Reactor pressure vessel protection method and system

Abstract

The present application relates to a reactor pressure vessel protection method and system, the method comprising the following steps: when the unit is in a downward state, judging whether the residual heat removal system is in an effective state, if the result is yes, generating a cold-state overpressure protection signal for activating a cold-state overpressure protection means; if the result is no, further judging whether the primary loop average temperature is less than a first conversion temperature, if the result is yes, generating the cold-state overpressure protection signal; when the unit is in an upward state, judging whether the primary loop average temperature is greater than a second conversion temperature, if the result is yes, generating a hot-state overpressure protection signal for activating a hot-state overpressure protection means; the present application can control the unit to switch between the cold-state overpressure protection state and the hot-state overpressure protection state according to the primary loop average temperature, thereby executing the corresponding overpressure protection means, and effectively preventing the reactor pressure vessel from breaking.

Description

Technical Field

[0001] This invention relates to the field of nuclear power plant equipment protection technology, and in particular to a reactor pressure vessel protection method and system. Background Technology

[0002] The primary loop of a nuclear power plant mainly consists of equipment such as the reactor pressure vessel, pressurizer, main pumps, steam generator, and main piping. The reactor pressure vessel contains the nuclear fuel that generates heat, while the other equipment contains the reactor coolant that conducts heat. Under all circumstances, nuclear power plants must monitor and ensure the integrity of the primary loop to prevent potential radioactive releases caused by reactor coolant leakage.

[0003] Of all primary circuit equipment ruptures, the reactor pressure vessel rupture often has the most severe consequences. This is because it contains highly radioactive nuclear fuel, and once ruptured, the nuclear power plant lacks any means to isolate the breach, leading to continuous leakage of coolant and radioactivity. Therefore, nuclear power plants impose the most stringent requirements on the manufacturing process of reactor pressure vessels to ensure their reliability. Simultaneously, the primary circuit pressure must be rigorously monitored during nuclear power plant operation to ensure it never exceeds the allowable pressure of the reactor pressure vessel (the highest pressure the equipment is allowed to withstand during operation). The allowable pressure of the reactor pressure vessel is temperature-dependent. Based on the material properties of the reactor pressure vessel, its allowable pressure decreases as the temperature decreases, especially when the temperature reaches the ductile-brittle transition temperature, where the allowable pressure drops sharply, making the reactor pressure vessel more susceptible to rupture.

[0004] Currently, although nuclear power plants have designed corresponding protection schemes for units in hot and cold states, these protection schemes differ greatly in their technical means and the correlation between them is weak. Furthermore, the design of protection settings for some equipment (such as safety valves) also varies greatly, leading to switching obstacles between hot and cold state protection schemes and reducing the safety of nuclear power plants. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a method and system for protecting reactor pressure vessels.

[0006] The technical solution adopted by this invention to solve its technical problem is: to construct a reactor pressure vessel protection method, comprising the following steps:

[0007] When the unit is in the downlink state, it is determined whether the waste heat removal system is in the active state. If the determination result is yes, a cold overpressure protection signal is generated to activate the cold overpressure protection means. If the determination result is no, it is further determined whether the average temperature of the primary loop is less than the first switching temperature. If the determination result is yes, the cold overpressure protection signal is generated.

[0008] When the unit is in the up-current state, it is determined whether the average temperature of the primary circuit is greater than the second switching temperature. If the determination result is yes, a hot overpressure protection signal is generated to activate the hot overpressure protection means.

[0009] Preferably, the cold overpressure protection method includes:

[0010] Set the safety valve opening threshold of the primary circuit voltage regulator to at least one of the following: cold safety threshold, closing a predetermined unnecessary device, and activating the safety valve of the waste heat discharge system.

[0011] Preferably, the hot overpressure protection means includes:

[0012] Set the safety valve opening threshold to at least one of the following: power safety threshold, opening the predetermined unnecessary device, and disabling the waste heat discharge system safety valve.

