Method for decoating coated fuel particles
The molten salt electrolysis method chemically removes the coatings of TRISO-type fuel particles, addressing reprocessing challenges by preventing mechanical scattering and wear, facilitating a smooth transition to the PUREX process.
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- KK TOSHIBA
- Filing Date
- 2024-10-30
- Publication Date
- 2026-05-15
AI Technical Summary
TRISO-type coated fuel particles used in high-temperature gas reactors are difficult to reprocess using the PUREX method due to their chemically stable and mechanically strong pyrolysis carbon and silicon carbide coatings, leading to issues like scattering of radioactive materials and equipment wear during mechanical crushing, and chemical methods require fuel fluorination.
A method involving molten salt electrolysis to anodically oxidize and dissolve the carbon and silicon carbide layers of the fuel particles, allowing for chemical removal of the coatings without mechanical operations, transitioning smoothly to reprocessing.
The method effectively removes the coatings without mechanical scattering or equipment wear, enabling easy transition to the PUREX reprocessing by preserving the nuclear fuel for reuse.
Smart Images
Figure 2026079082000001_ABST
Abstract
Description
[Technical Field]
[0001] Embodiments of the present invention relate to a method for decoating coated fuel particles. [Background technology]
[0002] In today's world, where a carbon-neutral society is becoming increasingly important, the importance of nuclear power generation, which does not produce CO2 during power generation, is being re-evaluated. While light water reactors are the mainstream method of nuclear power generation, various countries are developing safer next-generation innovative reactors. Among these next-generation innovative reactors is the high-temperature gas reactor, which uses chemically stable helium gas as a coolant and operates at high temperatures. High-temperature gas reactors not only achieve high power generation efficiencies of over 45%, but also have the advantage of being able to produce hydrogen using the heat generated. The fuel used in high-temperature gas reactors is TRISO (Tri-structural isotropic) coated fuel particles. Unlike general light water reactor fuel, these TRISO coated fuel particles have the nuclear fuel UO2 coated in multiple layers of heat-resistant ceramic material or carbon material with high thermal conductivity. Therefore, even in the event of an accident, the fuel is not damaged and the release of radioactive materials into the environment is unlikely.
[0003] In Japan, a country poor in resources and with a low energy self-sufficiency rate, as mentioned in the 6th Strategic Energy Plan, it is important not only to ensure a stable energy supply through nuclear power generation, but also to realize a nuclear fuel cycle that effectively utilizes resources by reprocessing spent fuel. In the reprocessing of typical light water reactor fuel, the PUREX method (Plutonium Uranium Redox Extraction) is used, in which the fuel rods housed in cladding tubes are sheared, dissolved in a nitric acid solution, and then various components are separated and purified for reuse as fuel raw material.
[0004] On the other hand, in high-temperature gas reactor fuel, the fuel itself is UO2, the same as in light water reactor fuel, but as mentioned above, the UO2 particles are covered with high-density pyrolysis carbon, low-density pyrolysis carbon, and silicon carbide, forming spherical fuel particles with a diameter of about 0.8 to 1.0 mm. The coating material of the fuel particles can contain radioactive materials even at 1600°C, has high chemical stability, and also has high mechanical strength, making it difficult to reprocess in its current state using the current PUREX method. [Prior art documents] [Patent Documents]
[0005] [Patent Document 1] Patent No. 3569659 [Patent Document 2] Patent No. 6534945 [Overview of the project] [Problems that the invention aims to solve]
[0006] TRISO-type coated fuel particles used in high-temperature gas reactors are made of UO2, so if the pyrolysis carbon (PyC) and silicon carbide (SiC) covering the UO2 can be removed from the fuel particles, they can be reprocessed using the PUREX method.
