High-temperature gas reactor fuel with chemical capture capabilities for radioactive cesium, and raw material powder used therein.

Abstract

This invention provides a high-temperature gas reactor fuel capable of immobilizing radioactive cesium, which is produced by nuclear fission from the fuel core of a high-temperature gas reactor and released from the coated fuel particles, within the fuel at a temperature range of room temperature to 1600°C, and a raw material powder used therefor. [Solution] The high-temperature gas reactor fuel having a chemical collection function for radioactive cesium is a high-temperature gas reactor fuel comprising: a fuel core 1 containing fissile material; a coated fuel particle 6 comprising at least a low-density pyrolysis carbon layer 2 covering the fuel core 1; and an outer high-density pyrolysis carbon layer 5 covering the low-density pyrolysis carbon layer 2; and a matrix base material 7 that holds the coated fuel particle 6 and is made of carbon or silicon carbide, wherein aluminum silicate is added to the raw material powder and / or surface of the matrix base material 7.

Description

[[Technical Field]]

[0001] The present invention relates to a high-temperature gas reactor fuel having a function of chemically trapping radioactive cesium, and a raw material powder used therefor. [[Background Art]]

[0002] Conventionally, many methods for trapping cesium in aqueous cesium compound solutions or solid cesium compounds using aluminum silicate have been developed for cesium trapping technologies, including a method of reducing cesium ions in an aqueous solution by bringing a cesium ion-containing aqueous solution into contact with aluminum silicate (Patent Document 1), and a method of performing gelation treatment on cesium-containing solid waste together with an aluminum silicate-containing solution (Patent Document 2). On the other hand, no trapping method targeting gaseous cesium (at 671°C or higher) in high-temperature environments has been developed so far.

[0003] Coated fuel particles, which are fuels for high-temperature gas reactors, have a structure in which fuel kernels with a diameter of several hundred micrometers (for example, uranium dioxide or uranium oxycarbide containing fissile materials) are coated with coating layers of carbon and silicon carbide (SiC), and fission products (FP) generated from the fuel kernels during reactor operation are physically confined, thereby suppressing radioactive contamination in the plant system. Therefore, when improving the FP confinement effect, conventional fuels adopt the method of thickening the coating layer to enhance physical confinement.

[0004] In a known study by the present applicant (Non-Patent Document 1), it was proposed that for cesium that cannot be physically confined by the conventional coating layers described above, cesium is trapped with antimony as a chemical confinement method, and a cesium-antimony compound is formed to immobilize cesium up to 1500°C. [[Prior Art Documents]] [[Patent Documents]]

[0005] [[Patent Document 1]] International Publication No. 2013 / 183742 [[Patent Document 2]] Patent No. 6210659 [Non-patent literature]

[0006] [Non-Patent Document 1] K. Sasaki, S. Miura, K. Fukumoto, M. Goto, H. Ohashi, “Development of Cesium Trap Material for Coated Fuel Particles in High Temperature Gas-Cooled Reactors”, Proceedings of the 28th International Conference on Nuclear Engineering, ICONE28-61765, August 4-6, 2021, Virtual, online. [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] As mentioned above, conventional fuels employ methods to enhance physical containment by increasing the thickness of the coating layer. However, even when fission products (FPs) are physically contained within the coating layer, some cesium, a major volatile metal FP, diffuses and migrates outside the fuel through defects within the coating layer, becoming the main radioactive deposition in the primary system of a high-temperature gas reactor plant. Therefore, methods other than physical containment are needed.

[0008] Furthermore, conventional chemical capture of cesium is limited to a maximum temperature of 1500°C due to the high-temperature stability of cesium-antimony compounds. This limitation prevents it from reaching the maximum allowable temperature of 1600°C for high-temperature gas reactor fuel during conceivable reactor accidents (such as abnormal transient changes during operation or the superposition of primary cooling system double-tube rupture and loss of reactor shutdown function). Therefore, there is a greater need for cesium capture materials that can immobilize cesium up to 1600°C.

