JP2017032408A - Transuranium element conversion fuel assembly, transuranium element conversion reactor core and method for designing transuranium element conversion fuel assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently enable the nuclear transformation of a TRU in a light water reactor.SOLUTION: A transuranium element conversion fuel assembly 1 includes a plurality of fuel rods 10, at least one water rod 3 and a channel box 2. Each fuel rod 10a comprises: a fuel pellet including a fissile material having a concentration satisfying the condition that an infinite neutron multiplication factor in the transuranium element conversion fuel assembly when a predetermined burnup is obtained is equal to or more than a predetermined value, and laminated in the longitudinal direction including a transuranium element; and a cladding tube including the fuel pellet. Each fuel rod 10a has a thermal neutron rate increase structure for suppressing the reduction of a neutron level in the inside of the fuel pellet by self shielding.SELECTED DRAWING: Figure 2

Description

  Embodiments described herein relate generally to a transuranium element conversion fuel assembly, a transuranium element conversion core, and a transuranium element conversion fuel assembly design method.

  In the case of a once-through cycle, the radiotoxicity of high-level radioactive waste generated with the use of nuclear power has been at a higher level than the natural world for over 100,000 years. After a lapse of about 500 years, the toxicity caused by the transuranium element (TRU) becomes dominant among these toxicities. The long-term toxicity of TRU nuclides is due to its long half-life. Therefore, in order to shorten the management period of radioactive waste and reduce the environmental load, it is effective to convert TRU nuclides to short-lived nuclides by nuclear transmutation.

  For transmutation of TRU nuclides, generally, a technique using high-energy fast neutrons, a technique using fuel not containing uranium, or a technique combining these is used.

  However, light water reactors are still considered to be the main reactor type used in future nuclear power plants. For this reason, a considerable amount of TRU accumulates by the time the TRU transmutation by the fast reactor is started, and the number of necessary fast reactors may increase to a number that cannot be practically introduced. Therefore, it is necessary to improve the transmutation efficiency of TRU in the light water reactor and to suppress the net TRU production amount in the light water reactor.

However, when transmutation is attempted in a light water reactor or the like, the transmutation efficiency of radionuclides such as plutonium 238 ( ²³⁸ Pu), plutonium 241 ( ²⁴¹ Pu), or americium 241 ( ²⁴¹ Am) has a large contribution to radiotoxicity. Since it is low, it is an issue to efficiently transmutate them. Further, when the shape of the fuel assembly is changed for the purpose of improving the transmutation efficiency, the reactivity of the fuel at the end of the core cycle changes. If the current core is loaded, it is required to maintain the same reactivity as standard fuel at the end of the core cycle. At this time, there is a problem that there is no method for determining the fuel specification.

US Patent Application Publication No. 2002/0025016 JP 2006-64678 A Japanese Patent No. 5524582 Japanese Patent No. 554581 Japanese Patent No. 5524573

  In the TRU transmutation method and design method for the above-mentioned combustion fuel, when using thermal neutrons or epithermal neutrons such as general light water reactors and heavy water reactors, transmutation of several nuclides that have a large contribution to radiotoxicity Low efficiency. For this reason, the radiotoxicity of TRU nuclides over a long period of time is less likely to decrease compared to the fast reactor.

  In addition, there is no fuel specification determination method for maintaining the same degree of reactivity as the standard fuel at the end of the core cycle when the fuel shape is changed.

  Embodiments of the present invention have been made to solve the above-described problems, and an object of the present invention is to enable TRU transmutation efficiently in a light water reactor.

  In order to achieve the above-described object, the present embodiment is a transuranium element conversion fuel assembly used in the core of a light water cooled nuclear reactor for transmutation of transuranium element, which extends in the longitudinal direction and is parallel to each other. A plurality of fuel rods arranged in a grid pattern and a channel box surrounding the radially outer side of the plurality of fuel rods, each of the fuel rods converting the transuranium element at a predetermined burn-up time A fuel pellet stacked in the longitudinal direction and containing a fissile material having a concentration satisfying a condition that the infinite neutron multiplication factor in the fuel assembly is equal to or higher than a predetermined value and containing a transuranium element, and the fuel pellet are included. And a thermal neutron ratio increasing structure that suppresses a decrease in the neutron level inside the fuel pellet due to self-shielding.

