Abstract:
The present invention provides a passive residual heat removal system and an atomic power plant comprising the same, the passive heat removal system comprising: a plate-type heat exchanger for causing heat exchange between a primary system fluid or a secondary system fluid which, in order to remove sensible heat from an atomic reactor cooling material system and residual heat from a reactor core, has received the sensible heat and the residual heat, and a cooling fluid which has been introduced from outside of a containment unit; and circulation piping for connecting the atomic reactor cooling material system to the plate-type heat exchanger, thereby forming a circulation channel of the primary system fluid, or connecting a steam generator, which is arranged at the boundary between the primary and secondary systems, to the plate-type heat exchanger, thereby forming a circulation channel of the secondary system fluid.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the invention 
         [0002]    The present disclosure relates to a passive residual heat removal system to which a plate type heat exchanger is applied and a nuclear power plant including the same. 
         [0003]    2. Description of the related art 
         [0004]    Reactors are divided into active reactors using active power such as a pump, and passive reactors using passive power such as a gravity force, a gas pressure or the like according to the configuration method of a safety system. Furthermore, reactors are divided into loop type reactors (for example, Korean pressurized water reactor) in which main components (a steam generator, a pressurizer, a pump impeller, etc.) are installed at an outside of the reactor vessel, and integrated type reactors (for example, SMART reactor) in which the main components are installed at an inside of the reactor vessel according to the installation location of the main component. 
         [0005]    A passive residual heat removal system has been employed as a system for removing heat in a reactor coolant system (sensible heat in the reactor coolant system and residual heat in the core) when an accident occurs in various nuclear power plants including an integral reactor. For a coolant circulation method of the passive residual heat removal system, two methods such as a method of directly circulating reactor primary coolant to cool a reactor (AP1000: U.S. Westinghouse) and a method of circulating secondary coolant using a steam generator to cool a reactor (SMART reactor: Korea) are mostly used, and a method of injecting primary coolant to a tank to directly condense it (CAREM: Argentina) is partially used. 
         [0006]    Furthermore, for a method of cooling an outside of a heat exchanger (condensation heat exchanger), a water-cooled method (AP1000), a partially air-cooled method (WWER 1000: Russia), and a water-air hybrid cooled method (IMR: Japan) have been used. A heat exchanger of the passive residual heat removal system performs a function of transferring heat received from a reactor to an outside (ultimate heat sink) through an emergency cooling tank or the like, and condensation heat exchangers using a steam condensation phenomenon with an excellent heat transfer efficiency have been mostly employed for a heat exchanger method. 
         [0007]    However, in general, a passive residual heat removal system may use primary coolant (reactor coolant system) or secondary coolant (steam generator) to perform the role of a pressure boundary to a primary system or secondary system, and a heat exchanger of the passive residual heat removal system may typically form a boundary to atmospheric environment outside the containment building, and when a pressure boundary is damaged, primary coolant or secondary coolant may be discharged to atmospheric environment, and therefore, maintaining a pressure boundary during an accident is a very important role. 
         [0008]    Accordingly, a method of enhancing the performance of a passive residual heat removal system may be taken into consideration to enhance the performance of a reactor. 
       SUMMARY OF THE INVENTION 
       [0009]    An object of the present disclosure is to provide a passive residual heat removal system for overcoming the coverage limit of a plate type heat exchanger and solving a problem such as flow instability or the like occurring in applying the plate type heat exchanger, and a nuclear power plant including the same. 
         [0010]    Another object of the present disclosure is to propose a passive residual heat removal system for effectively removing sensible heat in a reactor coolant system and residual heat in a core through a high heat exchange efficiency while maintaining a pressure boundary between heat exchange fluids in a passive manner, and a nuclear power plant including the same. 
         [0011]    In order to accomplish the foregoing object of the present disclosure, a passive residual heat removal system according to an embodiment of the present disclosure may include a plate type heat exchanger configured to exchange heat between primary system fluid or secondary system fluid that has received sensible heat in a reactor coolant system and residual heat in a core and cooling fluid introduced from an inside or outside of a containment to remove the sensible heat and residual heat, and a circulation line configured to connect the reactor coolant system to the plate type heat exchanger to form a circulation flow path of the primary system fluid or connect a steam generator disposed at a boundary between a primary system and a secondary system to the plate type heat exchanger to form a circulation flow path of the secondary system fluid. 
         [0012]    According to the present disclosure having the foregoing configuration, a plate type heat exchanger having high-density heat transfer performance and durability to high temperature and high pressure may be applicable to a passive residual heat removal system. According to the present disclosure, a closed flow path and an open flow path or partially open flow path may be selectively introduced to a plate type heat exchanger of a passive residual heat removal system to efficiently circulate and discharge cooling fluid or atmosphere, and a water cooling, air cooling or hybrid cooling method may be all applicable thereto. 
         [0013]    Furthermore, according to the present disclosure, a passive residual heat removal system having a collection of heat exchangers configured with a plurality of plate type heat exchangers may be provided by freely choosing a width and a height of the plate and freely selecting a number of plates. Accordingly, it may be possible to provide a passive residual heat removal system for mitigating a bottleneck phenomenon at an inlet of the plate type heat exchanger. 