[0013] Preferably, the reactor pressure vessel protection method further includes:

[0014] Perform cold overpressure analysis to calculate the cold safety threshold.

[0015] Preferably, the cold overpressure analysis includes:

[0016] The unit is placed in a cold state, and only the pressure regulator safety valve in the overpressure protection device is activated to increase the pressure of the primary circuit. The maximum withstand pressure of the pressure regulator safety valve under various transient conditions is collected. Based on the overpressure protection criteria for each transient condition, the corresponding protection threshold is calculated. Then, the corresponding protection threshold is selected as the cold-state safety threshold according to the transient condition type of the primary circuit. The transient conditions include Class II, Class III, and Class IV transient conditions.

[0017] Preferably, the overpressure protection criterion for the second type of transient operating condition is: the transient pressure of the voltage regulator does not exceed 105% of the design pressure of the primary circuit;

[0018] The overpressure protection criteria for the three types of transient operating conditions are: the transient pressure of the voltage regulator shall not exceed 120% of the design pressure;

[0019] The overpressure protection criterion for the four types of transient operating conditions is: the transient pressure of the voltage regulator shall not exceed 130% of the design pressure.

[0020] Preferably, in the cold overpressure analysis, the method of increasing the pressure of the primary loop includes:

[0021] By controlling the safety injection system in the unit, the lowest temperature liquid is injected into the primary circuit at the maximum flow rate; or

[0022] Mass energy is injected by controlling the chemical and volumetric control systems in the unit; or

[0023] The reactor coolant pumps in the unit are kept running continuously by controlling their operation.

[0024] Preferably, the calculation expression for the first conversion temperature is:

[0025]

[0026] Where T1 is the first conversion temperature, T NDT ΔRT is the ductile-brittle transition temperature of the reactor pressure vessel. NDT %Cu represents the ductile-brittle transition temperature increment of the reactor pressure vessel over time, %Cu represents the copper content of the reactor pressure vessel, %P represents the phosphorus content of the reactor pressure vessel, and f represents the current maximum neutron flux on the inner surface of the reactor pressure vessel.

[0027] Preferably, the second conversion temperature is equal to the sum of the first conversion temperature and the set hysteresis value.

[0028] Preferably, the reactor pressure vessel protection method further includes: limiting the cold safety threshold to a value less than the maximum pressure that the reactor pressure vessel can withstand at the ductile-brittle transition temperature.

[0029] The present invention also constructs a reactor pressure vessel protection system, comprising:

[0030] The first judgment unit is used to determine whether the waste heat removal system is in an active state when the unit is in a down-flow state, and to generate a cold overpressure protection signal for activating the cold overpressure protection means when the waste heat removal system is in an active state.

[0031] The second judgment unit is used to determine whether the average temperature of the primary loop is lower than the first conversion temperature when the waste heat removal system is not in operation, and to generate the cold overpressure protection signal when the average temperature of the primary loop is lower than the first conversion temperature; and

[0032] The third judgment unit is used to determine whether the average temperature of the primary circuit is greater than the second conversion temperature when the unit is in the upward state, and to generate a hot overpressure protection signal to activate the hot overpressure protection means when the average temperature of the primary circuit is greater than the second conversion temperature.

[0033] Preferably, the reactor pressure vessel protection system further includes:

[0034] The first execution unit is configured to execute the cold overpressure protection measures upon receiving the cold overpressure protection signal; the cold overpressure protection measures include at least one of setting the safety valve opening threshold of the primary circuit regulator to a cold safety threshold, closing a predetermined unnecessary device, and activating the safety valve of the waste heat removal system; and / or

[0035] The second execution unit is used to execute the hot overpressure protection means when the hot overpressure protection signal is received; the hot overpressure protection signal includes at least one of setting the safety valve opening threshold to the power safety threshold, opening the predetermined unnecessary device, and disabling the waste heat discharge system safety valve.

