Nuclear fuel assemblies for high-temperature gas reactors

Abstract

In high-temperature gas reactor fuel, this enables the core's power density while maintaining the core's criticality conditions. [Solution] According to the embodiment, the nuclear fuel assembly 100 for a high-temperature gas reactor comprises a plurality of fuel sections 110 having a columnar matrix extending in the axial direction and a plurality of coated fuel particles containing nuclear fuel material dispersed within the matrix, and a coolant flow path 12 formed to extend in the axial direction between each adjacent fuel section 110 among the plurality of fuel sections 110, for passing a coolant. The flow path volume ratio, which is the ratio of the sum of the volumes of each of the plurality of coolant flow paths 120 to the volume of the nuclear fuel assembly 100 for a high-temperature gas reactor, is 10% or more and 50% or less, and the fuel particle filling rate, which is the ratio of the volume of coated fuel particles to the sum of the volume of the matrix and the volume of the coated fuel particles, is above a predetermined boundary value and 60% or less.

Description

[Technical Field]

[0001] Embodiments of the present invention relate to nuclear fuel assemblies for high-temperature gas reactors. [Background technology]

[0002] Figure 11 is a cross-sectional view showing an example of the configuration of a conventional fuel assembly 1 for a high-temperature gas reactor.

[0003] The fuel assembly 1 comprises a hexagonal prism-shaped graphite matrix 3, a plurality of fuel compacts 2, and a plurality of flammable toxic elements 4. The plurality of fuel compacts 2 and the plurality of flammable toxic elements 4 are arranged within the graphite matrix 3 in a direction parallel to the axial direction of the graphite matrix 3.

[0004] Each fuel compact 2, in the cross-section shown in Figure 11, has an annular fuel section 2a and a structural member 2b positioned at the center of the fuel section 2a. Between the radially outer side of the fuel compact 2, i.e., the outside of the fuel section 2a, and the graphite base material 3, an annular coolant channel 2c is formed, which serves as a flow path for a coolant such as helium.

[0005] In a fuel assembly 1 with this configuration, heat generation is concentrated in each fuel compact 2, and heat removal from the fuel compact 2 is limited to heat transfer from its radially outer surface. Therefore, the power density of the fuel assembly 1 is 5 W / cm². 3 or 6 W / cm 3 It was to that extent.

[0006] Such power densities are about an order of magnitude lower than those of other reactor types of the same power scale, such as light water-cooled reactors. This low power density in high-temperature gas reactors leads to increased core volume and higher costs, worsening the economic viability of the reactor system. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] U.S. Patent Application Publication No. 2022 / 0301732 [Patent Document 2] U.S. Patent Application Publication No. 2023 / 0282373 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] The power density of the reactor core is limited by the fuel temperature. Therefore, in order to increase the power density of the reactor core, there is a challenge in that the cooling performance of the fuel must be improved. To solve this problem, attempts have been made to create flow channels within the fuel using additive manufacturing technology and to fabricate complex shapes.

[0009] On the other hand, while studies have been conducted on fuel production methods and specific flow path shapes, the systematic conditions necessary to achieve core criticality have not yet been clearly defined.

[0010] The problem to be solved by the present invention is to provide a nuclear fuel assembly for a high-temperature gas reactor that can increase the power density of the reactor core while maintaining critical conditions for the reactor core. [Means for solving the problem]

[0011] To achieve the above-mentioned objectives, the nuclear fuel assembly for a high-temperature gas reactor according to this embodiment comprises a plurality of fuel sections, each having a columnar matrix extending in the axial direction and a plurality of coated fuel particles dispersed in the matrix and containing nuclear fuel material, and a coolant flow path formed between each of the plurality of fuel sections adjacent to each other so as to extend in the axial direction and for passing a coolant, wherein the flow path volume ratio, which is the ratio of the sum of the volumes of each of the plurality of coolant flow paths to the volume of the nuclear fuel assembly for a high-temperature gas reactor, is 10% or more and 50% or less, and the fuel particle filling rate, which is the ratio of the volume of the coated fuel particles to the sum of the volume of the matrix and the volume of the coated fuel particles, is greater than or equal to a predetermined boundary value and 60% or less. [Brief explanation of the drawing]

