JP4786007B2 - Water-based reactor with built-in receptacle - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention is such that the vessel is equipped with a basin-shaped receptacle intended to receive a “corium” or solid or liquid piece derived from the reactor core upon accidental melting of the reactor core. Further, the present invention relates to a pressurized water reactor or a boiling water reactor.
[0002]
[Prior art]
With respect to nuclear reactors, a number of systems have been devised and developed over the last 20-30 years with the aim of limiting serious situations that cause partial or complete melting of the reactor core.
[0003]
In particular, as disclosed in French Patent Application No. 2 341 181 to prevent perforation of the vessel by corium formed at the bottom of the reactor vessel during accidental melting of the reactor core. It has already been proposed to arrange a suppression device. The restraining device comprises a plurality of horizontal plates that are spaced apart from one another. These plates are fixed to the wall of the vessel, and the plates are zigzag-shifted to form openings between the plates, and the edges of the plates are bent upwards. Yes. In the event of an accident, corium passes through the openings one after another, moves into a bell-shaped disperser located in the center of the horizontal plate, and stays at the bottom of the vessel.
[0004]
Also, as disclosed in U.S. Pat. No. 3,964,966, the idea of placing a corium receptacle inside a reactor vessel below a reactor core cooled by a liquid metal has already been proposed. Has been. This receptacle, made of steel, is directly suspended by a lower horizontal plate supporting the core. A heat exchange pipe projects upwardly from the bottom of the receptacle into the interior of the receptacle. These pipes are closed at the upper end and communicate with the interior of the receptacle through holes provided for communication purposes.
[0005]
[Problems to be solved by the invention]
Regardless of the geometry of the receptacle device that is intended to be incorporated inside the vessel of a water-based reactor, there is a risk that an accidental core melt may cause an explosion. As the corium flows toward the bottom of the vessel, the corium breaks up (breaks) into small particles. The water contained in the reactor vessel evaporates upon contact with small corium particles. A vapor film is formed around each particle. This creates favorable conditions for the explosion that causes high energy shock waves. Due to the origin of this type of explosion, this is usually referred to as a “steam explosion”.
[0006]
Currently, receptacle devices for receiving and holding corium at the bottom of a water-based reactor vessel are not configured to prevent steam explosions. Also, these receptacle devices are not protected from the effects of such explosions. Despite the low probability of interaction between water vapor and corium, the possibility of this type of explosion cannot be completely eliminated. Extremely strong pressure peaks generated by explosions impair the integrity of the receptacle device and hence the effectiveness of the receptacle device.
[0007]
[Means for Solving the Problems]
The subject of the present invention is precisely a pressurized water or boiling water reactor in which a corium receiving device is provided in the vessel, which prevents the occurrence of a steam explosion due to the unique configuration of the corium receiving device. Therefore, the integrity of the receiving device in the event of an accident can be maintained.
[0008]
In the present invention, a water-utilizing nuclear reactor comprising a vessel and a nuclear reactor core accommodated in the vessel, which is accommodated in the vessel below the core and at least a part of which is refractory. A basin-shaped receptacle formed from a material, which can receive corium formed upon accidental melting of the core, and within the receptacle by being mixed with the corium It is obtained by a nuclear reactor characterized in that the inside is filled with a porous inorganic material capable of lowering the corium temperature to an equilibrium temperature below the melting point of the refractory material.
[0009]
In the nuclear reactor thus configured, the corium formed when the core is melted penetrates into the porous inorganic material not containing moisture and splits there (splits into fragments). Therefore, it is possible to avoid a favorable condition for a steam explosion in which a steam film exists around each particle of corium. As a result, no steam explosion occurs and the integrity of the receptacle (being undamaged) is maintained.
[0010]
This effect is facilitated by the slow cooling of the corium as it moves down into the porous inorganic material housed in the receptacle. In this way, damage to the receptacle due to melting of the refractory material can be prevented.
[0011]
In a preferred embodiment of the present invention, a porous inorganic material in which a corium spreading means for spreading and dispersing corium, for example in the form of a grid or a horizontal perforated plate, is housed in the receptacle and the receptacle. And above.
[0012]
In this preferred embodiment of the invention, the porous inorganic material is a ceramic foam, preferably containing about 99% silica. The porosity of the porous inorganic material means that a volume sufficient to receive a large amount of corium is made available inside the receptacle. This porosity can in particular be about 63% to about 80%.
[0013]
Advantageously, the receptacle is isolated from the bottom of the vessel by a space that communicates upwards and ensures water circulation around the receptacle. This water circulation assists in cooling the receptacle and thus contributes to maintaining the integrity of the receptacle.
[0014]
The receptacle may be substantially hemispherical, if desired, or may comprise a substantially hemispherical portion and a substantially cylindrical portion connected to the upper side of the hemispherical portion. It can also be configured.
[0015]
In a preferred embodiment of the invention, the refractory material is in the form of bricks and is placed inside a steel casing that forms part of the receptacle.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Preferred embodiments of the present invention as non-limiting examples will now be described with reference to the accompanying drawings. FIG. 1 is a vertical sectional view showing a lower part of a water-utilizing nuclear reactor, and the reactor vessel is provided with a receptacle according to the present invention in consideration of an accident that may lead to melting of the reactor core. ing.
