Combined cooling structure of passive nuclear reactor pressure vessel lower sealing head

Abstract

The invention provides a combined cooling structure of a passive nuclear reactor pressure vessel lower sealing head, and belongs to the field of nuclear reactor safety. The problems of generation of a large number of steam bubbles and boiling critical phenomena caused by over-high temperature of the outer wall surface of the lower sealing head of the nuclear reactor pressure vessel are solved. The device comprises a pressure vessel lower sealing head, wherein a heat insulation air gap, a phase change material, a self-supporting frame layer and a heat preservation layer are sequentially arranged outside the pressure vessel lower sealing head; a water chamber is formed between the self-supporting type frame layer and the heat preservation layer, and the water chamber is provided with a water supplementing valve used for supplementing cooling water into the water chamber and an explosion valve used for releasing pressure in the water chamber. The cooling device is mainly used for cooling the lower end socket of the nuclear reactor pressure vessel.

Description

Technical Field

[0001] The invention belongs to the field of nuclear reactor safety, and in particular relates to a combined cooling structure of a lower head of a passive nuclear reactor pressure vessel. Background Art

[0002] When an accident occurs in a nuclear reactor, it is required to immediately insert the control rods into the nuclear reactor to shut down the nuclear reactor to ensure the safety of the reactor and the integrity of the pressure boundary. However, the residual power for a period of time after the reactor is shut down is still very considerable. If the residual power cannot be removed in a timely, continuous and effective manner, it may cause serious consequences such as the melting or even meltdown of the reactor core, such as the Three Mile Island nuclear accident, the Chernobyl nuclear accident and the Fukushima nuclear accident. The molten material formed after the core melts or melts down will fall into the lower head of the reactor pressure vessel under the action of gravity. If the heat transferred to the lower head of the pressure vessel by the molten material cannot be removed in time, the lower head of the pressure vessel may melt through, causing radioactive leakage.

[0003] In order to deal with this potential threat, the AP600 and AP1000 designed by Westinghouse Electric Company of the United States are equipped with a reactor cavity cooling system to prevent the core melt from melting through the lower head of the pressure vessel. The working principle of this system is that after a core meltdown accident occurs, external cooling water is directly injected into the reactor cavity between the lower head of the nuclear reactor pressure vessel and the insulation layer to form a water chamber, and the convection heat exchange and radiation heat exchange between the water in the reactor cavity and the lower head of the pressure vessel are used to cool the outer wall of the lower head of the pressure vessel to prevent the lower head of the nuclear reactor pressure vessel from being melted through. There may still be some problems with the system: first, after a serious core meltdown accident occurs, the temperature of the outer wall of the pressure vessel is too high. Since the insulation layer is a non-pressure-bearing structure, the cooling water in the water chamber is also in a low-pressure state. The excessive temperature difference will cause the outer wall of the lower head of the pressure vessel to easily produce a boiling critical phenomenon due to the accumulation of a large number of bubbles, thereby destroying the cooling capacity of the cooling water on the pressure vessel, resulting in the risk of the pressure vessel still melting through; second, the cooling water in the water chamber needs to be injected after the accident occurs, and the process of core meltdown in the reactor is generally relatively fast. The operating personnel need to have extremely fast judgment and operation capabilities, which may further aggravate the consequences of serious accidents due to human-induced accidents; third, the cooling water injection process involves the actions of pumps, valves, etc. If there are mechanical problems with the pumps, valves, etc., it will be difficult to control the consequences of the accident.

[0004] In order to solve the problems existing in the AP600 / A1000 reactor cavity cooling system, someone applied for a patent for a reactor pressure vessel external cooling system, proposing to lay metal gallium outside the pressure vessel to prevent steam explosions outside the nuclear reactor pressure vessel and melting of the lower head of the pressure vessel. However, how to ensure the phase state of metal gallium under normal operation is an important issue. In addition, metal gallium is highly corrosive to other metals, and metal gallium may destroy other metals and cause them to lose their due strength and rigidity. Furthermore, under accident conditions, relying solely on the melting heat of gallium to maintain the integrity of the lower head of the pressure vessel requires too much metal gallium, which is difficult to use in engineering.