[0012] [Figure 1] It is a cross-sectional view showing the configuration of a nuclear fuel body for a high-temperature gas reactor according to the first embodiment. [Figure 2] It is a longitudinal sectional view showing the configuration of a nuclear fuel body for a high-temperature gas reactor according to the first embodiment. [Figure 3] It is a perspective view conceptually showing the configuration of a fuel part of a nuclear fuel body for a high-temperature gas reactor according to the first embodiment. [Figure 4] It is a perspective view showing the configuration of coated fuel particles in a fuel part of a nuclear fuel body for a high-temperature gas reactor according to the first embodiment. [Figure 5] It is a partial longitudinal sectional view explaining the modeling of a nuclear fuel body for a high-temperature gas reactor according to the first embodiment. [Figure 6] It is a graph showing the dependence characteristics of the effective multiplication factor of a reactor core using a nuclear fuel body for a high-temperature gas reactor according to the first embodiment on the number of burn days. [Figure 7] It is a table showing the dependence characteristics of the relative value of the fuel loading amount of a nuclear fuel body for a high-temperature gas reactor according to the first embodiment on the coated particle fuel filling rate and the coolant channel volume ratio. [Figure 8] It is a graph showing the dependence characteristics of the boundary value of the coated particle fuel filling rate on the coolant channel volume ratio when the relative value of the fuel loading amount of a nuclear fuel body for a high-temperature gas reactor according to the first embodiment is 0.3000. [Figure 9] It is a table showing the dependence characteristics of the infinite multiplication factor of a nuclear fuel body for a high-temperature gas reactor according to the first embodiment on the coated particle fuel filling rate and the coolant channel volume ratio. [Figure 10] It is a cross-sectional view showing the configuration of a nuclear fuel body for a high-temperature gas reactor according to the second embodiment. [Figure 11] It is a cross-sectional view showing a configuration example of a conventional fuel body for a high-temperature gas reactor.

Mode for Carrying Out the Invention

[0013] Hereinafter, a nuclear fuel body for a high-temperature gas reactor according to an embodiment of the present invention will be described with reference to the drawings. Here, the same or similar parts are denoted by common reference numerals, and overlapping explanations are omitted.

[0014] [First Embodiment] Figure 1 is a cross-sectional view showing the configuration of the nuclear fuel assembly 100 for a high-temperature gas reactor according to the first embodiment. Figure 2 is a longitudinal cross-sectional view showing the configuration of the nuclear fuel assembly 100 for a high-temperature gas reactor according to the first embodiment. Figure 1 is a cross-sectional view taken along the line YY in Figure 2, and Figure 2 is a cross-sectional view taken along the line XX in Figure 1. Part A in Figure 2 will be explained later with reference to Figure 6. <Structure>

[0015] Each high-temperature gas reactor fuel assembly 100 has a hexagonal cross-section and a columnar shape extending in the longitudinal direction. The reactor core 50 (not shown) has a configuration in which multiple high-temperature gas reactor fuel assemblies 100 are arranged in parallel with each other. Each high-temperature gas reactor fuel assembly 100 has multiple fuel sections 110, multiple coolant channels 120 that serve as channels for a coolant such as helium, and two end plates 130.

[0016] Here, the direction in which the central axis of the high-temperature gas reactor nuclear fuel assembly 100 extends is called the axial direction, the direction extending radially outward from the central axis in a cross-section perpendicular to the axial direction is called the radial direction, and the direction perpendicular to the axial and radial directions, or the tangential direction that circles around the central axis in a cross-section, is called the circumferential direction. Note that in Figure 2, up and down are shown for convenience, but this does not mean that the direction is limited to up and down.

