An enhanced passive containment heat removal system based on secondary evaporative cooling

Abstract

The present invention discloses an enhanced passive containment heat removal system (PCS) based on secondary evaporative cooling, comprising: a PCS water tank, a PCS system circulation pipeline is connected to the lower part of the PCS water tank body, and the PCS system circulation pipeline includes a PCS system descending pipeline and a PCS system ascending pipeline; a steam-water separator is arranged in the PCS water tank, and the lower end of the steam-water separator is connected to the PCS system ascending pipeline; the PCS water tank is provided with an exhaust channel at the upper part of the body on one side of the steam-water separator, an exhaust device is arranged above the exhaust channel, and an air intake device is arranged at the upper part of the body on the other side. The present invention solves the contradiction that the temperature of the water tank outside the shell reaches a saturated state and the system operation performance is reduced in the long-term operation stage of the existing passive containment heat removal system, and the secondary evaporative cooling water tank on the surface of the water tank outside the shell reduces the temperature of the water tank outside the shell during long-term operation, improves the system operation characteristics, and achieves the purpose of enhancing the performance of the passive containment heat removal system.

Description

Technical Field

[0001] The present invention relates to the field of passive systems, and in particular to an enhanced passive containment heat removal system based on secondary evaporative cooling. Background Art

[0002] A passive system refers to a system that does not rely on an external power source, but only relies on natural characteristics such as gravity and stored pressure potential energy to achieve its functions. Since it does not require external power source support and will not lose its function due to power source failure, passive systems are generally considered to have high reliability and are increasingly widely used in nuclear power plants with extremely high reliability requirements. At the same time, reducing power source support is conducive to simplifying system design. For nuclear power plant design, it is conducive to improving safety while ensuring economy. As a representative of passive safety system design, AP1000 widely adopts a primary circuit passive residual heat removal system, a passive injection system, a passive containment heat removal system, etc. Hualong One, a third-generation pressurized water reactor nuclear power unit independently developed by my country, adopts an active + passive design concept, and adds a passive containment heat removal system and a secondary circuit passive residual heat removal system as a supplement to the active safety system, mainly used to cope with design expansion conditions. The passive containment heat removal system (PCS) of the existing third-generation nuclear power models in China consists of a heat exchanger inside the containment, a high-level water tank outside the containment, and pipes and valves connected in the middle, such as Figure 1 In the accident condition, when the mass-energy release in the containment leads to a high temperature and high pressure state and the active containment spray system fails, the PCS starts to operate, the heat exchanger in the shell absorbs the heat in the containment, heats the water in the system, forms a temperature difference and density difference in the upper and lower pipes of the PCS system, and generates a natural circulation driving pressure head under the action of gravity, which continuously removes the heat in the containment.

[0003] The system characteristics of the existing third-generation nuclear power model PCS in China are: in the early stage of PCS operation, when the temperature of the water tank outside the containment is low, the system circulation flow is large, the water temperature in the heat exchanger is low, and the system heat carrying capacity is high; in the long-term stage, when the temperature of the PCS water tank rises and reaches the saturation temperature (100°C), the PCS system maintains high-temperature operation, the heat carrying capacity is reduced, and the containment equilibrium pressure and temperature under PCS cooling are maintained at a high level. How to improve the operating characteristics of PCS and improve its long-term equilibrium heat carrying capacity is an important goal of the design improvement of the existing third-generation nuclear power model PCS system. Summary of the invention

[0004] In view of the defects existing in the prior art, the object of the present invention is to provide an enhanced passive containment heat removal system based on secondary evaporative cooling, so as to further reduce the temperature of the external water tank and improve the heat exchange capacity of the system.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] An enhanced passive containment heat removal system based on secondary evaporative cooling comprises: a PCS water tank, wherein the lower part of the PCS water tank body is connected with a PCS system circulation pipeline, wherein the PCS system circulation pipeline comprises a PCS system descending pipeline and a PCS system ascending pipeline; a steam-water separator is arranged in the PCS water tank, wherein the lower end of the steam-water separator is communicated with the PCS system ascending pipeline; the PCS water tank is provided with an exhaust channel at the upper part of the body on one side of the steam-water separator, an exhaust device is arranged above the exhaust channel, and an air intake device is arranged at the upper part of the body on the other side;

