Nuclear reactor pressure vessel bolt strain detection device based on optical fiber sensing technology

Abstract

A nuclear reactor pressure vessel bolt strain detection device based on optical fiber sensing technology comprises a probe light source configured to provide incident light; an optical fiber sensor embedded in the bolt body, with its optical fiber line penetrating through a strain beam and being fixed by a high-temperature-resistant material, the lower end of the strain beam suspending a deflectable gravity pendulum, the optical fiber sensor being configured to receive the incident light and transmit reflected light to the optical spectrum detection device; an optical spectrum detection device configured to receive the light signal; and a strain calculation electronic device configured to determine the strain degree of the bolt body according to the deviation of the gravity pendulum from the axis of the bolt body and the output light difference of the optical fiber sensor obtained by the optical spectrum detection device. The device can provide physical shielding for the sensing part of the optical fiber sensor, delay the damage of irradiation to the optical fiber core, and when the bolt is subjected to a small deformation, the deformation can be 100% non-destructively transmitted to the optical fiber sensor, realizing high-precision detection of the strain of the measured bolt, effectively improving the precision and reliability of the nuclear reactor pressure vessel bolt strain detection, and providing an important guarantee for the safe operation of the nuclear reactor.

Description

Technical Field

[0001] This application belongs to the field of physical sensing and measurement technology, and in particular relates to a strain detection device for bolts of nuclear reactor pressure vessels based on fiber optic sensing technology. Background Technology

[0002] The reactor pressure vessel (RPV) is one of the most critical pieces of equipment in a nuclear power plant reactor system. Its primary function is to contain the reactor core and coolant, and to maintain the reactor's safe operation under extreme conditions. As a major barrier for the nuclear reactor, the RPV not only withstands complex environments such as high temperature, high pressure, and strong neutron radiation, but also must possess extremely high structural strength, corrosion resistance, and long-term operational stability. The design and manufacturing quality of this equipment directly affects the overall safety, reliability, and service life of the nuclear power plant.

[0003] Nuclear reactor pressure vessel bolts (especially the main top cover bolts) are directly related to the reactor's safe sealing and structural integrity, therefore, extremely stringent requirements are placed on materials, manufacturing, inspection, and installation. Nuclear reactor pressure vessel bolts are critical fasteners for nuclear safety level (Level 1), with requirements far exceeding those of conventional industrial bolts. Materials must simultaneously meet requirements for high strength, high toughness, resistance to radiation embrittlement, and resistance to stress corrosion; manufacturing requires comprehensive quality control and rigorous non-destructive testing; and installation requires precise control of the uniformity and consistency of preload to ensure the integrity and sealing of the pressure boundary throughout the reactor's entire lifespan.

[0004] In terms of operational characteristics, RPVs need to possess the following key performance characteristics: resistance to high temperature and high pressure, typically operating at temperatures above 300°C and pressures above 15MPa; resistance to neutron irradiation embrittlement, as materials gradually become embrittled due to long-term exposure to neutron radiation, therefore RPV steel must undergo special treatment to slow down this process; sealing and structural integrity, the RPV's sealing system must ensure no coolant leakage while maintaining structural stability under extreme conditions; maintainability and monitorability, modern RPV designs typically integrate multiple monitoring sensors to monitor their operational status in real time and perform preventative maintenance.

[0005] However, in existing technologies, the main bolts of the top cover of nuclear reactor pressure vessels are often measured and inspected before and after the event, lacking effective in-service measurement methods. This is because in-service measurement requires sensor equipment to possess high strength, high toughness, resistance to radiation embrittlement, and resistance to stress corrosion. To address these issues, current technologies do not provide effective solutions, hence this invention. Summary of the Invention

[0006] This application provides a strain detection device for bolts of a nuclear reactor pressure vessel based on fiber optic sensing technology, which solves the technical problems existing in the prior art.

[0007] In a first aspect, embodiments of this application provide a strain detection device for bolts of a nuclear reactor pressure vessel based on fiber optic sensing technology, comprising: The detection light source is configured to provide incident light; An optical fiber sensor is embedded in the bolt body, with its optical fiber line passing through the strain beam and sealed and fixed by a high-temperature resistant material. A deflectable gravity pendulum is suspended at the lower end of the strain beam. The optical fiber sensor is configured to receive incident light and transmit reflected light to the spectral detection device. The spectral detection equipment is configured to receive optical signals; The strain calculation electronic device is configured to determine the degree of strain of the bolt body based on the deviation of the gravity pendulum from the axis of the bolt body, combined with the difference in reflected light from the fiber optic sensor obtained by the spectral detection device.

[0008] In a preferred embodiment, the optical fiber of the optical fiber sensor is a radiation-resistant quartz optical fiber. Preferably, the optical fiber of the optical fiber sensor is a metal-coated quartz optical fiber. Preferably, the optical fiber metal coating of the optical fiber sensor is either a nickel coating or a gold coating.

[0009] In a preferred embodiment, the device further includes an optical fiber connector partially embedded in the bolt body and a sealed housing that engages and seals with the optical fiber connector, wherein the optical fiber of the optical fiber sensor enters the sealed housing through the optical fiber connector. Preferably, the strain beam is fixedly installed inside the sealed housing, and an optical fiber through hole is formed on the strain beam. The optical fiber of the optical fiber sensor enters the optical fiber through hole inside the sealed housing after passing through the optical fiber connector, and is sealed and fixed with the optical fiber through hole. Preferably, the optical fiber through-hole is provided with a first sealing position and a second sealing position. The first sealing position is located at the entrance of the optical fiber sensor after the optical fiber line passes through the optical fiber connector and enters the optical fiber through-hole in the sealed housing; the second sealing position is located at the exit of the optical fiber through-hole.

[0010] In a preferred embodiment, when the fiber optic sensor is located in the bolt body and the fiber optic line is in the first sealing position, it is sealed by gold-tin eutectic brazing and then sealed a second time with high-temperature resistant adhesive. Preferably, when the fiber optic sensor is located in the bolt body and the fiber optic line is in the second sealing position, it is sealed by gold-tin eutectic brazing and then sealed a second time with high-temperature resistant adhesive. Preferably, the solder used for the gold-tin eutectic brazing seal is Au80Sn20 gold-tin eutectic solder; Preferably, the high-temperature resistant adhesive is a silicone adhesive, a polyimide adhesive, or an epoxy adhesive.