         [0014]    In addition, the present disclosure may maintain a safety function of a passive residual heat removal system for a long period of time (in a semi-permanent manner) through the employment of an air cooling or hybrid cooling method. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
           [0016]    In the drawings: 
           [0017]      FIG. 1  is a conceptual view illustrating a passive residual heat removal system and a nuclear power plant including the same associated with an embodiment of the present disclosure; 
           [0018]      FIG. 2  is a conceptual view illustrating a passive residual heat removal system and a nuclear power plant including the same associated with another embodiment of the present disclosure; 
           [0019]      FIG. 3  is a conceptual view illustrating an intermediate stage and a late stage of the accident in which time has passed after the occurrence of the accident in a passive residual heat removal system and a nuclear power plant including the same illustrated in  FIG. 2 ; 
           [0020]      FIG. 4  is a conceptual view illustrating a passive residual heat removal system and a nuclear power plant including the same associated with still another embodiment of the present disclosure; 
           [0021]      FIG. 5  is a conceptual view illustrating a passive residual heat removal system and a nuclear power plant including the same associated with yet still another embodiment of the present disclosure; 
           [0022]      FIG. 6  is a conceptual view illustrating a passive residual heat removal system and a nuclear power plant including the same associated with still yet another embodiment of the present disclosure; 
           [0023]      FIG. 7  is a conceptual view illustrating a passive residual heat removal system and a nuclear power plant including the same associated with yet still another embodiment of the present disclosure; 
           [0024]      FIGS. 8 through 14  are flow path conceptual views illustrating a plate type heat exchanger selectively applicable to the passive residual heat removal system in  FIGS. 1 through 7 ; 
           [0025]      FIG. 15  is a conceptual view illustrating a plurality of plate type heat exchangers selectively applicable to the passive residual heat removal system in  FIGS. 1 through 7 ; and 
           [0026]      FIG. 16  is a layout conceptual view illustrating a plurality of plate type heat exchangers illustrated in  FIG. 15 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0027]    Hereinafter, a passive residual heat removal system associated with the present disclosure will be described in more detail with reference to the accompanying drawings. Even in different embodiments according to the present disclosure, the same or similar reference numerals are designated to the same or similar configurations, and the description thereof will be substituted by the earlier description. Unless clearly used otherwise, expressions in the singular number used in the present disclosure may include a plural meaning. 
         [0028]    A plate type heat exchanger in the present disclosure may refer to all plate type heat exchangers as far as there is any difference in the processing method or bonding method of a plate thereof as well as a typical plate type heat exchanger and a printed circuit type heat exchanger, unless otherwise specified in particular. 
         [0029]      FIG. 1  is a conceptual view illustrating a passive residual heat removal system  100  and a nuclear power plant  10  including the same associated with an embodiment of the present disclosure. 
         [0030]    The nuclear power plant  10  illustrated in  FIG. 1  is illustrated as an integral reactor, but the present disclosure may not only be applicable to an integral reactor, but also be applicable to a loop type reactor. 
         [0031]    Referring to  FIG. 1 , for the sake of convenience of explanation, the passive residual heat removal system  100  and the nuclear power plant  10  including the same disclosed in the present disclosure are symmetrically illustrated around a reactor coolant system  12 . Furthermore, a normal operation of the nuclear power plant  10  is illustrated on the right of  FIG. 1 , and the occurrence of an accident at the nuclear power plant  10  is illustrated on the left. It is likewise in the other drawings illustrated below to be symmetrical to each other. 
         [0032]    The nuclear power plant  10  may include various systems maintaining the integrity of the nuclear power plant  10  in preparation for a normal operation and the occurrence of an accident, and further include structures such as the containment  11 , and the like. 
         [0033]    The containment  11  is formed to surround the reactor coolant system  12  at an outside of the reactor coolant system  12  to prevent the leakage of radioactive materials. The containment  11  performs the role of a final barrier for preventing the leakage of radioactive materials from the reactor coolant system  12  to external environment. 
         [0034]    The containment  11  is divided into a containment building (or referred to as a reactor building) configured with reinforced concrete, and a containment vessel and a safeguard vessel configured with steel containment. The containment vessel is a large sized vessel designed at a low pressure such as a containment building, and the safeguard vessel is a small-sized vessel designed with a small size by increasing a design pressure. According to the present disclosure, the containment  11  may collectively refer to a containment building, a reactor building, a containment vessel, a safeguard vessel, and the like, unless otherwise specified in particular. 
         [0035]    During a normal operation of the nuclear power plant  10 , when feedwater is supplied from a feedwater system  13  to a steam generator  12   b  through a main feedwater line  13   a,  steam is generated by the steam generator  12   b  using heat transferred from a reactor core  12   a.  The steam is supplied to a turbine system  14  through a main steam line  14   a,  and the turbine system  14  produces electricity using the supplied steam. Isolation valves  13   b,    14   b  installed at the main feedwater line  13   a  and main steam line  14   a  are open during a normal operation of the nuclear power plant  10 , but closed by an actuation signal during the occurrence of an accident. 
         [0036]    Primary system fluid is filled into the reactor coolant system  12 , and heat transferred from the reactor core  12   a  to the primary system fluid is transferred to secondary system fluid in the steam generator  12   b.  A primary system of the nuclear power plant  10  is a system for directly receiving heat from the reactor core  12   a  to cool the reactor core  12   a,  and a secondary system is a system for receiving heat from the primary system while maintaining a pressure boundary to the primary system to produce electricity using the received heat. In particular, a pressure boundary should be necessarily maintained between the primary system and the secondary system to ensure the integrity of a pressurized water nuclear power plant. 
         [0037]    A reactor coolant pump  12   c  for circulating primary system fluid, and a pressurizer  12   d  for suppressing the boiling of coolant and controlling an operating pressure are installed at the reactor coolant system  12 . The steam generator  12   b  is disposed at a boundary between the primary system and the secondary system to transfer heat between the primary system fluid and the secondary system fluid. 
