Criticality prevention method and criticality prevention device

Abstract

To allow for holding neutron absorbers in desired positions, ensuring the health of surrounding structural materials, and protecting fuel debris against falling objects.SOLUTION: A criticality prevention method is provided, comprising configuring a criticality prevention device comprising an elastic container expanded to accommodate a neutron absorber therein in a hermetically sealed condition, and placing the criticality prevention device on top of accumulated fuel debris.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a criticality prevention method and a criticality prevention device. [Background technology]

[0002] For example, Patent Document 1 discloses a neutron absorber that reliably injects neutron absorbing material near fuel debris to reduce reactivity and prevent a criticality accident during fuel debris removal work. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-156283 Summary of the Invention [Problem to be solved by the invention]

[0004] Before the retrieval operation, the fuel debris is cooled by circulating cooling water and is in a subcritical state. However, during the fuel debris retrieval operation, there is a possibility that the shredded fuel debris may mix with the cooling water and become critical. Here, if, for example, multiple granular neutron absorbers are injected into the fuel debris retrieval location to prevent such criticality, there is a risk that the granular neutron absorbers will flow out along with the cooling water and not reach the location of the shredded fuel debris. Therefore, it is desirable to hold the neutron absorbers in the desired position. Furthermore, there is a concern that if the neutron absorbers are exposed to neutrons or gamma rays, corrosive components that affect the integrity of surrounding structural materials may be generated and released into the cooling water.

[0005] On the other hand, if a heavy object on top of the fuel debris falls, it could shred the fuel debris, which could then mix with the cooling water and cause criticality. Therefore, in order to prevent criticality, measures must also be taken to prevent heavy objects from falling.

[0006] The present disclosure is intended to solve the above-mentioned problems, and aims to provide a criticality prevention method and criticality prevention device that can hold neutron absorbing materials in desired positions, ensure the integrity of surrounding structural materials, and protect fuel debris from falling objects. [Means for solving the problem]

[0007] In order to achieve the above-mentioned object, a criticality prevention method according to one aspect of the present disclosure includes the steps of constructing a criticality prevention device in which a neutron absorbing material is sealed and housed inside a stretchable container, and placing the criticality prevention device on top of a pile of fuel debris.

[0008] In order to achieve the above-mentioned object, a criticality prevention device according to one aspect of the present disclosure includes an expandable container and a neutron absorbing material contained inside the container, which is stretched and sealed. [Effects of the Invention]

[0009] The present disclosure can maintain neutron absorbing material in a desired position while ensuring the integrity of surrounding structural materials and protecting fuel debris from falling objects. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a configuration diagram showing a criticality prevention device according to an embodiment. [Figure 2] FIG. 2 is a flowchart of the criticality prevention method according to the embodiment. [Figure 3] FIG. 3 is a process diagram of the criticality prevention method according to the embodiment. [Figure 4] FIG. 4 is a process diagram of the criticality prevention method according to the embodiment. [Figure 5] FIG. 5 is a process diagram of the criticality prevention method according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. However, the present disclosure is not limited to these embodiments. Furthermore, the components in the following embodiments include those that are easily replaceable by those skilled in the art, or those that are substantially identical.

[0012] In a nuclear power plant, when the reactor core or other components inside the reactor vessel melt, molten material such as molten fuel accumulates at the bottom of the reactor vessel, or the reactor vessel itself melts and falls onto the concrete mat. In this case, the reactor containment vessel is cooled by supplying cooling water to the inside, and the molten material cools and solidifies into fuel debris. The fuel debris is removed and recovered from the reactor vessel. The criticality prevention method and criticality prevention device of this embodiment prevent criticality when removing fuel debris.

[0013] As shown in FIG. 1, the criticality prevention device 1 of the embodiment includes a vessel 2 and a neutron absorbing material 3 housed in the vessel 2.

[0014] The container 2 is made of a material such as rubber, plastic, vinyl, or polyester, and has the properties of plasticity, thermoplasticity, flexibility, and airtightness. The container 2 is formed, for example, in a bag shape. Furthermore, the container 2 is preferably made of a material with a melting point of approximately 100°C to 300°C so that it can burst if the heat generated by the criticality of the fuel debris 10 exceeds a predetermined temperature.