[0009] In view of the above problems, the present invention aims to provide a high-temperature gas reactor fuel capable of immobilizing radioactive cesium, which is produced by nuclear fission from the fuel nucleus of a high-temperature gas reactor and released from the coated fuel particles, within the high-temperature gas reactor fuel at a temperature range of room temperature to 1600°C, and a raw material powder used therefor. [Means for solving the problem]

[0010] To solve the above problems, the high-temperature gas reactor fuel having a radioactive cesium chemical collection function of the present invention comprises a fuel core (1) containing fissile material, a coated fuel particle (6) comprising at least a low-density pyrolysis carbon layer (2) covering the fuel core (1), and an outer high-density pyrolysis carbon layer (5) covering the low-density pyrolysis carbon layer (2), and a matrix base material (7) that holds the coated fuel particle (6) and is made of carbon or silicon carbide, wherein aluminum silicate is added to the raw material powder and / or surface of the matrix base material (7). Furthermore, the raw material powder of the present invention is the raw material powder used in the matrix base material (7), wherein the aluminum silicate powder is added to the carbon or silicon carbide powder. [Effects of the Invention]

[0011] According to the high-temperature gas reactor fuel having a radioactive cesium chemical capture function of the present invention, radioactive cesium produced by nuclear fission from the fuel core of a high-temperature gas reactor and released from the coated fuel particles can be immobilized within the high-temperature gas reactor fuel at a temperature range of room temperature to 1600°C. Furthermore, the raw material powder of the present invention makes it possible to easily provide high-temperature gas reactor fuel having a radioactive cesium chemical capture function. [Brief explanation of the drawing]

[0012] [Figure 1] This is an explanatory diagram showing the structure of high-temperature gas reactor fuel according to one embodiment of the present invention. [Figure 2] This is the result of a reaction test (elemental analysis) between aluminum silicate and cesium in carbon. [Figure 3]Figure 2 shows the results of a heat resistance test (elemental analysis results) at 1600°C for 1 hour in an argon gas atmosphere for the sample obtained by reacting aluminum silicate and cesium in carbon. [Figure 4] Figure 2 shows the results of a heat resistance test (elemental analysis results) at 1600°C for 1 hour in an air atmosphere for the sample obtained by reacting aluminum silicate and cesium in carbon. [Figure 5] Figure 3 shows the crystal structure analysis results (electron diffraction pattern) of the sample. [Figure 6] This shows the results of a heat resistance test (elemental analysis results) of a sample obtained by reacting aluminum silicate and cesium in silicon carbide, under an argon gas atmosphere at 1600°C for 1 hour. [Modes for carrying out the invention]

[0013] Hereinafter, embodiments of the present invention (hereinafter abbreviated as "examples") will be described based on the drawings. In the following drawings, common parts are denoted by the same reference numerals, and redundant explanations for parts with the same reference numerals will be omitted.

[0014] (Cesium capture reaction) The high-temperature gas reactor fuel having a chemical collection function for radioactive cesium according to the present invention is a compact fuel 8 or a pebble fuel 9, as shown in Figure 1. They consist of a fuel core 1 containing fissile material, a coated fuel particle 6 comprising at least a low-density pyrolysis carbon layer 2 covering the fuel core 1, and an outer high-density pyrolysis carbon layer 5 covering the low-density pyrolysis carbon layer 2, and a matrix base material 7 made of carbon or silicon carbide that holds the coated fuel particle 6.

[0015] As shown in Fig. 1, the coated fuel particle 6 is constituted by a fuel kernel 1 made of uranium or the like that is quadruple-coated with a low-density pyrolytic carbon layer 2, an inner high-density pyrolytic carbon layer 3, a silicon carbide layer 4, and an outer high-density pyrolytic carbon layer 5. There also exist coated fuel particles of a type in which the nuclear fuel kernel 1 is double-coated with the low-density pyrolytic carbon layer 2 and the outer high-density pyrolytic carbon layer 5, and the present invention can be applied to either type. In the present specification, the quadruple-coated type coated fuel particles will be described as a representative example.

[0016] Thousands to tens of thousands of the coated fuel particles 6 are held in a fuel matrix base material 7 made of carbon or silicon carbide, aggregated into the shape of a compact fuel 8 or a pebble-type fuel 9, and this is loaded into a reactor core as fuel for a high-temperature gas-cooled reactor.

[0017] In the present invention, radioactive cesium that is fission-generated from the fuel kernel 1 and released from the coated fuel particles 6 is brought into contact with aluminum silicate added in advance to the raw material powder and / or surface of the fuel matrix base material 7 located outside the coated fuel particles 6, whereby cesium aluminosilicate is formed to collect the radioactive cesium.

