Neutron radiography device and method for detecting a damaged nuclear fuel assembly

Abstract

The application relates to a neutron radiography device and method applied to the detection of damaged nuclear fuel assemblies, wherein the neutron radiography device comprises a neutron source, a shielding cover and a neutron imaging system arranged in a shielding environment respectively, the upper parts of the two sides of the shielding cover are respectively provided with a neutron beam inflow port and a neutron beam outflow port, the neutron source is close to the neutron beam inflow port, and the neutron imaging system is close to the neutron beam outflow port. Advantages: under the condition that the fuel assembly is not disassembled, the damaged fuel rods in the fuel assembly can be quickly and accurately identified, the operation is simple, and the detection is fast; whether the fuel rods are damaged can be identified by analyzing whether an image of water vapor is formed at the air cavity part, all the damaged fuel rods in the assembly can be detected through the images of two visual angles, and the detection time is greatly shortened.

Description

Technical Field

[0001] This invention relates to the field of neutron radiography, and in particular to a neutron radiography apparatus and method for detecting damaged nuclear fuel assemblies. Background Technology

[0002] Fuel assemblies are the core components and most important parts of a nuclear reactor. They are the site of nuclear reactions and energy generation, and also a crucial safety barrier, preventing radioactive materials from entering the coolant circuit. The integrity of fuel assemblies directly impacts the safety and economics of a nuclear power plant. Fuel assemblies operate under extreme conditions of high temperature, high pressure, high radioactivity, and hydraulic movement. With increasing service life, material aging, pressure between the inner and outer layers of the fuel rods, intense neutron and gamma-ray radiation, and wear from other objects inevitably lead to damage to some fuel rods. Nuclear power plants require the inspection of fuel assemblies for damage, including the location and size of any damage, and timely replacement of damaged fuel rods to minimize safety and economic problems caused by fuel assembly failure. Common methods for detecting and identifying damaged fuel assemblies in nuclear power plants include visual inspection, sip testing, ultrasonic testing, and eddy current testing. Due to limitations in their application, industrial applications primarily use combinations of the first three methods, but these methods suffer from low efficiency and operational complexity in practice.

[0003] Underwater cameras can be used to visually inspect fuel assemblies for damage, but this method only checks the outer fuel rods and cannot detect internal fuel rods. It also only detects very obvious defects, resulting in low accuracy. Nuclear power plants typically use a siphon technique to identify fuel assembly damage: the fuel assembly is placed in a sealed container, and by increasing or decreasing the internal pressure, if a fuel rod is damaged, the fission gas inside will rapidly leak into the water or air inside the container. By sampling the water or air in the sealed container and analyzing its radioactivity, the extent of damage can be determined. However, siphon testing can only determine if there are damaged fuel rods inside the assembly, not which rod (or rods) is damaged. In practice, if the inspected assembly leaks, the radioactivity of the water or air in the sealed container may not quickly return to background levels, potentially leading to misjudgments when inspecting the next assembly. For damaged fuel assemblies, ultrasonic testing is used to identify the damaged fuel rods. During testing, an ultrasonic probe is inserted into the row spacing of the fuel rod to collect signals. As the ultrasonic probe passes through the fuel rod, the ultrasonic waves emitted by the transmitting probe are reflected by the fuel rod and received by the receiving probe. Fuel rods that have leaked coolant due to damage will reflect less ultrasonic energy, while intact fuel rods will reflect almost all of the ultrasonic energy. By comparing the amplitude of the echo signals, it can be determined whether the fuel rod is damaged. Currently, ultrasonic testing equipment for fuel assembly inspection mainly relies on imports, which are expensive and have high maintenance costs. It also requires a high level of operational skills from the testing personnel. Only some nuclear power plants in China are equipped with such equipment. Eddy current testing is used to identify the location and size of the damage to fuel rods that have been identified as damaged. Eddy current testing requires disassembling the assembly and inspecting each fuel rod individually, which is time-consuming and uneconomical.