[0036] The present invention provides the following beneficial effects: It provides a reactor pressure vessel protection method; when the unit is in the down-flow state, if the residual heat removal system is active, a cold overpressure protection signal is generated to activate the cold overpressure protection mechanism, placing the unit in a cold overpressure protection state to prevent the reactor pressure vessel from becoming brittle due to temperature decrease in a cold state, thus preventing reactor pressure vessel rupture; furthermore, if the residual heat removal system is not active, a cold overpressure protection signal is generated when the primary loop average temperature drops below the first transition temperature; when the unit is in the up-flow state, if the primary loop average temperature is greater than the second transition temperature... Temperature readings generate a hot overpressure protection signal to activate hot overpressure protection measures, placing the unit in a hot overpressure protection state to prevent the reactor pressure vessel from rupturing due to excessive temperature in a cold state. This invention generates either a cold overpressure protection signal or a hot overpressure protection signal based on the average temperature of the primary loop, controlling the unit to switch between cold and hot overpressure protection states based on actual conditions. By leveraging the resilience characteristics of the reactor pressure vessel as it changes with temperature, it selectively executes corresponding overpressure protection measures, effectively preventing reactor pressure vessel rupture and playing a positive role in improving the safety of nuclear power plants. Attached Figure Description

[0037] The present invention will be further described below with reference to the accompanying drawings and embodiments. In the accompanying drawings:

[0038] Figure 1 This is a flowchart of a reactor pressure vessel protection method in some embodiments of the present invention;

[0039] Figure 2 This is a structural diagram of the reactor pressure vessel protection system in some embodiments of the present invention. Detailed Implementation

[0040] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0041] It should be noted that the flowcharts shown in the accompanying drawings are merely illustrative and do not necessarily include all content and operations / steps, nor do they necessarily have to be performed in the described order. For example, some operations / steps can be broken down, while others can be combined or partially combined; therefore, the actual execution order may change depending on the specific circumstances.

[0042] The block diagrams shown in the accompanying drawings are merely functional entities and do not necessarily correspond to physically independent entities. That is, these functional entities can be implemented in software, in one or more hardware modules or integrated circuits, or in different network and / or processor devices and / or microcontroller devices.

[0043] See Figure 1 The diagram shown is a flowchart of a reactor pressure vessel protection method in some embodiments of the present invention. This protection method is used to prevent the reactor pressure vessel from rupturing due to overpressure, and includes steps S1 and S2.

[0044] Step S1: When the unit is in the downlink state, determine whether the waste heat removal system is in the active state. If the determination result is yes, generate a cold overpressure protection signal to activate the cold overpressure protection means. If the determination result is no, further determine whether the average temperature of the primary loop is less than the first switching temperature. If the determination result is yes, generate a cold overpressure protection signal.

[0045] Step S2: When the unit is in the up-current state, determine whether the average temperature of the primary circuit is greater than the second switching temperature. If the determination result is yes, generate a hot overpressure protection signal to activate the hot overpressure protection method.

[0046] Downward movement refers to the process by which the unit begins to cool down and depressurize from its power operation state until it reaches the state of shutdown and refueling.

[0047] The up-state refers to the process by which the unit starts to heat up and pressurize from the state of shutdown and refueling until it reaches the power operation state.

[0048] Overpressure refers to the phenomenon in a system where the fluid pressure increases to or exceeds the design pressure due to thermal imbalance, excessive pump flow, or other similar phenomena.

[0049] Cold state refers to the state during reactor shutdown and refueling when the primary coolant loop is open to the atmosphere and the reactor coolant is at normal temperature and pressure. It should be noted that in a certain nuclear power plant, the reactor coolant can be considered to be in a cold state when the coolant temperature is below 180°C and the coolant pressure is less than 3.2 MPa·abs.

[0050] Hot state refers to the state in which the reactor coolant is in a high temperature and high pressure state during the generation of electricity in a nuclear power plant, so as to transfer the heat generated by the nuclear fuel to the secondary loop.

[0051] The residual heat removal system is used to remove residual heat from the reactor core in a cold state, ensuring that the primary loop temperature remains within the allowable range.