         [0038]    The passive residual heat removal system  100 , as one of major systems for securing the safety of the nuclear power plant  10  when an accident occurs, is a system for removing sensible heat in the reactor coolant system  12  and residual heat in the reactor core  12   a  to discharge them to an outside. 
         [0039]    Hereinafter, first, the composition of the passive residual heat removal system  100  will be described, and then the operation of the passive residual heat removal system  100  when an accident occurs at the nuclear power plant  10  will be described. 
         [0040]    The passive residual heat removal system  100  may include a plate type heat exchanger  110 , and a circulation line  120 , and further include an emergency cooling water storage section  130 . 
         [0041]    The plate type heat exchanger  110  may be installed at least one place of an inside and an outside of the containment  11 . The plate type heat exchanger  110  exchanges heat between primary system fluid or secondary system fluid that have received the sensible heat and residual heat and cooling fluid introduced from an outside of the containment  11  to remove sensible heat in the reactor coolant system  12  and residual heat in the reactor core  12   a.    
         [0042]    The plate type heat exchanger  110  illustrated in  FIG. 1  is installed at an outside of the containment  11 , and configured to exchange heat between secondary system fluid and cooling fluid outside the containment  11 . 
         [0043]    The circulation line  120  connects the reactor coolant system  12  to the plate type heat exchanger  110  or connects the steam generator  12   b  between the primary system and the secondary system to the plate type heat exchanger  110  to form a circulation flow path of the primary system fluid or secondary system fluid. The circulation line  120  connected between the steam generator  12   b  and the plate type heat exchanger  110  to form a circulation flow path of the secondary system fluid is illustrated in  FIG. 1 . 
         [0044]    The plate type heat exchanger  110  is arranged on a plate to be distinguished from each other to exchange heat between primary system fluid or secondary system fluid supplied through the circulation line  120  and cooling fluid while maintaining a pressure boundary, and may include a plurality of channels (not shown) for allowing the fluids to alternately pass therethrough. 
         [0045]    The plate type heat exchanger  110  may include at least one of a printed circuit type heat exchanger and a plate type heat exchanger. The printed circuit type heat exchanger is provided with channels formed by diffusion bonding and densely formed by a photochemical etching technique. On the contrary, the plate type heat exchanger extrudes a plate to form channels, and is formed to couple(or join) the plates using at least one of a gasket, a welding, and a brazing welding methods. 
         [0046]    The channels may include first flow paths (not shown) and second flow paths (not shown) for allowing different fluids to pass therethrough. The first flow paths are arranged to be separated from one another to allow cooling fluid for cooling primary system fluid or secondary system fluid to pass therethrough. A plurality of second flow paths are formed to allow the primary system fluid or the secondary system fluid to pass therethrough, and alternately arranged with the first flow paths to exchange heat while maintaining a pressure boundary to the cooling fluid. 
         [0047]    The plate type heat exchanger  110  of  FIG. 1  uses the circulation of secondary system fluid, and thus the secondary system fluid flows through the second flow path, and cooling fluid flowing through the first flow path cools the secondary system fluid. 
         [0048]    An inlet header  111   a,    112   a  and an outlet header  111   b ,  112   b  are formed at each inlet and outlet of the plate type heat exchanger  110 . The inlet header  111   a ,  112   a  is formed at an inlet of the first flow path and the second flow path to distribute fluids supplied to the plate type heat exchanger to each channel. The outlet header  111   b ,  112   b  is formed at an outlet of the first flow path and the second flow path to collect the fluids that have passed the each channel. The fluids supplied to the plate type heat exchanger  110  may include cooling fluid passing through the first flow path, primary system fluid or secondary system fluid passing through the second flow path. In particular, in the passive residual heat removal system  100  illustrated in  FIG. 1 , the fluids supplied to the plate type heat exchanger  110  are cooling fluid and secondary system fluid. 
         [0049]    In  FIG. 1 , the inlet header  111   a  and outlet header  111   b  of the second flow path are necessarily provided to maintain a pressure boundary. However, since the first flow path has a configuration in which the inlet and outlet thereof are open to the fluid of the emergency cooling water storage section, it is a configuration in which the inlet header  112   a  and outlet header  112   b  are selectively provided to efficiently perform inlet and outlet flow. Accordingly, the inlet header  112   a  and outlet header  112   b  may not be provided at the first flow path, and replaced by an inlet guide structure, an outlet guide structure, and the like in the form of being extended from the first flow path to an outside. 
         [0050]    The cooling fluid and secondary system fluid exchange heat while flowing in different directions, and thus the inlet of the first flow path is disposed adjacent to the outlet of the second flow path, and the outlet of the first flow path is disposed adjacent to the inlet of the second flow path. Furthermore, the inlet header  112   a  of the first flow path is disposed adjacent to the outlet header  111   b  of the second flow path, and the outlet header  112   b  of the first flow path is disposed adjacent to the inlet header  111   a  of the second flow path. 
         [0051]    The circulation line  120  may include a steam line  121  for supplying secondary system fluid to the plate type heat exchanger  110  and a feedwater line  122  for receiving secondary system fluid from the plate type heat exchanger  110 . 
         [0052]    The steam line  121  is branched from a main steam line  14   a  and connected to the inlet of the second flow path to receive the secondary system fluid from the main steam line  14   a  extended from an outlet of the steam generator  12   b.  The feedwater line  122  is branched from a main feedwater line  13   a  extended to the inlet of the steam generator  12   b  and connected to the outlet of the second flow path to transfer heat to the cooling fluid and recirculate the cooled secondary system fluid into the steam generator  12   b.    
         [0053]    The passive residual heat removal system  100  may include the emergency cooling water storage section  130 . 
