JP2869445B2 - Concrete cask - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cask for storing and managing spent nuclear fuel generated by the operation of a nuclear power plant.

[0002]

BACKGROUND OF THE INVENTION Spent nuclear fuel from nuclear power plants is either reprocessed or requires proper storage and management. The storage method includes a wet method using a storage pool or the like and a dry method using a cask or the like. If the size of the storage is small, it is economically advantageous to use a cask. As such, the United States uses casks to store spent nuclear fuel in nuclear power plants.

A variety of cask structures have been proposed, including a concrete cask.
Concrete is excellent as a neutron shielding material, it is cheaper than other shielding materials, and it can obtain the necessary strength as a structure,
Concrete casks have many advantages.

[0004]

On the other hand, concrete is susceptible to heat. The spent nuclear fuel stored in the cask is at a high temperature, and when the heat is transferred to the concrete and the local temperature exceeds the limit temperature (about 90 ° C), the local strength of the concrete decreases, causing cracks and other causes. Become.

[0005] Concrete generally has low heat transfer characteristics, so a concrete cask requires a heat removal system, including a ventilation type as shown in FIG. 5 and a heat pipe type as shown in FIG. is there. In the former, an air flow path is provided between a sealed container (MSB) 71 having a plurality of storage holes therein for sealing used nuclear fuel and a concrete cask 72 covering the outer periphery thereof, and an air inlet 73 provided at a lower portion is provided. An air outlet 74 provided at an upper portion cools the air by natural convection.

[0006] In the latter, a sealed basket for storing spent nuclear fuel is directly stored in a concrete cask having a metal liner such as stainless steel on the inner peripheral surface. A plurality of heat pipes are embedded in a concrete layer so as to be in contact with the liner. Thereby, the heat of the nuclear fuel is released from the liner to the outside of the cask mainly through the heat pipe.

[0007] However, since the ventilation type is provided with a large number of air inlets / outlets, it is difficult to form a multiple sealed structure. Further, in the heat pipe type, as shown in FIG. 4, a non-uniform temperature distribution occurs along the liner circumferential direction. May reach 140 ° C. To suppress such a rise in the local temperature, it is conceivable to arrange a large number of heat pipes densely, but in this case, the cost required for the cask is unavoidably increased.

It is an object of the present invention to provide a concrete cask provided with a heat removal system using a heat pipe, capable of suppressing a rise in a local temperature of a concrete layer.

Another object of the present invention is to provide a concrete cask capable of preventing an increase in local temperature of a concrete layer and a decrease in strength due to the increase in the local temperature without increasing the number of heat pipes.

[0010]

According to the first aspect of the present invention, there is provided a concrete cask in which a metal liner is provided on an inner periphery of a concrete layer forming an outer shell of a storage section for nuclear fuel. A heat insulating material layer interposed between the metal liner and the concrete layer, a heat receiving portion contacting the liner, and a heat radiating portion embedded in the heat insulating material layer so as to contact a cooling portion outside the cask. And a heat pipe.

Further, in the invention described in claim 2, in the above structure, the heat insulating material layer is made of a heat-resistant resin having a neutron shielding ability.

[0012]

The concrete cask according to the first aspect of the present invention comprises a metal liner, a heat insulating material layer, and a concrete layer which sequentially provide a storage space from the inside. It has a laminated structure in contact with it.

The storage space is a space for storing several tens of spent nuclear fuels, and the outer wall of the storage space has a good thermal conductivity and a high neutron capacity for releasing the heat of the nuclear fuel to the outside. Made of a material that is not damaged by irradiation due to, which may be combined with a liner or separate from the liner,
For example, it is good also as comprising a sealed basket container and having a multiple sealing structure.

The liner faces the inner surface of the concrete layer and is also made of a metal having good thermal conductivity. This liner has a function of diffusing the heat radiated from the storage portion to the entire surface of the liner by conduction.

