Nuclear power generation apparatus

Abstract

To provide a nuclear power generation apparatus capable of stably supplying power in a maintenance-free manner over a long period of time.SOLUTION: The nuclear power generation apparatus for generating power using a subcritical reactor comprises: a subcriticality reactor having a reactor core part including a neutron beam source, a nuclear fuel material and a moderator and a barrier covering the reactor core part; and a thermal photoelectric conversion part provided on the circumference of the subcritical reactor. The reactor core part includes a neutron beam source arranged in the center part, and mixture of the nuclear fuel material and the moderator arranged on the circumference of the neutron beam source. A thermoelectric transducer provided in the thermal photoelectric conversion part directly converts thermal output generated in the reactor core part into electricity and the power is outputted.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a nuclear power generation device, and more particularly to a nuclear power generation device that is maintenance-free over a long period of time and can stably supply power.

Background Art

[0002] In recent years, space exploration has entered a new stage, namely the deep space exploration stage, and space agencies such as JAXA are attempting to advance deep space exploration plans.

[0003] In such space exploration, conventionally, solar cells and nuclear batteries have been used as power sources. However, considering that deep space exploration is carried out on a scale exceeding 100 years, these are not satisfactory as power sources for deep space exploration, specifically, as power generation devices that are small, can provide high output over a long period of time.

[0004] That is, in the case of solar cells, the intensity of sunlight is inversely proportional to the square of the distance from the sun, and in the space outside Jupiter, almost no sunlight reaches, so sufficient solar energy cannot be expected, and it is difficult to obtain power with sufficient output. Also, in the case of nuclear batteries, due to the principle of generating electricity using the decay heat of radioactive elements, the output is low, and if an attempt is made to increase the output, the device inevitably becomes larger.

[0005] On the other hand, as a power generation device that is small, can provide high output stably over a long period of time, a power generation device using a nuclear reactor (nuclear power generation device) can be considered. However, in the case of a nuclear power generation device using a general turbine, there is a concern that mechanical parts such as a cooling mechanism and a control mechanism may be damaged by the impact during launch, or these mechanical parts may malfunction during navigation. It is difficult to handle such problems in space without human intervention, and a nuclear power generation device that is maintenance-free over a long period of time is required.

[0006] Therefore, a space nuclear reactor that maintains the reactor core in a subcritical state without using an external device during a period when control rods may not function, such as during launch, has been proposed (see Patent Document 1).

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] However, the technology disclosed in Patent Document 1 performs control using control rods after launch, and does not consider how to deal with failures in parts involving mechanical operations such as the cooling system and lubrication system during a voyage that can be considered to exceed 100 years. It cannot be said to be sufficient for long-term maintenance-free operation.

[0009] The demand for a nuclear power generation device that can be maintenance-free over a long period and stably supply power is not limited to use in space. The same applies to power generation devices installed in environments where maintenance is difficult, such as on planets like the moon and Mars, in polar regions, on high mountains, in the deep sea, and on uninhabited islands, for various measurements.

[0010] Therefore, an object of the present invention is to provide a nuclear power generation device that can be maintenance-free over a long period and stably supply power.

Means for Solving the Problems

[0011] The inventors of the present invention have intensively studied how to solve the above problems, and have found that the above problems can be solved by the invention described below, thus completing the present invention.

[0012] The invention according to claim 1 is A nuclear power generation device that generates electricity using a subcritical nuclear reactor, a neutron source, a nuclear fuel material, a reactor core portion including a moderator, and a subcritical nuclear reactor provided with a partition wall covering the reactor core portion, and a thermoelectric conversion portion provided on the outer periphery of the subcritical nuclear reactor, wherein the reactor core portion is composed of a neutron source disposed at the center and a mixture of a nuclear fuel material and a moderator disposed around the neutron source, and the heat output generated in the reactor core portion is directly converted into electricity by a thermoelectric conversion element provided in the thermoelectric conversion portion and output. A nuclear power generation device characterized by this.

[0013] The invention according to claim 2 is, The nuclear power generation device according to claim 1, wherein the neutron source is a spontaneous fission source.

