Step-by-step fluorination system and method applied to post-treatment of oxide spent fuel

Abstract

The invention discloses a step-by-step fluorination system and method applied to post-treatment of oxide spent fuel. The step-by-step fluorination system comprises a spent fuel assembly disassembling and core package separating device, a gas supply system, a first-stage fluidized bed, a second-stage fluidized bed and a third-stage fluidized bed, the spent fuel assembly disassembling and core package separating device is connected with a pretreatment tail gas system and the first-stage fluidized bed; according to the method, U and Pu are separated from other volatile fission elements in stages in the fluorination volatilization process, primary separation of various valuable nuclides such as Tc, Mo, U, Np and Pu can be achieved, and the subsequent purification treatment difficulty is greatly reduced. Spent fuel dry post-treatment is realized through simple gas-solid reaction, and the method has great advantages in treatment efficiency and economical efficiency and is also applicable to a high-burn-up fast reactor spent fuel treatment technology in the future.

Description

Technical Field

[0001] This invention relates to the field of spent fuel reprocessing technology, and more specifically to a stepwise fluorination system and method for oxide spent fuel reprocessing. Background Technology

[0002] While water-based reprocessing of spent fuel is currently mature and widely used, it faces significant limitations in the reprocessing of spent fuel from future Generation IV reactors, which operate under conditions of higher burnup and radioactivity. In contrast, dry reprocessing technologies, with their advantages of shorter processes, more compact equipment, smaller waste volume, and higher radiation resistance, hold greater promise. Fluorination volatilization technology, which separates high-valence fluorides of elements such as uranium, neptunium, and plutonium from a complex matrix based on differences in their physical properties (boiling point, sublimation point), offers advantages for large-scale, continuous operation. This method was proposed in the 1960s in the United States for the reprocessing of spent fuel from molten salt reactors, but its development stalled with the cancellation of molten salt reactor projects. Russia has established a fluorination volatilization process for spent fuel from the BOR-60 reactor, achieving uranium recovery rates greater than 99% and plutonium recovery rates of 89%–91%. The decontamination coefficients of UF6 and PuF6 for fragmented elements are 10. 7 and 10 3 ~10 4 In recent years, research on the fluorination volatilization process has been quite active, with the most outstanding research results coming from Japan, the Czech Republic, Russia, and France.

[0003] By comparison, the currently proposed fluorination volatilization technology routes are mainly based on one-step co-fluorination in a flame furnace. After fluorination of spent fuel, in addition to U, Np, and Pu, fission elements such as Mo, Tc, W, V, Ru, Rh, Te, and Nb are also fluorinated and volatilized. This results in the separation of UF6 containing volatile substances with similar properties. In particular, gaseous fluorides such as MoF6, TcF6, PuF6, and NpF6 mixed together are difficult to separate. To improve the purification coefficient of U metal for fission elements and transuranic elements, a more complex gas purification process must be added.

[0004] Therefore, this invention provides a stepwise fluorination process for oxide fuel reprocessing. This method differs from the one-step fluorination process described above. It allows for controlled reaction conditions to first fluorinate and volatilize volatile impurity elements such as Mo and Tc, then fluorinate and volatilize uranium, the main recoverable element in the spent fuel, and finally fluorinate and volatilize neptunium and plutonium. Through this simple fluorination process, uranium resources are recovered from spent fuel while simultaneously removing equally volatile fluoride impurities, thus improving the purity of the recovered uranium. Summary of the Invention

[0005] The purpose of this invention is to provide a stepwise fluorination system and method for the reprocessing of oxide spent fuel. This system can achieve the stepwise volatilization of different elements in spent fuel by controlling the reaction conditions between oxide spent fuel and F2. It mainly utilizes the differences in the thermodynamic and kinetic properties of the reaction between different elements in spent fuel and F2 to achieve the stepwise fluorination purpose. The fluorination process alone is used to recover and purify uranium metal from spent fuel. Compared with other volatilization methods at home and abroad, this method has a shorter process and greater operability.