The heat pipe is fixed to the outer peripheral surface of the liner. The fixing method may be welding such as brazing in addition to screwing. The heat diffused over the entire surface by the liner is released to the outside by the heat pipe.

The space between the liner and the concrete layer is filled with a heat insulating material layer. This heat insulating material layer is for blocking or suppressing the transfer of heat from the liner to the concrete layer.
° C). Therefore, the thickness of the heat insulating material layer should be larger than the mounting dimension (radial direction) of the heat pipe, and be sufficient to bury the heat pipe. In this regard, the material is appropriately selected as described below, but in any case, it is assumed that the material does not deteriorate due to neutron irradiation.

[0017] There is a correlation between the total calorific value of spent nuclear fuel or the amount of heat passing through the heat insulating material layer and the heat transfer capacity of the heat pipe.
° C) must be determined. As the total calorific value of the nuclear fuel increases and the amount of heat passing through the heat insulating material layer decreases, the heat pipe is required to have a higher heat transfer capability.

The amount of heat passing through the heat insulating material layer is determined by its thermal conductivity and the thickness of the layer. That is, the higher the heat conductivity or the thinner the thickness of the heat insulating material layer, the larger the amount of heat passing. Therefore, the amount of heat passing can be determined by appropriately combining these.

The heat insulating material layer needs to have heat resistance to at least the temperature of the liner. The material of the heat insulating material layer is not limited as long as the above conditions are satisfied.

The concrete layer may be ordinary concrete, and need not be special heavy concrete or serpentine concrete. This is advantageous in terms of cask manufacturing costs. The concrete layer forms the shell of the cask, maintains the strength of the cask and shields neutrons from nuclear fuel.

In the concrete cask according to the present invention, the heat pipe is embedded in the heat insulating material layer. The heat pipe includes a heat receiving section, a heat radiating section, and a heat insulating section, and transfers heat from the heat receiving section to the heat radiating section with high efficiency. It is desirable to arrange, for example, a mesh on the inner surface of the heat pipe in order to ensure the wettability of the working fluid in the heat receiving section. The heat transfer amount and operating environment of the heat pipe depend on the dimensions, material,
It is determined by the type of the hydraulic fluid and the like. Therefore, the specifications of these heat pipes need to be selected together with the number and arrangement of the heat pipes in consideration of the total heat generation of the nuclear fuel to be stored.

The heat pipe must be made of a material that can withstand heat and neutrons from nuclear fuel, but is preferably made of a material having a high thermal conductivity in order to improve heat conduction in the heat receiving portion. In order to satisfy this condition, it is conceivable that the surface of the heat pipe has a double structure. That is, the outside of the surface is made of a corrosion-resistant material such as stainless steel or titanium, and the inside is made of a material having high thermal conductivity such as copper or a copper alloy.

The heat pipe needs to have a length connecting the liner and the cooling portion provided outside the cask, but the cross-sectional shape is not particularly limited, and may be a circle, an ellipse, or a polygon.

The heat receiving portion of the heat pipe contacts the liner,
The heat radiating part is in contact with the cooling part. The structure of the cooling unit is determined according to the amount of heat radiation. For example, a heat radiation fin may be used. In this case, a fan that generates forced convection around the heat radiation fin may be provided to increase the amount of heat radiation.

Spent nuclear fuel still generates a high temperature, and the heat transfer is as follows. That is, heat from the nuclear fuel is transmitted from the storage space to the metal liner surrounding the outer periphery by radiation and conduction. The heat transfer from the liner to the concrete layer is small since the heat insulating material layer surrounds the outer periphery of the liner. Accordingly, the temperature of the concrete layer does not become high and the temperature distribution becomes relatively flat in the circumferential direction.

Since the heat receiving portion of the heat pipe is in contact with the liner, heat is efficiently released from the liner to the outside of the cask through the heat pipe.

