JP6727690B2 - Core melt holding device - Google Patents

Description

The present invention relates to a core melt holding device, and more particularly to a core melt holding device suitable for application to a boiling water reactor plant.

In a nuclear power plant such as a boiling water reactor plant and a pressurized water reactor plant, a reactor core is arranged in a reactor pressure vessel, and a nuclear fuel material containing a fissile material (for example, uranium 235) ( For example, a plurality of fuel assemblies including a plurality of fuel rods filled with UO ₂ ) are loaded. In the normal state of a nuclear power plant, the cooling water present in the reactor pressure vessel makes contact with the fuel rods in each loaded fuel assembly to cool these fuel rods.

In the unlikely event that a severe accident occurs in the nuclear power plant, the cooling water in the reactor pressure vessel is lost, and furthermore, the cooling water is not injected into the reactor pressure vessel by the emergency core cooling device. The temperature of the fuel rods rises and eventually the nuclear fuel material in each rod melts. A cladding tube made of zirconium alloy, which is a structural member of a fuel rod, and an upper tie plate and a lower tie plate made of zirconium alloy, which are structural members of a fuel assembly, filled with the nuclear fuel material as the nuclear fuel material melts. Etc., and further, the structural members made of stainless steel such as the lower core support plate and the control rod guide tubes are melted, and the fuel debris containing the nuclear fuel material and the melt of these structural members is generated. The molten fuel debris may fall on the upper surface of the lower mirror part which is the bottom of the reactor pressure vessel, and may melt this lower mirror part and fall on the bottom part of the reactor containment vessel surrounding the reactor pressure vessel. ..

If the molten fuel debris penetrated the bottom of the reactor pressure vessel and dropped to the bottom of the reactor containment vessel, it was provided at the bottom of the reactor containment vessel to maintain the airtightness of the reactor containment vessel. There is a possibility that the metal lining member is melted by the high temperature fuel debris that has fallen, and the concrete existing below the lining member is also melted, so that the airtightness of the reactor containment vessel is impaired.

Therefore, for the purpose of avoiding damage to the reactor containment vessel and maintaining the airtightness of the reactor containment vessel, the core melt holding device that receives the high temperature molten fuel debris falling from the reactor pressure vessel is It is located above the pedestal floor below the pressure vessel. An example of a core melt holding device is described in Patent Document 1.

The core melt holding device described in Patent Document 1 is arranged on the floor of the reactor containment vessel inside the pedestal below the reactor pressure vessel. This core melt holding device is configured as a hollow disc, and has a water supply container having an upper surface of a reverse cone shape that is inclined from an outer edge portion toward a central portion, and a reverse cone which is arranged above the water supply container and covers the upper surface thereof. The upper cover and the upper cover have an inverted conical heat insulating material layer, and a cooling channel inclined toward the center of the water supply container is formed between the water supply container and the upper cover. The heat-insulating material layer has a first heat-resistant layer and a second heat-resistant layer arranged upward from the upper lid, and a first joint portion between the upper lid and the first heat-resistant layer and between the first heat-resistant layer and the second heat-resistant layer. The second joint portion is arranged at the position. The first heat-resistant layer has higher thermal conductivity than the second heat-resistant layer, and the second joint has lower thermal conductivity than the first joint.

The molten fuel debris falls from the reactor pressure vessel onto the heat insulating material layer, specifically, the second heat resistant layer. Cooling water is poured onto the second heat-resistant layer where the fuel debris has fallen, and onto the fuel debris, and the cooling water is also supplied to the cooling channel. As a result, the dropped fuel debris is cooled.

JP, 2014-062859, A

As described in Patent Document 1, a conventional core melt holding device uses a heat-resistant layer (for example, a high-melting point oxide) that is a debris holding portion stacked. These layers are often separated by mechanical properties, melting points, etc. This is due to the fact that the molten fuel debris is heavy and has a high temperature near 2000 degrees. In particular, an oxide mainly composed of zirconia, which has a high melting point of 2700° C. and a high thermal shock resistance, is often used for the layer that is in direct contact with the debris. On the other hand, an oxide (for example, alumina) having a melting point lower than that of zirconia is used in a region that does not directly contact the debris in the initial state.

