A passive nuclear power plant steel containment cooling system and its working method

Abstract

The present invention belongs to the technical field of containment cooling in nuclear power plants, and provides a passive nuclear power plant steel containment cooling system and its working method, including: a steel containment; a plurality of air guide plates, which are arranged in layers at the middle and upper ends outside the steel containment; a shielding cover, which is arranged outside the plurality of air guide plates; an air inlet is arranged on the side surface of the upper end of the shielding cover, and an air outlet is arranged at the middle position of the top; an air auxiliary inlet is arranged on the side surface of the bottom end of the shielding cover; in the present invention, only air guide plates are arranged at the middle and upper ends outside the steel containment, and no air guide plates are arranged at the lower end; at the same time, an air auxiliary inlet is arranged on the side surface of the bottom end of the shielding cover, which is used to assist the air that has passed through the downcomer to flow into the air riser during a failure, so as to achieve the purpose of reducing the amount of air guide plates while ensuring the cooling capacity of the containment, shortening the project construction period, improving the convenience of installation, operation and maintenance, and reducing the equipment cost.

Description

Technical Field

[0001] The present invention belongs to the technical field of containment cooling in nuclear power plants, and particularly relates to a passive nuclear power plant steel containment cooling system and its working method. Background Art

[0002] As a dedicated safety facility of a nuclear power plant, the containment protects the reactor from external events and serves as the last barrier for radioactive containment of the nuclear reactor after an accident, protecting the public and the environment from excessive radiation. When a large amount of mass-energy is released into the containment, the temperature and pressure of the containment rise rapidly. To prevent the containment from overheating and overpressurizing, it is necessary to use the containment cooling system to promptly remove the heat from the containment.

[0003] The inventor found that in the existing passive containment cooling system, the air deflector is heavy and difficult to install and construct. Such an air deflector has extremely high requirements for airtightness and heat insulation ability, and its surface needs to be anodized, resulting in a high cost; during the construction stage of the power plant, the installation period of the air deflector is long, about 100 days, and there is a sequential logical relationship between the containment pressure test and the installation of the air deflector. It is necessary to conduct the containment pressure test first and then install the air deflector, which affects the critical path of construction and installation; during the operation stage of the power plant, when conducting regular tests on the water film coverage rate and in-service visual inspection of the containment welds, it is necessary to disassemble some air deflectors for operation, with complex operations and a large workload. Summary of the Invention

[0004] In order to solve the above problems, the present invention provides a passive nuclear power plant steel containment cooling system and its working method. The containment cooling system designed by the present invention can reduce the amount of air deflectors, shorten the project construction period, improve the convenience of installation, operation and maintenance, and reduce the equipment cost on the premise of ensuring the containment cooling capacity.

[0005] To achieve the above object, the present invention is realized by the following technical solutions:

[0006] In a first aspect, the present invention provides a passive nuclear power plant steel containment cooling system, comprising:

[0007] A steel containment;

[0008] A plurality of air deflectors, which are arranged in layers at the upper middle part outside the steel containment;

[0009] A shielding cover, which is arranged outside the plurality of air deflectors; an air inlet is opened on the side surface at the upper end of the shielding cover, and an air outlet is opened at the middle position of the top; an air auxiliary inlet is opened on the side surface at the bottom end of the shielding cover.

[0010] Furthermore, the air auxiliary inlets are multiple and are evenly distributed in the circumferential direction of the shielding cover.

[0011] Furthermore, the air-assisted inlet is divided into a first vertical section, a horizontal section communicating with the first vertical section, and a second vertical section communicating with the horizontal section; the first vertical section is arranged outside the shield, the air inlet of the first vertical section is vertically downward, the second vertical section is located between the shield and the steel containment, and the air outlet of the second vertical section is vertically upward.

[0012] Furthermore, a plurality of air guide plates divide the space between the shield and the steel containment into an air descending space and an air ascending space; the air inlet communicates with the air descending space.

[0013] Furthermore, no air guide plate is provided at the lower end of the steel containment.

[0014] Furthermore, the area of the air guide plate provided outside the steel containment is not greater than the total area outside the steel containment.

