Abstract:
The present invention relates to a passive heat removal system which circulates cooling fluid via a main water supply line, connected to the lower inlet of a steam generator, and a main steam pipe, connected to the top outlet of the steam generator, to the steam generator, in order to remove sensible heat of a reactor coolant system and residual heat of a core, the passive heat removal system comprising: supplementary equipment for receiving surplus cooling fluid or supplying supplementary cooling fluid in order to maintain the flow rate of the cooling fluid within a predetermined range, wherein the supplementary equipment comprises: a supplementary tank, installed at a predetermined height between the lower inlet and the top outlet of the steam generator, for receiving the surplus cooling fluid or supplying the supplementary cooling fluid, passively, depending on the flow rate of the cooling fluid; a first connection pipe, connected to the main steam pipe and the supplementary tank, for forming a flow path to allow the cooling fluid, exhausted to the main steam pipe from the steam generator, to flow to the supplementary tank; and a second connection pipe connected to the supplementary tank and the main water supply pipe for forming a supply flow path of the supplementary cooling fluid supplied from the supplementary tank.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the invention 
         [0002]    The present disclosure relates to a passive residual heat removal system used as one of emergency safety facilities and a nuclear power plant including the same. 
         [0003]    2. Description of the related art 
         [0004]    A residual heat removal system is an emergency safety facility for removing sensible heat in a reactor coolant system and residual heat in a reactor core during an accident. In particular, the passive residual heat removal system removes sensible heat and residual heat in a passive manner using natural forces. 
         [0005]    For a coolant circulation method of the passive residual heat removal system, two methods such as 1) a method of directly circulating reactor primary coolant to cool a reactor (AP1000: U.S. Westinghouse) and 2) a method of circulating secondary coolant using a steam generator to cool a reactor (SMART reactor: Korea) are mostly used. In, addition, 3) a method of injecting primary coolant to a tank to directly condense it (CAREM: Argentina) is partially used. 
         [0006]    For a passive residual heat removal system to which a secondary coolant circulation method is applied, two types such as 2-1) a mode to which a pressurized makeup tank is applied and both directions of a steam line and a feedwater line of the passive residual heat removal system are isolated with an isolation valve during a normal operation (Korean Patent Application No. 10-2000-0067089), and 2-2) a mode to which gravity water head makeup tank is applied and only one direction of a feedwater line of the passive residual heat removal system is isolated with an isolation valve (IRIS: U.S. Westinghouse, SMART reactor: Korea) are used. 
         [0007]    Furthermore, for a method of cooling an outside of a heat exchanger (condensation heat exchanger), 1) a water-cooled method (AP1000) applied to most reactors, 2) a partially air-cooled method (WWER 1000: Russia), and 3) 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 (atmosphere) through an emergency cooling tank (heat sink) or the like, and condensation heat exchangers using a steam condensation phenomenon with an excellent heat transfer efficiency have been mostly used for a heat exchanger method. 
         [0008]    A steam generator performs a function of receiving heat in a reactor coolant system to produce steam, and supplying the steam to a turbine system. Furthermore, a secondary side of the steam generator is used as a supply source for producing steam in a passive residual heat removal system. The passive residual heat removal system performs a very important function for removing sensible heat and residual heat in a reactor during an accident. However, the passive residual heat removal system is generally known to exhibit a big difference in the cooling performance according to a coolant flow of the secondary side including the steam generator. 
         [0009]    In particular, a once-through type steam generator configured to receive feedwater to a tube side to produce superheated steam in the tube may exhibit a large different secondary water level in the steam generator according to a power operation state of the nuclear power plant. Furthermore, a flow of the passive residual heat removal system during an accident is affected by a time point at which the discharge of steam is suspended or the supply of feedwater is suspended by related signals (valve closed or pump stopped) during the accident. As described above, a coolant flow at a secondary side including the steam generator is affected by an initial water level of the steam generator and a time point at which steam discharge or feedwater is stopped, and the like, and if the coolant flow is unable to maintain an appropriate flow level, it is difficult to accomplish the target performance of the passive residual heat removal system. 
         [0010]    Furthermore, a gravity or pressurized makeup tank is provided in the passive residual heat removal system, and those makeup tanks is provided to make up a flow when the flow is insufficient. However, a conventional makeup tank is configured to supply a flow even when the flow is sufficient in a system, thus rather acting as a cause of deteriorating the performance of the passive residual heat removal system. 
         [0011]    Non-condensable gas in connection with the present disclosure performs the role of preventing flow and condensation in a heat exchanger such as a condensation heat exchanger to act as a cause of significantly deteriorating the performance of the heat exchanger. A patent associated with a vent system in a passive residual heat removal system associated therewith is disclosed in KR Laid-open Patent Publication No. 2001-0076565. In this patent, it is disclosed a line valve connected to a line subsequent to a main steam isolation valve from an upper portion of a condensation heat exchanger to remove non-condensable gas. However, the patent does not disclose a specific pressure drop scheme, and a connection line is provided subsequent to the main steam isolation valve, and as a result, if the isolation valve of the exhaust line is not closed when the passive residual heat removal system is operated during an accident, then there is a possibility in which the coolant of the passive residual heat removal system is lost through the exhaust line to cause a serious accident. 
         [0012]    On the other hand, a steam line of the passive residual heat removal system has a relatively large volume, and the passive residual heat removal system to which a method of opening the steam line is applied is operated in a state that the steam line is open during a normal operation. Accordingly, as the normal operation of a nuclear power plant continues, light non-condensable gas may be accumulated in the steam line. As a result, when an accident requiring the operation of the passive residual heat removal system occurs, the accumulated non-condensable gas may flow into the condensation heat exchanger to prevent steam condensation to cause the performance degradation of the condensation heat exchanger. In consideration of the effect, it is designed in such a way that a condensation heat exchanger capacity of the passive residual heat removal system is conservatively large. However, as a high pressure (for example, SMART reactor: 17 MPa) facility, the passive residual heat removal system has a problem of significantly increasing the cost due to an increase of capacity. Furthermore, as a high-temperature high-pressure facility, the reactor restricts rapid cooling to alleviate thermal shock other than a normal operation and a partially restrictive accident. Accordingly, there is a limit in designing that the capacity of the condensation heat exchanger is conservatively too large. 
       SUMMARY OF THE INVENTION 
       [0013]    An object of the present disclosure is to prevent the dysfunction of a makeup tank associated with a flow at a secondary side of a passive residual heat removal system and alleviate the performance degradation and prediction uncertainty of a condensation heat exchanger due to non-condensable gas to overcome the foregoing problems in the related art. 
         [0014]    Another object of the present disclosure is to propose a passive residual heat removal system provided with a makeup facility for performing a function of accommodating excess fluid and compensating for the lack of fluid in a passive residual heat removal system. 
         [0015]    Still another object of the present disclosure is to overcome a problem of causing the performance degradation of a passive residual heat removal system due to non-condensable gas accumulated in a line of the system. 
         [0016]    In order to accomplish one object of the present disclosure, according to a passive residual heat removal system in accordance with an embodiment of the present disclosure, there is provided a passive residual heat removal system for circulating cooling fluid to a steam generator through a main feedwater line connected to a lower inlet of the steam generator and a main steam line connected to an upper outlet of the steam generator to remove sensible heat in a reactor coolant system and residual heat in a core during an accident, and the passive residual heat removal system may include a makeup facility configured to accommodate excess cooling fluid or supply makeup cooling fluid to maintain an amount of the cooling fluid within a preset range, wherein the makeup facility includes a makeup tank provided at a preset height between a lower inlet and an upper outlet of the steam generator to passively accommodate the excess cooling fluid or supply the makeup cooling fluid according to an amount of the cooling fluid; a first connection line connected to the main steam line and the makeup tank to form a flow path for flowing cooling fluid discharged from the steam generator to the main steam line through the makeup tank; and a second connection line connected to the makeup tank and the main feedwater line to form a supply flow path for supplying cooling fluid supplied from the makeup tank. 
         [0017]    According to an example associated with the present disclosure, an initial water level of the makeup tank may be set to either one of a first through a third water level, and the first water level may correspond to a level at which the cooling fluid is fully filled in the makeup tank to supply the makeup cooling fluid when the water level of the steam generator is lower than the water level of the makeup tank during an accident, and the second water level may correspond to a level at which cooling fluid is depleted in the makeup tank to accommodate the excess cooling fluid and supply the accommodated cooling fluid as the makeup cooling fluid during an accident, and the third water level may correspond to a level formed at a height between the first water level and the second water level to accommodate the excess cooling fluid and supply the cooling fluid using initially stored cooling fluid as the makeup cooling fluid. 
         [0018]    According to another example associated with the present disclosure, the first connection line may be connected to the main steam line through a steam line of the passive residual heat removal system to receive steam or the cooling fluid from the steam line. 