When the molten fuel debris contacts the first layer (upper layer) composed of zirconia in the debris holding portion of the core melt holding device, zirconium and uranium contained in the fuel debris reduce the first layer of zirconia, Further, if the reduction reaction proceeds during the long-term holding, the oxygen concentration in the first layer of zirconia decreases. Furthermore, a decrease in oxygen concentration in the first layer, zirconia, induces a reduction reaction in the second layer (lower layer). Even if zirconia, which is the first layer, is reduced to zirconium, the melting point is maintained at 1840 degrees. However, when a reduction reaction occurs in the second layer, for example, alumina having a melting point of 2050 degrees is reduced to aluminum having a melting point of 660 degrees. That is, the inventors have found that the performance of the core melt is greatly deteriorated because the second layer is easily melted.

The present invention has been made in view of the above problems, and an object thereof is a core melting capable of maintaining erosion resistance by suppressing a reduction reaction of a lower layer by an upper layer reduced by fuel debris. It is to provide an object holding device.

In order to solve the above-mentioned problems, the present invention provides a core melt holding device disposed on a pedestal bed below a reactor pressure vessel, in which an upper layer composed of a first oxide having zirconia as a substrate. A lower layer composed of a second oxide having as a substrate an oxide of a metal mainly composed of a first element having a higher free energy for oxide formation than zirconium obtained by reducing the zirconia; A metal mainly composed of zirconium and a second element having a higher free energy for oxide formation than the first element, or a third oxide having an oxide thereof as a substrate. And an intermediate layer.

When a severe accident occurs in the reactor containment vessel and the molten fuel debris falls from the reactor pressure vessel to the upper layer (the first oxide with zirconia as a substrate), the metallic elements contained in the fuel debris The first oxide is reduced, and the oxygen concentration in the first oxide decreases with the passage of time.

In the above-described present invention, when the intermediate layer is made of a metal mainly containing the second element, the intermediate layer does not contain oxygen, and therefore the first oxide having a reduced oxygen concentration does not reduce the intermediate layer. Absent. In addition, since the second element forming the intermediate layer has a higher free energy for oxide formation than that of the first element obtained by reducing the lower layer (second oxide) (it is difficult to combine with oxygen), the intermediate layer lowers the lower layer. Is never reduced.

On the other hand, in the above-described present invention, when the intermediate layer is composed of the third oxide, the second element containing oxygen in the intermediate layer and reducing the third oxide is higher than zirconium reducing the first oxide. Since it has free energy for oxide formation (it is difficult to combine with oxygen), the intermediate layer (third oxide) is reduced to the second element by the first oxide having a reduced oxygen concentration. However, the second element that has reduced the intermediate layer (third oxide) has a higher free energy for oxide formation than the first element that has reduced the lower layer (second oxide) (it is difficult to combine with oxygen). Also in this case, the lower layer is not reduced by the intermediate layer.

Thus, by disposing the intermediate layer composed of the metal mainly containing the second element or the third oxide, which is an oxide thereof, between the upper layer and the lower layer, the upper portion reduced by the fuel debris is reduced. Since the reduction reaction of the lower layer (second oxide) by the layer (zirconium) can be suppressed and the melting point of the lower layer can be kept high, it becomes possible to maintain the erosion resistance of the core melt holding device. ..

According to the present invention, it is possible to maintain the erosion resistance of the core melt holding device by suppressing the reduction reaction of the lower layer by the upper layer reduced by the fuel debris.

1 is a schematic configuration diagram of a boiling water reactor to which a core melt holding device according to the present invention is applied. It is sectional drawing of the core melt holding|maintenance apparatus which concerns on the 1st and 2nd Example of this invention. It is sectional drawing of the core melt holding|maintenance apparatus which concerns on the 3rd Example of this invention. It is a partially expanded sectional view of a core melt holding device according to a fourth embodiment of the present invention.

The inventors have made various studies on a core melt holding device capable of suppressing a reduction reaction due to fuel debris and preventing corrosion of a heat resistant layer. The results of this examination will be described below.

In order to ameliorate the above-mentioned problems found by the inventors, the inventors studied a countermeasure for suppressing a reduction reaction by molten fuel debris, particularly a reduction reaction by zirconium and uranium contained in the fuel debris. .. In this study, the inventors have found that, between the upper layer and the lower layer, which are heat-resistant layers made of a heat-resistant material, zirconium and the lower layer, which are the first oxide having zirconia as a substrate and constituting the upper layer, are reduced An intermediate layer composed of a metal mainly composed of a second element having a free energy of oxide formation higher than that of the first element obtained by reducing the second oxide constituting We came up with the idea that it should be installed. That is, the core melt holding device in this case is an upper layer composed of a first oxide having zirconia as a substrate, a lower layer composed of a second oxide, and a metal mainly composed of a second element or And an intermediate layer composed of a third oxide using the oxide as a substrate.