[0004] Since the early 1970s, neutron radiography has been used for quality control of nuclear fuel. At the 250kW Triga reactor in the thermal fuel inspection facility of Idaho National Laboratory, neutron radiography is used to inspect the internal characteristics of fuel rods. In the early 1990s, the nuclear industries of Japan and Spain conducted joint experimental projects to study high burnup samples. The neutron radiography facility at the Spallation Neutron Source SINQ in PSI, Switzerland, is equipped with a special device called NEURAP (Netron Radiographic Activated Probe) for the transfer and detection of actual fuel. Currently, neutron radiography is widely used in many countries, including Switzerland, France, Germany, the United States, Australia, Japan, India, and South Korea, as an effective means of nuclear fuel element research and quality control. Neutrons have strong penetrating power, a high reaction cross-section with uranium and hydrogen, and a low reaction cross-section with cladding materials, allowing for clear imaging of internal defects in fuel rods, such as pellet deformation and breakage, and the detection of the hydrogen polymerization state and content in the cladding. Currently, the use of neutron radiography for fuel assembly inspection both domestically and internationally is mainly based on reactor neutron sources, which are bulky, costly, and immobile. Neutron radiography based on reactors is only used for inspecting individual fuel rods. It employs a transillumination method to directly detect the location and size of damage. However, due to limitations in inspection accuracy, it requires disassembling the fuel assembly, making overall inspection impossible. Furthermore, conventional neutron radiography methods directly image the fuel rod through a transillumination lens, requiring multiple images of different parts of a single fuel rod before determining whether it is damaged, necessitating a significant amount of inspection time.

[0005] Chinese patent CN102280149B (application date 2011.06.28) discloses a neutron radiographic inspection device and method for pressurized water reactor nuclear fuel rods. It designs a shielded base for inspecting individual fuel rods and employs an indirect imaging method based on film or IP plates. This device is based on a reactor neutron source, but this method can only inspect one fuel rod at a time, and the indirect imaging method is slow. Chinese patent CN107170499 (application date 2017.05.31) discloses a nuclear fuel inspection device and method that uses a camera assembly to monitor the three-dimensional dimensions and deformation of fuel rods or fuel assemblies through visual inspection. For fuel assemblies, only the outer fuel rods can be observed; the internal fuel rods are difficult to observe. Chinese patent CN110033873A (application date 2019.04.25) discloses a method for analyzing and judging the damage of fuel assemblies. It analyzes and judges whether the nuclear fuel is damaged by analyzing the changing trend of radionuclides under different power operation states of the reactor. The detection and judgment period of this method is relatively long (more than 10 days), and it can only judge whether the fuel assembly is damaged, but cannot determine which fuel rod is damaged. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a medical self-service terminal that effectively overcomes the defects of the prior art.

[0007] The technical solution of the present invention to solve the above-mentioned technical problems is as follows:

[0008] A neutron radiography device for detecting damaged nuclear fuel assemblies includes a neutron source, a shield, and a neutron imaging system, each disposed within a shielded environment. The upper portions of both sides of the shield are provided with neutron beam inlet and neutron beam outlet, respectively. The neutron source is located near the neutron beam inlet, and the neutron imaging system is located near the neutron beam outlet.

[0009] Based on the above technical solution, the present invention can be further improved as follows.

[0010] Furthermore, the lower end of the aforementioned shield is provided with a drive mechanism to rotate it.

[0011] Furthermore, the aforementioned drive mechanism is a servo motor or a stepper motor.

[0012] Furthermore, a cryogenic gas flow purging device is provided outside the neutron beam inlet, blowing air towards its interior.

[0013] Furthermore, the aforementioned neutron imaging system includes a shielded chamber, a neutron conversion screen, a reflector, and a camera. The shielded chamber is open at both ends, and an optical path is formed between its two ends inside. One end of the shielded chamber is close to the neutron beam outlet. The neutron conversion screen is vertically mounted inside the open end of the shielded chamber. The camera is installed inside the other end of the shielded chamber. Multiple reflectors are provided and arranged on the optical path to reflect and change the path of the visible light, thereby directing the visible light toward the camera. The camera is sequentially connected to an image acquisition unit and an image processor.

[0014] Furthermore, the aforementioned shielding chamber forms a "Z"-shaped optical path, and two reflectors are provided, which are arranged in parallel at the two inflection points of the optical path and are respectively tilted.

[0015] Furthermore, the aforementioned reflector is a concave reflector.

[0016] Furthermore, the aforementioned shielding chamber is made of lead components.

[0017] Furthermore, the aforementioned camera is a CCD camera.

