A pressurized water reactor chemical shutdown system and method using cadmium nitrate as a shutdown solution

Abstract

The application discloses a pressurized water reactor chemical shutdown system and method using cadmium nitrate as a shutdown solution, which comprises a chemical shutdown system and a main loop; the chemical shutdown system adopts an active or passive mode and mainly comprises a concentrated cadmium nitrate storage tank, a high-pressure injection pump and an injection pipeline; after receiving a poison emergency injection signal or a cold shutdown maintenance signal, the injection pipeline inlet valve and the outlet valve are opened, the concentrated cadmium nitrate solution is driven in an active or passive mode, is injected from the concentrated cadmium nitrate storage tank to a nuclear reactor cold end main pipeline through the injection pipeline, is further stirred in a lower chamber of a pressure vessel, and is then distributed in the pressure vessel through flow redistribution. At the same temperature, cadmium nitrate has a greater solubility and a neutron absorption cross section than traditional neutron poison boric acid; the application enhances the reliability of reactor chemical shutdown under accident conditions, improves the safety of nuclear reactor shutdown, and reduces the arrangement and maintenance costs.

Description

Technical Field

[0001] This invention belongs to the field of nuclear power plant reactor safety design technology, specifically relating to a pressurized water reactor chemical shutdown system and method that uses cadmium nitrate as the shutdown solution. Background Technology

[0002] Nuclear power plants commonly use control rods as a shutdown method. However, in the event of an "Action-Temporary Transient Without Emergency Shutdown (ATWS)" accident, the depth of the control rod insertion is insufficient and cannot meet the negative reactivity requirements for shutdown. Therefore, the "HAF102-2016 Nuclear Power Plant Design Safety Regulations" include requirements such as "maintaining independence between different defense-in-depth layers as practically possible" and "at least one system must be able to independently maintain the reactor in a subcritical state with sufficient depth and high reliability." Thus, a chemical shutdown system is essential for nuclear power plants.

[0003] Boric acid is commonly used as a chemical shutdown solution in nuclear power plants. However, boric acid has low solubility at room temperature, which cannot meet the requirements for concentrated boron storage in nuclear power plants. An additional heater is needed to maintain the high concentration of boric acid solution required for shutdown.

[0004] Patent CN102881340 discloses an emergency shutdown system and method combining active and passive mechanisms. This system includes a control rod emergency shutdown subsystem and an emergency boron injection subsystem. Emergency shutdown is achieved by the control rods providing negative reactivity; if emergency shutdown fails or the core neutron flux is too high, the emergency boron injection subsystem injects concentrated boric acid to provide negative reactivity. However, based on the operational experience of existing nuclear power plants throughout their entire lifespan, control rod shutdown is the most common method, making the emergency boron injection subsystem extremely unlikely to function. Maintaining the emergency boron injection subsystem as a backup shutdown system requires high costs. Firstly, the temperature of the emergency boron injection subsystem needs to be maintained above the crystallization temperature limit of 9000 ppm boric acid solution, requiring high reliability of the electric heater. Secondly, the emergency boron injection subsystem consists of two series of concentrated boron tanks, each providing sufficient negative reactivity to meet the shutdown requirements of the entire reactor. Furthermore, considering that the concentrated boron tanks need to be located as close as possible to the nuclear reactor to provide a faster injection rate, the large volume of the concentrated boron tanks and their close proximity impose requirements on the layout of the nuclear reactor building. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing boric acid as a shutdown solution for pressurized water reactors by providing a chemical shutdown system and method for pressurized water reactors that uses cadmium nitrate as the shutdown solution.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution is disclosed. The system is actively driven and includes a chemical shutdown system 1 and a main loop 2. The chemical shutdown system 1 includes a concentrated cadmium nitrate storage tank 4, an electric heater 5 installed inside the concentrated cadmium nitrate storage tank 4 to prevent cadmium nitrate crystallization, a high-pressure gas cylinder 3 connected to the top of the concentrated cadmium nitrate storage tank 4, an injection pipeline 14 connected to the bottom of the concentrated cadmium nitrate storage tank 4, a high-pressure injection pump 10 installed on the injection pipeline 14, and inlet valves 8 and 13 respectively installed at the inlet and outlet of the injection pipeline 14. Upon receiving an emergency injection signal for toxic substances or a signal to maintain cold shutdown, the inlet valve 8 and outlet valve 13 of the injection pipeline 14 are opened. The concentrated cadmium nitrate solution, driven by the high-pressure injection pump 10 and / or the high-pressure gas cylinder 3, is injected from the concentrated cadmium nitrate storage tank 4 through the injection pipeline 14 into the cold end main pipeline 12 of the main loop 2. After further agitation in the lower chamber 11 of the pressure vessel, the flow rate is redistributed to various parts of the pressure vessel 9. At the same temperature, cadmium nitrate has a greater solubility and neutron absorption cross section compared to the traditional neutron poison boric acid. This invention enhances the reliability of reactor chemical shutdown under accident conditions, improves the safety of nuclear reactor shutdown, and reduces layout and maintenance costs.