         [0054]    The emergency cooling water storage section  130  is formed to store cooling fluid therewithin and installed at an outside of the containment  11 . The emergency cooling water storage section  130  is provided with an opening portion  131  at an upper portion thereof to dissipate heat transferred by evaporating the cooling fluid stored therewithin during a temperature increase due to heat transferred from the primary system fluid or the secondary system fluid to cooling fluid. 
         [0055]    At least part of the plate type heat exchanger  110  may be installed within the emergency cooling water storage section  130  to allow at least part thereof to be immersed into the cooling fluid. In this case, the steam line  121  and the feedwater line  122  may be connected to the main steam line  14   a  and the main feedwater line  13   a,  respectively, from an outside of the containment  11  through the emergency cooling water storage section  130 . 
         [0056]    As illustrated in  FIG. 1 , when the plate type heat exchanger  110  is completely immersed into the cooling fluid of the emergency cooling water storage section  130 , the plate type heat exchanger  110  cools secondary system fluid using the cooling fluid (coolant) of the emergency cooling water storage section  130  with a water cooling method. 
         [0057]    Next, the operation of the passive residual heat removal system  100  during the occurrence of an accident will be described. The left side of the drawing illustrated to be symmetric to each other in  FIG. 1  illustrates a state of the passive residual heat removal system  100  during the occurrence of an accident. 
         [0058]    When a loss of coolant accident or non-loss of coolant accident (steam line break or the like) occurs at the nuclear power plant  10 , isolation valves  13   b ,  14   b  installed at the main feedwater line  13   a  and the main steam line  14   a  are closed by related signals. Furthermore, an isolation valve  122   a  installed at the feedwater line  122  of the passive residual heat removal system  100  is open by related signals, and a check valve  122   b  installed at the steam line  121  is open by the flow of the secondary system fluid formed by opening the isolation valve  122   a . Accordingly, the supply of feedwater from the feedwater system  13  to the steam generator  12   b  is suspended, and secondary system fluid is circulated within the passive residual heat removal system  100 . 
         [0059]    The secondary system fluid sequentially passes through the feedwater line  122  and the main feedwater line  13   a  to be introduced to an inlet of the steam generator  12   b.  The secondary system fluid supplied to the steam generator  12   b  receives sensible heat from primary system fluid within the reactor coolant system  12  and residual heat in the reactor core  12   a  at the steam generator  12   b,  and the temperature of the secondary system fluid increases to evaporate at least part thereof. 
         [0060]    The secondary system fluid discharged through the outlet of the steam generator  12   b  flows upward along the main steam line  14   a  and the steam line  121  of the passive residual heat removal system  100  and is introduced to the second flow path of the plate type heat exchanger  110 . The cooling fluid within the emergency cooling water storage section  130  is introduced to the first flow path of the plate type heat exchanger  110 , and heat is transferred from the secondary system fluid to the cooling fluid in the plate type heat exchanger  110 . 
         [0061]    The secondary system fluid that has transferred heat to the cooling fluid is cooled and condensed and flows downward, and moves again along the feedwater line  122  to circulate through the steam generator  12   b.  The circulation of the secondary system fluid is generated by natural phenomenon due to a density difference, and thus the circulation of the secondary system fluid continues until sensible heat in the reactor coolant system  12  and residual heat in the reactor core  12   a  are almost removed and a density difference required for the circulation of the secondary system fluid almost disappears. 
         [0062]    When heat is transferred from the secondary system fluid to the cooling fluid, the temperature within the emergency cooling water storage section  130  gradually increases. At least part of the cooling fluid is evaporated and discharged to an outside through the opening portion  131 , and heat transferred to the cooling fluid is also discharged to the outside. 
         [0063]    In this manner, the passive residual heat removal system  100  may circulate secondary system fluid in a passive method due to a natural force to remove sensible heat in the reactor coolant system  12  and residual heat in the reactor core  12   a.  Furthermore, the plate type heat exchanger  110  may be configured to allow the secondary system fluid and the cooling fluid to pass through different channels to exchange heat, thereby preventing damage at a pressure boundary and inducing sufficient heat exchange through small flow paths. 
         [0064]    Hereinafter, another embodiment of the passive residual heat removal system will be described. 
         [0065]      FIG. 2  is a conceptual view illustrating a passive residual heat removal system  200  and a nuclear power plant  20  including the same associated with another embodiment of the present disclosure. 
         [0066]    At least part of a plate type heat exchanger  210  is immersed into the cooling fluid of an emergency cooling water storage section  230  to allow cooling fluid within the emergency cooling water storage section  230  and atmosphere outside a containment  21  to pass therethrough to a first flow path. An upper end portion of the plate type heat exchanger  210  may be formed in a protruding manner to an upper side of the emergency cooling water storage section  230  through the emergency cooling water storage section  230  to discharge cooling fluid evaporated by heat transfer with secondary system fluid and/or atmosphere to the outside. The other configuration is similar to the description of  FIG. 1 . 
         [0067]    The plate type heat exchanger  210  is formed in a relatively lengthy manner compared to the plate type heat exchanger  210  illustrated in  FIG. 1  to provide two heat exchange conditions of water cooling and air cooling methods to fluids that exchange heat in the plate type heat exchanger  210 . 
         [0068]    The left and the right of nuclear power plant  20  of  FIG. 2  are symmetrically illustrated, wherein the right side thereof illustrates a normal operation state, and the left side thereof illustrates an early stage of the occurrence of an accident. 