The first oxide forming the upper layer uses zirconia (ZrO ₂ ) as a substrate, and yttria (Y ₂ O ₃ ), calcia (CaO), etc. for keeping zirconia stable, and hafnia (HfO ₂ ) which is an unavoidable oxide. ), etc., and silicon dioxide (SiO ₂ ), alumina (Al ₂ O ₃ ), beryllia (BeO), etc. mixed as a binder material when manufactured through a sintering process.

The second oxide forming the lower layer uses, as a substrate, a metal oxide mainly composed of an element having an oxide formation energy higher than that of zirconium. Typical examples of the second oxide include oxides having alumina (Al ₂ O ₃ ), silicon dioxide (SiO ₂ ), calcia (CaO), magnesia (MgO), and beryllia (BeO) as substrates. Since these are used as substrates, oxides such as mullite composed of alumina (Al ₂ O ₃ ) and silicon dioxide (SiO ₂ ) are also included. Note that silicon (Si) here is silicon having a β-tin structure and is a metal.

The intermediate layer is a metal mainly composed of a second element having a free energy of oxide formation higher than that of the first element obtained by reducing the second oxide forming the lower layer and having a melting point higher than that of the first element. Composed. Representative examples of the metal mainly containing the second element include Fe-based alloys, Cr-based alloys, Si-based alloys, Cu-based alloys and Au-based alloys. In addition, the intermediate layer is one of oxides of these metals, that is, iron oxide (for example, Fe ₂ O ₃ ), chromium oxide (for example, Cr ₂ O ₃ ), silicon dioxide (SiO ₂ ), or the like, or It is composed of a combination of two or more. The oxide formation free energies of the main elements at 1000 K are zirconium: -820 kJ/mol, aluminum: -780 kJ/mol, silicon: -640 kJ/mol, iron: -340 kJ/mol. The material forming the intermediate layer is selected from a metal mainly composed of the second element having a higher free energy for oxide formation than an element obtained by reducing the second oxide, or a third oxide using the oxide as a substrate. To be done.

First, an upper layer composed of stabilized zirconia which is a first oxide, and a metal oxide (M1O) mainly composed of a first element (M1) having a higher free energy for oxide formation than zirconium obtained by reducing zirconia. ₂ ) as a substrate, a lower layer composed of a second oxide, and a second element having a higher free energy for oxide formation than the first element (M1) obtained by reducing the substrate (M1O ₂ ) of the second oxide. Consider a core melt holding device provided with an intermediate layer composed mainly of (M2) made of metal.

When the severe accident occurs, the upper layer with which the molten fuel debris dropped from the reactor pressure vessel comes into contact first is the stabilized zirconia that is the first oxide, and the zirconia is a metallic element (M0 ) (For example, zirconium or uranium), as shown in the following formula (1).
ZrO ₂ +M0→Zr+M0O ₂ (1)
Note that zirconium in the fuel debris reduces zirconia in the upper layer as shown in formula (1) for entropy stabilization.

Zirconia that constitutes the upper layer has a very high oxygen conductivity. Therefore, in the upper layer, oxygen diffuses from the lower region of the upper layer toward the vicinity of the debris contact surface where the oxygen concentration is lowered so as to keep the oxygen concentration spatially constant. On the other hand, at the debris contact surface, the reduction reaction shown in the equation (1) proceeds, so that the oxygen concentration in the entire upper layer decreases. As a result, a low oxygen concentration occurs in the lower region of the upper layer. Since this region contains unoxidized zirconium, it has a strong oxidizing power.

If this oxidizing region contacts the second oxide, the second oxide is reduced and the melting point drops significantly. For example, when the second oxide is made of alumina having a melting point of 2050 degrees, when it is reduced, it becomes aluminum having a melting point of 660 degrees and may be easily melted.

