High-temperature gas-cooled reactor fuel with a function for chemically capturing radioactive cesium, and raw powder used therein

Abstract

To provide a high-temperature gas-cooled reactor fuel capable of immobilizing radioactive cesium, which is generated by nuclear fission from fuel kernels in a high-temperature gas-cooled reactor and released from coated fuel particles, within the high-temperature gas-cooled reactor fuel in a temperature range from room temperature to 1600°C, and a raw material powder used therefor. [Solution] The high-temperature gas reactor fuel having a function of chemically capturing radioactive cesium is a high-temperature gas reactor fuel composed of a fuel kernel 1 containing fissile material, a coated fuel particle 6 comprising at least a low-density pyrolytic carbon layer 2 covering the fuel kernel 1, and an outer high-density pyrolytic carbon layer 5 covering the low-density pyrolytic carbon layer 2, and a matrix base material 7 which holds the coated fuel particle 6 and is composed of carbon or silicon carbide, and aluminum silicate is added to the raw material powder and / or the surface of the matrix base material 7.

Description

[Technical field]

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

[0002] Conventionally, many cesium capture technologies have been developed, including a method of contacting an aqueous solution containing cesium ions with aluminum silicate to reduce the amount of cesium ions in the aqueous solution (Patent Document 1), and a method of gelling cesium-containing solid waste together with an aluminum silicate-containing solution (Patent Document 2).However, no capture method has been developed to date that targets cesium in a gaseous state (671°C or higher) in a high-temperature environment.

[0003] Coated fuel particles, which are fuel for high-temperature gas-cooled reactors, are made by covering fuel kernels (such as uranium dioxide or uranium oxycarbide containing fissile material) with a diameter of several hundred μm with a coating layer of carbon and silicon carbide (SiC), and by physically containing fission products (FPs) generated from the fuel kernels during reactor operation, they are designed to suppress radioactive contamination within the plant system. Therefore, when improving the FP containment effect, a method of thickening the coating layer is used in conventional fuels to strengthen the physical containment.

[0004] In a publicly known study by the present applicant (Non-Patent Document 1), a method of chemically confining cesium, which cannot be physically confined by the above-mentioned conventional coating layers, was proposed in which cesium is captured using antimony 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. Summary of the Invention [Problem to be solved by the invention]

[0007] As mentioned above, in conventional fuel, the method of strengthening physical containment is to make the cladding layer thicker, but even if the FPs are physically contained by the cladding layer, some of the cesium, which is the main volatile metal FP, diffuses and migrates outside the fuel through defects inside the cladding layer, and this becomes the main deposited radioactivity in the primary system of a high-temperature gas-cooled reactor plant. Therefore, methods other than physical containment are needed.

[0008] Furthermore, the temperature at which chemical cesium capture using conventional technology works is limited to 1500°C due to the high-temperature stability of cesium-antimony compounds, and there was a restriction that it could not handle 1600°C, the maximum allowable temperature of high-temperature gas-cooled reactor fuel in the event of a possible reactor accident (abnormal transient changes during operation, or the combination of rupture of the double pipes of the primary cooling equipment and loss of reactor shutdown function, etc.). Therefore, there is a growing 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-cooled reactor fuel capable of immobilizing radioactive cesium produced by nuclear fission from fuel kernels in a high-temperature gas-cooled reactor and released from coated fuel particles within the high-temperature gas-cooled reactor fuel at temperatures ranging from room temperature to 1600°C, and a raw material powder to be used therefor. [Means for solving the problem]

[0010] In order to solve the above problems, the high-temperature gas reactor fuel having a function of chemically capturing radioactive cesium of the present invention is a high-temperature gas reactor fuel comprising: a fuel kernel (1) containing fissile material; coated fuel particles (6) comprising at least a low-density pyrolytic carbon layer (2) covering the fuel kernel (1); and an outer high-density pyrolytic carbon layer (5) covering the low-density pyrolytic carbon layer (2); and a matrix base material (7) which holds the coated fuel particles (6) and is made of carbon or silicon carbide, and aluminum silicate is added to the raw material powder and / or the surface of the matrix base material (7). Further, the raw material powder of the present invention is the raw material powder used for the matrix base material (7), and the aluminum silicate powder is added to the carbon or silicon carbide powder. Effect of the Invention