[0017] Multiple fuel sections 110 are arranged in layers concentrically, with radial spacing between them, within the high-temperature gas reactor nuclear fuel assembly 100. In a cross-section perpendicular to the axial direction, the radial outer surface and radial inner surface of each fuel section 110 are regular hexagons with the same center.

[0018] Each of the multiple coolant flow channels 120 is formed by fuel sections 110 that are radially adjacent to each other. That is, each coolant flow channel 120 has a width in the radial direction, and the radially inner wall and radially outer wall are formed with a regular hexagonal cross-section.

[0019] As shown in Figure 2, the two end plates 130 are provided on both the axial outer sides of the multiple fuel sections 110. Each end plate 130 is a regular hexagonal flat plate. Each end plate 130 has multiple flow path holes 131 formed at circumferential intervals at positions corresponding to the coolant flow path 120. Unlike the coolant flow path 120, the flow path holes 131 are not formed continuously along the circumferential direction. That is, the flow path holes 131 have portions that connect the radially inner and outer sides at multiple locations in the circumferential direction. The end plates 130 formed in this way function as inlet and outlet portions of the coolant flow path 120 by having flow path holes 131, and also have the function of supporting and fixing the multiple fuel sections 110 and maintaining the relative positional relationship of the multiple fuel sections 110 within the high-temperature gas reactor nuclear fuel assembly 100.

[0020] Figure 3 is a conceptual perspective view showing the configuration of the fuel section 110 of the nuclear fuel assembly 100 for a high-temperature gas reactor according to the first embodiment.

[0021] As shown in Figure 3, each fuel section 110 has a matrix 111 and a plurality of coated fuel particles 112 dispersed within the matrix 111. The matrix 111 forms the external shape of the fuel section 110.

[0022] Figure 4 is a perspective view showing the configuration of the coated fuel particles 112 in the fuel section 110 of the nuclear fuel assembly 100 for a high-temperature gas reactor according to the first embodiment.

[0023] The coated fuel particles 112 are, for example, spherical in shape with a particle size of about 1 mm. The coated fuel particles 112 are so-called TRISO fuel and consist of a spherical fuel core 112a and a coating portion 112s made of graphite and silicon carbide ceramic arranged on the outside thereof. The coating portion 112s is formed in layers radially outward from the side closest to the fuel core 112a and has four layers, for example, a low-density pyrolysis carbon layer 112b, an inner high-density silicon carbide layer 112c, a silicon carbide layer 112d, and an outer high-density silicon carbide layer 112e. Other configurations are also acceptable as long as they have the capacity to contain and retain fission product gases.

[0024] Fuel core 112a is a nuclear fuel material. Specifically, it is, for example, an oxide of enriched uranium. It is also possible that fuel core 112a exists in which natural uranium contains flammable toxins.

[0025] The high-temperature gas reactor nuclear fuel assembly 100 described above can be manufactured by assembling each fuel section 110 and the end plate 130. Alternatively, the high-temperature gas reactor nuclear fuel assembly 100 may be manufactured by additive manufacturing.

[0026] Figure 5 is a partial longitudinal cross-sectional view illustrating the modeling of the nuclear fuel assembly 100 for a high-temperature gas reactor according to the first embodiment. Figure 5 shows the model element 20 in section A in Figure 2.

[0027] Now, let's define the line extending on the longitudinal section of the radially central surface of the fuel section 110 as the centerline La, and the line extending on the longitudinal section of the radially central surface of the coolant flow path 120 as the centerline Lb. Since radially adjacent fuel sections 110 face each other via the coolant flow path 120, the centerline La and the centerline Lb can be considered as substantially adiabatic boundaries.

[0028] As shown in the cross-sectional view in Figure 5, the model element 20 has a radially inner surface 20a, a radially outer surface 20b, an upstream surface 20c for the coolant, and a downstream surface 20b for the coolant. The center line La passes through surface 20a, and the center line Lb passes through surface 20b.