However, when the intermediate layer is arranged between the upper layer and the lower layer, the lower region of the upper layer having strong oxidizing power contacts the intermediate layer, not the lower layer. Since the intermediate layer made of a metal mainly containing the second element (M2) does not contain oxygen, the upper layer having an oxidizing power cannot extract oxygen from the intermediate layer. Therefore, the reduction reaction that occurs in the debris holding unit will not be transmitted below the intermediate layer. Therefore, no matter how much the upper layer is reduced over a long period of time, no reduction reaction occurs in the lower layer and the melting point of the lower layer is maintained.

Next, an upper layer composed of stabilized zirconia that is the first oxide, and an oxide of a metal mainly composed of the first element (M1) having a higher free energy for oxide formation than zirconium obtained by reducing zirconia ( (M1O ₂ ), a lower layer composed of a second oxide having a substrate, and a second element (M2) having a higher free energy for oxide formation than zirconium and a metal having a first element (M1) as a main component. Given the core catcher device having a to an oxide of a metal (M2O ₂₎ it was composed of the third oxide to a substrate interlayer.

Thus even when the oxide of the metal second element a (M2) composed mainly of (M2O ₂₎ constitutes an intermediate layer in the third oxide to a substrate, and the metallic second element described above (M2) Similar to the case where the intermediate layer is made of the metal, the fuel debris causes a reduction reaction in the upper layer, and after a long time, the upper layer lower region mainly composed of zirconia has a strong oxidizing power.

Since the intermediate layer in this case is composed of an oxide (third oxide), zirconium having a strong oxidizing power in the upper layer lower region constitutes the intermediate layer as shown in the following formula (2). reducing substrate (M2O ₂₎ and the third oxide that is.
M2O ₂ +Zr→M2+ZrO ₂ (2)
Thus, the substrate of the third oxide constituting the intermediate layer (M2O ₂₎ is reduced by the upper layer, the state of the metal in which the second element a (M2) mainly. The second element (M2) of this intermediate layer has a higher oxide formation energy (combines with oxygen) than the first element (M1) obtained by reducing the substrate (M1O ₂ ) of the second oxide constituting the lower layer. Therefore, it does not cause a reduction reaction on the substrate (M2O ₂ ) of the third oxide of the lower layer. Therefore, no matter how much the upper layer is reduced over a long period of time, no reduction reaction occurs in the lower layer, and the melting point of the lower layer is maintained.

An embodiment of the present invention that reflects the above-described examination results will be described below in detail with reference to the drawings.

A core melt holding device according to a first embodiment of the present invention will be described with reference to FIGS. 1 and 2. FIG. 1 is a schematic configuration diagram of a boiling water reactor to which the core melt holding apparatus according to the present invention is applied, and FIG. 2 is a sectional view of the core melt holding apparatus according to the present embodiment.

As shown in FIG. 1, the boiling water reactor has a reactor pressure vessel 7 in which a core 11 is arranged. A plurality of fuel assemblies 12 having a plurality of fuel rods (not shown) having a nuclear fuel material filled therein are loaded in a core 11. The reactor pressure vessel 7 is surrounded by the reactor containment vessel 9 and is mounted inside the cylindrical pedestal 8 installed on the reactor containment vessel floor 10 in the reactor containment vessel 9. A plurality of control rods 14 are provided below the reactor pressure vessel 7 so that they can be inserted into the core 11.

The core 11 is arranged inside a cylindrical core shroud 13 installed in the reactor pressure vessel 7, and an annular downcomer is formed between the inner surface of the reactor pressure vessel 7 and the outer surface of the core shroud 13. A plurality of jet pumps (not shown) installed in the reactor pressure vessel 7 are arranged in the downcomer. One end of the recirculation system pipe having a recirculation pump is connected to the side wall of the reactor pressure vessel 7 and communicates with the downcomer. The other end of the recirculation pipe is connected to a nozzle (not shown) of the jet pump via a riser pipe which is an ascending pipe arranged in the downcomer.

The core melt holding device 1 is disposed below the reactor pressure vessel 7 and on a portion 6 of the reactor containment vessel floor 10 surrounded by a pedestal 8 (hereinafter referred to as “pedestal floor”) 6. The entire upper surface of the pedestal floor 6 and the inner peripheral surface of the lower end portion of the pedestal 8 are covered.

As shown in FIG. 2, the core melt holding device 1 includes a lower layer 3 arranged so as to cover the entire upper surface of the pedestal floor 6, and an intermediate layer 4 arranged so as to overlap the entire lower layer 3. , And an upper layer 2 which is arranged so as to be superposed on the intermediate layer 4.