[0015] The neutron absorber 3 is configured as a plurality of small spheres. Small spheres refer to granular bodies with a diameter of about 1 mm. The neutron absorber 3 configured as small spheres is formed, for example, from boron carbide or gadolinia. The neutron absorber 3 can also be configured as a liquid. The neutron absorber 3 configured as a liquid is made of, for example, high-concentration sodium pentaborate water. The neutron absorber 3 can also be configured as a gel. The neutron absorber 3 configured as a gel is a material form intermediate between the solid spheres described above and a liquid, and has both permeability and resistance to flow.

[0016] The neutron absorber 3 contained in the container 2 can be a mixture of multiple small spheres, a liquid, or a gel. The neutron absorber 3 contained in the container 2 can also be a mixture of at least two of multiple small spheres, a liquid, and a gel. The neutron absorber 3 contained in the container 2 can also be a mixture of multiple types of neutron absorber 3 with different neutron absorption properties.

[0017] The criticality prevention device 1 of the embodiment is configured by hermetically storing a neutron absorber 3 inside the vessel 2 in a manner that the vessel 2 is tensioned. When the vessel 2 is tensioned, a gas may be stored in the vessel 2 together with the neutron absorber 3.

[0018] As shown in FIG. 2, the criticality prevention method of the embodiment includes steps S1 to S11.

[0019] In the criticality prevention method, in step S1, the neutron absorbing material 3 is sealed and housed inside the expandable container 2 in a manner that the container 2 is stretched. That is, the criticality prevention device 1 described above is configured.

[0020] In step S2, the criticality prevention method places a criticality prevention device 1 on top of the pile of fuel debris 10. The criticality prevention device 1 has a neutron absorbing material 3 sealed in a container 2. Therefore, in the criticality prevention method, the neutron absorbing material 3 can prevent the generation of corrosive components that affect the integrity of surrounding structural materials, thereby ensuring the integrity of the surrounding structural materials.

[0021] In the criticality prevention method, if in step S3 a falling object 50 falls from above the fuel debris 10 as shown in Fig. 3 (step S3: Yes), and if in step S4 the load of the falling object 50 on the criticality prevention device 1 is less than a predetermined load (step S4: Yes), in step S5 the container 2 of the criticality prevention device 1 absorbs the impact of the falling object 50. Therefore, in the criticality prevention method, the criticality prevention device 1 can reduce the impact of the falling object 50 on the fuel debris 10, and the fuel debris 10 can be protected.

[0022] Furthermore, in the criticality prevention method, if a falling object 50 falls from above the fuel debris 10 in step S3 (step S3: Yes), and if the load of the falling object 50 on the criticality prevention device 1 is equal to or greater than a predetermined load in step S4 (step S4: No), in step S6, the container 2 of the criticality prevention device 1 ruptures, releasing the neutron absorbing material 3, as shown in Fig. 4. Therefore, even if the falling object 50 shreds the fuel debris 10, the criticality prevention method can suppress the criticality of the fuel debris 10 by the neutron absorbing material 3 released from the criticality prevention device 1.

[0023] In the criticality prevention method, if no falling object 50 has fallen from above the fuel debris 10 in step S3 (step S3: No), or after steps S5 and S6, the fuel debris 10 is removed in step S7.

[0024] 5, in the criticality prevention method, when the fuel debris 10 is removed or fragmented by falling objects 50, and the heat generated by criticality exceeds a predetermined temperature in step S8 (step S8: Yes), the container 2 of the criticality prevention device 1 ruptures in step S9, releasing the neutron absorber 3. Therefore, in the criticality prevention method, the neutron absorber 3 is released from the criticality prevention device 1 due to the heat generated by criticality caused by fragmentation of the fuel debris 10 during removal, quickly ending the criticality of the fuel debris 10 and transitioning it to a subcritical state.

[0025] Furthermore, in the criticality prevention method, if the heat generated by criticality is below a predetermined temperature in step S8 (step S8: No), or if, after step S9, removal of the fuel debris 10 is completed in step S10 (step S10: Yes), the criticality prevention method recovers the criticality prevention device 1 in step S11. If removal of the fuel debris 10 is not completed in step S10 (step S10: No), the criticality prevention method waits for removal of the fuel debris 10 to be completed.