[0029] The distance between surfaces 20a and 20b is represented by the radial width Δr. The distance between surfaces 20c and 20d is represented by the axial width Δz. The width in the direction perpendicular to Figure 5 (the depth direction in Figure 5) is represented by the horizontal width Δx.

[0030] Here, in model element 20, the axial width Δz and the lateral width Δx are set to the same dimensions as the radial width Δr. In other words, model element 20 is a cube.

[0031] In the following calculations, we use the example where the width of the fuel section 110 is 16 mm and the width of the coolant flow path 120 is 4 mm. In this case, the side length of the cubic model element 20 is (16 + 4) / 2, or 10 mm. In this case, the coolant flow path volume ratio CP, which will be described later, is 2 / 10, or 0.2 (20%).

[0032] The analysis was performed using a model with this model element 20. The conditions during the analysis were: surface temperature of fuel core 112a of 1200K, thermal conductivity of fuel section 110 of 50W / m / K, temperature of helium gas as coolant flowing through coolant channel 120 of 900K, and helium gas flow velocity of 8m / sec. The results of the analysis showed approximately 28W / cm 3 The results showed that even with this heat generation density, steady heat removal from the fuel section 110 is possible. In this case, the temperature difference within the matrix 111 of the fuel section 110 is also less than 50K.

[0033] Figure 6 shows the effective multiplier k of the reactor core 50 using the nuclear fuel assembly 100 for a high-temperature gas reactor according to the first embodiment. eff This graph shows the dependence of the fuel on burn-up days D. The horizontal axis represents the burn-up days D (days) of the high-temperature gas reactor fuel assemblies 100 loaded into the core. The vertical axis represents the effective multiplication factor k. eff That is the case.

[0034] The curves in the figure represent the following: the enrichment level of nuclear fuel in fuel core 112a is 19 wt%, the coolant flow channel volume ratio (CP) is 20%, and the power density is 30 W / cm³. 3 The figures show the burnup calculation results for different cases of cladding particle fuel packing ratio PF, under the condition that the core height HT is 400 mm or 300 mm. Note that only curve l05b in the figure has a core height HT of 300 mm; in the other cases, the core height HT is 400 mm.

[0035] Here, the coolant flow channel volume ratio CP is the ratio of the volume of the coolant flow channel 120 to the total volume of the portion of the high-temperature gas reactor nuclear fuel assembly 100 excluding the two end plates 130, i.e., the sum of the volume of the fuel section 110 and the volume of the coolant flow channel 120. Also, the coated particle fuel filling ratio PF is the ratio of the volume of coated fuel particles 112 to the total volume of the fuel section 110, i.e., the sum of the matrix 111 and coated fuel particles 112.

[0036] Currently, the effective multiplier for core 50 is k eff The boundary where the value becomes 1.0 is shown by the straight line L1, and the boundary where the number of burning days becomes one year (365 days) is shown by the straight line L2.

[0037] Here, the criticality condition for the reactor core is met when the burn-through period reaches one year (365 days), and the effective multiplication factor k is defined as the criticality condition for the reactor core. eff The condition is that is 1.0 or greater. This criticality condition means that among the curves shown in Figure 6, the curve exists in the criticality region that is above the line L1 and to the left of the line L2.

[0038] In Figure 6, there are three curves that satisfy the criticality conditions: the solid lines L04a, L05a, and L05b. Specifically, curve L04a corresponds to "19wt_HT400_PF0.400" with a core height of 400 mm and a clad particle fuel packing ratio PF of 0.4. Curve L05a corresponds to "19wt_HT400_PF0.500" with a core height of 400 mm and a clad particle fuel packing ratio PF of 0.5. Curve L05b corresponds to "19wt_PF0.500" with a core height of 300 mm and a clad particle fuel packing ratio PF of 0.5.

[0039] As these three cases demonstrate, the coated particle fuel packing ratio PF in cases where the criticality conditions are met is 0.4 and 0.5.