The upper layer 2 includes a bottom portion 2a that is arranged on the intermediate layer 4 in an overlapping manner, and an annular portion 2b that is erected from the outer edge portion of the bottom portion 2a and that is arranged along the inner peripheral surface of the lower end portion of the pedestal 8. Have. By providing the annular portion 2b in this way, even if the fuel debris 5 spreads on the bottom portion 2a, the fuel debris 5 is prevented from coming into contact with the inner peripheral surface of the pedestal 8, and the fuel debris 5 moves from the pedestal 8 to the pedestal 8. It is possible to delay the heat transfer of.

The upper layer 2 is composed of a first oxide mainly containing zirconia which is an oxide of zirconium. The lower layer 3 is composed of a second oxide mainly composed of an oxide of the first metal having a higher free energy of oxide formation than zirconium. The intermediate layer 4 is composed of a metal mainly composed of zirconium and a second element having a higher free energy for oxide formation than that of the first element, or a third oxide having the oxide thereof as a substrate.

Each of the upper layer 2 and the lower layer 3 of the core melt holding device 1 is made of ceramics. Since ceramics are manufactured through a sintering process, their size cannot generally be increased. Therefore, in the core melt holding device 1 having a diameter of the order of 10 m, the upper layer 2 and the lower layer 3 cannot be made of a single ceramic. Therefore, the upper layer 2 and the lower layer 3 are manufactured by spreading small tile materials (plate materials) or block materials, which are ceramics, and further laminating the tile materials or block materials.

According to this example, the oxide higher than zirconium obtained by reducing the substrate (zirconia) of the first oxide forming the upper layer 2 and the first element obtained by reducing the substrate of the second oxide forming the lower layer 3 is used. By disposing an intermediate layer composed of a metal mainly containing a second element having a free energy of formation or a third oxide having an oxide thereof as a substrate between the upper layer 2 and the lower layer 3, fuel debris can be obtained. Since the reduction reaction of the lower layer 3 (second oxide) by the upper layer 2 (zirconium) reduced by the above can be suppressed and the melting point of the lower layer 3 can be kept high, the erosion resistance of the core melt holding device 1 It is possible to retain the sex.

The lower layer 3 does not necessarily have to be composed of only the second oxide, and may have a multilayer structure in which a plurality of materials are stacked. For example, when the oxygen transferability of the second oxide is not high, the second oxide is disposed only in the upper layer portion of the lower layer 3 which is in contact with the intermediate layer 4, and the second oxide is disposed in the lower layer portion excluding the upper layer portion. By disposing an oxide having a thermal conductivity lower than that of the object, it becomes possible to delay the heat transfer from the fuel debris 5 to the pedestal bed 6.

A core melt holding device according to a second embodiment of the present invention will be described. The core melt holding device according to the present embodiment uses alumina-based ceramics as the second oxide forming the lower layer 3. Since the structure of the core melt holding device 1 according to this embodiment is the same as that described in the first embodiment (shown in FIG. 2), the description thereof is omitted here.

Alumina (Al ₂ O ₃ ) which is a substrate of alumina-based ceramics has a high melting point of 2000 degrees and is suitable as a second oxide forming the lower layer 3, but when reduced, it becomes aluminum having a melting point of 660 degrees. , May dissolve easily. SUS is one of the materials suitable for the intermediate layer 4 in the case where the lower layer 3 is composed of such alumina-based ceramics. SUS is zirconium obtained by reducing the substrate (zirconia) of the first oxide forming the upper layer 2 and the first element (alumina) obtained by reducing the substrate (alumina) of the second oxide (alumina-based ceramics) forming the lower layer 3. It is a metal mainly composed of a second element having a higher free energy for oxide formation than aluminum).

According to this embodiment, the intermediate layer 4 made of SUS is arranged between the upper layer 2 made of the first oxide having zirconia as a substrate and the lower layer 3 made of alumina-based ceramics. As a result, the reduction reaction of the lower layer 3 (alumina) by the upper layer 2 (zirconium) reduced by the fuel debris can be suppressed, and the melting point of the lower layer 3 can be kept high. It becomes possible to maintain erosion.

Moreover, since the melting point of SUS is 1400 degrees or more, by configuring the intermediate layer 4 of SUS, it becomes possible to improve the corrosion resistance of the core melt holding device 1. Further, since SUS has excellent corrosion resistance, configuring the intermediate layer 4 from SUS is effective in ensuring the reliability of the core melt holding device 1 which may be maintained for a long period of several decades. is there.