[0005] In another patent for a metal insulation device that can achieve external cooling of a reactor pressure vessel, a metal insulation layer structure is used to promptly conduct heat from the nuclear reactor pressure vessel after a serious accident. However, this method does not solve the problem faced by the AP600 / AP1000 reactor cavity cooling system, namely, excessively high outer wall temperature of the nuclear reactor pressure vessel will lead to the generation of a large number of bubbles and the appearance of boiling criticality, making it difficult to truly solve the problem of ensuring the integrity of the pressure vessel after a serious accident. Summary of the invention

[0006] In view of this, the present invention aims to propose a combined cooling structure for the lower head of a passive nuclear reactor pressure vessel to solve the problem of a large number of bubbles generated and boiling critical phenomenon caused by the excessively high temperature of the outer wall of the lower head of the nuclear reactor pressure vessel.

[0007] To achieve the above-mentioned object, the present invention adopts the following technical scheme: a combined cooling structure of a lower head of a passive nuclear reactor pressure vessel, comprising a lower head of a pressure vessel, on the outside of which a heat-insulating air gap, a phase change material, a self-supporting frame layer and a heat-insulating layer are sequentially arranged;

[0008] A water chamber is formed between the self-supporting frame layer and the thermal insulation layer, and the water chamber is provided with a water replenishment valve for replenishing cooling water therein and a bursting valve for releasing the pressure therein.

[0009] Furthermore, the thickness of the thermal insulation air gap is 5-10 mm.

[0010] Furthermore, the phase change material is a metal material and has a melting point between 60-260°C.

[0011] Furthermore, the self-supporting frame layer is a frame structure.

[0012] Furthermore, reinforcing ribs are arranged inside the self-supporting frame layer.

[0013] Furthermore, the self-supporting frame layer is made of metal and has a melting point higher than that of the phase change material.

[0014] Furthermore, cooling water is pre-installed in the water chamber.

[0015] Furthermore, the water replenishment valve is arranged at the bottom of the water chamber.

[0016] Furthermore, the bursting valve is arranged at the upper part of the water chamber.

[0017] Furthermore, the bursting valves are provided in a plurality and are evenly distributed around the water chamber.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] This structure reduces the heat loss of the reactor during normal operation by setting up an insulating air gap, ensuring that the phase change material will not melt during normal operation of the reactor. When a core meltdown accident occurs, the phase change material reaches the melting point temperature and begins to melt under the action of the thermal radiation of the lower head. The melted phase change material gradually fills the insulating air gap. During this process, the phase change material will be in direct contact with the lower head. The phase change material will absorb the heat of the lower head through heat conduction and heat convection, thereby reducing the heat load imposed on the lower head by the melted core, controlling the temperature rise of the lower head, and extending the effective time of the lower head. At the same time, the cooling water reserved in the water chamber will absorb the heat of the phase change material, extend the heat sinking time of the phase change material, and improve its thermal control performance. When the cooling water begins to boil, the pressure in the water chamber gradually begins to rise. When the safety value of the burst valve is reached, the burst valve will automatically open to release the pressure.

[0020] At the end of the accident, the operator can open the water supply valve to achieve long-term cooling of the lower head. The temperature difference between the phase change material and the cooling water inside the self-supporting frame layer structure forms a heat exchange channel, which not only limits the melting speed of the phase change material, but also further reduces the temperature of the lower head, ensuring the integrity of the lower head of the nuclear reactor pressure vessel and preventing the lower head of the nuclear reactor pressure vessel from boiling. It has high reliability. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the accompanying drawings:

[0022] Figure 1 It is a schematic diagram of the overall structure of a combined cooling structure for the lower head of a passive nuclear reactor pressure vessel according to the present invention;

[0023] Figure 2 It is a partial structural cross-sectional view of a combined cooling structure of a lower head of a passive nuclear reactor pressure vessel according to the present invention;

[0024] Figure 3 State diagram of the existing solution before water injection;

[0025] Figure 4 State diagram after water injection for the existing solution;

[0026] Figure 5 The present invention is a schematic diagram of a specific implementation process of a combined cooling structure for a lower head of a passive nuclear reactor pressure vessel.

[0027] The lower head of the pressure vessel 1; the heat-insulating air gap 2; the phase change material 3; the self-supporting frame layer 4; the water chamber 5; the thermal insulation layer 6; the water supply valve 7; and the bursting valve 8. DETAILED DESCRIPTION

[0028] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely explain the technical solutions in the embodiments of the present invention. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict, and the described embodiments are only part of the embodiments of the present invention, not all of the embodiments.