[0007] A known method for decoating TRISO-type coated fuel particles involves mechanically destroying the coating and extracting UO2 from within. This method involves introducing fuel particles into the gap between a pair of opposing rotary discs to destroy the coating. While it has been reported that this method can crush only the coating and recover the fuel portion, it presents problems such as the scattering of radioactive materials, adhesion to the equipment, and wear and tear on the crushing components.
[0008] Also, as a chemical approach, a method of reacting coated fuel particles with a fluoride gas at about 600 °C is known. This method converts the coating component, fuel component, and fission product component into volatile fluoride gases, and separates and recovers each component by utilizing the difference in vapor pressure of each fluoride. The recovered nuclear fuel component and fission product component are reacted with steam, added as oxides, and processed by the PUREX method. In this method, it is possible to chemically remove the coating material, but the fuel component is fluorinated, and it is necessary to perform oxide conversion in order to introduce it into the PUREX method.
[0009] Embodiments of the present invention have been made in consideration of such circumstances, and an object thereof is to provide a method for removing the coating component of coated fuel particles without going through mechanical operations and easily transitioning to a reprocessing process when reprocessing coated fuel particles.
Means for Solving the Problems
[0010] The method for removing the coating of coated fuel particles according to an embodiment of the present invention includes a molten salt immersion step of immersing coated fuel particles in the form of particles in which nuclear fuel is coated with four layers of a buffer layer, an inner carbon layer, a silicon carbide layer, and an outer carbon layer in a molten salt containing a molten halide, an outer carbon anodic oxidation step of anodizing the outer carbon layer in the molten salt, a silicon carbide dissolution step of alternately repeating electrolysis of reduction and oxidation with respect to the silicon carbide layer to dissolve the silicon carbide layer, an inner carbon anodic oxidation step of anodizing the inner carbon layer, and a buffer layer anodic oxidation step of anodizing the buffer layer.
Effects of the Invention
[0011] According to an embodiment of the present invention, there is provided a method for removing the coating of coated fuel particles that can remove the coating component without going through mechanical operations and can easily transition to a reprocessing process when reprocessing coated fuel particles.
Brief Description of the Drawings
[0012] [Figure 1]Cross-sectional view showing a coated fuel particle. [Figure 2] Flowchart showing a method for removing the coating of a coated fuel particle. [Figure 3] Explanatory drawing showing the configuration of an electrolytic cell in a carbon recovery process. [Figure 4] Explanatory drawing showing the configuration of an electrolytic cell in a silicon recovery process.
Mode for Carrying Out the Invention
[0013] Hereinafter, embodiments of a method for removing the coating of a coated fuel particle will be described in detail while referring to the drawings.
[0014] Reference numeral 1 in FIG. 1 is a coated fuel particle. This coated fuel particle 1 is the object to be processed by the coating removal method of the present embodiment. The coated fuel particle 1 has a particulate (spherical) shape and is a particle having a diameter D of about 0.8 to 1.0 mm. A large number of coated fuel particles 1 are loaded into a nuclear reactor in a state of being accommodated in a predetermined container (not shown). The method for removing the coating of the coated fuel particle 1 of the present embodiment relates to coating removal, which is a pre-step mainly for reprocessing the coated fuel particle 1 used in a high-temperature gas reactor.
[0015] First, the structure of the coated fuel particle 1 will be described. FIG. 1 is a schematic structural diagram of a TRISO-type coated fuel particle 1 that is currently mainstream.
[0016] At the center of the coated fuel particle 1 is a nuclear fuel 10 containing uranium dioxide (UO2), called the fuel kernel. This nuclear fuel 10 is covered by a buffer layer 11 made of low-density pyrolysis carbon. Furthermore, the buffer layer 11 is covered by an inner carbon layer 12 made of inner high-density pyrolysis carbon (IPyC). Furthermore, the inner carbon layer 12 is covered by a silicon carbide layer 13 made of silicon carbide (SiC). Furthermore, the silicon carbide layer 13 is covered by an outer carbon layer 14 made of outer high-density pyrolysis carbon (OPyC). In other words, the coated fuel particle 1 has a structure in which the central nuclear fuel 10 is covered by four layers: the buffer layer 11, the inner carbon layer 12, the silicon carbide layer 13, and the outer carbon layer 14. The method for decoating the coated fuel particle 1 in this embodiment is to remove each of these layers by molten salt electrolysis.
