Patent Publication Number: US-2021193340-A1

Title: Nuclear reactor long-term cooling system and nuclear plant having the same

Description:
TECHNICAL FIELD 
     The present invention relates to a nuclear reactor long-term cooling system capable of enhancing the safety of a nuclear plant, and a nuclear plant having the same. 
     BACKGROUND ART 
     A nuclear reactor is categorized into a separate nuclear reactor (e.g., a commercial nuclear reactor: domestic) where main apparatuses (a steam generator, a pressurizer, a pump, etc.) are installed outside a nuclear reactor vessel, and an integral nuclear reactor (e.g., a SMART nuclear reactor: domestic) where main apparatuses are installed inside the nuclear reactor vessel, according to an installation position of the main apparatuses. 
     Especially, the integral nuclear reactor has a characteristic to exclude weaknesses which may occur at connection portions among systems. More specifically, in the conventional separate nuclear reactor, a heat exchanger, a cooling device, a steam generator, and a pressurizer are connected to each other by pipes. And the integral nuclear reactor has been devised to improve the weaknesses such as a breakdown of pipes. 
     The nuclear reactor is categorized into an active nuclear reactor and a passive nuclear reactor according to an implementation method of a safety system. The active nuclear reactor uses an active device such as a pump operated by a power of an emergency generator, etc. so as to drive a safety system. On the other hand, the passive nuclear reactor uses a passive device operated by a passive force such as a gravitational force or a gas pressure, so as to drive a safety system. 
     In the passive nuclear reactor, a passive safety system can safely maintain the nuclear reactor, only with a natural force mounted therein, without a driving source or an alternating current (AC) power source of a safety class such as an emergency diesel generator, for a predetermined time (three days, 72 hours) requested by restriction requirements when an accident occurs. And the passive safety system can safely maintain its power source or an emergency direct current (DC) power source by utilizing a driving source or a non-safety system after 72 hours. 
     However, even if three days requested by restriction requirements lapse after an accident occurrence, residual heat is generated from a core of the nuclear reactor of a nuclear power plant for a considerable time. Different from a general thermal power plant where heat generation is stopped if fuel supply is stopped, even if a nuclear fission reaction is stopped at a core (nuclear fuel) into which a control rod has been inserted, residual heat is generated from the core, by nuclear fission products produced and accumulated during a normal operation. Accordingly, various safety systems for removing residual heat of a core when an accident occurs are installed at a nuclear plant. 
     A passive nuclear plant developed or being developed for enhanced safety of a nuclear plant (US Westinghouse AP1000, Korean SMART) can safely process a nuclear accident by introducing a passive force such as a gas pressure or a gravitational force, in order to exclude an active device such as a pump which requires a large amount of electricity. 
     Especially, a nuclear plant may perform a long-term cooling operation for removing residual heat when an accident occurs. The conventional nuclear reactor has performed a long-term cooling operation with large-scale cooling facilities using sea water. However, such large-scale cooling facilities using sea water resulted in increasing construction costs of a nuclear plant. 
     Further, a long-term cooling operation using sea water can be performed only when there is sea water nearby, which restricted on conditions of a site of a nuclear plant. Further, a long-term cooling operation using sea water should be provided with a pump to use sea water as a cooling source. Here, if the pump cannot be operated, a long-term cooling operation cannot be performed. This may cause a major incident such as a fuel meltdown or a hydrogen explosion. 
     Accordingly, the present invention needs a nuclear reactor long-term cooling system capable of excluding the conventional large-scale cooling facilities using sea water when a nuclear accident occurs. 
     DISCLOSURE OF INVENTION 
     Technical Problem 
     An object of the present disclosure is to provide a nuclear reactor long-term cooling system and a nuclear plant having the same, the nuclear reactor long-term cooling system capable of enhancing the safety by performing a long-term cooling operation by excluding large-scale facilities for sea water cooling, and by utilizing a safety system inside a nuclear plant. 
     Solution to Problem 
     To achieve the above purpose, a nuclear reactor long-term cooling system according to the present invention comprises: a lower containment area formed to enclose a reactor coolant system, and configured to prevent steam containing radioactive substances generated from the reactor coolant system from leaking to a path other than a discharge unit; an In-Containment Refueling Water Storage Tank (IRWST) disposed outside the lower containment area, and having refueling water stored therein; and a discharge pipe configured to connect the lower containment area to the IRWST, and to discharge steam of the lower containment area to the refueling water when an accident occurs. 
     In an embodiment of the present invention, the nuclear reactor long-term cooling system may further comprise an injection portion formed at an end part of the discharge pipe and configured to inject the steam which flows along the discharge pipe to the refueling water. 
     In an embodiment of the present invention, one side of the discharge pipe may be connected to the first lower containment area which is at a higher position than a level of the refueling water, and another side thereof may be connected to a lower part of the IRWST which is at a lower position than the level. And the steam may be discharged to the refueling water by a difference between an internal pressure of the lower containment area and a pressure of said another side of the discharge pipe. 
     In an embodiment of the present invention, the injection portion may be arranged near a bottom surface of the IRWST, be extended in parallel to the bottom surface, and be provided with a plurality of injection holes. 
     In an embodiment of the present invention, a check valve may be installed at the discharge pipe so as to prevent a backflow of the refueling water from the IRWST to the lower containment area. 
     In an embodiment of the present invention, the lower containment area may include: a first lower containment area formed to enclose the reactor coolant system; and a second lower containment area communicated with the first lower containment area, and formed to enclose a safety injection system for safely injecting emergency cooling water to the reactor coolant system when an accident occurs. 
     In an embodiment of the present invention, the safety injection system may be provided with at least one of a safety injection tank and a core makeup tank each connected to the reactor coolant system. 
     In an embodiment of the present invention, the second lower containment area may be disposed at a higher position than the first lower containment area. An automatic depressurization system for lowering a pressure of the reactor coolant system may be accommodated in the second lower containment area. And one side of the automatic depressurization system may be connected to an upper part of the reactor coolant system, another side thereof may be extended to an upper part of the second lower containment area, and the automatic depressurization system may be configured to discharge steam of the reactor coolant system to the second lower containment area from the first lower containment area. 
