WO2005076284A1

WO2005076284A1 - Device and process for cooling molten core material released from a reactor vessel

Info

Publication number: WO2005076284A1
Application number: PCT/KR2005/000368
Authority: WO
Inventors: Hwan-Yeol Kim; Jin-Ho Song; Beong-Tae Min; Hee-Dong Kim
Original assignee: Korea Atomic Energy Research Institute; Korea Hydro & Nuclear Power Co., Ltd.
Priority date: 2004-02-10
Filing date: 2005-02-07
Publication date: 2005-08-18
Also published as: KR20050080668A; CN100375200C; KR100549862B1; CN1771567A

Abstract

An apparatus and a method for effectively retaining and cooling molten core material releasing from a reactor vessel when a severe accident occurs in a nuclear power plant. The apparatus includes a glass material mixing part, cooling-water retention tanks, and a cooling-water supply part. The cooling-water retention tanks are stacked below the glass material mixing part. The cooling- water supply part is provided at laterial side of the glass material mixing part and the cooling-water retention tanks. The glass-material mixing part includes a support plate, a glass material layer, and a blocking wall. The cooling-water supply part has a cooling-water storage tank, a passage, and a valve. The apparatus and method can lower the heat generation density of the molten core material and prevent the steam explosion and effectively cool down the molten core material.

Description

Description DEVICE AND PROCESS FOR COOLING MOLTEN CORE MATERIAL RELEASED FROM A REACTOR VESSEL Technical Field

[1] The present invention relates to an apparatus and a method for retaining and cooling molten core material in a reactor cavity, and more particularly, to an apparatus and a method for stably retaining and cooling molten core material inside a reactor cavity, by which when the molten core material releases out of a reactor vessel at a severe accident of meltdown in a nuclear power plant, the released molten core material does not erode the reactor cavity contaminating neighboring soil or threaten safety of a containment building.

[2] Background Art

[3] Generally, a nuclear power plant includes hundreds of systems having separate functions. These systems are roughly classified into a nuclear steam supply system (NSSS) having a nuclear reactor as its main element, a turbine/generator system having a generator driven by supplied steam and other auxiliary equipments. Here, the nuclear reactor controls large amount of energy instantaneously produced during a nuclear reaction to be slowly released so that nuclear energy can be utilized for real life.

[4] However, when molten core material releasing from a damaged reactor vessel during a severe accident in the nuclear power plant is not effectively retained and cooled, the released molten core material may unfortunately corrode a reactor cavity, contaminate neighboring soil, and threaten the soundness of a containment building.

[5] Therefore, an apparatus and a method for retaining and properly cooling the molten core material have been constantly studied.

[6] As a conventional apparatus for cooling the molten core material, an apparatus for contacting molten core material of high temperature directly with cooling water has been suggested.

[7] A method for cooling the molten core material by directly supplying the cooling water to the molten core material has high efficiency, but it is problematic in that the molten core material of high temperature directly reacts to the cooling water in a closed space to cause rapid steam generation thereby potentially creating steam explosion.

[8] Further, to prevent the steam explosion caused by direct interaction between the molten core material of high temperature and the cooling water, a core catcher having elongated molten core catch parts was proposed in Japanese Patent Publication No. 1996-43575. As illustrated in FIG. 1, the conventional apparatus has a plurality of molten core catcher parts 110 below a reactor vessel 100. In the apparatus, molten core released from the reactor vessel 100 during a severe accident in a nuclear power plant is retained in the molten core catcher parts 110, and the retained molten core is cooled by cooling water 120 filled around the molten core catcher parts 110.

[9] However, the above-described apparatus cools the molten core of high temperature by bringing the molten core into indirect contact with the cooling water. Accordingly, heat transfer efficiency between the molten core and the cooling water is undesirably reduced and thus a very large cooling water tank is required.