According to the second aspect of the present invention, the first aspect is provided.
The material of the heat insulating material layer is specified, and a heat-resistant resin having a neutron shielding ability, for example, an epoxy resin is used. Neutrons from the nuclear fuel are shielded by the resin of the thermal insulation layer, and heat transmitted from the liner to the concrete layer is also blocked.

[0028]

FIG. 1 is a perspective view of a concrete cask according to an embodiment of the present invention, in which the structure of the cask is partially cut away. The basic structure of the cask according to this embodiment is based on the structure of a heat pipe type concrete cask having a conventional heat removal system shown in FIG.

The cask 1 is cylindrical and has a laminated structure composed of a storage space 2, a liner 3, a resin layer 4 and a concrete layer 5 in order from the center.
A heat pipe 6 is embedded in the resin layer 4 in parallel with the central axis of the cask. The heat insulating material layer in the claims corresponds to the resin layer 4 in this embodiment.

The storage space 2 provided by the liner 3 has a cylindrical shape with an inner diameter of about 1.5 m to 1.8 m,
In storage, a number of storage basket containers 21 are provided so that the spent nuclear fuels 10 do not come into contact with each other. The storage basket container 21 has good thermal conductivity and releases the heat of the spent nuclear fuel 10 to the outside. In addition, if the storage basket container 21 is configured to be further placed in the sealed basket container 71 in FIG. 5, neutrons can be effectively shielded.

The liner 3 is made of stainless steel and is a good heat conductor. Heat pipe 6 outside liner 3
Is attached. This state will be described with reference to FIG. FIG. 2A shows the layer structure of the concrete cask of FIG. 1 for a quarter portion of the cross section, and the remaining three quarter portions have the same layer structure. Also,
FIG. 2B is an enlarged view of a portion A in FIG.

A total of 16 heat pipes 6 are used.
It is arranged at equal angular intervals on the outer peripheral surface of the liner 3. The heat pipes 6 are screwed to the liner 3 one by one by a fixture 61.

The fixture 61 has a hole 62 slightly larger than the outer diameter of the heat pipe 6, and has flanges 63 on the left and right. The flange 63 has a mounting hole (not shown). The attachment 61 is made of a metal having a good thermal conductivity, and is attached to the liner 3 and the heat pipe 6 in close contact. Therefore, the heat of the liner 3 is efficiently transmitted from the flange 63 to the heat pipe 6.

Here, the description returns to FIG. The heat pipe 6 includes a heat receiving section 62, a heat insulating section 63, and a heat radiating section 64,
The heat receiving section 62 is attached to the fixture 61. Radiator 64
Is attached to the cooling unit 65 outside the cask. The heat insulating portion 63 is a portion where the heat pipe 6 penetrates the concrete layer 5.

For this reason, the heat pipe 6 receives the heat in the heat receiving section 62, carries the heat to the heat radiating section 64 by the action thereof, and transfers the heat to the cooling section 65. In the heat insulating part 63, there is no transfer of heat to the outside, and no heat is transferred to and from the concrete layer 5. Therefore, from the heat pipe 6 to the concrete layer 5
No heat is transferred. In the case where the penetrating portion of the concrete layer 5 becomes the heat radiating portion 64 of the heat pipe 6, there is a method of sandwiching a heat insulating material between the two.

The cooling unit is a radiation fin not shown. When the amount of heat radiation is large, there is a method of forcibly convection the vicinity of the heat radiation fin with a fan or the like.

The specifications of the heat pipe 6 are determined by the total calorific value of the nuclear fuel 1 stored in the cask. For example, when the total power generation of spent nuclear fuel is 20 kW, about 10 heat pipes having an outer diameter of 20 to 30 mm are sufficient for heat removal. The heat pipe 6 has a double structure, in which an outer pipe is made of a corrosion-resistant material such as stainless steel or titanium, and a copper or copper alloy is used as an inner pipe.