         [0019]    According to a still another example associated with the present disclosure, the makeup facility may further include a circulation line connected to the main steam line and the makeup tank to form a first circulation flow path to prevent non-condensable gas from being accumulated in the makeup tank along with the first connection line. 
         [0020]    The first connection line may be connected to the main steam line at a position closer to the steam generator than to the circulation line, and the circulation line may be connected to the main steam line at a position farther from the steam generator than the first connection line to form a flow of steam circulating through the first circulation flow path based on a phenomenon in which a pressure gradually decreases as being further away from the steam generator. 
         [0021]    The makeup facility may further include a first pressure drop structure configured to locally cause a decrease of pressure based on a principle in which a pressure relatively decreases according to a speed increase of fluid, and the first pressure drop structure may be provided at a connection portion of the circulation line and the main steam line to form a flow of steam circulating through the first circulation flow path so as to cause a local pressure drop. 
         [0022]    The makeup tank, the first connection line and the circulation line may be insulated by an insulator to limit the energy loss of steam passing through the first circulation flow path during the normal operation of a nuclear power plant. 
         [0023]    The passive residual heat removal system may include a condensation heat exchanger configured to discharge sensible heat in the reactor coolant system and residual heat in the core received through the circulation of the cooling fluid to an outside; a feedwater line connected to the condensation heat exchanger and the main feedwater line to form a flow path for supplying the cooling fluid from the condensation heat exchanger to the steam generator; a steam line connected to the main steam line and the condensation heat exchanger to form a flow path for supplying cooling fluid discharged from the steam generator to the condensation heat exchanger; and a vent line connected to the steam line and the main steam line to form a second circulation flow path for preventing non-condensable gas from being accumulated in the makeup tank or the steam line. 
         [0024]    The passive residual heat removal system may further include an inflow structure configured to induce at least part of a flow of steam circulating through the first circulation flow path and second circulation flow path to a preset flow path, and the inflow structure may include at least one of a first inflow structure extended from the first connection line and inserted into the main steam line to allow an inlet of an internal flow path to face steam flowing through the main steam line so as to induce at least part of steam flowing through the steam line to the first connection line; and a second inflow structure extended from the vent line and inserted into the steam line to allow an inlet of an internal flow path to face steam flowing through the steam line so as to induce at least part of steam flowing through the steam line to the vent line. 
         [0025]    The steam line may be connected to the main steam line at a position closer to the steam generator than to the vent line, and the vent line may be connected to the main steam line at a position farther from the steam generator than the steam line to form a flow of steam circulating through the second circulation flow path based on a principle in which a pressure gradually decreases as being further away from the steam generator. 
         [0026]    The passive residual heat removal system may further include a second pressure drop structure configured to locally cause a decrease of pressure based on a principle in which the pressure relatively decreases according to a speed increase of fluid, and the second pressure drop structure may be provided at an internal flow path of a connection portion of the vent line and the main steam line to form a flow of steam circulating through the second circulation flow path so as to cause a local pressure drop. 
         [0027]    An isolation valve that is open by related signals during an accident may be provided at the feedwater line to initiate the operation of the passive residual heat removal system, and the isolation valve may be provided in duplicate or in parallel or provided along with a check valve for preventing the backflow of feedwater from the main feedwater line, and the second connection line may be connected to the feedwater line at a position between the two isolation valves provided in duplicate or between the isolation valve and the check valve, and connected to the main feedwater line through the feedwater line to form a supply flow path of excess cooling fluid to the makeup tank. 
         [0028]    The feedwater line may be connected to the makeup tank to form a flow path for supplying cooling fluid discharged from the condensation heat exchanger to the makeup tank, and the second connection line may be connected to the main feedwater line to form a flow path for supplying cooling fluid received through the feedwater line to the steam generator, and the feedwater line may be connected to the main feedwater line through the makeup tank and the second connection line. 
         [0029]    The makeup facility further may include a flow resistance portion, and the flow resistance portion may include at least one of a first flow resistance portion provided at the first connection line to adjust a flow of cooling fluid introduced from the main steam line to the makeup tank; a second flow resistance portion provided at the second connection line to adjust a flow of makeup cooling fluid supplied from the makeup tank to the main feedwater line; and a third flow resistance portion provided at the feedwater line to adjust a flow of cooling fluid supplied from the condensation heat exchanger to the feedwater line. 
         [0030]    In order to accomplish the foregoing task, the present disclosure discloses a nuclear power plant having a passive residual heat removal system. The nuclear power plant may include a steam generator provided at a boundary between a primary system and a secondary system; a main feedwater line connected to a lower inlet of the steam generator to supply feedwater from a feedwater system to the steam generator during a normal operation; a main steam line connected to an upper outlet of the steam generator to supply steam from the steam generator to a turbine system during a normal operation; a passive residual heat removal system configured to circulate cooling fluid to a steam generator through the main feedwater line and the main steam line to remove sensible heat in a reactor coolant system and residual heat in a core during an accident; and a makeup facility configured to accommodate excess cooling fluid or supply makeup cooling fluid to maintain an amount of the cooling fluid within a preset range, wherein the makeup facility includes a makeup tank provided at a preset height between a lower inlet and an upper outlet of the steam generator to passively accommodate the excess cooling fluid or supply the makeup cooling fluid according to a water level of the cooling fluid; a first connection line connected to the main steam line and the makeup tank to form a flow path for flowing cooling fluid discharged from the steam generator to the main steam line through the makeup tank; and a second connection line connected to the makeup tank and the main feedwater line to form a supply flow path for supplying cooling fluid supplied from the makeup tank. 
         [0031]    According to the present disclosure having the foregoing configuration, a makeup tank is provided at an appropriate position between a lower inlet and a upper outlet of a steam generator, and an upper portion of the makeup tank is filled with steam and a lower portion thereof is filled with makeup cooling fluid. when cooling fluid is excessive in a passive residual heat removal system, the makeup tank may accommodate excess cooling fluid, and on the contrary, when cooling fluid is insufficient in the passive residual heat removal system, the makeup tank may perform a buffer action for supplying makeup cooling fluid. 
         [0032]    Furthermore, according to the present disclosure, a steam line of the passive residual heat removal system and a circulation line of the makeup tank may be connected to a main steam line to continuously circulate a small amount of steam during the normal operation of a nuclear power plant. Accordingly, non-condensable gas may not be accumulated in the makeup tank, thereby preventing the performance degradation of a condensation heat exchanger due to non-condensable gas during the operation of the passive residual heat removal system. 
         [0033]    When a makeup facility according to the present disclosure is applied to a nuclear power plant, a water level of the steam generator may be maintained within an optimal range to allow the makeup tank to induce an optimal performance of the passive residual heat removal system during an accident. Furthermore, a leak flow rate of the passive residual heat removal system may be compensated by the makeup tank, thereby allowing the passive residual heat removal system to achieve an optimal performance. In addition, the present disclosure may remove non-condensable gas that can be accumulated in the passive residual heat removal system, thereby preventing the performance degradation of the condensation heat exchanger. 
         [0034]    Ultimately, the present disclosure may present a scheme capable of removing design uncertainty in the passive residual heat removal system and implementing an optimal performance to facilitate the performance prediction of the passive residual heat removal system. Accordingly, the prediction uncertainty of the passive residual heat removal system may decrease, thereby designing a condensation heat exchanger in an appropriate size and reducing the cost of a condensation heat exchanger facility. A excessive cooling problem of a nuclear power plant according to a conservative condensation heat exchanger design may be alleviated due to a supplicated performance prediction, thereby enhancing safety. Moreover, the design of a more sophisticated condensation heat exchanger may be allowed through the removal of the accumulation of non-condensable gas and the uncertainty of the water level to overcome design difficulties to allow the configuration of a safer passive residual heat removal system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    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. 
