Abstract:
A hydraulic system for driving a control rod drive mechanism includes a control rod drive water pump, a charging line connected to a discharge side of the control rod drive water pump and including a pressure gauge on the discharge side thereof, a purge line connected to the discharge side of the control rod drive water pump for feeding a coolant to a control rod drive mechanism, and a plurality of hydraulic control units connected to the charging line through charging headers and the purge line, each of said hydraulic control units including an accumulator for storing pressurized water using high-pressure gas and a scrum valve through which the accumulator is connected to the control rod drive mechanism. A charging pump is further connected to the charging line so as to independently charge the accumulators with water at a predetermined pressure.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a control rod drive hydraulic system for driving a control rod drive mechanism of an advanced boiling water reactor (ABWR) so as to perform emergency insertion of control rods (i.e. scram) by hydraulic pressure, and particularly, to a hydraulic system for driving control rod mechanism (which may be called hereafter “control rod drive hydraulic system”) including a pumping system having an improved configuration serving as a pressurizing source for pressurizing water for driving the control rods.  
         [0003]     2. Related Art  
         [0004]     The ABWR employs a fine motion control rod drive mechanism (FMCRD: which will simply be referred to as “control rod drive mechanism” hereafter) for controlling insertion/withdrawal of control rods. This enables power (output) control of a reactor and functions thereof such as operation, shutdown, and so forth.  
         [0005]     The control rod drive mechanism has two types of drive mechanisms. One is a normal-operation drive mechanism for power control by a driving force of an electric motor. The other one is an emergency drive mechanism for scram which utilizes pressurized water. That is, the control rod drive mechanism has two types of sources of the driving force. One is an electric motor which is used as a power control driving source for normal operation. The other one is a control rod drive hydraulic system (CRD hydraulic drive system) serving as a source of pressurized water for driving the control rods. The control rod drive hydraulic system is made up of an accumulator (HCU accumulator), a control rod drive water pump (CRD pump) and the like, which is used as an emergency drive source for insertion of the control rods in a case of scram.  
         [0006]     The control rod drive hydraulic system has a charging line including a pressure gauge provided on the discharge side of the control rod drive water pump. The charging line is connected to multiple hydraulic control units (HCUs) through a charging header. Each hydraulic control unit has an HCU accumulator for storing pressurized water using high-pressure gas. The HCU accumulator is connected to the control rod drive mechanism through a scram valve. In a case of emergency, the HCU accumulator supplies pressurized water to the control rod drive mechanism. This enables scram operation in several seconds or less.  
         [0007]     After the scram reset, the HCU accumulator is charged with pressurized water again. Conventionally, high-head and high-pressure control rod drive water pump (control rod drive water pump) is employed as means for charging the HCU accumulator with pressurized water at a necessary predetermined pressure again after the scram reset (for example, refer to Japanese Patent Laid-open (KOKAI) Publication Nos. HEI 8-62366 and HEI 10-31088).  
         [0008]     The control rod drive water pump has various functions such as a function for supplying cooling water to the control rod drive mechanism through a CRD purge line (FMCRD cooling water supply function), a function for supplying purge water to a reactor internal pump/reactor water clean-up pump (RIP/CUW) and so on in addition to an HCU accumulator charging function which is a function of charging the HCU accumulator with pressurized water through a charging line. Conventional control rod drive water pumps need to have pump head for each function of the control rod drive mechanism as follows, for example:  
         [0009]     About 8 MPa for the FMCRD cooling water supply function;  
         [0010]     Approximately 15 MPa for the HCU accumulator charging function; and  
         [0011]     Approximately 8 MPa for the RIP/CUW pump purge water supply function.  
         [0012]     As described above, the HCU accumulator charging function requires extremely high pump head of approximately 15 MPa for satisfactory operation. Furthermore, the control rod drive water pump needs to operate so as to supply FMCRD cooling water and RIP/CUW pump purge water during the normal (steady) operation. Such functions require an extremely high pump head, high pressurizing, and full-time-operation of the control rod drive water pump. Accordingly, long-time operation of conventional ABWR plants leads to a problem of deterioration in the control rod drive water pump.  
         [0013]     Furthermore, a pump with special specifications is employed as such an ABWR control rod drive water pump. Accordingly, the initial cost of the ABWR control rod drive water pump is higher than that of a BWR plant control rod drive water pump. Furthermore, such an ABWR control rod drive water pump requires great electric power, also leading to a problem of high running cost.  