[0014] The invention according to claim 3 is, wherein the spontaneous fission source is, 252 Cf (Californium), 240 Pu (Plutonium), 250 Cm (Curium), and is any one of them. The nuclear power generation device according to claim 2, characterized by this.

[0015] The invention according to claim 4 is, The nuclear power generation device according to claim 1, wherein the neutron source is an α-ray source that generates neutrons by α-rays generated from an (α, n) reaction.

[0016] The invention according to claim 5 is, The nuclear power generation device according to claim 4, wherein the α-ray source is an α-decaying nuclide.

[0017] The invention according to claim 6 is, wherein the α-decaying nuclide is, 238 Pu (Plutonium), 239 Pu (Plutonium), 241 Am (Americium), 244 Cm (Curium),226 Ra (radium), 227 The nuclear power generation device according to claim 5, characterized in that it is any one of Ac (actinium).

[0018] The invention according to claim 7 is The nuclear power generation device according to any one of claims 1 to 6, characterized in that the nuclear fuel material is a thermonuclear fissionable material or a mixture of a thermonuclear fissionable material and a transuranium element.

[0019] The invention according to claim 8 is The thermonuclear fissionable material is, if necessary 235 U (uranium) enriched with U (uranium), or 239 Pu (plutonium), 233 U (uranium), and is the nuclear power generation device according to claim 7.

[0020] The invention according to claim 9 is The nuclear power generation device according to any one of claims 1 to 5, characterized in that the moderator is C (carbon) or Be (beryllium).

[0021] The invention according to claim 10 is The nuclear power generation device according to any one of claims 1 to 5, characterized in that the material constituting the partition wall is any one of Cu (copper), W (tungsten), B (boron), or a combination thereof.

[0022] The invention according to claim 11 is The nuclear power generation device according to any one of claims 1 to 5, characterized in that a heat radiating plate that radiates the heat output not used for the electric conversion to the outside of the system is connected to the subcritical nuclear reactor on the outer surface of the partition wall.

[0023] The invention according to claim 12 is The nuclear power generation device according to claim 11, characterized in that the heat radiating plate is made of stainless steel.

[0024] The invention according to claim 13 The nuclear power generation device according to any one of claims 1 to 5, characterized in that the effective multiplication factor in the subcritical nuclear reactor is 0.99 to 0.999.

[0025] The invention according to claim 14 The nuclear power generation device according to any one of claims 1 to 5, characterized in that it is a nuclear power generation device for planetary exploration or deep space exploration.

[0026] The invention according to claim 15 The nuclear power generation device according to any one of claims 1 to 5, characterized in that it is a nuclear power generation device used in polar regions, high mountains, deep seas, and uninhabited islands.

Advantages of the Invention

[0027] According to the present invention, it is possible to provide a nuclear power generation device that is maintenance-free over a long period of time and can stably supply power.

Brief Description of the Drawings

[0028]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Embodiments for Carrying Out the Invention

[0029] 1. Nuclear power generation device according to this embodiment First, a nuclear power generation device according to this embodiment (hereinafter, also simply referred to as a "power generation device") will be described.

[0030] (1) Configuration FIG. 1 is a schematic cross-sectional view of the power generation device according to this embodiment. In FIG. 1, 1 is the power generation device, 2 is the nuclear reactor, 3 is the thermoelectric conversion unit, and 4 is the heat dissipation plate.

[0031] As shown in FIG. 1, the power generation device 1 includes a nuclear reactor 2, a thermoelectric conversion unit 3, and a heat dissipation plate 4. The nuclear reactor 2 has a reactor core 21 inside, and its periphery is surrounded by a partition wall 24. A neutron beam source 22 is arranged at the center of the reactor core 21, and a mixture 23 of nuclear fuel material and moderator is arranged around it. The thermoelectric conversion unit 3 is arranged outside the partition wall 24 so as to cover the outer surface of the partition wall 24, and further, the heat dissipation plate 4 is provided outside the thermoelectric conversion unit 3.

