Patent Description:
When a severe accident occurs in a nuclear power plant, a core is exposed and finally melt due to a coolant loss of a primary loop system of the nuclear power plant, and the core corium will eventually collapse into a lower head of RPV (reactor pressure vessel). If the core corium cannot be cooled in time, the corium will eventually melt through a wall of the lower head of the RPV because of the core decay heat, thus the core corium falls in a pit and may possibly melt through a basemat of a containment, resulting in a large leakage of radioactive substance in the end.

A reactor pit flooding system is generally adopted in the prior art to realize a pit water injection in domestic and foreign pressurized water reactor nuclear power plant, and a forced or natural circulation cooling is generated on an outer wall surface of a reactor pressure vessel to take away decay heat, so that the lower head is prevented from being melted through. However, under the severe accident conditions initiating in a large or medium break event, a coolant loss speed of the reactor pressure vessel is fast due to a boundary break area of the primary loop system is large. Due to factors such as an uncertainty of a core melt process and a delayed operation time of severe accident mitigation systems, the possibility of the core corium melting through the RPV still exists.

Two or more accumulators are utilized in current pressurized water reactor nuclear power plant. Each accumulator is divided into a gas space (about <NUM><NUM>) and a boric acid water space (about <NUM><NUM>). When a pressure of the primary loop system of the nuclear power plant is less than a specific value, generally <NUM>-<NUM>. 0MPa, an electric valve is automatically started, and the boric acid water is quickly injected into the reactor pressure vessel (RPV) through a cold pipe at a time. However on other conditions, there is no passive injection water available in the accumulator, resulting in an occurrence of the accident of the core being melted.

Examples of safety injection systems for nuclear power plants are disclosed in <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT>.

A technical problem to be solved in the present disclosure is to provide a safety system for dealing with a severe accident of a nuclear power plant and a control method therefor to improve the safety of the nuclear reactor.

A technical solution adopted by the present disclosure to solve the technical problem is to provide a safety system for dealing with a severe accident of a nuclear power plant, wherein the safety system includes at least one in-reactor water injection system for injecting water into a reactor pressure vessel;.

each in-reactor water injection system includes a multi-stage accumulator, and a first water injection pipeline connected to a cold pipe of a primary loop system; an interior space of the multi-stage accumulator includes a gas-phase space, a first-stage water injection space, and a second-stage water injection space distributed from top to bottom in sequence; the multi-stage accumulator is provided with a first flow pipeline and a second flow pipeline having a pipe diameter less than that of the first flow pipeline, the first flow pipeline communicates the first-stage water injection space with the first water injection pipeline, and the second flow pipeline communicates the second-stage water injection space with the first water injection pipeline.

The safety system for dealing with the severe accident of the nuclear power plant further includes at least one outer-reactor cooling water injection system for injecting water into a pit; and
each outer-reactor cooling water injection system includes a high-level water injection tank disposed above the reactor pressure vessel, and a second water injection pipeline connected between the high-level water injection tank and the pit.

Preferably, the first flow pipeline has a pipe diameter of ≥ <NUM>; the second flow pipeline has a pipe diameter of <NUM>-<NUM>, and is configured to supply water at a water injection flow rate of <NUM><NUM>/h-<NUM><NUM>/h.

Preferably, the gas-phase space is configured to have a pressure of <NUM>. 0MPa-<NUM>.

Preferably, each in-reactor water injection system further includes a first check valve disposed on the first water injection pipeline, a first power valve disposed on the first flow pipeline, and a second power valve disposed on the second flow pipeline.

Preferably, each in-reactor water injection system further includes a liquid level gauge disposed on the multi-stage accumulator corresponding to the first-stage water injection space; and
when a liquid level in the multi-stage accumulator drops to a preset value, the safety system is configured to trigger a signal to close the first power valve, to close the first power valve, and to disconnect the communication between the first flow pipeline and the first-stage water injection space; a position where the preset value is located is higher than a connection position of the first flow pipeline on the multi-stage accumulator.