[0018] Thereby, even at the maximum allowable temperature of 1600°C for high-temperature gas-cooled reactor fuel, which is assumed under reactor accident conditions (such as abnormal transients during operation, or the superposition of primary cooling facility double pipe rupture and loss of reactor shutdown function), cesium is immobilized, and radioactive contamination due to cesium in other high-temperature gas-cooled reactor plant areas is reduced.

[0019] As shown in Chemical Formula 1 below, aluminum silicate (xAl₂O₃·ySiO₂), which is a cesium collector, has a plurality of chemical formulas with different x and y. In any aluminum silicate, it reacts with cesium (+ floating oxygen O₂ in the fuel) to form cesium aluminosilicate CsAlSi₂O₆ (+ by-product Al₂O₃). Note that xAl₂O₃·ySiO₂ is a chemical formula of any aluminum silicate that an operator desires to use.

[0020]

Chemical Formula

[0021] (Required amount of cesium collecting material) To fully realize the effects of the present invention, the amount of aluminum silicate required is greater than or equal to the amount that can convert the total amount of cesium produced by nuclear fission in the reactor into cesium aluminosilicate.

[0022] The amount of aluminum silicate (xAl2O3·ySiO2) required to collect cesium z[mol] is 2×z / y[mol], based on the molar ratio of aluminum silicate to cesium shown in chemical formula 1 above.

[0023] (Placement of cesium collecting material) As mentioned above, the matrix base material 7 is composed of carbon and silicon carbide, but aluminum silicate powder is added to these powders to create the raw material powder for the fuel matrix base material 7.

[0024] By dispersing this raw material powder, aluminum silicate, within the fuel matrix base material 7, which is the migration pathway for cesium, the chemical capture function for radioactive cesium is most effectively achieved.

[0025] In addition to mixing aluminum silicate into the raw material powder as described above, other methods for adding aluminum silicate to the fuel matrix base material 7 include coating the surface of coated fuel particles, the surface of compact fuel 8 formed by assembling and molding coated fuel particles, and the space between the fuel matrix base material 7 and the unfueled hull 10 of pebble fuel 9. However, if coating is used, separate technological development will be required for the coating method and durability. It is also considered possible to chemically capture cesium and immobilize it at that location by coating other parts that cesium may come into contact with (for example, the surface of internal reactor structures, the inner surface of the reactor pressure vessel, or the inner surface of primary system structures). However, in that case, the range of cesium radioactive contamination will expand, reducing the benefits to improving plant safety and economic efficiency, and separate technological development will be required for the coating method and durability. Therefore, mixing aluminum silicate into the fuel matrix base material powder is preferred. [Examples]

[0026] The present invention will be described in more detail below based on the following examples, but the present invention is not limited thereto. (Specific examples of the amount of aluminum silicate to be added) For example, for one compact fuel unit with a 20% enrichment level, if the fission yield of cesium produced by fissioning all U235 is 20%, the required amount of aluminum silicate (Al2O3·3SiO2) to be added can be determined from the specific conditions shown in the table below. In this example, a 100% fuel failure rate is assumed, and the case where the entire amount of fission-generated cesium is used as the collection target is shown; however, the amount of cesium to be collected is determined by the operator, so this is not always the case. The cesium (2.39 × 10) generated in one compact fuel under the conditions shown in the table below. -3 The amount of aluminum silicate (Al2O3·3SiO2) required to collect [mol] is 2 × 2.39 × 10, based on the molar ratio of aluminum silicate to cesium according to chemical formula 1 above. -3 / 3, that is, 1.59 × 10 -3 The result is [mol](0.45[g]). If necessary, the design should reflect a correction for the volume increase of the fuel matrix base material 7 due to the addition of aluminum silicate. Furthermore, when actually manufacturing the raw material powder for the fuel matrix base material 7 with added aluminum silicate, prepare an amount equal to the planned production compact number multiplied by the quantity.

[0027] [Table 1]

[0028] (Specific examples of methods for adding aluminum silicate) The effects of the present invention are obtained by adding aluminum silicate to the fuel matrix base material 7. Specifically, in the conventional high-temperature gas reactor fuel manufacturing process, aluminum silicate is added in advance to the raw material powder of the fuel matrix base material 7 that is coated on each coated fuel particle 6, before the aggregation of the coated fuel particles 6.

[0029] When aluminum silicate is dispersed and added to a graphite fuel matrix base material 7, it is preferable to add it during the preparation process of the adjusted graphite powder (a mixed powder of natural graphite powder, artificial graphite powder, and phenolic resin binder), which is the raw material for the fuel matrix base material 7, in order to ensure homogeneity. Furthermore, the amount of aluminum silicate added should be adjusted so that the aluminum silicate is incorporated into the raw material powder without excess or deficiency.