[0040] Incidentally, the result that the coating particle fuel filling ratio PF in the case satisfying the above criticality establishment condition is 0.4 or more varies depending on parameters such as the core height and the coolant flow path volume ratio CP. As a result of separate evaluation, by expanding the range of parameters, the lower limit of the coating particle fuel filling ratio PF has been obtained as 0.3.

[0041] Therefore, in order to satisfy the criticality establishment condition that the effective multiplication factor k eff becomes 1.0 or more at the time when the burn-up days are 1 year (365 days), a certain amount of fissile nuclides is required. Specifically, in the case of the above analysis conditions, the coating particle fuel filling ratio PF needs to be 0.3 or more.

[0042] In the case where the coating particle fuel filling ratio PF is small, since the moderator ratio is large, the transition ratio to the thermal neutron region is large, and the ratio of resonance absorption in the intermediate energy region decreases. Therefore, the initial effective multiplication factor is high. As a result of the high initial effective multiplication factor, the fuel loading amount can be small. As a result, the amount of fissile nuclides decreases, so the effective multiplication factor k eff falls below 1.0 at an early stage. From this, it can be seen that the coating particle fuel filling ratio PF has a lower limit as described above.

[0043] FIG. 7 is Table 141 showing the dependence characteristics of the fuel loading amount relative value FR of the nuclear fuel body 100 for a high-temperature gas reactor according to the first embodiment on the coating particle fuel filling ratio PF and the coolant flow path volume ratio CP.

[0044] As described above, the coating particle fuel filling ratio PF has a value from 0.05 to 0.6, and the coolant flow path volume ratio CP has a value from 0.1 to 0.8. For each combination of the value of the coating particle fuel filling ratio PF and the value of the coolant flow path volume ratio CP, the value of the fuel loading amount relative value FR is shown. The value of this fuel loading amount relative value FR varies in the range from 0.0100 to 0.5400.

[0045] In Table 141 of Figure 7, cells with values ​​between 0.01 and 0.10 display both large and small diagonal filled circles. No circles are displayed for intermediate values. Cells with values ​​between 0.20 and 0.54 display both small and large open white circles. In other words, the values ​​increase sequentially in the order of large diagonal filled circle, small diagonal filled circle, no circle, small open white circle, and large open white circle.

[0046] As mentioned above, referring to Figure 6, the effective multiplication factor k is reached when the number of burning days becomes one year (365 days). eff In order to satisfy the criticality condition, which is that the ratio is 1.0 or higher, the cladding particle fuel packing ratio PF must be 0.3 or higher. Here, one year is an example of the length of operation in an operating cycle. After one year of operation, the high-temperature gas reactor fuel assemblies 100 that have completed their final cycle are replaced with new high-temperature gas reactor fuel assemblies 100 in the core 50 at a rate corresponding to a predetermined number of fuel batches. By replacing them with new high-temperature gas reactor fuel assemblies 100, criticality conditions are ensured for the next operating period. Therefore, it is sufficient for criticality conditions to be ensured for one operating period of a given length.

[0047] As shown in Table 141 of Figure 7, when the coolant flow channel volume ratio CP is 20% and the cladding particle fuel packing rate PF is 0.3, the relative fuel load value FR is 0.2400. The relative fuel load value FR is the value obtained by multiplying the cladding particle fuel packing rate PF by (1 - coolant flow channel volume ratio CP). Therefore, when the value of the coolant flow channel volume ratio CP is zero, the relative fuel load value FR is at its maximum, and the value of the coolant flow channel volume ratio CP matches the value of the cladding particle fuel packing rate PF. In this case, a margin of safety is ensured for criticality conditions. Based on the above, the lower limit of the relative fuel load value FR is set to 0.3000 as a condition for maintaining the critical state of core 50 throughout the operating cycle.