  Further, the present embodiment is a transuranium element conversion core of a light water cooled nuclear reactor, and a plurality of transuranium element conversion fuel assemblies arranged in a grid in parallel with each other and extending in the vertical direction, and the transuranium element A control rod for controlling the degree of reactivity arranged in parallel with the element conversion fuel assembly, and each of the transuranium element conversion fuel assemblies extends in the longitudinal direction and is arranged in a grid in parallel with each other A plurality of fuel rods, and a channel box surrounding the radially outer side of the plurality of fuel rods, each of the fuel rods in the transuranium element conversion fuel assembly at a predetermined burnup securing time point A fuel pellet laminated in the longitudinal direction and containing a fissile material having a concentration satisfying a condition that the infinite neutron multiplication factor is a predetermined value or more, and a cladding tube containing the fuel pellet; And having, with suppressing heat neutron fraction increased structural deterioration of the internal neutron level of the fuel pellet due to self-shielding, characterized in that.

  Further, the present embodiment is a design method of a transuranium element conversion fuel assembly for transmutation of transuranium element in the core of a light water cooled nuclear reactor, and a geometric shape parameter of the transuranium element conversion fuel assembly A geometric shape parameter setting step for setting, a fuel composition parameter setting step for setting a fuel composition parameter of a fissile material of the transuranium element conversion fuel assembly, and a combustion calculation step for calculating the combustion of fuel in the core As a result of the combustion calculation, it is determined whether or not the infinite neutron multiplication factor in the transuranium element conversion fuel assembly at a predetermined burn-up time securing point is a predetermined value or more, and is determined to be a predetermined value or more. The fuel composition parameter is changed when it is determined in the first determination step that is terminated if it is not greater than a predetermined value in the first determination step and the first determination step. It is determined whether or not it is possible to change, and if it is determined that the change is possible, the second determination step that shifts to the fuel composition parameter setting step, and the case where the change is not determined in the second determination step And determining whether the geometric shape parameter can be changed. If it is determined that the geometric shape parameter can be changed, the method further includes a third determination step that shifts to the geometric shape parameter setting step.

  According to the embodiment of the present invention, TRU can be transmuted efficiently in a light water reactor.

It is a horizontal sectional view showing the configuration of the transuranium element conversion core according to the first embodiment. It is a horizontal sectional view showing the configuration of the transuranium element conversion fuel assembly according to the first embodiment. It is a horizontal sectional view showing the configuration of the hollow fuel rod of the transuranium element conversion fuel assembly according to the first embodiment. It is a horizontal sectional view showing the composition of the solid fuel rod of the transuranium element conversion fuel assembly according to the first embodiment. It is a flowchart which shows the procedure of the transuranium element conversion fuel assembly design method which concerns on 1st Embodiment. It is a graph for supplementing description of the transuranium element conversion fuel assembly design method according to the first embodiment. It is a graph which shows the neutron spectrum in the fuel rod in a fuel assembly at the time of not applying this embodiment for explaining the effect of a 1st embodiment. It is a graph which shows the neutron flux distribution in the fuel assembly at the time of not applying this embodiment for demonstrating the effect of 1st Embodiment. It is a graph which shows the influence on the neutron spectrum in the fuel rod in the fuel assembly of the hollow part ratio of the fuel pellet for demonstrating the effect of 1st Embodiment. It is a graph which shows the influence on the neutron flux level of the hollow part ratio of the fuel pellet for demonstrating the effect of 1st Embodiment. It is a graph which shows the influence on the transmutation rate of the fuel volume for demonstrating the effect of 1st Embodiment. It is a conceptual graph which shows the example of the reaction absorption cross section of a transuranium element. It is a horizontal sectional view showing the composition of the fuel rod of the transuranium element conversion fuel assembly according to the second embodiment. It is a horizontal sectional view showing the composition of the fuel rod of the transuranium element conversion fuel assembly according to the third embodiment. It is a horizontal sectional view showing the composition of the fuel rod of the transuranium element conversion fuel assembly according to the fourth embodiment. It is a horizontal sectional view showing the composition of the fuel rod of the transuranium element conversion fuel assembly according to the fifth embodiment.

  Hereinafter, a transuranium element conversion fuel assembly 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 redundant description is omitted. In the following, it is assumed that the transuranium element conversion fuel assembly and its elements are in the direction in which they are installed in the core for convenience of explanation.