[0006] The technical solution of the present invention is as follows: A stepwise fluorination system for the reprocessing of spent oxide fuel includes a spent fuel assembly dismantling and core separation device, a gas supply system, a first-stage fluidized bed, a second-stage fluidized bed, and a third-stage fluidized bed; the spent fuel assembly dismantling and core separation device is connected to a pre-treatment tail gas system and the first-stage fluidized bed; the first-stage fluidized bed is connected to a Tc and Mo fluoride trapping cold trap and a second-stage fluidized bed, and the second-stage fluidized bed is connected to the third-stage fluidized bed; the first-stage fluidized bed, the second-stage fluidized bed, and the third-stage fluidized bed are respectively connected to a Tc and Mo fluoride trapping cold trap, a UF6 trapping cold trap, and a hexafluoride thermal decomposition device, the hexafluoride thermal decomposition device is connected to an NpF6 adsorption device, and the Tc and Mo fluoride trapping cold trap, the UF6 trapping cold trap, and the NpF6 adsorption device are all connected to the tail gas system; the gas supply system is respectively connected to the first-stage fluidized bed, the second-stage fluidized bed, and the third-stage fluidized bed to provide fluorinating agent.

[0007] The temperature inside the first-stage fluidized bed is 300–350℃; the temperature inside the second-stage fluidized bed is maintained at 450–600℃; the temperature inside the third-stage fluidized bed needs to be controlled at 600–650℃; and the temperature of the hexafluoride thermal decomposition device is maintained at 200–350℃.

[0008] Before use, the first-stage fluidized bed, the second-stage fluidized bed, and the third-stage fluidized bed are all replaced with inert gas.

[0009] The fluorination residue treatment system recovers fluorination residues through molten salt electrodeposition.

[0010] The NpF6 adsorption device is filled with MgF2.

[0011] The fluorinating agent is any one of F2, NF3, and BrF3.

[0012] A stepwise fluorination method for oxide spent fuel reprocessing includes the following steps:

[0013] S1: The oxide spent fuel assembly is pretreated by the spent fuel assembly dismantling and core separation device to obtain solid product powder and radioactive tail gas. The tail gas is captured, purified and discharged by the pretreated tail gas system, and the solid product powder enters the first-stage fluidized bed fluorination treatment.

[0014] S2: The solid product powder is conveyed to the first-stage fluidized bed, and the first-stage fluidized bed is replaced with inert gas before use. Pure fluorine gas is introduced for first-stage fluorination, converting the easily fluorinated substances into gaseous substances and entering the Tc and Mo fluoride trap for freezing and collection. The exhaust gas is finally sent to the exhaust gas treatment system.

[0015] S3: The solid product powder after fluorination in the first fluidized bed in S2 is conveyed to the second fluidized bed. Before use, the second fluidized bed is replaced with an inert gas, and the generated UF6 is collected by freezing through a UF6 trap.

[0016] S4: The fluorinated residue after the second-stage fluidized bed fluorination reaction in S3 is transported to the third-stage fluidized bed. Before use, the third-stage fluidized bed is replaced with an inert gas. Then, the tail gas of the third-stage fluidized bed is passed into the hexafluoride thermal decomposition device, and the temperature is maintained at 200℃. Then, the NpF6 adsorption device is used for adsorption.

[0017] S5: The fluorinated slag produced by the third-stage fluidized bed is transported to the fluorinated residue treatment system 9.

[0018] In step S2, the temperature of the first-stage fluidized bed is maintained at 300°C; in step S3, the temperature of the second-stage fluidized bed is maintained at 450°C; in step S4, the temperature of the third-stage fluidized bed is controlled at 650°C; and the temperature of the hexafluoride thermal decomposition device is maintained at 200°C.

[0019] A stepwise direct fluorination method for oxide spent fuel reprocessing includes the following steps:

[0020] S1: The oxide spent fuel assembly is pretreated by the spent fuel assembly dismantling and core separation device to obtain solid product powder and radioactive tail gas. The tail gas is captured, purified and discharged by the pretreated tail gas system, and the solid product powder enters the first-stage fluidized bed for fluorination treatment.

[0021] S2: The solid product powder is conveyed to the first-stage fluidized bed. Before use, the first-stage fluidized bed is replaced with an inert gas and the bed temperature is maintained at 350°C. A fluorine-argon mixture is introduced to carry out the first-stage fluorination, converting the easily fluorinated substances into gaseous substances that enter the Tc and Mo fluoride trap for cryogenic capture.