[0007] When the PCS is running, water flows out of the PCS water tank from the PCS system downpipe, enters the PCS heat exchanger in the containment, is heated by the air in the containment, and flows back to the PCS water tank through the PCS system uppipe;

[0008] When the temperature of the PCS water tank is low, the PCS loop is a single-phase water cycle. When the temperature of the PCS water tank rises to a certain level, the water in the rising pipe of the PCS system will exceed the saturation temperature corresponding to the atmospheric pressure. As the fluid flows upward in the rising pipe of the PCS system, the pressure decreases, and a portion of the water will flash into steam and be discharged through the steam-water separator. After the water vapor overflows the water surface, it enters the exhaust channel and is discharged into the atmosphere through the exhaust device.

[0009] Furthermore, in the system as described above, due to the low density of water vapor, when the steam is discharged through the exhaust passage, a chimney effect is formed, and air is sucked into the PCS water tank from the air intake device. The air passes over the water surface of the PCS water tank, promoting surface evaporation, taking away the steam generated by evaporation, and being discharged through the exhaust passage and the exhaust device.

[0010] Furthermore, the system as described above achieves the effect of enhancing the chimney effect by reasonably setting the height and flow area of ​​the exhaust channel; the higher the height of the exhaust channel, that is, the greater the height difference between the air inlet of the air intake device and the exhaust port of the exhaust device, the stronger the chimney effect is and the greater the flow speed of the gas, so that the air inlet can inhale more air, thereby reducing the air humidity in the upper space of the PCS water tank and increasing the evaporation rate.

[0011] Furthermore, for the system as described above, the flow areas of the air inlet and the exhaust port and the height of the exhaust port are calculated based on a comprehensive consideration of the total power level of the PCS system, the cooling temperature required to be achieved by the PCS water tank, and the area of ​​the PCS water tank, and are based on the natural circulation Bernoulli equation and the water surface evaporation-diffusion equation, and the factors of engineering feasibility are taken into consideration.

[0012] Furthermore, in the system as described above, a plurality of air inlets or air inlet flow distribution channels are provided on the upper part of the PCS water tank body according to a certain area ratio, and the air outlets are located at both ends of the PCS water tank body.

[0013] Furthermore, in the system as described above, the exhaust passage is arranged just above the steam-water separator, ensuring that the saturated steam discharged through the steam-water separator can directly enter the exhaust passage without flowing in the upper part of the PCS water tank.

[0014] Furthermore, in the system as described above, an appropriate distance is provided between the PCS system descending pipe and the PCS system ascending pipe, so that the high-temperature water flowing from the PCS system ascending pipe into the PCS water tank can fully flow to various areas of the water tank surface and be fully cooled.

[0015] Furthermore, for the system as described above, the PCS water tank structure design should take into account the reasonable allocation of tank depth and area.

[0016] The beneficial effects of the present invention are as follows: the present invention solves the contradiction that the temperature of the water tank outside the shell reaches a saturated state and the system operating performance is reduced during the long-term operation stage of the existing passive containment heat removal system. The secondary evaporation cooling water tank on the surface of the water tank outside the shell can significantly reduce the temperature of the water tank outside the shell during long-term operation, improve the heat carrying capacity of the PCS in the long-term stage, improve the system operating characteristics, further reduce the temperature and pressure of the containment under the operation of the PCS, and achieve the purpose of enhancing the performance of the passive containment heat removal system. Through the present invention, it is expected that the water temperature of the water tank outside the shell can be controlled at 80-90°C during the long-term cooling stage, and the heat exchange power of the PCS can be increased by 20-80% under the same temperature and pressure conditions in the shell. Based on the principle of the present invention, the PCS box structure is optimized, etc., which can further reduce the temperature of the water tank outside the shell and improve the heat exchange capacity of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A schematic diagram of the structure of the Hualong One passive containment heat removal system provided in an embodiment of the present invention;

[0018] Figure 2 It is a structural schematic diagram of an enhanced passive containment heat removal system based on secondary evaporative cooling provided in an embodiment of the present invention.

[0019] Figure 3 , Figure 4 , Figure 5 , Figure 6 Four PCS water tanks and inlet and exhaust structures are provided in the embodiments of the present invention.