In addition, as a material suitable for the intermediate layer 4 in the case where the second oxide is an alumina-based ceramics, a material obtained by solidifying triiron tetroxide (Fe ₃ O ₄ ) powder with a silica (SiO ₂ ) adhesive can be mentioned. Since silicon (Si) obtained by reducing silica (SiO ₂ ) also has a high melting point exceeding 1300° C., the erosion resistance of the core melt holding device 1 can be improved by forming the intermediate layer 4 from the material. It will be possible. Table 1 shows examples of materials (second element or third oxide) suitable for the intermediate layer when the second oxide is alumina-based ceramics, including the materials described above.

A core melt holding device according to a third embodiment of the present invention will be described with reference to FIG. FIG. 3 is a sectional view of the core melt holding device according to the present embodiment. Hereinafter, differences from the core melt holding device 1 (shown in FIG. 2) according to the first or second embodiment will be mainly described.

The core melt holding device 1A according to the present embodiment is applied to a boiling water reactor. The core melt holding device 1B is provided on the pedestal floor 6 located below the reactor pressure vessel 7 in the same manner as the core melt holding devices 1 (shown in FIG. 2) according to the first and second embodiments. It is installed and covers the entire pedestal floor 6 and the inner peripheral surface of the lower end portion of the pedestal 8.

The core melt holding device 1 (shown in FIG. 2) according to the first embodiment has a three-layer structure in which the intermediate layer 4 is added between the upper layer 2 and the lower layer 3, so that the conventional core melt It may be thicker than the holding device. On the other hand, in the boiling water reactor, as shown in FIG. 1, since the devices such as the control rod 14 provided in the lower portion of the reactor pressure vessel 7 extend to the vicinity of the pedestal floor 6, the thick core melting It may not be possible to install an object holding device. This embodiment is applicable to a boiling water reactor in which the space on the pedestal floor 6 is limited by thinning the core melt holding device.

As shown in FIG. 3, the core melt holding device 1A according to the present embodiment is composed of only the upper layer 2 and the intermediate layer 4, and does not include the lower layer 3 (shown in FIG. 2).

When the core melt holding device was provided only with the upper layer 2 as the heat resistant layer, silica (SiO ₂ ) and calcia (CaO), which are the main components of the concrete forming the pedestal floor 6, were reduced by the fuel debris. The upper layer 2 (zirconium) reduces the melting point of the pedestal bed 6 to lower the melting point.

On the other hand, according to the core melt holding device 1A of the present embodiment, the second element having a higher free energy for oxide formation than the element obtained by reducing the oxide contained in the concrete forming the pedestal floor 6. Concrete which constitutes the pedestal floor 6 by arranging the intermediate layer 4 composed of the metal mainly composed of or a third oxide whose substrate is an oxide thereof between the upper layer 2 and the pedestal floor 6. Since the reduction reaction can be suppressed and the melting point of the pedestal bed 6 can be kept high, the erosion resistance of the pedestal bed 6 can be maintained. The materials (second element or third oxide) suitable for forming the intermediate layer 4 are the same as those listed in the second embodiment (shown in Table 1).

A core melt holding device according to a fourth embodiment of the present invention will be described with reference to FIG. FIG. 4 is a partially enlarged cross-sectional view of the core melt holding device 1B according to this embodiment. Hereinafter, differences from the core melt holding device 1 (shown in FIG. 2) according to the first or second embodiment will be mainly described.

In the core melt holding device 1 (shown in FIG. 2) according to the first or second embodiment described above, when the intermediate layer 4 reaches the melting point and is melted, the member constituting the intermediate layer 4 is the upper part. There is a risk that the upper layer 2 and the lower layer 3 may come into contact with each other by flowing out from the gap between the layer 2 and the lower layer 3, and the lower layer 3 may be reduced by the upper layer 2. On the other hand, in the core melt holding device 1B according to the present embodiment, the upper layer 2 and the lower layer 3 when the intermediate layer 4 reaches the melting point and melts and flows out from the gap between the upper layer 2 and the lower layer 3. By reducing the contact area with the upper layer 2 (zirconia) reduced by the fuel debris, the reduction reaction of the lower layer 3 is suppressed. Hereinafter, differences from the first or second embodiment will be mainly described.