           [0036]    In the drawings: 
           [0037]      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; 
           [0038]      FIG. 2  is a conceptual view illustrating the detailed structure of portion “A” illustrated in  FIG. 1 ; 
           [0039]      FIG. 3  is a conceptual view illustrating another example of portion “A” illustrated in  FIG. 1 ; 
           [0040]      FIG. 4  is a conceptual view illustrating a normal operation state of the nuclear power plant including the passive residual heat removal system illustrated in  FIG. 1 ; 
           [0041]      FIG. 5  is a conceptual view illustrating a valve operation during an accident of the nuclear power plant including the passive residual heat removal system illustrated in  FIG. 1 ; 
           [0042]      FIG. 6  is a conceptual view illustrating a water level equilibrium state according to the progression of an accident in the nuclear power plant including the passive residual heat removal system illustrated in  FIG. 1 ; 
           [0043]      FIG. 7  is a conceptual view for accommodating excess cooling fluid into a makeup tank according to the progression of an accident in the nuclear power plant including the passive residual heat removal system illustrated in  FIG. 1 ; 
           [0044]      FIG. 8  is a conceptual view for supplying makeup cooling fluid from a makeup tank according to the progression of an accident in the nuclear power plant including the passive residual heat removal system illustrated in  FIG. 1 ; 
           [0045]      FIG. 9  is a conceptual view illustrating a modified example of the nuclear power plant including the passive residual heat removal system illustrated in  FIG. 1 ; 
           [0046]      FIG. 10  is an another conceptual view illustrating a modified example of the nuclear power plant including the passive residual heat removal system illustrated in  FIG. 1 ; 
           [0047]      FIG. 11  is a still another conceptual view illustrating a modified example of the nuclear power plant including the passive residual heat removal system illustrated in  FIG. 1 ; 
           [0048]      FIG. 12  is a yet still another conceptual view illustrating a modified example of the nuclear power plant including the passive residual heat removal system illustrated in  FIG. 1 ; 
           [0049]      FIG. 13  is a still yet another conceptual view illustrating a modified example of the nuclear power plant including the passive residual heat removal system illustrated in  FIG. 1 ; 
           [0050]      FIG. 14  is a conceptual view illustrating the detailed structure of an inflow structure illustrated in  FIG. 13 ; 
           [0051]      FIG. 15  is another conceptual view of an inflow structure illustrated in  FIG. 13 ; 
           [0052]      FIG. 16  is a still another conceptual view illustrating a modified example of the nuclear power plant including the passive residual heat removal system illustrated in  FIG. 1 ; 
           [0053]      FIG. 17  is a yet still another conceptual view illustrating a modified example of the nuclear power plant including the passive residual heat removal system illustrated in  FIG. 1 ; 
           [0054]      FIG. 18  is a still another conceptual view illustrating a modified example of the nuclear power plant including the passive residual heat removal system illustrated in  FIG. 1 ; 
           [0055]      FIG. 19  is a yet still another conceptual view illustrating a modified example of the nuclear power plant including the passive residual heat removal system illustrated in  FIG. 1 ; 
           [0056]      FIG. 20  is a conceptual view illustrating another passive residual heat removal system and a nuclear power plant including the same; 
           [0057]      FIG. 21  is a conceptual view illustrating a modified example of the nuclear power plant including the passive residual heat removal system illustrated in  FIG. 20 ; and 
           [0058]      FIG. 22  is a still another conceptual view illustrating a passive residual heat removal system and a nuclear power plant including the same according to another embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0059]    Hereinafter, a nuclear power plant including 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. 
         [0060]    In the specification, in case where it is mentioned that an element is “connected” to another element, it should be understood that an element may be directly connected to another element, but another element may exist therebetween. On the contrary, in case where it is mentioned that an element is “directly connected” to another element, it should be understood that any other element does not exist therebetween. It is mentioned to be “indirectly connected” when another element exists therebetween. 
         [0061]      FIG. 1  is a conceptual view illustrating a passive residual heat removal system  100   a  and a nuclear power plant  10   a  including the same associated with an embodiment of the present disclosure. 
         [0062]    The nuclear power plant  10   a  may include a reactor coolant system  11   a,  a core  12   a,  a steam generator  13   a,  a reactor coolant pump  14   a,  and a pressurizer  15   a.  In addition to the constituent elements illustrated in  FIG. 1 , the nuclear power plant  10   a  may include systems for a normal operation and various systems for securing safety. 
         [0063]    The reactor coolant system  11   a  is a coolant system for transferring and transporting thermal energy generated by fuel fission. An inside of the reactor coolant system  11  a is filled with primary fluid. When an accident such as a loss of coolant accident occurs, primary fluid(steam) may be released from the reactor coolant system  11   a,  and a containment (not shown) prevents radioactive materials from being leaked to an outside. 
         [0064]    The steam generator  13   a  is located at a boundary between a primary system and a secondary system. A lower inlet of the steam generator  13   a  is connected to a main feedwater line  16   a   1 , and an upper outlet of the steam generator  13   a  is connected to a main steam line  17   a   1 . During a normal operation of the nuclear power plant  10   a,  working fluid is supplied to the steam generator  13   a  through the main feedwater line  16   a   1  from a feedwater system  16   a.  Feedwater becomes steam by receiving the heat of the core  12   a  from primary fluid while passing through the steam generator  13   a.  The steam is supplied from the steam generator  13   a  to a turbine system  17   a.    
         [0065]    The reactor coolant pump  14   a  is provided to form a flow of primary fluid entering into the core  12   a.    
         [0066]    The pressurizer  15   a  maintains a pressurized state exceeding a saturation pressure to suppress the boiling of coolant in the core  12   a  of a pressurized water reactor, and the pressurizer  15   a  controls a pressure of the reactor coolant system  11   a.    
         [0067]    The passive residual heat removal system  100   a  is an emergency safety facility for removing sensible heat in the reactor coolant system  11   a  and residual heat in the core  12   a  during an accident, and in particular, remove sensible heat and residual heat in a passive manner using natural forces. The passive residual heat removal system  100   a  circulates cooling fluid to the steam generator  13   a  through the main feedwater line  16   a   1  and main steam line  17   a   1 . 
         [0068]    The passive residual heat removal system  100   a  may include a condensation heat exchanger  110   a,  a feedwater line  140   a,  and a steam line  130   a.    
         [0069]    The condensation heat exchanger  110   a  discharges sensible heat in the reactor coolant system  11  a and residual heat in the core  12   a  received through the circulation of cooling fluid to an outside. The condensation heat exchanger  110   a  may be provided within the emergency cooling tank  120   a,  and coolant is filled into the emergency cooling tank  120   a.  Heat is exchanged between cooling fluid (vapor phase) and the coolant of the emergency cooling tank  120   a  in the condensation heat exchanger  110   a.  Heat is transferred to the coolant of the emergency cooling tank  120   a  from cooling fluid (vapor phase). The coolant of the emergency cooling tank  120   a  is evaporated as the temperature gradually increases. Steam formed by evaporating the coolant is discharged to an outside of the emergency cooling tank  120   a.  The passive residual heat removal system  100   a  may remove sensible heat in the reactor coolant system  11   a  and residual heat in the core  12   a  through the repetition of the process. 
         [0070]    The feedwater line  140   a  is connected to the condensation heat exchanger  110   a  and main feedwater line  16   a   1  to form a flow path for supplying cooling fluid to the steam generator  13   a  from the condensation heat exchanger  110   a.  An isolation valve  142   a  that is open by related signals during an accident is provided at the feedwater line  140   a  to initiate the operation of the passive residual heat removal system  100   a.  The isolation valve  142   a  is provided in duplicate or in parallel or provided along with a check valve  143   a  for preventing the backflow of feedwater from the main feedwater line  16   a   1 . The isolation valve  142   a  is open by related signals when an accident occurs, and the check valve  143   a  is open by a flow of cooling fluid formed during an accident. In particular, referring to  FIG. 1 , the isolation valve  142   a  and check valve  143   a  are provided together at the feedwater line  140   a.    
         [0071]    Furthermore, a flow resistance portion  141   a  for adjusting a flow of cooling fluid (liquid phase) supplied from the condensation heat exchanger  110   a  to the steam generator  13   a  may be provided at the feedwater line  140   a.  The flow resistance portion  141   a  described herein is referred to as a third flow resistance portion  141   a  to be distinguished from another flow resistance portion that will be described below. The third flow resistance portion  141   a  may include an orifice or venturi, for example. The third flow resistance portion  141   a  is provided at an internal flow path of the feedwater line  140   a  to form a flow resistance so as to adjust a flow of cooling fluid supplied to the steam generator  13   a.    
         [0072]    The steam line  130   a  is connected to the main steam line  17   a   1  and condensation heat exchanger  110   a  to form a flow path for transferring cooling fluid (vapor phase) discharged from the steam generator  13   a  to the condensation heat exchanger  110   a.  Referring to  FIG. 1 , the steam line  130   a  may be branched from the main steam line  17   a   1  and connected to an upper portion of the condensation heat exchanger  110   a.  The condensation heat exchanger  110   a,  feedwater line  140   a,  main feedwater line  16   a   1 , steam generator  13   a,  main steam line  17   a   1  and steam line  130   a  form a circulation flow path for circulating cooling fluid. The circulation flow path described herein is referred to as a third circulation flow path to be distinguished from another circulation flow path which will be described later. 
         [0073]    The cooling fluid receives sensible heat in the reactor coolant system  11   a  and residual heat in the core  12   a  while circulating through the third circulation flow path, and transfers heat from the condensation heat exchanger  110   a  to cooling water within the emergency cooling tank  120   a.  Due to the repetition of the process, the passive residual heat removal system  100   a  removes sensible heat in the reactor coolant system  11  a and residual heat in the core  12   a.    