[0018] The beneficial effects of this invention are: it enables rapid and accurate identification of damaged fuel rods in the fuel assembly without disassembling the fuel assembly; it is simple to operate and fast to detect; it only images the upper cavity of the fuel rod in the fuel assembly, and identifies whether the fuel rod is damaged by analyzing whether water vapor is formed in the cavity; all damaged fuel rods in the assembly can be detected by images from two perspectives, without the need to scan the entire fuel assembly, which greatly shortens the detection time.

[0019] A neutron radiography method for detecting damaged fuel assemblies is also provided, utilizing a neutron radiography apparatus for detecting damaged nuclear fuel assemblies, comprising the following steps:

[0020] S1. Place the fuel assembly to be tested inside the shielding cover, ensuring that the fuel rod of the fuel assembly is immersed in the liquid inside the shielding cover, wherein the upper end of the fuel rod is above the liquid surface. If the fuel rod is damaged, the liquid enters the interior of the damaged fuel rod and reacts with the fuel pellet inside the fuel rod to vaporize. The water vapor moves upward to the internal cavity at the upper end of the damaged fuel rod.

[0021] S2. Wait for the water vapor to condense on the cavity wall of the internal cavity at the upper end of the damaged fuel rod, forming droplets;

[0022] S3. The neutron source generates fast or thermal neutrons that irradiate and penetrate the upper cavity of the fuel rod.

[0023] S4. The neutron imaging system acquires the neutron beam that penetrates the upper cavity of the fuel rod and performs imaging to obtain a neutron photographic image of the fuel assembly from one perspective.

[0024] S5. Rotate the shielding cover by a certain angle and repeat the above steps S3 and S4 to obtain neutron radiographs from different perspectives.

[0025] S6. Combining the images obtained from different perspectives in steps S4 and S5, and finding that the same defect appears at the intersection of two parallel beams, the location of the damaged fuel rod in the entire fuel assembly is determined. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the neutron radiography device of the present invention for detecting damaged nuclear fuel assemblies;

[0027] Figure 2 This is a structural distribution diagram of the fuel assembly in the neutron radiography apparatus of the present invention for detecting damaged nuclear fuel assemblies.

[0028] Figure 3 This is a schematic diagram of the fuel rod structure in the neutron radiography device for detecting damaged nuclear fuel assemblies according to the present invention;

[0029] Figure 4 This is a schematic diagram of multi-view imaging in the neutron radiography method for detecting damaged fuel assemblies according to the present invention;

[0030] Figure 5 This is a diagram showing the arrangement of fuel rods in a neutron radiography simulation for water vapor detection in a fuel assembly, as described in the neutron radiography method for detecting damaged fuel assemblies of the present invention.

[0031] Figure 6 The figure shows the simulation results of neutron radiography for detecting water vapor in fuel assemblies in the neutron radiography method for detecting damaged fuel assemblies according to the present invention.

[0032] The attached diagram lists the components represented by each number as follows:

[0033] 1. Shielding enclosure; 2. Neutron source; 3. Shielding cover; 4. Fuel assembly; 5. Neutron imaging system; 6. Drive mechanism; 7. Cryogenic gas purging device; 31. Neutron beam inlet port; 32. Neutron beam outlet port; 41. Fuel rod; 51. Shielding chamber; 52. Neutron conversion screen; 53. Reflector; 54. Camera. Detailed Implementation

[0034] The principles and features of the present invention are described below with reference to the accompanying drawings. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0035] Example

[0036] like Figure 1 As shown, the neutron radiography device for detecting damaged nuclear fuel assemblies in this embodiment includes a neutron source 2, a shield 3, and a neutron imaging system 5, which are respectively set in a shielded environment. The upper parts of both sides of the shield 3 are respectively provided with a neutron beam inlet 31 and a neutron beam outlet 32. The neutron source 2 is close to the neutron beam inlet, and the neutron imaging system 5 is close to the neutron beam outlet.

[0037] In this embodiment, both the neutron beam inlet 31 and the neutron beam outlet 32 ​​are strip-shaped openings extending circumferentially along the shield 3.

[0038] The method for detecting and locating damage to nuclear fuel assemblies using this neutron radiography device is as follows:

[0039] S1. Place the fuel assembly 4 to be tested inside the shield 3, ensuring that the fuel rod 41 of the fuel assembly 4 is immersed in the liquid inside the shield 3, wherein the upper end of the fuel rod 41 is above the liquid surface. If the fuel rod 41 is damaged, the liquid enters the interior of the damaged fuel rod 41 and reacts with the fuel pellet inside the fuel rod 41 to vaporize. The water vapor moves upward to the internal cavity at the upper end of the damaged fuel rod 41.