[0032] As is clear from FIG. 1, the power generation device according to this embodiment has no moving parts, unlike conventional power generation devices. That is, in a conventional power generation device, critical control is performed by inserting and removing control rods, cooling of the nuclear reactor and deceleration of neutrons are performed by injecting cooling water, and power generation is performed by operating a power generation turbine, using mechanical elements with moving parts. In contrast, in the power generation device according to this embodiment, no moving parts are provided and no mechanical control is performed. Therefore, there is no risk of failure, and power can be stably supplied without maintenance for a long period of over 100 years.

[0033] (2) Principle of power generation Next, the principle of power generation in the power generation device according to this embodiment will be described.

[0034] The power generation device according to this embodiment uses a subcritical nuclear reactor formed by a combination of a neutron source and nuclear fuel, and its basic principle is to directly convert the generated heat into electricity by means of a thermoelectric conversion element provided in the thermoelectric conversion section.

[0035] (a) Regarding the thermal output of the nuclear reactor In a nuclear reactor, a nuclear fission reaction occurs when nuclear fuel absorbs neutrons generated from a neutron source, and fission fragments (two fission products), two to three neutrons per fission, and γ-rays are emitted (nuclear fission reaction). Then, a part of the emitted neutrons is moderated by a moderator and returns to the nuclear fuel in the system, causing the same nuclear fission reaction as before. The remaining neutrons and gamma rays are absorbed and extinguished by B and W in the partition outside the system and converted into thermal energy. By repeating this, the nuclear fission reaction proceeds in a chain.

[0036] At this time, it is important to maintain an appropriate balance between the number of neutrons returning to the system and the number of neutrons discharged outside the system for controlling nuclear power generation. That is, if the ratio (effective multiplication factor keff) of the number of neutrons returning to the system and inducing nuclear fission to the number of neutrons generated by the nuclear fission reaction is less than 1.0, the nuclear fission reaction will tend to end and the output of the nuclear reactor will decrease, so safe operation is possible (subcritical state). However, if it exceeds 1.0, the nuclear fission reaction will increase and the output of the nuclear reactor will rise, eventually leading to the explosion of the nuclear reactor (supercritical state). In the case of 1.0, the output of the nuclear reactor will be constant (critical state).

[0037] In order to maintain the above-mentioned critical state (keff = 1.0), conventional power generation devices used to perform control by inserting and removing control rods, so a movable part was required for this purpose and it could not be made maintenance-free. On the other hand, in the power generation device according to this embodiment, as described above, it is in a subcritical state and does not require control, so there is no need to provide a movable part. Therefore, it can supply power stably and maintenance-free for a long period of over 100 years.

[0038] And the output P of the nuclear reactor in the subcritical state (keff < 1.0) can be expressed as follows using the effective multiplication factor keff described above. It can be seen that in order to obtain a high output, it is more advantageous that the effective multiplication factor keff is closer to 1. Therefore, in this embodiment, the target is keff = 0.99 to 0.999. P = 1 / (1 - keff)

[0039] Specifically, in this embodiment, as shown in FIG. 1, first, a neutron source 22 is arranged at the center of the core 21.

[0040] At this time, as the neutron source 22, a spontaneous fission neutron source, for example, 252 Cf (californium), or 240 Pu (plutonium), 250 Cm (curium), etc. are preferably used. Among these, from the viewpoints of the half-life length and the neutron generation amount per unit weight, 250 Cm is particularly preferable. Note that 250 The usage amount of Cm is preferably at least about 1 Kg or more, and the more the better.

[0041] Alternatively, neutrons may be generated by an (α, n) reaction that emits neutrons by bombarding light elements such as Be (beryllium) or B (boron) which are targets for emitting α particles (α rays). As such an α-ray source, for example, 238 Pu (plutonium) or 241 Am (americium), 244 Cm (curium), 226 Ra (radium), 227 α-emitting nuclides such as Ac (actinium) are preferably used.

[0042] Next, a mixture 23 of a nuclear fuel material and a moderator is arranged so as to cover the neutron source 22. In this embodiment, as the nuclear fuel material, a thermonuclear fissionable material, for example, 235 Highly enriched U (uranium) in which U (uranium) is enriched, or 239Pu (Plutonium), 233 It is preferable to use U (Uranium) and the like. At this time, it is also preferable to mix transuranic elements such as Np (Neptunium), Am (Americium), and Cm (Curium) recovered after reprocessing of spent fuel into the nuclear fuel material from the viewpoint of adjusting the output coefficient (a coefficient indicating the change in reactivity accompanying the increase in output). Further, as the moderator, it is preferable to use C (Carbon), Be (Beryllium), etc., which are light nuclides with high melting points.