Preferably, each in-reactor water injection system further includes a pressure gauge and a high-pressure gas source connecting with the gas-phase space; a third power valve is provided on a connection pipeline connected between the high-pressure gas source and the gas-phase space; when a gas pressure of the multi-stage accumulator is less than <NUM>. 0MPa, the safety system is configured to trigger a signal to open the third power valve, to open the third power valve, and the high-pressure gas source is configured to supply gas into the multi-stage accumulator.

Preferably, the interior space of the multi-stage accumulator further includes at least one third-stage water injection space disposed below the second-stage water injection space; the multi-stage accumulator is further provided with at least one third flow pipeline, and each third flow pipeline is communicated with each third-stage water injection space and has a pipe diameter less than that of the first flow pipeline.

Preferably, each outer-reactor cooling water injection system further includes a third flow pipeline connected between the high-level water injection tank and the second water injection pipeline; a pipe diameter of the third flow pipeline is less than that of the second water injection pipeline, and a connection position of the third flow pipeline on the high-level water injection tank is lower than that of the second water injection pipeline on the high-level water injection tank.

Preferably, each outer-reactor cooling water injection system further includes a fourth power valve and a second check valve disposed sequentially on the second water injection pipeline in a direction from the high-level water injection tank to the pit, and a fifth power valve disposed on the third flow pipeline; and
a connection position of the third flow pipeline on the second water injection pipeline is located between the fourth power valve and the second check valve.

The present disclosure further provides a control method for dealing with a severe accident of a nuclear power plant, wherein the control method adopts the safety system for dealing with the severe accident of the nuclear power plant in any one of the above; the control method for dealing with the severe accident of the nuclear power plant includes:
before the reactor being under a severe accident condition, when a pressure of the primary loop system of the reactor being less than a first preset value, the at least one in-reactor water injection system quickly injecting water into the reactor pressure vessel through the first flow pipeline, the first water injection pipeline and the cold pipe, to flood the core; when the pressure of the primary loop system being less than a second preset value, the at least one in-reactor water injection system injecting water into the reactor pressure vessel through the second flow pipeline, the first water injection pipeline and the cold pipe, to re-flood the core; the first preset value being larger than the second preset value.

The control method further includes: when the reactor being under a severe accident condition, at least one outer-reactor cooling water injection system injecting water into a pit through a second water injection pipeline.

The present disclosure has the following beneficial effects: both in-reactor and outer-reactor cooling may be conducted by the in-reactor water injection system in conjunction with the outer-reactor cooling water injection system, to effectively prevent a large scale of core melt, and to reduce the risk of core corium melting through the RPV when a severe accident occurs. Wherein, the in-reactor water injection system injects water in a manner of large flow rate combined with small flow rate, thereby the time of in-reactor water injection is prolonged, the core in a flooding state for a long time is ensured, the possibility of core degradation or being melted caused by a coolant loss when an accident occurs is significantly reduced, the process of the severe accident is mitigated, and maintaining the integrity of the RPV is achieved when a severe accident occurs.

The present disclosure will now be further described with reference to the accompanying drawings and embodiments, and in the drawings:.

To clearly understand the technical features, objectives and effects of the disclosure, specific embodiments of the disclosure will now be described in detail with reference to the accompanying drawings.

As shown in <FIG>, a safety system for dealing with a severe accident of a nuclear power plant in an embodiment of the present disclosure is disposed in a containment, and the safety system includes at least one in-reactor water injection system <NUM> for injecting water into a reactor pressure vessel <NUM> and at least one outer-reactor cooling water injection system <NUM> for injecting water into a pit <NUM>.