     In an embodiment of the present invention, the nuclear reactor long-term cooling system may further comprises: a containment formed to enclose the lower containment area, and serving as a final containment to reduce radioactive substances; and an emergency cooling tank having therein a heat exchanger for heat-exchanging with steam transmitted from the lower containment area. 
     In an embodiment of the present invention, the emergency cooling tank may be arranged at a higher position than the IRWST outside the containment. 
     In an embodiment of the present invention, the emergency cooling tank may store therein emergency cooling water for heat-exchange with the heat exchanger. And the emergency cooling tank may further include therein a cooling water supplement pipe for supplementing emergency cooling water supplied from the outside. 
     In an embodiment of the present invention, the nuclear reactor long-term cooling system may further comprise: a steam pipe configured to connect the lower containment area to the heat exchanger; and a first recovery pipe configured to connect the heat exchanger to the IRWST, and disposed at a higher position than the injection portion from a bottom surface of the IRWST. 
     In an embodiment of the present invention, the discharge pipe may discharge the steam from the lower containment area to the refueling water at an early stage of an accident. And the steam may be discharged to the refueling water through the discharge pipe and the first recovery pipe, respectively in a preset time after the early stage of the accident. 
     In an embodiment of the present invention, the steam of the lower containment area may be discharged to the refueling water through the discharge pipe to thus be condensed, and the steam discharge through the discharge pipe may be stopped as a water level of the IRWST is increased. And the steam of the lower containment area may be introduced into the heat exchanger along the steam pipe to thus be condensed by heat exchange. 
     In an embodiment of the present invention, the nuclear reactor long-term cooling system may further comprise a second recovery pipe configured to connect the heat exchanger to a lower part of the lower containment area, extended by passing through the IRWST, and configured to heat-exchange steam condensed by the heat exchanger with the refueling water and collect the steam to the lower part of the lower containment area. 
     In an embodiment of the present invention, the nuclear reactor long-term cooling system may further comprise: a radioactive substance reduction tank disposed at a higher position than the IRWST in a spaced manner in the containment, and configured to store therein cooling water; and a steam intake pipe having one side connected to an upper space of the IRWST, and another side extended to inside of the radioactive substance reduction tank, and configured to introduce steam discharged to the upper space from the IRWST into the cooling water of the radioactive substance reduction tank. 
     In an embodiment of the present invention, the steam intake pipe may include: a first steam intake pipe upward extended from a bottom surface of the radioactive substance reduction tank, above a water surface of the radioactive substance reduction tank; and a second steam intake pipe having one side connected to an upper end of the first steam intake pipe, and another side downward extended to the bottom surface of the radioactive substance reduction tank so as to be adjacent to the bottom surface, and communicated with the inside of the radioactive substance reduction tank. 
     In an embodiment of the present invention, the nuclear reactor long-term cooling system may further comprise a third recovery pipe configured to connect the heat exchanger to the radioactive substance reduction tank, and to discharge steam condensed by the heat exchanger to the radioactive substance reduction tank. 
     In the present invention, there is provided a nuclear plant, comprising: a reactor coolant system; a lower containment area formed to enclose the reactor coolant system, and having a first space for accommodating the reactor coolant system therein; a containment formed to enclose the lower containment area, and having a second space for accommodating the lower containment area therein; an In-Containment Refueling Water Storage Tank (IRWST) disposed at a lower part of a second space between the lower containment area and the containment, and configured to store refueling water therein; and a reactor long-term cooling system configured to discharge steam containing radioactive substances generated from the lower containment area when an accident occurs, to outside of the lower containment area, and to condense the steam. 
     In an embodiment of the nuclear plant according to the present invention, the reactor long-term cooling system may include a discharge pipe having one side connected to one side of the lower containment area which is at a higher position than a bottom surface of the IRWST, and another side adjacent to and connected to the bottom surface of the IRWST, the discharge pipe configured to discharge the steam inside the lower containment area to the refueling water when an accident occurs. 
     In an embodiment of the nuclear plant according to the present invention, the reactor long-term cooling system may further include an emergency cooling tank disposed outside the containment, having therein a heat exchanger, and configured to cool the steam transmitted from the lower containment area by heat-exchanging the steam with emergency cooling water. 
     In an embodiment of the nuclear plant according to the present invention, the reactor long-term cooling system may further include a first recovery pipe configured to connect the heat exchanger to the IRWST. The reactor long-term cooling system may discharge steam generated from the lower containment area to the refueling water of the IRWST through the discharge pipe, at an early stage of an accident. And the reactor long-term cooling system may discharge the steam of the lower containment area to the refueling water through the discharge pipe, and may collect the steam condensed by the heat exchanger to the IRWST through the first recovery pipe, in a preset time after the early stage of the accident. 
     In an embodiment of the nuclear plant according to the present invention, the steam of the lower containment area may be discharged to the refueling water of the IRWST through the discharge pipe and the first recovery pipe, the steam discharge through the discharge pipe may be stopped as a water level of the IRWST is increased, and the steam introduced into the heat exchanger may be condensed by the emergency cooling water of the emergency cooling tank. 
     In an embodiment of the nuclear plant according to the present invention, the reactor long-term cooling system may further include a second recovery pipe connected to a lower part of the lower containment area from the heat exchanger by passing through the IRWST. And the reactor long-term cooling system may heat-exchange the steam condensed by the heat exchanger with the refueling water through the second recovery pipe, and may collect condensation water to the lower containment area, in a preset time after the accident. 
     Advantageous Effects of Invention 
     The reactor long-term cooling system and the nuclear plant having the same according to the present disclosure may have the following effects. 
     Firstly, the discharge pipe is connected to the lower containment area and the IRWST, and steam inside the lower containment area is discharged to the refueling water of the IRWST through the discharge pipe by a difference between an internal pressure of the lower containment area and a water pressure of the IRWST. This can allow a long-term cooling operation of the nuclear plant, without using the conventional large-scale seawater cooling facilities when a nuclear accident such as a steam pipe breakdown accident occurs. This can enhance the safety of the nuclear plant, because there is no long-term cooling inability due to a breakdown of the conventional seawater pump. Since the conventional large-scale cooling facilities using sea water are not required, construction costs of the nuclear plant can be saved. Further, even when there is no sea water, the nuclear plant can be constructed, and limited conditions of a site of the nuclear plant can be mitigated. 