[10] Also, in some cases, the apparatus for cooling the molten core material of the related art is designed to forcibly cool down the molten core material using an active device such as a pump. However, this not only degrades the reliability when a severe accident occurs but also decreases cooling ability of the pump as time goes on so that decay heat coming from the molten core material cannot be effectively removed. Accordingly, it is particularly problematic in a long-term cooling.

[11] Therefore, an apparatus for retaining and cooling the molten core material capable of solving the above-described problems has been required in the art.

[12] Disclosure of Invention Technical Problem

[13] The object of the present invention is to provide an apparatus and a method for effectively removing decay heat as well as guaranteeing safety of neighboring buildings by preventing steam explosion during cooling process that uses interaction between molten core material and cooling water while effectively retaining the molten core material within a containment building.

[14] Technical Solution

[15] In order to achieve the above objects, the present invention provides an apparatus for retaining and cooling molten core material releasing from a reactor vessel due to meltdown, the apparatus comprising: a plate-shaped glass-material mixing part formed in a lower portion of the reactor vessel and having a glass material layer on its upper surface, such that molten core material that has passed through the reactor vessel mixes with glass material to lower a heat generation density; and a plurality of cooling- water retention tanks stacked in form of multi-layers below the glass-material mixing part, with cooling water containing inert gases being filled inside the cooling-water retention tanks, for cooling down the molten core material of high temperature.

[16] In another aspect of the present invention, there is provided an apparatus for retaining and cooling molten core material for performing a long-term cooling, which includes: a cooling- water supply part provided in a lateral side of the glass-material mixing part and the cooling-water retention tanks, the cooling-water supply part including a cooling-water storage tank, and a passage and a valve formed at one side of the cooling- water storage tank, for removing decay heat by additionally supplying the cooling water to the molten core material cooled down by the cooling water in the cooling-water retention tanks.

[17] The glass-material mixing part includes: a support plate installed at a lower portion of the reactor vessel, inclined at a predetermined angle, and formed with material not molten even if it contacts the molten core material; a glass material layer formed with a predetermined thickness on an upper surface of the support plate; and a blocking wall installed at a lower end of the inclined support plate, and formed of material that melts when it contacts the molten core material, for preventing the glass material formed on an upper surface of the support plate from flowing down.

[18] The glass material layer may be made of glass or graphite.

[19] Each of the cooling- water retention tanks may have a body made of material that melts when it contacts the molten core material of high temperature.

[20] The cooling-water supply part may further include a filtering unit on an upper portion of a cooling- water storage tank so that water from the condensing steam generated by interaction between the cooling water and the molten core material can flow back to the cooling- water storage tank.

[21] In yet another aspect of the present invention, there is provided a method for retaining and cooling down molten core material releasing from a reactor vessel due to meltdown, the method including steps of: (1) retaining the molten core material in a cavity of a containment building to restrict a passage thereof and mixing the molten core material with glass material to lower a heat generation density; and (2) causing the molten core material mixed with the glass material to flow down to cooling-water retention tanks stacked at a lower portion and contacting the mixture with cooling water containing inert gases in the cooling- water retention tanks, thereby cooling the molten core material.

[22] Further, the method may further include the step 3 of, after the step 2: opening a valve of a cooling-water supply part to supply cooling water through a passage from a cooling- water storage tank to remove decay heat from the molten core material, thereby achieving a long-term cooling.

[23] Here, the method may further include the step of: allowing water from the condensing steam generated by interaction between the cooling water and the molten core material to flow back to the cooling-water storage tank, thereby recycling the cooling water. [24] In the step 1, the molten core material may be mixed with glass material in a glass material layer formed on an upper surface of a support plate inclined at a predetermined angle below the reactor vessel.