[0030] (Cesium capture function of aluminum silicate) A reaction test was conducted between aluminum silicate and cesium. Specifically, a mixed powder of carbon and aluminum silicate (Al2O3·3SiO2) (molar ratio 10:1) and cesium were placed in an alumina reaction vessel to simulate a state in which aluminum silicate is dispersed in a carbon fuel matrix base material 7. The reaction test between aluminum silicate and cesium was then carried out by heating at 670°C for 1 hour in an argon gas atmosphere. The elemental analysis results in Figure 2 confirm the detection of cesium in the sample after the reaction test.

[0031] A heat resistance test was conducted on the sample shown in Figure 2, which was obtained by reacting aluminum silicate in carbon with cesium, at 1600°C for 1 hour, the allowable fuel temperature for a high-temperature gas reactor. Here, the abnormal transient change environment during operation was simulated by using an inert gas (argon gas) as the heat resistance test atmosphere, and the severe accident environment due to the superposition of primary cooling system double-wall rupture and loss of reactor shutdown function was simulated by using air as the heat resistance test atmosphere. As shown in Figures 3 and 4, the detection of cesium was confirmed in the sample after the heat resistance test, confirming that the cesium collection function can be maintained even under the fuel temperature environment during a high-temperature gas reactor accident.

[0032] As shown in Figure 5, the crystal structure analysis results (electron diffraction pattern) of the sample in Figure 3 identified the stable compound formed by the reaction of aluminum silicate and cesium as CsAlSi2O6, and the aforementioned chemical formula 1 in the cesium capture reaction was revealed.

[0033] To confirm the cesium capture function of aluminum silicate in the silicon carbide fuel matrix base material 7, a sample simulating a state where aluminum silicate is dispersed in the silicon carbide fuel matrix base material 7 was prepared. A mixed powder of silicon carbide and aluminum silicate (Al2O3·3SiO2) (molar ratio 10:1) and cesium were placed in an alumina reaction vessel, and a reaction test of aluminum silicate and cesium was conducted by heating at 670°C for 1 hour in an argon gas atmosphere. The sample obtained from this test was subjected to a heat resistance test at 1600°C for 1 hour in an argon gas atmosphere. As shown in Figure 6, cesium was detected in the sample after the heat resistance test, indicating that the cesium capture method using aluminum silicate is effective even when the fuel matrix base material 7 is made of silicon carbide.

[0034] The experimental results above confirm that contacting radioactive cesium with aluminum silicate in the fuel matrix base material forms aluminosilicate cesium, which has a chemical capture effect on radioactive cesium. Therefore, it is clear that the high-temperature gas reactor fuel with the radioactive cesium chemical capture function of the present invention can immobilize radioactive cesium produced by nuclear fission from the fuel nucleus of a high-temperature gas reactor and released from the coated fuel particles within the high-temperature gas reactor fuel at temperatures ranging from room temperature to 1600°C.

[0035] The high-temperature gas reactor fuel having the chemical collection function for radioactive cesium described above, and the raw material powder used therein, are merely examples, and their configurations can be modified as appropriate without departing from the spirit of the invention. [Explanation of Symbols]

[0036] 1 fuel nucleus 2. Low-density pyrolysis carbon layer 3. Inner high-density pyrolysis carbon layer 4. Silicon carbide layer 5 Outer high-density pyrolytic carbon layer 6. Coated fuel particles 7 Fuel Matrix Base Material 8 Compact fuel 9. Pebble-type fuel 10 Unfueled hull

Claims

1. A fuel nucleus (1) containing fissile material, A low-density pyrolysis carbon layer (2) covering the fuel core (1), The outer high-density pyrolysis carbon layer (5) covers the low-density pyrolysis carbon layer (2), A coated fuel particle (6) comprising at least the following: A matrix base material (7) made of carbon or silicon carbide holds the coated fuel particles (6), High-temperature gas reactor fuel composed of, A high-temperature gas reactor fuel having a chemical capture function for radioactive cesium, characterized in that aluminum silicate is added to the raw material powder and / or surface of the matrix base material (7).

2. The raw material powder used in the matrix base material (7) according to claim 1, characterized in that the aluminum silicate powder is added to the carbon or silicon carbide powder.

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