[0008] Preferably, the electric heater 5, inlet valve 8, high-pressure injection pump 10 and outlet valve 13 are all connected to the nuclear reactor shutdown protection system, and can be remotely started and shut down by the nuclear reactor shutdown protection system, or manually started and shut down.

[0009] Preferably, the high-pressure injection pump 10 is a multi-stage centrifugal pump with multiple impellers connected in series, which can gradually increase the pressure to meet the 15.5MPa high-pressure injection conditions in the main loop 2 during an emergency reactor shutdown.

[0010] Preferably, the high-pressure gas cylinder 3 can be a high-pressure gas such as a nitrogen cylinder or a helium cylinder.

[0011] The described shutdown method for a pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution involves the following steps: Upon receiving an emergency injection signal for toxic substances or a signal to maintain a cold shutdown, the reactor shutdown protection system automatically or manually opens the inlet valve 8 and outlet valve 13 of the injection pipeline 14. Concentrated cadmium nitrate solution, driven by a high-pressure injection pump 10 and / or a high-pressure gas cylinder 3, is injected from the concentrated cadmium nitrate storage tank 4 through the injection pipeline 14 into the cold-end main pipeline 12 of the main circuit. After further agitation in the lower chamber 11 of the pressure vessel, the flow is redistributed throughout the pressure vessel 9. The concentrated cadmium nitrate solution is uniformly mixed with the coolant and ultimately converges in the upper chamber 6 of the pressure vessel, flowing out of the pressure vessel 9 through the hot-end main pipeline 7. The coolant circulates throughout the pressurized water reactor main circuit via the hot-end main pipeline 7, then returns to the cold-end main pipeline 12 and enters the pressure vessel 9. Finally, coolant containing cadmium nitrate flows throughout the pressurized water reactor main circuit.

[0012] A pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution is disclosed. The system is passively driven and includes a chemical shutdown system 1 and a main loop 2. The chemical shutdown system 1 includes a concentrated cadmium nitrate storage tank 4, an electric heater 5 installed inside the concentrated cadmium nitrate storage tank 4 to prevent cadmium nitrate crystallization, an injection line 14 connected to the bottom of the concentrated cadmium nitrate storage tank 4, an inlet valve 8 installed on the injection line 14, an equalization line 16 connected to the top of the concentrated cadmium nitrate storage tank 4, and an equalization valve 17 installed on the equalization line 16. The concentrated cadmium nitrate storage tank 4 has a vertical height greater than that of a pressure vessel 9 in the main loop 2. Upon receiving an emergency injection signal for toxic substances or a signal to maintain cold shutdown, the equalization valve 17 is opened. After the concentrated cadmium nitrate storage tank 4 is pressurized with the main loop, the inlet valve 8 of the injection line 14 is opened, and the concentrated cadmium nitrate solution is injected into the cold end main pipeline 12 of the main loop from the concentrated cadmium nitrate storage tank 4 through the injection line 14 under gravity.