[0017]
The embodiment shown in FIG. 1 relates to a pressurized water reactor. However, as already pointed out, the present invention is not limited to this type of reactor, but generally relates to all water-based reactors. Therefore, the present invention can be applied to a boiling water reactor.
[0018]
In FIG. 1, reference numeral 10 indicates a reactor vessel. More specifically, only the lower part of the vessel is shown. The vessel 10 includes a nuclear reactor core 12 in the central portion, and includes internal equipment related to the core.
[0019]
The core 12 is typically formed from a number of nuclear fuel assemblies juxtaposed in the vertical direction. The nuclear fuel assembly is mounted on the lower horizontal plate 14. The plate 14 is perforated in the vicinity of each nuclear fuel assembly so that the cooling water retained in the vessel 10 can circulate inside the nuclear fuel assembly. The perforated distribution / distribution plate 16 preferably has a convex lower part, and is fixed below the plate 14 over the entire plate 14.
[0020]
The reactor core and associated internal equipment such as the horizontal lower plate 14 and the perforated plate 16 are supported on the vertical cylindrical wall of the vessel 10 via a support guide device 18.
[0021]
It should be noted that within the scope of the present invention, the core 12 and associated internal equipment may be of any shape other than that shown.
[0022]
In the present invention, as shown in FIG. 1, a basin-shaped receptacle 20 is disposed inside the vessel 10 below the reactor core 12. More specifically, the receptacle 20 is disposed between the perforated plate 16 and the convex and generally hemispherical base of the vessel 10.
[0023]
The receptacle 20 is configured to receive corium upon accidental melting of the reactor core 12. The term “corium” refers to the molten mass that would be produced during such an accident, and corium generally includes nuclear fuel, nuclear fuel cladding, control rod cladding, and core. The internal structure related to 12 is contained. The receptacle 20 is arranged and configured to receive the entire amount of corium in the event of a serious accident that could lead to complete melting of the reactor core 12.
[0024]
In the preferred embodiment of the present invention shown in FIG. 1, the receptacle 20 is a portion of the space communicating from the base of the vessel 10 to the upper side with respect to the gap between the upper outer periphery of the receptacle and the vessel 10. , Isolated. This space 22 allows the circulation of the water retained in the reactor vessel 10 as indicated by the arrows in the figure. When an accident occurs, water circulation occurs in this space 22 by natural convection. The effect brought about by this water circulation is the cooling of the receptacle 20.
[0025]
The receptacle 20 includes at least one brick layer 24 formed from a refractory material. This material is selected to have the highest possible melting point and to have good chemical compatibility with corium. The material can in particular be a ceramic material based on zirconium, which also has the advantage of being widely available in the industrial market.
[0026]
As shown in FIG. 1, the bricks 24 are juxtaposed and preferably have complementary edge shapes. The edge shape can be a U-shape or a wide dovetail shape that allows mutual interconnection.
[0027]
The receptacle 20 also includes a metal casing 26 having a brick 24 disposed therein. This metal casing 26 can in particular be formed from stainless steel. The metal casing 26 includes an inner skin and an outer skin that completely cover the brick 24. The casing 26 also includes an upper flange for connecting the upper edges of the inner skin and outer skin of the receptacle 20 to each other.
[0028]
The upper flange of the metal casing 26 can be used to suspend the receptacle 20 relative to the vessel 10. In this case, the receptacle 20 is supported on a support 28 provided inside the vessel 10 as shown in FIG.
[0029]
As a modification, a radial reinforcing member (not shown) oriented in the radial direction with respect to the vertical axis of the vessel 10 is interposed in the space 22 that separates the receptacle 20 and the vessel 10. Can do. In this case, the radial reinforcing member is provided with a plurality of holes for assisting the circulation of water in the space 22.
[0030]
In the illustrated embodiment, the receptacle 20 has a substantially hemispherical shape that is concentric with the shape of the base of the vessel 10.
[0031]
In a modification (not shown) of this embodiment, the receptacle 20 can be extended upward by connecting a substantially cylindrical portion having a center line on the vertical axis of the vessel 10 to the hemispherical shape. . This configuration is employed especially when the total volume of corium expected to be formed in the event of a serious accident is expected to exceed the available volume in the receptacle 20 if it is a hemispherical receptacle 20. can do. The estimation of the volume that can be used in the receptacle 20 is performed in consideration of the presence of a porous inorganic material 30 to be described later filled in the receptacle.
[0032]
In order to effectively receive the entire amount of corium produced by the core 12 in the event of a serious accident into the receptacle 20, a ring collector 32 can be placed above the upper edge of the receptacle as shown. . This collector 32 can in particular be supported on the upper flange of the receptacle 20 via the cross beam structure 33.
[0033]
The upper surface of the collector 32 is curved inward and has a hopper shape that faces upward near the wall of the vessel 10. More specifically, a clearance limited to a few centimeters is provided between the collector 32 and the vessel 10 wall. This is to prevent the flow of debris from the core at the time of melting from entering the space 22 between the receptacle 20 and the base of the vessel 10.