         [0074]    As mentioned above as a problem in the background of the invention, the performance of the passive residual heat removal system  100   a  is affected by an amount of cooling fluid. Accordingly, it may be possible to maximize the performance of the passive residual heat removal system  100   a  when an amount of cooling fluid is maintained within an optimal range according to the characteristics of the passive residual heat removal system  100   a  of each nuclear power plant  10   a.    
         [0075]    The present disclosure may include a makeup facility for deriving the maximum performance of the passive residual heat removal system  100   a.  The makeup facility may accommodate excess cooling fluid or supply makeup cooling fluid to maintain an amount of cooling fluid within a preset range. For the purpose of this, the makeup facility may include a makeup tank  160   a,  a first connection line  170   a  and a second connection line  180   a.    
         [0076]    The makeup tank  160   a  is provided at a preset height between a lower inlet and an upper outlet of the steam generator  13   a  to passively accommodate excess cooling fluid or supply makeup cooling fluid according to an amount of the cooling fluid. When the passive residual heat removal system  100   a  is operated, the water levels and pressures of the makeup tank  160   a,  steam generator  13   a  and condensation heat exchanger  110   a,  respectively, may form an equilibrium. Furthermore, a change of temperature or pressure and a leakage according to the operation of the passive residual heat removal system  100   a  have an effect on an amount of cooling fluid. Accordingly, the installation height, water level and pressure of the makeup tank  160   a  are important factors for maintaining an amount of cooling fluid in an optimal range. 
         [0077]    An initial water level of the makeup tank  160   a  may be set to any one of a first through a third water level. The first through the third water level are referred to distinguish them from one another, but do not denote absolute values. 
         [0078]    The first water level corresponds to a level at which cooling fluid is fully filled in the makeup tank  160   a  to supply makeup cooling fluid when the water level of the steam generator  13   a  is less than the water level of the makeup tank  160   a  during an accident. The makeup tank  160   a  set to the first water level performs only a makeup function, and supplies makeup cooling fluid only when the water level of the steam generator  13   a  is less than that of the makeup tank  160   a  during an accident. The first water level may be applicable to a case where an amount of cooling fluid of the passive residual heat removal system  100   a  is insufficient in all conditions during an accident or a case of the nuclear power plant  10   a  having a characteristic capable of sufficiently performing the performance even though an amount of cooling fluid is somewhat large as a capacity of the condensation heat exchanger  110   a  is designed to be large enough. 
         [0079]    The second water level corresponds to a level at which cooling fluid is depleted in the makeup tank  160   a  to accommodate excess cooling fluid and afterward supply the accommodated cooling fluid as makeup cooling fluid during an accident. The makeup tank  160   a  set to the second water level may further accommodate excess cooling fluid when the excess cooling fluid is additionally generated during a continuous operation subsequent to an accident as well as an early stage of the accident of the passive residual heat removal system  100   a.  The second water level may be applicable to a case of the nuclear power plant  10   a  having a characteristic in which an amount of cooling fluid of the passive residual heat removal system  100   a  is excessive in all conditions during an accident. 
         [0080]    The third water level corresponds to a level formed at a height between the first water level and the second water level to accommodate the excess cooling fluid and supply cooling fluid using initially stored cooling fluid as makeup cooling fluid during an accident. The makeup tank  160   a  set to the third water level may further accommodate excess cooling fluid when the excess cooling fluid is additionally generated during a continuous operation of the passive residual heat removal system  100   a.  The third water level may be applicable to a case of the nuclear power plant  10   a  having a characteristic in which a cooling fluid flow of the passive residual heat removal system  100   a  is insufficient as well as it is excessive according to a condition during an accident. 
         [0081]    An initial water level of the makeup tank  160   a  illustrated in  FIG. 1  is set to the third water level. During a normal operation of the nuclear power plant  10   a,  the water level of the makeup tank  160   a  is maintained by a circulation flow due to the first connection line  170   a  and circulation line  190   a.  Furthermore, the makeup tank  160   a  accommodates excess cooling fluid during an accident, and supplies cooling fluid using initially stored cooling fluid as makeup cooling fluid. 
         [0082]    The makeup tank  160   a  is insulated by an insulator  161   a  to limit the energy loss of steam passing through the first circulation flow path during a normal operation of nuclear power plant  10   a.  When a case without the makeup tank  160   a  and a case with the makeup tank  160   a  are compared, a flow path of steam may increase and the energy loss of steam flowing into the turbine system  17   a  may increase due to the existence of the makeup tank  160   a.  In order to compensate this disadvantage, the insulator  161   a  is installed at an outer circumferential surface of the makeup tank  160   a  to reduce the energy loss of steam. 
         [0083]    The first connection line  170   a  is connected to the main steam line  17   a   1  and makeup tank  160   a  to form a flow path of flowing cooling fluid discharged from the steam generator  13   a  to the main steam line  17   a   1 . Referring to  FIG. 1 , the first connection line  170   a  is branched from the main steam line  17   a   1  and connected to an upper portion of the makeup tank  160   a.    
         [0084]    A manual valve  171   a  and a first flow resistance portion  172   a  may be provided at a flow path of the first connection line  170   a.    
         [0085]    The manual valve  171   a  may be manually closed at a time point at which it should be isolated for the maintenance or the like of related facilities subsequent to being open during a normal operation. The first flow resistance portion  172   a  is provided at an internal flow path of the first connection line  170   a  to adjust a flow of cooling fluid introduced into the makeup tank  160   a  from the main steam line  17   a   1 . 
         [0086]    The first connection line  170   a  is also insulated by an insulator  173   a  to limit the energy loss of steam flowing into the turbine system during a normal operation of the nuclear power plant  10   a  similarly to the makeup tank  160   a.    
         [0087]    The second connection line  180   a  is connected to the makeup tank  160   a  and main feedwater line  16   a   1  to form a supply flow path for supplying cooling fluid supplied from the makeup tank  160   a.  Referring to  FIG. 1 , the second connection line  180   a  is branched from the feedwater line  140   a  and coupled to a lower portion of the makeup tank  160   a.    
         [0088]    A second flow resistance portion  181   a  and a manual valve  182   a  may be provided at a flow path of the second connection line  180   a.  The second flow resistance portion  181   a  is provided within the second connection line  180   a  to adjust a flow of makeup cooling fluid supplied from the makeup tank  160   a  to the main feedwater line  16   a   1 . The passive valve  182   a  may be manually closed at a time point at which it should be isolated for the maintenance or the like of related facilities subsequent to being open during a normal operation. 
         [0089]    The second connection line  180   a  may be connected to the feedwater line  140   a  at a position between the isolation valve  142   a  and the check valve  143   a  of the feedwater line  140   a.  The second connection line  180   a  is connected to the main feedwater line  16   a   1  through the feedwater line  140   a.  Accordingly, when makeup cooling fluid is supplied from the makeup tank  160   a,  the makeup cooling fluid is introduced into the feedwater line  140   a  through the second connection line  180   a.  When excess cooling fluid is introduced into the second connection line  180   a,  the excess cooling fluid is introduced into the makeup tank  160   a  through the feedwater line  140   a  and second connection line  180   a.  When makeup cooling fluid is supplied from the makeup tank  160   a,  the makeup cooling fluid and the cooling fluid from the condensation heat exchanger  110   a  through the feedwater line  140   a  join together, and then they are supplied to the main feedwater line  16   a   1 . 
         [0090]    A makeup facility may further include a circulation line  190   a  and a pressure drop structure  191   a.    
         [0091]    The circulation line  190   a  is connected to the main steam line  17   a   1  and makeup tank  160   a  to form a circulation loop for preventing the accumulation of non-condensable gas and maintaining the water level of the makeup tank  160   a  along with the first connection line  170   a.  The circulation line  190   a  is connected to a preset height of the makeup tank  160   a.  The circulation loop described herein is referred to as a first circulation loop to be distinguished from another circulation loop. The main steam line  17   a   1 , first connection line  170   a,  makeup tank  160   a  and circulation line  190   a  form the first circulation loop. 
         [0092]    During a normal operation of the nuclear power plant  10   a,  non-condensable gas may be accumulated within the makeup tank  160   a.  However, when the first circulation loop is formed by the circulation line  190   a  and first connection line  170   a  connected to the makeup tank  160   a,  a small amount of steam or a small amount of cooling fluid (two phases of water and vapor) may be continuously circulated through the first circulation loop. As a result, it may be possible to prevent the accumulation of non-condensable gas and prevent the performance degradation of the condensation heat exchanger  110   a  due to the non-condensable gas along with the first connection line  170   a.  Furthermore, when the water level increases over a connection portion of the circulation line  190   a,  a small amount of cooling fluid may be discharged along the circulation line  190   a  to maintain a preset water level of the makeup tank  160   a.    
         [0093]    An arrow shown in the drawing indicates a flow of fluid. A relatively large sized arrow indicates a relatively large flow, and a relatively small sized arrow indicates a relatively small flow. On the drawing, it is seen that a flow of steam or cooling fluid (water or two phases) circulating through the first circulation loop is much smaller than that of fluid flowing through the main steam line  17   a   1 . 