[0026] As described above, the criticality prevention method of the embodiment includes the steps of constructing a criticality prevention device 1 in which a neutron absorbing material 3 is sealed and housed inside by stretching an expandable container 2, and placing the criticality prevention device 1 on top of a pile of fuel debris 10.

[0027] According to this criticality prevention method, the criticality prevention device 1, which has neutron absorbers 3 sealed and housed inside the vessel 2, is placed above the pile of fuel debris 10, so that the neutron absorbers 3 can be held in a desired position without the neutron absorbers 3 being washed away by the cooling water. Furthermore, according to this criticality prevention method, the criticality prevention device 1, which has neutron absorbers 3 sealed and housed inside the vessel 2, is placed above the pile of fuel debris 10, so that the generation of corrosive components from the neutron absorbers 3 into the surrounding area can be prevented, and the integrity of the surrounding structural materials can be ensured. Furthermore, according to this criticality prevention method, the criticality prevention device 1, which has neutron absorbers 3 sealed and housed inside the vessel 2, is placed above the pile of fuel debris 10 in a manner that stretches the expandable vessel 2, so that even if a falling object 50 occurs, the impact can be absorbed to protect the fuel debris 10. Moreover, according to this criticality prevention method, the criticality prevention device 1, which contains a neutron absorbing material 3 sealed inside a vessel 2, is placed above the pile of fuel debris 10, so that the criticality prevention device 1 can be visually confirmed and managed using a camera or the like.

[0028] In addition, the criticality prevention method of the embodiment further includes a step in which, when a load greater than a predetermined level is applied to the criticality prevention device 1 by a falling object 50 from above the fuel debris 10, the container 2 of the criticality prevention device 1 ruptures and releases the neutron absorbing material 3.

[0029] According to this criticality prevention method, if a load that shreds the fuel debris 10 is applied by a falling object 50, the load will cause the vessel 2 of the criticality prevention device 1 to rupture and release the neutron absorber 3, thereby preventing a situation in which the shredded fuel debris 10 mixes with the cooling water and causes criticality. Moreover, according to this criticality prevention method, the load will cause the vessel 2 of the criticality prevention device 1 to rupture and release the neutron absorber 3, so it is possible to passively detect the falling object 50 and prevent criticality without requiring any control or human operation.

[0030] In addition, the criticality prevention method of the embodiment further includes a step of rupturing the container 2 of the criticality prevention device 1 and releasing the neutron absorbing material 3 when heat of a predetermined temperature or higher is applied to the criticality prevention device 1 due to heat generated by the criticality caused by the fragmentation of the fuel debris 10.

[0031] According to this criticality prevention method, even if criticality occurs due to the removal of fuel debris 10 or the fuel debris 10 being shredded by falling objects 50 and the shredded fuel debris 10 mixing with cooling water, the criticality can be terminated because the heat generated by criticality causes the vessel 2 of the criticality prevention device 1 to rupture and release the neutron absorber 3. Moreover, according to this criticality prevention method, the heat generated by criticality causes the vessel 2 of the criticality prevention device 1 to rupture and release the neutron absorber 3, so criticality can be detected and terminated passively without the need for control or human operation.

[0032] Moreover, the criticality prevention method of the embodiment further includes a step of recovering the criticality prevention device 1 after the fuel debris 10 is removed.

[0033] According to this criticality prevention method, the criticality prevention device 1 is recovered after the fuel debris 10 is removed, so it is possible to prevent the neutron absorbing material 3 from remaining behind.

[0034] The criticality prevention device 1 of the embodiment includes a flexible container 2 and a neutron absorbing material 3 housed inside the container 2 that has been stretched and sealed.