[0029] It should be noted that the descriptions of the present invention regarding directions such as "left", "right", "left side", "right side", "upper", "lower", "top", "bottom", etc. are all defined based on the relationship between the orientations or positions shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the structure described must be constructed and operated in a specific orientation, and therefore, cannot be understood as a limitation on the present invention. In the description of the present invention, the meaning of "multiple" is more than two, unless otherwise clearly and specifically defined.

[0030] In the description of the present invention, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a direct connection, or an indirect connection through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0031] Referring to the accompanying drawings, this embodiment is described. A combined cooling structure for the lower head of a passive nuclear reactor pressure vessel is characterized in that it includes: a lower head of a pressure vessel 1, an insulating air gap 2, a phase change material 3, a self-supporting frame layer 4 and a thermal insulation layer 6 arranged in sequence from the inside to the outside; the phase change material 3 is generally made of metal tin, tin-bismuth alloy or tin-bismuth-indium alloy. This is not exhaustive, and other phase change materials that are conducive to achieving the specific functions of this application can also be used in this application, all within the spirit of the invention of this application. The thermal insulation layer 6 can be made of existing materials that are conducive to thermal insulation, and a reasonable selection is made according to the use requirements.

[0032] A water chamber 5 is formed between the self-supporting frame layer 4 and the thermal insulation layer 6 , and the water chamber 5 is provided with a water replenishment valve 7 for replenishing cooling water therein and a bursting valve 8 for releasing the pressure therein.

[0033] Among them, the water replenishment valve 7 is arranged at the bottom of the water chamber 5, and water is replenished from bottom to top. Under normal circumstances, it is always in a closed state. In the event of an accident, the valve is opened to replenish the cooling water in the water chamber 5, thereby realizing long-term cooling of the nuclear reactor. The specific position of the water replenishment valve 7 at the bottom is selected according to actual needs. The setting methods that help replenish cooling water, ensure flow rate and cooling effect can be used in this application. There are three bursting valves 8 and they are evenly distributed in a circle relative to the water chamber 5. Specifically, they can be set at the top of the water chamber 5. When the bursting valve 8 explodes, it helps to discharge the pressure upward. Here, setting the bursting valve 8 to three is a better solution. Other numbers or valve body types that can meet the gas discharge function are within the spirit of the present invention.

[0034] In this embodiment, the thickness of the heat insulating air gap 2 is specifically 5-10 mm. When the nuclear reactor is operating normally, the temperature of the phase change material 3 is limited by the heat insulating effect of the heat insulating air gap, ensuring that the temperature of the phase change material 3 is lower than its melting point.

[0035] In this embodiment, the phase change material 3 is specifically a phase change material layer. The phase change material should be a metal material with high thermal conductivity, high specific heat, and high melting heat. The melting point of the metal material is between 60-260°C.

[0036] In this embodiment, the self-supporting frame layer 4 is a frame structure. Reinforcing ribs are provided inside, and the reinforcing ribs are arranged in a horizontal and vertical staggered manner. It can form a certain supporting effect, and any metal layer with a self-supporting frame structure can be used in this application, mainly to form a supporting and wrapping effect on the phase change material 3 inside it. When the phase change material 3 melts, it can continue to support it and provide a convection space after the phase change material melts. The heat exchange channel formed by the temperature difference between the cooling water in the water chamber 5 and the phase change material 3 not only limits the melting speed of the phase change material 3, but also can further reduce the temperature of the lower head. In addition, the phase change material 3 and the cooling water in the water chamber 5 will form a natural circulation inside each of them. The double natural circulation can further enhance the heat exchange capacity of the lower head 1 of the pressure vessel and ensure the integrity of the lower head of the nuclear reactor pressure vessel.

[0037] In this embodiment, the self-supporting frame layer 4 is made of metal and has a higher melting point than the phase change material 3. This setting is mainly to support the phase change material 3 after the phase change material 3 melts, so that the temperature difference effect between the phase change material 3 and water can achieve natural circulation and reduce the temperature. For the self-supporting frame layer 4, a structure that is conducive to providing wrapping and support for the phase change material 3 is adopted. In order to improve the support, the interior is set as a frame structure, and some horizontal and vertical staggered support ribs can be set according to actual conditions to improve the structural strength. The self-supporting frame layer 4 can meet the working temperature requirements and strength requirements. It can be reasonably set according to actual needs. The self-supporting frame layer 4 structure that can meet the requirements of this application can be used in this application.