[0013] Preferably, the electric heater 5, the inlet valve 8, and the equalizing valve 17 are all connected to the nuclear reactor shutdown protection system, and can be remotely started and shut down by the nuclear reactor shutdown protection system, or manually started and shut down.

[0014] The described shutdown method for a pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution involves the following steps: Upon receiving an emergency injection signal for toxic substances or a signal to maintain a cold shutdown, the reactor shutdown protection system automatically or manually opens the equalization valve 17. After equalization of the pressure in the concentrated cadmium nitrate storage tank 4 and the main circuit, the inlet valve 8 of the injection pipeline 14 is opened. Under the influence of gravity, the concentrated cadmium nitrate solution is injected from the concentrated cadmium nitrate storage tank 4 through the injection pipeline 14 into the cold end main pipeline 12 of the main circuit. After further turbulence in the lower chamber 11 of the pressure vessel, the flow is redistributed to various parts of the pressure vessel 9. The concentrated cadmium nitrate solution is uniformly mixed with the coolant and finally converges in the upper chamber 6 of the pressure vessel, flowing out of the pressure vessel 9 through the hot end main pipeline 7. The coolant circulates throughout the pressurized water reactor main circuit through the hot end main pipeline 7, then returns to the cold end main pipeline 12 and enters the pressure vessel 9. Finally, the coolant containing cadmium nitrate flows throughout the pressurized water reactor main circuit.

[0015] Preferably, in the actual layout of a nuclear power plant, it includes two or more independent chemical shutdown systems 1, each of which can independently meet the shutdown capacity requirements and ensure safety redundancy.

[0016] Preferably, the main circuit 2 includes a pressure vessel 9, an upper chamber 6 located above the pressure vessel 9, a lower chamber 11 located below the pressure vessel 9, a hot-end main pipe 7 connected to the upper chamber 6, and a cold-end main pipe 12 connected to the lower chamber 11.

[0017] Preferably, the concentrated cadmium nitrate storage tank 4 can be connected to an external circulation pipeline, pressure gauge and temperature gauge, and the concentrated cadmium nitrate storage tank 4 can be checked regularly to see if it meets the shutdown requirements, and the solution can be replaced regularly.

[0018] Preferably, if the chemical shutdown system 1 is driven by an active means, the chemical shutdown system 1 can be arranged inside or outside the containment; if the chemical shutdown system 1 is driven by a passive means, the chemical shutdown system 1 can be arranged inside the containment.

[0019] Compared with the prior art, the present invention has at least the following advantages:

[0020] (1) This invention uses cadmium nitrate as a chemical shutdown solution to replace the traditional boric acid as a chemical shutdown solution, thus enriching the types of chemical shutdown solutions for nuclear power plants.

[0021] (2) The high solubility of cadmium nitrate solution can effectively save the volume of storage tanks, which is beneficial to the layout of nuclear power plant buildings, especially the pressurized water reactor of the marine nuclear floating platform.

[0022] (3) The high solubility of cadmium nitrate solution avoids the strong demand for electric heaters and reduces the electricity cost of maintaining electric heaters.

[0023] (4) The high neutron absorption capacity of cadmium nitrate solution can quickly absorb excess neutrons in the reactor, thereby enabling rapid reactor shutdown, increasing the speed of the chemical shutdown system and improving its reliability.

[0024] (5) The high solubility and high neutron absorption capacity of cadmium nitrate solution reduce the volume of shutdown poison solution that needs to be injected into the reactor under high pressure, reduce the reliability test of the high pressure injection pump, and reduce the high pressure injection time.

[0025] (6) This invention is not only applicable to nuclear power plant design, but also beneficial to the retrofitting of chemical shutdown of existing nuclear power plants, with minimal changes and increased shutdown capacity redundancy.

[0026] (7) This invention provides two driving modes for the shutdown system during the nuclear reactor design stage: active and passive, providing options for adapting to different nuclear reactor building layouts. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of a pressurized water reactor chemical shutdown system that uses cadmium nitrate as the shutdown solution and employs an active approach.