As shown <FIG> and <FIG>, the in-reactor water injection system <NUM> includes a multi-stage accumulator <NUM> and a first water injection pipeline <NUM> connected to a cold pipe <NUM> of a primary loop system <NUM>. An interior space of the multi-stage accumulator <NUM> includes a gas-phase space <NUM>, a first-stage water injection space <NUM> and a second-stage water injection space <NUM> distributed in sequence from top to bottom. The gas-phase space <NUM> is configured as a nitrogen space with a pressure of <NUM>. 0MPa-<NUM>. The first-stage water injection space <NUM> and the second-stage water injection space <NUM> are liquid-phase spaces for storage of cooling water (boron water such as boric acid water), and configured to inject water in a medium pressure stage and a low-pressure stage respectively. When the first-stage water injection space <NUM> starts water injection, the corresponding pressure of the primary loop system of the nuclear power plant is <NUM>. 0MPa-<NUM>. That is, when the pressure of the primary loop system of the nuclear power plant is <NUM>. 0MPa-<NUM>. 0MPa, the first-stage water injection space <NUM> starts injecting water to submerge the reactor core. When the second-stage water injection space <NUM> starts water injection, the corresponding pressure of the primary loop system of the nuclear power plant is <NUM>. 4MPa-<NUM>. That is, when the pressure of the primary loop system of the nuclear power plant is <NUM>. 4MPa-<NUM>. 0MPa, the second-stage water injection space <NUM> starts injecting water to submerge the reactor core. The multi-stage accumulator <NUM> is provided with a first flow pipeline <NUM> and a second flow pipeline <NUM>. The first flow pipeline <NUM> communicates the first-stage water injection space <NUM> with the first water injection pipeline <NUM>, and the second flow pipeline <NUM> communicates the second-stage water injection space <NUM> with the first water injection pipeline <NUM>. A pipe diameter of the first flow pipeline <NUM> is greater than that of the second flow pipeline <NUM>, so that the first flow pipeline <NUM> is a large flow pipe, and the cooling water in the first-stage water injection space <NUM> may be quickly injected into a core of the reactor. The second flow pipeline <NUM> is a small flow pipe, so that the primary loop system <NUM> can inject water into the core under a low-pressure state.

Alternatively, the first flow pipeline <NUM> has a pipe diameter of ≥ <NUM>, and a flow rate of ≥ <NUM><NUM>/h; the second flow pipeline <NUM> has a pipe diameter of <NUM>-<NUM>, and a water injection flow rate of <NUM><NUM>/h-<NUM><NUM>/h. The first flow pipeline <NUM> may be the same as the first water injection pipeline <NUM>.

The first flow pipeline <NUM> may correspond to a lower end of the first-stage water injection space <NUM>, one end of first flow pipeline <NUM> is connected to the multi-stage accumulator <NUM> and communicated with the first-stage water injection space <NUM>, and another end of first flow pipeline <NUM> is connected to the first water injection pipeline <NUM> and communicated therewith. The second flow pipeline <NUM> may correspond to the second-stage water injection space <NUM>, one end of second flow pipeline <NUM> is connected to a lower end or a bottom portion of the multi-stage accumulator <NUM>, and another end of second flow pipeline <NUM> is connected to the first water injection pipeline <NUM> and communicated therewith.

Furthermore, the in-reactor water injection system <NUM> further includes a first check valve <NUM> disposed on the first water injection pipeline <NUM>, a first power valve <NUM> disposed on the first flow pipeline <NUM>, and a second power valve <NUM> disposed on the second flow pipeline <NUM>. The first check valve <NUM> disposed on the first water injection pipeline <NUM> is configured to prevent the cooling water from flowing back. The first power valve <NUM> is configured to control the open-close of the first flow pipeline <NUM>, and the second power valve <NUM> is configured to control the open-close of the second flow pipeline <NUM>.

Alternatively, the first power valve <NUM> and the second power valve <NUM> adopt electric valves and are connected to an instrument control system of the nuclear power plant, respectively. The opening and closing of the first power valve <NUM> or the second power valve <NUM> is controlled by the instrument control system through an automatic signal, to realize an automatic injection of cooling water into the reactor. When the nuclear power plant is powered off or the like, the first power valve <NUM> or the second power valve <NUM> can be opened manually to realize water injection.