[0038] In this embodiment, cooling water is pre-installed in the water chamber 5. The cooling water can cool the self-supporting frame layer 4 and the phase change material 3, and realize natural circulation under the action of temperature difference, and can also shield the radiation generated during the operation of the reactor, reducing the radiation dose of the reactor to the surrounding environment.

[0039] For existing technical solutions, such as Figure 3 As shown, only an insulation layer 6 is arranged outside the lower head 1 of the pressure vessel, and the space between the two is called a reactor cavity. Under normal circumstances, there is no cooling water in the reactor cavity, and only air is used for heat insulation. The insulation layer 6 is used to reduce the heat dissipation from the reactor core to the environment under normal operating conditions.

[0040] Traditional solution: In the early stage of the accident during the core melt, when the operator did not perform water injection into the reactor cavity, the core decay heat Q 堆芯 Mainly divided into Q RPV and Q 空气 Two parts, Q RPV Indicates the heat absorbed by the pressure vessel, Q 空气 It indicates the heat carried away by natural convection of air near the pressure vessel. However, since the amount of heat that can be carried away by natural convection of air is very small, while the decay heat of the core is very high, most of the heat will be absorbed by the pressure vessel, causing the temperature of the pressure vessel to rise rapidly. When the temperature exceeds the melting point of the pressure vessel material, the pressure vessel will gradually burn and melt through, thereby losing its containment function for the core, and eventually causing a large amount of radioactive material to leak. Similar accidents include the Three Mile Island nuclear accident in the United States. Due to misoperation by the operator, a large amount of core coolant was lost, which eventually led to the core meltdown. During the accident, a certain amount of radioactive material was leaked into the environment, causing widespread panic among residents.

[0041] Q 堆芯 =Q RPV +Q 空气 =m RPV c RPV ΔtRPV +h 空气 A 下封头 ΔT1

[0042] ΔT1=T W -T 空气

[0043] Where: Q 堆芯 is the decay heat generated by the core; Q RPV The heat absorbed by the pressure vessel; Q 空气 , the heat removed by air through natural convection; m RPV is the mass of the pressure vessel; c RPV is the specific heat of the pressure vessel; Δt RPV is the temperature rise of the pressure vessel; h 空气 is the convective heat transfer coefficient of air to the wall; A 下封头 is the surface area of ​​the outer wall of the lower head of the pressure vessel; ΔT1 is the temperature difference between the outer wall temperature of the lower head of the pressure vessel and the air temperature; Tw is the outer wall temperature of the lower head; T 空气 is the air temperature;

[0044] When the operator performs the water injection operation normally, the reactor cavity will be filled with water to form a water chamber 5. Figure 4 In the middle and late stages of the accident, the decay heat of the core will be absorbed by the cooling water, and the pressure vessel will continue to be effectively cooled by natural circulation. 水 Equal to the decay heat Q generated by the core 堆芯 However, due to the large decay heat generated by the core, the heat flux density q on the outer wall of the lower head is 下封头 Extremely high heat flux density will cause a large number of bubbles to form near the heat transfer surface. When the bubbles gather to a certain extent, the heat transfer coefficient h 水 The pressure drops rapidly, the heat taken away by the cooling water decreases sharply, the temperature of the pressure vessel rises rapidly, and eventually the pressure vessel is burned out.

[0045] Q 堆芯 =Q 水 =h 水 A 下封头 ΔT2

[0046] ΔT2=T W -T 水

[0047] Q 水 =q 下封头 A 下封头 =h 水 ΔT2A 下封头

[0048] q 下封头 ∝α

[0049]

[0050] Where: Q 堆芯 is the decay heat generated by the core; Q 水 The heat removed by cooling water through convection; h 水 A is the heat transfer coefficient of cooling water to the outer wall of the lower head; 下封头 is the surface area of ​​the outer wall of the lower head; ΔT2 is the temperature difference between the outer wall temperature of the lower head and the cooling water temperature; Tw is the outer wall temperature of the lower head; T 水 is the cooling water temperature; q 下封头 is the heat flux density on the outer surface of the lower head; α is the cavitation fraction on the outer surface of the lower head.