[0017] The outer high-density pyrolysis carbon (OPyC) is a pyrolysis carbon material with a density at least higher than the inner high-density pyrolysis carbon (IPyC). The inner high-density pyrolysis carbon (IPyC) is a pyrolysis carbon material with a density at least higher than the low-density pyrolysis carbon. The low-density pyrolysis carbon is the pyrolysis carbon material with the lowest density among the four layers mentioned above.
[0018] As shown in Figure 3, the decoating system 20 for performing the decoating method of coated fuel particles 1 according to this embodiment includes an electrolytic cell 21, a basket 22, a first electrode 23, and a second electrode 24. In this example in Figure 3, the first electrode 23 is the anode and the second electrode 24 is the cathode. However, the decoating system 20 can also have the first electrode 23 as the cathode and the second electrode 24 as the anode. In other words, the electrodes (cathode and anode) of the first electrode 23 and the second electrode 24 can be switched as appropriate. For example, the cathode and anode may be switched alternately within a single process.
[0019] The electrolytic cell 21 is a predetermined container that holds molten salt 25. The basket 22 is made of a conductive, mesh-like or net-like metal member. This basket 22 contains aggregates (aggregates) of coated fuel particles 1. The basket 22 is submerged in the molten salt 25 in the electrolytic cell 21. By submerging the basket 22, the aggregates of coated fuel particles 1 are immersed in the molten salt 25. The tip of the first electrode 23 (anode) is connected to the basket 22. The second electrode 24 (cathode) is located away from the basket 22. The tip of the second electrode 24 (cathode) is submerged in the molten salt 25 in the electrolytic cell 21.
[0020] Next, the method for decoating coated fuel particles 1 using the decoating system 20 will be explained using the flowchart in Figure 2. Refer to the previously mentioned diagrams as appropriate. Furthermore, each step included in the following flowchart represents at least some of the steps in this decoating method, and other steps may also be included in the flowchart below.
[0021] First, in step S1, the decoating system 20 (Figure 3) performs a molten salt immersion process in which the coated fuel particles 1 (Figure 1) are immersed in a molten salt 25 containing molten halogens. For example, an operator places an aggregate of coated fuel particles 1 into a basket 22 and immerses it in the molten salt 25 stored in the electrolytic cell 21.
[0022] In the next step S2, the decoating system 20 performs an outer carbon anodizing step in which the outer carbon layer 14 is anodized in molten salt 25. Here, the decoating system 20 applies a voltage to the first electrode 23 (anode) and the second electrode 24 (cathode) and performs molten salt electrolysis with the basket 22 containing the aggregate of coated fuel particles 1 as the anode.
[0023] Through this molten salt electrolysis, the outer carbon layer 14 is gradually dissolved into the molten salt 25 and removed from the coated fuel particles 1.
[0024] In the next step S3, the decoating system 20 performs a silicon carbide dissolution process in which the silicon carbide layer 13 is dissolved by repeatedly and alternately performing reduction and oxidation electrolysis on the silicon carbide layer 13.
[0025] As mentioned above, when the outer carbon layer 14 is removed in the outer carbon anodizing process, the silicon carbide layer 13 is exposed on the surface. SiO2 is generated on the outermost surface of this silicon carbide layer 13, which inhibits the dissolution and removal of SiC by anodizing. Therefore, the decoating system 20 dissolves the silicon carbide layer 13 by electrolysis, which alternates between cathode reduction and anodizing, and removes it from the coated fuel particles 1.
[0026] In the next step S4, the decoating system 20 performs an inner carbon anodizing step to anodize the inner carbon layer 12.