[0052] In this embodiment, when the unit is in the downtrend state, if the residual heat removal system is active, it indicates that the reactor has been shut down and the residual heat of the reactor core is being released, meaning that the reactor pressure vessel is rapidly cooling down (relative to natural cooling). To prevent the reactor pressure vessel from rupturing due to a sudden drop in temperature to the ductile-brittle transition temperature, a cold overpressure protection signal needs to be generated to activate the cold overpressure protection mechanism, putting the unit in a cold overpressure protection state. This prevents the reactor pressure vessel from becoming brittle as the temperature decreases in a cold state, which could lead to rupture. Furthermore, if the residual heat removal system is not active, a cold overpressure protection signal is generated when the primary loop average temperature drops below the first transition temperature to prevent the reactor pressure vessel from rupturing.

[0053] When the unit is in the up-state, if the average temperature of the primary loop is greater than the second conversion temperature, a hot overpressure protection signal is generated to activate the hot overpressure protection mechanism, so that the unit is in the hot overpressure protection state to prevent the reactor pressure vessel from rupturing due to excessive temperature under hot conditions.

[0054] This embodiment generates a cold overpressure protection signal or a hot overpressure protection signal based on the average temperature of the primary loop, so as to control the unit to switch between cold overpressure protection state and hot overpressure protection state according to the actual situation. Based on the toughness characteristics of the reactor pressure vessel as it changes with temperature, the corresponding overpressure protection measures are implemented in a targeted manner, which effectively prevents the reactor pressure vessel from rupturing and plays a positive role in improving the safety of nuclear power plants.

[0055] In an optional embodiment, the cold overpressure protection means in step S1 includes at least one of setting the safety valve opening threshold of the primary circuit regulator to a cold safety threshold, closing a predetermined unnecessary device, and activating the safety valve of the waste heat discharge system.

[0056] In this embodiment, the safety valve opening threshold refers to the gas pressure threshold at which the safety valve in the primary circuit pressurizer opens, used to prevent overpressure in the pressurizer. When the unit is in a hot state, the safety valve opening threshold is generally set to the power safety threshold (which can be 17 MPa.abs). However, when the cold overpressure protection measures are implemented, it means that the unit is in a cold state, and the reactor pressure vessel will become embrittled. Therefore, the cold safety threshold is less than the power safety threshold, which can prevent the reactor pressure vessel from rupturing due to overpressure after cooling and embrittlement.

[0057] The term "non-essential equipment" refers to equipment that may inject mass energy into the primary circuit and will not be used after the unit enters the down-state. Such equipment includes safety injection system pressurization lines and other equipment that may increase the temperature or pressure of the primary circuit. This can help to avoid unexpected increases in the temperature or pressure of the primary circuit during the down-state.

[0058] The waste heat removal system safety valve refers to the safety valve connected to the primary loop in the waste heat removal system (its connection is independent of whether the waste heat removal system is active). Activating the waste heat removal system safety valve is a crucial protective measure against primary loop overheating and overpressure, preventing reactor pressure vessel rupture due to overpressure. It can be used in conjunction with the safety valve in the primary loop pressurizer to enhance the protection of the reactor pressure vessel. It should be noted that the waste heat removal system only becomes active after the unit is in a cold state; therefore, the opening threshold of the waste heat removal system safety valve is lower than the power safety threshold.

[0059] In an alternative embodiment, the reactor pressure vessel protection method further includes performing a cold overpressure analysis to calculate a cold safety threshold.

[0060] In this embodiment, the role of cold overpressure analysis is to calculate a suitable cold safety threshold by analyzing the pressure changes of the reactor pressure vessel when the unit is in a cold state and combining this with the relevant protection criteria of the nuclear power plant.

[0061] Furthermore, in one specific embodiment, the cold overpressure analysis includes:

[0062] The unit is kept in a cold state, and only the pressure regulator safety valve in the overpressure protection device is activated to increase the pressure of the primary circuit. The maximum pressure that the pressure regulator safety valve can withstand under various transient conditions is collected. Based on the overpressure protection criteria for various transient conditions, the corresponding protection threshold is calculated. Then, the corresponding protection threshold is selected as the cold state safety threshold according to the transient condition type of the primary circuit. The transient conditions include Class II, Class III and Class IV transient conditions.