[First Embodiment]
FIG. 1 is a horizontal sectional view showing the configuration of the transuranium element conversion core according to the first embodiment. The transuranium element conversion core 100 includes a plurality of transuranium element conversion fuel assemblies 1 and a plurality of control rods 21. The transuranium element conversion fuel assemblies 1 are arranged in parallel with each other in a lattice shape and extend in the vertical direction. Four transuranium element conversion fuel assemblies 1 constitute a cell. The control rod 21 is arranged at the center of each cell, is arranged in parallel with the transuranium element conversion fuel assembly 1, and the position in the height direction is controlled by a control rod driving device (not shown).

  In addition, although the case where all the fuel assemblies in the transuranium element conversion core 100 are the transuranium element conversion fuel assemblies 1 is shown, the present invention is not limited to this. For example, a part of a uranium fuel assembly (not shown) using normal enriched uranium as a fissile material may be the transuranium element conversion fuel assembly 1.

  FIG. 2 is a horizontal sectional view showing a configuration of the transuranium element conversion fuel assembly according to the first embodiment. The transuranium element conversion fuel assembly 1 includes a plurality of fuel rods 10, two water rods 3, and a channel box 2 provided on the radially outer side thereof.

  The plurality of fuel rods 10 are arranged in a lattice shape in parallel to each other and extend in the longitudinal direction. The plurality of fuel rods 10 have hollow fuel rods 10a as thermal neutron ratio increasing structures. Here, the thermal neutron ratio increasing structure refers to a structure for increasing the ratio of thermal neutrons in the neutron energy spectrum by suppressing the decrease of thermal neutrons due to self-shielding. As will be described later, the hollow fuel rod 10a is a fuel rod for TRU combustion.

  The two water rods 3 are arranged at the center in the channel box 2 and extend in parallel to the fuel rods 10. Here, the arrangement positions of the fuel rods 10 in the channel box 2 are defined as a position A, a position B, a position C, and a position D from the side closer to the control rod center 20.

  Although FIG. 2 shows a case where the fuel rods in the fuel rods 10 are only the hollow fuel rods 10a as the thermal neutron ratio increasing structure, the present invention is not limited to this. The fuel rod 10 may include a solid fuel rod 10b (FIG. 4) for TRU combustion. In this case, the solid fuel rod 10b is disposed at a position A close to the gap between the reactor coolant as a moderator, that is, the control rod center 20 with a large amount of cooling water, or a position D close to the water rod 3 or the like. . Or you may have a normal uranium fuel rod.

  FIG. 3 is a horizontal sectional view showing the configuration of the hollow fuel rod 10a of the transuranium element conversion fuel assembly. The hollow fuel rod 10 a has a hollow fuel pellet 13 and a cladding tube 11 that encloses the hollow fuel pellet 13. The hollow fuel pellet 13 has a through hole 13a formed in the center portion in the radial direction of the cylindrical pellet. The hollow region 18 inside the through hole 13a is, for example, a space where argon gas exists. The hollow fuel pellets 13 are stacked in the central axis direction. In addition, the cross-sectional shape of the through-hole 13a is not limited to the circular shape in FIG.

  FIG. 4 is a horizontal sectional view showing the configuration of the solid fuel rods of the transuranium element conversion fuel assembly. The solid fuel rod 10 b has a solid fuel pellet 12 and a cladding tube 11 that encloses the solid fuel pellet 12. The solid fuel pellets 12 are stacked in the central axis direction.

The solid fuel pellet 12 and the hollow fuel pellet 13 have a fuel material. The fuel material includes a fissile material. Further, the fuel material includes TRU. The TRU may be a fissile material such as Pu ²⁴¹ or may not be a fissile material such as Pu ²⁴⁰ , for example.

Although the volume of the fuel material containing the fissile material is reduced by providing the hollow fuel pellet 13, it is required to ensure the same performance as the conventional one. FIG. 5 is a flowchart showing the procedure of the design method of the transuranium element conversion fuel assembly according to the present embodiment. FIG. 6 is a graph for supplementing the explanation of the design method of the transuranium element conversion fuel assembly. The horizontal axis of the graph of FIG. 6 is the cumulative generated energy E (GWD) of the noticed transuranium element conversion fuel assembly 1, and the vertical axis is the neutron infinite multiplication factor k _∞ in the superuranium element conversion fuel assembly 1 of interest. is there.