[0027] As mentioned above, when the silicon carbide layer 13 is removed in the silicon carbide dissolution process, the inner carbon layer 12 is revealed. This inner carbon layer 12 is the same material as the outer carbon layer 14. Therefore, just like the outer carbon layer 14, the inner carbon layer 12 can be removed by anodic oxidation.
[0028] In the next step S5, the decoating system 20 performs a buffer layer anodizing step to anodize the buffer layer 11.
[0029] As mentioned above, when the inner carbon layer 12 is removed in the inner carbon anodizing process, the buffer layer 11 is revealed. Since this is also composed of pyrolysis carbon, the buffer layer 11 can be removed from the coated fuel particles 1 by anodizing.
[0030] Once the buffer layer 11 is removed, the nuclear fuel 10 remains. The worker lifts the assembly of this remaining nuclear fuel 10, along with the basket 22, from the molten salt 25, thus completing the decoating method for the coated fuel particles 1. In this way, the decoated nuclear fuel 10 can be removed without going through the mechanical crushing operation.
[0031] Alternatively, the process may be repeated by returning to step S1 from step S5, and the decoating method for the coated fuel particles 1 may be repeated. In this case, the molten salt 25 accumulated in the electrolytic cell 21 can be reused any number of times.
[0032] Next, the detailed reaction of this embodiment will be described. Here, the molten halide contained in the molten salt 25 is a dissolved oxide of the cation that constitutes the halide.
[0033] When electrolysis is performed in molten halide with a carbon material as the anode, halogen gas is generated if the halide is pure. However, in actual electrolysis, trace amounts of water and air are mixed into the molten halide, and oxide ions (O) are present in the molten halide. 2- ) exists. The anodic reaction in a molten halide containing this oxide ion is as follows (see Equation 1).
[0034] (Equation 1) C+2O 2- =CO2+4e -
[0035] This reaction is undesirable in industrial electrolysis, such as metal refining, because it leads to the consumption of carbon electrodes. However, the decoating method for coated fuel particles 1 in this embodiment actively introduces oxide ions (O) into the molten halide to remove the coating component made of carbon. 2- We are adding )
[0036] For example, LiCl-KCl with a eutectic composition is used as a molten halide. In this case, the cations constituting the molten salt 25 and the same type of oxide, i.e., Li2O, are added to the molten salt 25 beforehand. When electrolysis is performed with the coated fuel particles 1 as the anode in this state, the buffer layer 11, the inner carbon layer 12, and the outer carbon layer 14 can be removed as carbon dioxide (CO2 gas) through the aforementioned reaction.
[0037] Furthermore, sufficient oxide ions (O) are present in the molten salt 25. 2-) exists, and if the contact between the generated gas and the oxide ions is sufficient, the carbonate ion (CO3 2- ) can trap the carbon component in the molten salt 25 as (see Equation 2).
[0038] (Equation 2) CO2 + O 2- = CO3 2-
[0039] Regarding the removal of the silicon carbide layer 13, it is necessary to remove not only SiC itself but also SiO2 that exhibits the high corrosion resistance of SiC. Since SiC and SiO2 have high resistance to anodic oxidation, they are converted to metallic Si by a reduction reaction (see Equation 3).
[0040] (Equation 3) SiC + 2O 2- + 4e - = Si + CO2 SiO2 + 4e - = Si + O 2-
[0041] The reduction reaction of SiO2 may be an indirect electrolytic reduction through the electrodeposition of an active metal instead of a direct electrolytic reduction (see Equation 4).
[0042] (Equation 4) Li + + e - = Li 4Li + SiO2 = 2Li2O + Si
[0043] If metallic Si is generated, it can be dissolved as ions by electrolysis using this as the anode (see Equation 5).
[0044] (Equation 5) Si = Si 4+ + 4e -
[0045] However, simultaneously with the progress of this dissolution reaction, SiO2 is generated by a competing reaction, so the reaction stops halfway (see Equation 6).
