Method for decoating coated fuel particles

Abstract

The present invention provides a method for decoating coated fuel particles that allows for the removal of coating components without mechanical operation during reprocessing of coated fuel particles, and enables easy transition to the reprocessing process. [Solution] The method for decoating coated fuel particles 1 includes a molten salt immersion step in which coated fuel particles 1, which are particulate in which nuclear fuel 10 is coated with four layers: a buffer layer 11, an inner carbon layer 12, a silicon carbide layer 13, and an outer carbon layer 14, are immersed in a molten salt 25 containing a molten halide; an outer carbon cathode reduction step in which the outer carbon layer 14 is cathode-reduced in the molten salt 25; a silicon carbide dissolution step in which the silicon carbide layer 13 is dissolved by repeatedly performing alternating reduction and oxidation electrolysis on the silicon carbide layer 13; an inner carbon cathode reduction step in which the inner carbon layer 12 is cathode-reduced; and a buffer layer cathode reduction step in which the buffer layer 11 is cathode-reduced.

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] Another known chemical approach involves reacting coated fuel particles with fluoride gas at approximately 600°C. This method converts the coating components, fuel components, and fission product components into volatile fluoride gases, and then separates and recovers each component by utilizing the difference in vapor pressure of the respective fluorides. The recovered nuclear fuel components and fission product components are reacted with water vapor to form oxides, which are then processed using the PUREX method. While this method allows for the chemical removal of the coating material, it also fluorinates the fuel components, requiring oxide conversion before they can be introduced into the PUREX method.

[0009] The embodiments of the present invention have been made in consideration of these circumstances, and aim to provide a method for decoating coated fuel particles that can remove the coating component without mechanical operation when reprocessing coated fuel particles, and that can easily proceed to the reprocessing process. [Means for solving the problem]

[0010] A method for decoating coated fuel particles according to an embodiment of the present invention includes: a molten salt immersion step of immersing coated fuel particles, which are particulate 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 a molten halide; an outer carbon cathode reduction step of cathodely reducing the outer carbon layer in the molten salt; a silicon carbide dissolution step of dissolving the silicon carbide layer by alternately repeating reduction and oxidation electrolysis on the silicon carbide layer; an inner carbon cathode reduction step of cathodely reducing the inner carbon layer; and a buffer layer cathode reduction step of cathodely reducing the buffer layer. [Effects of the Invention]

[0011] According to embodiments of the present invention, a method for decoating coated fuel particles is provided that allows for the removal of coating components without mechanical operation during reprocessing of coated fuel particles, and enables easy transition to the reprocessing process. [Brief explanation of the drawing]

[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 an electrolytic cell in a carbon recovery process. [Figure 4] Explanatory drawing showing an electrolytic cell in a silicon recovery process. [Figure 5] Explanatory drawing showing an electrolytic cell in a preliminary electrolysis 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 with reference 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 cathode and the second electrode 24 is the anode. However, the decoating system 20 can also have the first electrode 23 as the anode and the second electrode 24 as the cathode. 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 (cathode) is connected to the basket 22. The second electrode 24 (anode) is located away from the basket 22. The tip of the second electrode 24 (anode) 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 coated fuel particles 1 (Figure 1) are immersed in a molten salt 25 containing a molten halide. 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. The halide used here is selected to have an alkali metal or alkaline earth metal as its cation.

[0022] In the next step S2, the decoating system 20 performs an outer carbon cathode reduction step in which the outer carbon layer 14 is cathode-reduced in molten salt 25. Here, the decoating system 20 applies a voltage to the first electrode 23 (cathode) and the second electrode 24 (anode) and performs molten salt electrolysis with the basket 22 containing the aggregate of coated fuel particles 1 as the cathode.

[0023] This electrolytic operation causes the outer carbon layer 14 covering the outermost periphery of the coated fuel particle 1 to react with alkali metals or alkaline earth metals, generating alkali metal carbides or alkaline earth metal carbides. As these carbides dissolve in the molten salt 25, the outer carbon layer 14 is gradually dissolved into the molten salt 25 and removed from the coated fuel particle 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 dissolved and removed as carbide in the outer carbon cathode reduction process, the silicon carbide layer 13 appears 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 anodic oxidation. Therefore, the decoating system 20 dissolves the silicon carbide layer 13 by electrolysis, which alternates between cathode reduction and anodic oxidation, and removes it from the coated fuel particles 1.

[0026] In the next step S4, the decoating system 20 performs an inner carbon cathode reduction step to cathode-reduce 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, similar to the outer carbon layer 14, the inner carbon layer 12 can be removed by carbide generation through cathode reduction and subsequent carbide dissolution.

[0028] In the next step S5, the decoating system 20 performs a buffer layer cathode reduction step to cathode-reduce the buffer layer 11.

[0029] As mentioned above, when the inner carbon layer 12 is removed in the inner carbon cathode reduction process, the buffer layer 11 is revealed. Since this is also composed of pyrolysis carbon, the buffer layer 11 can be dissolved as carbide in the cathode reduction and removed from the coated fuel particles 1.

[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 an alkali metal halide or an alkaline earth metal halide, or a mixture thereof.

[0033] The cathode reaction that occurs during molten salt electrolysis of alkali metal halides, alkaline earth metal halides, or mixtures thereof is a deposition reaction of alkali metals or alkaline earth metals. A specific example is molten CaCl2. When the precipitated metallic Ca comes into contact with carbon, calcium carbide (CaC2) is produced (see Equation 1).

[0034] (Equation 1) Ca 2+ +2e - =Ca Ca + 2C = CaC2

[0035] Furthermore, even when electrolysis is performed at a potential nobler than the potential at which metallic Ca precipitates, it is possible to produce CaC2, albeit at a slower reaction rate (see Equation 2).

