JPH08146175A - Subcritical nuclear reactor - Google Patents

Abstract

PURPOSE: To always put a reactor core in a subcritical state so as to have high safety by heightening the use efficiency of natural uranium as an energy source. CONSTITUTION: A subcritical nuclear reactor is formed by the combination of an accelerator 10 for producing proton or deuteron, and a subcritical reactor core 12. The subcritical reactor core includes a target substance 14 for generating neutron by proton or deuteron supplied from the accelerator, a reactor core part 18 which has solid nuclear fuel and generates heat by fission reaction using neutron generated from the target substance, and a neutron reflector 20 surrounding the above members. Pin type fuel or particle fuel is used as the solid nuclear fuel. In the case of the pin type fuel, it is better to use liquid sodium as a coolant, and in the case of particle fuel, it is better to use helium gas as a coolant. Natural uranium or plutonium enriched fuel is used as nuclear fuel.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention combines an accelerator and a subcritical core, controls the generation of neutrons in the reactor by an accelerator installed outside, and uses the generated neutrons to cause a fission reaction in the reactor. By doing so, the present invention relates to a nuclear reactor that safely burns the fuel loaded in the core.

[0002]

2. Description of the Related Art A nuclear power generation system is a system for obtaining electric power by using uranium or the like as a nuclear fuel and converting the energy released by the fission reaction into electric energy. A fission reaction is a reaction that occurs when neutrons are absorbed by nuclear fuel, and conventional nuclear reactors use neutrons that are generated at the same time as energy during a fission reaction as neutrons for causing this reaction. In other words, a stable energy is always obtained by stably causing a process of obtaining neutrons necessary for fission reaction from fission reaction (this is called fission chain reaction). The state where this fission chain reaction occurs stably (the number of fission is constant over time) is called critical.

In order for a nuclear reactor to become critical, a certain amount of fissile material is required in the core. On the other hand, if the amount is less than this fixed amount, the fission chain reaction does not occur and the energy cannot be extracted. A conventional nuclear reactor is a device that can control the fission reaction so that a neutron absorber such as a control rod can maintain a critical state by loading a larger amount of fissile material than that required for criticality into the core. Yes,
This is a system that makes it possible to extract a certain amount of energy with respect to time.

[0004]

As described above, when the nuclear reactor is in a subcritical state, no fission chain reaction occurs and energy cannot be taken out. Therefore, it is necessary to completely burn nuclear fuel like oil and coal. I can't. In order to secure a “fixed amount” of fissile material, for example, when using uranium as a fuel in a light water reactor, a process called enrichment is necessary, and in the operation of a nuclear reactor, periodical replenishment of fissile material is necessary. Refueling is required.

Further, in the operation of the conventional nuclear reactor, since it is necessary to continuously take out energy for a certain period of time (the above-mentioned refueling work is uneconomical because the reactor must be stopped), the supercriticality is set to some extent. The design is different, that is, the control rod is used to suppress the supercriticality. This is one of the factors that reduce safety. That is, if the control rods or the like are lost from the core, the safety of the nuclear reactor will be significantly impaired.

In order to maintain the criticality of the nuclear reactor, it is necessary to take out the fuel with the lowered criticality from the nuclear reactor. However, since the extracted fuel contains substances that can be nuclear fuel, the once-through method of discarding this as it is,
The utilization rate of natural uranium resources is low. For example, in the light water reactor once through system, it is about 0.5%. Therefore, in order to improve the utilization efficiency of natural uranium, a process of reprocessing spent fuel is required. This process has the potential to enhance nuclear diffusivity, as it involves the transfer of nuclear material and plutonium alone. Further, the high radioactive level waste generated by the reprocessing contains transuranium elements and fission products (FP) having a long half-life, so it is necessary to fully consider the environmental impact.

An object of the present invention is to provide a nuclear reactor system capable of satisfying the requirements of increasing the utilization efficiency of natural uranium as an energy source, keeping the core in a subcritical state at all times and having high safety. Is.