[25] Further, in the step 2, the molten core material mixed with the glass material may fall down to the cooling-water retention tanks, which have a predetermined slope with respect to a bottom of a containment building and are stacked in form of multi-layers at a lower portion, and the mixture may sequentially flow along the predetermined slope to melt the cooling-water retention tanks and contact the cooling water containing inert gases inside the cooling-water retention tanks. Brief Description of the Drawings

[26] FIG. 1 is a schematic view of a non-contact type apparatus for cooling molten core material of a related art;

[27] FIG. 2 is a schematic view of an apparatus for cooling molten core material according to the present invention;

[28] FIG. 3 is a detailed view of a glass-material mixing part of the present invention;

[29] FIGs. 4A and 4B are cross-sectional views of stacked cooling-water retention tanks of the present invention; and

[30] FIG. 5 is a transversal, cross-sectional view of the cooling- water retention tank illustrated in FIG. 4.

[31] Best Mode for Carrying Out the Invention

[32] Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the attached drawings.

[33] FIG. 2 is a schematic view of an apparatus for cooling molten core material according to the present invention, which will be described as follows.

[34] To retain and cool down molten core material 5 generated due to meltdown in a reactor when a severe accident occurs in a unclear plant, a reactor vessel 10 is arranged in an upper portion of a containment building 1 of a predetermined size, and a glass- material mixing part 20 and cooling-water retention tanks 30 are provided in a lower cavity inside the containment building 1. The glass-material mixing part 20 is intended to mix the molten core material 5 releasing from the reactor vessel with glass material first in order to lower a heat generation density. Referring to FIG. 2, the glass-material mixing part is shaped as a plate inclined at a predetermined angle α.

[35] The cooling-water retention tanks 30 are stacked in form of multi-layers below the glass-material mixing part 20. The molten core material 5 mixed with the glass material in the glass-material mixing part 20 falls down to the cooling- water retention tanks 30, where the mixture interacts with the cooling water containing inert gases inside the cooling-water retention tanks 30, whereby the mixture is cooled down. A body of the cooling-water retention tanks 30 may be made of material that can be molten by the molten core material of high temperature.

[36] As described above, the molten core material 5 is mixed with the glass material first so that the heat generation density is lowered and further a steam explosion due to interaction with the cooling water can be suppressed using the inert gases, thus heat transfer efficiency by a direct contact with the cooling water can be increased and risk of steam explosion can be remarkably reduced.

[37] The cooling-water retention tanks 30 stacked in multi-layers may be inclined at a predetermined angle β with respect to a bottom of the containment building 1 so that the molten core material 5 may sequentially flows down.

[38] Further, the present invention can include a cooling-water supply part 40 having a cooling- water storage tank 43, a cooling- water passage 42 and a valve 41 formed on one side of the cooling-water storage tank 43 in order to remove decay heat after the cooling by the cooling water of the cooling-water retention tanks 30. When a severe accident occurs in a nuclear power plant and the valve 41 of the cooling- water supply part 40 is opened, the cooling water is supplied through the cooling- water passage 42 from the cooling-water storage tank 43 by gravity, so that the molten core material 5 can be further cooled down after the cooling by the cooling water of the cooling-water retention tanks 30 and thus its decay heat can be removed.

[39] In addition, the present invention further includes a filtering device 44 at an upper portion of the cooling- water supply part 40. Steam generated by interaction between the cooling water and the molten core material is condensed into water, and the water flows back to the cooling- water storage tank 43, whereby the cooling water is recycled. Therefore, a long-term cooling can be effectively performed.

[40] Of course, a water-supply line (not shown) for maintaining supplied water to an appropriate level all the time is connected to the cooling- water storage tank 43.

[41] FIG. 3 is a detailed view of a glass-material mixing part of the present invention. The present invention has the glass-material mixing part 20 below the reactor vessel, which serves to mix the molten core material 5 with the glass material first in order to lower a heat generation density. The glass-material mixing part 20 includes a support plate 21 preferably inclined at a predetermined angle α. The support plate 21 is made of material that does not melt even if it contacts the molten core material 5. Such configuration is intended for allowing the molten core material 5 to flow toward a lower portion of the containment building after the material 5 that has passed through the reactor vessel is mixed with the glass material.