[0028] Figure 2 This is a schematic diagram of a pressurized water reactor chemical shutdown system that uses cadmium nitrate as the shutdown solution and employs a passive method.

[0029] In the diagram, 1 is the chemical shutdown system, 2 is the main loop, 3 is the high-pressure gas cylinder, 4 is the concentrated cadmium nitrate storage tank, 5 is the electric heater, 6 is the upper chamber of the pressure vessel, 7 is the hot-end main pipeline, 8 is the inlet valve, 9 is the pressure vessel, 10 is the high-pressure injection pump, 11 is the lower chamber of the pressure vessel, 12 is the cold-end main pipeline, 13 is the outlet valve, 14 is the injection line, 15 is the containment vessel, 16 is the equalization line, and 17 is the equalization valve. Detailed Implementation

[0030] The technical solutions in the embodiments of the present invention will be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the protection scope of the present invention.

[0031] Example 1

[0032] This embodiment provides a pressurized water reactor chemical shutdown system and method using cadmium nitrate as the shutdown solution. Figure 1 This is a schematic diagram of a pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution, employing an active approach according to the present invention. The system in this embodiment includes a chemical shutdown system 1 and a main loop 2. The chemical shutdown system 1 includes a high-pressure gas cylinder 3, a concentrated cadmium nitrate storage tank 4, an electric heater 5, an inlet valve 8, a high-pressure injection pump 10, an outlet valve 13, and an injection pipeline 14. The electric heater 5 is installed inside the concentrated cadmium nitrate storage tank 4 to prevent cadmium nitrate from crystallizing. The high-pressure gas cylinder 3 is connected to the top of the concentrated cadmium nitrate storage tank 4, and the injection pipeline 14 is connected to the bottom of the concentrated cadmium nitrate storage tank 4. The high-pressure injection pump 10 is installed on the injection pipeline 14, and inlet valve 8 and outlet valve 13 are respectively installed at the inlet and outlet of the injection pipeline 14. The main loop 2 includes a pressure vessel 9, an upper chamber 6 located above the pressure vessel 9, a lower chamber 11 located below the pressure vessel 9, a hot-end main pipeline 7 connected to the upper chamber 6, and a cold-end main pipeline 12 connected to the lower chamber 11.

[0033] Upon receiving an emergency injection signal for toxic substances or a signal to maintain a cold shutdown, the inlet valve 8 and outlet valve 13 of the injection pipeline 14 are opened. Concentrated cadmium nitrate solution, driven by a high-pressure injection pump 10 and / or a high-pressure gas cylinder 3, is injected from the concentrated cadmium nitrate storage tank 4 through the injection pipeline 14 into the cold-end main pipeline 12 of the main circuit 2. After further turbulence in the lower chamber 11 of the pressure vessel, the flow is redistributed throughout the pressure vessel 9. At the same temperature, cadmium nitrate has a greater solubility and neutron absorption cross-section compared to the traditional neutron poison boric acid. This invention enhances the reliability of reactor chemical shutdown under accident conditions, improves the safety of nuclear reactor shutdown, and reduces layout and maintenance costs.

[0034] Pressurized water reactors are typically thermal neutron reactors. The equivalent thermal neutron absorption cross section of natural boron (B) is 764 barns, while that of natural cadmium (Cd) is 2529 barns. Theoretically, given the same molar ratio of boron to cadmium, cadmium's neutron absorption capacity is 3.3 times that of boron. Modifying the existing chemical shutdown method in power plants—replacing the boric acid solution with a cadmium nitrate solution (i.e., replacing concentrated boron storage tanks with concentrated cadmium nitrate storage tanks)—doubles the neutron absorption capacity while maintaining the same tank volume. This increases the negative reactivity provided by chemical shutdown and reduces shutdown time.

