tests are carried out with pressure control.

7. The internal pressure fatigue test method for irradiated fuel rod cladding tubes according to claim 1, characterized in that: The criterion for fatigue failure in the test is that the peak pressure drops by 20% after reaching the maximum value or the specimen fails.

8. The internal pressure fatigue test method for irradiated fuel rod cladding tubes according to claim 1, characterized in that: When the cladding tube sample is pressure loaded, the pressurized medium is injected into the cladding tube sample through the hydraulic pump, and the loading of the test waveform is started. After the waveform is stable, the cycle number is recorded.

9. The internal pressure fatigue test method for irradiated fuel rod cladding tubes according to claim 1, characterized in that: Strain control adjusts the peak pressure in real time during the unstable stage of the test. When the test stress amplitude reaches the set value and stabilizes for a certain period of time, the test control is converted to pressure control.

10. A system for internal pressure fatigue testing of irradiated fuel rod cladding tubes, for implementing the internal pressure fatigue testing method of irradiated fuel rod cladding tubes according to any one of claims 1 to 9, characterized in that: It includes a vacuum system, a heating system, a deformation measurement system, a pressure loading system and a data acquisition system. The vacuum system is used to evacuate the high-temperature vacuum furnace, the heating system is used to maintain the test temperature in the high-temperature vacuum furnace, the deformation measurement system is used to monitor the change in the diameter of the cladding tube sample in real time, the pressure loading system is used to cyclically pressurize the cladding tube sample, and the data acquisition system is used to collect relevant test data during the test.

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