  First, geometric parameters such as the shape and dimensions of the fuel assembly are set based on the basic specifications (step S01). Here, the basic specifications are, for example, the outer shape of the channel box of the fuel assembly, the number of fuel rods arranged, the presence or absence of water rods, or the nuclear and thermal conditions. The geometric parameters are, for example, fuel rod outer dimensions, fuel pellet outer diameter and inner diameter, density, water rod shape, outer dimensions, and the like.

  Next, fuel composition parameters such as the fuel material enrichment and TRU enrichment of the fuel pellets are set (step S02). Here, when the nuclide is not limited, the ratio of the total weight of uranium 235 and uranium 233, fissile plutonium, that is, plutonium 239, plutonium 241 and the like to the weight of the entire fuel material is also called enrichment.

  The conditions set in steps S01 and S02 are displayed as initial conditions IC1 in FIG.

  Next, combustion calculation is performed (step S03). Combustion calculation is performed for the entire transuranium element conversion core. Here, the transuranium element conversion core has a transuranium element conversion fuel assembly 1, a normal fuel assembly (not shown), and a control rod (not shown). The combustion calculation is performed for a period from when the noticed transuranium element conversion fuel assembly 1 is loaded until the transuranium element conversion fuel assembly 1 achieves a predetermined burnup.

In Figure 6, the initial conditions IC1, as combustion progresses, while the burnup progresses along the broken line ICA, the transuranic conversion fuel assembly 1 of the neutron infinite multiplication factor k _∞ decreases. If not containing burnable poison, the neutron infinite multiplication factor k _∞ decreases substantially monotonically.

Now, the dependence of the infinite neutron multiplication factor k _∞ on the accumulated generated energy E is approximately expressed as the following equation (1).
k _∞ = aE + b (1)

At this time, a and b are functions of the fuel composition parameter e such as TRU enrichment and the geometric parameter g, and can be expressed by the following equations (2) and (3), respectively.
a = fa (e, g) (2)
b = fb (e, g) (3)

Next, it is determined whether or not a target state has been established (step S04). Target state, when the burnup of transuranic conversion fuel assembly 1 of interest has reached the predetermined value E _g, the transuranic conversion fuel assembly 1 of the neutron infinite multiplication factor k _∞ is a predetermined value A state where k _{∞ g} or more is assumed. Note that a time point or state other than this may be defined as the target state. When it is determined that the target state has been established (YES in step S04), a formation solution is obtained. If it is not determined that the target state is established (NO in step S04), the process proceeds to the next step S05.

In the case shown in FIG. 6, when the transuranic conversion fuel assembly 1 of burnup reaches a predetermined value E _g, the transuranic conversion fuel neutron infinite multiplication factor k _∞ of assembly 1 is k1 _Yes, it is not over the predetermined value _k∞g . Therefore, since it corresponds to the case where it is not determined that the target state is established (NO in step S04), the process proceeds to the next step S05.

  Next, it is determined whether or not the fuel composition parameter e such as the TRU enrichment can be changed (step S05). If it is determined that the fuel composition parameter e can be changed (YES in step S05), steps S02 and after are repeated. If it is not determined that the fuel composition parameter e can be changed (NO in step S05), the process proceeds to the next step S06.

In the case shown in FIG. 6, the fuel composition parameter e is changed (step S02), for example, the enrichment is increased, and the process proceeds to the initial condition IC2. Combustion calculation is performed using this as a new initial condition (step S03). The state of the noticed transuranium element conversion fuel assembly 1 changes along the alternate long and short dash line ICB. The neutron infinite multiplication factor k _{∞ of the} transuranium element conversion fuel _assembly 1 when the burnup of the transuranium element conversion fuel _assembly 1 of _interest reaches a predetermined value E _g is greater than or equal to the predetermined value k _∞g It is determined whether or not there is (step S04). In the case of FIG. 6, the neutron infinite multiplication factor k _∞ is k2 and is not greater than or equal to the predetermined value k _∞g, and thus corresponds to the case of proceeding to step S06.