[0015] Furthermore, there are a plurality of air inlets, which are evenly distributed in the circumferential direction of the shield.

[0016] Furthermore, a drainage system is provided at the upper end of the shield, and the outlet pipe of the drainage system is located directly above the steel containment.

[0017] Furthermore, the top of the shield is set to be ellipsoidal.

[0018] In a second aspect, the present invention also provides a working method for a passive nuclear power plant steel containment cooling system, which adopts the passive nuclear power plant steel containment cooling system as described in the first aspect, including: in the event of an accident, the drainage system pours water on the top of the steel containment to form a water film covering the outer surface of the steel containment, and the heat inside the steel containment is carried out through the evaporation of the water film and the natural convection heat transfer of air; the external air enters the air descending section through the air inlet, the air flows through the descending section and then turns to enter the air ascending section, and is discharged into the atmosphere at the air outlet, and the air located at the lower part of the steel containment enters the space between the shield and the steel containment through the auxiliary inlet, and the air after passing through the descending section turns to enter the air ascending section.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0020] The present invention only provides air guide plates at the middle and upper ends outside the steel containment, and does not provide air guide plates at the lower end; at the same time, an air-assisted inlet is provided on the side surface at the bottom end of the shield, which is used to assist the air after passing through the descending section to turn to enter the air ascending section in the event of a design basis accident, so as to achieve the purpose of reducing the amount of air guide plates on the premise of ensuring the cooling capacity of the containment, shortening the project construction period, improving the convenience of installation, operation and maintenance, and reducing the equipment cost. Brief Description of the Drawings

[0021] The accompanying drawings forming a part of this embodiment are used to provide a further understanding of this embodiment. The schematic embodiments and descriptions thereof of this embodiment are used to explain this embodiment and do not constitute an improper limitation to this embodiment.

[0022] Figure 1 Schematic structural diagram of the prior art in Embodiment 1 of the present invention;

[0023] Figure 2 Schematic structural diagram of Embodiment 1 of the present invention;

[0024] Figure 3 Simulation diagram of the air diversion phenomenon in the tail position area of the deflector in Embodiment 1 of the present invention;

[0025] Figure 4 Air heat-carrying capacity curve of Embodiment 1 of the present invention;

[0026] Figure 5 Schematic diagram of the design of the containment cooling capacity verification test system in Embodiment 1 of the present invention;

[0027] Figure 6 Analysis of the peak pressure of the containment in Embodiment 1 of the present invention;

[0028] Figure 7 Analysis of the air cooling capacity of the containment in Embodiment 1 of the present invention;

[0029] Wherein, 1, steel containment; 2, shielding cover; 21, air inlet; 22, air auxiliary inlet; 221, first vertical section; 222, horizontal section; 223, second vertical section; 23, air descending space; 24, air ascending space; 3, air deflector. Detailed Description of the Embodiment

[0030] The present invention will be further described below in conjunction with the drawings and embodiments.

[0031] It should be noted that the following detailed descriptions are all exemplary and are intended to provide further explanations for this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the technical field to which this application belongs.

[0032] Embodiment 1:

[0033] This embodiment provides a passive nuclear power plant steel containment cooling system, including a steel containment 1, a shielding cover 2, and air deflectors 3; a plurality of air deflectors 3 are provided and are arranged in layers at the upper middle part outside the steel containment 1, and no air deflectors are provided at the lower end; the shielding cover 2 is arranged outside the plurality of air deflectors 3; an air inlet 21 is provided on the side surface of the upper end of the shielding cover 2, and an air outlet is provided at the middle position of the top; an air auxiliary inlet 22 is provided on the side surface of the bottom end of the shielding cover 2, and the shielding cover 2 can be set as a shielding building. In this embodiment, air deflectors are only provided at the upper middle part outside the steel containment 1, and no air deflectors are provided at the lower end; at the same time, an air auxiliary inlet 22 is provided on the side surface of the bottom end of the shielding cover 2, which is used to assist the air deflected after passing through the downcomer 23 to enter the air riser 24 in case of a failure, so as to achieve the purpose of reducing the amount of air deflectors used on the premise of ensuring the cooling capacity of the containment, shortening the project construction period, improving the convenience of installation, operation and maintenance, and reducing the equipment cost.