[0063] Overpressure protection equipment refers to equipment used to prevent overpressure in the primary circuit, mainly including various safety valves, such as pressure regulator safety valves and waste heat discharge system safety valves.

[0064] Type II transient operating conditions refer to operating conditions where there is a disturbance in the primary circuit under normal operating conditions. The overpressure protection criterion for this operating condition is: the transient pressure of the voltage regulator shall not exceed 105% of the design pressure of the primary circuit.

[0065] The third type of transient operating condition refers to the primary circuit being in an emergency operating condition. The overpressure protection criterion for this condition is: the transient pressure of the voltage regulator does not exceed 120% of the design pressure of the primary circuit.

[0066] The four types of transient operating conditions refer to the primary circuit being in an accident condition or multiple accident sequence. The overpressure protection criterion for this condition is: the transient pressure of the voltage regulator does not exceed 130% of the design pressure of the primary circuit.

[0067] To prevent equipment damage caused by overpressure, overpressure protection of the unit is necessary, such as setting overpressure protection criteria for different operating conditions and designing overpressure prevention devices (commonly such as safety valves). In this embodiment, the cold-state overpressure analysis first puts the unit in a cold state, activating only the pressure regulator safety valve, i.e., other equipment in the overpressure protection system fails, preparing for subsequent targeted analysis of the pressure regulator safety valve's pressure-bearing capacity. Then, by gradually increasing the pressure in the primary loop, the pressure regulator safety valve is simulated to operate under various transient conditions, collecting the maximum pressure it can withstand under these conditions. Based on the overpressure protection criteria for each transient condition, the corresponding protection threshold is calculated. Finally, a cold-state safety threshold is selected according to the current transient condition type to ensure the effectiveness of the cold-state overpressure protection measures.

[0068] In some embodiments, the pressure of the primary loop can be increased by: injecting a liquid (water or coolant) at the lowest temperature into the primary loop at a maximum flow rate through a safety injection system in the unit; or by injecting mass energy through a chemical and volume control system in the unit; or by continuously operating the reactor coolant pumps in the unit to increase the coolant flow rate, thereby increasing the pressure of the primary loop.

[0069] In an optional embodiment, the hot overpressure protection means in step S1 includes at least one of setting the safety valve opening threshold to a power safety threshold, opening a predetermined unnecessary device, and shutting down the safety valve of the waste heat discharge system.

[0070] In an optional embodiment, the calculation expression for the first conversion temperature is:

[0071]

[0072] Where T1 is the first transition temperature, T NDT ΔRT is the ductile-brittle transition temperature of the reactor pressure vessel. NDT %Cu represents the ductile-brittle transition temperature increment of the reactor pressure vessel over time, %Cu represents the copper content of the reactor pressure vessel, %P represents the phosphorus content of the reactor pressure vessel, and f represents the current maximum neutron flux on the inner surface of the reactor pressure vessel.

[0073] Furthermore, in one specific embodiment, the reactor pressure vessel protection method further includes limiting the cold safety threshold to a value less than the maximum pressure the reactor pressure vessel can withstand at the ductile-brittle transition temperature. Additionally, the maximum pressure value can be calculated based on the first transition temperature obtained from material analysis of the reactor pressure vessel. Optionally, the cold safety threshold is 8 MPa.

[0074] In one optional embodiment, the second switching temperature is equal to the sum of the first switching temperature and a set hysteresis value. Furthermore, the set hysteresis value can be determined based on the range and accuracy of the instrument monitoring the first switching temperature, and can be 1% of the instrument range. Optionally, the set hysteresis value is 3°C.

[0075] See Figure 2 As shown, the present invention also provides a reactor pressure vessel protection system, which includes: a first judgment unit 1, a second judgment unit 2 and a third judgment unit 3.