[0011] In a preferred embodiment, the device further includes an optical fiber connector partially embedded in the bolt body and a sealed housing that engages and seals with the optical fiber connector, wherein the optical fiber lines of the first optical fiber sensor and the second optical fiber sensor enter the sealed housing through the optical fiber connector. Preferably, the sealed housing is filled with inert gas; Preferably, the inert gas is selected from one or more of helium, argon, nitrogen, neon, and krypton.

[0012] In a preferred embodiment, the strain beam is fixedly installed inside the sealed housing. At least two optical fiber through holes are opened on the strain beam. The optical fibers of the first optical fiber sensor and the second optical fiber sensor are arranged opposite to each other on both sides of the strain beam. After passing through the optical fiber connector, the optical fibers of the first optical fiber sensor and the second optical fiber sensor respectively enter the optical fiber through holes in the sealed housing and are adhered and fixed to the optical fiber through holes. Preferably, the optical fibers of the first and second optical fiber sensors are bonded and fixed to the optical fiber through-holes using high-temperature resistant adhesive. Preferably, the high-temperature resistant adhesive is selected from adhesives with a temperature resistance range of 400°C or higher.

[0013] In a preferred embodiment, the sensing portions of the first and second fiber optic sensors are located at the points where the fiber optic lines of the first and second fiber optic sensors enter the strain beam. Preferably, the optical fibers of the first and second optical fiber sensors terminate at the lower end face of the strain beam after entering the strain beam. Preferably, the sensing parts of the first fiber optic sensor and the second fiber optic sensor are at the same height level.

[0014] In a preferred embodiment, a first gap is provided between the lower end face of the strain beam and the upper end face of the gravity pendulum, and the distance between the optical fiber lines of the first optical fiber sensor and the lower end face of the strain beam after entering the strain beam is greater than or equal to the first gap. Preferably, the suspension connection between the gravity pendulum and the strain beam is located at the center of the lower end face of the strain beam.

[0015] In a preferred embodiment, the sealed shell is a blind end at the lower end of the gravity pendulum, and the cavity of the blind end is filled with inert gas. Preferably, the sealed housing is provided with a sealing cover at the lower end of the gravity pendulum, the sealing cover is filled with inert gas, and the lower end of the gravity pendulum is free.

[0016] In a preferred embodiment, the upper end of the strain beam is fixed to the optical fiber connector; Preferably, the side of the strain beam is clearance-fitted with the sealed shell; Preferably, the center of the upper end face of the gravity pendulum and the center of the lower section of the strain beam are located on the axis of the sealed shell, and the area of ​​the upper end face of the gravity pendulum is less than or equal to the area of ​​the lower section of the strain beam. Preferably, the upper end face of the gravity pendulum and the lower cross-section of the strain beam are concentric circles when projected onto the lower cross-section of the strain beam, and the area of ​​the upper end face of the gravity pendulum is smaller than the area of ​​the lower cross-section of the strain beam.

[0017] In a preferred embodiment, the device further includes a third optical fiber sensor, wherein the optical fibers of the first optical fiber sensor, the second optical fiber sensor, and the third optical fiber sensor extend along the axis of the sealed housing and are uniformly distributed circumferentially according to the axis of the sealed housing.

[0018] In a preferred embodiment, the device further includes an optical fiber coupler configured to transmit the detection light emitted by the detection light source to the optical fiber lines of the first optical fiber sensor and the second optical fiber sensor, and to transmit the reflected light from the optical fiber lines to the spectral detection device.

[0019] In a preferred embodiment, the strain calculation electronic device is an electronic device equipped with a strain calculation module. Preferably, the fiber optic coupler is an optical circulator.

[0020] In this technical solution, after the optical fiber enters the fiber optic through-hole, it is sealed and fixed to the through-hole using high-temperature resistant adhesive and gold-tin eutectic brazing. This prevents micro-bending loss in the optical fiber. The sealing structure (especially the combination of metal-coated silica optical fiber and gold-tin brazing) provides physical shielding for the sensing component, delaying irradiation damage to the fiber core. When the bolt expands or contracts due to heat or is under pressure, the optical fiber may randomly bend or twist, resulting in severe "micro-bending loss" during transmission, preventing the spectral detection equipment from acquiring a valid signal. In reactor operation or vibration environments, gravity pendulums generate mechanical tensile or shear forces. A secure fixation prevents the optical fiber from shifting, breaking, or fatigue-damaged under long-term stress. Through sealing (such as using high-temperature resistant adhesive or gold-tin eutectic brazing), the optical fiber sensor is firmly bonded to the bolt body or the internal strain beam. When the bolt undergoes slight deformation under stress, the deformation is transmitted to the optical fiber sensor 100% without loss, ensuring that the data acquired by the spectral detection equipment accurately reflects the strain level of the bolt and guarantees the accuracy of strain transmission.

[0021] This invention significantly improves the detection accuracy of minute strains in bolts of nuclear reactor pressure vessels by introducing a collaborative working mechanism of fiber optic sensors and a gravity pendulum. Specifically, a first and a second fiber optic sensor are embedded in the bolt body and connected to the gravity pendulum via a strain beam, forming a highly sensitive detection system. When the bolt experiences minute strain, the strain beam bends due to the change in force on the gravity pendulum, causing the pendulum to deviate from its initial position. This deviation causes a change in the reflected light signal from the fiber optic sensor, which is captured by a spectral detection device and transmitted to a strain calculation electronics device for analysis. Furthermore, this invention employs a non-contact detection principle similar to line laser measurement technology, enabling precise capture of micron-level strains, further improving the detection resolution and accuracy. Current verification experimental results show that this technology can effectively detect strains smaller than 10 micrometers in practical applications, demonstrating significant advantages over traditional methods.

[0022] This invention incorporates numerous optimizations in structural design and material selection to enhance the long-term stable operation of the device and reduce the impact of environmental factors on the detection results. First, the core component—the fiber optic sensor—utilizes a special radiation-resistant fiber optic material that is resistant to high temperatures and corrosion, ensuring stable performance even under the high-temperature, high-pressure, and strong radiation environment inside a nuclear reactor. Second, the sealed housing design effectively protects the fiber optic connectors and internal optical components from external contamination and mechanical damage. In particular, filling the sealed housing with an inert gas (such as helium or argon) not only reduces the risk of oxidation but also enhances the device's anti-interference capabilities, ensuring the accuracy and consistency of the detection results. Attached Figure Description

[0023] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly introduced below. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0024] Figure 1 This is a schematic diagram of the structure of a nuclear reactor pressure vessel according to an embodiment of this application.