[0046] (Equation 6) Si+2O 2- =SiO2+4e -
[0047] To avoid this problem, the silicon carbide dissolution process alternately repeats reduction and oxidation, continuously causing the reduction of SiO2 to metallic Si, the oxidative dissolution of metallic Si, and the generation of metallic Si by the re-reduction of SiO2, thereby gradually dissolving the silicon carbide layer 13.
[0048] When the decoating method for coated fuel particles 1 of this embodiment is performed, carbon and silicon, which are the coating components of the coated fuel particles 1, accumulate in the molten salt 25. In order to reuse the molten salt 25 repeatedly over a long period of time, it is necessary to remove and recover these components from the molten salt 25.
[0049] Next, the recovery and removal of carbon components will be explained using Figure 3. In this example in Figure 3, the first electrode 23 is the anode and the second electrode 24 is the cathode. As for carbon, it is either discharged from the system as carbon dioxide (CO2 gas) or as oxide ions (O2). 2- If ) is present, CO3 2- CO3 is trapped in molten salt 25. 2- The ions can be reduced at the cathode and precipitated as solid carbon-26 (see Equation 7).
[0050] (Equation 7) CO3 2- +4e - =C+3O 2-
[0051] In other words, the decoating method for coated fuel particles 1 of this embodiment includes a carbon recovery step in which the carbon components removed in at least one of the steps described above are recovered from the molten salt 25 as carbon dioxide or solid carbon 26 by a reduction reaction at the cathode. Here, each step includes an outer carbon anodizing step, a silicon carbide dissolution step, an inner carbon anodizing step, and a buffer layer anodizing step.
[0052] The carbon recovery process may be performed in parallel with at least one of the aforementioned processes, between processes, before the molten salt immersion process, or after the buffer layer anodizing process. Here, each process includes the molten salt immersion process, the outer carbon anodizing process, the silicon carbide dissolution process, the inner carbon anodizing process, and the buffer layer anodizing process.
[0053] Next, the recovery and removal of silicon components will be explained using Figure 4. In this example in Figure 4, the first electrode 23 is the anode and the second electrode 24 is the cathode. For Si, 4+ When dissolved in molten salt 25, it can be recovered through two routes. The first is to precipitate solid silicon 27 at the cathode, similar to carbon (see Equation 8).
[0054] (Equation 8) Si 4+ +4e - =Si
[0055] The second method involves volatilizing and removing the low-boiling-point SiCl4 gas (boiling point 57.6°C) from the molten salt 25 (see Equation 9).
[0056] (Equation 9) Si 4+ +4M - =SiCl4
[0057] This reaction is effective with chloride-based molten salts that do not contain fluoride.
[0058] In other words, the decoating method for coated fuel particles 1 in this embodiment includes a silicon recovery step in which the silicon component removed in the silicon carbide dissolution step is recovered as metallic silicon at the cathode by molten salt electrolysis, or the silicon component is removed from the molten salt 25 as a volatile silicon compound.
[0059] The silicon recovery step may be performed in parallel with at least one of the steps described above, or between the steps, or before the molten salt immersion step, or after the buffer layer anodizing step. Here, each step includes the molten salt immersion step, the outer carbon anodizing step, the silicon carbide dissolution step, the inner carbon anodizing step, and the buffer layer anodizing step.
[0060] When each layer of the coating is removed by anodizing, once the coating is gone and UO2 is exposed, depending on the potential, UO2 may dissolve as ions in the molten salt 25 (see Equation 10).
[0061] (Equation 10) UO2 = UO2 2+ +2e -
[0062] This reaction causes a loss of fuel components, so it should be avoided as much as possible. However, if UO2 2+ Even in such cases, molten salt electrolysis can be used to recover UO2 as solid UO2 at the cathode (see Equation 11).