[42] Further, a glass material layer 22 formed in a predetermined thickness is provided to an upper surface of the support plate 21. The molten core material that has passed through the reactor vessel is mixed with the glass material in the glass material layer 22. At this point, the glass material layer 22 may be made of glass or graphite.

[43] Further, the glass or the graphite may be prepared in form of grains or powder.

[44] The glass material is mixed with the molten core material 5 to lower the heat generation density.

[45] A blocking wall 23 is installed at a lower end of the inclined support plate 21. The blocking wall 23 may be made of material that melts when it contacts the molten core material 5 of high temperature releasing from the reactor vessel. Then, the blocking wall 23 prevents the glass material formed on the upper surface of the support plate 21 from flowing down in normal times and is molten by the molten core material 5 of high temperature when a severe accident occurs in the nuclear power plant so that the material 5 mixed with the glass material can flow down to the lower portion of the containment building.

[46] FIGs. 4 and 5 are views illustrating a cross-section of stacked cooling-water retention tanks and a transversal, cross-section of a cooling-water retention tank.

[47] To prevent steam explosion, the present invention has the cooling- water retention tanks 30 including the cooling water containing inert gases to cool down the molten core material 5. The cooling- water retention tanks 30 can be stacked in multi-layers below the glass-material mixing part 20. At this point, an arrangement direction of the cooling-water retention tanks 30 of each layer can be the same as or perpendicular to that of an adjacent layer of the retention tanks 30. For example, referring to FIG. 4A, all of the cooling-water retention tanks 30 are arranged in the same direction and stacked in multi-layers. Alternatively, the cooling-water retention tanks 30 can be stacked in such a way that the tanks 30 of one layer are arranged perpendicular with those of adjacent layers as illustrated in FIG. 4B.

[48] At this point, the cooling-water retention tanks 30 may be so stacked as to form a predetermined angle β with respect to the bottom of the containment building. With such configuration, the molten core material 5 is cooled down by the cooling- water retention tanks 30 on the first layer while flowing at the predetermined angle β and then the molten core material 5 is sequentially cooled down again by the cooling- water retention tanks 30 on the second layer this time.

[49] Further, referring to FIG. 5, a body 31 of such a cooling- water retention tank may be made of material having a low melting point to melt when it contacts the molten core material 5 of high temperature. The cooling water 33 containing inert gases 32 is filled inside the cooling- water retention tanks 30. After the bodies 31 of the cooling- water retention tanks 30 are molten due to a contact with the molten core material 5 of high temperature, the molten core material 5 reacts to the cooling water 33 containing the inert gases 32. Therefore, the molten core material 5 directly interacts with the cooling water so that cooling efficiency is increased and steam explosion can be remarkably reduced.

[50] Operation of the apparatus having the above-described construction for cooling the molten core material and the method thereof will be described below with reference to FIGs. 2 to 5.

[51] The method includes the step 1 of retaining the molten core material in a cavity of the containment building to restrict a passage thereof and mixing the molten core material with the glass material to lower a heat generation density.

[52] The step 1 is performed, at a severe accident in the nuclear power plant, by retaining the molten core material 5 releasing below the reactor vessel 10 due to meltdown of the core in a cavity of the containment building 1 to restrict a passage thereof and mixing the molten core material 5 with the glass material to lower the heat generation density first.

[53] For that purpose, referring to FIG. 2, the glass-material mixing part 20 has the support plate 21 installed below the reactor vessel 10. At this point, the support plate 21 may be inclined at a predetermined angle α so that the molten core material 5 that has passed through the reactor vessel 10 may be mixed with the glass material of the glass material layer 22 and then flow down to the lower portion of the containment building 1. Therefore, the glass material layer 22 formed in a predetermined thickness is provided to an upper surface of the support plate 21 so that the molten core material 5 may be mixed with the glass material.