The in-reactor water injection system <NUM> further includes a liquid level gauge <NUM> disposed on the multi-stage accumulator <NUM> corresponding to the first-stage water injection space <NUM>, for monitoring a liquid level of the first-stage water injection space <NUM>. When the liquid level of the first-stage water injection space <NUM> in the multi-stage accumulator <NUM> drops to a preset value, a liquid level alarm is triggered, a signal to close the first power valve <NUM> is triggered, the first power valve <NUM> is automatically closed, and the communication between the first-stage water injection space <NUM> and the first flow pipeline <NUM> is disconnected, so as to prevent the gas in the multi-stage accumulator <NUM> from leaking through the first flow pipeline <NUM>, and thereby to avoid an insufficient back pressure when the second stage starts due to the gas leakage in the multi-stage accumulator <NUM>. The position where the preset value of the liquid level is located is higher than the connection position of the first flow pipeline <NUM> on the multi-stage accumulator <NUM>, ensuring that gas in the multi-stage accumulator <NUM> will not leak.

When the pressure of the primary loop system <NUM> of the reactor is less than a first preset value (e.g. <NUM>. 0MPa-<NUM>. 0MPa), the first power valve <NUM> is opened by the instrument control system of the nuclear power plant through an automatic signal, water is quickly injected into the reactor pressure vessel (RPV) through the first flow pipeline <NUM> and the cold pipe <NUM>, and thereby a core re-flooding is achieved under the accident condition. When the liquid level in the multi-stage accumulator <NUM> is below the preset value, a liquid level alarm is triggered, a signal to close the first power valve <NUM> is triggered, and the first power valve <NUM> is automatically closed. When the pressure of the primary loop system <NUM> continues to drop to less than a second preset value (<NUM>. 4MPa-1MPa), a signal to open the second power valve <NUM> is triggered, the second power valve <NUM> is automatically opened, water is injected into the reactor pressure vessel through the second flow pipeline <NUM>, and re-flooding by the primary loop system <NUM> is achieved under the low-pressure state.

According to needs, the interior space of the multi-stage accumulator <NUM> may further include at least one third-stage water injection space (not shown in the drawings) disposed below the second-stage water injection space <NUM>, configured as a water injection space for subsequent stage. Correspondingly, the multi-stage accumulator <NUM> is provided with at least one third flow pipeline (not shown in the drawings), and the third flow pipeline is communicated with the third-stage water injection space and has a pipe diameter less than that of the first flow pipeline <NUM>. The pipe diameter and flow rate of the third flow pipeline may be the same as that of the second flow pipeline <NUM>. Similarly, a power valve is disposed on the third flow pipeline to control the open-close thereof. When the third-stage water injection space starts water injection, the corresponding pressure of the primary loop system of the nuclear power plant is less than that when the second-stage water injection space starts water injection.

Furthermore, the in-reactor water injection system <NUM> further includes a pressure gauge (not shown in the drawings) and a high-pressure gas source <NUM> (e.g. a high-pressure gas tank) connecting with the gas-phase space <NUM>. A third power valve <NUM> is provided on a connection pipeline <NUM> which connects the high-pressure gas source <NUM> with the gas-phase space <NUM>. The third power valve <NUM> may adopt an electric valve, the pressure gauge and the third power valve <NUM> are connected to the instrument control system of the nuclear power plant. The opening and closing of the third power valve <NUM> are controlled by the instrument control system through a pressure signal, so as to charge and boost the gas-phase space <NUM>. When the gas pressure of the multi-stage accumulator <NUM> is less than <NUM>. 0MPa, a signal to open the third power valve <NUM> is triggered, the third power valve <NUM> is opened, and the high-pressure gas source <NUM> supplies gas into the multi-stage accumulator <NUM>.

Two or more in-reactor water injection systems <NUM> may be provided, and each in-reactor water injection system <NUM> is connected to a corresponding cold pipe <NUM> respectively.

As shown in <FIG>, the outer-reactor cooling water injection system <NUM> includes a high-level water injection tank <NUM> disposed above the reactor pressure vessel <NUM>, and a second water injection pipeline <NUM> connected between the high-level water injection tank <NUM> and the pit <NUM>. The high-level water injection tank <NUM> is configured to store cooling water (boron water), and the cooling water can be injected into the pit <NUM> by gravity force without a power pump.