  In the next step S06, it is determined whether or not the geometric parameter g can be changed. If it is determined that the geometric shape parameter g can be changed (YES in step S06), steps S01 and after are repeated. If it is not determined that the geometric parameter g can be changed (NO in step S06), a successful solution cannot be obtained.

In the case shown in FIG. 6, the geometric parameter g is changed (step S01), and the process proceeds to the initial condition IC3. Combustion calculation is performed using this as a new initial condition (step S03). The state of the transuranium element conversion fuel assembly 1 of interest changes along the solid line ICC. The neutron infinite multiplication factor k _{∞ of the} transuranium element conversion fuel _assembly 1 when the burnup of the transuranium element conversion fuel _assembly 1 of _interest reaches a predetermined value E _g is greater than or equal to the predetermined value k _∞g It is determined whether or not there is (step S04). In the case of FIG. 6, the neutron infinite multiplication factor k _∞ is because it is a is a predetermined value k _∞G more k3, satisfied solution is obtained (step S04 YES).

  FIG. 7 is a graph showing a neutron spectrum at the fuel rod in the fuel assembly when the present embodiment for explaining the effect of the first embodiment is not applied. The horizontal axis represents the energy (eV) of neutrons incident on the fuel assembly, and the vertical axis represents the neutron flux. The unit of the vertical axis is arbitrary. The four curves LA, LB, LC, and LD show the neutron spectra at positions A, B, C, and D, respectively.

  As described above, in FIG. 2, the locations of the transuranium element conversion fuel assemblies 1 in the channel box 2 are A, B, C, and D from the side closer to the control rod center 20 side. Now, assume that a TRU derived from spent water from a light water reactor is loaded on a light water reactor fuel assembly that has an ordinary geometric shape. That is, a case is assumed in which a transuranium element conversion fuel assembly 1 is loaded in a core constituted by a fuel assembly having UO2 fuel containing enriched uranium as shown in FIG.

  FIG. 7 shows the neutron spectrum at each location in this case. The horizontal axis is the incident neutron energy, and the vertical axis is the neutron flux. The neutron spectra at positions A, B, C, and D are not significantly different from each other, but the position A is different from the other positions in that there is a peak in the thermal neutron region. This is because the fuel rod arranged at the position A is located on the outermost periphery of the fuel assembly and is located at the corner of the fuel assembly where a lot of water serving as a moderator exists around. That is, the fuel rods arranged at other positions have less moderator around the fuel rods arranged at the position A.

  Here, the thermal neutron means a neutron in equilibrium with the thermal motion of surrounding atoms. The energy distribution of thermal neutrons in this equilibrium state follows Maxwell-Boltzmann distribution. For example, the average energy at room temperature is about 0.025 eV. Here, considering this distribution, for example, neutrons in the region of 1 eV or less are referred to as thermal neutrons.

  FIG. 8 is a graph showing the neutron flux distribution in the fuel assembly when the present embodiment is not applied to explain the effects of the present embodiment. The horizontal axis represents positions A, B, C and D, and the vertical axis represents the amount of thermal neutrons having energy of 1 eV or less. As shown in FIG. 7, the position A is the largest. Next, the position D close to the water rod 3 (FIG. 2) having the moderator is large. The fuel rods at positions A and D have a larger neutron flux than the fuel rods at positions C and B because the amount of water that is the moderator or reactor coolant around each fuel rod is large. by.

  Regarding the positions B and C that are away from the control rod and the water rod, the fuel rod disposed at the position B near the control rod is slightly larger than the position C. This is mainly due to the self-shielding effect in the fuel assembly with respect to the thermal neutron flux from the outside of the fuel assembly.

  TRU nuclides generally have a large neutron absorption cross section in the thermal neutron region (see FIG. 12), and the average thermal neutron flux of the fuel assembly decreases. On the other hand, as shown in FIG. 7, a large thermal neutron flux is obtained for the fuel rod arranged at the position A. This is because the existence ratio of the coolant as the moderator is large around the position A. If the same level of thermal neutron flux is obtained for the fuel rods arranged at other positions, the conversion efficiency of the TRU will greatly increase.

  FIG. 9 is a graph showing the influence of the hollow portion ratio of the fuel pellet on the neutron spectrum at the fuel rod in the fuel assembly for explaining the effect of the present embodiment. The horizontal axis is the incident neutron energy, and the vertical axis is the neutron flux. FIG. 9 shows the neutron spectrum at position B.