[0048] As shown in Table 141 of Figure 7, when the coated particle fuel filling rate PF is 0.3 or higher, in order to obtain a relative fuel load value FR of 0.3000, the coolant flow path volume ratio CP must be 50% or less. Thus, in order to satisfy the criticality conditions, the coated particle fuel filling rate PF must be 0.3 or higher and the coolant flow path volume ratio CP must be 50% or less.

[0049] Figure 8 is a graph showing the dependence of the boundary value PFBV of the coated particle fuel filling rate PF on the coolant flow channel volume ratio CP when the relative fuel load value FR of the high-temperature gas reactor nuclear fuel assembly 100 according to the first embodiment is 0.3000. The horizontal axis represents the coolant flow channel volume ratio CP, and the vertical axis represents the coated particle fuel filling rate PF.

[0050] As mentioned above, a relative fuel load value (FR) of 0.3000 is a condition required for the core 50 to maintain criticality throughout the operating cycle.

[0051] Here, we will explain the "boundary value PFBV of the coated particle fuel filling rate PF when the relative fuel load value FR is 0.3000," referring to Table 141 shown in Figure 7.

[0052] In Table 141, when the coolant flow path volume ratio CP is, for example, 0.2 (20%), the relative fuel load value FR is 0.2400 when the coated particle fuel filling rate PF is 0.3, and the relative fuel load value FR is 0.3200 when the coated particle fuel filling rate PF is 0.4. Linear interpolation between these two cases yields the value of the coated particle fuel filling rate PF that gives the relative fuel load value FR 0.3000, i.e., the boundary value PFBV. In this case, the boundary value PFBV is 0.375. In this way, the dependency characteristic of the boundary value PFBV of the coated particle fuel filling rate PF on the coolant flow path volume ratio CP is obtained.

[0053] The curve PFB shown in Figure 8 represents the dependency characteristic of the boundary value PFBV of the coated particle fuel filling rate PF obtained in this manner on the coolant flow channel volume ratio CP, and is expressed by the following equation (1). PFBV = 1.1019 CP 3 -0.0139CP 2 +0.3316CP+0.2997 ...(1)

[0054] The region on and above the curve PFB, i.e., the region where the coated particle fuel filling rate PF is greater than or equal to the boundary value PFBV, is the region where the relative fuel load value FR is 0.3000 or greater.

[0055] Region F shown in Figure 8 will be explained later.

[0056] Figure 9 shows the infinite multiplication factor k of the nuclear fuel assembly 100 for a high-temperature gas reactor according to the first embodiment. inf Table 142 shows the dependency characteristics on the coated particle fuel filling rate PF and the coolant flow path volume ratio CP.

[0057] The coated particle fuel filling rate PF is between 0.05 and 0.6, and the coolant flow path volume ratio CP is between 0.1 and 0.8. For each combination of the coated particle fuel filling rate PF and the coolant flow path volume ratio CP, the infinite multiplication factor k is set. inf The value obtained for these combinations is the infinite multiplication factor k. inf The value of varies within the range of 9.46E-01 to 1.42E-00. Similar to Figure 7, Figure 9 also displays circles indicating relative magnitudes instead of specific values.

[0058] In Figure 9, cells with values ​​from 9.46E-01 to 9.94E-01 are displayed with diagonal-filled circles of a large diameter, followed by diagonal-filled circles of a small diameter. Intermediate values ​​are not displayed with a circle. Cells with values ​​from 1.00E-00 to 1.42E-00 are displayed with white circles of a small diameter, followed by white circles of a large diameter. In other words, the values ​​increase sequentially in the order of diagonal-filled circle of a large diameter, diagonal-filled circle of a small diameter, no circle, white circle of a small diameter, and white circle of a large diameter.

[0059] In Table 142 of Figure 9, referring to Table 141 of Figure 7 mentioned above, if we examine the range where the coated particle fuel filling rate PF is 0.3 or higher and the coolant flow path volume ratio CP is 50% or lower, we can conclude the following.