[0035] When designing pressurized water reactor nuclear power plants, the tank volume remains constant, and cadmium nitrate is used as the chemical shutdown solution. The higher chemical shutdown neutron absorption capacity can match the neutron flux density under higher nuclear fuel enrichment. High-enriched nuclear fuel can provide high economic efficiency, long refueling cycles, and high heat source output.

[0036] When designing a pressurized water reactor nuclear power plant, to ensure that the neutron absorption capacity provided by the poison remains unchanged, the volume of the concentrated cadmium nitrate storage tank is less than half the volume of the concentrated boron storage tank, which is beneficial to the layout of the nuclear reactor building and other related facilities.

[0037] The existing concentrated boric acid storage tank stores 8000 ppm of concentrated boric acid. Converted to the same thermal neutron absorption capacity, concentrated cadmium nitrate storage tank 4 stores 2400 ppm. The concentrated boric acid storage tank needs to be maintained above 25°C to ensure boric acid solubility. At 25°C, the solubility of cadmium nitrate corresponds to 650,000 ppm, which fully meets the chemical shutdown requirements of the existing pressurized water reactor. Optionally, an electric heater 5 can be used to ensure that the internal temperature of concentrated cadmium nitrate storage tank 4 is higher than the 2400 ppm cadmium nitrate crystallization temperature, further increasing the internal temperature of concentrated cadmium nitrate storage tank 4 and raising the concentration of cadmium nitrate solution to the corresponding saturation solubility. Under the premise of ensuring the same chemical shutdown capacity, the volume of concentrated cadmium nitrate storage tank 4 can be further reduced.

[0038] Preferably, in this embodiment, the electric heater 5, inlet valve 8, high-pressure injection pump 10 and outlet valve 13 are all connected to the nuclear reactor shutdown protection system, and can be remotely started and shut down by the nuclear reactor shutdown protection system, or manually started and shut down.

[0039] Preferably, in the actual nuclear power plant layout, this embodiment includes three independent chemical shutdown systems 1, corresponding to the three loops of the 100,000 kW pressurized water reactor unit. Each chemical shutdown system 1 can independently meet the shutdown capacity requirements, ensuring safety redundancy.

[0040] Preferably, in this embodiment, the high-pressure injection pump 10 is a multi-stage centrifugal pump with multiple impellers connected in series, which can gradually increase the pressure to meet the 15.5MPa high-pressure injection conditions in the main loop 2 during an emergency reactor shutdown.

[0041] Preferably, in this embodiment, the high-pressure gas cylinder 3 can be a high-pressure gas such as a nitrogen cylinder or a helium cylinder.

[0042] Preferably, in this embodiment, the concentrated cadmium nitrate storage tank 4 can be externally connected to a circulation pipeline, pressure gauge, and temperature gauge to periodically check whether the concentrated cadmium nitrate storage tank 4 meets the shutdown requirements, and to periodically replace the solution. In this embodiment, the concentrated cadmium nitrate storage tank 4 and the injection pipeline 14 are made of 316L austenitic stainless steel, which can effectively prevent corrosion from the concentrated cadmium nitrate solution.

[0043] Preferably, this embodiment employs a chemical shutdown system 1 driven by an active method, which can be located inside or outside the containment.

[0044] This embodiment describes a shutdown method for a pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution. Upon receiving an emergency injection signal for toxic substances or a signal to maintain a cold shutdown, the nuclear reactor shutdown protection system automatically or manually opens the inlet valve 8 and outlet valve 13 of the injection pipeline 14. The concentrated cadmium nitrate solution is driven by a high-pressure injection pump 10 and / or a high-pressure gas cylinder 3, and injected from the concentrated cadmium nitrate storage tank 4 through the injection pipeline 14 into the cold end main pipeline 12 of the main loop. After further turbulence in the lower chamber 11 of the pressure vessel, the flow is redistributed to various parts of the pressure vessel 9. The concentrated cadmium nitrate solution is uniformly mixed with the coolant and finally converges in the upper chamber 6 of the pressure vessel, flowing out of the pressure vessel 9 through the hot end main pipeline 7. The coolant circulates throughout the pressurized water reactor main loop through the hot end main pipeline 7, and then returns to the cold end main pipeline 12 and enters the pressure vessel 9. Finally, the coolant containing cadmium nitrate flows throughout the pressurized water reactor main loop.