[0076] The first judgment unit 1 is used to determine whether the waste heat removal system is in effect when the unit is in the down-flow state, and to generate a cold overpressure protection signal to activate the cold overpressure protection means when the waste heat removal system is in effect.

[0077] The second judgment unit 2 is used to determine whether the average temperature of the primary loop is less than the first conversion temperature when the waste heat removal system is not in an active state, and to generate a cold overpressure protection signal when the average temperature of the primary loop is less than the first conversion temperature.

[0078] The third judgment unit 3 is used to determine whether the average temperature of the primary circuit is greater than the second conversion temperature when the unit is in the upward state, and to generate a hot overpressure protection signal to activate the hot overpressure protection means when the average temperature of the primary circuit is greater than the second conversion temperature.

[0079] like Figure 2 As shown, the reactor pressure vessel protection system further includes a first execution unit 4. The first execution unit 4 is used to execute cold overpressure protection measures when a cold overpressure protection signal is received; the cold overpressure protection measures include at least one of setting the safety valve opening threshold of the primary circuit pressurizer to a cold safety threshold, closing predetermined unnecessary devices, and activating the safety valve of the residual heat removal system.

[0080] like Figure 2 As shown, the reactor pressure vessel protection system further includes a second execution unit 5. The second execution unit 5 is used to execute hot overpressure protection measures when a hot overpressure protection signal is received; the hot overpressure protection signal includes at least one of setting the safety valve opening threshold to the power safety threshold, opening a predetermined unnecessary device, and shutting down the residual heat removal system safety valve.

[0081] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple; relevant parts can be referred to the method section.

[0082] Those skilled in the art will further recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of both. To clearly illustrate the interchangeability of hardware and software, the components and steps of the various examples have been generally described in terms of functionality in the foregoing description. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementations should not be considered beyond the scope of this invention.

[0083] The steps of the methods or algorithms described in conjunction with the embodiments disclosed herein can be implemented directly by hardware, a software module executed by a processor, or a combination of both. The software module can be located in random access memory (RAM), memory, read-only memory (ROM), electrically programmable ROM, electrically erasable programmable ROM, registers, hard disk, removable disk, CD-ROM, or any other form of storage medium known in the art.

[0084] It is understood that the above embodiments only illustrate preferred embodiments of the present invention, and their descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can freely combine the above technical features without departing from the concept of the present invention, and can also make several modifications and improvements, all of which fall within the protection scope of the present invention. Therefore, all equivalent transformations and modifications made with respect to the scope of the claims of the present invention should fall within the scope of the claims of the present invention.

Claims

1. A reactor pressure vessel protection method characterized by, The method comprises the following steps: When the unit is in a downward state, it is determined whether the residual heat removal system is in an effective state, and if the determination result is yes, a cold-state overpressure protection signal for activating a cold-state overpressure protection means is generated; if the determination result is no, it is further determined whether the primary loop average temperature is less than a first conversion temperature, and if the determination result is yes, the cold-state overpressure protection signal is generated; wherein the cold-state overpressure protection means comprises at least one of setting a safety valve opening threshold of a primary loop pressurizer to a cold-state safety threshold, closing predetermined unnecessary devices, and enabling a residual heat removal system safety valve; When the unit is in an upward state, it is determined whether the primary loop average temperature is greater than a second conversion temperature, and if the determination result is yes, a hot-state overpressure protection signal for activating a hot-state overpressure protection means is generated; Cold-state overpressure analysis is performed to calculate the cold-state safety threshold; wherein the cold-state overpressure analysis comprises: placing the unit in a cold state, enabling only the pressurizer safety valve in the overpressure protection equipment, increasing the pressure of the primary loop, collecting the maximum bearing pressure of the pressurizer safety valve under various transient conditions, combining the overpressure protection criteria of various transient conditions to calculate the corresponding protection threshold, and then selecting the corresponding protection threshold as the cold-state safety threshold according to the type of transient condition of the primary loop; wherein the transient conditions include two types of transient conditions, three types of transient conditions, and four types of transient conditions.