         [0094]    The circulation line  190   a  is also insulated by an insulator  193   a  to limit the energy loss of steam flowing into the turbine system during a normal operation of the nuclear power plant  10   a  similarly to the makeup tank  160   a,  first connection line  170   a.  During a normal operation of the nuclear power plant  10   a,  the insulator  193   a  may limit the energy loss of steam circulating through the first connection line  170   a,  makeup tank  160   a  and circulation line  190   a.    
         [0095]    A manual valve  192   a  and a flow resistance portion (not shown) are also provided at the circulation line  190   a.  The manual valve  192   a  may be manually closed at a time point at which it should be isolated for the maintenance or the like of related facilities subsequent to being open during a normal operation. 
         [0096]    Though not shown in the drawing, a flow resistance portion (not shown) may be provided at the circulation line  190   a.  The flow resistance portion may include an orifice or venturi that forms a flow resistance at an internal flow path of the circulation line  190   a.  The flow resistance portion is provided to suppress a large amount of flow (water or two phases of water and vapor) from being abruptly discharged and introduced into the turbine system  17   a  according to a variation of water level during a normal operation of the nuclear power plant  10   a,  and limit the circulation flow of steam to a design flow. 
         [0097]    The pressure drop structure  191   a  is configured to locally cause a decrease of pressure based on a principle in which a pressure relatively decreases according to a speed increase of fluid. The pressure drop structure  191   a  described herein is referred to as a first pressure drop structure  191   a  to be distinguished from another pressure drop structure. The first pressure drop structure  191   a  is provided at a connection portion between the circulation line  190   a  and the main steam line  17   a   1  to form a flow of steam or cooling fluid (water or two phases) circulating through the first circulation loop so as to cause a local pressure drop. 
         [0098]    Since a pressure drop is locally induced at a position at which the first pressure drop structure  191   a  is provided, a circulation flow of steam or cooling fluid (water or two phases) may be formed by a pressure difference induced at the first circulation loop. 
         [0099]    The detailed description of the first pressure drop structure  191   a  will be described with reference to  FIGS. 2 and 3 . 
         [0100]      FIG. 2  is a conceptual view illustrating the detailed structure of portion “A” illustrated in  FIG. 1 . 
         [0101]    An upper outlet of the steam generator  13   a  (refer to  FIG. 1 ) is connected to one end portion of the main steam line  17   a   1  (an end portion disposed when continuously extended to the left side of the main steam line  17   a   1  in  FIG. 2 ), and the turbine system  17   a  (refer to  FIG. 1 ) is connected to the other end portion of the main steam line  17   a   1  (an end portion disposed when continuously extended to the right side of the main steam line  17   a   1  in  FIG. 2 ). 
         [0102]    The first pressure drop structure  191   a  may include an orifice or venturi provided at an internal flow path of the main steam line  17   a   1 . The pressure drop structure  191   a  forms a flow resistance at an internal flow path of the main steam line  17   a   1 , and at least part thereof is open not to block a flow path connected to the circulation line  190   a.    
         [0103]    High-pressure steam flows into the main steam line  17   a   1 . The speed increases while the high-pressure steam passes through the first pressure drop structure  191   a.  The first pressure drop structure  191   a  locally induces a drop of pressure. Accordingly, steam or cooling fluid within the circulation line  190   a  flows to a low pressure side, and the circulation flow of steam or cooling fluid is formed at the first circulation loop. 
         [0104]      FIG. 3  is a conceptual view illustrating another example of portion “A” illustrated in  FIG. 1 . 
         [0105]    An upper outlet of the steam generator  13   a  (refer to  FIG. 1 ) is connected to one end portion of the main steam line  17   a   1  (an end portion disposed when continuously extended to the left side of the main steam line  17   a   1  in  FIG. 2 ), and the turbine system  17   a  (refer to  FIG. 1 ) is connected to the other end portion of the main steam line  17   a   1  (an end portion disposed when continuously extended to the right side of the main steam line  17   a   1  in  FIG. 2 ). 
         [0106]    The first pressure drop structure  191   a  may be provided at the circulation line  190   a.  The first pressure drop structure  191   a  is provided at an end portion of the circulation line  190   a,  and protruded within the main steam line  17   a   1  and exposed at an inner flow path of the main steam line  17   a   1 . Referring to  FIG. 3 , the first pressure drop structure  191   a  is configured to form a partially narrow flow path within the main steam line  17   a   1 . The first pressure drop structure  191   a  is configured to form a flow path of steam or cooling fluid (fluid that has passed through the first circulation flow path) in a direction (a direction of flowing from the bottom to the top) crossing a flow direction (a direction of flowing from the left to the right) of high-pressure steam, and the steam or cooling fluid and the high-pressure steam join together. It may be configured to introduce steam or cooling fluid eve in an opposite direction (a direction of flowing from the top to the bottom) to the flow direction of steam or cooling fluid. 
         [0107]    The speed of high-pressure steam increases while passing through a narrow flow path formed by the pressure drop structure  191   a.  Furthermore, a pressure difference between different portions of the first circulation flow path is formed due to a local pressure drop induced by the first pressure drop structure  191   a.  Steam or cooling fluid may flow through the first circulation flow path due to the pressure difference. 
         [0108]    Hereinafter, an operation in case the nuclear power plant  10   a  including the passive residual heat removal system  100   a  illustrated in  FIG. 1  is in a normal operation or an anticipated accident state will be described. 
         [0109]      FIG. 4  is a conceptual view illustrating a normal operation state of the nuclear power plant  10   a  including the passive residual heat removal system  100   a  illustrated in  FIG. 1 . 
         [0110]    During a normal operation of the nuclear power plant  10   a,  an isolation valve  17   a   2  provided at the main steam line  17   a   1  and an isolation valve  16   a   2  provided at the main feedwater line  16   a   1  are open. Feedwater is supplied from the feedwater system  16   a  to the steam generator  13   a  through the main feedwater line  16   a   1 . The check valve  143   a  for allowing only a uni-directional flow to pass therethrough is provided at the feedwater line  140   a.  During a normal operation of the nuclear power plant  10   a,  an internal pressure of the main feedwater line  16   a   1  is higher than that of the feedwater line  140   a,  and thus the check valve  143   a  is not open. Accordingly, the check valve  143   a  may prevent feedwater from flowing backward from the main feedwater line  16   a   1  to the feedwater line  140   a.    
         [0111]    Feedwater exchanges heat with primary fluid while passing through the steam generator  13   a.  The feedwater is heated to become steam by the heat of the core  12   a  received from primary fluid. Steam formed by heating feedwater is discharged through an upper outlet of the steam generator  13   a,  and supplied to the turbine system  17   a  through the main steam line  17   a   1 . 
         [0112]    During a normal operation of the nuclear power plant  10   a,  the manual valve  171   a,    192   a  provided at the first connection line  170   a  and circulation line  190   a  are open, respectively. Most of steam passing through the main steam line  17   a   1  is supplied to the turbine system  17   a.  A small amount of steam flowing through the main steam line  17   a   1  is supplied to the makeup tank  160   a  through the first connection line  170   a,  and discharged to the main steam line  17   a   1  through the circulation line  190   a  again. Actuating power that circulates steam is provided from a pressure difference formed by the first pressure drop structure  191   a.  non-condensable gas is prevented from being accumulated in the makeup tank  160   a  while a small amount of steam continuously circulates through the first connection line  170   a  and circulation line  190   a  connected to the makeup tank  160   a.    
         [0113]    During a normal operation of the nuclear power plant  10   a,  the water level of the makeup tank  160   a  is maintained below a height at which the circulation line  190   a  is connected to makeup tank  160   a.  The water level formed above a connection height of the circulation line  190   a  is sprayed and discharged to the main steam line  17   a   1  through the circulation line  190   a.    
         [0114]      FIG. 5  is a conceptual view illustrating a valve operation during an accident of the nuclear power plant  10   a  including the passive residual heat removal system  100   a  illustrated in  FIG. 1 . 
         [0115]    When an accident requiring the operation of the passive residual heat removal system  100   a  occurs in the nuclear power plant  10   a,  isolation valves  16   a   2 ,  17   a   2  provided at the main feedwater line  16   a   1  and main steam line  17   a   1  are closed by related signals. Accordingly, the supply of feedwater from the feedwater system  16   a  is stopped, and the supply of steam to the turbine system  17   a  is also stopped. When an accident requiring the operation of the passive residual heat removal system  100   a  occurs in the nuclear power plant  10   a,  the isolation valve  142   a  provided at the feedwater line  140   a  is open by relates signals. 
