High-temperature gas-cooled reactor fuel having function of chemically trapping radioactive cesium and raw material powder to be used for same

Abstract

[Problem] To provide: a high-temperature gas-cooled reactor fuel capable of immobilizing radioactive cesium, which is produced by nuclear fission in fuel kernels of a high-temperature gas-cooled reactor and released from coated fuel particles, within the high-temperature gas-cooled reactor fuel in a temperature range of room temperature to 1600°C; and a raw material powder to be used for the high-temperature gas-cooled reactor fuel. [Solution] A high-temperature gas-cooled reactor fuel having the function of chemically trapping radioactive cesium comprises: coated fuel particles 6 each including at least a fuel kernel 1 containing a fissile material, a low-density pyrolytic carbon layer 2 coating the fuel kernel 1, and an outer high-density pyrolytic carbon layer 5 coating the low-density pyrolytic carbon layer 2; and a matrix material 7 configured to retain the coated fuel particles 6 and composed of carbon or silicon carbide. Aluminum silicate is added to a raw material powder of the matrix material 7 and / or to a surface of the matrix base material 7.

Description

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

[0001] This invention relates to high-temperature gas reactor fuel having a chemical collection function for radioactive cesium, and to a raw material powder used therefor.

[0002] Conventionally, many cesium collection technologies have been developed that involve collecting aqueous solutions of cesium compounds or solid cesium compounds using aluminum silicate, such as a method of reducing the amount of cesium ions in an aqueous solution by contacting it with aluminum silicate (Patent Document 1), or a method of gelling solid cesium-containing waste together with an aluminum silicate-containing solution (Patent Document 2). On the other hand, no collection method has been developed for cesium in a gaseous state (above 671°C) in high-temperature environments.

[0003] The clad fuel particles used in high-temperature gas reactors consist of fuel nuclei (such as uranium dioxide or uranium oxycarbide containing fissile material) with a diameter of several hundred micrometers, coated with a layer of carbon and silicon carbide (SiC). This structure suppresses radioactive contamination within the plant system by physically containing fission products (FPs) generated from the fuel nuclei during reactor operation. Therefore, to improve FP containment, conventional fuels employ the method of increasing the thickness of the cladding layer to strengthen physical containment.

[0004] In a previously published study by the applicant (Non-Patent Document 1), it was proposed that, in response to cesium that cannot be physically contained by the conventional coating layer described above, cesium be collected with antimony as a chemical containment method, and that cesium-antimony compounds be formed to immobilize cesium up to 1500°C.

[0005] International Publication No. 2013 / 183742, Patent No. 6210659

[0006] 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.

[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 by the coating layer, some cesium, the main volatile metal FP, diffuses and migrates outside the fuel through defects within the coating layer, and this becomes the main radioactive deposition in the primary system of the 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 the cesium-antimony compound. 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.

[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 low-density pyrolysis carbon layer (2) covering the fuel core (1); an outer high-density pyrolysis carbon layer (5) covering the low-density pyrolysis carbon layer (2) comprising coated fuel particles (6); and a matrix base material (7) that holds the coated fuel particles (6) and is composed 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 aluminum silicate powder is added to the carbon or silicon carbide powder.

[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.

[0012] This is an explanatory diagram showing the structure of high-temperature gas reactor fuel according to one embodiment of the present invention. This is the result of a reaction test (elemental analysis result) of aluminum silicate in carbon and cesium. This is the result of a heat resistance test (elemental analysis result) of the sample in Figure 2, in which aluminum silicate in carbon and cesium were reacted, at 1600°C for 1 hour in an argon gas atmosphere. This is the result of a heat resistance test (elemental analysis result) of the sample in Figure 2, in which aluminum silicate in carbon and cesium were reacted, at 1600°C for 1 hour in an air atmosphere. This is the result of a crystal structure analysis (electron diffraction pattern) of the sample in Figure 3. This is the result of a heat resistance test (elemental analysis result) of the sample in which aluminum silicate in silicon carbide and cesium were reacted, at 1600°C for 1 hour in an argon gas atmosphere.

[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 Collection 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 Figure 1, the coated fuel particle 6 is constructed by coating a fuel core 1, composed of uranium or the like, with four layers: a low-density pyrolysis carbon layer 2, an inner high-density pyrolysis carbon layer 3, a silicon carbide layer 4, and an outer high-density pyrolysis carbon layer 5. There are also coated fuel particles of the type in which the nuclear fuel core 1 is coated with two layers: the low-density pyrolysis carbon layer 2 and the outer high-density pyrolysis carbon layer 5. The present invention can be applied to either type, but in this specification, the four-layer coated fuel particle will be described as a representative example.

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

[0017] In this invention, radioactive cesium produced by nuclear fission from the fuel core 1 and released from the coated fuel particles 6 is brought into contact with aluminum silicate, which is added beforehand to the raw material powder and / or surface of the fuel matrix base material 7 located outside the coated fuel particles 6, to form aluminosilicate cesium and collect the radioactive cesium.

[0018] This will immobilize cesium even at the maximum allowable fuel temperature of 1600°C, which is assumed to occur during a reactor accident (such as abnormal transient changes during operation or the superposition of a double-walled primary cooling system rupture and loss of reactor shutdown function), and reduce radioactive contamination by cesium in other areas of the high-temperature gas reactor plant.

[0019] As shown in Chemical Formula 1 below, aluminum silicate (xAl) is a cesium collecting material. ２ O ３ ・ySiO ２In ( ), there are multiple types of chemical formulas of x and y. However, in any aluminum silicate, cesium ( + floating oxygen O in the fuel) ２ reacts with ) to form cesium aluminosilicate CsAlSi ２ O ６ ( + by-product Al ２ O ３ ). Note that xAl ２ O ３ ・ySiO ２ is the chemical formula of any aluminum silicate that the operator wants to use.