[0045] In this embodiment, if the expected transient that fails to shut down the reactor in an emergency (ATWS), or if the control rod insertion fails or is insufficient, the control system provides an emergency injection signal, and the chemical shutdown system 1 quickly takes effect to reduce the reactivity of the nuclear reactor and ensure the safety of the nuclear reactor.

[0046] Example 2

[0047] like Figure 2As shown in the figure, this embodiment describes a pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution. This system is passively driven and includes a chemical shutdown system 1 and a main loop 2. The chemical shutdown system 1 includes a concentrated cadmium nitrate storage tank 4, an electric heater 5, an inlet valve 8, an injection line 14, an equalization line 16, and an equalization valve 17. The electric heater 5 is installed inside the concentrated cadmium nitrate storage tank 4 to prevent cadmium nitrate crystallization. The injection line 14 is connected to the bottom of the concentrated cadmium nitrate storage tank 4, and the inlet valve 8 is located at the inlet of the injection line 14. The equalization line 16 is connected to the top of the concentrated cadmium nitrate storage tank 4, and the equalization valve 17 is installed on the equalization line 16. The main loop 2 includes a pressure vessel 9, an upper chamber 6 located above the pressure vessel 9, a lower chamber 11 located below the pressure vessel 9, a hot-end main pipe 7 connected to the upper chamber 6, and a cold-end main pipe 12 connected to the lower chamber 11.

[0048] Preferably, in this embodiment, the electric heater 5, inlet valve 8, and equalizing valve 17 are all connected to the nuclear reactor shutdown protection system, and can be remotely started and shut down by the nuclear reactor shutdown protection system, or manually started and shut down.

[0049] Preferably, in the actual nuclear power plant layout, this embodiment includes three independent chemical shutdown systems 1, corresponding to the three loops of the 100,000 kW pressurized water reactor unit. Each chemical shutdown system 1 can independently meet the shutdown capacity requirements, ensuring safety redundancy.

[0050] Preferably, in this embodiment, the concentrated cadmium nitrate storage tank 4 can be connected to an external circulation pipeline, pressure gauge, and temperature gauge to periodically check whether the concentrated cadmium nitrate storage tank 4 meets the shutdown requirements, and to periodically replace the solution. In this embodiment, the concentrated cadmium nitrate storage tank 4 and the injection pipeline (14) are made of 316L austenitic stainless steel, which can effectively prevent corrosion of the concentrated cadmium nitrate solution.

[0051] Preferably, this embodiment employs a passively driven chemical shutdown system 1, which can be located inside the containment vessel.

[0052] In this embodiment, the vertical height of the concentrated cadmium nitrate storage tank 4 is greater than that of the pressure vessel 9.

[0053] Preferably, in this example, the equalizing valve 17 is a shut-off valve.

[0054] This embodiment describes a shutdown method for a pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution. Upon receiving an emergency injection signal for toxic substances or a signal to maintain a cold shutdown, the nuclear reactor shutdown protection system automatically or manually opens the equalization valve 17. After the concentrated cadmium nitrate storage tank 4 is pressurized with the main loop, the inlet valve 8 of the injection pipeline 14 is opened. Under the action of gravity, the concentrated cadmium nitrate solution is injected from the concentrated cadmium nitrate storage tank 4 through the injection pipeline 14 into the cold end main pipeline 12 of the main loop. After being further stirred in the lower chamber 11 of the pressure vessel, the flow is redistributed to various parts of the pressure vessel 9. The concentrated cadmium nitrate solution and coolant are uniformly mixed and finally converge in the upper chamber 6 of the pressure vessel, and then flow out of the pressure vessel 9 through the hot end main pipeline 7. The coolant circulates throughout the pressurized water reactor main loop through the hot end main pipeline 7, and then returns to the cold end main pipeline 12 and enters the pressure vessel 9. Finally, the coolant containing cadmium nitrate flows throughout the pressurized water reactor main loop.