As shown in FIG. 4( a ), a large number of recesses 3 a are formed on the upper surface of the heat-resistant material block forming the uppermost part of the lower layer 3. The number, shape and arrangement of the recessed portions 3a can be selected at any time. In the example shown in FIG. 4(a), since the intermediate layer 4 is made of a plate material, the interior of the recess 3a is hollow, but the material of the intermediate layer 4 is an oxide and powder. In this case, the material forming the intermediate layer 4 is also filled inside the recess 3a.

According to the core melt holding device 1B of the present embodiment, the following effects are obtained in addition to the same effects as those of the first or second embodiment.

By forming the recessed portion 3a on the upper surface of the lower layer 3, as shown in FIG. 4B, even after the intermediate layer 4 melts and flows out from the gap between the upper layer 2 and the lower layer 3, the upper layer Since the contact area between the lower layer 3 and the lower layer 3 can be reduced, it is possible to suppress the reduction reaction of the lower layer 3 (second oxide) by the upper layer 2 (zirconia) reduced by the fuel debris. .. Further, at this time, since the melt of the intermediate layer 4 is filled in the recess 3a, it is possible to improve the structural strength of the core melt holding device 1B as compared with the case where the recess 3a is hollow.

In the case of the core melt holding device 1A (shown in FIG. 3) according to the third embodiment having no lower layer, the intermediate layer 4 is melted by forming the recessed portion on the upper surface of the pedestal floor 6. Even when it flows out from the gap between the upper layer 2 and the pedestal floor 6, the contact area between the upper layer 2 and the lower layer 3 can be reduced, so that the reduction reaction of the pedestal floor 6 by the upper layer 2 is suppressed. It becomes possible.

Although the embodiments of the present invention have been described above in detail, the present invention is not limited to the above-described embodiments and includes various modifications. For example, the above-described embodiments have been described in detail in order to explain the present invention in an easy-to-understand manner, and are not necessarily limited to those having all the configurations described. It is also possible to add a part of the configuration of another embodiment to the configuration of a certain embodiment, delete a part of the configuration of a certain embodiment, or replace it with a part of another embodiment. It is possible.

Further, since the respective core melt holding devices according to the first to fourth examples can be installed below the reactor pressure vessel 7 without substantially changing the structure of the reactor containment vessel 9, It can be applied not only to a new nuclear power plant but also to an existing nuclear power plant.

In addition, the core melt holding devices according to the first to fourth embodiments can be applied to a pressurized water reactor.

1, 1A, 1B... Core melt holding device, 2... Upper layer, 2a... Bottom portion, 2b... Annular portion, 3... Lower layer, 3a... Recessed portion, 4... Intermediate layer, 5... Fuel debris, 6... Pedestal floor , 7... Reactor pressure vessel, 8... Pedestal, 9... Reactor containment vessel, 10... Reactor containment floor, 11... Reactor core, 12... Fuel assembly, 13... Reactor shroud, 14... Control rod.

Claims (4)

In the core melt holding device arranged on the pedestal floor below the reactor pressure vessel,
An upper layer composed of a first oxide having zirconia as a substrate,
A lower layer composed of a second oxide whose substrate is an oxide of a metal mainly composed of a first element having a higher free energy for oxide formation than zirconium obtained by reducing zirconia;
A third oxide, which is disposed between the upper layer and the lower layer, is mainly composed of zirconium and a second element having a higher free energy for oxide formation than the first element, or a metal oxide thereof. Core melt holding device having an intermediate layer composed of a material. The core melt holding device according to claim 1,
The oxide of the metal mainly composed of the first element is alumina,
The core melt holding device, wherein the metal mainly containing the second element is an Fe-based alloy, a Cr-based alloy, a Si-based alloy, a Cu-based alloy, or an Au-based alloy. In the core melt holding device arranged on the pedestal floor below the reactor pressure vessel,
An upper layer composed of a first oxide having zirconia as a substrate,
Arranged between the upper layer and the pedestal floor, having a higher free energy for oxide formation than zirconium reduced in the zirconia and an element reduced in the oxide contained in the concrete forming the pedestal floor. A core melt holding device comprising: an intermediate layer composed of a metal mainly containing two elements or an oxide thereof as a substrate; and a third oxide. The core melt holding device according to claim 1 or 2,
A core melt holding device, wherein a recess is formed on the upper surface of the lower layer.

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