[0063] (Equation 11) UO2 2+ +2e - =UO2
[0064] The UO2 recovered at the cathode can then be processed using the PUREX method, just like the UO2 in nuclear fuel 10.
[0065] In other words, the decoating method for coated fuel particles 1 in this embodiment includes a fuel component recovery step in which fuel components dissolved in molten salt 25 are recovered by molten salt electrolysis, which precipitates them as fuel oxides at the cathode.
[0066] The fuel component recovery process may be performed in parallel with at least one of the aforementioned processes, between processes, before the molten salt immersion process, or after the buffer layer anodizing process. Here, each process includes the molten salt immersion process, the outer carbon anodizing process, the silicon carbide dissolution process, the inner carbon anodizing process, and the buffer layer anodizing process.
[0067] According to the embodiments described above, the method for decoating the coated fuel particles 1 includes an outer carbon anodizing step, a silicon carbide dissolution step, an inner carbon anodizing step, and a buffer layer anodizing step. This allows the coating components to be removed without mechanical operation when reprocessing the coated fuel particles 1, and enables a smooth transition to the reprocessing process.
[0068] The decoating method for coated fuel particles 1 includes a step of chemically and electrochemically removing the coating material of coated fuel particles 1 used in a high-temperature gas reactor in a molten salt 25, and a step of extracting oxides containing UO2, which is the nuclear fuel 10. In this way, when reprocessing coated fuel particles 1, the coating components can be removed without mechanical operation, thereby suppressing the scattering of radioactive materials and wear and tear on equipment. Furthermore, since the nuclear fuel 10 can be separated and recovered as oxides, it is possible to easily proceed to the PUREX reprocessing process.
[0069] While several embodiments of the present invention have been described, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These embodiments can be carried out in a variety of other forms, and various omissions, substitutions, modifications, and combinations are possible without departing from the spirit of the invention. These embodiments or their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Explanation of Symbols]
[0070] 1...Covered fuel particles, 10...Nuclear fuel, 11...Buffer layer, 12...Inner carbon layer, 13...Silicon carbide layer, 14...Outer carbon layer, 20...Decorating system, 21...Electrolyzer, 22...Basket, 23...First electrode, 24...Second electrode, 25...Molten salt, 26...Solid carbon, 27...Solid silicon.
Claims
1. A molten salt immersion step involves immersing coated fuel particles, which are particulate matter in which nuclear fuel is coated with four layers: a buffer layer, an inner carbon layer, a silicon carbide layer, and an outer carbon layer, in a molten salt containing molten halogens. An outer carbon anodizing step in which the outer carbon layer is anodized in the molten salt, A silicon carbide dissolution step is performed by repeatedly and alternately reducing and oxidizing the silicon carbide layer to dissolve the silicon carbide layer, The inner carbon anodizing step involves anodizing the inner carbon layer, A buffer layer anodizing step in which the buffer layer is anodized, including, A method for decoating coated fuel particles.
2. The molten halide is one in which the oxide of the cation constituting the halide is dissolved. A method for decoating coated fuel particles according to claim 1.
3. The process includes a carbon recovery step in which the carbon components removed in at least one of the steps of the outer carbon anodizing step, the silicon carbide dissolution step, the inner carbon anodizing step, and the buffer layer anodizing step are recovered from the molten salt as carbon dioxide or solid carbon by a reduction reaction at the cathode. A method for decoating coated fuel particles according to claim 1 or claim 2.
4. The process includes a silicon recovery step in which the silicon component removed in the silicon carbide dissolution step is recovered as metallic silicon at the cathode by molten salt electrolysis, or the silicon component is removed from the molten salt as a volatile silicon compound. A method for decoating coated fuel particles according to claim 1 or claim 2.
5. The process includes a fuel component recovery step in which the fuel components dissolved in the molten salt are recovered by molten salt electrolysis, by precipitating them as fuel oxides at the cathode. A method for decoating coated fuel particles according to claim 1 or claim 2.