[0034]
In an essential feature of the present invention, as described above, the receptacle 20 is completely filled with the porous inorganic material 30. This porous inorganic material can be mixed with the corium formed during the accidental melting of the core 12 so that the temperature of the corium can be lowered to an equilibrium temperature lower than the melting point of the refractory material forming the brick 24. So that it is selected. By being mixed with corium, the non-moisture material 30 prevents the generation of preconditions for a steam explosion. This is because the material 30 having no moisture prevents the vapor film from being formed around each corium particle formed by the splitting of the corium when the corium moves down in the receptacle 20. is there. Thus, the risk of receptacle damage due to a steam explosion is actually eliminated.
[0035]
The porous inorganic material 30 filling the receptacle 20 is advantageously a ceramic foam containing about 99% silica. With this type of material, the corium can be gradually cooled during the downward movement of the corium in the receptacle 20. Therefore, the temperature of corium is stabilized below the melting point of the refractory material forming the brick 24. In this way, damage to the receptacle 20 due to melting of the refractory brick can be avoided.
[0036]
The porosity of the material 30 is selected so that the internal volume of the receptacle 20 is large enough to receive the entire amount of corium formed by melting the core 12. From this point of view, at the same time, a material having a porosity of about 63% to about 80% is considered satisfactory from the viewpoint of deceleration and cooling of the corium during the downward movement.
[0037]
Preferably, as shown in FIG. 1, means for spreading and dispersing the corium is disposed on the receptacle 20 and the porous inorganic material 30 filled in the receptacle. This means can in particular be formed from a horizontal perforated plate 34 or a horizontal grid.
[0038]
Corium passages 36 created by the melting of the core 12 in the event of a serious accident are shown. When corium falls onto the perforated plate 34, it spreads across the entire width of the receptacle 20. Corium then enters the receptacle 20 through the hole in the plate 34. Thereafter, the corium is split in the porous inorganic material 30 that does not contain moisture. As mentioned above, this material 30 eliminates the risk of a steam explosion. As the corium is decelerated by the material 30, the corium is cooled to a temperature below the melting point of the refractory brick 24. The refractory brick 24 is cooled by the water circulation formed by natural convection in the space 22, and the entire receptacle 20 is cooled. Thus, the integrity of the receptacle 20 is maintained under the best conditions.
[0039]
It is obvious that the present invention is not limited to the illustrated embodiment. As already pointed out, the present invention can be applied to any type of water-based nuclear reactor, regardless of the configuration of the core and the associated internal structure. Further, the shape and structure of the receptacle may be different from those illustrated. Furthermore, although it is preferable to provide the space 22, the corium expanding means, and the ring collector 32, it may be omitted depending on circumstances. Finally, the porous inorganic material is preferably a ceramic foam based on silica, but may be other in some cases.
[Brief description of the drawings]
FIG. 1 is a vertical cross-sectional view showing a lower part of a water-based nuclear reactor, and includes a receptacle according to the present invention.
[Explanation of symbols]
10 Vessel 12 Reactor Core 20 Receptacle 22 Space 24 Refractory Material 26 Metal Casing 30 Porous Inorganic Material 34 Horizontal Perforated Plate (Corium Spreading Means)

Claims (10)

A water-utilizing nuclear reactor comprising a vessel and a reactor core housed in the vessel,
A receptacle in the shape of a basin, housed under the core in the vessel and at least partially formed from a refractory material;
The receptacle can receive corium formed upon accidental melting of the core;
The receptacle is a porous inorganic material capable of lowering the corium temperature to an equilibrium temperature not higher than the melting point of the refractory material by being melted and mixed with the corium and having no moisture. A nuclear reactor characterized in that it is completely filled with a conductive inorganic material . The nuclear reactor according to claim 1,
A nuclear reactor characterized in that a corium expanding means for expanding and dispersing corium is disposed above the receptacle and the porous inorganic material accommodated in the receptacle. The nuclear reactor according to claim 2, wherein
The reactor characterized in that the corium spreading means includes a grid or a horizontal perforated plate. The nuclear reactor according to claim 1,
A nuclear reactor, wherein the porous inorganic material is a ceramic foam. The nuclear reactor according to claim 4, wherein
A nuclear reactor, wherein the ceramic foam contains about 99% silica. The nuclear reactor according to claim 1,
A nuclear reactor characterized in that the porous inorganic material has a porosity of about 63% to about 80%. The nuclear reactor according to claim 1,
The nuclear reactor according to claim 1, wherein the receptacle is isolated from the bottom of the vessel by an upper communication space for water circulation. The nuclear reactor according to claim 1,
A nuclear reactor characterized in that the receptacle is substantially hemispherical. The nuclear reactor according to claim 1,
The nuclear reactor according to claim 1, wherein the receptacle includes a substantially hemispherical portion and a substantially cylindrical portion connected to an upper side of the hemispherical portion. The nuclear reactor according to claim 1,
The receptacle comprises a metal casing;
A nuclear reactor characterized in that the refractory material is disposed inside the metal casing.

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