[0022] S3: The powder tailings from S2, which have been processed by the first-stage fluidized bed, are directly sent to the third-stage fluidized bed. Before use, the third-stage fluidized bed is replaced with an inert gas and the temperature is raised to 600°C. The fluorinated volatile gas is then introduced into the hexafluoride thermal decomposition device, and the temperature is maintained at 350°C. Finally, the tail gas is introduced into the NpF6 adsorption device.

[0023] In step S2, a fluorine-argon mixture is introduced for the first stage of fluorination, with the fluorine and argon gases introduced in a 1:1 ratio.

[0024] The significant advantages of this invention are as follows: The stepwise fluorination process for oxide spent fuel reprocessing described in this invention is an optimization and improvement of the existing volatilization method. Through experiments, the fluorination volatilization rate of key fluorinating agents at specific process temperatures was obtained. During the fluorination volatilization process, U and Pu are separated from other volatile fissile elements in stages, and preliminary separation of multiple valuable nuclides such as Tc, Mo, U, Np, and Pu can be achieved, greatly reducing the difficulty of subsequent purification treatment. Currently, F2 has been used as a fluorinating agent to verify the stepwise fluorination of impurities such as MoO3, Re2O7, TeO2, and Nb2O5 in U3O8. At relatively low temperatures (~300℃), the volatilization rate of the dopant elements can reach over 90%, while uranium does not volatilize. Stepwise fluorination experiments of uranium-plutonium oxide mixed materials have also been completed, and the results show that the volatilization rate of uranium oxide is much greater than that of plutonium oxide. This process achieves dry reprocessing of spent fuel through a simple gas-solid reaction, which has significant advantages in terms of processing efficiency and economy, and is also applicable to future high burnup fast reactor spent fuel processing technologies. Attached Figure Description

[0025] Figure 1 Curve showing the change in fluorination conversion rate of key elements with temperature;

[0026] Figure 2 Flowchart of the stepwise fluorination process for spent fuel in this invention;

[0027] In the diagram: 1. Spent fuel assembly dismantling and core separation device; 2. Pre-treatment exhaust gas system; 3. Gas supply system; 4. First-stage fluidized bed; 5. Tc and Mo fluoride trapping cold trap; 6. UF6 trapping cold trap; 7. Second-stage fluidized bed; 8. Third-stage fluidized bed; 9. Fluoride residue treatment system; 10. Hexafluoride thermal decomposition device; 11. NpF6 adsorption device. Detailed Implementation

[0028] Many specific details are set forth in the following description to provide a full understanding of this application. However, this application can be implemented in many other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of this application; therefore, this application is not limited to the specific embodiments disclosed below.

[0029] The terminology used in one or more embodiments of this application is for the purpose of describing particular embodiments only and is not intended to limit the scope of one or more embodiments of this application. The singular forms “a,” “the,” and “the” used in one or more embodiments of this application and in the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” used in one or more embodiments of this application refers to and includes any or all possible combinations of one or more associated listed items.

[0030] It should be understood that although the terms first, second, etc., may be used to describe various information in one or more embodiments of this application, such information should not be limited to these terms. These terms are only used to distinguish information of the same type from one another. For example, first may also be referred to as second without departing from the scope of one or more embodiments of this application, and similarly, second may also be referred to as first.

[0031] The specific technical content of the present invention will now be described with reference to the accompanying drawings;

[0032] The volatile dry reprocessing process mainly utilizes the difference in volatility between the fluoride products of uranium and plutonium and other elements in spent fuel to achieve the separation and recovery of uranium and plutonium. According to previous research, the fluorides of key elements in spent fuel can be divided into volatile elements and non-volatile elements, including volatile elements (U, Pu, Np, Mo, Tc, Te, Sb, etc.) and non-volatile elements (Am, Cm, FP, etc.), as detailed in Table 1 below.

[0033] Table 1. Volatility of Fluorides, the Main Components in Spent Fuel

[0034]

[0035] Within the grouping of volatile elements, experimental studies have revealed significant differences in the thermodynamics and kinetics of the reactions between different substances and F2. This allows for the differentiation of readily fluorinated elements such as Mo and Tc from less readily fluorinated elements such as U, Np, and Pu. This characteristic enables the controlled fluorination conditions for the stepwise fluorination of spent fuels.