[0008]

The present invention comprises a combination of an accelerator for producing protons or deuterons and a subcritical core, the subcritical core being a neutron produced by the protons or deuterons supplied from the accelerator. In a sub-critical nuclear reactor comprising a target material that generates, a core that generates heat by a fission reaction using neutrons generated from the target material that has a solid nuclear fuel, and a neutron reflector that surrounds them. is there. Pin-type fuel or particulate fuel is used as the solid nuclear fuel. In the case of pin type fuel,
Liquid sodium is used as the coolant, and in the case of particulate fuel,
Helium gas is preferably used as the coolant. As the nuclear fuel, natural uranium or plutonium-rich fuel is used.

[0009]

[Operation] In the nuclear power generation system, the energy generated by the fission reaction of uranium or the like is used to generate electricity.
Since the fission reaction occurs when neutrons are absorbed by uranium or the like, neutrons are necessary to cause this reaction. A feature of the nuclear reactor of the present invention is that the neutrons necessary for causing a fission reaction are not obtained only by the fission reaction. The protons or deuterons generated in the accelerator collide with the target material to generate neutrons, which contribute to the fission reaction. From this, the nuclear reactor of the present invention does not require the concept of criticality, and the nuclear fuel in the nuclear reactor may be a subcritical amount. Also, no process such as uranium enrichment is required.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is an overall configuration diagram showing an embodiment of a subcritical nuclear reactor according to the present invention, which is an example of using a pin type fuel. This nuclear reactor system includes an accelerator 10 that produces protons or deuterons, and a subcritical core 12 that uses solid fuel. As shown in FIG. 2, the structure of the subcritical core 12 is a target material 1 that produces neutrons by protons or deuterons supplied from the accelerator 10.
4, a core portion 18 including a fuel assembly 16 that generates heat by fission reaction by neutrons generated from the target material 14, and a neutron reflector 20 surrounding the core portion 18.
Have.

As shown in FIG. 3, for example, the fuel assembly 16 includes a large number of pin type fuels 22 and an entrance nozzle 2.
4, a ductless structure (a structure without a trumpet tube) supported by the grid 26 and the handling head 28 is used to avoid waste of neutrons due to the structural material as much as possible.

As shown in FIG. 1, the primary sodium of the coolant heated by the nuclear fission reaction in the subcritical core 12 exchanges heat with the secondary sodium through the intermediate heat exchanger 32.
After cooling, the primary circulation pump 34 returns to the subcritical core 12. The secondary sodium heated in the intermediate heat exchanger 32 is
It exchanges heat with water through the steam generator 36, is cooled, and returns to the intermediate heat exchanger 32 by the secondary circulation pump 38. The steam heated by the steam generator 36 turns the generator 42 by the turbine 40, becomes water, and returns to the steam generator 36 by the pump 44.

Neutrons for causing the fission reaction are specifically obtained by a combination of the accelerator 10 and the target material 14 such as uranium, bismuth, lead or tungsten. That is, protons or deuterons are produced by the proton synchrotron or linac accelerator. Then, these protons or deuterons are guided to a target material 14 arranged in the core through a proton conduit and irradiated, and neutrons are generated by a nuclear fragmentation reaction. The reason why neutrons are generated by introducing protons or deuterons into the core is that neutrons cannot be directly introduced into the core by a magnetic method because they have no electric charge.

By irradiating the nuclear fuel in the core with neutrons generated from the target material 14, the parent material (U-238
Etc.) into fissionable substances (Pu-239 etc.) or cause fission reaction. In this way, in principle, all nuclear fuel in the reactor can undergo a nuclear reaction, and natural uranium utilization efficiency of almost 100% can be achieved.

FIG. 4 is an overall configuration diagram showing another embodiment of the subcritical nuclear reactor according to the present invention, which is an example in the case of using particulate fuel. This nuclear reactor system includes an accelerator 50 that produces protons or deuterons, and a subcritical core 52 that uses solid fuel. The configuration of the subcritical core 52 is
As shown in FIG. 5, the target material 54 that generates neutrons by the protons or deuterons supplied from the accelerator
And a core portion 58 having a fuel assembly 56 that generates heat by fission reaction by neutrons generated from the target material 54, and a neutron reflector 60 surrounding the core portion 58.

For example, as shown in FIG. 6, the fuel assembly 56 has a structure in which a cylindrical basket 64 is filled with a particulate fuel 62 in which carbon is made to contain a nuclear fuel substance and is in the form of particles. Try to avoid eating as much as possible. The coolant is allowed to freely flow in the vicinity of the particulate fuel 62 through the holes 65 provided in the basket 64.