[0034] As Figure 3 shown, the space formed between the steel containment 1 and the shielding cover 2 is a cooling flow channel, and air deflectors 3 are installed in the middle and upper regions of the cooling flow channel. The plurality of air deflectors 3 include a top panel, a middle panel, and a bottom panel; in the traditional system, the number of layers of the top panel and the bottom panel is 1 each, and the number of layers of the middle panel is 8. In the system designed by this solution, the number of layers of the middle panel can be reduced to 4 layers or less, and the area where the air deflectors 3 need to be installed can be reduced by more than 50%.

[0035] At the bottom of the cooling flow channel, a plurality of groups of pipes that penetrate the shielding building and connect the cooling flow channel and the atmospheric environment are added along the circumferential direction as the air auxiliary inlet 22. Specifically, the air auxiliary inlet 22 is divided into a first vertical section 221, a horizontal section 222 communicating with the first vertical section 221, and a second vertical section 223 communicating with the horizontal section 222; the first vertical section 221 is arranged outside the shielding cover 2, the air inlet of the first vertical section 221 is vertically downward, the second vertical section 223 is located between the shielding cover 2 and the steel containment 1, and the air outlet of the second vertical section 222 is vertically upward; the cross-section of the air auxiliary inlet 22 can be circular or rectangular and is arranged in a Z shape or other forms.

[0036] In order to demonstrate the performance of the steel containment cooling system in this embodiment, this embodiment also proposes a performance demonstration method for the design solution, which specifically includes:

[0037] Research on the heat rejection mechanism of the design solution. With the help of fluid dynamics software (CFD), study the flow and heat transfer phenomena of air / steam in the cooling flow channel of the design solution from the mechanism, master the key phenomena affecting the heat rejection of the design solution, and provide theoretical guidance for subsequent verification tests and safety evaluations.

[0038] Experimental verification of the design solution. Based on the mechanism research, verification tests are carried out to verify the flow and heat transfer phenomena simulated by CFD, verify the applicability of the heat transfer and mass transfer relationships in the safety analysis program, and determine the envelope factor of the heat transfer and mass transfer relationships as the input to the safety analysis program.

[0039] Safety evaluation of the containment for the design solution. Based on the input determined by the mechanism research and verification tests of the design solution, a containment safety analysis program approved by the safety review party is used to carry out the analysis of the cooling capacity of the containment after the design basis accident to ensure that the acceptance criteria for the containment safety analysis are met.

[0040] For the design of the PCS cooling channel of a conventional passive nuclear power plant, the air deflector 3 covers the outer surface of the steel containment at full height. As Figure 1 shown, it is a schematic diagram of the design of the cooling channel of the steel containment of a conventional passive nuclear power plant. The air deflector 3 is composed of a series of panels supported on the steel containment 1 and suspended in the annulus cavity. It consists of 9 panels from top to bottom. The top is 1 panel forming the diffuser section of the deflector. The middle 6 panels are rectangular structures, forming most of the height of the air deflector. The bottom panel deflects the air by 180° and flows it into the inner annulus cavity.

[0041] As Figure 2 shown, it is a schematic diagram of the design solution of the cooling channel of the steel containment of the passive nuclear power plant proposed in this embodiment. The air deflector 3 is composed of multiple panels from top to bottom. The top panel is connected to the outer wall surface of the containment through a support member to form the diffuser section of the air deflector. The middle several panels are rectangular structures, forming the main part of the air deflector. The bottom panel deflects the air by 180° and flows it into the rising section. The design solution proposed in this embodiment significantly reduces the number of middle panels, which can be reduced by more than 4 layers, and the area for installing the air deflector can be reduced by more than 50%. At the bottom of the cooling channel, multiple groups of pipes penetrating the shield building and connecting the cooling channel and the atmospheric environment are added along the circumferential direction.

[0042] When accidents such as a break accident or a main steam pipe rupture occur, the air flows in the air descending space 23 and is split at the bottom position of the air deflector 3. Part of the air deflects by 180° and enters the air rising space 24. The other part of the air continues to flow downward for a certain distance and then deflects by 180° and flows upward. Then, after mixing with the first part of the air at the bottom of the air deflector 3, it enters the rising section. At the same time, the ambient air can also enter the lower annulus cavity through the air auxiliary inlet 22, and after mixing with the first part of the air at the bottom of the air deflector 3, it enters the air rising space 24.