The second water injection pipeline <NUM> is a pipe with a pipe diameter ≥ <NUM>, and can quickly fill up the pit <NUM> with cooling water.

In the present disclosure, the outer-reactor cooling water injection system <NUM> further includes a third flow pipeline <NUM> connected between the high-level water injection tank <NUM> and the second water injection pipeline <NUM>.

A pipe diameter of the third flow pipeline <NUM> is smaller than that of the second water injection pipeline <NUM>, and a connection position of the third flow pipeline <NUM> on the high-level water injection tank <NUM> is lower than that of the second water injection pipeline <NUM> on the high-level water injection tank <NUM>. For example, the second water injection pipeline <NUM> may be connected to a middle portion or a lower end of the high-level water injection tank <NUM>, and the cooling water is injected into the pit <NUM> through the second water injection pipeline <NUM> until the liquid level of the high-level water injection tank <NUM> drops to below a water inlet end of the second water injection pipeline <NUM>. The third flow pipeline <NUM> is connected to a bottom of the high-level water injection tank <NUM>, subsequently the cooling water can be continuously supplied for pit <NUM> through the third flow pipeline <NUM> with a relatively small flow rate. The flow rate of the third flow pipeline <NUM> may be <NUM><NUM>/h-<NUM><NUM>/h.

The outer-reactor cooling water injection system <NUM> further includes a fourth power valve <NUM> and a second check valve <NUM> which are disposed sequentially on the second water injection pipeline <NUM> in a direction from the high-level water injection tank <NUM> to the pit <NUM>, and a fifth power valve <NUM> disposed on the third flow pipeline <NUM>. The connection position of the third flow pipeline <NUM> on the second water injection pipeline <NUM> is located between the fourth power valve <NUM> and the second check valve <NUM>. The second check valve <NUM> disposed on the second water injection pipeline <NUM> is configured to prevent the cooling water from flowing back. The fourth power valve <NUM> is configured to control the open-close of one end of the second water injection pipeline <NUM> connecting to the high-level water injection tank <NUM>, and the fifth power valve <NUM> is configured to control the open-close of the third flow pipeline <NUM>.

As an option, the fourth power valve <NUM> and the fifth power valve <NUM> adopt electric valves and are connected to the instrument control system of the nuclear power plant, and the open and close of the fourth power valve <NUM> or the fifth power valve <NUM> is controlled by the instrument control system through an automatic signal to realize an automatic injection of cooling water into the pit <NUM>. When the nuclear power plant is power off or the like, the fourth power valve <NUM> and the fifth power valve <NUM> can be opened manually to realize water injection.

Furthermore, the safety system for dealing with a severe accident of a nuclear power plant of the present disclosure further includes a reactor pit natural circulation system <NUM> disposed between the reactor pressure vessel <NUM> and the pit <NUM>. The reactor pressure vessel <NUM> is suspended in the pit <NUM>, and an outer periphery of the reactor pressure vessel <NUM> is provided with a thermal insulation guide layer <NUM>. The pit <NUM> is formed by enclosing a shield wall.

As shown in <FIG> and <FIG>, the reactor pit natural circulation system <NUM> includes a cooling water flow channel <NUM> defined between the reactor pressure vessel <NUM> and the thermal insulation guide layer <NUM>, a pit water injection space <NUM> defined between the thermal insulation guide layer <NUM> and an inner wall surface of the pit <NUM>, an annular water tank <NUM> disposed on an outer periphery of an upper end of the reactor pressure vessel <NUM>, and a water return flow channel <NUM> disposed in the pit wall (shield wall) and communicating the annular water tank <NUM> with the pit water injection space <NUM>.

Wherein, a bottom of the thermal insulation guide layer <NUM> is provided with a water inlet <NUM> communicating the cooling water flow channel <NUM> with the pit water injection space <NUM>, so that cooling water in the pit water injection space <NUM> is able to enter the cooling water flow channel <NUM> through the water inlet <NUM>. The water inlet <NUM> of the thermal insulation guide layer <NUM> remains closed when the reactor is under a normal operating condition; and the water inlet <NUM> is opened when the reactor is under a severe accident condition. An exhaust outlet (not shown in the drawings) is disposed on an upper end of the thermal insulation guide layer <NUM> and communicates the cooling water flow channel <NUM> with the annular water tank <NUM>. The exhaust outlet is always above the liquid level of the annular water tank <NUM>, and a distance between a center of the exhaust outlet and the liquid level may be <NUM>-<NUM>.