[0011] According to the high-temperature gas-cooled reactor fuel of the present invention having a function of chemically capturing radioactive cesium, radioactive cesium generated by nuclear fission from fuel kernels in a high-temperature gas-cooled reactor and released from coated fuel particles can be immobilized within the high-temperature gas-cooled reactor fuel in the temperature range from room temperature to 1600° C. Furthermore, the raw material powder of the present invention can easily provide a high-temperature gas-cooled reactor fuel having a function of chemically capturing radioactive cesium. [Brief description of the drawings]

[0012] [Figure 1] 1 is an explanatory diagram showing a structure of a high-temperature gas reactor fuel according to an embodiment of the present invention; [Diagram 2] These are the reaction test results (elemental analysis results) of aluminum silicate and cesium in carbon. [Diagram 3]This shows the results of a heat resistance test (elemental analysis results) of the sample in Figure 2, which was made by reacting aluminum silicate and cesium in carbon, at 1600℃ for 1 hour in an argon gas atmosphere. [Figure 4] This shows the results of a heat resistance test (elemental analysis results) performed on the sample in Figure 2, which was made by reacting aluminum silicate and cesium in carbon, at 1600℃ for 1 hour in an air atmosphere. [Diagram 5] 4 shows the crystal structure analysis results (electron diffraction pattern) of the sample in FIG. 3. [Figure 6] These are the results of a heat resistance test (elemental analysis results) conducted at 1600℃ for 1 hour in an argon gas atmosphere on a sample in which aluminum silicate in silicon carbide was reacted with cesium. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] Hereinafter, embodiments of the present invention (hereinafter, abbreviated as examples) will be described with reference to the drawings. In the following drawings, the same reference numerals are used to designate common parts, and duplicated explanations of the parts with the same reference numerals will be omitted.

[0014] (Cesium capture reaction) The high temperature gas-cooled reactor fuel having a function of chemically capturing radioactive cesium according to the present invention is a compact-type fuel 8 or a pebble-type fuel 9, as shown in Fig. 1. They are composed of a fuel kernel 1 containing fissile material, a coated fuel particle 6 comprising at least a low-density pyrolytic carbon layer 2 that coats the fuel kernel 1, and an outer high-density pyrolytic carbon layer 5 that coats the low-density pyrolytic carbon layer 2, and a matrix base material 7 that holds the coated fuel particle 6 and is made of carbon or silicon carbide.

[0015] 1, a coated fuel particle 6 is constructed by quadruple-coating a fuel kernel 1 made of uranium or the like 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 are also coated fuel particles of a type in which the nuclear fuel kernel 1 is doubly coated with a low-density pyrolytic carbon layer 2 and an outer high-density pyrolytic carbon layer 5, and the present invention can be applied to either type, but in this specification, a quadruple-coated type 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 or silicon carbide and assembled into the shape of compact fuel 8 or pebble-shaped fuel 9, which are then loaded into the core as fuel for a high-temperature gas-cooled reactor.

[0017] In the present invention, radioactive cesium produced by nuclear fission from the fuel kernels 1 and released from the coated fuel particles 6 is brought into contact with aluminum silicate that is 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 to form aluminosilicate, thereby capturing the radioactive cesium.

[0018] This will immobilize cesium even at the maximum allowable temperature of 1,600°C, which is assumed to be the result of a reactor accident (such as abnormal transient changes during operation or the combination of rupture of the double pipes of the primary cooling equipment and loss of reactor shutdown function), thereby reducing radioactive contamination by cesium in other areas of the HTGR plant.