[0020] In the attached figure: 1-PCS water tank, 2-PCS system descending pipe, 3-PCS system ascending pipe, 4-steam-water separator, 5-exhaust channel, 6-exhaust port, 7-air inlet, 8-external high-level water tank, 9-internal heat exchanger. DETAILED DESCRIPTION

[0021] In order to make the technical problems solved by the present invention, the technical solutions adopted and the technical effects achieved more clearly, the technical solutions of the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0022] Evaporation on the surface of liquid is a natural phenomenon. The evaporation rate of the water surface is closely related to the temperature of the water body and the surface air pressure, temperature, humidity and other parameters. Generally speaking, when the water temperature rises above 60°C, the evaporation on the water surface will be more significant. The present invention makes full use of the physical phenomenon of evaporation on the water surface, and by setting a reasonable gas flow channel and utilizing the density difference between the high steam content fluid and the air, drives the flow of air and water vapor, forms an air flow rate and humidity that are conducive to evaporation on the water surface, forms secondary evaporation on the water surface of the PCS water tank, utilizes the latent heat of vaporization carried away by evaporation, and cools the PCS water tank, thereby achieving the effect of improving the operating characteristics of the PCS and enhancing the capabilities of the PCS system.

[0023] like Figure 2 As shown, an embodiment of the present invention provides an enhanced passive containment heat removal system based on secondary evaporative cooling, comprising: a PCS water tank 1, the lower part of the PCS water tank 1 is connected to a PCS system circulation pipeline, the PCS system circulation pipeline includes a PCS system down pipe 2 and a PCS system up pipe 3; a steam-water separator 4 is arranged in the PCS water tank 1, the lower end of the steam-water separator 4 is connected to the PCS system up pipe 3; the PCS water tank 1 is provided with an exhaust channel 5 at the upper part of the box on one side of the steam-water separator 4, an exhaust port 6 is arranged above the exhaust channel 5, and an air inlet 7 is arranged at the upper part of the box on the other side. When the PCS is running, water flows out of the PCS water tank 1 from the PCS system down pipe 2, enters the PCS heat exchanger in the containment, and flows back to the PCS water tank 1 through the PCS system up pipe 3 after being heated by the air in the shell. When the temperature of the PCS water tank 1 is low, the PCS loop is a single-phase water cycle. When the temperature of the PCS water tank 1 rises to a certain level, the water in the PCS system riser pipe 3 will exceed the saturation temperature corresponding to the atmospheric pressure. As the water flows upward in the PCS system riser pipe 3, the pressure decreases, and a portion of the water will flash into steam and be discharged through the steam-water separator 4. After the water vapor overflows the water surface, it enters the exhaust channel 5 and is discharged into the atmosphere through the exhaust port 6. Due to the low density of water vapor, when the steam is discharged through the exhaust channel 5, a chimney effect is formed, and air is sucked into the PCS water tank 1 from the air inlet. The air passes over the water surface of the PCS water tank 1, promoting surface evaporation, taking away the steam generated by evaporation, and is discharged through the exhaust channel 5 and the exhaust port 6.

[0024] The existing PCS shell water tank design of Hualong One has only one steam discharge channel with the atmospheric space, and it is difficult for the external atmosphere to enter the upper space inside the water tank. Therefore, the upper gas space inside the water tank is in a saturated state, which inhibits the evaporation of the water surface. The present invention can maintain the humidity of the upper gas space in the water tank within a certain range by setting the air intake and exhaust channels, through the continuous inflow of air and the discharge of the mixed gas containing water vapor, to ensure surface evaporation. Generally speaking, the lower the humidity of the upper gas space, the greater the evaporation rate.

[0025] In addition to being related to the humidity in the upper air space, the evaporation rate of the water surface in the PCS water tank 1 is also affected by the upper gas flow rate. A larger flow rate is conducive to increasing the evaporation rate. By reasonably setting the height and flow area of ​​the exhaust channel 5, the chimney effect can be enhanced. In theory, the greater the height of the exhaust channel 5, that is, the greater the height difference between the water surface of the PCS water tank 1 and the exhaust port 6, the stronger the chimney effect and the greater the flow rate of the gas. At the same time, a larger gas flow rate can inhale more air from the air inlet, which is conducive to reducing the air humidity in the upper space of the water tank and increasing the evaporation rate. The flow area of ​​the air inlet and exhaust device and the height of the exhaust port need to be comprehensively considered based on the total power level of the PCS system, the cooling temperature required to be achieved by the outer shell water tank, the area of ​​the outer shell water tank, etc., and are calculated based on the natural circulation Bernoulli equation (the balance of buoyancy force and flow resistance generated by the gas density difference), the water surface evaporation diffusion equation, etc., and consider factors such as engineering feasibility.