As shown in FIG. 4, the helium gas, which is the coolant heated in the subcritical core 52 by the nuclear fission reaction, exchanges heat with the air through the heat exchanger 72, is cooled, and is circulated by the circulation pump 74. Then, the process returns to the subcritical core 52. Heat exchanger 62
The air heated in 1 rotates the generator 78 in the turbine 76, is cooled, and returns to the heat exchanger 72 in the pump 79.

Since the structure of the accelerator 50 may be the same as that of the above-mentioned embodiment, the description thereof will be omitted.
The advantage of using particulate fuel is that only particulate fuel can be extracted from the fuel assembly. The nuclear reactor of the present invention operates for an extremely long period of time. Therefore, it is expected that the structural material of the reactor will deteriorate due to the irradiation of radiation. If the nuclear fuel is to be used over the service life of the structural material, it is necessary to take out the fuel. Particulate fuel can only be transferred to a new reactor system for the reasons given above, ensuring long-term sound operation.

FIG. 7 is a 1 / 2RZ sectional view of the subcritical core used in the simulation. The described core dimensions are based on the prototype reactor class with an output of about 400 MW. Figure 7
As shown in, the subcritical core 82 is a target material 8 that generates neutrons by protons or deuterons from an accelerator.
4, a core portion 88 having a solid nuclear fuel that generates heat by a nuclear fission reaction composed of nuclear fuel, and a target material 84
And a neutron reflector 90 such as stainless steel surrounding the core portion 88. The target material in the center has a cylindrical shape, and the target material in the periphery has a cylindrical shape. The neutron reflector 90 returns neutrons to the core portion 88 to reduce the burden on the accelerator and also functions as a shield.

In the core size shown in FIG. 7, the reactor power 40
FIG. 8 shows the secular change in the neutron source intensity required to obtain 0 MW (constant). In order to obtain a constant output, a relatively high neutron source intensity is required at the start of operation, but thereafter, the neutron source intensity may be about 10 ¹³ to 10 ¹⁴ n / cm ³ / sec. However, a considerably high neutron source intensity is required at the end of operation. This is because extraneous neutrons are needed because neutrons are absorbed by fission products generated during reactor operation. For reference, the neutron source intensity required when the fission product (FP) generated during operation is removed is also shown.

In the core shown in FIG. 7, it is assumed that the volume of the target material for supplying neutrons from the outside is about 5 × 10 ⁵ cm ³ . Using the known data that when one 1.5 GeV proton collides with a target material (lead / bismuth), about 40 neutrons are generated, the number of protons required to obtain the above neutron source intensity is used. Is 2.5 × 10 ¹² ~
It becomes 10 ¹³ n / cm ³ / sec. From these facts, the current value required for the accelerator is (5 × 10 ⁵ ) × (2.5 × 10 5
^{12 ~10 13) × (1.6 ×} 10 -19) = 200~200
It becomes 0 mA. A current value of several hundred mA can be sufficiently realized. Regarding the upper limit, it is considered that it is difficult to realize at present, but in such a case, when one proton is collided with the target by increasing the energy of the proton or increasing the target density. The problem can be solved by increasing the neutron generation amount or decreasing the reactor power.

As described above, in the present invention, the number of neutrons generated from the target material 84 is controlled and the output of the nuclear reactor is controlled by controlling the concentration of accelerating particles from the accelerator installed outside the nuclear reactor. . That is, the intensity of the external neutron source is controlled by the current value of the accelerator. Therefore, if the operation of the accelerator is stopped for some reason, neutrons are not supplied and the operation of the reactor is naturally stopped.

In FIG. 8, in the nuclear reactor, the nuclear fuel material is concentrated in the initial stage of operation (less than 5 to 10 years), and thereafter, reprocessing and fuel processing are performed, and the final stage (60 to 65 years)
It can be seen that waste treatment is being carried out after the year. At the end of operation, if the reactor output is reduced because it is a waste treatment, the neutron source strength may be small. If plutonium-enriched fuel is used, the neutron source strength at the initial stage of operation can be kept low. Therefore, any surplus plutonium produced in a reactor system other than the reactor of the present invention can be utilized.