[0043] By configuring the core part 21 in this way, even without movable parts such as control rods, at the start of operation, as shown in FIGS. 2 and 4 described later, since the temperature coefficient is positive at low temperatures, the temperature of the core rises and the output increases. Then, near a certain set temperature (output), the temperature coefficient turns negative and the output increase stops. At this time, as shown in FIGS. 3 and 5 described later, since the effective multiplication factor keff is less than 1 in all temperature regions, it does not run away. As described above, a stable thermal output can be obtained even without movable parts.

[0044] Next, the outer periphery of the mixture 23 of the nuclear fuel material and the moderator is covered with a partition wall 24 made of a shielding material. The energy of the fission products generated by the fission reaction is transmitted to the partition wall 24 after being absorbed by the substance in the mixture 23. Also, the radiation (neutrons and γ-rays) generated in the mixture 23 diffuses to the partition wall 24, and the γ-rays are absorbed by the shielding material of the partition wall 24 and converted into thermal energy. Also, neutrons decelerated to a certain extent in the mixture 23 are similarly absorbed by the partition wall 24 and converted into thermal energy. As the shielding material, in addition to the effect of shielding γ-rays and neutrons, a material with excellent thermal conductivity is preferable. For example, it is preferable to use Cu (Copper), W (Tungsten), B (Boron), etc., and these may be used in combination. Note that the thickness of the partition wall 24 is preferably about several tens of cm.

[0045] The content described above will be further explained based on FIGS. 2 to 5.

[0046] First, FIG. 2 is a diagram for explaining the relationship between the output coefficient and the output in the power generation device according to the present embodiment. In FIG. 2, the horizontal axis represents the output, and the vertical axis represents the output coefficient. As shown in FIG. 2, in the present embodiment, the output coefficient is positive at low output and decreases as the output increases. Then, the fuel is combined so that the output coefficient becomes 0 when reaching near the target output of 100 kWth in the present embodiment, and becomes negative thereafter. By adopting such a configuration, a desired output can be obtained only with the configuration of the neutron source and the fuel.

[0047] Next, FIG. 3 is a diagram for explaining the relationship between the effective multiplication factor and the output in the power generation device according to the present embodiment. In FIG. 3, the horizontal axis represents the output, and the vertical axis represents the effective multiplication factor keff. As shown in FIG. 3, in the present embodiment, the effective multiplication factor keff initially increases toward 1.0 as the output increases (the progress of the nuclear fission reaction). Then, the fuel is combined so that the effective multiplication factor keff, which has become close to 1.0 when reaching near the target output of 100 kWth in the present embodiment, reverses to a decreasing trend. By adopting such a configuration, a desired output can be obtained only with the configuration of the neutron source and the fuel.

[0048] Next, FIG. 4 is a diagram for explaining the relationship between the output coefficient and the core temperature in the power generation device according to an embodiment of the present invention, where the horizontal axis represents the core temperature and the vertical axis represents the output coefficient. Further, FIG. 5 is a diagram for explaining the relationship between the effective multiplication factor and the core temperature in the power generation device according to the present embodiment, where the horizontal axis represents the core temperature and the vertical axis represents the output coefficient.

[0049] The change in the core temperature is related to the change in the output. As the output increases (the progress of the nuclear fission reaction), the core temperature gradually rises. Therefore, in FIGS. 4 and 5, the same relationship as in FIGS. 2 and 3 can be observed.

[0050] (b) Conversion from heat to electricity In the present embodiment, the conversion from heat to electricity is directly converted into electrical energy in the thermoelectric conversion unit 3 installed outside the heated partition wall 24.

[0051] The thermoelectric conversion unit 3 is configured using thermoelectric conversion elements. At this time, since the conversion efficiency of the thermoelectric conversion elements is currently about 0.1 (10%), and the electrical output per element is not sufficient, it is preferable to stack them on the entire outer periphery of the partition wall 24 and arrange them in series to obtain the required electrical output.