         [0116]    As the isolation valves  16   a   2 ,  17   a   2  provided at the main feedwater line  16   a   1  and main steam line  17   a   1  are closed and the isolation valve  142   a  provided at the feedwater line  140   a  is open, a flow path of fluid circulating through the steam generator  13   a  is changed to form a third circulation flow path connected to the condensation heat exchanger  110   a,  feedwater line  140   a,  main feedwater line  16   a   1 , steam generator  13   a,  main steam line  17   a   1 , steam line  130   a  and condensation heat exchanger  110   a.  As a result, the passive residual heat removal system  100   a  starts the operation. 
         [0117]    Cooling fluid (liquid phase) is supplied to the steam generator  13   a  while sequentially passing through the feedwater line  140   a  and main steam line  17   a   1 , and the cooling fluid (liquid phase) is heated to become steam by receiving sensible heat in the reactor coolant system  11   a  and residual heat in the core  12   a  at the steam generator  13   a.  The cooling fluid (steam phase) is discharged to an upper outlet of the steam generator  13   a,  and transferred to the condensation heat exchanger  110   a  through the steam line  130   a.  The cooling fluid (steam phase) transfers heat to the coolant of the emergency cooling tank  120   a  from the condensation heat exchanger  110   a  and condenses. The cooling fluid that has transferred heat to the coolant of the emergency cooling tank  120   a  and condensed is supplied to the steam generator  13   a  again through the feedwater line  140   a  and main feedwater line  16   a   1 . Sensible and residual heat in the reactor coolant system  11  a can be removed by the circulation of the cooling fluid. 
         [0118]    Since the manual valve  182   a  provided at the second connection line  180   a  is open, part of cooling fluid supplied from the condensation heat exchanger  110   a  through the feedwater line  140   a  may flow to the makeup tank  160   a  through the second connection line  180   a.  Furthermore, the manual valve  171   a  provided at the first connection line  170   a  and the manual valve  192   a  provided at the circulation line  190   a  are also open, and thus part of steam generated by the evaporation of cooling fluid may be supplied to the makeup tank  160   a  through the first connection line  170   a  prior to flowing into the condensation heat exchanger  110   a  and then discharged to the main steam line  17   a   1  through the circulation line  190   a  again. However, since a flow of steam formed in the circulation line  190   a  during an accident is very small compared to that formed during a normal operation, the circulation line  190   a  during an accident cannot perform the non-condensable gas removal and a water level maintenance function of the makeup tank  160   a,  which is performed during a normal operation. 
         [0119]    Ps indicates a pressure of the steam generator  13   a,  and Hs indicates a water level of the steam generator  13   a.  Pm indicates a pressure of the makeup tank  160   a,  and Hm indicates a water level of the makeup tank  160   a.  Pc indicates a pressure of the condensation heat exchanger  110   a,  and Hc indicates a water level of the condensation heat exchanger  110   a.  At the time point of starting the operation of the passive residual heat removal system  100   a,  the pressure (Ps), water level (Hs) of the steam generator  13   a,  the pressure (Pm), water level (Hm) of the makeup tank  160   a,  and the pressure (Pc), water level (Hc) of the condensation heat exchanger  110   a  are not in an equilibrium state. 
         [0120]      FIG. 6  is a conceptual view illustrating a water level equilibrium state according to the progression of an accident in the nuclear power plant  10   a  including the passive residual heat removal system  100   a  illustrated in  FIG. 1 . 
         [0121]    When the operation of the passive residual heat removal system  100   a  starts and time passes, excess cooling fluid (excess cooling fluid at an upper portion higher than an initial water level of the makeup tank  160   a,  provided that a water level in consideration of pressure equilibrium) within cooling fluid circulating through the passive residual heat removal system  100   a  is returned to the makeup tank  160   a.  Furthermore, the pressure (Ps) and water level (Hs) of the steam generator  13   a,  the pressure (Pm) and water level (Hm) of the makeup tank  160   a,  and the pressure (Pc) and water level (Hc) of the condensation heat exchanger  110   a  form an equilibrium state. 
         [0122]    In a state that the operation of the passive residual heat removal system  100   a  continues, a large flow of cooling fluid does not occur. Furthermore, the makeup tank  160   a,  first connection line  170   a  and circulation line  190   a  are insulated by the insulator  161   a,    173   a,    193   a  (refer to  FIG. 1 ), no significant heat loss occurs. As a result, the pressure (Ps) of the steam generator  13   a  and the pressure (Pm) of the makeup tank  160   a  are maintained at similar pressures and thus a water level in consideration of a loss of pressure and a change of density difference due to flow resistance is similarly maintained. 
         [0123]      FIG. 7  is a conceptual view for accommodating excess cooling fluid into a makeup tank according to the progression of an accident in the nuclear power plant  10   a  including the passive residual heat removal system  100   a  illustrated in  FIG. 1 . 
         [0124]    A temperature or pressure change according to the operation of the passive residual heat removal system  100   a  and a flow change of cooling fluid according to the leakage are accommodated in the makeup tank  160   a.  Excess cooling fluid is introduced into the makeup tank  160   a  to increase the water level of the makeup tank  160   a.  As a result, the pressure (Ps) and water level (Hs) of the steam generator  13   a,  the pressure (Pm) and water level (Hm) of the makeup tank  160   a,  and the pressure (Pc) and water level (Hc) of the condensation heat exchanger  110   a  form an equilibrium state. 
         [0125]      FIG. 8  is a conceptual view for supplying makeup cooling fluid from a makeup tank according to the progression of an accident in the nuclear power plant  10   a  including the passive residual heat removal system  100   a  illustrated in  FIG. 1 . 
         [0126]    When cooling fluid circulating through the second connection line  180   a  is insufficient, the makeup tank  160   a  supplies makeup cooling fluid to the steam generator  13   a  through the second connection line  180   a.  As a result, the pressure (Ps) and water level (Hs) of the steam generator  13   a,  the pressure (Pm) and water level (Hm) of the makeup tank  160   a,  and the pressure (Pc) and water level (Hc) of the condensation heat exchanger  110   a  continuously form an equilibrium state. 
         [0127]    During the operation of the passive residual heat removal system  100   a  for a long period of time, cooling fluid circulating through the passive residual heat removal system  100   a  may decrease due to a small amount of leakage. A decrease of cooling fluid may be checked through a water level measurement of the makeup tank  160   a  or the like. In this case, makeup water may be additionally injected into the passive residual heat removal system  100   a  to continuously maintain an appropriate water level. 
         [0128]      FIG. 9  is a conceptual view illustrating a modified example of the nuclear power plant  10   b  including the passive residual heat removal system  100   b  illustrated in  FIG. 1 . 
         [0129]    Isolation valves  142   b,    144   b  may be provided in duplicate or in parallel (not shown) at a feedwater line  140   b.  Furthermore, a check valve  143   b  may be also provided at the feedwater line  140   b  along with the isolation valves  142   b,    144   b  provided in duplicate. Furthermore, a second connection line  180   b  is connected to the feedwater line  140   b  at a position between two isolation valves  142   b,    144   b,  and the second connection line  180   b  is connected to the a main feedwater line  16   b   1  through the feedwater line  140   b.  The remaining configuration will be substituted by the earlier description of  FIG. 1 . 
         [0130]      FIG. 10  is an another conceptual view illustrating a modified example of the nuclear power plant  10   c  including the passive residual heat removal system  100   c  illustrated in  FIG. 1 . 
         [0131]    Referring to  FIG. 1  prior to examining  FIG. 10 , the first connection line  170   a  and steam line  130   a  are sequentially connected to the main steam line  17   a   1  in a direction of being further away from the steam generator  13   a.  The passive residual heat removal system  100   c  and nuclear power plant  10   c  illustrated in  FIG. 10  have a different connection sequence from that of  FIG. 1 . 
         [0132]    Referring to  FIG. 10 , a first connection line  170   c,  a circulation line  190   c  and a steam line  130   c  are sequentially connected to a main steam line  17   c   1  in a direction of being further away from a steam generator  13   c.  If a connection sequence of  FIG. 10  is referred to as a forward connection, then a connection sequence of  FIG. 1  may be referred to as a backward connection. 
         [0133]    A configuration in which steam is introduced into a makeup tank  160   c  through a first connection line  170   c,  and returned to the main steam line  17   c   1  through a circulation line  190   c  will be substituted by the earlier description. Furthermore, due to this, it may be possible to prevent non-condensable gas from being accumulated in the makeup tank  160   c  and maintain a water level of the makeup tank  160   c.    
         [0134]      FIG. 11  is a still another conceptual view illustrating a modified example of the nuclear power plant  10   d  including the passive residual heat removal system illustrated  100   d  in  FIG. 1 . 