[0020]

[0021] (Required amount of cesium capture material) The aluminum silicate for fully exerting the effects of the present invention needs to be an amount sufficient to convert all the cesium generated by nuclear fission in the reactor into cesium aluminosilicate.

[0022] The amount of aluminum silicate (xAl ２ O ３ ・ySiO ２ ) required to capture z [mol] of cesium is 2 × z / y [mol] from the relationship of the molar ratio of aluminum silicate and cesium according to the above chemical formula 1.

[0023] (Arrangement of cesium capture material) The matrix base material 7 is composed of carbon or silicon carbide as described above. However, a powder of aluminum silicate is added to those powders to prepare a raw material powder for the fuel matrix base material 7.

[0024] By dispersing and arranging this raw material powder with aluminum silicate in the fuel matrix base material 7 which is the migration path of cesium, the chemical capture function of radioactive cesium can be most effectively exerted.

[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 its 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 its durability. Therefore, mixing aluminum silicate into the fuel matrix base material powder is preferred.

[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 example of the amount of aluminum silicate added) As an example, for one compact fuel with an enrichment level of 20%, if the fission yield of cesium produced by fissioning all of U235 is 20%, then aluminum silicate Al ２ O ３ 3SiO ２ The required amount of additive is determined from the conditions of the specific example shown in the table below. In this example, a fuel failure rate of 100% is assumed, and the total amount of cesium produced by nuclear fission is used as the collection target. However, the amount of cesium to be collected is determined by the operator, so this is not limited to this example. The amount of cesium generated in one compact fuel under the conditions shown in the table below is (2.39 × 10⁻⁶). －３ [mol]) is necessary to collect aluminum silicate (Al ２ O ３ 3SiO ２ The amount of ) is 2 × 2.39 × 10, based on the molar ratio relationship between aluminum silicate and cesium according to the above chemical formula 1. －３ / 3, that is, 1.59 × 10 －３It is found to be [mol] (0.45 [g]). If necessary, the correction of the volume increase of the fuel matrix base material 7 due to the addition of aluminum silicate is reflected in the design. Also, when actually manufacturing the raw material powder for the fuel matrix base material 7 to which aluminum silicate is added, a quantity multiplied by the number of compacts scheduled for production is prepared.

[0027]

[0028] (Specific example of the method of adding aluminum silicate) The effect of the present invention is obtained by adding aluminum silicate to the fuel matrix base material 7. Specifically, in the conventional high-temperature gas furnace fuel manufacturing process, aluminum silicate is added in advance to the raw material powder of the fuel matrix base material 7 that coats each coated fuel particle 6 before the aggregation of the coated fuel particles 6.

[0029] When dispersedly adding aluminum silicate to the graphite-based fuel matrix base material 7, it is preferable to add it in the blending process of the adjusted graphite powder (mixed powder of natural graphite powder, artificial graphite powder, and phenolic resin binder), which is the raw material of the fuel matrix base material 7, from the viewpoint of ensuring homogeneity. Also, the addition amount of aluminum silicate is adjusted so that aluminum silicate is incorporated into the raw material powder without excess or deficiency.

[0030] (Cesium capture function of aluminum silicate) A reaction test between aluminum silicate and cesium was conducted. Specifically, as a sample simulating a state in which aluminum silicate is dispersed in the carbon-based fuel matrix base material 7, carbon and aluminum silicate (Al ２ O ３ ·3SiO ２ ) mixed powder (molar ratio 10 to 1), and cesium were put into an alumina reaction vessel, and a reaction test between aluminum silicate and cesium was carried out by heating at 670 °C for 1 hour in an argon gas atmosphere. In the elemental analysis result of Figure 2, the detection of cesium can be confirmed from the sample after the reaction test.

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

[0032] As shown in Fig. 5, from the results of the crystal structure analysis (electron beam diffraction pattern) of the sample in Fig. 3, the stable compound formed by the reaction of aluminum silicate and cesium was identified as CsAlSiO ２ O ６ , and Chemical Formula 1 described above in the cesium collection reaction was clarified.

[0033] To confirm the cesium collection function by aluminum silicate in the fuel matrix base material 7 made of silicon carbide, as a sample simulating the state where aluminum silicate was dispersed in the fuel matrix base material 7 made of silicon carbide, silicon carbide and aluminum silicate (Al ２ O ３ ·3SiO ２ [[ID=1)6]]), a mixed powder (molar ratio 10:1) and cesium were put into an alumina reaction vessel, and a reaction test of aluminum silicate and cesium by heating at 670 °C for 1 hour in an argon gas atmosphere was conducted. For the sample obtained in this test, a heat resistance test was conducted at 1600 °C for 1 hour in an argon gas atmosphere. As shown in Fig. 6, since cesium detection was confirmed from the sample after the heat resistance test, it can be seen that the cesium collection method by 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 to form aluminosilicate cesium 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 nuclei 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 composition can be modified as appropriate without departing from the spirit of the invention.

[0036] 1. Fuel core 2. Low-density pyrolysis carbon layer 3. Inner high-density pyrolysis carbon layer 4. Silicon carbide layer 5. Outer high-density pyrolysis carbon layer 6. Coating fuel particles 7. Fuel matrix base material 8. Compact fuel 9. Pebble fuel 10. Unfueled shell

Claims

1. A high-temperature gas reactor fuel having a chemical capture function for radioactive cesium, comprising: a fuel core (1) containing fissile material; a low-density pyrolysis carbon layer (2) covering the fuel core (1); an outer high-density pyrolysis carbon layer (5) covering the low-density pyrolysis carbon layer (2); a coated fuel particle (6) comprising at least the above; 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).

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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