[0035] According to this criticality prevention device 1, by sealingly accommodating the neutron absorber 3 inside the vessel 2, when the criticality prevention device 1 is placed on top of the accumulation of fuel debris 10, the neutron absorber 3 can be held in a desired position without the neutron absorber 3 being washed away by the cooling water. Furthermore, according to this criticality prevention device 1, by sealingly accommodating the neutron absorber 3 inside the vessel 2, when the criticality prevention device 1 is placed on top of the accumulation of fuel debris 10, it is possible to prevent the generation of corrosive components from the neutron absorber 3 into the surroundings, thereby ensuring the integrity of the surrounding structural materials. Furthermore, according to this criticality prevention device 1, by sealingly accommodating the neutron absorber 3 inside the expandable vessel 2 in a manner that causes it to be stretched, when the criticality prevention device 1 is placed on top of the accumulation of fuel debris 10, it is possible to absorb the impact of a falling object 50 and protect the fuel debris 10. Moreover, according to this criticality prevention device 1, since the neutron absorbing material 3 is housed inside the vessel 2, the state of the fuel debris 10 placed on top of the pile can be visually confirmed and managed using a camera or the like.

[0036] In the criticality prevention device 1 of the embodiment, the vessel 2 bursts when a load equal to or greater than a predetermined value is applied.

[0037] According to this criticality prevention device 1, if a load that shreds the fuel debris 10 is applied by a falling object 50, the load will cause the vessel 2 to rupture, releasing the neutron absorbing material 3, thereby preventing a situation in which the shredded fuel debris 10 mixes with the cooling water and causes criticality. Moreover, according to this criticality prevention device 1, because the load causes the vessel 2 to rupture, releasing the neutron absorbing material 3, it is possible to passively detect the falling object 50 and prevent criticality without requiring any control or human operation.

[0038] In the criticality prevention device 1 of the embodiment, the container 2 bursts when heat equal to or greater than a predetermined level is applied.

[0039] According to this criticality prevention device 1, even if criticality occurs due to the removal of fuel debris 10 or the fuel debris 10 being shredded by falling objects 50 and mixing of the shredded fuel debris 10 with cooling water, the criticality can be terminated because the heat generated by criticality causes the vessel 2 to rupture and release the neutron absorber 3. Moreover, according to this criticality prevention device 1, the heat generated by criticality causes the vessel 2 to rupture and release the neutron absorber 3, so criticality can be detected and terminated passively without the need for control or human operation.

[0040] In the criticality prevention device 1 of the embodiment, the neutron absorbing material 3 includes a plurality of granular bodies.

[0041] According to this criticality prevention device 1, the neutron absorbing material 3 is made into a plurality of granular bodies, so that the neutron absorbing material 3 can penetrate into the cracks where the fuel debris 10 has been shredded and be retained therein, thereby preventing criticality.

[0042] In the criticality prevention device 1 of the embodiment, the neutron absorbing material 3 includes a liquid.

[0043] According to this criticality prevention device 1, by using a liquid neutron absorbing material 3, the neutron absorbing material 3 can be inserted into cracks created by shredding the fuel debris 10, thereby suppressing criticality. Moreover, according to this criticality prevention device 1, by using a liquid neutron absorbing material 3, it is possible to prevent the neutron absorbing material 3 from clogging the cooling water filter.

[0044] In the criticality prevention device 1 according to the embodiment, the neutron absorbing material 3 includes a gel.

[0045] According to this criticality prevention device 1, by making the neutron absorber 3 a gel, the neutron absorber 3 can be inserted into cracks created by shredding the fuel debris 10, thereby preventing criticality. Moreover, according to this criticality prevention device 1, by making the neutron absorber 3 a gel, flow is suppressed, and the neutron absorber 3 can be held in a desired position without being washed away by the cooling water.

[0046] In the criticality prevention device 1 of the embodiment, the neutron absorbing material 3 includes a combination of a plurality of granular materials and a liquid.

[0047] This criticality prevention device 1 provides the advantages of both granular and liquid materials.

[0048] In the criticality prevention device 1 of the embodiment, the neutron absorber 3 includes a combination of multiple types of materials with different neutron absorption characteristics.

[0049] According to this criticality prevention device 1, the neutron absorbing material 3 is a combination of a plurality of types with different neutron absorption characteristics, so that various measures against criticality are possible.