     Secondly, the emergency cooling tank is communicated with the upper part of the lower containment area through the steam pipe, and accommodates therein a heat exchanger and emergency cooling water. The emergency cooling tank heat-exchanges steam of the lower containment area with the emergency cooling water through the heat exchanger, even when the discharge pipe cannot be operated due to a level increase of the refueling water during a long-term cooling operation. This can allow a long-term cooling operation of the nuclear plant. 
     Thirdly, the first recovery pipe is connected to the heat exchanger and the IRWST, and steam of the lower containment area condensed by the heat exchanger is discharged to the refueling water. This can allow a long-term cooling operation of the nuclear plant. 
     Fourthly, the second recovery pipe is connected to the heat exchanger and the lower containment area, and is extended by passing through the IRWST. And steam of the lower containment area condensed by the heat exchanger is heat-exchanged with the refueling water, and then is collected into the lower containment area. As a result, a cooling circulation of the steam is repeated, and a long-term cooling operation of the nuclear plant can be performed. 
     Fifthly, the radioactive substance reduction tank is arranged above the IRWST, and accommodates therein cooling water and the steam intake pipe communicated with the upper space of the IRWST. And non-condensed steam of the IRWST is discharged to the IRWST through the steam intake pipe. This can reduce radioactive substances. 
     Sixthly, the third recovery pipe is connected to the heat exchanger and the radioactive substance reduction tank, and steam of the lower containment area condensed by the heat exchanger is collected to the radioactive substance reduction tank. This can reduce radioactive substances contained in steam, and can perform a long-term cooling circulation. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a conceptual view showing a nuclear plant having a nuclear reactor long-term cooling system according to the present disclosure; 
         FIG. 1B  is an operational state view showing that steam of a lower containment area is discharged to an In-Containment Refueling Water Storage Tank (IRWST) through a discharge pipe at an early stage of a nuclear accident in  FIG. 1A ; 
         FIG. 1C  is an operational state view showing that steam of a lower containment area is discharged to an In-Containment Refueling Water Storage Tank (IRWST) through a discharge pipe and a first recovery pipe in a predetermined time after a nuclear accident in  FIG. 1A ; 
         FIG. 1D  is an operational state view showing that steam of a lower containment area is discharged to an In-Containment Refueling Water Storage Tank (IRWST) through a second recovery pipe via an emergency cooling tank (ECT) in three days after a nuclear accident in  FIG. 1A ; 
         FIG. 2  is a graph showing a change of a pressure according to a time at the time of cooling by a nuclear reactor long-term cooling system of the present disclosure; and 
         FIG. 3  is an operational state view showing that steam of a lower containment area is discharged to a radioactive substance reduction tank through a third recovery pipe via an emergency cooling tank (ECT) when a severe nuclear accident occurs in  FIG. 1A . 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Description will now be given in detail according to exemplary embodiments disclosed herein, with reference to the accompanying drawings. For the sake of brief description with reference to the drawings, the same or equivalent components may be provided with the same or similar reference numbers, and description thereof will not be repeated. In describing the present disclosure, if a detailed explanation for a related known function or construction is considered to unnecessarily divert the gist of the present disclosure, such explanation has been omitted but would be understood by those skilled in the art. The accompanying drawings are used to help easily understand the technical idea of the present disclosure and it should be understood that the idea of the present disclosure is not limited by the accompanying drawings. The idea of the present disclosure should be construed to extend to any alterations, equivalents and substitutes besides the accompanying drawings. 
     It will be understood that although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are generally only used to distinguish one element from another. 
     A singular representation may include a plural representation unless it represents a definitely different meaning from the context. 
     Terms such as “include” or “has” are used herein and should be understood that they are intended to indicate an existence of several components, functions or steps, disclosed in the specification, and it is also understood that greater or fewer components, functions, or steps may likewise be utilized. 
     Before explaining a long-term cooling system of a nuclear plant according to the present invention, a nuclear plant  100  according to the present disclosure may be explained. 
       FIG. 1A  is a conceptual view showing a nuclear plant  100  having a nuclear reactor long-term cooling system according to the present disclosure. 
     Referring to  FIG. 1A , the nuclear plant  100  includes a containment  10 , a lower containment area  20 , an In-Containment Refueling Water Storage Tank (IRWST)  30 , an Emergency Cooling Tank (ECT)  40 , and a radioactive substance reduction tank  50 . 
     The containment  10  forms a final containment which encloses the lower containment area  20  so as to prevent leakage of a radioactive substance. The containment  10  of the present disclosure refers to a containment building, a nuclear reactor building, a containment vessel, a safety protection vessel, etc. The containment  10  may be referred to as an Upper Containment Area (UCA) for reducing radioactive substances. 
     The lower containment area  20  may be referred to as a Lower Containment Area (LCA) for reducing radioactive substances. The lower containment area  20  is provided in the containment  10 . The lower containment area  20  may separate a first space  20   a  and a second space  20   b  from each other. The first space  20   a  accommodates a reactor coolant system  21  therein. The second space  20   b  is formed between the first space  20   a  and the containment  10 . More specifically, the second space  20   b  may be formed as a large space when compared to the first space  20   a . Further, the first space  20   a  may be referred to as the inside of the lower containment area  20 , and the second space  20   b  may be referred to as the outside of the lower containment area  20 . The second space  20   b  is formed in the containment  10 . 
     The lower containment area  20  may include a first lower containment area  201  and a second lower containment area  202 . The first lower containment area  201  accommodates the reactor coolant system  21  therein. And the second lower containment area  202  is formed to be communicated with an upper part of the first lower containment area  201 . 
     The reactor coolant system  21  is accommodated in a refueling pool. An opening is formed at an upper part of the refueling pool. A CPRSS cover is installed to open and close the opening of the refueling pool. The CPRSS is a Containment Pressure and Radioactivity Suppression System. 