Specifically, the thermal insulation guide layer <NUM> includes a guide plate and a thermal insulation layer disposed sequentially outside the reactor pressure vessel <NUM>.

The second water injection pipeline <NUM> of the outer-reactor cooling water injection system <NUM> is connected and communicated with the annular water tank <NUM> or the pit water injection space <NUM>, to fill the cooling water into the pit <NUM>, and to fill up the cooling water flow channel <NUM>, the pit water injection space <NUM> and the annular water tank <NUM>.

A buoyancy opening member <NUM> is disposed at a water outlet of the water return flow channel <NUM> communicated with the pit water injection space <NUM>. The water outlet of the water return flow channel <NUM> remains closed when the reactor is under a normal operating condition so as to reduce the vent system bypass, and may be automatically opened by the buoyancy opening member <NUM> when the pit is filled up with water under a severe accident condition. The buoyancy opening member <NUM> may be a buoyancy ball or a buoyancy cover plate, and when the cooling water is injected into the pit water injection space <NUM>, the buoyancy ball floats up to open the water outlet. The water inlet <NUM> of the thermal insulation guide layer <NUM> is further provided with a passive open-close member, such as a buoyancy cover plate or the like, floating up to open the water inlet <NUM> when there is water, and kept to close the water inlet <NUM> without water.

When the reactor is under a severe accident condition, the cooling water is heated outside the reactor pressure vessel <NUM> to generate a steam-water two-phase flow, the steam-water two-phase flow flows upward along the cooling water flow channel <NUM> and then is separated to steam and water when flowing through the exhaust outlet, and the liquid water falls into the annular water tank <NUM>; the cooling water in the annular water tank <NUM> flows back into the pit water injection space <NUM> through the water return flow channel <NUM>, thereby to form a natural circulation loop. A plurality of the water-return flow channels <NUM> are provided, and are spaced distributed in the pit wall (shield wall) of the pit <NUM> along a circumference direction of the reactor pressure vessel <NUM>, thereby not occupying the pit space, meanwhile, the natural circulation of the pit will not be affected by deformation or leakage or installation clearance of the thermal insulation guide layer <NUM>.

In the reactor pit natural circulation system <NUM>, the natural circulation is formed by utilized a water density difference between the pit <NUM> and the water return flow channel <NUM>, and the flow rate of the natural circulation may be up to <NUM><NUM>/h above, thus the cooling ability of an outer wall surface of the reactor pressure vessel <NUM> is improved; meanwhile, the steam-water two-phase flow discharged from the exhaust outlet is automatically separated to steam and water above the annular water tank <NUM>, and the steam is discharged into a large space inside the containment through a space between a main channel of an upper end of the reactor pressure vessel <NUM> and shield wall of the pit.

A control method for dealing with a severe accident of a nuclear power plant of the present disclosure adopts the above safety system for dealing with a severe accident of a nuclear power plant. As shown in <FIG>, the control method for dealing with a severe accident of the nuclear power plant may include the following steps.

Before the reactor is under a severe accident condition, when a pressure of the primary loop system <NUM> of the reactor is less than a first preset value (e.g. <NUM>. 0MPa-<NUM>. 0MPa), the in-reactor water injection system <NUM> quickly injects water into the reactor pressure vessel <NUM> through the first flow pipeline <NUM>, the first water injection pipeline <NUM> and the cold pipe <NUM>, to flood the core; when the pressure of the primary loop system <NUM> is less than a second preset value (e. 4MPa-<NUM>. 0MPa), the in-reactor water injection system <NUM> injects water into the reactor pressure vessel <NUM> through the second flow pipeline <NUM>, the first water injection pipeline <NUM> and the cold pipe <NUM>, to re-flood the core. Wherein, the first preset value is larger than the second preset value.