[0036] (Formula 2) Ca 2+ +2e - +2C=CaC2

[0037] The CaC2 thus produced dissolves in the molten salt 25 (see Formula 3).

[0038] (Formula 3) CaC2=Ca 2+ +C2 2-

[0039] Since the outer carbon layer 14, the inner carbon layer 12, and the buffer layer 11, which are the cladding materials of the nuclear fuel 10, are all carbon, these cladding materials can be removed as carbide ions C2 2- into the molten salt 25.

[0040] 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 silicon (Si) by a reduction reaction (see Formula 4).

[0041] (Formula 4) SiC+2O 2- +4e - =Si+CO2 SiO2+4e - =Si+O 2- <000019​​​​​​​​​​​​​​​​​​​​

[0045] (Equation 6) Si = Si 4+ +4e -

[0046] However, as this dissolution reaction proceeds, SiO2 is produced in a competing reaction, causing the reaction to stop midway (see Equation 7).

[0047] (Equation 7) Si+2O 2- =SiO2+4e -

[0048] 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.

[0049] When the decoating method for coated fuel particles 1 of this embodiment is performed, the carbon, which is the coating component of coated fuel particles 1, becomes carbide ions C2 2- These components 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.

[0050] 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 cathode and the second electrode 24 is the anode. C2 2- It can be precipitated as solid carbon-26 by an oxidation reaction at the anode (see Equation 8).

[0051] (Equation 8) C2 2- = 2C + 2e -

[0052] 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 solid carbon 26 by an oxidation reaction at the anode. Here, each step includes an outer carbon cathode reduction step, a silicon carbide dissolution step, an inner carbon cathode reduction step, and a buffer layer cathode reduction step.

[0053] The carbon 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 cathode reduction step. Here, each step includes the molten salt immersion step, the outer carbon cathode reduction step, the silicon carbide dissolution step, the inner carbon cathode reduction step, and the buffer layer cathode reduction step.

[0054] When the decoating method for coated fuel particles 1 of this embodiment is performed, silicon, which is the coating component of the coated fuel particles 1, accumulates 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.

[0055] 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 9).

[0056] (Equation 9) Si 4+ +4e - =Si

[0057] 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 10).

[0058] (Equation 10) Si 4+ +4M - =SiCl4

[0059] This reaction is effective with chloride-based molten salts that do not contain fluoride.

[0060] 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.

[0061] 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 cathode reduction step. Here, each step includes the molten salt immersion step, the outer carbon cathode reduction step, the silicon carbide dissolution step, the inner carbon cathode reduction step, and the buffer layer cathode reduction step.

[0062] Among the coating components of coated fuel particle 1, carbon is converted into carbide ions C2 2- In order to dissolve and remove them, it is necessary to generate readily soluble alkali metal carbides or alkaline earth metal carbides in the molten salt 25 through a reduction reaction. However, if cations other than alkali metals or alkaline earth metals, such as chromium or zirconium, are present, precipitation reactions of these components will occur, making it impossible to generate the desired alkali metal or alkali earth metal carbides.

[0063] To prevent this, as shown in Figure 5, the worker immerses the tip of the spare electrode 30, which will be used as the cathode, in the molten salt 25. In this example in Figure 5, the first electrode 23 is not used, the second electrode 24 is used as the anode, and the spare electrode 30 is used as the cathode. Then, by performing pre-electrolysis at a potential nobler than the potential at which alkali metals and alkaline earth metals are deposited using this spare electrode 30, other inhibiting cations 28 can be deposited on the spare electrode 30 and removed.

[0064] In other words, the decoating method for coated fuel particles 1 in this embodiment includes a pre-electrolysis step in which cationic components other than alkali metals or alkaline earth metals (inhibiting cations 28) are deposited at the cathode by pre-electrolysis and removed.

[0065] The pre-electrolysis 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 cathode reduction step. Here, each step includes the molten salt immersion step, the outer carbon cathode reduction step, the silicon carbide dissolution step, the inner carbon cathode reduction step, and the buffer layer cathode reduction step. The pre-electrolysis step is performed in at least one of the steps described above before carbon components, alkali metal carbides, and alkaline earth metal carbides are generated in the molten salt 25.

[0066] According to the embodiments described above, the method for decoating the coated fuel particles 1 includes an outer carbon cathode reduction step, a silicon carbide dissolution step, an inner carbon cathode reduction step, and a buffer layer cathode reduction 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.

[0067] 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.

[0068] 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]

[0069] 1...Coated fuel particles, 10...Nuclear fuel, 11...Buffer layer, 12...Inner carbon layer, 13...Silicon carbide layer, 14...Outer carbon layer, 20...Decoating system, 21...Electrolyzer, 22...Basket, 23...First electrode, 24...Second electrode, 25...Molten salt, 26...Solid carbon, 27...Solid silicon, 28...Inhibiting cation, 30...Reserve electrode.

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 cathode reduction step in which the outer carbon layer is cathode-reduced 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 cathode reduction step involves reducing the inner carbon layer at the cathode, A buffer layer cathode reduction step for cathode reduction of the buffer layer, including, A method for decoating coated fuel particles.

2. The molten halide is an alkali metal halide or an alkaline earth metal halide, or a mixture thereof. A method for decoating coated fuel particles according to claim 1.

3. The carbon recovery step includes recovering the carbon components removed in at least one of the steps of the outer carbon cathode reduction step, the silicon carbide dissolution step, the inner carbon cathode reduction step, and the buffer layer cathode reduction step, as solid carbon from the molten salt by an oxidation reaction at the anode. 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 pre-electrolysis step in which cationic components other than alkali metals or alkaline earth metals are deposited and removed at the cathode by pre-electrolysis. A method for decoating coated fuel particles according to claim 1 or claim 2.

Images