[0025] Figure 2 This is a schematic diagram illustrating the application scenario of the strain detection device for bolts of a nuclear reactor pressure vessel according to an embodiment of this application.

[0026] Figure 3 A schematic diagram of the specific structure of the strain detection device for the bolts of a nuclear reactor pressure vessel provided in this embodiment of the application.

[0027] Figure 4 for Figure 3 BB-direction cross-section view.

[0028] Figure 5 for Figure 3 Enlarged view of point A.

[0029] Figure 6 The diagram shows the state of the nuclear reactor pressure vessel bolt strain detection device before strain, as provided in the embodiments of this application.

[0030] Figure 7 A diagram showing the strain state of a nuclear reactor pressure vessel bolt strain detection device provided in an embodiment of this application.

[0031] Figure 8 This is a schematic diagram of the strain calculation electronic device for the strain detection device of the nuclear reactor pressure vessel bolts provided in the embodiments of this application. Detailed Implementation

[0032] The features and exemplary embodiments of various aspects of this application will be described in detail below. To make the objectives, technical solutions, and advantages of this application clearer, the application will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are only intended to explain this application and not to limit it. For those skilled in the art, this application can be implemented without some of these specific details. The following description of the embodiments is merely to provide a better understanding of this application by illustrating examples.

[0033] like Figure 1 As shown, Figure 1A structural schematic diagram of a nuclear reactor pressure vessel is provided. Structurally, a nuclear reactor pressure vessel is typically made of high-strength low-alloy steel, and its main body includes a cylindrical shell 1, a top cover 2, end caps 3, and nozzles for connecting various pipes and control devices. The inner wall of the shell is usually covered with a stainless steel weld overlay to enhance corrosion resistance. The reactor core is housed inside the vessel and surrounded by a coolant (such as light water, heavy water, or liquid metal) to facilitate heat conduction and transfer. The design of the RPV must meet stringent nuclear safety standards to ensure effective prevention of radioactive material leakage under normal operation, abnormal conditions, and even accident scenarios. During long-term service, the main bolts 4 of the top cover may experience significant strain due to transient conditions such as temperature changes and pressure fluctuations, which can affect the sealing effect at the flange connection. Therefore, accurate strain detection of the main bolts 4 of the top cover is of significant engineering importance.

[0034] like Figure 2 The diagram illustrates an application scenario of the nuclear reactor pressure vessel bolt strain detection device 5 of this invention. The device is installed within the bolt body of the main top cover bolt 4. During operation, the main bolts of the nuclear reactor pressure vessel face a complex mechanical environment. Especially under transient conditions, the temperature hysteresis effect leads to a significant increase in the alternating stress amplitude of the main bolts, resulting in problems such as accumulated plastic deformation and changes in flange separation. These problems not only affect sealing performance but may also jeopardize the overall safety of the reactor. Currently, traditional physical measurement methods typically rely on external sensors or mechanical measuring tools, making real-time monitoring during bolt use difficult. Post-operative physical testing requires disassembling the bolts for offline analysis, failing to reflect the strain under operating conditions in a timely manner. Testing must be performed while the system is shut down, increasing maintenance costs and potentially causing production interruptions. Therefore, developing a technology capable of real-time and accurate monitoring of bolt strain under complex operating conditions is crucial. This invention's nuclear reactor pressure vessel bolt strain detection device 5 provides a detection device based on fiber optic sensing technology. By introducing a highly sensitive fiber optic sensor and an intelligent data processing module, it effectively overcomes the shortcomings of traditional detection methods. This invention's detection device employs fiber optic sensing technology. By embedding a first and second fiber optic sensor within the bolt body and incorporating a strain beam and gravity pendulum structure, combined with a detection light source, spectral detection equipment, and strain calculation electronics, it achieves high-precision detection of the bolt's strain. Specifically, when the bolt experiences strain, the gravity pendulum on the strain beam deviates from the bolt's axis, causing a difference in the reflected light signals from the first and second fiber optic sensors. The strain calculation electronics determine the degree of strain based on this difference. This technical solution effectively improves the accuracy and reliability of strain detection in nuclear reactor pressure vessel bolts, providing crucial assurance for the safe operation of nuclear reactors.

[0035] like Figures 3-7The diagram shows a structural schematic of a strain detection device 5 for a nuclear reactor pressure vessel bolt based on fiber optic sensing technology. It includes: a detection light source 51 configured to provide incident light; a fiber optic sensor embedded in the bolt body, the fiber optic sensor passing through a fixed strain beam 55, with a deflectable gravity pendulum 56 suspended at the lower end of the strain beam 55; the fiber optic line of the fiber optic sensor passing through the strain beam sealed with a high-temperature resistant material, configured to receive incident light and transmit reflected light to the spectral detection device; a spectral detection device 52 configured to receive optical signals; and a strain calculation electronics device 57 configured to determine the strain degree of the bolt body based on the deviation of the gravity pendulum from the bolt body's axis and the difference in reflected light from the fiber optic sensor obtained by the spectral detection device. The embedded design of the fiber optic sensor is a key element in achieving high-precision strain detection based on fiber optic sensing technology. This embedded design not only effectively ensures the strain transfer efficiency between the fiber optic sensor and the bolt body but also minimizes the impact on the bolt's structural integrity, thereby meeting the safe operation requirements of nuclear reactor pressure vessel bolts under high stress conditions. In this device, a fiber optic sensor is embedded along the central axis of the bolt body, penetrating its entire length to ensure comprehensive sensing of strain changes during stress application. The fiber optic sensor passes through a fixed strain beam and is sealed with a high-temperature resistant material to ensure stable signal transmission even in complex environments.

[0036] In a preferred embodiment of the present invention, the optical fiber of the fiber optic sensor is a radiation-resistant quartz optical fiber; preferably, the optical fiber of the fiber optic sensor is a metal-coated quartz optical fiber; preferably, the metal coating of the optical fiber of the fiber optic sensor is one of nickel plating or gold plating. The fiber optic sensor uses radiation-resistant quartz optical fiber or metal-coated quartz optical fiber to enhance its stability and durability in a nuclear radiation environment. In the strain detection device for bolts of a nuclear reactor pressure vessel, the sealing design of the optical fiber at the strain beam is one of the key aspects to ensure detection accuracy. Since the operating environment of a nuclear reactor is usually accompanied by high temperatures, the selection of sealing materials must focus on their high-temperature resistance, sealing effect, and impact on signal transmission. Encapsulating the sensing part of the fiber optic sensor within the strain beam prevents other substances from affecting it during strain detection, thus ensuring detection accuracy. Radiation-resistant quartz optical fiber, due to its high purity and uniform refractive index distribution, can maintain stable optical performance under extreme conditions; while metal-coated quartz optical fiber further improves the reliability of the sensor by enhancing surface hardness and bending resistance. Experimental results show that the sensor using metal-coated quartz fiber exhibits lower signal drift rate and higher durability under high temperature and high pressure conditions.