         [0069]    When an accident occurs such as a loss of coolant accident or the like, secondary system fluid discharged from an outlet of the steam generator  22   b  is introduced into an inlet of the second flow path of the plate type heat exchanger  210  through a main steam line  24   a  and a steam pipe  221 . During an early stage of the occurrence of an accident, cooling fluid is sufficiently stored within the emergency cooling water storage section  230 , and at least part of the plate type heat exchanger  210  is immersed into the cooling fluid, and the heat exchange performance of a water cooling method is significantly higher than that of an air cooling method, and thus the secondary system fluid is cooled by the water cooling method. 
         [0070]    The secondary system fluid cooled in the plate type heat exchanger  210  and discharged from an outlet of the second flow path is circulated again into the steam generator  22   b  through a feedwater pipe  222  and a main feedwater line  23   a  to remove sensible heat in the reactor coolant system  22  and residual heat in the reactor core  22   a  through a continuous circulation. 
         [0071]      FIG. 3  is a conceptual view illustrating an intermediate stage and a late stage of the accident in which time has passed after the occurrence of the accident in a passive residual heat removal system  200  and a nuclear power plant  20  including the same illustrated in  FIG. 2 . 
         [0072]    In  FIG. 3 , the left side thereof illustrates an intermediate stage of the accident and the right side thereof illustrates a late stage of the accident around a symmetric drawing. 
         [0073]    First, referring to the drawing illustrating an intermediate stage of the accident, it is seen that a water level is decreased due to the evaporation of the cooling fluid of the emergency cooling water storage section  230  compared to an early stage of the accident. As a water level of the cooling fluid of the emergency cooling water storage section  230  is reduced, the cooling fluid of the emergency cooling water storage section  230  and atmosphere outside the containment  21  are introduced to the first flow path of the plate type heat exchanger  210  to cool the secondary system fluid with a water-air hybrid cooled method. 
         [0074]    Next, referring to a drawing illustrating a late stage of the accident on the right, it is seen that the water level is further decreased due to the evaporation of most cooling fluid of the emergency cooling water storage section  230  compared to an intermediate stage of the accident. Accordingly, atmosphere outside of the containment  21  is introduced to the first flow path of the plate type heat exchanger  210  to cool the secondary system fluid with an air cooled method. 
         [0075]    The cooling method of the plate type heat exchanger  210  formed as described above may vary according to the water level of the cooling fluid stored in the emergency cooling water storage section  230  and the passage of time subsequent to the occurrence of an accident. It uses a characteristic in which residual heat in the reactor core  22   a  is gradually reduced as time has passed subsequent to the occurrence of an accident. A water cooling method, a hybrid method mixed with a water cooling method and an air cooling method may be sequentially employed and configured to be switched to an appropriate cooling method according to residual heat reduction to enhance cooling efficiency and maintain cooling durability. Accordingly, the passive residual heat removal system  200  may continuously remove sensible heat in the reactor coolant system  22  and residual heat in the reactor core  22   a.    
         [0076]      FIG. 4  is a conceptual view illustrating a passive residual heat removal system  300  and a nuclear power plant  30  including the same associated with yet still another embodiment of the present disclosure. The right side of a drawing symmetrically illustrated in  FIG. 4  illustrates a normal operation of the nuclear power plant  30 , and the left side thereof illustrates the occurrence of an accident at the nuclear power plant  30 . 
         [0077]    The passive residual heat removal system  300  cools secondary system fluid only with an air cooling method without any emergency cooling water storage section contrary to the passive residual heat removal system  100 ,  200  illustrated in  FIGS. 1 through 3 . 
         [0078]    Atmosphere outside a containment  31  is introduced to a first flow path of a plate type heat exchanger  310 , and secondary system fluid supplied from a steam generator  32   b  is introduced to a second flow path thereof. Heat is transferred to atmosphere from secondary system fluid passing through each flow path, and the atmosphere is discharged to an outside of the plate type heat exchanger  310 . Accordingly, sensible heat and residual heat transferred from a reactor coolant system  32  and a reactor core  32   a  may be discharged to external atmosphere. 
         [0079]      FIG. 5  is a conceptual view illustrating a passive residual heat removal system  400  and a nuclear power plant  40  including the same associated with still yet another embodiment of the present disclosure. 
         [0080]    A plate type heat exchanger  410  is installed in an inner space of a containment  41 , and an emergency cooling water storage section  430  is installed at an outside of the containment  41 . The plate type heat exchanger  410  is connected to the cooling water storage section  430  by connection lines  441 ,  442  on which an inlet and an outlet of the first flow path pass through the containment  41 , respectively, to allow cooling fluid within the cooling water storage section  430  through the first flow path. 
         [0081]    Secondary system fluid is supplied to a second flow path of the plate type heat exchanger  410  through a main steam line  44   a  and a steam pipe  421  to exchange heat with cooling fluid supplied to the first flow path of the plate type heat exchanger  410  from the cooling water storage section  430 . Accordingly, the secondary system fluid is cooled by a water cooling method. Both the secondary system fluid and cooling fluid continuously circulate through the plate type heat exchanger  410 . 
         [0082]    The cooling fluid of the cooling water storage section  430  is supplied to the plate type heat exchanger  410  through the connection line  441 , but flows through a flow path distinguished from the secondary system fluid, and thus a pressure boundary is not damaged at the plate type heat exchanger  410 . The cooling fluid of the cooling water storage section  430  receives heat from the secondary system fluid while circulating through the plate type heat exchanger  410  to increase the temperature thereof, and is introduced again to the cooling water storage section  430  through the connection line  442 . When the temperature increases, the cooling fluid of the cooling water storage section  430  is evaporated to discharge the received heat to an outside. 
         [0083]    Isolation valves  441   a,    442   a  and a check valve  441   b  installed at the connection lines  441 ,  442  are normally open, but closed only when required for maintenance. 