[54] The blocking wall 23 installed at a lower end of the inclined support plate 21 prevents the glass material formed on the upper surface of the support plate 21 from flowing down in normal times, but when a severe accident occurs, is molten by the molten core material 5 of high temperature so that the molten core material 5 mixed with the glass material may flow down to the lower portion.

[55] Next, the method for cooling the molten core material includes the step 2 of causing the molten core material mixed with the glass material to flow down to cooling-water retention tanks 30 stacked at the lower portion to contact cooling water 33 containing inert gases 32 in the cooling-water retention tanks 30, thereby cooling the mixed molten core material.

[56] The step 2 is a cooling step by the inert gases 32 and the cooling water 33. The molten core material 5 mixed with the glass material falls down to the cooling- water retention tanks 30 stacked below the glass-material mixing part 20, the body 31 of the cooling-water retention tanks 30 is molten when it contacts the molten core material 5 of high temperature, and the molten core material is cooled down through interaction with the cooling water 33 containing the inert gases 32 in the cooling-water retention tanks 30. [57] Here, the cooling-water retention tanks 30 form a predetermined angle β with respect to the bottom of the containment building 1, and are stacked in multi-layers. Accordingly, while flowing down, the molten core material 5 may be cooled down by the cooling-water retention tanks 30 in the first layer and cooled down again by the cooling-water retention tanks 30 in the second layer, and cooled down step by step by subsequently underlying layers of the cooling-water retention tanks.

[58] As described above, the present invention mixes the molten core material 5 with the glass material first to lower the heat generation density and then has the molten core material touched with the stacked cooling-water retention tanks 30 to cool down its temperature. Therefore, the cooling can be efficiently performed and a danger of steam explosion can be remarkably reduced.

[59] After the step 2 where the molten core material 5 is cooled down while sequentially passing through the stacked cooling-water retention tanks 30, the invention can additionally proceed with the step 3 of: opening the valve 41 of the cooling- water supply part 40 and supplying the cooling water through the passage 42 from the cooling- water storage tank 43, thereby removing decay heat from the molten core material.

[60] In the step 3, as the valve 41 of the cooling- water supply part 40 is opened, the cooling water is supplied through the passage 42 from the cooling- water storage tank 43, so that the decay heat of the molten core material 5 can be completely removed.

[61] Unlike a forcible (active) system for supplying the cooling water using a pump according to the related art, the cooling and the decay heat removal processes by the cooling water are naturally (passively) carried out according to the present invention, whereby complete treatment can be taken with improved reliability when a severe accident occurs.

[62] Steam generated by interaction between the cooling water and the molten core material 5 during the above-described cooling process is discharged to atmosphere from the reactor cavity inside the containment building 1 and condensed on a surface of the containment building 1 above the reactor vessel 10. Then, the condensed water can flow down along the surface of the containment building 1 above the reactor vessel 10, pass through the filtering device 44, and flow back to the cooling- water storage tank 43.

[63] As described above, the present invention has a cooling-water supply part 40 to effectively remove decay heat, and further, recycles the cooling water, which is very effective for a long-term cooling.

[64] Although the preferred embodiments of the present invention have been disclosed for illustrative purpose, those skilled in the art will appreciate that various modifications and equivalents to the embodiments can be made without departing from the scope and spirit of the invention as defined in the accompanying claims. In particular, while various material changes, functional additions, shape changes or size changes to the components of the invention can be addressed, it is apparent that those fall within the scope of right of the invention.

[65] Industrial Applicability

[66] The apparatus and method for cooling the molten core material can lower the heat generation density of the molten core material 5 by mixing the molten core material 5 that has passed through the reactor vessel 10 with the glass material first, and prevent the steam explosion and effectively cool down the molten core material 5 by having the molten core material 5 flow to the cooling-water retention tanks 30 containing the inert gases.