The outside of the liner 3 is surrounded by a resin layer 4. A heat pipe 6 is embedded in the resin layer 4,
The resin layer 4 is also formed outside the heat pipe 6. For this reason, the concrete layer 5 outside the resin layer 4 and the heat pipe 6 do not come into contact with each other. Although the thickness of the resin layer 4 is determined by the temperature distribution of the entire cask,
It is about several tens to 200 mm.

In this embodiment, the resin layer 4 is coated with an epoxy resin. Since this resin has a low thermal conductivity, most of the heat of the nuclear fuel 10 transmitted to the liner 3 is released from the heat receiving portion of the heat pipe 6 to the outside, and the amount of heat transmitted from the resin layer 4 to the concrete layer 5 decreases. The resin layer 4 needs to have heat resistance to the temperature of the liner 3, and the epoxy resin satisfies this condition.

FIG. 3 shows the temperature distribution in this state. In FIG. 3, since the resin layer 4 has a thickness of 100 mm, the middle part of the resin layer 4 in contact with the concrete layer 5 is kept uniformly at about 100 ° C., and the outer surface of the resin layer 4 is 90 ° C. It is as follows. In FIG. 4 showing the temperature distribution when the resin layer 4 is not included, the outer surface temperature of the liner is locally 140 ° C., and a comparison between the two shows a change in heat transfer due to the resin layer 4. That is, the local temperature of any part of the concrete layer in FIG. 3 can be suppressed to the allowable temperature (about 90 ° C.) or less.

The epoxy resin serving as the resin layer 4 further has a neutron absorbing ability. The neutrons emitted from the spent nuclear fuel 10 are blocked from leaking by the storage basket 21, the liner 3 and the concrete layer 5, but the neutrons can be further reduced to the concrete layer 5 by the resin layer 4. The burden of neutron shielding can be reduced. For this reason, the temperature rise of the concrete layer 5 due to neutron irradiation and irradiation damage can be eliminated. The concrete layer 5 is made of ordinary concrete.

[0042]

As described above, according to the first aspect of the present invention, the rise in the local temperature of the concrete layer can be suppressed by the heat insulating material layer inside the concrete layer. The lowering can be prevented, and the life of the cask can be extended.

According to the second aspect of the present invention, since the neutrons are absorbed by the heat insulating material layer, the concrete is not damaged by irradiation with neutrons.
In that it can prevent a decrease in concrete strength,
The life of the cask can be further extended.

[Brief description of the drawings]

FIG. 1 is a perspective view of a heat pipe type concrete cask according to an embodiment of the present invention, with a part of the structure cut away.

2A is a transverse sectional view showing a quarter structure of a layer structure of a cask according to the embodiment of FIG. 1, and FIG. 2B is an enlarged sectional view of a portion A in FIG.

FIG. 3 is an explanatory diagram showing an example of a circumferential temperature distribution in a layer structure of a cask according to the embodiment of FIG. 1;

FIG. 4 is an explanatory diagram showing an example of a temperature distribution in a circumferential direction in a layer structure of a cask according to a conventional technique.

FIG. 5 is a partially cutaway perspective view showing the structure of a ventilation-type concrete cask according to the related art.

FIG. 6 is a partially cutaway perspective view showing the structure of a heat pipe type concrete cask according to the related art.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Cask 2 ... Storage part 3 ... Liner 4 ... Resin layer 5 ... Concrete layer 6 ... heat pipe

Claims (2)

(57) [Claims] 1. A concrete cask in which a metal liner is provided on an inner periphery of a concrete layer forming an outer periphery of a storage section for nuclear fuel, and a heat insulating material layer interposed between the metal liner and the concrete layer. A heat pipe embedded in the heat insulating material layer such that a heat receiving portion is in contact with the metal liner and a heat radiating portion is in contact with a cooling portion outside the cask. 2. The concrete cask according to claim 1, wherein the heat insulating material layer is made of a heat-resistant resin having a neutron shielding ability.