[0055] In this embodiment, if the expected transient that fails to shut down the reactor in an emergency (ATWS), or if the control rod insertion fails or is insufficient, the control system provides an emergency injection signal, and the chemical shutdown system 1 quickly takes effect to reduce the reactivity of the nuclear reactor and ensure the safety of the nuclear reactor.

[0056] The present invention maintains the cold shutdown signal after the nuclear reactor has been successfully shut down using control rods. Considering that the nuclear reactor may return to criticality due to the consumption of xenon poison in the iodine pit, the chemical and volume control systems of commercial nuclear power plants may not have sufficient boric acid reserves, and marine nuclear power reactors may not use neutron poisons to regulate reactivity during normal operation, the chemical shutdown system 1 can be used to inject poisons to keep the nuclear reactor in a long-term shutdown state.

[0057] The above description is only used to illustrate the present invention and should not be construed as limiting the specific embodiments of the present invention to this. Any modifications made based on the technical solution shall fall within the protection scope of the claims of the present invention.

Claims

1. A pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution, characterized in that: The system is driven by an active mechanism and includes a chemical shutdown system (1) and a main loop (2). The chemical shutdown system (1) includes a concentrated cadmium nitrate storage tank (4), an electric heater (5) installed inside the concentrated cadmium nitrate storage tank (4) to ensure that cadmium nitrate does not crystallize, a high-pressure gas cylinder (3) connected to the top of the concentrated cadmium nitrate storage tank (4), an injection pipeline (14) connected to the bottom of the concentrated cadmium nitrate storage tank (4), and a high-pressure injection pump (10) installed on the injection pipeline (14), with inlets at the inlet and outlet of the injection pipeline (14) respectively. Valve (8) and outlet valve (13); upon receiving an emergency injection signal for toxic substances or a signal to maintain cold shutdown, the inlet valve (8) and outlet valve (13) of the injection pipeline (14) are opened. The concentrated cadmium nitrate solution is driven by the high-pressure injection pump (10) and / or the high-pressure gas cylinder (3) and injected from the concentrated cadmium nitrate storage tank (4) through the injection pipeline (14) into the cold end main pipeline (12) of the main circuit (2). After being further stirred by the lower chamber (11) of the pressure vessel, the flow rate is redistributed to various parts of the pressure vessel (9).

2. A pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution as described in claim 1, characterized in that: In actual nuclear power plant layout, there are two or more independent chemical shutdown systems (1), each of which can independently meet the shutdown capacity requirements and ensure safety redundancy.

3. A pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution as described in claim 1, characterized in that: The main circuit (2) includes a pressure vessel (9), an upper chamber (6) of the pressure vessel located above the pressure vessel (9), a lower chamber (11) of the pressure vessel located below the pressure vessel (9), a hot end main pipe (7) connected to the upper chamber (6) of the pressure vessel, and a cold end main pipe (12) connected to the lower chamber (11) of the pressure vessel.

4. A pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution as described in claim 1, characterized in that: The electric heater (5), inlet valve (8), high-pressure injection pump (10) and outlet valve (13) are all connected to the nuclear reactor shutdown protection system and can be remotely started and shut down by the nuclear reactor shutdown protection system, or manually started and shut down.