         [0084]      FIG. 6  is a conceptual view illustrating a passive residual heat removal system  500  and a nuclear power plant  50  including the same associated with yet still another embodiment of the present disclosure. 
         [0085]    A plate type heat exchanger  510  is installed in an inner space of a containment  51 , but an emergency cooling water storage section last plot view icon  530  is not installed. The plate type heat exchanger  510  is formed such that an inlet and an outlet of the first flow path communicate with an outside of the containment  51  by connection lines  541 ,  542  passing through the containment  51 . 
         [0086]    External atmosphere is introduced into the plate type heat exchanger  510  through the connection lines  541 ,  542  by natural circulation and flows along the first flow path. Accordingly, secondary system fluid flowing along the second flow path is cooled with an air cooling method. 
         [0087]    Atmosphere introduced from an outside of the containment  51  is supplied to the plate type heat exchanger  510  through the connection lines  541 ,  542 , but flows through a flow path distinguished from the secondary system fluid, and thus a pressure boundary is not damaged at the plate type heat exchanger  510 . 
         [0088]      FIG. 7  is a conceptual view illustrating a passive residual heat removal system  600  and a nuclear power plant  60  including the same associated with still yet another embodiment of the present disclosure. 
         [0089]    The passive residual heat removal system  600  is configured to remove sensible heat in a reactor coolant system  62  and residual heat in a reactor core  62   a  using primary system fluid contrary to the passive residual heat removal system illustrated in  FIGS. 1 through 6 . An emergency cooling water storage section  630  is installed at an outside of a containment  61 , and a plate type heat exchanger  610  is immersed into the cooling fluid of the emergency cooling water storage section  630 . 
         [0090]    A circulation line  620  may include a steam line  621  and an injection line  622 . 
         [0091]    The steam line  621  is connected to the reactor coolant system  62  and an inlet of the second flow path through the containment  61  to receive primary system fluid from the reactor coolant system  62  and transfer it to the plate type heat exchanger  610 . The injection line  622  is an outlet of the second flow path and the reactor coolant system  62  through the containment  61  to transfer heat to the cooling fluid and reinject the cooled primary system fluid to the reactor coolant system  62 . 
         [0092]    The cooling fluid of the emergency cooling water storage section  630  flows into the first flow path of the plate type heat exchanger  610 , and primary system fluid flows into the second flow path to carry out cooling with a water cooling method, and the passive residual heat removal system  600  circulates primary system fluid to remove sensible heat in the reactor coolant system  62  and residual heat in the reactor core  62   a.    
         [0093]    The primary system fluid and the cooling fluid flow through flow paths distinguished from each other, and thus the passive residual heat removal system  600  may exchange heat without damaging a pressure boundary. Unless the pressure boundary is damaged, the plate type heat exchanger  610  may be installed within the containment  61  contrary to the illustration. Furthermore, it may employ a circulation composition of the primary system fluid instead of the secondary system fluid in  FIGS. 1 through 6 . 
         [0094]    In the above, a composition of the passive residual heat removal system and the operation of the passive residual heat removal system due to natural circulation have been described, but in actuality when the plate type heat exchanger is applied to the passive residual heat removal system, problems such as flow instability in a two phase flow region, bottleneck phenomenon at a heat exchanger inlet, and the like may occur, and thus it is required to resolve them. Hereinafter, a structure of the plate type heat exchanger proposed by the present disclosure to enhance the problems will be described. 
         [0095]    The following description will be described without distinguishing a first flow path from a second flow path, and unless the description thereof is only limited to either one of the first flow path and the second flow path, the description of the first flow path will be also applicable to that of the second flow path, and the description of the second flow path will be also applicable to that of the first flow path. 
         [0096]    Hereinafter, the detailed structure of a plate type heat exchanger  710  applicable to a passive residual heat removal system  100 ,  200 ,  300 ,  400 ,  500 ,  600  illustrated in  FIGS. 1 through 7  will be described. 
         [0097]      FIGS. 8 through 14  are flow path conceptual views illustrating a plate type heat exchanger  710  selectively applicable to the passive residual heat removal system  100 ,  200 ,  300 ,  400 ,  500 ,  600  illustrated in  FIGS. 1 through 7 . 
         [0098]    When a fabrication technique of a printed circuit type heat exchanger is applied to the plate type heat exchanger  710 , it has a structure capable of allowing a dense flow path arrangement by a photochemical etching technology and removing a welding between the plates of the heat exchanger using a diffusion bonding technology, and allows a typical plate type heat exchanger to have a dense flow path arrangement. The plate type heat exchanger  710  may include channels  715 ,  716  distinguished from each other on a plate to exchange heat between the atmosphere of the containment  11 ,  21 ,  31 ,  41 ,  51 ,  61  (refer to  FIGS. 1 through 7 ) and the cooling fluid of the emergency cooling water storage section  130 ,  230 ,  430 ,  630  (refer to  FIGS. 1 through 3, 5, and 7 ) and exchange heat between fluids while maintaining a pressure boundary. 
         [0099]    The channels  715 ,  716  may include a first flow path  715  for allowing cooling fluid to pass therethrough, and a second flow path  716  for allowing primary system fluid or secondary system fluid to pass therethrough, and each channel  715 ,  716  corresponds to either one of the first flow path  715  and the second flow path  716 . 
         [0100]    The shape of the first flow path  715  and second flow path  716  may be a closed flow path in the shape of allowing cooling fluid or atmosphere to pass therethrough only in one direction and allowing primary system fluid or secondary system fluid to pass therethrough only in a direction opposite to the one direction. 