         [0135]    The water level of the makeup tank  160   d  is set to a second water level. A cooling fluid flow of the passive residual heat removal system  100   d  during an accident may be different according to the design characteristics of the nuclear power plant  10   d.  The second water level corresponds to a level at which cooling fluid is depleted in the makeup tank  160   d  to accommodate excess cooling fluid and supply the accommodated cooling fluid as makeup cooling fluid during an accident. The makeup tank  160   d  set to the second water level may further accommodate excess cooling fluid when the excess cooling fluid is additionally generated during a continuous operation subsequent to an accident as well as an early stage of the accident of the passive residual heat removal system  100   d.  The second water level may be applicable to a case of the nuclear power plant  10   d  having a characteristic in which a cooling fluid flow of the passive residual heat removal system  100   d  is excessive in all conditions during an accident. 
         [0136]    A circulation line  190   d  is branched from a feedwater line  140   d  and connected to a main steam line  17   d   1 . Accordingly, the circulation line  190   d  is connected to the makeup tank  160   d  through the feedwater line  140   d.    
         [0137]      FIG. 12  is a yet still another conceptual view illustrating a modified example of the nuclear power plant  10   e  including the passive residual heat removal system  100   e  illustrated in  FIG. 1 . 
         [0138]    Referring to  FIG. 12 , the sequence of connecting a first connection line  170   e  and a circulation line  190   e  to a main steam line  17   e   1  corresponds to a forward connection described in  FIG. 10 . When the first connection line  170   e  and circulation line  190   e  are sequentially provided between the steam generator  13   e  and an isolation valve of the main steam line  17   e   1 , and a flow resistance region  17   e   1 ′ is formed at the main steam line  17   e   1 , a circulation flow may be formed using a pressure drop according to a line length of the main steam line  17   e   1 . 
         [0139]    As located away from the steam generator  13   e,  a pressure of the main steam line  17   e   1  gradually decreases. Based on such a principle, when the first connection line  170   e  is connected to the main steam line  17   e   1  at a position closer to the steam generator  13   e  than to the circulation line  190   e,  and the circulation line  190   e  is connected to the main steam line  17   e   1  at a position farther from the steam generator  13   e  than the first connection line  170   e,  a circulation flow may be formed without a first pressure drop structure  191   e.    
         [0140]    In particular, the flow resistance region  17   e   1 ′ may be formed between a connection portion of the first connection line  170   e  and a connection portion of the circulation line  190   e  in the main steam line  17   e   1  to induce the formation of a circulation flow. The flow resistance region  17   e   1 ′ may induce the flow of a small amount of steam to the first connection line  170   e.    
         [0141]      FIG. 13  is a still yet another conceptual view illustrating a modified example of the nuclear power plant  10   f  including the passive residual heat removal system  100   f  illustrated in  FIG. 1 . 
         [0142]    A first connection line  170   f  and a circulation line  190   f  have a forward connection structure. The first connection line  170   f,  makeup tank  160   f  and circulation line  190   f  form a first circulation flow path. 
         [0143]    The passive residual heat removal system  100   f  may further include an inflow structure  174   f  configured to induce at least part of a flow steam or cooling fluid (water or two phases) circulating through the first circulation loop to a preset flow path. The inflow structure  174   f  may be provided at any position between a portion connected to a steam generator  13   f  and an installation portion of an isolation valve  17   f   2  in a main steam line  17   f   1  to induce a flow. 
         [0144]    The inflow structure described herein is referred to as a first inflow structure  174   f  to be distinguished from an inflow structure disposed at another place, and the inflow structure disposed at another place is referred to as a second inflow structure. The detailed structure of the first inflow structure  174   f  will be described with reference to  FIGS. 14 and 15 . 
         [0145]      FIG. 14  is a conceptual view illustrating the detailed structure of the inflow structure  174   f  illustrated in  FIG. 13 . 
         [0146]    A first inflow structure  174   f ′ is extended from the first connection line  170   f  and inserted into the main steam line  17   f   1  to induce at least part of steam flowing through the main steam line  17   f   1  to the first connection line  170   f.  An inlet of an internal flow path of the first inflow structure  174   f ′ faces steam flowing through the main steam line  17   f   1 . Due to such a structure, a small amount of steam within steam flowing through the main steam line  17   f   1  flows to the side of the first connection line  170   f.    
         [0147]      FIG. 15  is another conceptual view of the inflow structure  17   f  illustrated in  FIG. 13 . 
         [0148]    A first inflow structure  174   f ″ may be bent in a curved shape on the contrary to the first inflow structure  174   f ′ illustrated in  FIG. 14 . An inlet of the first inflow structure  174   f ″ faces a flow of steam flowing through the mode switching intention  17   f   1  at the front side, thereby efficiently inducing the flow of steam to the side of the first connection line  170   f.    
         [0149]      FIG. 16  is a still another conceptual view illustrating a modified example of the nuclear power plant  10   g  including the passive residual heat removal system  100   g  illustrated in  FIG. 1 . 
         [0150]    A feedwater line  140   g  is connected to a makeup tank  160   g  to form a flow path for supplying cooling fluid discharged from a condensation heat exchanger  110   g  to the makeup tank  160   g.  The feedwater line  140   g  is connected to a main feedwater line  16   g   1  through the makeup tank  160   g  and second connection line  180   g.  A manual valve  145   g  is provided at the feedwater line  140   g  to be manually closed at a time point that requires maintenance. A check valve  183   g  is provided at a second connection line  180   g  to prevent feedwater from flowing backward from the main feedwater line  16   g   1  to the makeup tank  160   g.    
         [0151]    The second connection line  180   g  is connected to the main feedwater line  16   g   1  to form a flow path for supplying cooling fluid stored therewithin and cooling fluid received through the feedwater line  140   g  to a steam generator  13   g.    
         [0152]    According to the foregoing configuration, the passive residual heat removal system  100   g  is operated as a mechanism of collecting cooling fluid supplied from the condensation heat exchanger  110   g  to the makeup tank  160   g  and supplying the collected cooling fluid to the steam generator  13   g  again. 
         [0153]      FIG. 17  is a yet still another conceptual view illustrating a modified example of the nuclear power plant  10   h  including the passive residual heat removal system  100   h  illustrated in  FIG. 1 . 
         [0154]    Similarly to  FIG. 16 , in  FIG. 17 , a feedwater line  140   h  is connected to a main feedwater line  16   h   1  through a makeup tank  160   h  and a second connection line  180   h.  The feedwater line  140   h  is connected to the makeup tank  160   h  to form a flow path for supplying cooling fluid discharged from a condensation heat exchanger  110   h  to the makeup tank  160   h.  A check valve  183   a  and an isolation valve  184   h  may be respectively provided at the feedwater line  140   h.  The isolation valve  184   h  is open by related signals at a time point that requires the operation of the passive residual heat removal system  100   h.  The check valve  183   h  prevents feedwater from flowing backward from a main steam line  17   h   1  to the makeup tank  160   h,  and open by a flow of makeup cooling fluid when the makeup cooling fluid is supplied from the makeup tank  160   h.    
         [0155]      FIG. 18  is a still another conceptual view illustrating a modified example of the nuclear power plant  10   i  including a passive residual heat removal system  100   i  illustrated in  FIG. 1 . 
         [0156]    Various shapes such as a spherical shape, a cylindrical shape, a rectangular shape both end portions of which are formed in a hemispherical shape or the like may be applicable to the shape of a makeup tank  160   i.  However, assuming that it is configured with the same volume, the shape of the makeup tank  160   i  with a low height and a large area is advantageous for minimizing a variation of the water level and maintaining an optimal water level of a steam generator  13   i.  In case where the makeup tank  160   i  in a rectangular shape is provided, it is more advantageous to have a lying-down shape. 
         [0157]    Referring to  FIG. 18 , the makeup tank  160   i  is formed in a shape in which both end portions thereof is formed in a hemispherical shape with a rectangular lying-down shape. 
         [0158]      FIG. 19  is a yet still another conceptual view illustrating a modified example of the nuclear power plant  10   j  including the passive residual heat removal system  100   j  illustrated in  FIG. 1 . 
         [0159]    A first connection line  170   j  may be connected to an appropriate position of a steam line  130   j  according to the convenience of the installation. 
         [0160]    The first connection line  170   j  is connected to a main steam line  17   j   1  through the steam line  130   j  to receive steam from the steam line  130   j  of the passive residual heat removal system  100   j.  The steam line  130   j  is branched from the main steam line  17   j   1 , and the first connection line  170   j  is branched from the steam line  130   j  and connected to a makeup tank  160   j.  Steam flowing through the main steam line  17   j   1  is introduced into the steam line  130   j,  and the steam is introduced into the first connection line  170   j  again, and supplied to the makeup tank  160   j  through the first connection line  170   j.    
         [0161]    The foregoing structure is formed for the purpose of forming a circulation flow of steam or cooling fluid in a first circulation loop to maintain a water level of the makeup tank  160   j  as well as prevent non-condensable gas from being accumulated in the makeup tank  160   j.    
         [0162]      FIG. 20  is a conceptual view illustrating another passive residual heat removal system  200   a  and a nuclear power plant  20   a  including the same. 