[0019] As shown in the following chemical formula 1, the cesium adsorption material, aluminum silicate (xAl 2 O 3 ·ySiO 2 There are several types of chemical formulas for aluminum silicate with different x and y, but in all cases, the cesium (+ the oxygen O suspended in the fuel) 2 ) to form cesium aluminosilicate, CsAlSi 2 O 6 (+By-product Al 2 O 3 ) is formed. 2 O 3 ·ySiO2 is the chemical formula of any aluminum silicate that the operator wishes to use.

[0020] [ka]

[0021] (Required amount of cesium collection material) In order to fully exert the effects of the present invention, the amount of aluminum silicate must be at least that which is sufficient to convert the total amount of cesium produced by nuclear fission in a nuclear reactor into cesium aluminosilicate.

[0022] The amount of aluminum silicate (xAl) required to capture cesium z [mol] 2 O 3 ·ySiO 2 ) is 2 × z / y [mol] based on the molar ratio of aluminum silicate to cesium in Chemical Formula 1 above.

[0023] (Placement of cesium collection material) The matrix base material 7 is made of carbon or silicon carbide as described above, and aluminum silicate powder is added to these powders to prepare raw material powder for the fuel matrix base material 7.

[0024] By dispersing this raw material powder in the fuel matrix base material 7, which has aluminum silicate as a migration path for cesium, the chemical capture function of radioactive cesium can be most effectively achieved.

[0025] The aluminum silicate can be added to the fuel matrix base material 7 by mixing it with the raw material powder as described above, or by coating the surface of the coated fuel particles, the surface of the compact fuel 8 formed by assembling and molding the coated fuel particles, or between the fuel matrix base material 7 and the fuel-free shell 10 of the pebble-type fuel 9. However, when coating, separate technological development is required for the coating method and durability. It is also possible to chemically capture cesium and immobilize it at that position by coating it on other parts that may come into contact with cesium (for example, the surface of the reactor internal structure, the surface inside the reactor pressure vessel, the surface inside the primary system structure, etc.). However, in that case, the range of cesium radioactive contamination will be expanded, and the benefits of improving the safety and economy of the plant will be reduced, and separate technological development is required for the coating method and durability. Therefore, it is preferable to add aluminum silicate by mixing it with the raw material powder of the fuel matrix base material. EXAMPLES

[0026] The present invention will be described in more detail below based on examples, but the present invention is not limited to these examples. (Specific examples of amounts of aluminum silicate added) For example, for one compact fuel with an enrichment level of 20%, if the fission yield of cesium produced by fissioning all of the U235 is 20%, then aluminum silicate Al 2 O 3 3SiO 2 The required amount of added is calculated based on the specific conditions shown in the table below. In this example, we assume a 100% fuel damage rate and show a case where the total amount of cesium generated by nuclear fission is the capture target, but this is not the only case, as the amount of cesium to be captured is determined by the operator. The cesium (2.39 x 10 -3 The amount of aluminum silicate (Al [mol]) required to capture 2 O 3 ·3SiO 2 ) is calculated as 2 × 2.39 × 10 based on the molar ratio of aluminum silicate and cesium in the above formula 1. -3 / 3 or 1.59 × 10-3 [mol] (0.45 [g]). If necessary, the design should be adjusted to account for the increase in volume of the fuel matrix base material 7 due to the addition of aluminum silicate. When actually manufacturing raw powder for the fuel matrix base material 7 to which aluminum silicate has been added, the amount is multiplied by the number of compacts to be produced.

[0027] [Table 1]

[0028] (Specific example of how to add 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-cooled reactor fuel manufacturing process, aluminum silicate is added in advance to the raw material powder of the fuel matrix base material 7 that coats each of the coated fuel particles 6 before the coated fuel particles 6 are assembled.