[0043] For the design scheme of the passive nuclear power plant steel containment cooling flow path, this embodiment takes the evaluation of the water film cooling capacity of the containment in a typical large-break loss-of-coolant accident (LOCA) and the evaluation of the air cooling capacity of the containment during refueling outage as examples, and conducts performance demonstration through three steps. The specific steps are as follows:

[0044] S1. Research on the heat rejection mechanism of the design scheme:

[0045] S1.1. Heat rejection process of the containment after the accident:

[0046] During the water film cooling process of the containment in a large-break LOCA accident, it is assumed that a large-break LOCA accident occurs during the operation of the nuclear power plant at rated power. A large amount of high-energy fluid sprays out from the break and is released into the containment. After reaching the high-pressure set value of the containment, the PCS system is automatically triggered to be put into operation. Under the action of gravity, the cooling water flows out from the water storage tank in the drainage system set at the upper end of the shield, and after being distributed by the flow distribution device, a water film is formed on the outer surface of the steel containment. Using the steel containment shell as the heat transfer medium, the steam condenses on the inner surface of the containment and heats the inner surface, and then transfers the heat to the outer surface of the steel shell through heat conduction.

[0047] Most of the heated outer surface of the steel containment 1 is covered by the water film, and the heat is carried away by the evaporation on the water film surface. A small part of the outer surface of the steel containment 1 not covered by the water film transfers heat to the air through natural convection and radiation. The evaporation of the water film on the outer surface of the steel containment 1 and natural convection heat the air in the air rising space 24, and the air density decreases. Driven by the air density difference among the air rising space 24, the air descending space 23 and the ambient air, the air from the environment enters the air descending space 23 through the air inlet 21. At the tail position of the air deflector 3, the air shows a splitting phenomenon. After the air splits, a part of the air turns 180° and enters the air rising space 24; the other part of the air continues to flow downward for a certain distance and then turns 180° upward, and then after mixing with the first part of the air at the bottom of the air deflector 3, it enters the air rising space 24. These two parts of air and the steam generated by the evaporation of the water film finally return to the environment through the air outlet at the middle position of the top of the shield building. The water film that is not evaporated accumulates at the bottom of the cooling flow path and is discharged to the external environment through the flow path penetrating the shield building.

[0048] During the air cooling process of the containment during refueling outage: It is assumed that during the refueling outage of the nuclear power plant, when the spent fuel pool cooling system (SFS) is unavailable, the passive containment cooling water tank can provide cooling water source for the SFS. The refueling outage strategy requires that the natural convection air cooling capacity of the containment should be able to discharge the initial decay heat of the core of 7.12 MW within 3 days after the accident. In the analysis of the air cooling capacity of the containment, it is conservatively assumed that the in-containment refueling water storage tank (IRWST) loses the RNS cooling.

[0049] As the decay heat of the reactor core heats the IRWST water tank, after the water temperature rises and approaches the saturation state, the evaporation rate increases significantly, and the temperature and pressure of the steel containment 1 rise significantly. The natural convection on the outer surface of the steel containment 1 heats the air in the air rising space 24, and the air density decreases. Driven by the air density difference among the air rising space 24, the air descending space 23, and the ambient air, the air from the environment enters the air descending space 23. At the tail position of the air deflector, air diversion occurs. The diverted air can carry out the heat of the containment through convective heat transfer to ensure the cooling capacity of the steel containment 1. At the same time, the ambient air enters the lower annular cavity through the pipeline at the bottom of the cooling channel, mixes with the air at the bottom of the air deflector and the first part of the air, and then enters the air rising space 24.