[0052] (c) Waste heat As described above, since the conversion efficiency of the thermoelectric conversion elements is currently about 10%, about 90% of the heat that was not used for thermoelectric conversion needs to be cooled by removing heat (waste heat) outside the system to suppress excessive temperature rise in the nuclear power plant and its surroundings (if heat accumulates and the temperature rises excessively, there is a risk of melting the reactor). In this embodiment, the cooling inside the reactor is configured such that a heat dissipation plate 4 is provided so as to be connected to the outside of the thermoelectric conversion unit 3, and heat is dissipated to the outside of the system by radiation from the surface of the heat dissipation plate 4.

[0053] At this time, considering that the surface temperature of the heat dissipation plate rises to about 1000°C, the heat dissipation plate is preferably made of a metal with excellent emissivity and heat resistance. For example, stainless steel such as SUS310 can be preferably used. And regarding the size and number, they can be appropriately determined in consideration of the heat energy to be radiated and the emissivity of the heat dissipation plate.

[0054] In an environment such as the deep sea where sufficient cooling can be considered possible as it is, the heat dissipation plate may be arranged as necessary.

[0055] (3) Advantages of the nuclear power plant according to this embodiment The nuclear power plant according to this embodiment directly obtains electric power using the thermoelectric conversion elements provided in the thermoelectric conversion unit from the thermal energy generated in the subcritical nuclear reactor, so it does not require mechanical moving parts such as a circulation pump mechanism and a power generation turbine mechanism like conventional nuclear power plants. Therefore, even in an environment where humans cannot work, such as in space or the deep sea, stable power supply can be achieved without maintenance for a long period of over 100 years.

[0056] And, since it does not require mechanically movable parts, it is possible to easily miniaturize the entire nuclear power generation device.

[0057] 2. Specific Embodiments Hereinafter, as a specific embodiment, the present invention will be described in more detail by taking a nuclear power generation device for deep space exploration assuming a thermal output of 100 kWth, an electrical output of 10 kWe, and a service life of 100 years as an example. Note that the subscripts th and e after kW indicate thermal output and electrical output, respectively.

[0058] Here, it is assumed that a subcritical nuclear reactor (thermal output: approximately 100 kWth) with a surface temperature of the reactor of about 1000 °C, keff of 0.999, and a total weight (including the thermoelectric conversion section and the heat sink) of less than 1 ton is used to obtain an electrical output of about 10 kWe with a thermoelectric conversion efficiency of 10% (in series arrangement) (about 90 kWth of the difference is exhausted to the outside of the system by the heat sink), and the nuclear power generation device shown in FIG. 1 is configured.

[0059] Specifically, first, as a spontaneous fission neutron source, 1000 g of 250 Cm processed into a sphere is placed at the center of the reactor core.

[0060] Next, the outside of the spontaneous fission neutron source ( 250 Cm) is covered with a mixture of enriched uranium (enrichment: about 50%) as nuclear fuel and carbon as a moderator (mixing ratio: U:C = 50:50) (in the case of a sphere, with a radius of about 20 cm).

[0061] Next, further, the surrounding is covered with copper with a thickness of 20 to 30 cm to serve as a partition wall.

[0062] Next, outside the partition wall, as a thermoelectric conversion section, a plurality of thermoelectric conversion elements are stacked and arranged in series. Thereby, a total electrical output of 10 kWe (thermoelectric conversion efficiency: 10%) can be obtained.

[0063] Finally, a heat sink is arranged to discharge 90% of the heat output that has not been converted into electricity (about 100 kWth - about 10 kWe = about 90 kWth) outside the system.

[0064] At this time, the heat dissipation energy E can be expressed by the following formula according to Stefan - Boltzmann's law. E = εσT 4 ε: Emissivity σ: Stefan - Boltzmann constant (5.68×10 -8 W·m - 2 ·K -4 ) T: Temperature (K)

[0065] From the above formula, it can be seen that an appropriate heat sink and size can be determined from the emissivity obtained based on the heat dissipation energy E (about 90 kWth in this embodiment) and the furnace temperature (about 1000 °C in this embodiment). In this embodiment, a stainless - steel plate (SUS304) about 5 m square is arranged as the heat sink.