[0051] The principle of this structure: In the early and middle stages of an accident, when the operator does not discover the core melt and does not perform water injection operations, the decay heat Q 堆芯 It is mainly divided into three parts: RPV Indicates the heat absorbed by the pressure vessel, Q PCM represents the heat absorbed by the phase change material, Q 水 Indicates the amount of heat absorbed by cooling water. The biggest difference from the original solution is that the phase change material and cooling water have been added to absorb heat, effectively controlling the temperature rise of the pressure vessel and ensuring the integrity of the pressure vessel in the early stages of an accident.

[0052] Specifically, the heat absorbed by the phase change material from the core can be divided into two parts: sensible heat and latent heat. The former is manifested as a rise in the temperature of the phase change material, and the latter is manifested as the melting of the phase change material. The same is true for cooling water. The heat absorbed is divided into two parts: sensible heat and latent heat. The former is manifested as a rise in the temperature of the cooling water, and the latter is manifested as the vaporization of the cooling water. With this new cooling solution, in the early stage of the core meltdown accident, the lower head of the pressure vessel is effectively cooled in a completely passive manner, further extending the time that the temperature of the lower head of the pressure vessel is within a controllable range, and buying a certain amount of reaction time for the operator to discover the accident.

[0053] Q 堆芯 =Q RPV +Q 空气

[0054] Q 堆芯 =Q RPV +Q PCM +Q 水

[0055] Q RPV =m RPV c RPV Δt RPV

[0056] QPCM =m PCM c PCM Δt PCM +m PCM H PCM

[0057] Q 水 =m 水 c 水 Δt 水 +m 水 H 水

[0058] Where: Q 堆芯 is the decay heat generated by the core; Q RPV The heat absorbed by the pressure vessel; Q PCM is the heat absorbed by the phase change material; Q 水 The heat absorbed by cooling water; m PCM is the mass of phase change material; c PCM is the specific heat of the phase change material; Δt PCM is the temperature rise of phase change material; H PCM is the melting latent heat of the phase change material; m 水 is the cooling water quality; c 水 is the specific heat of cooling water; Δt 水 is the cooling water temperature rise; H 水 is the latent heat of vaporization of cooling water;

[0059] At the end of the accident, after the operator discovered the core melt accident and promptly injected water into the water chamber 5, the decay heat Q generated by the core 堆芯 Heat is indirectly transferred through the phase change material and eventually carried away by the cooling water through convection heat transfer. 堆芯 =Q 水 , and since the phase change material is arranged outside the pressure vessel, the heat exchange surface is expanded from the outer wall of the original pressure vessel lower head 1 to the outer wall of the phase change material, so A PCM Area larger than A 下封头 , and the decay heat of the core is the same, so the heat flux density q on the outer surface of the phase change material PCM Smaller than the heat flux density q on the outer surface of the original pressure vessel lower head 下封头 According to relevant research, the cavitation fraction near the wall of the heat exchange surface is positively correlated with the heat flux density. Therefore, the cooling scheme proposed in this paper generates fewer bubbles near the wall, allowing the cooling water to more stably carry away the decay heat generated by the core.

[0060] Q 堆芯 =Q 水 =q PCM A PCM =h 水 A PCM ΔT2

[0061] ΔT2=T W -T 水

[0062] A PCM >>A 下封头

[0063]

[0064] Figure 5 The present invention is a schematic diagram of a specific implementation process of a combined cooling structure for a passive nuclear reactor pressure vessel lower head. Stage 1 represents the situation when the nuclear reactor is operating normally. Since the air gap has a certain heat insulation effect, the phase change material 3 will not melt.

[0065] When a partial melting or meltdown accident of the core occurs, the molten material falls to the lower head 1 area of ​​the reactor pressure vessel under the action of gravity. Due to the extremely high temperature of the molten material, the temperature of the lower head 1 area of ​​the reactor pressure vessel rises. Relying on the convection and heat conduction of the insulating air gap 2 and the radiation heat exchange of the lower head 1 of the reactor pressure vessel through the insulating air gap 2, the heat is transferred to the phase change material 3. When the temperature of the phase change material 3 reaches its melting point, it begins to melt. During the melting process, the volume of the phase change material 3 gradually increases, and it falls to the bottom under the action of gravity and gradually fills the insulating air gap. This stage is stage two.