[0026] The present invention mainly utilizes the heat from the evaporation of the outer water tank surface, and the evaporation amount is closely related to the water surface area in the water tank. When the total water storage capacity is constant, a deeper tank body and a smaller water surface area are not conducive to the total evaporation amount. Therefore, the design of the outer water tank structure should consider the reasonable distribution of the tank body depth and area.

[0027] The present invention mainly utilizes the heat brought by evaporation from the surface of the water tank outside the shell, and the evaporation amount is closely related to the effective evaporation area of ​​the water surface in the water tank. The effective evaporation area refers to the humidity and flow rate in the gas space above the water surface that can meet the expected evaporation conditions, that is, the water surface that the air can reach to ensure that the water surface is not in a saturated state, which is the effective evaporation area. Therefore, the air intake device and the exhaust device should be reasonably arranged to facilitate air to reach various positions in the upper space of the water tank. A reasonable way is to set multiple air intake devices or air intake flow distribution channels in the upper part of the box body according to a certain area ratio, and the exhaust devices are at both ends of the box body, such as Figure 2As shown. The exhaust channel 5 should be set just above the steam-water separator 4 to ensure that the saturated steam discharged from the steam-water separator 4 can directly enter the exhaust channel 5 without flowing in the upper part of the water tank. Generally, the wider the range of the saturated steam discharged from the steam-water separator 4 flowing or diffusing on the surface of the water tank, the greater the humidity of the gas on the surface of the water body in the corresponding area, which is less conducive to evaporation on the surface of the water body.

[0028] The PCS water inlet pipe 2 and the drainage pipe 3 should be at an appropriate distance to facilitate the high-temperature water flowing from the drainage pipe 3 into the outer shell water tank to fully flow to various areas of the water tank surface and be fully cooled.

[0029] like Figure 3 , Figure 4 , Figure 5 , Figure 6 The figure shows four PCS water tanks and inlet and exhaust structures that can be used in different sites. Including rectangular, annular, fan-shaped and trapezoidal PCS water tanks, all of which can meet the requirements of uniform arrangement of the air inlet 7 and the exhaust channel 5. The number of PCS water tanks can be selected according to the needs.

[0030] The air intake and exhaust device structures of the present invention are only used to describe the core technology of the present invention, i.e., setting appropriate air intake and exhaust channels and establishing the required height difference for natural circulation (chimney effect), etc., and do not mean that the structural forms different from the present invention are out of the core technology scope to be protected by the present invention. For example, various air intake and exhaust hoods with mature existing technologies are used.

[0031] When the present invention is applied and implemented, necessary measures and means are taken in the air intake device and the exhaust device according to factors such as preventing foreign matter from entering, avoiding freezing of the water tank in extremely cold areas or seasons, and reducing the reduction of water volume caused by evaporation from the surface of the water tank during normal storage, such as adding a foreign matter filter, setting a damper, setting an inverted U-shaped structure in the air intake device, and heat preservation. The PCS system design further optimized based on the present invention can meet the cooling capacity requirements for coping with design basis accidents.

[0032] The present invention provides an enhanced passive containment heat removal system based on secondary evaporation. Without changing the main structure of the existing Hualong One passive containment heat removal system, the upper structure of the external water tank is partially modified, and air intake devices and exhaust devices are arranged at both ends of the external water tank.

[0033] The air inlet of the air intake device is located at a lower position on the upper part of the water tank. The exhaust device is an exhaust channel extending a certain length upward from the upper part of the water tank. The position of the exhaust port forms a significant height difference with the water surface of the water tank. The air reaches the water surface in the water tank and the gas space at the top of the water tank from the air intake device. The natural convection and evaporation on the surface of the water pool take away the heat, and the high-temperature and high-humidity gas formed is discharged through the exhaust port on the upper part of the exhaust device. Through secondary evaporative cooling on the surface of the outer shell water tank, the temperature of the outer shell water tank during long-term operation is reduced, the system operation characteristics are improved, and the performance of the passive containment heat removal system can be significantly enhanced.