[0036] like Figure 1 As shown, a stepwise fluorination system for the reprocessing of spent oxide fuel includes a spent fuel assembly dismantling and core separation device 1, a gas supply system 3, a first-stage fluidized bed 4, a second-stage fluidized bed 7, and a third-stage fluidized bed 8. The spent fuel assembly dismantling and core separation device 1 is connected to the pre-treatment tail gas system 2 and the first-stage fluidized bed 4. The first-stage fluidized bed 4 is connected to the Tc and Mo fluoride trapping cold trap 5 and the second-stage fluidized bed 7, and the second-stage fluidized bed 7 is connected to the third-stage fluidized bed 8. The first-stage fluidized bed 4, the second-stage fluidized bed 7, and the third-stage fluidized bed 8 are respectively connected to the Tc and Mo fluoride trapping cold trap 5, the UF6 trapping cold trap 6, and the hexafluoride thermal decomposition device 10. The hexafluoride thermal decomposition device 10 is connected to the NpF6 adsorption device 11. The Tc and Mo fluoride trapping cold trap 5, the UF6 trapping cold trap 6, and the NpF6 adsorption device 11 are all connected to the tail gas system 12. The gas supply system 3 is connected to the first-stage fluidized bed 4, the second-stage fluidized bed 7, and the third-stage fluidized bed 8 respectively, providing fluorinating agent.

[0037] The spent fuel assembly dismantling and core separation device 1 performs pretreatment such as dismantling, shearing, core pulverization and core separation of spent fuel assemblies, and obtains solid product powder and radioactive tail gas. The tail gas is captured, purified and discharged through the pretreatment tail gas system 2, and the solid product powder enters the first-stage fluidized bed 4 for fluorination treatment.

[0038] Specifically, the solid product powder is mainly composed of U3O8, and also contains small amounts of fission products and transuranic elements. The fission products mainly include Mo and Tc, while the transuranic elements mainly include Np, Pu, Am, and Cm. Mo, Tc, U, Np, and Pu are easily volatile after forming fluorides, while Am, Cm, and FP are difficult to volatilize after forming fluorides. Furthermore, Mo and Tc are readily fluorinated, while Np and Pu are not easily fluorinated, and U has a moderate level of fluorination difficulty.

[0039] Specifically, the radioactive exhaust gas contains 3 H, Kr, Xe, I, CO2, etc.

[0040] The fluorinating agent provided by the gas supply system 3 is any one of F2, NF3, and BrF3.

[0041] Preferably, the fluorinating agent and argon are introduced in a 1:1 ratio.

[0042] Before use, the first-stage fluidized bed 4 is replaced with an inert gas to remove oxygen and other substances inside. The bed temperature is maintained at 300-350℃, and easily fluorinated substances such as Tc and Mo are converted into gaseous MoF6, TcF6 and other substances, which are then frozen and captured in the Tc and Mo fluoride trap 5. The tail gas after freezing and capture is finally sent to the tail gas treatment system 12.

[0043] Specifically, the exhaust gas treatment system 12 includes a dust filter, a radioactive gas adsorption trap, a charcoal trap, an activated carbon trap, and a scrubbing tower;

[0044] Specifically, the Tc and Mo fluoride trap 5 can trap substances that are volatile after forming fluorides, including Tc and Mo.

[0045] Preferably, the inert gas is Ar;

[0046] Preferably, the inert gas is a fluorine-argon mixture;

[0047] Before use, the second-stage fluidized bed 7 is purged with an inert gas to remove oxygen and other substances, maintaining the bed temperature at 450–600°C. The second-stage fluidized bed 7 ensures that most of the U metal (>95%) undergoes fluorination to UF6 and volatilizes during this stage, while less fluorinated elements such as Np and Pu remain in the fluorination residue. The generated UF6 is collected by freezing in the UF6 trapping cold trap 6, and then the UF6 gas is heated and volatilized to fill storage tanks.

[0048] Specifically, the temperature of the UF6 cold trap 6 is around -40℃.

[0049] Before use, the third-stage fluidized bed 8 is replaced with an inert gas to remove oxygen and other substances. The temperature needs to be controlled at 600-650℃ to fluorinate Np and Pu in the fluorination residue. The product contains PuF6 and NpF6 gas. First, the thermal instability of PuF6 and NpF6 is used to pass the tail gas into the hexafluoride thermal decomposition device 10 to decompose PuF6 into PuF4 and collect the powder. Then, the NpF6 is adsorbed by the adsorption trap of the NpF6 adsorption device 11 filled with MgF2.