[0066] When the melted phase change material 3 fills the insulating air gap, the phase change material 3 is in direct contact with the lower head 1 of the pressure vessel. At this time, the phase change material 3 can absorb the heat of the lower head 1 of the pressure vessel through heat conduction and convection heat transfer, suppress the temperature rise of the lower head 1 of the pressure vessel, and ensure that the temperature of the lower head 1 of the pressure vessel does not exceed the allowable limit in a passive manner in the early stage of the accident. This stage is stage three.

[0067] While the phase change material 3 absorbs the core decay heat, the cooling water reserved in the water chamber 5 synchronously absorbs the heat in the phase change material 3. The cooling water can greatly extend the heat sinking time of the phase change material 3 and improve the thermal control performance of the phase change material 3. This stage is stage four.

[0068] After a period of time, as the cooling water temperature increases and the heat is continuously extracted, the cooling water in the water chamber 5 begins to boil, and the pressure in the water chamber 5 begins to gradually increase until the pressure increases to the bursting pressure of the bursting valve 8, and the bursting valve 8 opens. During this period, the structure provides the function of extracting the heat of the molten material in a passive manner in the middle of the accident, limiting the temperature of the lower head of the pressure vessel to not exceed the allowable limit. After the bursting valve 8 is opened, the pressure in the water chamber 5 is reduced to the ambient pressure, and the water supply valve 7 is opened to replenish water to the water chamber 5, providing long-term cooling of the lower head of the pressure vessel after the accident. This stage is stage five.

[0069] The combined cooling structure of the lower head of a passive nuclear reactor pressure vessel proposed by the present invention operates completely in a passive manner in the early and middle stages after a partial core melting or meltdown accident occurs in the reactor. After the accident, an active water replenishment method is used for a long time to achieve effective cooling of the lower head of the pressure vessel, thereby ensuring the integrity of the pressure vessel after the accident.

[0070] The embodiments of the present invention disclosed above are only used to help illustrate the present invention. The embodiments do not describe all the details in detail, nor do they limit the invention to the specific implementation methods described. According to the content of this specification, many modifications and changes can be made. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the present invention, so that those skilled in the art can understand and use the present invention well.

Claims

1. A combined cooling structure for a lower head of a passive nuclear reactor pressure vessel, comprising a lower head of the pressure vessel (1), characterized in that: A heat-insulating air gap (2), a phase-change material (3), a self-supporting frame layer (4) and a heat-insulating layer (6) are sequentially arranged outside the lower end cover (1) of the pressure vessel; A water chamber (5) is formed between the self-supporting frame layer (4) and the thermal insulation layer (6), and the water chamber (5) is provided with a water replenishment valve (7) for replenishing cooling water therein and a bursting valve (8) for releasing the pressure therein.

2. The combined cooling structure of the lower head of a passive nuclear reactor pressure vessel according to claim 1, characterized in that: The thickness of the heat insulating air gap (2) is 5-10 mm.

3. A combined cooling structure for a lower head of a passive nuclear reactor pressure vessel according to claim 1 or 2, characterized in that: The phase change material (3) is a metal material and has a melting point between 60-260°C.

4. The combined cooling structure of the lower head of a passive nuclear reactor pressure vessel according to claim 3, characterized in that: The self-supporting frame layer (4) is a frame structure.

5. The combined cooling structure of the lower head of a passive nuclear reactor pressure vessel according to claim 4, characterized in that: The self-supporting frame layer (4) is provided with reinforcing ribs inside.

6. The combined cooling structure of the lower head of a passive nuclear reactor pressure vessel according to claim 4, characterized in that: The self-supporting frame layer (4) is made of metal and has a higher melting point than the phase change material (3).

7. A combined cooling structure for a lower head of a passive nuclear reactor pressure vessel according to claim 1, 2, 4, 5 or 6, characterized in that: The water chamber (5) is pre-filled with cooling water.

8. The combined cooling structure of the lower head of a passive nuclear reactor pressure vessel according to claim 7, characterized in that: The water replenishment valve (7) is arranged at the bottom of the water chamber (5).

9. The combined cooling structure of the lower head of a passive nuclear reactor pressure vessel according to claim 8, characterized in that: The bursting valve (8) is arranged at the upper part of the water chamber (5).

10. The combined cooling structure of the lower head of a passive nuclear reactor pressure vessel according to claim 9, characterized in that: A plurality of bursting valves (8) are provided and are evenly distributed around the circumference of the water chamber (5).

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