[0037] In a preferred embodiment of the present invention, the device further includes an optical fiber connector partially embedded in the bolt body and a sealed housing that engages and seals with the optical fiber connector. The optical fiber of the optical fiber sensor enters the sealed housing through the optical fiber connector. Preferably, the strain beam is fixedly installed inside the sealed housing, and an optical fiber through-hole is formed on the strain beam. The optical fiber of the optical fiber sensor enters the optical fiber through-hole inside the sealed housing after passing through the optical fiber connector and is sealed and fixed with the optical fiber through-hole. Preferably, a first sealing position 541 and a second sealing position 542 are provided on the optical fiber through-hole. The first sealing position is located at the entrance of the optical fiber sensor's optical fiber through the optical fiber connector into the optical fiber through-hole inside the sealed housing; the second sealing position is located at the exit of the optical fiber through-hole. Through the sealing structure (especially the combination of metal-coated quartz optical fiber and gold-tin brazing), physical shielding can be provided for the sensing part, delaying the damage of radiation to the optical fiber core.

[0038] In a preferred embodiment, when the fiber optic sensor is located within the bolt body and the fiber optic cable is in the first sealing position, it is sealed by gold-tin eutectic brazing followed by a secondary seal using high-temperature adhesive. Preferably, when the fiber optic sensor is located within the bolt body and the fiber optic cable is in the second sealing position, it is sealed by gold-tin eutectic brazing followed by a secondary seal using high-temperature adhesive. Preferably, the solder used for the gold-tin eutectic brazing is Au80Sn20 gold-tin eutectic solder. Preferably, the high-temperature adhesive is silicone adhesive, polyimide adhesive, or epoxy adhesive. Gold-tin eutectic brazing, as a highly efficient sealing technology, plays a crucial role in fiber optic cable sealing. This process utilizes the eutectic structure formed by Au80Sn20 gold-tin eutectic solder at a specific temperature to achieve a reliable connection between the fiber optic cable and the sealing structure. Au80Sn20 solder, due to its low melting point (approximately 280°C), good wettability, and excellent mechanical properties, is an ideal choice for fiber optic sealing in high-temperature environments. In practical applications, the gold-tin eutectic brazing process, by precisely controlling parameters such as heating temperature, holding time, and cooling rate, can effectively prevent fiber damage caused by thermal stress, while ensuring the long-term stability of the sealed joint. The advantages of the gold-tin eutectic brazing process are not only reflected in its superior sealing effect but also in its adaptability to complex structures. For example, during the process of fiber optic cables passing through strain gauge beams, traditional sealing methods often struggle to achieve a complete seal due to the presence of fiber optic vias. The gold-tin eutectic brazing process, with its excellent flowability and filling capacity, effectively solves this problem, ensuring that the sealing quality of the fiber optic cable at the strain gauge beam meets design requirements. Furthermore, this process exhibits high repeatability and reliability, making it suitable for large-scale production applications.

[0039] In a specific embodiment of the present invention, the device includes: a detection light source 51 and a spectral detection device 52 disposed outside the bolt body; the detection light source 51 is configured to provide incident light; the spectral detection device 52 is configured to receive optical signals; a first fiber optic sensor 53 and a second fiber optic sensor 54 embedded in the bolt body are configured to receive incident light and transmit reflected light to the spectral detection device; a strain beam 55 is disposed between the first fiber optic sensor 53 and the second fiber optic sensor 54, the first fiber optic sensor 53 and the second fiber optic sensor 54 are fixed to the surface of the strain beam 55, and a deflectable gravity pendulum 56 is suspended from the lower end of the strain beam 55; a strain calculation electronic device 57 is configured to determine the strain degree of the bolt body based on the deviation of the gravity pendulum from the axis of the bolt body and the difference in reflected light obtained by the spectral detection device 52 from the first fiber optic sensor 53 and the second fiber optic sensor 54. This invention provides a strain detection device for bolts of nuclear reactor pressure vessels based on fiber optic sensing technology. This device achieves high-precision real-time monitoring of bolt strain through the coordinated operation of the detection light source, the spectral detection device, the fiber optic sensor, the strain beam, and the gravity pendulum. Its design fully considers the extreme environments of nuclear reactors, such as high temperature, high pressure, and strong neutron radiation, and possesses characteristics such as high strength, high toughness, resistance to radiation embrittlement, and resistance to stress corrosion. Experimental results show that this detection device can effectively solve the problems in existing technologies, providing important protection for the safe operation of nuclear power plants. Furthermore, its technical principles can be extended to the monitoring of equipment in other similar complex environments.

[0040] In a specific embodiment of the present invention, the device further includes an optical fiber connector 532 partially embedded in the bolt body and a sealed housing 531 that engages and seals with the optical fiber connector 532. The optical fibers of the first optical fiber sensor 53 and the second optical fiber sensor 54 enter the sealed housing 531 through the optical fiber connector 532. The optical fiber connector is partially embedded in the bolt body and connected to the sealed housing through an engaging and sealing structure. In a specific embodiment, the sealed housing is filled with an inert gas. The design of the sealed housing not only effectively protects the internal optical fiber sensors and other key components but also further enhances the stability and reliability of the device by filling it with an inert gas. Specifically, the inert gas filling the sealed housing can be one or more mixed gases selected from helium, argon, nitrogen, neon, or krypton. Filling the sealed housing with an inert gas, such as helium or argon, provides good environmental isolation and heat dissipation performance. The filling pressure of the inert gas is controlled between 0.2 MPa and 0.5 MPa to prevent external media from penetrating and improve the radiation resistance of the housing. These inert gases possess excellent chemical and thermal stability, maintaining their performance even under harsh environments such as high temperature and high pressure, thereby reducing the impact of environmental factors on the detection results. Furthermore, the interlocking and sealing structure between the fiber optic connector and the enclosed housing effectively prevents the ingress of external impurities, avoiding signal attenuation or measurement errors caused by contamination. Experimental results show that, with this preferred embodiment, the detection accuracy and long-term stability of the device are significantly improved.