[0040] S2. Wait for the water vapor to condense on the cavity wall of the internal cavity at the upper end of the damaged fuel rod 41, forming droplets;

[0041] S3, neutron source 2 generates fast or thermal neutrons that irradiate and penetrate the upper cavity of the fuel rod 41.

[0042] S4. The neutron imaging system 5 acquires the neutron beam that penetrates the upper cavity of the fuel rod 41 and performs imaging to obtain a neutron photographic image of the fuel assembly 4 from one perspective.

[0043] S5. Rotate the shielding cover 3 by a certain angle and repeat the above steps S3 and S4 to obtain neutron radiograph images from different perspectives.

[0044] S6. Combining the images obtained from different perspectives in steps S4 and S5, and finding that the same defect appears at the intersection of two parallel beams, the location of the damaged fuel rod 41 in the entire fuel assembly 4 is determined.

[0045] It should be noted that:

[0046] Fuel assembly 4 is a product of existing technology, generally consisting of more than 200 fuel rods 41 arranged in a square pattern of 15×15, 16×16, 17×17 or 18×18 (e.g., Figure 2 As shown), the components are fixed by a positioning grid, and the fuel rods 41 are arranged as follows. Figure 5 As shown, if there is a damaged fuel rod 41 in the fuel assembly 4, the cooling water in the shield 3 will seep into the damaged fuel rod 41. Upon encountering the high temperature of the fuel pellets inside the fuel rod 41, it will vaporize. At this time, the water vapor inside the fuel rod 41 will mainly adhere to the internal cavity at the upper end of the fuel rod 41. Neutron imaging is performed on the gas chamber of the upper tube seat of the fuel assembly 4. By identifying whether water vapor forms in the upper part of different fuel rods 41 in the neutron radiograph images, it can be determined whether there is a damaged fuel rod 41. The neutron radiograph simulation results are as follows: Figure 6 The image shows the differences in images indicating whether water vapor is present in the upper cavity of a fuel rod 41 within the fuel assembly, which can be distinguished using neutron radiography. The fuel assembly 4 is imaged from different perspectives, with each imaging session lasting less than 5 minutes. The location of the damaged fuel rod 41 within the fuel assembly 4 is located using images from three perspectives (at different rotation angles). A schematic diagram of the three-view imaging method is shown below (e.g., ...). Figure 4 (As shown).

[0047] More specifically: A first-view image of the fuel assembly 4 is obtained from one angle, and a second-view image of the fuel assembly 4 is obtained from another angle. The position of the damaged fuel rod 41 in the entire fuel assembly 4 is located by analyzing the images from the first and second angles, and the position of the damaged fuel rod 41 is verified by the imaging results from the third-view. The first-view is perpendicular to the second-view or at other angles, and the third-view is at 45 degrees or other angles to the second-view.

[0048] It is also important to emphasize that directly obtained neutron radiographs are prone to image distortion, requiring spatial distortion correction and grayscale value restoration. The method for correcting distorted images involves dividing an image into a series of quadrilateral regions covering the entire image. A distortion function related to angles is established using constraint points, defining the pixel positional relationship x = F(θ,x',y'). The image is gradually restored from the inside out to ensure better correction accuracy. After the function relationship is established at fixed points within the quadrilateral regions, the internal pixels are repaired according to the same function relationship. After spatial distortion correction, grayscale values ​​are interpolated to obtain the final corrected image. For the entire fuel assembly 4, fuel rod defects in the central axis region are difficult to distinguish, and the image edge contrast is significant. An edge-synchronous contrast enhancement algorithm is used to improve image contrast and enhance the ability to identify defects (such as water vapor areas within damaged fuel rods 41).

[0049] In this embodiment, neutron source 2 is a portable, high-current, small neutron source employing the T(d,n) reaction. The T(d,n) reaction produces a significantly higher neutron yield at lower incident ion beam energies (150 keV) than... 7 Li(p,n), 7 Li(d,n), 9 Be(p,n) and 9 Reactions such as Be(d,n) can produce 10 volts of electricity using the electrostatic acceleration method of the DT neutron source. 11 -10 13 n / s neutrons. In this embodiment, the source intensity of the DT reaction is relatively high. The imaging time for detecting fuel assembly 4 is critical. Fuel assembly 4 may burn out due to insufficient cooling caused by prolonged exposure to the water surface, so rapid detection is necessary. The small neutron source includes an ion source, a target, a high-voltage system, and auxiliary systems (control, vacuum, water cooling). The ion source is used for ionization, generation, and extraction of a high-current deuterium ion beam (this is prior art and will not be described in detail here).