  Now, in the cross section of the hollow fuel pellet 13a of the hollow fuel rod 10a, the ratio of the cross-sectional area of the hollow fuel pellet 13a to the area of the circle having the outer diameter of the hollow fuel pellet 13a is referred to as a solid rate. In this case, as a limit, the solid fuel pellet 12 also has a solid rate of 100%.

  A curve L10 indicates a case where the solid rate is 100%, a curve L6 indicates a case where the solid rate is 60%, and L2 indicates a case where the solid rate is 20%. Compared with the curve L10 having a solid rate of 100%, the ratio of neutron flux in the region of about 1 keV or more decreases as the curve L6 has a solid rate of 60% and further the curve L2 has a solid rate of 20%. The proportion of neutron flux below about 1 eV increases. That is, as the solid rate decreases, the neutron spectrum shifts to the thermal neutron side, and the ratio of thermal neutron flux increases.

  In general, thermal neutrons that flow into the fuel pellet from the outside of the fuel pellet and travel toward the fuel center are absorbed by the fuel and gradually decay, and the thermal neutrons are greatly reduced near the center of the fuel pellet as compared to the surface. Therefore, the thermal neutron ratio increasing structure can increase the ratio of thermal neutrons in the fuel pellets by removing the region where the thermal neutrons inside the fuel pellets are extremely small and reducing the self-shielding effect.

  FIG. 10 is a graph showing the influence of the hollow portion ratio of the fuel pellet on the neutron flux level for explaining the effect of the present embodiment. The horizontal axis is the solid rate of fuel pellets. The vertical axis is the ratio of the thermal neutron flux at the hollow fuel pellet at position B to the thermal neutron flux at the hollow fuel pellet at position A. In this trial calculation example, for simplicity, all the fuel rods 10 of the transuranium element conversion fuel assembly 1 are calculated as the same hollow fuel rod 10a.

  That is, FIG. 9 shows the neutron flux at each energy when the solid rate is 100%, 60%, and 20%. In FIG. 10, the result of integrating the neutron flux in the thermal neutron region is the solid rate. Shows how it depends on As shown in FIG. 10, the ratio of the thermal neutron flux tends to increase rapidly as the solid rate decreases.

  FIG. 11 is a graph showing the influence of the fuel volume on the transmutation rate for explaining the effect of the present embodiment. Now, the transmutation rate is defined as the amount of TRU extracted relative to the amount of TRU loaded. As shown in FIG. 11, when the fuel region cross-sectional area decreases and the fuel volume VF decreases, the transmutation rate increases.

FIG. 12 is a conceptual graph showing an example of the reaction absorption cross section of the transuranium element. Details of the cross-sectional area are omitted. Further, among the transuranium elements, the case of Pu ²⁴¹ is shown. The horizontal axis represents neutron energy (eV), and the vertical axis represents the cross-sectional area (barns) of the reaction with neutrons. 1 barn is 10 ⁻²⁸ m ² .

The one-dot chain line CF indicates the cross-sectional area σ _f of the fission reaction, the two-dot chain line CC indicates the cross-sectional area σ _C of the neutron capture reaction, and the solid line CT indicates the cross-sectional area σ _{T of} all reactions including the other scattering cross-sections. Indicates. As shown in FIG. 12, in the region where the neutron energy is about 1 eV or less, the cross-sectional area σ _f of the fission reaction and the cross-sectional area σ _C of the neutron capture reaction greatly increase as the neutron energy decreases. This tendency is almost the same for other TRU nuclides.

  Therefore, the shift of the neutron energy spectrum to the thermal neutron side by the hollow fuel pellet 13 has an effect of increasing the reaction with neutrons that TRU transmutates, that is, fission reaction and neutron capture reaction.

  As described above, according to the present embodiment, TRU nuclear transmutation can be efficiently performed in a light water reactor.

[Second Embodiment]
FIG. 13 is a horizontal sectional view showing the configuration of the fuel rods of the transuranium element conversion fuel assembly according to the second embodiment. This embodiment is a modification of the first embodiment, and the configuration of the fuel rod is different.

  The moderator-containing fuel rod 10c as the thermal neutron ratio increasing structure in the present embodiment includes a hollow fuel pellet 13, a speed reduction member 15, and a cladding tube 11 that includes these. The hollow fuel pellet 13 is formed with a through hole 13a in a central portion in the radial direction of a cylindrical fuel pellet. The hollow fuel pellets 13 are stacked in the central axis direction.