[0041] In a specific embodiment of the present invention, the strain beam 55 is fixedly installed inside the sealed housing 531. At least two fiber optic through-holes 533 are formed on the strain beam 55. The fiber optic cables of the first fiber optic sensor 53 and the second fiber optic sensor 54 are arranged opposite each other on both sides of the strain beam 55. After passing through fiber optic connectors 532, the fiber optic cables of the first and second fiber optic sensors respectively enter the fiber optic through-holes 533 within the sealed housing and are adhered and fixed to the through-holes. Specifically, the strain beam 55 is installed inside the sealed housing by fasteners to ensure that it will not loosen or shift during long-term use. Furthermore, at least two fiber optic through-holes are formed on the strain beam for fixing the fiber optic cables of the first and second fiber optic sensors. After entering the fiber optic through-holes, the fiber optic cables are adhered and fixed to the through-holes using high-temperature resistant adhesive, thereby further enhancing the stability of the device. The present invention recommends using adhesives with a temperature resistance range of over 400°C to meet the operational requirements of nuclear reactor pressure vessels in high-temperature environments and to maintain long-term stability in the high-temperature environment of nuclear reactors. Experiments show that fixing the optical fiber with high-temperature resistant adhesive results in superior performance under extreme temperature conditions, and significantly improves the signal transmission stability of the optical fiber sensor. This fixing method not only enhances the mechanical strength of the device but also reduces signal interference caused by vibration or temperature changes, thus providing strong support for high-precision detection.

[0042] Two fiber optic sensors are symmetrically embedded along the central axis of the bolt body, with their sensing units located within the bolt's effective tensile region to directly detect strain changes during stress. The embedding depth of the fiber optic sensors is designed based on the bolt's specific dimensions and stress characteristics, typically ranging from 2 / 3 to 4 / 5 of its total length, to ensure sufficient coverage of the bolt's high-stress areas. During embedding, the fiber optic sensors are fixed using high-temperature resistant epoxy resin to prevent displacement or loosening during use. Since nuclear reactor operation typically involves high-temperature conditions, the selection of sealing materials must prioritize their high-temperature resistance, sealing effectiveness, and impact on signal transmission. Silicone sealant, polyimide sealant, and epoxy sealant are preferred high-temperature resistant sealing materials, reducing stress concentration caused by thermal expansion and protecting the fiber optic cables from mechanical damage.

[0043] In a specific embodiment of the present invention, the sensing portion 540 of the first optical fiber sensor 53 and the second optical fiber sensor 54 is located at the portion where the optical fibers of the first and second optical fiber sensors enter the strain beam; more preferably, the optical fibers of the first and second optical fiber sensors terminate at the lower end face of the strain beam after entering it; specifically preferably, the sensing portions 540 of the first and second optical fiber sensors are at the same height level. The sensing portions 540 of the first and second optical fiber sensors are located at the portion where their optical fibers enter the strain beam, and the optical fibers terminate at the lower end face of the strain beam after entering it. To ensure that the sensing portions 540 are at the same height level, this patent employs precision machining and calibration processes, and ensures the consistency of the two sensing portions 540 in the height direction by strictly controlling the length and position of the optical fibers. The importance of this design is that when the bolt is strained, the strain beam will uniformly transmit the stress to the first and second optical fiber sensors, and the fact that the sensing portions 540 are at the same height level can effectively avoid measurement errors caused by height differences. Experimental results show that after adopting this preferred embodiment, the detection accuracy of the device is significantly improved, especially in the detection of small strains. The first and second fiber optic sensors are set to be two radiation-resistant fiber optic sensors of the same specification. It is preferable to use radiation-resistant quartz fiber optic sensors. This is also to avoid the influence of temperature fluctuations on the measurement results. The temperature of the other fiber optic sensor can be calibrated by the first or second fiber optic sensor.

[0044] In a specific embodiment of the present invention, a first gap D1 is provided between the lower end face of the strain beam and the upper end face of the gravity pendulum. The distance between the fiber optic cables of the first and second fiber optic sensors and the lower end face of the strain beam after entering the strain beam is greater than or equal to the first gap D1. Preferably, the suspension connection 534 between the gravity pendulum and the strain beam is located at the center of the lower end face of the strain beam. The present invention provides a detailed design for the gap between the lower end face of the strain beam and the upper end face of the gravity pendulum. Specifically, a first gap is provided between the lower end face of the strain beam and the upper end face of the gravity pendulum, and the distance between the fiber optic cables of the first and second fiber optic sensors and the lower end face of the strain beam after entering the strain beam is greater than or equal to the first gap. This design effectively prevents the fiber optic cables from being squeezed or stretched during the deflection of the gravity pendulum, thereby avoiding signal distortion caused by mechanical stress. Furthermore, the symmetrical design, with the suspension connection 534 between the gravity pendulum and the strain beam located at the center of the lower end face of the strain beam, ensures that the gravity pendulum remains in a balanced state during deflection, thus providing a more accurate reference for strain detection. Experimental results show that the strain detection accuracy of the device is significantly improved after adopting this preferred embodiment, especially its detection performance under complex working conditions is even better.

[0045] In a specific embodiment of the invention, the sealed housing is a blind end at the lower end of the gravity pendulum, and the cavity of the blind end is filled with inert gas. In a more specific embodiment, a sealing cover is provided at the lower end of the sealed housing, the sealing cover is filled with inert gas, and the lower end of the gravity pendulum is free. Regarding the blind end or sealing cover configuration of the sealed housing at the lower end of the gravity pendulum, the invention presents two optional solutions: one is to adopt a blind end structure and fill the cavity of the blind end with inert gas to enhance the sealing and stability of the device; the other is to adopt a sealing cover structure, the sealing cover is filled with inert gas, and the lower end of the gravity pendulum remains free to allow the gravity pendulum to deflect flexibly under external forces. Furthermore, the connection and fit between the strain beam, the fiber optic connector, and the sealed housing are also carefully designed, and the influence of mechanical stress on the fiber optic sensor is reduced by optimizing the connection structure. Regarding the positional relationship between the gravity pendulum and the strain beam, this invention ensures that the center of the upper end face of the gravity pendulum and the center of the lower end face of the strain beam are located on the axis of the sealed shell, and that the area of ​​the upper end face of the gravity pendulum is less than or equal to the area of ​​the lower end face of the strain beam, thereby avoiding measurement errors caused by excessive deflection range of the gravity pendulum.