[0034] The air intake device and the exhaust device have a simple structure and can operate continuously for a long time without the need for an external power source, and do not change the passive characteristics of the original system. Through the present invention, the water temperature of the outer water tank of the passive containment heat removal system in the long-term operation stage can be further reduced by 10 to 20°C from 100°C (the saturation temperature corresponding to the outer water tank pressure) to increase the system's heat carrying capacity by 20-80% under the same temperature and pressure conditions in the containment, thereby improving the heat carrying capacity of the passive containment heat removal system in the long-term operation stage and significantly reducing the equilibrium temperature and pressure in the containment during the long-term stage. After adopting the enhanced passive containment heat removal system, the containment heat removal requirements of the design basis accidents can be met through this system.

[0035] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. An enhanced passive containment heat removal system based on secondary evaporative cooling, characterized in that: include: A PCS water tank, wherein the lower part of the PCS water tank body is connected with a PCS system circulation pipeline, and the PCS system circulation pipeline includes a PCS system descending pipeline and a PCS system ascending pipeline; a steam-water separator is arranged in the PCS water tank, and the lower end of the steam-water separator is communicated with the PCS system ascending pipeline; the PCS water tank is provided with an exhaust passage at the upper part of the body on one side of the steam-water separator, an exhaust device is arranged above the exhaust passage, and an air intake device is arranged at the upper part of the body on the other side; the exhaust passage is arranged at the upper position of the steam-water separator, so as to ensure that the saturated steam discharged through the steam-water separator can directly enter the exhaust passage without flowing in the upper part of the PCS water tank; When the PCS is running, water flows out of the PCS water tank from the PCS system downpipe, enters the PCS heat exchanger in the containment, is heated by the air in the containment, and flows back to the PCS water tank through the PCS system uppipe; When the temperature of the PCS water tank is low, the PCS loop is a single-phase water cycle. When the temperature of the PCS water tank rises to a certain level, the water in the rising pipe of the PCS system will exceed the saturation temperature corresponding to the atmospheric pressure. As the flow in the rising pipe of the PCS system moves upward, the pressure decreases, and a portion of the water will flash into steam and be discharged through the steam-water separator. After the water vapor overflows the water surface, it enters the exhaust channel and is discharged into the atmosphere through the exhaust device. Due to the low density of water vapor, when the steam is discharged through the exhaust channel, a chimney effect is formed, and air is sucked into the PCS water tank from the air intake device. The air passes over the water surface of the PCS water tank, promoting surface evaporation, taking away the steam generated by evaporation, and is discharged through the exhaust channel and the exhaust device. A plurality of air inlets or an air inlet flow distribution channel are arranged on the upper part of the PCS water tank body according to a certain area ratio, and the air outlets are located at both ends of the PCS water tank body.

2. The system according to claim 1, characterized in that By reasonably setting the height and flow area of ​​the exhaust channel, the chimney effect can be enhanced; the higher the height of the exhaust channel, that is, the greater the height difference between the PCS water tank and the exhaust port of the exhaust device, the stronger the chimney effect is and the greater the flow speed of the gas, so that the air inlet can inhale more air to reduce the air humidity in the upper space of the PCS water tank and increase the evaporation rate.

3. The system according to claim 2, characterized in that The flow areas of the air inlet and the exhaust port and the height of the exhaust port are calculated based on a comprehensive consideration of the total power level of the PCS system, the cooling temperature required to be achieved by the PCS water tank, and the area of ​​the PCS water tank, and are based on the natural circulation Bernoulli equation and the water surface evaporation-diffusion equation, and the factors of engineering feasibility are taken into consideration.

4. The system according to claim 1, characterized in that An appropriate distance is set between the PCS system descending pipe and the PCS system ascending pipe to facilitate the high-temperature water flowing from the PCS system ascending pipe into the PCS water tank to fully flow to various areas of the water tank surface and be fully cooled.

5. The system according to any one of claims 1 to 4, characterized in that: The PCS water tank structural design should consider the reasonable distribution of tank depth and area.

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