[0050] In a preferred embodiment, the lower end of the shield 3 is provided with a drive mechanism 6 for rotating it.

[0051] In the above implementation scheme, the entire shield 3 is rotated by the drive mechanism 6, thereby adjusting the incident orientation of the neutron beam relative to the neutron beam inlet 31 of the shield 3, thus obtaining image results from different perspectives. The operation is very simple and convenient.

[0052] More specifically, the aforementioned drive mechanism 6 can be a servo motor or a stepper motor.

[0053] As a preferred embodiment, the above-mentioned neutron beam inlet 31 is provided with a cryogenic gas flow purging device 7 that blows air toward its interior.

[0054] In the above implementation scheme, the upper cavity of the fuel rod 41 is cooled by the low-temperature gas purging device 7 through the neutron beam inlet 31. The cooling gas reaches the surface of the fuel assembly 4 through the neutron beam inlet 31, which accelerates the condensation of water vapor in the fuel rod 41 and helps to shorten the cycle of the entire detection process.

[0055] like Figure 3 As shown, the specific structure of the fuel rod 41 is as follows: It generally includes a casing tube and a fuel pellet filled in the casing tube. A cavity is formed between the fuel pellet and the top wall of the casing tube. A spring for pressing the fuel pellet is also connected between the upper end of the fuel pellet and the top wall of the casing tube. If the fuel rod 41 is damaged, the cooling water in the shield 3 will enter the casing tube through the damaged part and vaporize with the fuel pellet at high temperature.

[0056] It should be noted that in this embodiment, the shielding environment consists of multiple shielding bodies surrounding the neutron source 2, the shielding cover 3, and the neutron imaging system 5. The shielding body 1 uses devices commonly used in nuclear reaction devices. The specific structure will not be described in detail here, but can be referred to the existing technology for configuration.

[0057] In a preferred embodiment, the neutron imaging system 5 includes a shielded chamber 51, a neutron conversion screen 52, a reflector 53, and a camera 54. The shielded chamber 51 is open at both ends, and an optical path is formed between its two ends inside. One end of the shielded chamber 51 is close to the neutron beam outlet 32. The neutron conversion screen 52 is vertically mounted inside the open end of the shielded chamber 51. The camera 54 is installed inside the other end of the shielded chamber 51. Multiple reflectors 53 are provided and arranged on the optical path to reflect and change the path of the visible light, so that the visible light is directed towards the camera 54. The camera 54 is sequentially connected to an image acquisition unit and an image processor.

[0058] In the above implementation scheme, neutron source 2 produces energy of 14 MeV and a source strength of 10. 12DT neutrons at a rate of n / s are collimated to form a uniformly distributed parallel beam, which then flows through the neutron beam inlet 31 and irradiates the cavity at the upper end of the fuel rod 41. The neutron intensity distribution changes as it penetrates the sample, reacting with the polypropylene in the neutron conversion screen 52 to excite the ZnS(Ag) fluorescent material, converting the neutrons into visible light. The light signal is focused onto the chip of the camera 54 after passing through the reflector 53, forming image data. The data is transmitted to the image processing system via the image acquisition system, which records and displays the spatial distribution of the transmitted neutron fluence, thus obtaining information about the internal structure of the cavity at the upper end of the fuel rod 41.

[0059] In addition, in the above implementation scheme, the reflector 53 is a concave reflector. The design of the concave reflector concentrates the imaging light of the large field of view conversion screen into the camera 54, which greatly improves the imaging field of view and light sensitivity of the camera 54.

[0060] More specifically, the shielding chamber 51 has a "Z"-shaped optical path inside, and two reflectors 53 are provided, which are arranged in parallel at the two inflection points of the optical path and are respectively tilted.

[0061] In the above scheme, a "Z"-shaped optical path design based on dual reflectors is adopted to reduce the influence of scattered neutrons and prevent the camera 54 from being directly exposed to neutrons. The camera 54 is placed in a shielded room 51 to reduce the radiation damage to the camera 54 caused by stray neutrons and accompanying gamma rays. At the same time, the Z-shaped optical path for neutron imaging, combined with the shielded lead room, can achieve better shielding of radiation and prevent it from affecting the quality of the image.