[0050] Specifically, the NpF6 adsorption device 11 is filled with MgF2.

[0051] Specifically, the temperature of the hexafluoride thermal decomposition device 10 is maintained at 200–350°C.

[0052] The fluorinated residue treatment system 9 recovers long-lived radioactive elements (i.e., Am, Cm, etc., which are difficult to volatilize after forming fluorides) through molten salt electrodeposition, transforming the waste into a form that is easy to landfill.

[0053] In one embodiment, the first-stage fluidized bed 4 is directly connected to the third-stage fluidized bed 8.

[0054] The stepwise fluorination method is described below through specific embodiments.

[0055] Example 1

[0056] S1: The oxide spent fuel assembly undergoes a pretreatment process including dismantling, shearing, pellet pulverization, and core separation using the spent fuel assembly dismantling and core separation device 1, yielding solid product powder (mainly U3O8, containing small amounts of fission products and transuranic elements) and radioactive tail gas (containing... 3 H, Kr, Xe, I, CO2, etc.), of which the exhaust gas is captured, purified and discharged through the pretreatment exhaust gas system 2, and the solid product powder enters the first-stage fluidized bed 4 for fluorination treatment.

[0057] S2: The solid product powder is conveyed to the first-stage fluidized bed 4. Before use, it is replaced by an inert gas to remove oxygen and other substances inside. The bed temperature is maintained at 300°C. A fluorinating agent is introduced for the first-stage fluorination. In this stage, easily fluorinated substances such as Tc and Mo are mainly converted into gaseous MoF6 and TcF6, which are then frozen and captured in the Tc and Mo fluoride trap 5. The exhaust gas is finally sent to the exhaust gas treatment system 12.

[0058] S3: The solid product powder after fluorination in the first fluidized bed 4 in S2 is transported to the second fluidized bed 7. The second fluidized bed 7 is also subjected to atmosphere replacement and heating operation in advance, but the temperature needs to be maintained at 450℃. Fluorinizing agent is introduced for second-stage fluorination. In this stage, it is ensured that most of the U metal (>95%) is fluorinated into UF6 and volatilized and transferred in this stage. Difficult-to-fluorinate elements such as Np and Pu remain in the fluorination residue. The generated UF6 is collected by freezing through UF6 trap 6, and then the UF6 gas is filled and stored by heating and volatilization. The tail gas is finally sent to the tail gas treatment system 12.

[0059] S4: The fluorinated residue (approximately 5% of the initial material mass) after the fluorination reaction in the second-stage fluidized bed 7 in S3 is conveyed to the third-stage fluidized bed 8. The third-stage fluidized bed 8 also undergoes atmosphere replacement and heating operations in advance. The temperature in this stage needs to be controlled at around 650℃. A fluorinating agent is introduced to carry out the third-stage fluorination, transferring Np and Pu in the fluorinated residue. The volatilized products contain gases such as PuF6 and NpF6. Taking advantage of the thermal instability of PuF6, the tail gas of the third-stage fluidized bed 8 is introduced into the hexafluoride thermal decomposition device 10, and the temperature is maintained at 200℃. PuF6 is decomposed into PuF4 and the powder is collected. Then, the NpF6 is adsorbed by the NpF6 adsorption device 11. The tail gas is finally sent to the tail gas treatment system 12.

[0060] S5: The fluorinated slag produced by the third-stage fluidized bed 8 is transported to the fluorinated residue treatment system 9, where long-lived radioactive elements (i.e., Am, Cm, etc., which are difficult to volatilize after forming fluorides) are recovered through molten salt electrodeposition, transforming the waste into a form that is easy to landfill.

[0061] Example 2

[0062] S1: The oxide spent fuel assembly undergoes a pretreatment process including dismantling, shearing, pellet pulverization, and core separation using the spent fuel assembly dismantling and core separation device 1, yielding solid product powder (mainly U3O8, containing small amounts of fission products and transuranic elements) and radioactive tail gas (containing... 3 H, Kr, Xe, I, CO2, etc.), of which the exhaust gas is captured, purified and discharged through the pretreatment exhaust gas system 2, and the solid product powder enters the first-stage fluidized bed 4 for fluorination treatment.