[0046] To prevent mechanical damage to the fiber optic sensor during embedding, a flexible metal sheath is wrapped around the fiber optic cable. This sheath not only provides additional mechanical protection but also enhances the sensor's radiation resistance. Furthermore, after entering the bolt body, the fiber optic cable gradually transitions to a straight state through a transition region to reduce bending loss during optical signal transmission. To ensure good adhesion between the fiber optic sensor and the bolt body, the inner surface of the bolt is sandblasted before embedding to improve surface roughness and enhance bond strength. Experimental results show that the fiber optic sensor fixed using the above embedding method exhibits excellent stability and sensitivity during long-term operation, meeting the high requirements for strain detection of bolts in nuclear reactor pressure vessels.

[0047] In a specific embodiment of the present invention, the device further includes a third optical fiber sensor. The optical fibers of the first, second, and third optical fiber sensors extend along the axis of the sealed housing and are uniformly distributed circumferentially along the axis of the sealed housing. The present invention also proposes an additional third optical fiber sensor, where the optical fibers of the first, second, and third optical fiber sensors extend along the axis of the sealed housing and are uniformly distributed circumferentially along the axis of the sealed housing, thereby further improving detection accuracy.

[0048] In a specific embodiment of the present invention, the upper end of the strain beam is fixed to the optical fiber connector; preferably, the side of the strain beam is clearance-fitted with the sealed shell. The strain beam adopts a cantilever beam structure, and its length, width, and thickness are designed according to the specific bolts. This strain beam can produce bending deformation when the bolts experience minor strain, thereby amplifying the strain signal and transmitting it to the optical fiber sensor. The strain beam is made of high-strength low-alloy steel, which has excellent resistance to radiation embrittlement and stress corrosion, and can maintain long-term stability in the complex environment of a nuclear reactor. In addition, the surface of the strain beam is precision polished to reduce the impact of surface roughness on optical signal transmission.

[0049] Preferably, the center of the upper end face of the gravity pendulum and the center of the lower end face of the strain beam are located on the axis of the sealed shell, and the area of ​​the upper end face of the gravity pendulum is less than or equal to the area of ​​the lower end face of the strain beam; preferably, the upper end face of the gravity pendulum and the lower end face of the strain beam are concentric circles projected onto the lower end face of the strain beam, and the area of ​​the upper end face of the gravity pendulum is less than the area of ​​the lower end face of the strain beam. The sensing parts of the first fiber optic sensor and the second fiber optic sensor are located in the middle or lower region of the strain beam to maximize the sensing of strain changes. The fiber optic sensors are bonded and fixed to the strain beam with high-temperature resistant adhesive, and the thickness of the adhesive layer is controlled within 0.1 mm to reduce the influence of the adhesive layer on strain transmission. The gravity pendulum is suspended from the center of the lower end face of the strain beam by a flexible metal strip. The design of the suspension point takes into account the balance and sensitivity of the gravity pendulum to ensure that it can respond quickly when the strain beam undergoes slight deformation. Experimental results show that the strain beam designed above exhibits good linear response characteristics and high sensitivity under simulated working conditions, which can meet the requirements of strain detection of bolts in nuclear reactor pressure vessels.

[0050] The sensitivity principle of the gravity pendulum is based on the high sensitivity of its center of mass position to the bending deformation of the strain beam. When the bolt experiences axial tensile or compressive strain, the suspension point of the gravity pendulum shifts, causing a corresponding bending deformation in the strain beam. Since the center of mass of the gravity pendulum is fixed, there is a clear mathematical relationship between its swing angle and the amount of bending deformation of the strain beam. The swing angle of the gravity pendulum can be measured by the change in the fiber optic sensor, allowing for the indirect calculation of the bending deformation of the strain beam, and thus the deduction of the bolt's strain level. Experimental results show that the gravity pendulum design can achieve high-precision detection within a micro-strain range, with a sensitivity of up to 0.01 mm / m, meeting the high requirements for strain detection of bolts in nuclear reactor pressure vessels.

[0051] In a preferred embodiment of the present invention, the device further includes an optical fiber coupler 58, which is configured to transmit the probe light emitted by the probe light source 51 to the optical fiber lines of the first optical fiber sensor 53 and the second optical fiber sensor 54, and to transmit the reflected light from the optical fiber lines to the spectral detection device 52. The strain calculation electronics is an electronic device equipped with a strain calculation module; specifically, preferably, the optical fiber coupler 58 is an optical circulator. The present invention also introduces an optical fiber coupler for transmitting the probe light emitted by the probe light source to the optical fiber lines of the first optical fiber sensor and the second optical fiber sensor, and for transmitting the reflected output light from the optical fiber lines to the spectral detection device. The application of the optical fiber coupler not only simplifies the optical path design but also improves signal transmission efficiency, thus providing strong support for the overall performance optimization of the device. The probe light source uses a narrow-linewidth laser as the light source, and the emitted probe light has high monochromaticity and coherence, which can significantly improve detection accuracy. The spectral detection device has a built-in high-sensitivity photodetector and a signal processing module, which can convert the received optical signal into an electrical signal and perform preliminary processing, providing a reliable data foundation for subsequent strain calculations.

[0052] During operation, the probe light emitted by the detection source is first transmitted to the incident ends of the first and second fiber optic sensors via fiber optic couplers, and then coupled into the fiber optic cable and propagates along it. Since the fiber optic sensors are embedded within the bolt body, when the bolt experiences strain, the optical signal in the fiber optic cable undergoes wavelength or intensity changes due to the strain effect. These changing optical signals are ultimately transmitted to the spectral detection equipment, which demodulates and analyzes the signals. By comparing the difference between the input and output light, the bolt's strain information can be accurately obtained. Furthermore, to further improve the system's anti-interference capability, a shielded cable is used to connect the detection source and the spectral detection equipment, and an electromagnetic shielding layer is installed on the equipment casing to reduce the impact of electromagnetic radiation on signal transmission.

[0053] When probe light passes through an optical fiber sensor, parameters such as the intensity, wavelength, and phase of the optical signal undergo minute changes due to variations in the external environment. These changes are captured by the spectral detection device and converted into electrical signals for subsequent analysis. Specifically, the sensing element in the optical fiber sensor is located near the strain beam. When probe light passes through this region, the presence of the strain beam and its potential minute strain cause phenomena such as scattering, refraction, or absorption of the optical signal, resulting in a significant difference between the output and input light. Furthermore, the spectral detection device, through high-precision spectral analysis technology, can accurately measure the changes in the spectral characteristics of the output light, providing reliable data support for subsequent strain calculations.