     In the lower containment area  20 , may be accommodated the reactor coolant system  21  having a core  211  and a steam generator, an Automatic Depressurization System (ADS)  22 , a Safety Injection Tank (SIT)  23 , a Core Makeup Tank (CMT)  24 , a discharge pipe  25  and a check valve  25   a . The discharge pipe  25  and the check valve  25   a  may be arranged at one side of the lower containment area  20 . For instance, the discharge pipe  25  may be arranged between one side of the lower containment area  20  and the IRWST  30 . 
     The discharge pipe  25  may be arranged between the first space  20   a  and the second space  20   b , or may be arranged at the second space  20   b.    
     The reactor coolant system  21  is configured to remove heat from the core  211  and the inner structures by circulating cooling water, and to transmit the heat generated from the core  211  to the steam generator. The reactor coolant system  21  refers to a reactor vessel in case of an integral reactor. A reactor coolant pump  212  may be provided in the reactor vessel. The reactor coolant pump  212  may transmit cooling water to a primary flow path of the steam generator through a forcible circulation. A pressurizer may be installed at an upper part of the reactor vessel. The pressurizer serves to uniformly maintain a pressure of the reactor coolant system  21 . 
     The second lower containment area  202  is configured to accommodate therein the Automatic Depressurization System (ADS)  22 , the Safety Injection Tank (SIT)  23  and the Core Makeup Tank (CMT)  24 . The second lower containment area  202  is upward extended from an upper part of the first lower containment area  201 . A lower side of the second lower containment area  202  is connected to the upper part of the first lower containment area  201  so as to be communicated with each other. 
     The automatic depressurization system  22 , the safety injection tank  23  and the core makeup tank  24  may form a passive safety injection system by being operated reciprocally when an accident occurs at the nuclear plant  100 . 
     The automatic depressurization system  22  is formed to lower an internal pressure of the reactor coolant system  21  into a value less than a predetermined level, so as to prevent damage of the reactor coolant system  21  due to an overpressure when an overpressure accident occurs at the nuclear plant  100 . The automatic depressurization system  22  may include a depressurization pipe  221  and a plurality of valves. One side of the automatic depressurization system  22  is connected to an upper end of a reactor vessel so as to be communicated with each other, and another side of the automatic depressurization system  22  is extended from an upper space of the first lower containment area  201  so as to be connected to an upper space of the second lower containment area  202 . The automatic depressurization system  22  transmits steam of the reactor coolant system  21  to the upper space of the second lower containment area  202  when a pressure of the reactor coolant system  21  is increased into a level more than a preset value. 
     A safety injection system  26  is configured to inject safety injection water into the reactor coolant system  21  when an accident occurs. The safety injection tank  23  and the core makeup tank  24  are configured to inject safety injection water such as a boric acid solution, into the reactor coolant system  21 . 
     Each of the safety injection tank  23  and the core makeup tank  24  may be connected to the reactor coolant system  21  by a pipe, in parallel or in series. In this embodiment, the safety injection tank  23  and the core makeup tank  24  are connected thereto in series. 
     The safety injection system  26  includes the safety injection tank  23 , the core makeup tank  24 , a pressure equilibrium pipe  231 , and a safety injection pipe  241 . 
     The safety injection tank  23  may be arranged above the core makeup tank  24 . The safety injection tank  23  and the core makeup tank  24  may be connected to each other by a connection pipe  232  so as to be communicated with each other. Safety injection water (a boric acid solution) stored in the safety injection tank  23  may be injected into the core makeup tank  24  through the connection pipe  232 . 
     One side of the pressure equilibrium pipe  231  may be connected to an upper end of the reactor coolant system  21  (reactor vessel in case of an integral reactor) so as to be communicated with each other, and another side thereof may be connected to an upper part of the safety injection tank  23  so as to be communicated with each other. The pressure equilibrium pipe  231  may serve to maintain a pressure equilibrium state between the reactor coolant system  21  and the safety injection tank  23 . The safety injection tank  23  and the core makeup tank  24  may maintain a pressure equilibrium state through the connection pipe  232 . 
     The safety injection pipe  241  is configured to connect a lower part of the core makeup tank  24  to the reactor vessel so as to be communicated with each other. Safety injection water may be injected into the reactor vessel from the core makeup tank  24  by gravity, through the safety injection pipe  241 . 
     The safety injection tank  23  is disposed at a higher position than the reactor vessel. The discharge pipe  25  is configured to discharge steam of the lower containment area  20  to refueling water  30 ′ accommodated in the In-Containment Refueling Water Storage Tank (IRWST)  30 . One side of the discharge pipe  25  is communicated with the lower containment area  20 , and another side of the discharge pipe  25  is communicated with the IRWST  30 . 
     One side of the discharge pipe  25  is arranged at a higher position than another side of the discharge pipe  25 . One side of the discharge pipe  25  is connected to an upper part of the first lower containment area  201  so as to be communicated with each other, and another side of the discharge pipe  25  is connected to a lower part of the IRWST  30  so as to be communicated with each other. 
     The check valve  25   a  may be installed at an upper side of the discharge pipe  25 . The check valve  25   a  is operated by a difference between an internal pressure of the lower containment area  20  and a pressure of the refueling water of the IRWST  30 . If the internal pressure of the lower containment area  20  is higher than the pressure of the IRWST  30 , the check valve  25   a  is open. On the other hand, if the internal pressure of the lower containment area  20  is lower than the pressure of the IRWST  30 , the check valve  25   a  is closed. The check valve  25   a  limits a flowing direction of steam inside the lower containment area  20 , into one direction. The check valve  25   a  restricts steam inside the lower containment area  20  to move to the IRWST  30 . That is, the check valve  25   a  prevents the refueling water of the IRWST  30  from moving to the lower containment area  20  along the discharge pipe  25 . 