[0029] When aluminum silicate is dispersed and added to the graphite fuel matrix base material 7, it is preferable from the viewpoint of ensuring homogeneity 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 of the fuel matrix base material 7. Also, the amount of aluminum silicate added is adjusted so that the aluminum silicate is incorporated into the raw material powder without excess or deficiency.

[0030] (Cesium collection function of aluminum silicate) A reaction test between aluminum silicate and cesium was carried out. Specifically, a sample of carbon and aluminum silicate (Al 2 O 3 ·3SiO 2 A mixed powder of aluminum silicate and cesium (molar ratio 10:1) was placed in an alumina reaction vessel, and a reaction test of aluminum silicate and cesium was carried out by heating at 670℃ for 1 hour in an argon gas atmosphere. The elemental analysis results in Figure 2 show that cesium was detected in the sample after the reaction test.

[0031] A heat resistance test was carried out for the sample shown in Figure 2, which was made by reacting aluminum silicate in carbon with cesium, at 1600℃ for 1 hour, which is the allowable value for the fuel temperature of a high-temperature gas-cooled reactor. Here, the heat resistance test atmosphere was inert gas (argon gas) to simulate an abnormal transient change environment during operation, and the heat resistance test atmosphere was air to simulate a severe accident environment caused by the rupture of the double pipe of the primary cooling equipment and the loss of the reactor shutdown function. As shown in Figures 3 and 4, cesium was detected in the sample after the heat resistance test, and it was confirmed that the cesium collection function was maintained even under the fuel temperature environment during an accident in a high-temperature gas-cooled reactor.

[0032] As shown in Figure 5, the crystal structure analysis (electron diffraction pattern) of the sample in Figure 3 shows that the stable compound formed by the reaction of aluminum silicate and cesium is CsAlSi 2 O 6 and clarified the previously mentioned chemical formula 1 for the cesium capture reaction.

[0033] In order to confirm the cesium capture function of aluminum silicate in the silicon carbide fuel matrix base material 7, a sample was prepared by dispersing aluminum silicate in the silicon carbide fuel matrix base material 7. 2 O 3 ·3SiO 2 A mixed powder of cesium and aluminum silicate (molar ratio 10:1) was placed in an alumina reaction vessel and heated in an argon gas atmosphere at 670°C for 1 hour to conduct a reaction test between aluminum silicate and cesium. A heat resistance test was conducted on the sample obtained in this 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, which shows that the cesium collection method using aluminum silicate is effective even when the fuel matrix base material 7 is made of silicon carbide.

[0034] From the above experimental results, it was confirmed that the formation of cesium aluminosilicate by contacting radioactive cesium with aluminum silicate in the fuel matrix base material has an effect of chemically capturing radioactive cesium. Therefore, it is clear that the radioactive cesium generated by nuclear fission from the fuel kernel of the HTGR and released from the coated fuel particles can be immobilized in the HTGR fuel in the temperature range from room temperature to 1600°C by the HTGR fuel having the function of chemically capturing radioactive cesium of the present invention.

[0035] The above-mentioned high-temperature gas reactor fuel having a function of chemically capturing radioactive cesium and the raw material powder used therein are merely examples, and the configurations thereof can be modified as appropriate without departing from the spirit of the invention. [Explanation of symbols]

[0036] 1 fuel nucleus 2. Low density pyrolytic carbon layer 3. Inner high density pyrolytic 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 Fuel-free shells

Claims

1. A fuel kernel (1) containing fissile material, a low-density pyrolytic carbon layer (2) covering the fuel kernel (1); an outer high density pyrolytic carbon layer (5) covering the low density pyrolytic carbon layer (2); A coated fuel particle (6) comprising at least a matrix base material (7) that holds the coated fuel particles (6) and is made of carbon or silicon carbide; A high-temperature gas-cooled reactor fuel consisting of: A high-temperature gas-cooled reactor fuel having a function of chemically capturing radioactive cesium, characterized in that aluminum silicate is added to the raw material powder and / or surface of the matrix base material (7).

2. 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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