[0050] S1.2 Post-accident Containment Heat Removal CFD Simulation Results:

[0051] The air diversion phenomenon and the heat-carrying process of the air in the area at the tail position of the air deflector are significantly different from the PCS cooling channels of conventional passive nuclear power plants. Using the CFD analysis tool, simulate the air flow behavior in the area at the tail position of the deflector of the steel containment 1 cooling channel after a hypothetical accident. The simulation shows that there is an obvious air diversion phenomenon in the area at the tail position of the air deflector. As Figure 3 shown, the air velocity vector diagram and streamline diagram of the area at the bottom of the air deflector are given. The simulation results show that after the air in the air descending space 23 flows through the bottom position of the air deflector, about 90% of the volume of a part of the air turns through 180° and directly enters the air rising space 24; about 10% of the volume of another part of the air continues to flow downward for a certain distance, then turns through 180°, and flows upward along the outer wall surface of the steel containment 1. After mixing with the first part of the air at the tail of the air deflector, it enters the air rising space 24.

[0052] Using the CFD analysis tool, simulate the heat-carrying capacity of the air and the containment wall surface in the area with the deflector and the area without the deflector in the post-accident containment cooling channel. As Figure 4 shown, the heat-carrying capacity of the air along the height direction of the containment is given. The simulation results show that under the action of the air diversion phenomenon, the air in the area without the deflector can effectively carry out the heat of the containment. For details, see the curve in the 0m - 22m section in Figure 4 It should be noted that due to the entrance effect of convective heat transfer, at the entrance of the air rising space 24, the heat transfer capacity of the air increases significantly and then gradually decreases. For details, see the curve in the 22m - 34m section in Figure 4 the curve in the 22m - 34m section in

[0053] S2 Design Scheme Test Verification:

[0054] The research on the heat rejection mechanism of the design shows that there are phenomena of flow turning, flow splitting, and flow confluence at the end of the flow deflector. The split air continues to flow downward, then turns 180° and directly enters the air rising space 24, and takes out the heat of the containment through convective heat transfer. In order to verify the conclusion of the theoretical research, a verification test of the passive containment cooling capacity was planned and carried out.

[0055] As Figure 5 shown, the flow chart of the test bench and the photos of the test bench are given. The test bench mainly includes the test body, the test loop system, and the auxiliary system. The general descriptions of these systems are as follows:

[0056] The test body is used to simulate the outer wall surface of the containment and the air flow channel annulus, and is composed of a heating plate, a cooling plate, and an observation plate, enclosing a square cavity structure. The test body section is fixed on the bracket, which can make the test section form a 90° vertical to the ground.

[0057] The test loop system includes: an oil supply system, a water supply system, an air supply system, and a PIV system. The oil supply system provides a heating heat source for the test main board; the water supply system provides a cooling water source for the water film cooling test condition; the air supply system provides the required air volume for the test body; the PIV system is used to observe the air flow field in the PCS annulus.

[0058] The auxiliary system includes a water supply system, a circulating cooling water system, a low-voltage AC power supply system, and a lift platform, etc.

[0059] S3. Safety assessment of the containment in the design solution:

[0060] The passive nuclear power plant steel containment cooling flow channel design solution proposed in this embodiment has a certain impact on the safety assessment of the containment, and safety assessment is required. After evaluation, the design solution affects the analysis of the air cooling capacity of the containment and the analysis of the peak pressure of the containment. Taking these two types of design basis accidents as examples, the safety assessment of the design solution is given.

[0061] Analysis of the peak pressure of the containment:

[0062] Based on the research on the heat rejection mechanism of the design solution, using the input determined by the verification test, the peak pressure analysis of the containment in the large break LOCA accident was carried out. The analysis shows that the water film evaporation phenomenon in the PCS is still the most important heat rejection path of the containment. As Figure 6 shown, the peak pressure analysis results are given. The analysis shows that the peak pressure of the containment is less than the design pressure of the containment. The pressure of the containment should be reduced to less than half of the peak pressure within 24 hours after the accident, and the pressure of the containment should be maintained at a low level during the long-term core cooling stage, meeting the requirements of the safety analysis of the containment.