FIGS. 9 and 10 show the criticality and the secular change of the fuel material when neutrons having the neutron source intensity shown in FIG. 8 are supplied to the core (only natural uranium is used as the fuel) constructed as described above. Shown respectively. As can be seen from FIG. 9, the core is always in a subcritical state (effective multiplication factor is less than 1) during the combustion period. As shown in FIG. 10, 1
U-238 loaded with 0000 kg will decrease to about 30 kg after 70 years, and can burn out almost completely (up to 99.7%). In other words, if it can be operated for about 70 years, the utilization efficiency of natural uranium will be almost 100%.

By the way, it is preferable that the nuclear reactor of the present invention continues to operate for a long period of several tens of years. Therefore, the core structural material will receive a large amount of neutron irradiation, and it is expected that swelling and strength deterioration due to irradiation damage will occur. Therefore, it is necessary to develop and use core structure materials with excellent durability, and it is also necessary to consider that a backup system that allows easy replacement can be taken. To do this, for example, reactor power and core size (fuel inventory)
It is conceivable to match the number of operating days with the life of plant equipment by properly designing. In this case, the reactor can be decommissioned when the operation ends. Alternatively, a reactor vessel provided with a target material and a neutron reflector, and a plurality of cooling systems are installed, before the life of the structural material and plant equipment of the reactor system in use expires, and It is necessary to take measures such as refilling another new basket with the particulate fuel, introducing it into the reactor vessel, and continuing operation.

[0026]

INDUSTRIAL APPLICABILITY As described above, the present invention does not obtain the neutrons necessary for the fission reaction from the fission reaction, but emits neutrons by irradiating the target material in the core with protons or deuterons introduced from the outside. Since it is configured to cause a fission reaction, the following effects can be obtained. Natural uranium can be used as nuclear fuel. The fissile material undergoes fission as it is, and the parent material absorbs neutrons and transforms into fissile material to undergo fission. Therefore, if it is operated for a long period of time, the nuclear fuel can be burned almost completely in the nuclear reactor, and it is possible to achieve a natural uranium utilization efficiency of almost 100%. The enrichment process is not required and the nuclear fuel cycle can be simplified. Also, if the operation is continued for a long period of time, the reprocessing process is unnecessary. In that case, because it does not involve the transfer of nuclear material or the single treatment of plutonium,
Nuclear diffusion resistance can be improved. Since the nuclear reactor is always operated in a subcritical state, there is no danger of nuclear runaway and safety is significantly improved.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram showing an embodiment of a subcritical reactor system according to the present invention.

FIG. 2 is an explanatory diagram showing an example of the core configuration.

FIG. 3 is an explanatory diagram showing an example of a fuel assembly used therein.

FIG. 4 is an overall configuration diagram showing another embodiment of the subcritical reactor system according to the present invention.

FIG. 5 is an explanatory diagram showing an example of the core configuration.

FIG. 6 is an explanatory view showing an example of a fuel assembly used therein.

FIG. 7 is a cross-sectional view of the nuclear reactor used for the simulation.

FIG. 8 is a diagram showing a secular change in neutron source intensity with a constant reactor output.

FIG. 9 is a diagram showing a change over time in criticality at a constant reactor output.

FIG. 10 is a diagram showing a secular change in the weight of fuel nuclides in the reactor at a constant reactor output.

[Explanation of symbols] 10 accelerator 12 subcritical core 14 target material 16 fuel assembly 18 core part 20 neutron reflector 32 intermediate heat exchanger 36 steam generator 40 turbine

Claims (5)

[Claims] 1. A combination of an accelerator for producing protons or deuterons and a subcritical core, the subcritical core comprising a target material for generating neutrons by the protons or deuterons supplied from the accelerator, and a solid. A core part that generates heat by a fission reaction using neutrons generated from the target material with nuclear fuel,
A subcritical nuclear reactor comprising a neutron reflector surrounding them. 2. A pin type fuel is used as the solid nuclear fuel,
The subcritical reactor according to claim 1, wherein a liquid metal is used as the coolant. 3. The subcritical reactor according to claim 1, wherein a particulate fuel is used as the solid nuclear fuel, and a gas is used as the coolant. 4. The use of natural uranium as a nuclear fuel.
The subcritical nuclear reactor according to any one of claims 1 to 3. 5. The subcritical nuclear reactor according to claim 1, wherein plutonium-enriched fuel is used as the nuclear fuel.