A speed reducing member 15 is provided on the radially inner side of the hollow fuel pellet 13. As a material of the speed reducing member 15, for example, beryllium oxide, boron carbide, zirconium hydride, hafnium hydride, spinel (for example, MgAl ₂ O ₄ ), or the like can be used. The speed reducing member 15 may be provided inside each hollow fuel pellet 13 and may be integrated with the hollow fuel pellet 13. Alternatively, the speed reduction member 15 may have a rod shape, and the hollow fuel pellets 13 may be stacked and accommodated in the through hole 13a communicating with the center in the radial direction.

  In the present embodiment, not only the radial center side of the fuel pellet having a small thermal neutron ratio is removed, but also the ratio of thermal neutrons is increased more than in the first embodiment by providing the speed reducing member 15. Can be made.

[Third Embodiment]
FIG. 14 is a horizontal sectional view showing the configuration of the fuel rods of the transuranium element conversion fuel assembly according to the third embodiment.

  This embodiment is a modification of the second embodiment. The fuel rod with coolant channel 10d as the thermal neutron ratio increasing structure in the present embodiment includes a hollow fuel pellet 13, and an outer cladding tube 11a and an inner cladding tube 11b that house the stacked hollow fuel pellets 13. The outer cladding tube 11a and the inner cladding tube 11b are arranged concentrically and extend vertically. The upper end of the outer cladding tube 11a and the upper end of the inner cladding tube 11b are joined by a perforated disc-shaped end plate (not shown). Similarly, the lower end of the outer cladding tube 11a and the lower end of the inner cladding tube 11b are joined by a perforated disc-shaped end plate (not shown).

  The inside of the inner cladding tube 11b forms part of the flow path of the reactor coolant 16 in the transuranium element conversion fuel assembly 1. Since the water that is the reactor coolant 16 has a moderating effect, this embodiment also provides the same effect as that of the second embodiment.

[Fourth Embodiment]
FIG. 15 is a horizontal sectional view showing the structure of the fuel rods of the transuranium element conversion fuel assembly according to the fourth embodiment. The transuranium element conversion fuel assembly 1 as the thermal neutron ratio increasing structure in the present embodiment has a small-diameter fuel rod 10e. The small-diameter fuel rod 10e has a small-diameter solid fuel pellet 12 and a small-diameter small-diameter cladding tube 11c. The volume ratio of the coolant to the fuel is increased by reducing the fuel rod diameter while maintaining the spacing between the small diameter fuel rods 10e. As a result, the neutron moderating effect is increased, the thermal neutron ratio is increased, and the same effects as those of the first to third embodiments can be obtained.

[Fifth Embodiment]
FIG. 16 is a horizontal sectional view showing the configuration of the fuel rods of the transuranium element conversion fuel assembly according to the fifth embodiment. The low density fuel rod 10 f as the thermal neutron ratio increasing structure in the present embodiment has a low density pellet 14. The low density pellet 14 increases the ratio of coolant to fuel by reducing the density of uranium 235 and TRU without changing the size of each part, and obtains the same effect as in the fourth embodiment. is there.

  Since only the fuel pellets are changed, there is little influence on manufacturing and the like.

[Other Embodiments]
As mentioned above, although some embodiment of this invention was described, these embodiment is shown as an example and is not intending limiting the range of invention. For example, in the embodiment, the transuranium element conversion fuel assembly 1 has been shown to include two types of fuel rods, the solid fuel rod 10b and the hollow fuel rod 10a. But you can.

  Moreover, you may combine the characteristic of each embodiment. Furthermore, these embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the invention described in the claims and the equivalents thereof.