[0066] In this way, by configuring a nuclear power generation device to directly obtain electric power from the thermal energy generated in a sub - critical nuclear reactor, stable electric power can be supplied without maintenance for a long period of over 100 years even in an environment where people cannot work.

Industrial Applicability

[0067] Different from conventional nuclear power generation devices, the nuclear power generation device according to the present invention is provided without moving parts, is small in size, and can obtain high output for a long time. Therefore, it can be preferably used as a maintenance - free power source for deep - space exploration on a scale exceeding 100 years. Also, it can be preferably used as a maintenance - free power source in environments where maintenance cannot be easily performed, such as exploration and various measurements in the deep sea, high mountains, polar regions, lunar surface, outer space, uninhabited islands, etc.

[0068] As described above, the present invention has been explained based on the embodiments. However, the present invention is not limited to the above embodiments. Various modifications can be made to the above embodiments within the same and equivalent scope as the present invention.

Explanation of Reference Numerals

[0069] 1 Power generation device (nuclear power generation device) 2 Nuclear reactor 3 Thermoelectric conversion unit 4 Heat sink 21 Core 22 Neutron source 23 Mixture of nuclear fuel material and moderator 24 Partition wall

Claims

1. A nuclear power generation device that generates electricity using a subcritical nuclear reactor, A subcritical nuclear reactor including a core section including a neutron source, nuclear fuel material, and a moderator, and a partition wall covering the core section; A thermoelectric conversion unit provided on the outer periphery of the subcritical reactor, The core section is composed of a neutron source disposed at the center and a mixture of nuclear fuel material and a moderator disposed around the neutron source, A nuclear power generation system, characterized in that thermal power generated in the reactor core is directly converted into electricity by a thermoelectric conversion element provided in the thermoelectric conversion unit and outputted.

2. 2. The nuclear power plant according to claim 1, wherein the neutron source is a spontaneous fission source.

3. The spontaneous fission source is ２５２ Cf (Californium), ２４０ Pu (plutonium), ２５０ 3. The nuclear power plant according to claim 2, wherein the hydrogen is selected from the group consisting of fluorine, argon, argon (Cm), and argon (Cu).

4. 2. The nuclear power generation apparatus according to claim 1, wherein the neutron source is an α-ray source that generates neutrons by α-rays produced from an (α, n) reaction.

5. 5. The nuclear power plant according to claim 4, wherein the alpha radiation source is an alpha decay nuclide.

6. The alpha decay nuclide is ２３８ Pu (plutonium), ２３９ Pu (plutonium), ２４１ Am (Americium), ２４４ Cm (curium), ２２６ Ra (radium), ２２７ 6. The nuclear power plant according to claim 5, wherein the metal is selected from the group consisting of fluorine, fluorine, fluorine-containing ...

7. 7. A nuclear power plant according to claim 1, wherein the nuclear fuel material is a thermofissile material or a mixture of a thermofissile material and a transuranium element.

8. The thermofissile material may optionally be ２３５ U enriched with U, or ２３９ Pu (plutonium), ２３３ 8. The nuclear power plant according to claim 7, characterized in that the fuel is U (uranium).

9. 6. The nuclear power plant according to claim 1, wherein the moderator is C (carbon) or Be (beryllium).

10. 6. The nuclear power plant according to claim 1, wherein the material constituting the partition wall is any one of Cu (copper), W (tungsten), B (boron), or a combination thereof.

11. 6. The nuclear power plant according to claim 1, wherein a heat sink for radiating the thermal output not used for the electrical conversion to the outside of the system is connected to the subcritical reactor on an outer surface of the partition.

12. 12. The nuclear power plant according to claim 11, wherein the heat sink is made of stainless steel.

13. 6. The nuclear power plant according to claim 1, wherein an effective multiplication factor in the subcritical reactor is 0.99 to 0.

999.

14. 6. The nuclear power generation system according to claim 1, which is a nuclear power generation system for planetary exploration or deep space exploration.

15. 6. The nuclear power generation system according to claim 1, which is a nuclear power generation system used in polar regions, high mountains, deep seas, or uninhabited islands.

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