[67] Further, the present invention has the cooling- water supply part 40 to completely remove decay heat from the molten core material 5.

[68] In addition, the cooling and the decay heat removal processes by the cooling water are naturally (passively) performed, whereby measures can be taken with high reliability when a severe accident occurs.

Claims

[1] An apparatus for retaining and cooling molten core material releasing from a reactor vessel due to meltdown, the apparatus comprising: a plate-shaped glass-material mixing part formed in a lower portion of the reactor vessel and having a glass material layer on its upper surface, such that molten core material that has passed through the reactor vessel mixes with glass material to lower a heat generation density; and a plurality of cooling- water retention tanks stacked in form of multi-layers below the glass-material mixing part, with cooling water containing inert gases being filled inside the cooling-water retention tanks, for cooling down the molten core material of high temperature.

[2] The apparatus of claim 1, further comprising: a cooling- water supply part provided in a lateral side of the glass-material mixing part and the cooling- water retention tanks, the cooling-water supply part including a cooling- water storage tank, and a passage and a valve formed at one side of the cooling- water storage tank, for removing decay heat by additionally supplying the cooling water to the molten core material cooled down by the cooling water in the cooling- water retention tanks, thereby performing a long-term cooling.

[3] The apparatus of claim 1, wherein the glass-material mixing part comprises: a support plate installed at a lower portion of the reactor vessel, inclined at a predetermined angle, and formed with material not molten even if it contacts the molten core material; a glass material layer formed with a predetermined thickness on an upper surface of the support plate; and a blocking wall installed at a lower end of the inclined support plate, and formed of material that melts when it contacts the molten core material, for preventing the glass material formed on an upper surface of the support plate from flowing down.

[4] The apparatus of claim 3, wherein the glass material layer is made of glass or graphite.

[5] The apparatus of claim 1, wherein each of the cooling-water retention tanks has a body made of material that melts when it contacts the molten core material of high temperature.

[6] The apparatus of claim 1, wherein the cooling- water retention tanks are stacked in form of multi-layers at a predetermined slope with respect to a bottom of a containment building.

[7] The apparatus of claim 2, wherein the cooling-water supply part is so configured as to remove decay heat when the valve is opened and the cooling water is supplied to the cooled-down molten core material through a passage formed at one side of the cooling- water storage tank, and to allow condensed water formed on an upper portion of a cooling- water storage tank to flow back to the cooling- water storage tank through a filtering device .

[8] A method for retaining and cooling down molten core material releasing from a reactor vessel due to meltdown, the method comprising steps of: (1) retaining the molten core material in a cavity of a containment building to restrict a passage thereof and mixing the molten core material with glass material to lower a heat generation density; and (2) causing the molten core material mixed with the glass material to flow down to cooling- water retention tanks stacked at a lower portion and contacting the mixture with cooling water containing inert gases in the cooling- water retention tanks, thereby cooling the molten core material.

[9] The method of claim 8, further comprising the step of, after the step (2): (3) opening a valve of a cooling-water supply part to supply cooling water through a passage from a cooling-water storage tank to remove decay heat from the molten core material, thereby achieving a long-term cooling.

[10] The method of claim 9, further comprising a step of: allowing water from the condensing steam generated by interaction between the cooling water and the molten core material to flow back to the cooling- water storage tank, thereby recycling the cooling water.

[11] The method of claim 8, wherein the step (1) is performed to mix the molten core material with the glass material of a glass material layer formed on an upper surface of a support plate inclined at a predetermined angle below the reactor vessel.

[12] The method of claim 8, wherein the step (2) is performed by causing the molten core material mixed with the glass material to flow down to the cooling- water retention tanks, which have a predetermined slope with respect to a bottom of a containment building and are stacked in form of multi-layers at a lower portion of the containment building, and allowing the mixture to sequentially flow along the predetermined slope to melt the cooling-water retention tanks and contact the cooling water containing inert gases inside the cooling-water retention tanks.