[0063] Analysis of the air cooling capacity of the containment:

[0064] Based on the input determined by CFD analysis results or tests, the analysis of the containment air cooling capacity was carried out. The analysis shows that the convective and radiative heat transfer processes on the outer wall of the containment are the main ways to discharge the heat inside the containment. After canceling a certain number of flow deflectors, the heat discharge capacity of the containment without covered flow deflectors is weakened to some extent, but the radiation capacity is enhanced. As Figure 7 shown, the analysis results of the containment air cooling capacity are given. The analysis shows that the peak pressure of the containment is less than the design pressure of the containment, meeting the requirements of the containment safety analysis.

[0065] The design scheme and performance demonstration method of the steel containment cooling channel in this embodiment. The cooling capacity of the containment in this scheme is not lower than that of the containment of a conventional passive nuclear power plant, and it can shorten the project construction period, improve the convenience of installation, operation and maintenance, reduce the equipment cost, and has significant design optimization value and economic benefits.

[0066] Embodiment 2:

[0067] This embodiment provides a working method for the passive nuclear power plant steel containment cooling system, which adopts the passive nuclear power plant steel containment cooling system described in Embodiment 1, including: in case of an accident, the drainage system pours water on the top of the steel containment, so that a water film covers the outer surface of the steel containment, and the heat inside the steel containment is taken out through the evaporation of the water film and the natural convection heat transfer of the air; the external air enters the air descending section at the air inlet, and after flowing through the descending section, the air deflects and enters the air ascending section, and is discharged into the atmosphere at the air outlet. The air located at the lower part of the steel containment enters the space between the shield and the steel containment through the auxiliary inlet, and the air after flowing through the descending section deflects and enters the air ascending section.

[0068] The above are only the preferred embodiments of this embodiment and are not used to limit this embodiment. For those skilled in the art, various changes and modifications can be made to this embodiment. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of this embodiment shall be included within the protection scope of this embodiment.

Claims

1. A passive nuclear power plant steel containment cooling system, characterized in that, it includes: a steel containment; a plurality of air deflectors, which are arranged in layers at the middle and upper ends outside the steel containment; a shield, which is arranged outside the plurality of air deflectors; an air inlet is provided on the side surface of the upper end of the shield, and an air outlet is provided at the middle position of the top; an air auxiliary inlet is provided on the side surface of the bottom end of the shield; the plurality of air deflectors divide the space between the shield and the steel containment into an air descending space and an air ascending space; the air inlet is communicated with the air descending space; no air deflector is provided at the lower end outside the steel containment.

2. The passive nuclear power plant steel containment cooling system according to claim 1, characterized in that, the air auxiliary inlets are multiple and are evenly distributed in the circumferential direction of the shield.

3. The passive nuclear power plant steel containment cooling system according to claim 2, characterized in that, the air auxiliary inlet is divided into a first vertical section, a horizontal section communicated with the first vertical section, and a second vertical section communicated with the horizontal section; the first vertical section is arranged outside the shield, the air inlet of the first vertical section is vertically downward, the second vertical section is located between the shield and the steel containment, and the air outlet of the second vertical section is vertically upward.

4. The passive nuclear power plant steel containment cooling system according to claim 1, characterized in that, the area of the air deflector arranged outside the steel containment is not greater than the total area outside the steel containment.

5. The passive nuclear power plant steel containment cooling system according to claim 1, characterized in that, the air inlets are multiple and are evenly distributed in the circumferential direction of the shield.

6. The passive nuclear power plant steel containment cooling system according to claim 1, characterized in that, a drainage system is provided at the upper end of the shield, and the water outlet pipe of the drainage system is located directly above the steel containment.

7. The passive nuclear power plant steel containment cooling system according to claim 6, characterized in that, the top of the shield is set to be ellipsoidal.

8. A working method of a passive nuclear power plant steel containment cooling system, characterized in that, it adopts the passive nuclear power plant steel containment cooling system according to any one of claims 1-7, including: in case of an accident, the drainage system pours water on the top of the steel containment to make the water film cover the outer surface of the steel containment, and the heat inside the steel containment is carried out through the evaporation of the water film and the natural convection heat transfer of the air; the external air enters the air descending section at the air inlet, the air flows through the descending section and then deflects into the air ascending section, and is discharged into the atmosphere at the air outlet, and the air located at the lower part of the steel containment enters the space between the shield and the steel containment through the auxiliary inlet, and the air after assisting through the descending section deflects into the air ascending section.

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