[0060] In the case of the successful scenario shown in Figure 6, where the critical conditions are a coated particle fuel filling rate PF of 0.4 and a coolant flow path volume ratio CP of 20%, the infinite multiplication factor k is as shown in Table 142 of Figure 9. inf This is 9.58E-01.

[0061] As shown in Table 142 of Figure 9, in the range where the coated particle fuel filling rate PF is 0.3 or higher and the coolant flow path volume ratio CP is 50% or lower, the infinite multiplication factor k inf The value falls below 9.58E-01 when the coated particle fuel filling ratio PF is 0.4 and the coolant flow path volume ratio CP is 20%. In this case, the infinite multiplication factor k inf The value is 9.55E-01, slightly lower than 9.58E-01.

[0062] In the reactor core, the high-temperature gas reactor fuel assemblies 100 that make up the core are divided into multiple groups, and the timing of loading them into the core is staggered. Therefore, the infinite multiplication factor k inf This level of difference poses no practical problem.

[0063] Therefore, as a criticality condition, Table 141 in Figure 7 shows that the range in which the coated particle fuel filling ratio PF is 0.3 or higher and the coolant flow channel volume ratio CP is 50% or lower is also acceptable as confirmed in Table 142 in Figure 9. <Conditions for success>

[0064] To summarize the results shown above, we can conclude the following: In order for the reactor core 50 to be established, the cladding particle fuel packing ratio PF and the coolant flow channel volume ratio CP must satisfy all of the following conditions (1) to (3). Here, the establishment of the reactor core 50 means that the criticality conditions of the reactor core 50 are ensured throughout the operating cycle period of the high-temperature gas reactor, and that the operation of the high-temperature gas reactor can be continued.

[0065] (1) The coolant flow channel volume ratio CP must be 10% or more and 50% or less. If the coolant flow channel volume ratio CP is less than 10%, cooling capacity cannot be secured. If the coolant flow channel volume ratio CP exceeds 50%, the fuel inventory will decrease, and the amount of fissile nuclides will decrease, making it impossible to meet criticality conditions early on.

[0066] (2) The cladding particle fuel packing ratio PF is greater than or equal to a predetermined boundary value PFBV. Here, PDBV is the value obtained by equation (1) above. If it is less than the boundary value PFBV, the amount of fissile nuclides decreases, and the criticality condition will not be met early on.

[0067] (3) The coated particle fuel packing ratio PF shall be 60% or less. When single-particle coated fuel particles 112 are dispersed in the matrix 111, the upper limit of the packing ratio is, for example, 66%. For this reason, the upper limit of a reproducible and stable packing ratio is set at 60%.

[0068] The region F shown in Figure 8 is the area that satisfies the conditions (1), (2), and (3) described above. Note that region F includes each of its boundaries.

[0069] As described above, this embodiment clarifies the conditions for the cladding particle fuel packing ratio PF and the coolant flow channel volume ratio CP required for the reactor core 50 to be established.

[0070] [Second Embodiment] Figure 10 is a cross-sectional view showing the configuration of a nuclear fuel assembly 100a for a high-temperature gas reactor according to the second embodiment.

[0071] This embodiment is a modification of the first embodiment. In this embodiment, instead of the fuel section 110 of the high-temperature gas reactor nuclear fuel assembly 100 in the first embodiment, the high-temperature gas reactor nuclear fuel assembly 100a has multiple regions, specifically a first fuel section 110a and a second fuel section 110b. The number of multiple regions may be three or more.

[0072] The first fuel section 110a is the radially inner region, and the second fuel section 110b is the radially outer region. The first fuel section 110a and the second fuel section 110b differ from each other in at least one of the following: nuclear fuel composition, including the amount of TRU added, cladding fuel filling rate, and flow path volume ratio.

[0073] Furthermore, there may be differences in the presence or absence of elements such as boron, gadolinium, erbium, and europium added as flammable poisons in each region.