         [0101]    Furthermore, contrary to the second flow path  716 , the shape of the first flow path  715  may be also an open flow path or partially open flow path in the shape of allowing cooling fluid or atmosphere to pass therethrough even in a direction crossing the one direction. The first flow path for allowing cooling fluid or atmosphere to pass therethrough may selectively employ an open flow path or partially open flow path for cooling with an air cooling method or with an air cooling method and a hybrid cooling method in the plate type heat exchanger  710  in a relatively long length. However, when the open flow path is employed in case of the second flow path  716 , a pressure boundary may be damaged, and thus the open flow path cannot be applied thereto. 
         [0102]    First, referring to  FIG. 8 , the plate type heat exchanger  710  illustrated in the drawing shows a cross-section of the first flow path  715  through which cooling fluid flows. The plate type heat exchanger  710  may include an inlet region  710   a,  a main heat transfer region  710   b,  and an outlet region  710   c.  The inlet region  710   a  is a region for distributing cooling fluid supplied to the plate type heat exchanger  710  to each first flow path  715 , and the main heat transfer region  710   b  is a region for carrying out substantial heat exchange between cooling fluid and primary system fluid, cooling fluid and secondary system fluid, and the outlet region  710   c  is a region for collecting and discharging fluids that have completed heat exchange from the first flow path  715 . The main heat transfer region  710   b  is connected between the inlet region  710   a  and the outlet region  710   c,  and formed between the inlet region  710   a  and the outlet region  710   c.    
         [0103]    The temperature of the cooling fluid is lower than that of the primary system fluid or secondary system fluid, and thus the cooling fluid receives heat from the primary system fluid or secondary system fluid while passing through the plate type heat exchanger  710  to increase the temperature. When the temperature of the cooling fluid increases, the density thereof decreases, and thus the cooling fluid flows upward within the plate type heat exchanger  710 . 
         [0104]    Next, referring to  FIG. 9 , the flow paths may be formed in such a manner that a flow resistance of the inlet region  710   a  is relatively larger than that of the main heat transfer region  710   b  connected between the inlet region  710   a  and the outlet region  710   c  to mitigate flow instability due to two phase flow. 
         [0105]    There may be various methods of forming a relatively large flow resistance, but the plate type heat exchanger  710  illustrated in  FIG. 9  employs a method in which a flow path in the inlet region  710   a  is formed with a smaller width than that of the main heat transfer region  710   b  and extended in a lengthy manner. 
         [0106]    A flow path  715   a  of the inlet region  710   a  is formed in a zigzag shape to have a relatively larger flow resistance than that of a straight flow path and connected to the main heat transfer region  710   b.  Specifically, it is formed in a shape in which the flow path of the inlet region  710   a  is alternatively and repetitively connected in a length direction and a width direction of the plate type heat exchanger  710 , and extended to the main heat transfer region  710   b.  As a flow resistance of the inlet region  710   a  is formed to be larger than that of the main heat transfer region  710   b,  it may be possible to reduce a flow instability occurrence probability in two phase flow. 
         [0107]    A flow expansion section  715   b  is formed between the inlet region  710   a  and the main heat transfer region  710   b,  and formed in such a manner that a width of the flow path gradually increases toward an extension direction from a flow path size of the inlet region  710   a  to a flow path size of the main heat transfer region  710   b.  The flow resistance relatively decreases while passing the flow expansion section  715   b,  and the relatively small flow resistance is maintained on the flow path of the subsequent main heat transfer region  710   b  and outlet region  710   c.    
         [0108]      FIGS. 10 through 12B  are conceptual views illustrating the plate type heat exchanger  710  having a header at an inlet and an outlet, respectively. 
         [0109]    First, referring to  FIG. 10 , an inlet header  712   a  for distributing a fluid to each flow path and an outlet header  712   b  for collecting a fluid from each flow path may be installed at the plate type heat exchanger  710 . The inlet header  712   a  and outlet header  712   b  are structures that should be necessarily installed to prevent a pressure boundary damage when the plate type heat exchanger is installed at an inside of the containment  11 ,  21 ,  31 ,  41 ,  51 ,  61  (refer to  FIGS. 1 through 7 ), but they are not structures that should be necessarily installed when installed at an outside of the containment, and may not be installed or replaced with a flow path guide structure for efficiently carrying out the flow of the inlet and outlet. 
         [0110]    The inlet header  712   a  is installed at an inlet of the flow path to distribute cooling fluid supplied from the emergency cooling water storage section  130 ,  230 ,  430 ,  630  (refer to  FIGS. 1 through 3, 5, and 7 ) or atmosphere supplied from an outside of the containment to each first flow path  715 . Furthermore, the outlet header  712   b  is installed at an outlet of the first flow path  715  to collect cooling fluid that has passed the first flow path  715  and return it to the emergency cooling water storage section or discharge it to an outside. 
         [0111]    The installation location of the inlet header  712   a  and outlet header  712   b  may vary according to the design of the plate type heat exchanger  710 . In particular, when a fabrication technique of a printed circuit type heat exchanger is applied to the plate type heat exchanger  710 , it may be fabricated by a photochemical etching technology to freely select the structure of channels  715 ,  716 , and a typical plate type heat exchanger may have a very free flow path structure, and thus the location of the inlet header  712   a  and outlet header  712   b  may also vary. 
         [0112]    Referring to  FIGS. 11 through 12B , the inlet header  711   a,    712   a  and outlet header  711   b ,  712   b  are installed at a lateral surface of the plate type heat exchanger  710 , respectively, and each flow path  715 ,  716  is bent in the inlet region  710   a  and outlet region  710   b  or formed to have a curved flow path and extended to the inlet header  711   a ,  712   a  or outlet header  711   b ,  712   b.    