5. A shutdown method for a pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution as described in any one of claims 1 to 4, characterized in that: Upon receiving an emergency injection signal for toxic substances or a signal to maintain a cold shutdown, the inlet valve (8) and outlet valve (13) of the injection pipeline (14) are automatically or manually opened by the nuclear reactor shutdown protection system. The concentrated cadmium nitrate solution is driven by the high-pressure injection pump (10) and / or the high-pressure gas cylinder (3) and injected from the concentrated cadmium nitrate storage tank (4) through the injection pipeline (14) into the cold end main pipeline (12) of the main circuit. After being stirred in the lower chamber (11) of the pressure vessel, the flow is redistributed to various parts of the pressure vessel (9). The concentrated cadmium nitrate solution is uniformly mixed with the coolant and finally merges into the upper chamber (6) of the pressure vessel and flows out of the pressure vessel (9) through the hot end main pipeline (7). The coolant circulates throughout the pressurized water reactor main circuit through the hot end main pipeline (7) and then returns to the cold end main pipeline (12) to enter the pressure vessel (9). Finally, the coolant containing cadmium nitrate flows throughout the pressurized water reactor main circuit.

6. A pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution, characterized in that: The system is driven passively and includes a chemical shutdown system (1) and a main loop (2). The chemical shutdown system (1) includes a concentrated cadmium nitrate storage tank (4), an electric heater (5) installed inside the concentrated cadmium nitrate storage tank (4) to prevent cadmium nitrate from crystallizing, an injection line (14) connected to the bottom of the concentrated cadmium nitrate storage tank (4), an inlet valve (8) installed on the injection line (14), an equalization line (16) connected to the top of the concentrated cadmium nitrate storage tank (4), and an equalization line (16) installed on the equalization line. (16) equalizing valve (17); the concentrated cadmium nitrate storage tank (4) is a pressure vessel (9) with a vertical height greater than that of the main circuit (2); when a toxic emergency injection signal or a cold shutdown signal is received, the equalizing valve (17) is opened, and after the concentrated cadmium nitrate storage tank (4) is equalized with the main circuit, the inlet valve (8) of the injection pipeline (14) is opened, and the concentrated cadmium nitrate solution is injected into the cold end main pipeline (12) of the main circuit from the concentrated cadmium nitrate storage tank (4) through the injection pipeline (14) under the action of gravity.

7. A pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution as described in claim 6, characterized in that: In actual nuclear power plant layout, there are two or more independent chemical shutdown systems (1), each of which can independently meet the shutdown capacity requirements and ensure safety redundancy.

8. A pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution as described in claim 6, characterized in that: The main circuit (2) includes a pressure vessel (9), an upper chamber (6) of the pressure vessel located above the pressure vessel (9), a lower chamber (11) of the pressure vessel located below the pressure vessel (9), a hot end main pipe (7) connected to the upper chamber (6) of the pressure vessel, and a cold end main pipe (12) connected to the lower chamber (11) of the pressure vessel.

9. A pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution as described in claim 6, characterized in that: The electric heater (5), inlet valve (8), and equalizing valve (17) are all connected to the nuclear reactor shutdown protection system and can be remotely started and shut down by the nuclear reactor shutdown protection system, or manually started and shut down.

10. A shutdown method for a pressurized water reactor chemical shutdown system using cadmium nitrate as the shutdown solution as described in any one of claims 6 to 9, characterized in that: Upon receiving an emergency injection signal for toxic substances or a signal to maintain a cold shutdown, the nuclear reactor shutdown protection system automatically or manually opens the equalization valve (17). After the concentrated cadmium nitrate storage tank (4) and the main circuit are equalized, the inlet valve (8) of the injection pipeline (14) is opened. Under the action of gravity, the concentrated cadmium nitrate solution is injected from the concentrated cadmium nitrate storage tank (4) through the injection pipeline (14) into the cold end main pipeline (12) of the main circuit. After being further stirred in the lower chamber (11) of the pressure vessel, the flow is redistributed to various parts of the pressure vessel (9). The concentrated cadmium nitrate solution and the coolant are evenly mixed and finally converge in the upper chamber (6) of the pressure vessel and flow out of the pressure vessel (9) through the hot end main pipeline (7). The coolant circulates throughout the pressurized water reactor main circuit through the hot end main pipeline (7) and then returns to the cold end main pipeline (12) to enter the pressure vessel (9). Finally, the coolant containing cadmium nitrate flows throughout the pressurized water reactor main circuit.

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