[0062] In this embodiment, the shielding chamber 51 is made of lead.

[0063] In this embodiment, the camera 54 is a cooled, scientific-grade, back-illuminated CCD camera. The selection of CCD chip materials focuses on CCD spectral coupling, quantum efficiency, dark current and other technical indicators.

[0064] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0065] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0066] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0067] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0068] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0069] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A neutron radiography method for detecting damaged nuclear fuel assemblies, characterized in that, The detection of damaged nuclear fuel assemblies was carried out using a neutron radiography device. The neutron radiography device includes a neutron source (2), a shield (3), and a neutron imaging system (5) respectively set in a shielded environment. The upper part of both sides of the shield (3) is provided with a neutron beam inlet (31) and a neutron beam outlet (32). The neutron source (2) is close to the neutron beam inlet, and the neutron imaging system (5) is close to the neutron beam outlet. The neutron radiography method for detecting damaged nuclear fuel assemblies includes the following steps: S1. Place the fuel assembly (4) to be tested inside the shield (3) to ensure that the fuel rod (41) of the fuel assembly (4) is immersed in the liquid inside the shield (3), wherein the upper end of the fuel rod (41) is above the liquid surface. If the fuel rod (41) is damaged, the liquid enters the interior of the damaged fuel rod (41) and reacts with the fuel core inside the fuel rod (41) to vaporize. The water vapor moves upward to the internal cavity at the upper end of the damaged fuel rod (41). S2. Wait for the water vapor to condense on the cavity wall of the internal cavity at the upper end of the damaged fuel rod (41) to form droplets; S3, the neutron source (2) generates fast or thermal neutrons that irradiate and penetrate the upper cavity of the fuel rod (41); S4. The neutron imaging system (5) acquires the neutron beam that penetrates the upper cavity of the fuel rod (41) for imaging, and acquires a neutron photographic image of the fuel assembly (4) from one angle. S5. Rotate the shield (3) by a certain angle and repeat the above steps S3 and S4 to obtain neutron photography images from different perspectives. S6. Combining the images obtained from different perspectives in steps S4 and S5, the location of the damaged fuel rod (41) in the entire fuel assembly (4) is determined by the presence of the same defect at the intersection of the two parallel beams.

2. The neutron radiography method for detecting damaged nuclear fuel assemblies according to claim 1, characterized in that: The lower end of the shield (3) is provided with a drive mechanism (6) that drives it to rotate.

3. The neutron radiography method for detecting damaged nuclear fuel assemblies according to claim 2, characterized in that: The drive mechanism (6) is a servo motor or a stepper motor.

4. The neutron radiography method for detecting damaged nuclear fuel assemblies according to claim 1, characterized in that: The neutron beam inlet (31) is provided with a cryogenic gas flow purging device (7) that blows air into it.

5. A neutron radiography method for detecting damaged nuclear fuel assemblies according to any one of claims 1 to 4, characterized in that: The neutron imaging system (5) includes a shielded chamber (51), a neutron conversion screen (52), a reflector (53), and a camera (54). The shielded chamber (51) is open at both ends, and an optical path is formed between its two ends inside. One end of the shielded chamber (51) is close to the neutron beam outlet (32). The neutron conversion screen (52) is vertically mounted inside the open end of the shielded chamber (51). The camera (54) is installed in the other end port of the shielded chamber (51). Multiple reflectors (53) are provided and are arranged on the optical path to reflect and change the path of visible light, so that the visible light is directed toward the camera (54). The camera (54) is sequentially connected to an image acquisition unit and an image processor.

6. A neutron radiography method for detecting damaged nuclear fuel assemblies according to claim 5, characterized in that: The shielding chamber (51) forms a "Z"-shaped optical path inside. Two reflectors (53) are provided and are arranged in parallel at the two inflection points of the optical path and are respectively tilted.

7. A neutron radiography method for detecting damaged nuclear fuel assemblies according to claim 5, characterized in that: The reflector (53) is a concave reflector.

8. A neutron radiography method for detecting damaged nuclear fuel assemblies according to claim 5, characterized in that: The shielding chamber (51) is made of lead.

9. A neutron radiography method for detecting damaged nuclear fuel assemblies according to claim 5, characterized in that: The camera (54) is a CCD camera.

Images