2. The reactor pressure vessel protection method of claim 1, wherein, The hot-state overpressure protection means comprises: At least one of setting the safety valve opening threshold to a power safety threshold, opening the predetermined unnecessary devices, and disabling the residual heat removal system safety valve.

3. The reactor pressure vessel protection method of claim 1, wherein, The overpressure protection criterion of the two types of transient conditions is that the transient pressure of the pressurizer does not exceed 105% of the design pressure of the primary loop; The overpressure protection criterion of the three types of transient conditions is that the transient pressure of the pressurizer does not exceed 120% of the design pressure; The overpressure protection criterion of the four types of transient conditions is that the transient pressure of the pressurizer does not exceed 130% of the design pressure.

4. The reactor pressure vessel protection method of claim 1, wherein, In the cold-state overpressure analysis, the way of increasing the pressure of the primary loop comprises: By controlling the safety injection system in the unit to inject the liquid of the lowest temperature into the primary loop at the maximum flow rate; or By controlling the chemical and volume control system in the unit to inject energy; or By controlling the reactor coolant pump in the unit to run continuously.

5. The reactor pressure vessel protection method of claim 1, wherein, The calculation expression of the first conversion temperature is: ； wherein, is the first transition temperature, is the ductile-to-brittle transition temperature of the reactor pressure vessel, is the ductile-to-brittle transition temperature increment of the reactor pressure vessel with increasing service time, is the copper content of the reactor pressure vessel, is the phosphorus content of the reactor pressure vessel, is the maximum neutron fluence of the current inner surface of the reactor pressure vessel.

6. The reactor pressure vessel protection method of claim 5, wherein, The second conversion temperature is equal to the sum of the first conversion temperature and a set back value.

7. The reactor pressure vessel protection method of claim 5, wherein, Further comprising: Limiting the cold-state safety threshold to be less than the maximum pressure value that the reactor pressure vessel can withstand at the ductile-brittle transition temperature.

8. A reactor pressure vessel protection system characterized by, Comprise: A first determination unit is configured to determine whether the residual heat removal system is in an effective state when the unit is in a downward state, and generate a cold-state overpressure protection signal for activating a cold-state overpressure protection means when the residual heat removal system is in an effective state; The second judging unit is configured to judge whether the average temperature of the loop is less than a first conversion temperature when the residual heat removal system is not in an effective state, and generate the cold-state overpressure protection signal when the average temperature of the loop is less than the first conversion temperature. The third judging unit is configured to judge whether the average temperature of the loop is greater than a second conversion temperature when the unit is in an uplink state, and generate a hot-state overpressure protection signal for activating a hot-state overpressure protection means when the average temperature of the loop is greater than the second conversion temperature. The first executing unit is configured to execute the cold-state overpressure protection means when the cold-state overpressure protection signal is received; the cold-state overpressure protection means includes at least one of setting a safety valve opening threshold of a loop stabilizer to a cold-state safety threshold, shutting down predetermined unnecessary devices, and enabling a residual heat removal system safety valve. The cold-state overpressure analysis includes: placing the unit in a cold state, enabling only a stabilizer safety valve in the overpressure protection equipment, increasing the pressure of the loop, collecting the maximum bearing pressure of the stabilizer safety valve under various transient working conditions, calculating corresponding protection thresholds in combination with overpressure protection criteria of various transient working conditions, and then selecting a corresponding protection threshold as the cold-state safety threshold according to the type of transient working condition of the loop; wherein the transient working condition includes a second-type transient working condition, a third-type transient working condition, and a fourth-type transient working condition.

9. The reactor pressure vessel protection system in accordance with claim 8, wherein, The second executing unit is configured to execute the hot-state overpressure protection means when the hot-state overpressure protection signal is received. The hot-state overpressure protection signal includes at least one of setting the safety valve opening threshold to a power safety threshold, opening the predetermined unnecessary devices, and disabling the residual heat removal system safety valve. ​

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