[0050] The present disclosure includes the following inventions. [Invention 1] A step of constructing a criticality prevention device in which a neutron absorbing material is sealed and housed inside a stretchable container in a manner that stretches the container; placing the criticality prevention device above the pile of fuel debris; A method for preventing criticality, comprising: [Invention 2] The criticality prevention method according to Invention 1, further comprising the step of rupturing the container of the criticality prevention device to release the neutron absorbing material when a load equal to or greater than a predetermined value is applied to the criticality prevention device by something falling from above the fuel debris. [Invention 3] The criticality prevention method according to Invention 1 or 2, further comprising the step of rupturing the container of the criticality prevention device to release the neutron absorber when heat of a predetermined temperature or more is applied to the criticality prevention device due to heat generated by the criticality caused by fragmentation of the fuel debris. [Invention 4] 4. The criticality prevention method according to any one of Inventions 1 to 3, further comprising the step of recovering the criticality prevention device after removing the fuel debris. [Invention 5] A container having elasticity; a neutron absorbing material contained inside the container, the container being tensioned and sealed; A criticality prevention device comprising: [Invention 6] 6. The criticality prevention device according to claim 5, wherein the container bursts when a load equal to or greater than a predetermined value is applied thereto. [Invention 7] 7. The criticality prevention device according to claim 5 or 6, wherein the container bursts when a predetermined amount of heat or more is applied thereto. [Invention 8] 8. The criticality prevention device according to any one of claims 5 to 7, wherein the neutron absorbing material includes a plurality of granular bodies. [Invention 9] 8. The criticality prevention device according to any one of claims 5 to 7, wherein the neutron absorbing material includes a liquid. [Invention 10] 8. The criticality prevention device according to any one of claims 5 to 7, wherein the neutron absorbing material includes a gel. [Invention 11] The criticality prevention device according to any one of inventions 5 to 7, wherein the neutron absorbing material includes a combination of a plurality of granular materials and a liquid. [Invention 12] 12. The criticality prevention device according to any one of claims 5 to 11, wherein the neutron absorbing material includes a combination of a plurality of types having different neutron absorption properties. [Explanation of symbols]

[0051] 1 Criticality prevention device 2 containers 3. Neutron absorbers 10 Fuel debris 50 Falling Objects

Claims

1. A step of constructing a criticality prevention device in which a neutron absorbing material is sealed and housed inside a stretchable container in a manner that stretches the container; placing the criticality prevention device above the pile of fuel debris; recovering the criticality prevention device after removing the fuel debris; A method for preventing criticality, comprising:

2. 2. The criticality prevention method according to claim 1, further comprising the step of rupturing the container of the criticality prevention device to release the neutron absorbing material when a load equal to or greater than a predetermined value is applied to the criticality prevention device by something falling from above the fuel debris.

3. 2. The criticality prevention method according to claim 1, further comprising the step of, when heat of a predetermined temperature or more is applied to the criticality prevention device due to heat generated by criticality caused by fragmentation of the fuel debris, rupturing the container of the criticality prevention device to release the neutron absorbing material.

4. A container having elasticity; a neutron absorbing material contained inside the container, the container being tensioned and sealed; Including, The criticality prevention device, wherein the neutron absorbing material includes a liquid.

5. A container having elasticity; a neutron absorbing material contained inside the container, the container being tensioned and sealed; Including, The criticality prevention device, wherein the neutron absorbing material includes a gel.

6. A container having elasticity; a neutron absorbing material contained inside the container, the container being tensioned and sealed; Including, The criticality prevention device, wherein the neutron absorbing material includes a combination of a plurality of granular materials and a liquid.

7. 7. The criticality prevention device according to claim 4, wherein the container bursts when a load equal to or greater than a predetermined value is applied to the container.

8. 7. The criticality prevention device according to claim 4, wherein the container bursts when a predetermined amount of heat or more is applied to the container.

9. The criticality prevention device according to claim 4 , wherein the neutron absorbing material includes a combination of a plurality of types having different neutron absorption characteristics.

10. A container having elasticity; a neutron absorbing material contained inside the container, the container being tensioned and sealed; Including, The neutron absorber includes a combination of multiple types of neutron absorbers having different neutron absorption properties, A criticality prevention device that causes the container to burst when a load greater than a predetermined level is applied by something falling from above the fuel debris, or when heat greater than a predetermined temperature is applied due to heat generated by criticality caused by the fragmentation of the fuel debris.

11. The criticality prevention device according to claim 10 , wherein the neutron absorbing material includes a plurality of granules.

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