When the reactor is under a severe accident condition, the outer-reactor cooling water injection system <NUM> injects water into the pit <NUM> through the second water injection pipeline <NUM>.

Wherein, under a severe accident condition, when the outer-reactor cooling water injection system <NUM> injects water into the pit <NUM> to a target liquid level, sustainable suppling the water to the pit <NUM> through the third flow pipeline <NUM>; and the flow rate for suppling the water is of <NUM><NUM>/h-<NUM><NUM>/h.

Specifically, when the reactor is under a severe accident condition, the cooling water in the pit water injection space <NUM> injected into the pit <NUM> enters the cooling water flow channel <NUM> through the water inlet of the thermal insulation guide layer <NUM>, and is heated outside the reactor pressure vessel <NUM> to generate a steam-water two-phase flow, the steam-water two-phase flow flows upward along the cooling water flow channel <NUM> and then is separated to steam and water when flowing through the exhaust outlet, and the liquid water falls into the annular water tank <NUM>; the cooling water in the annular water tank <NUM> flows back into the pit water injection space <NUM> through the water return flow channel <NUM>, so as to circulate.

In the present disclosure, the in-reactor and outer-reactor cooling are carried out through the in-reactor water injection system in conjunction with the outer-reactor cooling water injection system. The reactor is cooled by the in-reactor water injection system before the reactor is under a severe accident condition. Meanwhile, the start time of the outer-reactor cooling water injection system can be prolonged meanwhile.

For example, if the volume of the multi-stage accumulator <NUM> is increased by <NUM><NUM>, the start time of the water injection of the outer-reactor cooling water injection system can be prolonged to more than <NUM> hours later when the accident occurs, the corresponding outer-reactor water injection flow rate may be between <NUM><NUM>/h-<NUM><NUM>/h, and the start time and the outer-reactor water injection flow rate are mutual coupling. For the pit with a free volume of <NUM><NUM>, the larger the outer-reactor water injection flow rate is, the shorter the filled-up time is, the current filled-up water injection time is <NUM>, with a flow rate of <NUM><NUM>/h correspondingly, and the start time can be prolonged to <NUM> hours later when the accident occurs; if the outer-reactor water injection flow rate is <NUM><NUM>/h, the filled-up water injection time is <NUM> hour, the start time is <NUM> hours later. Obviously, the above start time is greatly prolonged compared with <NUM>-<NUM> without an in-reactor water injection condition, and very sufficient time for judgment and operation of the site accident dealing with personnel is thus provided.

Claim 1:
A safety system for dealing with a severe accident of a nuclear power plant, wherein the safety system comprises at least one in-reactor water injection system (<NUM>) for injecting water into a reactor pressure vessel (<NUM>); and
each in-reactor water injection system (<NUM>) comprises a multi-stage accumulator (<NUM>), and a first water injection pipeline (<NUM>) connected to a cold pipe (<NUM>) of a primary loop system (<NUM>); an interior space of the multi-stage accumulator (<NUM>) comprises a gas-phase space (<NUM>), a first-stage water injection space (<NUM>), and a second-stage water injection space (<NUM>) distributed from top to bottom in sequence; the multi-stage accumulator (<NUM>) is provided with a first flow pipeline (<NUM>) and a second flow pipeline (<NUM>) having a pipe diameter less than that of the first flow pipeline (<NUM>), the first flow pipeline (<NUM>) communicates the first-stage water injection space (<NUM>) with the first water injection pipeline (<NUM>), and the second flow pipeline (<NUM>) communicates the second-stage water injection space (<NUM>) with the first water injection pipeline (<NUM>),
wherein the safety system further comprises at least one outer-reactor cooling water injection system (<NUM>) for injecting water into a pit (<NUM>); and
each outer-reactor cooling water injection system (<NUM>) comprises a high-level water injection tank (<NUM>) disposed above the reactor pressure vessel (<NUM>), and a second water injection pipeline (<NUM>) connected between the high-level water injection tank (<NUM>) and the pit (<NUM>).