[0054] When the bolt experiences strain, the strain beam, as a key component, drives the movement of the fiber optic sensors and the gravity pendulum, resulting in changes in the optical signal. The specific mechanism of this change can be explained through dynamic analysis of the strain beam and the gravity pendulum. First, the strain beam is designed to generate elastic strain when subjected to external stress. This strain is directly transmitted to the first and second fiber optic sensors fixed to the surface of the strain beam. Due to the extremely high sensitivity of fiber optic sensors to mechanical strain, the refractive index distribution between the core and cladding changes with strain, thereby causing changes in the phase, intensity, or polarization state of the optical signal. Simultaneously, the presence of the gravity pendulum further enhances the system's ability to sense strain. When the bolt experiences axial or radial strain, the lower end of the strain beam deflects, causing the gravity pendulum to deviate from its initial equilibrium position. This deviation leads to changes in the relative positions between the fiber optic sensors, thus amplifying the differences in the optical signal.

[0055] The strain calculation electronic device accurately calculates the strain of a bolt by analyzing the differences in optical signals acquired by a spectral detection device and considering the specific circumstances of the gravity pendulum's deviation from the bolt axis. Its core algorithm is based on the relationship between the phase difference of the optical signals and the deflection angle of the gravity pendulum. First, the spectral detection device measures and records the output optical signals of the first and second fiber optic sensors, including their wavelength, intensity, and phase information. Since the two fiber optic sensors are relatively fixed and close together, the difference in their output optical signals mainly originates from the motion of the strain beam and the gravity pendulum. Second, the strain calculation electronic device uses the principle of interferometry to determine the strain of the strain beam by analyzing the phase difference of the output optical signals. Specifically, the phase difference of the optical signals is proportional to the strain of the strain beam, and this proportionality coefficient can be obtained through experimental calibration. Furthermore, the deflection angle of the gravity pendulum is geometrically related to the strain of the strain beam. For example, when the gravity pendulum deviates from the bolt axis by a certain angle, the strain of the strain beam can be calculated using trigonometric functions. Finally, the strain calculation electronic device, combining data from the optical signal phase difference and the deflection angle of the gravity pendulum, calculates the strain degree of the bolt using the following formula: ε = KΔφ / L, where ε represents the strain, K is the proportionality coefficient, Δφ is the optical signal phase difference, and L is the effective length of the strain beam. Through this method, the detection device can achieve high-precision measurement of bolt strain, providing crucial assurance for the safe operation of nuclear reactors.

[0056] like Figure 8As shown, the strain calculation electronic device of the present invention includes: a memory 220 for storing instructions; and a processor 210 for executing the instructions to enable the device to implement the strain calculation method described above. More specifically, the electronic device also includes a communication module. In this solution, the processor may include one or more processing units. For example, the processor may include one or more of the following: application processor (AP), modem, graphics processing unit (GPU), image signal processor (ISP), controller, video codec, digital signal processor (DSP), baseband processor, and / or neural-network processing unit (NPU). Different processing units may be independent devices or integrated into one or more processors.

[0057] The memory 220 may store a program, which can be executed by the processor 210, causing the processor 210 to perform the methods provided in this application. The memory 220 may also store data. The processor 210 may read the data stored in the memory 220. The memory 220 and the processor 210 may be configured separately. Optionally, the memory 220 may also be integrated into the processor 210.

[0058] The communication module 230 may include at least one of a mobile communication module and a wireless communication module. When the communication module 230 includes a mobile communication module, it can provide wireless communication solutions, including 2G / 3G / 4G / 5G, for use on the electronic device 200. Examples include Global System for Mobile Communications (GSM), General Packet Radio Service (GPRS), Code Division Multiple Access (CDMA), Wideband Code Division Multiple Access (WCDMA), Time Division Code Division Multiple Access (TD-SCDMA), Long Term Evolution (LTE), and New Radio (NR).

[0059] In the above embodiments, implementation can be achieved, in whole or in part, through software, hardware, firmware, or any combination thereof. When implemented using software, it can be implemented, in whole or in part, as a computer program product. The computer program product includes one or more computer instructions. When the computer program instructions are loaded and executed on a computer, all or part of the processes or functions described in the embodiments of this application are generated. The computer can be a general-purpose computer, a special-purpose computer, a computer network, or other programmable device. The computer instructions can be stored in a computer-readable storage medium or transmitted through the computer-readable storage medium. The computer instructions can be transmitted from one website, computer, server, or data center to another website, computer, server, or data center via wired (e.g., coaxial cable, fiber optic, digital subscriber line (DSL)) or wireless (e.g., infrared, wireless, microwave, etc.) means. The computer-readable storage medium can be any available medium accessible to a computer or a data storage device such as a server or data center that integrates one or more available media. The available medium can be a magnetic medium (e.g., floppy disk, hard disk, magnetic tape), an optical medium (e.g., DVD), or a semiconductor medium (e.g., solid-state disk (SSD)).

[0060] This detection device significantly enhances its environmental adaptability by employing high-strength materials in the manufacture of key components and incorporating inert gas protection technology. For example, the design of the strain beam and fiber optic sensor fully considers the radiation resistance of the materials, avoiding material embrittlement caused by long-term neutron radiation. Furthermore, the inert gas (such as helium or argon) filling the sealed casing effectively isolates the strain beam and fiber optic sensor from the external environment, further enhancing the device's resistance to stress corrosion. These designs enable the detection device to maintain high accuracy and reliability in the complex operating environment of a nuclear reactor, thus meeting practical application requirements. Due to the high sensitivity and electromagnetic interference resistance of the fiber optic sensor, it can accurately capture changes in optical signals caused by bolt strain, providing reliable data support for subsequent calculations. This detection device has the function of real-time monitoring of bolt strain, which is of great significance for ensuring the safe operation of a nuclear reactor. During the operation of a nuclear power plant, bolt strain may gradually accumulate due to factors such as temperature fluctuations, pressure changes, or material aging. If not detected and addressed in time, it may lead to sealing failure or even structural damage. By introducing real-time monitoring capabilities, this detection device can immediately issue an alarm when bolt strain exceeds a safety threshold, thereby providing timely decision support for maintenance personnel.