[0063] S2: The above solid product powder is transported to the first-stage fluidized bed 4. Before use, it is replaced by an inert gas to remove oxygen and other substances inside. The bed temperature is maintained at 350°C. A fluorine-argon mixture (F2:Ar = 1:1) is introduced to carry out the first-stage fluorination, converting easily fluorinated substances such as Tc and Mo into gaseous MoF6, TcF6 and other substances, which are then frozen and captured in the Tc and Mo fluoride trap 5. The exhaust gas is finally sent to the exhaust gas treatment system 12.

[0064] S3: The powder tailings processed by the first-stage fluidized bed 4 in S2 are directly sent to the third-stage fluidized bed 8. Before use, the third-stage fluidized bed 8 is replaced by an inert gas to remove oxygen and other substances inside, and the temperature is raised to about 600°C. In this stage, U, Np, Pu and other substances are co-fluorinated together. The fluorinated volatile gas is passed into the hexafluoride thermal decomposition device 10, and the temperature is maintained at about 350°C to decompose PuF6 into PuF4 and collect the powder. Then, the tail gas of the pyrolysis furnace is passed into the NpF6 adsorption device 11 to adsorb NpF6. The adsorbed tail gas is further passed into the tail gas treatment system 12 and the UF6 capture cold trap 6. This process is simpler and more convenient than Case 1.

[0065] It should be noted that the fluorination device in the above examples is a temperature-controlled resistance heating reactor, which is not limited to a fluidized bed, but can also be a fixed bed, a rotary reactor, etc., and has the function of rapid loading and unloading of fluorinated residue; the fluorinating agent is not limited to F2, but can also be a strong fluorinating agent such as NF3 and BrF3.

[0066] It should be noted that the gaseous fluoride capture device in the above examples is not limited to a single cold trap, but can also be a combination of an adsorption trap and a cold trap, an adsorption trap in series, a cold trap in series, etc.

[0067] It should be noted that the reaction temperature, time, and freezing temperature in the above examples are only reference values ​​and can be adjusted according to the actual reaction conditions.

[0068] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

[0069] It should be noted that, for the sake of simplicity, the foregoing method embodiments are all described as a series of actions. However, those skilled in the art should understand that this application is not limited to the described order of actions, as some steps may be performed in other orders or simultaneously according to this application. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and modules involved are not necessarily essential to this application.

[0070] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions of other embodiments.

[0071] The preferred embodiments disclosed above are merely illustrative of this application. The optional embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this application. These embodiments are selected and specifically described in this application to better explain the principles and practical applications of this application, thereby enabling those skilled in the art to better understand and utilize this application.

Claims

1. A stepwise fluorination system for oxide spent fuel reprocessing, characterized in that: The system includes a spent fuel assembly dismantling and core separation device (1), a gas supply system (3), a first-stage fluidized bed (4), a second-stage fluidized bed (7), and a third-stage fluidized bed (8); the spent fuel assembly dismantling and core separation device (1) is connected to a pre-treatment exhaust gas system (2) and a first-stage fluidized bed (4); the first-stage fluidized bed (4) is connected to a Tc and Mo fluoride trap (5) and a second-stage fluidized bed (7), and the second-stage fluidized bed (7) is connected to a third-stage fluidized bed (8); the first-stage fluidized bed (4), the second-stage fluidized bed (7), and the third-stage fluidized bed (8) are connected to each other. The first-stage fluidized bed (8) is connected to the Tc and Mo fluoride trap (5), the UF6 trap (6), and the hexafluoride thermal decomposition device (10), respectively. The hexafluoride thermal decomposition device (10) is connected to the NpF6 adsorption device (11). The Tc and Mo fluoride trap (5), the UF6 trap (6), and the NpF6 adsorption device (11) are all connected to the tail gas system (12). The gas supply system (3) is connected to the first-stage fluidized bed (4), the second-stage fluidized bed (7), and the third-stage fluidized bed (8), respectively, to provide fluorinating agents.

2. The stepwise fluorination system for oxide spent fuel reprocessing according to claim 1, characterized in that: The temperature inside the first-stage fluidized bed (4) is 300-350℃; the temperature inside the second-stage fluidized bed (7) is maintained at 450-600℃; the temperature inside the third-stage fluidized bed (8) needs to be controlled at 600-650℃; and the temperature inside the hexafluoride thermal decomposition device (10) is maintained at 200-350℃.