     An injection portion  251  may be provided at a lower side of the discharge pipe  25 . The injection portion  251  is formed at the end of the discharge pipe  25 . The injection portion  251  is configured to inject steam inside the lower containment area  20  which flows along the discharge pipe  25 , to the refueling water. The injection portion  251  may be arranged near a bottom surface of the IRWST  30 . The injection portion  251  may be extended in parallel to the bottom surface of the IRWST  30 . The injection portion  251  is provided with a plurality of injection holes  252 . The injection portion  251  may be configured to be immerged into the refueling water. 
     Thus, steam inside the lower containment area  20  may be discharged to the refueling water  30 ′ accommodated in the IRWST  30  along the discharge pipe  25 , by a pressure difference between the first space  20   a  and the second space  20   b  when an accident occurs at the nuclear plant  100 . Here, the discharge pipe  25  is provided with the check valve  25   a , and may prevent the refueling water  30 ′ accommodated in the IRWST  30  from backflowing to the first space  20   a.    
     The lower containment area  20  is installed in the containment  10 , and separates the first space  20   a  for accommodating the reactor coolant system  21  therein, from the second space  20   b  formed between the lower containment area  20  and the containment  10 . Further, the lower containment area  20  is formed to have a sealing structure to prevent steam and radioactive substances from leaking to the second space  20   b  when an accident occurs at the nuclear plant  100 . 
     Thus, the first space  20   a  inside the lower containment area  20  may include steam or radioactive substances discharged when an accident such as a steam pipe breakdown accident or a coolant loss accident occurs at the nuclear plant  100 . Here, the lower containment area  20  is formed to have a design pressure high enough to endure a pressure of vaporized steam when an accident occurs. 
     The refueling water  30 ′ may be accommodated in the IRWST  30 . When the nuclear plant  100  performs a refueling operation, a refueling water supplying pipe (not shown) for making refueling water flow between the IRWST  30  and the reactor coolant system  21  is connected. The refueling water supplying pipe may supply refueling water into the reactor coolant system  21 . 
     The emergency cooling tank (ECT)  40  is arranged outside the containment  10 . Emergency cooling water is stored in the ECT  40 . An emergency cooling water injection pipe  42  may be connected to the ECT  40 . One side of the emergency cooling water injection pipe  42  may be connected to an upper part of the ECT  40 , and another side of the emergency cooling water injection pipe  42  may be connected to an external emergency cooling water supplying unit. Emergency cooling water may be injected into the ECT  40  through the emergency cooling water injection pipe  42 . 
     The ECT  40  is configured to lower an inner temperature of the lower containment area  20  by condensing steam generated when an accident occurs at the nuclear plant  100 . Steam generated when an accident occurs at the nuclear plant  100  is cooled by being heat-exchanged with emergency cooling water  40 ′ accommodated in the ECT  40 . 
     More specifically, steam generated when an accident occurs at the nuclear plant  100  is heat-exchanged with the emergency cooling water  40 ′ by a heat exchanger  41  provided at the ECT  40 . The heat exchanger  41  may form condensation water by cooling steam generated from the lower containment area  20  when an accident occurs, through a heat exchange. Thus, as steam generated when an accident occurs is cooled by losing heat, the inner temperature of the lower containment area  20  may be lowered. Further, cooling water may be supplemented to the ECT  40  by charging an external cooling water source with the emergency cooling water injection pipe  42 . 
     The radioactive substance reduction tank  50  may be provided in the containment  10 . The radioactive substance reduction tank  50  is provided at the second space  20   b  of the containment  10 . Further, the radioactive substance reduction tank  50  is arranged above the IRWST  30 . 
     The radioactive substance reduction tank  50  may be provided with a steam intake pipe  51  therein. 
     The steam intake pipe  51  may consist of a first steam intake pipe and a second steam intake pipe. The first steam intake pipe is upward extended from a bottom surface of the radioactive substance reduction tank  50 , above a water surface. The second steam intake pipe is downward extended to the bottom surface of the radioactive substance reduction tank  50 , from an upper part of the first steam intake pipe. The first and second steam intake pipes are connected to each other by a connection pipe having a shape of a reversed ‘U’. 
     A lower side of the first steam intake pipe is connected to an upper space of the IRWST  30 . An upper side of the first steam intake pipe is connected to an upper side of the second steam intake pipe. A lower side of the second steam intake pipe is connected to the inside of the radioactive substance reduction tank  50 . 
     The upper space of the IRWST  30  may be sealed by being covered by a sealing cover. 
     Thus, steam or radioactive substances inside the IRWST  30  having its pressure increased when an accident occurs at the nuclear plant  100  may be injected into cooling water  50 ′ accommodated in the radioactive substance reduction tank  50 , while moving along the steam intake pipe  51 . 
     Thus, the steam or radioactive substances above the IRWST  30  may be condensed by the cooling water  50 ′. Especially, radioactive substances may be solved by the cooling water  50 ′ to thus be collected. Accordingly, the steam condensed by being injected to the cooling water  50 ′ may be introduced into the second space  20   b , thereby cooling the lower containment area  20  and lowering a concentration of the radioactive substances. 
     Further, in order to effectively reduce the radioactive substances in the cooling water  50 ′, the cooling water  50 ′ may be formed to accommodate therein refueling water of a pH more than a preset value, so as to prevent the volatility of the radioactive substances (especially, iodine). More specifically, the cooling water  50 ′ may be formed to have the alkalinity. Further, a preset pH of the cooling water  50 ′ may be 7, and may be preferably 7.5˜10. 
     In addition, the containment  10 , the lower containment area  20 , the IRWST  30 , the ECT  40 , and the radioactive substance reduction tank  50  may be connected to one another by pipes. 
     More specifically, the nuclear plant  100  may be provided with a steam pipe  60  and first to third recovery pipes  70 ˜ 90 , and the first to third recovery pipes are provided with valves. 
     More specifically, the steam pipe  60  is configured to connect the lower containment area  20  to the heat exchanger  41 . A steam valve  60   a  is provided on the steam pipe  60 . 
     One side of the first recovery pipe  70  is connected to the heat exchange  41 , and another side of the first recovery pipe  70  is connected to the inside of the IRWST  30 . A first valve  70   a  is installed at the first recovery pipe  70  so as to open and close the first recovery pipe  70 . The first recovery pipe  70  is extended from the heat exchanger  41 , and is configured to discharge steam and condensation water to the refueling water  30 ′ accommodated in the IRWST  30 . 