  DESCRIPTION OF SYMBOLS 1 ... Transuranium element conversion fuel assembly, 2 ... Channel box, 3 ... Water rod, 10 ... Fuel rod, 10a ... Hollow fuel rod (fuel rod), 10b ... Solid fuel rod (fuel rod), 10c ... Moderator Contained fuel rod (fuel rod), 10d ... Fuel rod with coolant channel (fuel rod), 10e ... Small diameter fuel rod (fuel rod), 10f ... Low density fuel rod (fuel rod), 11 ... Cladding tube, 11a ... Outer cladding tube (cladding tube), 11b ... Inner cladding tube (cladding tube), 11c ... Small diameter cladding tube (cladding tube), 12 ... Solid fuel pellet (fuel pellet), 13 ... Hollow fuel pellet (fuel pellet), 13a DESCRIPTION OF SYMBOLS ... Through-hole, 14 ... Low density pellet (fuel pellet), 15 ... Deceleration member, 16 ... Reactor coolant, 18 ... Hollow region, 20 ... Control rod center, 21 ... Control rod, 100 ... Transuranium element conversion core

Claims (7)

A transuranium element conversion fuel assembly for transmutation of transuranium elements used in the core of light water cooled nuclear reactors,
A plurality of fuel rods extending in the longitudinal direction and arranged in a grid parallel to each other;
A channel box surrounding a radially outer side of the plurality of fuel rods;
With
Each of the fuel rods
Stacked in the longitudinal direction containing a fissile material having a concentration satisfying a condition that the infinite neutron multiplication factor in the transuranium element conversion fuel assembly at a predetermined burnup securing point is not less than a predetermined value and containing the transuranium element Fuel pellets,
A cladding tube containing the fuel pellets;
And having
Having a thermal neutron ratio increasing structure that suppresses a decrease in the neutron level inside the fuel pellet due to self-shielding,
A transuranium element conversion fuel assembly characterized by that.   2. The transuranium element conversion fuel assembly according to claim 1, wherein the thermal neutron ratio increasing structure has a hollow region formed in a radially central portion of the fuel pellet. 3.   2. The transuranium element conversion fuel assembly according to claim 1, wherein the thermal neutron ratio increasing structure includes a speed reducing member disposed at a radial center of the fuel pellet. 3.   2. The transuranium element conversion fuel assembly according to claim 1, wherein the thermal neutron ratio increasing structure has a reactor coolant passage formed in a radially central portion of the fuel pellet. 3.   The supermarket according to any one of claims 1 to 4, further comprising at least one water rod that is arranged in a central region of the plurality of fuel rods and allows the reactor cooling water to pass therethrough. Uranium element conversion fuel assembly. A transuranium element conversion core of a light water cooled nuclear reactor,
A plurality of transuranium element conversion fuel assemblies arranged in a grid in parallel with each other and extending in the vertical direction;
A control rod for controlling the reactivity arranged in parallel with the transuranium element conversion fuel assembly;
Comprising
Each of the transuranium element conversion fuel assemblies is
A plurality of fuel rods extending in the longitudinal direction and arranged in a grid parallel to each other;
A channel box surrounding a radially outer side of the plurality of fuel rods;
With
Each of the fuel rods
Stacked in the longitudinal direction containing a fissile material having a concentration satisfying a condition that the infinite neutron multiplication factor in the transuranium element conversion fuel assembly at a predetermined burnup securing point is not less than a predetermined value and containing the transuranium element Fuel pellets,
A cladding tube containing the fuel pellets;
And having
Having a thermal neutron ratio increasing structure that suppresses a decrease in the neutron level inside the fuel pellet due to self-shielding,
A transuranic element conversion core characterized by this. A design method of a transuranium element conversion fuel assembly for transmutation of transuranium element in the core of a light water cooled nuclear reactor,
A geometric parameter setting step for setting a geometric parameter of the transuranium element conversion fuel assembly;
A fuel composition parameter setting step for setting a fuel composition parameter of a fissile material of the transuranium element conversion fuel assembly;
A combustion calculation step for calculating the combustion of fuel in the core;
As a result of the combustion calculation, it is determined whether or not the infinite neutron multiplication factor in the transuranium element conversion fuel assembly at the time of securing the predetermined burnup is greater than or equal to a predetermined value. Is a first determination step for ending, and
If it is determined in the first determination step that the fuel composition parameter is not greater than or equal to a predetermined value, it is determined whether or not the fuel composition parameter can be changed. If it is determined that the fuel composition parameter can be changed, the fuel composition parameter setting step is performed. A second determination step for transition;
If it is not determined that the change can be made in the second determination step, it is determined whether the geometric parameter can be changed. If it is determined that the change is possible, the process proceeds to the geometric parameter setting step. A determination step of
A transuranium element conversion fuel assembly design method comprising:

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