[0074] Furthermore, if loading a large amount of transuranic elements leads to the problem of being unable to expect a significant Doppler coefficient, a section using only uranium as the nuclear fuel material may be set up, and the temperature in that section may be made to rise more easily to facilitate the effect of Doppler reactivity.

[0075] This embodiment, configured in this way, makes it possible to adjust the output distribution and neutron spectrum.

[0076] According to the embodiments described above, it is possible to provide a gas-cooled reactor and a method for controlling the excess reactivity of a gas-cooled reactor that can suppress excess reactivity in the reactor core while maintaining a flat power distribution within the core.

[0077] [Other embodiments] Although embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. Furthermore, the features of each embodiment may be combined. Moreover, the embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. Embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents. [Explanation of Symbols]

[0078] 1…Fuel assembly, 2…Fuel compact, 2a…Fuel section, 2b…Structural material, 2c…Coolant channel, 3…Graphite matrix, 4…Combustible toxic element, 20…Model element, 50…Core, 100, 100a…High-temperature gas-cooled reactor nuclear fuel assembly, 110…Fuel section, 110a…First fuel section, 110b…Second fuel section, 111…Matrix, 112…Covered fuel particles, 112a…Fuel core, 112b…Low-density pyrolysis carbon layer, 112c…Inner high-density silicon carbide layer, 112d…Silicon carbide layer, 112e…Outer high-density silicon carbide layer, 112s…Covering section, 120…Coolant channel, 130…End plate, 131…Channel hole, 201…First compartment, 202…Second compartment

Claims

1. A plurality of fuel sections having a columnar matrix extending in the axial direction and a plurality of coated fuel particles containing nuclear fuel material dispersed within the matrix, A coolant flow path is formed between adjacent fuel sections of each of the multiple fuel sections, extending in the axial direction, and through which the coolant passes; A nuclear fuel assembly for a high-temperature gas reactor, comprising: The flow path volume ratio, which is the ratio of the sum of the volumes of each of the multiple coolant flow paths to the volume of the nuclear fuel assembly for the high-temperature gas reactor, is 10% or more and 50% or less. The fuel particle filling rate, which is the ratio of the volume of the coated fuel particles to the sum of the volume of the matrix and the volume of the coated fuel particles, is greater than or equal to a predetermined boundary value and less than or equal to 60%. A nuclear fuel assembly for high-temperature gas reactors characterized by the following features.

2. The nuclear fuel assembly for a high-temperature gas reactor according to claim 1, characterized in that when the flow path volume ratio is expressed by CP and the predetermined boundary value by PFBV, the predetermined boundary value PFBV is a value obtained by the following formula (1). PFBV =1.1019CP ３ -0.0139CP ２ +0.3316CP+0.2997 ・・・（１）

3. The nuclear fuel assembly for a high-temperature gas reactor according to claim 1, characterized in that, in a cross-section perpendicular to the axial direction, the radial outer surface and radial inner surface of each of the plurality of fuel portions are regular hexagons with the same center.

4. The nuclear fuel material for a high-temperature gas reactor according to claim 1 is characterized in that the nuclear fuel material contains transuranic elements in addition to uranium.

5. The nuclear fuel assembly for a high-temperature gas reactor according to claim 4, characterized in that some or all of the plurality of coated fuel particles contain uranium and transuranic elements in the nuclear fuel material.

6. The nuclear fuel assembly for a high-temperature gas reactor according to claim 4, characterized in that, in regions where it is necessary to ensure a margin of safety in the limiting conditions for fuel temperature, coated particle fuel using only uranium as the nuclear fuel material is arranged.

7. The nuclear fuel assembly for a high-temperature gas reactor according to claim 5, characterized in that the flow path is arranged such that the temperature in the region using only uranium as the nuclear fuel material tends to rise.

8. The nuclear fuel assembly for a high-temperature gas reactor according to any one of claims 1 to 7, characterized in that the fuel particle filling rate is spatially distributed in the radial direction in a plurality of fuel sections.

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