     Further, another side of the first recovery pipe  70  is arranged so that a separation distance from the bottom surface of the IRWST  30  is longer than that of the aforementioned injection portion  251  formed at the end of the discharge pipe  25 . Accordingly, steam or condensation water condensed at the heat exchanger  41  may be discharged to the IRWST  30 , through the first recovery pipe  70 , even by a pressure difference smaller than a pressure difference for operating the check valve  25   a  of the discharge pipe  25 . 
     One side of the second recovery pipe  80  may be connected to the heat exchanger  41 , and another side of the second recovery pipe  80  may be horizontally extended to pass through the IRWST  30  and may be connected to the inside of the lower containment area  20 . A second valve  80   a  is installed at the second recovery pipe  80  so as to open and close the second recovery pipe  80 . 
     Steam or condensation water condensed at the heat exchanger  41  downward moves along the second recovery pipe  80 , and may be cooled by being heat-exchanged with the refueling water at the time of passing through the IRWST  30 . The steam or condensation water cooled by passing through the IRWST  30  may be collected to the lower containment area  20 . 
     Another side of the first pipe may be positioned so that its height from the bottom surface of the IRWST  30  is higher than the second recovery pipe  80 . 
     One side of the third recovery pipe  90  is connected to the heat exchanger  41 , and another side of the third recovery pipe  90  is connected to the inside of the radioactive substance reduction tank  50 . A third valve  90   a  may be installed at the third recovery pipe  90  so as to open and close the third recovery pipe  90 . Steam or condensation water condensed at the heat exchanger  41  may be discharged to the cooling water  50 ′ stored in the radioactive substance reduction tank  50 . The aforementioned check valve  25   a , steam valve  60   a , first valve  70   a , second valve  80   a , and third valve  90   a  are selectively open and closed in a preset order when an accident occurs at the nuclear plant  100 . 
     The first to third recovery pipes  70 ,  80 ,  90  may be independently separated from the heat exchanger  41  to thus be connected to the IRWST  30 , the lower containment area  20 , and the radioactive substance reduction tank  50 , respectively. Alternatively, the first to third recovery pipes may be extended from the heat exchanger  41  as a single pipe, and may be respectively diverged at positions having different heights from the bottom surface of the IRWST  30 . In this embodiment, the first to third recovery pipes  70 ,  80 ,  90  are extended from the heat exchanger  41  as a single pipe, and are respectively diverged at positions having different heights from the bottom surface of the IRWST  30 . 
     Hereinafter, an operation of the long-term cooling system of the nuclear plant  100 , performed when an accident occurs at the nuclear plant  100  will be explained.  FIG. 1B  is an operational state view showing that steam of the lower containment area  20  is discharged to the IRWST  30  through the discharge pipe  25  at an early stage of a nuclear accident in  FIG. 1A .  FIG. 1C  is an operational state view showing that steam of the lower containment area  20  is discharged to the IRWST  30  through the discharge pipe and the first recovery pipe in a predetermined time after an accident which occurred at the nuclear plant  100  in  FIG. 1A . 
       FIG. 1D  is an operational state view showing that steam of the lower containment area  20  is discharged to the IRWST  30  through the second recovery pipe  80  via the emergency cooling tank (ECT), in three days after an accident which occurred at the nuclear plant  100  in  FIG. 1A . 
     In a case where a steam pressure inside the lower containment area  20  is increased to a level more than a preset value at an early stage of a design basis accident on the nuclear plant  100 , the check valve  25   a  of the discharge pipe  25  is open by a difference between the steam pressure inside the lower containment area  20  and a pressure of the refueling water operated at the injection portion  251 . 
     If the steam pressure inside the lower containment area  20  is higher than the pressure of the refueling water operated at the injection portion  251 , the check valve  25   a  is open, and steam inside the first lower containment area  201  downward moves towards the injection portion  251  along the discharge pipe  25 . 
     The steam of the lower containment area  20  is injected to the refueling water through the injection holes  252  of the injection portion  251 . 
     If a pressure of the reactor coolant system  21  is increased to a level more than a preset value in a predetermined time after an accident which occurred at the nuclear plant  100 , the automatic depressurization system  22  is operated. As a result, steam of the reactor coolant system  21  upward moves along the depressurization pipe  221  to thus move to an upper space of the second lower containment area  202 . 
     The steam of the second lower containment area  202  moves along the steam pipe  60  connected to the second lower containment area  202 , thereby being introduced into the heat exchanger  41 . The heat exchanger  41  inside the emergency cooling tank  40  condenses the steam of the second lower containment area  202  and the cooling water of the emergency cooling tank  40  by a heat-exchange, thereby cooling the steam of the second lower containment area  202 . 
     The steam or the condensation water condensed at the heat exchanger  41  moves to the IRWST  30  along the first recovery pipe  70 , thereby being collected to the refueling water. 
     If the steam of the second lower containment area  202  is discharged to the refueling water through the discharge pipe  25  and the first recovery pipe  70 , a steam pressure of the lower containment area  20  is lowered and a level of the refueling water is increased. If the level of the refueling water is increased, the pressure of the refueling water of the IRWST  30  is increased. As a result, the steam discharge through the discharge pipe  25  is decreased, and may be stopped after all. Here, the steam discharge may be performed only through the first recovery pipe  70 . 
     In a predetermined time (e.g., three days) after an accident which occurred at the nuclear plant  100 , the steam or the condensation water condensed at the heat exchanger  41  downward moves along the second recovery pipe  80 , and moves along the second recovery pipe  80  which passes through the IRWST  30 . As a result, the steam and the refueling water are heat-exchanged with each other. 
     The heat-exchanged steam is cooled to be collected to the inside of the first lower containment area  201 . 
     In three days after an accident occurrence, the steam or the condensation water condensed at the heat exchanger  41  downward moves along the third recovery pipe  90 , and is collected to the radioactive substance reduction tank  50 . 