[0061] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

Claims

1. A strain detection device for bolts of a nuclear reactor pressure vessel based on fiber optic sensing technology, characterized in that, include: The detection light source is configured to provide incident light; An optical fiber sensor is embedded in the bolt body, with its optical fiber line passing through the strain beam and sealed and fixed by a high-temperature resistant material. A deflectable gravity pendulum is suspended at the lower end of the strain beam. The optical fiber sensor is configured to receive incident light and transmit reflected light to the spectral detection device. The spectral detection equipment is configured to receive optical signals; A strain calculation electronic device is configured to determine the degree of strain of the bolt body based on the difference in reflected light from the fiber optic sensor acquired by the spectral detection device when the gravity pendulum deviates from the bolt body axis.

2. The strain detection device for bolts of a nuclear reactor pressure vessel according to claim 1, characterized in that, The optical fiber sensor uses radiation-resistant quartz optical fiber. Preferably, the optical fiber of the optical fiber sensor is a metal-coated quartz optical fiber. Preferably, the optical fiber metal coating of the optical fiber sensor is either a nickel coating or a gold coating.

3. The strain detection device for nuclear reactor pressure vessel bolts according to claim 2, characterized in that, Preferably, the device further includes an optical fiber connector partially embedded in the bolt body and a sealed housing that engages and seals with the optical fiber connector, wherein the optical fiber of the optical fiber sensor enters the sealed housing through the optical fiber connector; Preferably, the strain beam is fixedly installed inside the sealed housing, and an optical fiber through hole is formed on the strain beam. The optical fiber of the optical fiber sensor enters the optical fiber through hole inside the sealed housing after passing through the optical fiber connector, and is sealed and fixed with the optical fiber through hole. Preferably, the optical fiber through-hole is provided with a first sealing position and a second sealing position. The first sealing position is located at the entrance of the optical fiber sensor through-hole via the optical fiber connector; the second sealing position is located at the exit of the optical fiber through-hole.

4. The strain detection device for bolts of a nuclear reactor pressure vessel according to claim 3, characterized in that, The fiber optic sensor located inside the bolt body is sealed by gold-tin eutectic brazing at the first sealing position and then sealed a second time with high-temperature resistant adhesive. Preferably, the fiber optic sensor located within the bolt body is sealed at the second sealing position by gold-tin eutectic brazing and then a second sealing with high-temperature resistant adhesive. Preferably, the solder used for the gold-tin eutectic brazing seal is Au80Sn20 gold-tin eutectic solder; Preferably, the high-temperature resistant adhesive is a silicone adhesive, a polyimide adhesive, or an epoxy adhesive.

5. The strain detection device for nuclear reactor pressure vessel bolts according to claim 3, characterized in that, The detection light source and the spectral detection device are disposed outside the bolt body; Preferably, the fiber optic sensor includes a first fiber optic sensor and a second fiber optic sensor, the strain beam is disposed between the first fiber optic sensor and the second fiber optic sensor, and the strain calculation electronic device determines the strain degree of the bolt body based on the difference in reflected light between the first fiber optic sensor and the second fiber optic sensor obtained by the spectral detection device. Preferably, the device further includes an optical fiber connector partially embedded in the bolt body and a sealed housing that engages and seals with the optical fiber connector, wherein the optical fiber lines of the first optical fiber sensor and the second optical fiber sensor enter the sealed housing through the optical fiber connector; Preferably, the sealed housing is filled with inert gas; Preferably, the inert gas is selected from one or more of helium, argon, nitrogen, neon, and krypton; The strain beam is fixedly installed inside the sealed housing. At least two optical fiber through holes are opened on the strain beam. The optical fiber lines of the first optical fiber sensor and the second optical fiber sensor are arranged opposite to each other on both sides of the strain beam. After passing through the optical fiber connector, the optical fiber lines of the first optical fiber sensor and the second optical fiber sensor respectively enter the optical fiber through holes in the sealed housing and are adhered and fixed to the optical fiber through holes. Preferably, the optical fibers of the first and second optical fiber sensors are bonded and fixed to the optical fiber through-holes using high-temperature resistant adhesive. Preferably, the high-temperature resistant adhesive is selected from adhesives with a temperature resistance range of 400°C or higher.

6. The strain detection device for nuclear reactor pressure vessel bolts according to claim 5, characterized in that, The sensing parts of the first and second fiber optic sensors are located at the portions where the fiber optic lines of the first and second fiber optic sensors enter the strain beam. Preferably, the optical fibers of the first and second optical fiber sensors terminate at the lower end face of the strain beam after entering the strain beam. Preferably, the sensing parts of the first and second fiber optic sensors are at the same height level. A first gap is provided between the lower end face of the strain beam and the upper end face of the gravity pendulum, and the distance between the optical fiber lines of the first optical fiber sensor and the lower end face of the strain beam after entering the strain beam is greater than or equal to the first gap. Preferably, the suspension connection between the gravity pendulum and the strain beam is located at the center of the lower end face of the strain beam; The sealed shell is a blind end at the lower end of the gravity pendulum, and the cavity of the blind end is filled with inert gas. Preferably, the sealed housing is provided with a sealing cover at the lower end of the gravity pendulum, the cavity inside the sealing cover is filled with inert gas, and the lower end of the gravity pendulum is free.

7. The strain detection device for nuclear reactor pressure vessel bolts according to claim 5, characterized in that, The upper end of the strain beam is fixed to the optical fiber connector; Preferably, the side of the strain beam is clearance-fitted with the sealed shell; Preferably, the center of the upper end face of the gravity pendulum and the center of the lower end face of the strain beam are located on the axis of the sealed shell, and the area of ​​the upper end face of the gravity pendulum is less than or equal to the area of ​​the lower end face of the strain beam. Preferably, the upper end face of the gravity pendulum and the lower end face of the strain beam are concentric circles when projected onto the lower end face of the strain beam, and the area of ​​the upper end face of the gravity pendulum is smaller than the area of ​​the lower end face of the strain beam.

8. The strain detection device for nuclear reactor pressure vessel bolts according to claim 5, characterized in that, The device further includes a third optical fiber sensor, wherein the optical fibers of the first optical fiber sensor, the second optical fiber sensor and the third optical fiber sensor extend along the axis of the sealed housing and are uniformly distributed circumferentially according to the axis of the sealed housing.

9. The strain detection device for bolts of a nuclear reactor pressure vessel according to claim 1, characterized in that, The device further includes an optical fiber coupler configured to transmit the probe light emitted by the probe light source to the optical fiber line of the optical fiber sensor, and to transmit the reflected light from the optical fiber line to the spectral detection device.

10. The strain detection device for bolts of a nuclear reactor pressure vessel according to claim 9, characterized in that, The strain calculation electronic device is an electronic device equipped with a strain calculation module; preferably, the fiber optic coupler is an optical circulator.

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