3. A stepwise fluorination system for oxide spent fuel reprocessing according to claim 2, characterized in that: Before use, the first-stage fluidized bed (4), the second-stage fluidized bed (7), and the third-stage fluidized bed (8) are all replaced with inert gas.

4. A stepwise fluorination system for oxide spent fuel reprocessing according to claim 1, characterized in that: The fluorinated residue treatment system (9) recovers fluorinated residues by molten salt electrodeposition.

5. A stepwise fluorination system for oxide spent fuel reprocessing according to claim 1, characterized in that: The NpF6 adsorption device (11) is filled with MgF2.

6. A stepwise fluorination system for oxide spent fuel reprocessing according to claim 1, characterized in that: The fluorinating agent is any one of F2, NF3, and BrF3.

7. A stepwise fluorination method for oxide spent fuel reprocessing, using the stepwise fluorination system for oxide spent fuel reprocessing as described in claim 2, characterized in that: The following steps are involved: S1: The oxide spent fuel assembly is pretreated by the spent fuel assembly dismantling and core separation device (1) to obtain solid product powder and radioactive tail gas. The tail gas is captured, purified and discharged by the pretreated tail gas system (2), and the solid product powder enters the first-stage fluidized bed (4) for fluorination treatment. S2: The solid product powder is transported to the first-stage fluidized bed (4), and the first-stage fluidized bed (4) is replaced by an inert gas before use. Pure fluorine gas is introduced for first-stage fluorination, and the easily fluorinated substances are converted into gaseous substances and enter the Tc and Mo fluoride trap (5) for freezing and trapping. The tail gas is finally sent to the tail gas treatment system (12). S3: The solid product powder after fluorination in the first fluidized bed (4) in S2 is transported to the second fluidized bed (7). The second fluidized bed (7) is replaced with an inert gas before use. The generated UF6 is collected by freezing through the UF6 trap cold trap (6). S4: The fluorinated residue after the fluorination reaction in the second fluidized bed (7) in S3 is transported to the third fluidized bed (8). Before use, the third fluidized bed (8) is replaced with an inert gas. Then, the tail gas of the third fluidized bed (8) is introduced into the hexafluoride thermal decomposition device (10) and the temperature is maintained at 200°C. Then, the NpF6 adsorption device (11) is used for adsorption. S5: The fluorinated slag produced by the third-stage fluidized bed (8) is transported to the fluorinated residue treatment system 9.

8. A stepwise fluorination method for oxide spent fuel reprocessing according to claim 7, characterized in that: In S2, the temperature of the first-stage fluidized bed (4) is maintained at 300°C; in S3, the temperature of the second-stage fluidized bed (7) is maintained at 450°C; in S4, the temperature of the third-stage fluidized bed (8) is controlled at 650°C; and the temperature of the hexafluoride thermal decomposition device (10) is maintained at 200°C.

9. A stepwise direct fluorination method for oxide spent fuel reprocessing, characterized in that: The following steps are involved: S1: The oxide spent fuel assembly is pretreated by the spent fuel assembly dismantling and core separation device (1) to obtain solid product powder and radioactive tail gas. The tail gas is captured, purified and discharged by the pretreated tail gas system (2), and the solid product powder enters the first-stage fluidized bed (4) for fluorination treatment. S2: The solid product powder is transported to the first-stage fluidized bed (4). Before use, the first-stage fluidized bed (4) is replaced with an inert gas and the bed temperature is maintained at 350°C. A fluorine-argon mixture is introduced for fluorination, and the easily fluorinated substances are converted into gaseous substances and enter the Tc and Mo fluoride trap (5) for freezing and trapping. S3: The powder tailings processed by the first-stage fluidized bed (4) in S2 are directly sent to the third-stage fluidized bed (8). Before use, the third-stage fluidized bed (8) is replaced with an inert gas and the temperature is raised to 600°C. A fluorine-argon mixture is introduced for fluorination. The fluorinated volatile gas is then introduced into the hexafluoride thermal decomposition device (10) and the temperature is maintained at 350°C. Finally, the tail gas is introduced into the NpF6 adsorption device (11).

10. A stepwise direct fluorination method for oxide spent fuel reprocessing according to claim 9, characterized in that: In step S2, a fluorine-argon mixture is introduced for the first stage of fluorination, with the fluorine and argon gases introduced in a 1:1 ratio.

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