     In the long-term cooling system of the nuclear plant  100  according to the present disclosure, steam inside the lower containment area  20  is moved along the discharge pipe  25 , the first recovery pipe  70 , the second recovery pipe  80 , and the third recovery pipe  90 . This may allow a long-term cooling operation to be performed sequentially without large-scale cooling facilities of sea water, thereby enhancing a safety of the nuclear plant  100 . 
     Long-term cooling of steam through the discharge pipe  25  and the first to third recovery pipes  70 ,  80 ,  90  may be performed by a passive driving power by a heat source occurring when a design basis accident occurs at the nuclear plant  100 . Thus, the long-term cooling system of the nuclear plant  100  according to the present disclosure may exclude an additional power source such as an electric power. 
     Further, since long-term cooling of steam through the discharge pipe  25  and the first to third recovery pipes  70 ,  80 ,  90  is performed by an air cooling, construction costs of the nuclear plant  100  can be saved by excluding large-scale facilities for sea water cooling when constructing the nuclear plant  100 . Further, even when there is no sea water near the nuclear plant  100 , the nuclear plant  100  can be constructed, and limited conditions of a site of the nuclear plant  100  can be mitigated. 
     A long-term cooling method of the nuclear plant  100  may include a first cooling step, a second cooling step and a third cooling step. 
     Referring to  FIG. 1B , in the first cooling step, steam inside the lower containment area  20  may be discharged to the refueling water  30 ′ accommodated in the IRWST  30 , thereby cooling the inside of the lower containment area  20 . 
     More specifically, steam generated from the inside the lower containment area  20  when a design basis accident occurs at the nuclear plant  100  may be steam generated due to damage of the reactor coolant system  21  or the pipes. And the steam may be steam containing radioactive substances. Hereinafter, steam generated when an accident occurs at the nuclear plant  100  means pure steam or steam containing radioactive substances. 
     Steam generated when a design basis accident occurs at the nuclear plant  100  is discharged to the inside of the lower containment area  20 . Especially, since the steam is discharged to an upper part of the lower containment area  20  through the automatic depressurization system  22 , a large amount of steam may be formed at the upper part of the lower containment area  20 . 
     A pressure difference between the first space  20   a  (the inside of the lower containment area  20 ) and the IRWST  30 , due to steam generated when a design basis accident occurs at the nuclear plant  100 . As a result, steam which is at the upper part of the first lower containment area  201  is discharged to the refueling water  30 ′ accommodated in the IRWST  30 , thereby cooling the inside of the lower containment area  20 . As a result, an inner temperature of the lower containment area  20  may be lowered, and an internal pressure of the lower containment area  20  may be also lowered. 
     That is, as the check valve  25   a  is open, the steam inside the lower containment area  20  is condensed at the refueling water  30 ′ to thus cool the inside of the lower containment area  20 . Further, the steam discharged to the refueling water  30 ′ may be condensed to form condensation water. The condensation water formed by passing through the refueling water may be accommodated in the IRWST  30 . 
     Steam or radioactive substances not condensed at the refueling water  30 ′ may be introduced into the cooling water of the radioactive substance reduction tank  50  through the steam intake pipe  51 , and may be condensed. Further, the radioactive substances may be collected after being dissolved by the cooling water of the radioactive substance reduction tank  50 . 
     Referring to  FIG. 1C , in the second cooling step, steam inside the lower containment area  20  is heat-exchanged with emergency cooling water, after passing through the heat exchanger  41 . Then, the steam may be discharged to the refueling water  30 ′ along the first recovery pipe  70 , thereby cooling the lower containment area  20 . As a result, an inner temperature of the lower containment area  20  may be lowered, and an internal pressure of the lower containment area  20  may be also lowered. 
     The second cooling step is for cooling the first space  20   a  by operating the internal pressure of the lower containment area  20  at 160 kPa, and thereby the internal pressure of the lower containment area  20  is drastically lowered. A change of the internal pressure of the lower containment area  20  according to the long-term cooling system of the nuclear plant  100  will be explained with reference to  FIG. 2 . In the second cooling step, steam not condensed at the refueling water  30 ′ may be condensed at the radioactive substance reduction tank  50 , similar to the aforementioned first cooling step. Further, radioactive substances may be collected after being dissolved in the radioactive substance reduction tank  50 . 
     For the second cooling step, the steam valve  60   a  and the first valve  70   a  are open. Accordingly, the steam inside the lower containment area  20  may be heat-exchanged at the heat exchanger  41  to thus be condensed. Then, the steam may be discharged to the refueling water  30 ′, thereby cooling the inside of the lower containment area  20 . As a result, an inner temperature of the lower containment area  20  may be lowered, and an internal pressure of the lower containment area  20  may be also lowered. 
     The steam which passes through the heat exchanger  41  in the second cooling step is heat-exchanged with the emergency cooling water  40 ′ accommodated in the ECT  40 . Thus, the temperature of the emergency cooling water  40 ′ may be increased. Since the emergency cooling water  40 ′ having its temperature increased is discharged to the air through the ECT  40  and is cooled, steam generated when an accident occurs may be cooled continuously. 
     At the early stage of the second cooling step, the first cooling step may be performed simultaneously. As a result, the temperature and the pressure inside the lower containment area  20  may be lowered effectively. However, as the first cooling step is performed continuously, steam generated when an accident occurs may be directly condensed at the refueling water  30 ′ and thus a level of the refueling water  30 ′ may be increased. This may cause the pressure of the second space  20   b  to be increased. 
     A design pressure of the containment  10  which accommodates the second space  20   b  is set to be lower than a design pressure of the lower containment area  20 . Thus, in order to prevent steam and radioactive substances generated when an accident occurs from leaking to the outside, a pressure of a predetermined level needs to be maintained. If the level of the refueling water  30 ′ is increased as the steam is directly condensed at the refueling water  30 ′, the check valve  25   a  is closed. That is, as the check valve  25   a  is closed, the steam discharge through the injection portion  251  is stopped. 
     More specifically, at the early stage of the second cooling step, the steam discharge is performed through the injection portion  251  and the first recovery pipe  70 . However, if the level of the refueling water  30 ′ is increased, the steam discharge is performed only through the first recovery pipe  70 . 