         [0113]    An extension direction of the flow path  715 ,  716  in the inlet region  710   a  and an extension direction of the flow path  715 ,  716  in the outlet region  710   c  may be the same direction as illustrated in  FIG. 11 , or may be opposite directions to each other as illustrated in  FIGS. 12A and 12B , and vary according to the design of the passive residual heat removal system. 
         [0114]      FIGS. 12A and 12B  illustrate the first flow path  715  and second flow path  716  of the plate type heat exchanger  710 , respectively. The first flow path  715  receives heat while cooling fluid or external atmosphere passes therethrough to increase the temperature or evaporates to decrease the density, and the second flow path  716  transfers heat to the cooling fluid or atmosphere while primary system fluid or secondary system fluid passes therethrough to decrease the temperature or condenses to increase the density. 
         [0115]      FIGS. 13 and 14  are flow path conceptual views illustrating the plate type heat exchanger  710  having an open flow path or partially open flow path, respectively. 
         [0116]    Referring to  FIG. 13 , the plate type heat exchanger  710  may include an open flow path formed to introduce cooling fluid or atmosphere from a lateral surface to join cooling fluid and atmosphere passing through the first flow path so as to mitigate a bottleneck phenomenon at the inlet while maintaining a pressure boundary between fluids. Furthermore, referring to  FIG. 14 , the plate type heat exchanger  710  may include a partially open flow path in which a flow path is formed in an open shape only at part of the main heat transfer region  710   b.    
         [0117]    The plate type heat exchanger  710  having an open flow path or partially open flow path may include a longitudinal flow path  715  and a transverse flow path  717  forming the open flow path or partially open flow path. The longitudinal flow path  715  is connected between the inlet region  710   a  at an upper end portion of the plate type heat exchanger  710  and the outlet region  710   c  at a lower end portion thereof. The transverse flow path  717  is formed to flow the cooling fluid or atmosphere in or out through an inlet and an outlet formed at both side sections of the plate type heat exchanger  710  and cross the longitudinal flow path  715  so as to mitigate a bottleneck phenomenon of the inlet. 
         [0118]    In particular, the plate type heat exchanger  710  formed with an open flow path may form a passive residual heat removal system with only an air cooling method for cooling primary system fluid or secondary system fluid with only atmosphere. Furthermore, the plate type heat exchanger  710  may form a passive residual heat removal system with a hybrid method (water-air hybrid) for cooling primary system fluid or secondary system fluid with atmosphere and cooling fluid. The plate type heat exchanger  710  for cooling primary system fluid or secondary system fluid with an air cooling or hybrid method may be preferably formed in a relatively long length. 
         [0119]    The plate type heat exchanger  710  formed with a partially open flow path is to alleviate the overcooling problem of the reactor coolant system  12 ,  22 ,  32 ,  42 ,  52 ,  62  (refer to  FIGS. 1 through 7 ), and the partially open flow path is configured to operate in a water cooling method at an early stage of the accident so as to facilitate the circulation of cooling fluid, and suppress an additional cooling rate increase due to the introduction of atmosphere. 
         [0120]    In the plate type heat exchanger  710  of the present disclosure, the open flow path or partially open flow path may be formed only on the first flow path  715  for allowing cooling fluid or atmosphere to pass therethrough. It is because the second flow path  716  circulates a closed circuit to prevent a pressure boundary from being damaged. 
         [0121]      FIG. 15  is a conceptual view illustrating a plurality of plate type heat exchangers  810  selectively applicable to the passive residual heat removal system  100 ,  200 ,  300 ,  400 ,  500 ,  600  (refer to  FIGS. 1 through 7 ) in  FIGS. 1 through 7 . 
         [0122]      FIGS. 15A, 15B, 15C and 15D  illustrate a plan view, a left side view, a front view, and a right side view of the plurality of plate type heat exchangers  810 , respectively. The plurality of plate type heat exchangers  810  is surrounded by a casing  813 , respectively, and a cooling fin  818  for expanding a heat transfer area is installed at the casing  813 . 
         [0123]    The primary system fluid or secondary system fluid is distributed to each plate type heat exchanger  810  through a steam line  821 , and distributed to each second flow path (not shown) by an inlet header  811   a  within the each plate type heat exchanger  810 . The primary system fluid or secondary system fluid that has passed through the second flow path is collected by an outlet header  811   b  and joins again an injection line (primary system fluid circulation method) or feedwater line  822  (secondary system fluid circulation method). The cooling fluid or atmosphere is also distributed to each first flow path (not shown) by an inlet header  812   a,  and the cooling fluid or atmosphere that has passed through the first flow path is collected by an outlet header  812   b.  However, as described above, when the heat exchanger is installed at an outside of the containment, the inlet and outlet header  812   a,    812   b  are not essential structures. 
         [0124]      FIG. 16  is a layout conceptual view illustrating a plurality of plate type heat exchangers  910  illustrated in  FIG. 15 . 
         [0125]    Referring to  FIG. 16A , the plurality of plate type heat exchangers  910  may be transversely arranged to form a collection of heat exchangers, and disposed within an emergency cooling water storage section  930 . 
         [0126]    Referring to  FIG. 16B , the plurality of plate type heat exchangers  910  may be arranged in a lattice shape to form a collection of heat exchangers, and disposed within an emergency cooling water storage section  930 . 
         [0127]    The configurations and methods according to the above-described embodiments will not be applicable in a limited way to the foregoing passive residual heat removal system and a nuclear power plant including the same, and all or part of each embodiment may be selectively combined and configured to make various modifications thereto. 
         [0128]    The present disclosure may be used to enhance the performance of a passive residual heat removal system in the nuclear power plant industry.