         [0163]    The passive residual heat removal system  200   a  may further include a steam line  230   a  and a vent line  250   a  connected to a main steam line  27   a   1  to form a circulation loop for preventing non-condensable gas from being accumulated in a makeup tank  260   a  or steam line  230   a  and maintaining a water level of the makeup tank  260   a.  The circulation loop described herein is referred to as a second circulation loop to be distinguished from the foregoing first circulation loop. 
         [0164]    The vent line  250   a  is branched from the steam line  230   a.  Referring to  FIG. 20 , the vent line  250   a  is connected to the makeup tank  260   a  through a first connection line  270   a,  and connected to the main steam line  27   a   1  through the makeup tank  260   a  and a circulation line  290   a.  A small amount of steam flowing the main steam line  27   a   1  through the vent line  250   a  is introduced into the makeup tank  260   a,  and therefore it may be possible to prevent non-condensable gas from being accumulated in the makeup tank  260   a  through the process. The foregoing configuration is also used for the purpose of preventing non-condensable gas from being accumulated in the makeup tank  260   a  and steam line  230   a.  The circulation line  290   a,  vent line  250   a  and makeup tank  260   a  are integrated into one system. 
         [0165]    Steam flowing through the main steam line  27   a   1  is introduced into the steam line  230   a,  and a small amount of steam is supplied to the makeup tank  260   a  through the vent line  250   a.  The vent line  250   a  may be insulated by an insulator  251   a  to prevent the heat loss of steam during a normal operation of the nuclear power plant  20   a.  Steam supplied to the makeup tank  260   a  or cooling fluid in the makeup tank  260   a  is transferred to the main steam line  27   a   1  again through the circulation line  290   a.  Due to the foregoing configuration, it may be possible to prevent the accumulation of non-condensable gas and maintain a water level of the makeup tank  260   a  by a circulation flow formed at a first and a second circulation loop. 
         [0166]    The foregoing inflow structure  134   f    134   f ′,  134   f ″ (refer to  FIGS. 13 and 14 ) may be also applicable to the passive residual heat removal system  200   a.  The inflow structure  134   f    134   f ′,  134   f ″ (refer to  FIGS. 13 and 14 ) is provided within a portion shown as “B” on the drawing. The inflow structure described herein is referred to as a second inflow structure (not shown) to be distinguished from the foregoing inflow structure. 
         [0167]    The second inflow structure (not shown) is extended from the vent line  250   a  and inserted into the steam line  230   a  to induce at least part of steam flowing through the steam line  230   a  to the vent line  250   a.  An inlet of an internal flow path of the second inflow structure (not shown) faces steam flowing through the steam line  230   a.  The detailed structure of the second inflow structure (not shown) is illustrated with reference to  FIGS. 14 and 15 . 
         [0168]    Forming a flow of steam circulating through the second circulation flow path based on a principle in which a pressure gradually decreases as being further away from the steam generator  23   a  may be also applicable to the passive residual heat removal system  200   a  in  FIG. 20 . For example, on the contrary to the illustration of  FIG. 20 , the steam line  230   a  may be connected to the main steam line  27   a   1  at a position closer to the steam generator  23   a  than to the vent line  250   a,  and the vent line  250   a  may be connected to the main steam line  27   a   1  at a position farther from the steam generator  23   a  than the steam line  230   a.  Furthermore, a flow resistance region (not shown) may be formed between a connection portion of the steam line  230   a  and a connection portion of the vent line  250   a  in the main steam line  27   a   1 . 
         [0169]      FIG. 21  is a conceptual view illustrating a modified example of the nuclear power plant  20   b  including the passive residual heat removal system  200   b  illustrated in  FIG. 20 . 
         [0170]    A first connection line  270   b  and a vent line  250   b  are formed as individual constituent elements. 
         [0171]    The passive residual heat removal system  200   b  may further include a second pressure drop structure  252   b  configured to locally cause a decrease of pressure based on a principle in which the pressure relatively decreases according to a speed increase of fluid. Referring to  FIG. 21 , the second pressure drop structure  252   b  is provided along with a first pressure drop structure  291   b.    
         [0172]    The second pressure drop structure  252   b  is provided at an internal flow path of a connection portion of the vent line  250   b  and the main steam line  27   b   1  to form a flow of steam circulating through the second circulation flow path to cause a local pressure drop. The detailed structure of the second pressure drop structure  252   b  is illustrated with reference to  FIGS. 2 and 3 . 
         [0173]      FIG. 22  is a still another conceptual view illustrating a passive residual heat removal system  300  and a nuclear power plant  30  including the same according to another embodiment of the present disclosure. 
         [0174]    Referring to  FIG. 22 , a makeup tank  360  is formed in a rectangular shape both end portions of which are formed in a hemispherical shape. Furthermore, a water level of the makeup tank  360  is determined as a first water level. 
         [0175]    The first water level corresponds to a level at which cooling fluid is fully filled in the makeup tank  360  to supply makeup cooling fluid when the water level of the steam generator  33   a  is less than the water level of the makeup tank  360  during an accident. The makeup tank  360  set to the first water level performs only a makeup function, and supplies makeup cooling fluid only when the water level of the steam generator  33   a  is less than that of the makeup tank  360  during an accident. The first water level may be applicable to a case where a flow of cooling fluid of the passive residual heat removal system  300  is insufficient in all conditions during an accident or a case of the nuclear power plant  30  having a characteristic capable of sufficiently performing the performance even though an amount of cooling fluid is somewhat large as a capacity of the condensation heat exchanger  310   a  is designed to be large enough. 
         [0176]    When the water level of the makeup tank  360  is determined as a first water level, it may be possible to remove the circulation line (not shown) described in another drawing in the above. 
         [0177]    The present disclosure illustrates only a case where a water-cooling condensation heat exchanger is applied thereto using an emergency cooling tank, but the present disclosure may be also applicable to a case where an air-cooling condensation heat exchanger is applied thereto by increasing the capacity of the condensation heat exchanger, and further applicable to a case where a water and air hybrid cooling condensation heat exchanger is applied thereto. 
         [0178]    The present disclosure may maintain an optimal amount of cooling fluid of the passive residual heat removal system, and the passive residual heat removal system may exhibit an optimal performance for a long period of time. Furthermore, the present disclosure may prevent non-condensable gas from being accumulated in the passive residual heat removal system. As a result, the present disclosure may enhance accuracy on the performance prediction of the passive residual heat removal system. In addition, the present disclosure may optimize a facility of the passive residual heat removal system to enhance economic efficiency, and alleviate the supercooling phenomenon of the nuclear power plant during an accident through an optimal design, and provide the safety enhancement of the nuclear power plant through the accuracy enhancement of performance prediction. 
         [0179]    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. 
       REFERENCE NUMERALS 
       [0180]      10   a,    10   b,    10   c,    10   d,    10   e,    10   f,    10   g,    10   h,    10   i,    10   j,    20   a,    20   b,    30 : Nuclear power plant 
         [0181]      100   a,    100   b,    100   c,    100   d,    100   e,    100   f,    100   g,    100   h,    100   i,    100   j,    200   a,    200   b,    300 : Passive residual heat removal system 
         [0182]      110   a,    110   b,    110   c,    110   d,    110   e,    110   f,    110   g,    110   h,    110   i,    110   j,    210   a,    210   b,    300 : Condensation heat exchanger 
         [0183]      120   a,    120   b,    120   c,    120   d,    120   e,    120   f,    120   g,    120   h,    120   i,    120   j,    220   a,    220   b,    320 : Emergency cooling tank 
         [0184]      130   a,    130   b,    130   c,    130   d,    130   e,    130   f,    130   g,    130   h,    130   i,    130   j,    230   a,    230   b,    330 : Steam line 
         [0185]      140   a,    140   b,    140   c,    140   d,    140   e,    140   f,    140   g,    140   h,    140   i,    140   j,    240   a,    240   b,    340 : Feedwater line 
         [0186]      250   a,    250   b:  Vent line 
         [0187]      160   a,    160   b,    160   c,    160   d,    160   e,    160   f,    160   g,    160   h,    160   i,    160   j,    260   a,    260   b,    360 : Makeup tank 
         [0188]      170   a,    170   b,    170   c,    170   d,    170   e,    170   f,    170   g,    170   h,    170   i,    170   j,    270   a,    270   b,    370 : First connection line 
         [0189]      180   a,    180   b,    180   c,    180   d,    180   e,    180   f,    180   g,    180   h,    180   i,    180   j,    280   a,    280   b,    380 : Second connection line 
         [0190]      190   a,    190   b,    190   c,    190   d,    190   e,    190   f,    190   g,    190   h,    190   i,    190   j,    290   a,    290   b:  Circulation line 
         [0191]    The present disclosure may be used in the nuclear power plant industry including a passive residual heat removal system.