     Referring to  FIG. 1D , in the third cooling step, steam inside the lower containment area  20  is heat-exchanged, after passing through the heat exchanger  41 . Then, the steam is re-cooled by being heat-exchanged with the second recovery pipe which passes through the refueling water  30 ′. Thus, the steam condensed by a heat exchange with the heat exchanger  41  and the refueling water  30 ′ is re-supplied into the first space  20   a , thereby cooling the inside of the lower containment area  20 . As a result, the inner temperature of the lower containment area  20  may be lowered, and the internal pressure of the lower containment area  20  may be also lowered. 
     Especially, the third cooling step is performed in three days after a design basis accident, for cooling the inside of the lower containment area  20 . The time in three days after a design basis accident is a time requested by restriction requirements when an accident occurs. For three days after a design basis accident, the reactor needs to be safely maintained only with a natural force, without a driving source or without an operation of a system which utilizes an emergency generator. 
     However, in three days after a design basis accident, a power system, a driving source or the like may be normally operated, for operation of a safety system utilizing them. Thus, in the third cooling step, a closed loop is formed by the second recovery pipe which passes through the heat exchanger  41  and the refueling water  30 ′, thereby sufficiently cooling the inside of the lower containment area  20  by a heat exchange. 
     For the third cooling step, the steam valve  60   a  and the second valve  80   a  are open. As a result, steam inside the lower containment area  20  may be condensed by being heat-exchanged at the heat exchanger  41  and the refueling water  30 ′, and then may be resupplied into the lower containment area  20  to thus cool the inside of the lower containment area  20 . Accordingly, the inner temperature of the lower containment area  20  may be lowered, and the internal pressure of the lower containment area  20  may be also lowered. 
     Referring to  FIG. 2 , illustrated is a graph showing a change of a pressure of each region according to a time at the time of cooling the nuclear plant  100  by the nuclear reactor long-term cooling system of the present disclosure when a design basis accident occurs at the nuclear plant  100 . 
     When a design basis accident occurs at the nuclear plant, the first space  20   a  indicated as ‘LCA’ on the graph has a drastic pressure increase. Then, the pressure inside the IRWST  30  indicated as ‘IRWST’, and the pressure of the second space  20   b  indicated as ‘UCA’ are formed in order. The pressure of ‘RRT’ is formed similar to the pressure of the second space  20   b  indicated as ‘UCA’. That is, the pressure increased when a design basis accident occurs at the nuclear plant is the highest at the first space  20   a  indicated as ‘LCA’, the IRWST  30  indicated as ‘IRWT’, and the second space  20   b  indicated as ‘UCA’, in order. This is related to a sealing structure of the lower containment area  20  for preventing steam and radioactive substances from leaking to the second space  20   b  when an accident occurs at the nuclear plant  100 . 
     If the first space  20   a  has a drastic pressure increase at an early stage of a design basis accident, the first cooling step is performed and the pressure is continuously increased. And when the pressure inside the lower containment area  20  is 160 kPa, the second cooling step may be performed. As a result, the pressure inside the lower containment area  20 , drastically increased at an early stage of a design basis accident which occurred at the nuclear plant may be stably lowered. 
     At an early stage of the second cooling step, the first cooling step and the second cooling step shown in  FIGS. 1B and 1C  may be simultaneously performed to prevent the drastic pressure increase of the first space  20   a . However, in this case, hazards of the nuclear plant  100  may be increased due to a level increase of the refueling water  30 ′. 
     Accordingly, the second cooling step is performed, and the first cooling step is terminated when the pressure inside the lower containment area  20  is lowered to about 190 kPa. Since only the second cooling step shown in  FIG. 1C  is performed, a small amount of pressure increase occurs until three days after the occurrence of an accident. 
     Then, in three days after the accident, a driving source is used or a system utilizing an emergency generator is operated. As a result, a long-term cooling having the third cooling step may be performed. 
       FIG. 3  is an operational state view showing that steam of the lower containment area  20  is discharged to the radioactive substance reduction tank  50  through the third recovery pipe  90  via the emergency cooling tank (ECT)  40  when a severe accident occurs at the nuclear plant  100  in  FIG. 1A . 
     Referring to  FIG. 3 , when a severe accident occurs at the nuclear plant  100 , the steam valve  60   a  and the third valve  90   a  may be open to perform a long-term cooling operation for cooling the first space  20   a , the inside of the lower containment area  20 . When a severe accident occurs at the nuclear plant  100  and when the aforementioned design basis accident occurs, additionally-generated hydrogen may be accumulated with steam and radioactive substances generated at the first space  20   a.    
     That is, when a severe accident occurs at the nuclear plant  100 , steam, radioactive substances and hydrogen of the first space  20   a  are heat-exchanged at the heat exchanger  41 , and are discharged to the cooling water  50 ′ accommodated in the radioactive substance reduction tank  50 . This can prevent the steam, radioactive substances and hydrogen from being discharged into the IRWST  30 . 
     If hydrogen is discharged to the refueling water  30 ′ accommodated in the IRWST  30  as the steam valve and the first valve are open similar to the first cooling step when a severe accident occurs at the nuclear plant  100 , the hydrogen is accumulated on the IRWST  30 , resulting in increasing a probability of explosion. 
     Referring to  FIG. 3  back, when a severe accident occurs at the nuclear plant  100 , the steam valve  60   a  and the third valve  90   a  may be open to perform a cooling operation for discharging steam, radioactive substances and hydrogen condensed at the heat exchanger  41  to a large space such as the second space  20   b.    
     More specifically, steam can be condensed at the heat exchanger  41  to thus be cooled, thereby lowering a temperature and a pressure. Radioactive substances can be dissolved at the cooling water  50 ′ representing the alkalinity, thereby having its amount reduced. Further, hydrogen can be discharged to the second space  20   b , a larger space than the first space  20   a , thereby reducing a probability of explosion. 
     It will be apparent to those skilled in the art that the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. 
     Also, all changes and modifications that fall within the metes and bounds of the claims, or equivalents of such metes and bounds are therefore intended to be embraced by the appended claims.