         [0014]     Furthermore, with conventional arrangements, the high discharge pressure of the control rod drive water pump causes great difference in pressure at a CRD system flow control valve, flow regulator/orifice of the RIP/CUW pump purge line, control rod drive water pump mini flow line orifice or control rod drive water pump mini flow line manual needle valve, and the like, also leading to a problem of erosion thereof in some cases.  
       SUMMARY OF THE INVENTION  
       [0015]     Accordingly, an object of the present invention is to substantially eliminate defects or drawbacks encountered in the prior art mentioned above and to provide a hydraulic system for driving a control rod driving mechanism, which aims that a dedicated low-pressure pump for charging an HCU accumulator or a booster pump is used for handling all or a part of the water charging operation of charging the HCU accumulator of the hydraulic control unit with pressurized water for enabling the control rod drive water pump to maintain suitable performance for a long time, as well as relaxing the specifications required thereof such as high pump head.  
         [0016]     Another object of the present invention is to provide a hydraulic system for driving a control rod drive mechanism capable of improving reliability, i.e., suppressing deterioration in the control rod drive water pump with advantages of reduced costs, reduced power consumption, and so forth.  
         [0017]     A further object of the present invention is to provide a hydraulic system for driving a control rod drive mechanism capable of requiring no full-time operation of the control rod drive water pump at the high discharge pressure so as to suppress erosion occurring at various valves or orifices and charging HCU accumulator with water, which is used for driving the control rods in a sure manner after scram reset.  
         [0018]     These and other objects can be achieved according to the present invention by providing, in one aspect, a hydraulic system for driving a control rod drive mechanism comprising:  
         [0019]     a control rod drive water pump;  
         [0020]     a charging line connected to a discharge side of the control rod drive water pump and including a pressure gauge on the discharge side thereof;  
         [0021]     a purge line connected to the discharge side of the control rod drive water pump for feeding a coolant to a control rod drive mechanism;  
         [0022]     a plurality of hydraulic control units connected to the charging line through a charging header and the purge line, each of the hydraulic control units including an accumulator for storing pressurized water using high-pressure gas and a scrum valve through which the accumulator is connected to the control rod drive mechanism; and  
         [0023]     a charging pump connected to the charging line so as to independently charge the accumulators with water at a predetermined pressure.  
         [0024]     In this aspect, the pressure gauge may be disposed on a downstream side of the charging pump so as to monitor a pressure for accumulating the water in a manner such that in a case of detection of a reduction in the pressure, the charging pump is started and in a case of detection of a completion of the charging operation, the charging pump is stopped.  
         [0025]     The hydraulic system may further include a flow control valve provided downstream of the charging pump, the pressure gauge being disposed on a downstream side of the flow control valve so as to monitor a pressure for charging the water to the accumulator, the flow control valve being operated in a manner such that in a case of detection of a reduction in the pressure, the flow control valve is automatically opened and the charging pump is started so as to charge the accumulator with water at a predetermined pressure, and in a case of detection of a completion of the pressure charging operation, the flow control valve is automatically closed and the charging pump is automatically stopped.  
         [0026]     It is desirable that the charging pump is a pump dedicated for a hydraulic control unit accumulator having a pump head of more than 15 MPa.  
         [0027]     In another aspect of the present invention, there is also provided a hydraulic system for driving a control rod drive mechanism comprising:  
         [0028]     a control rod drive water pump;  
         [0029]     a charging line connected to a discharge side of the control rod drive water pump and including a pressure gauge on the discharge side thereof;  
         [0030]     a purge line connected to the discharge side of the control rod drive water pump for feeding a coolant to a control rod drive mechanism;  
         [0031]     a plurality of hydraulic control units connected to the charging line through a charging header and the purge line, each of the hydraulic control units including an accumulator for storing pressurized water using high-pressure gas and a scrum valve through which the accumulator is connected to the control rod drive mechanism; and  
         [0032]     a charging pump incorporated in the charging line so as to charge the accumulators with water at a predetermined pressure in a simultaneous operation with the control rod drive water pump.  
         [0033]     In this aspect, it is desirable that the charging line includes a bypass line for bypassing the charging pump.  
         [0034]     The pressure gauge may be disposed on a downstream side of the charging pump so as to monitor a pressure for accumulating the water in a manner such that in a case of detection of a reduction in the pressure, the charging pump is started and in a case of detection of a completion of the charging operation, the charging pump is stopped.  
         [0035]     The hydraulic system may further include a flow control valve disposed on a downstream side of the charging pump.  
         [0036]     It is desirable that the charging pump is a booster pump dedicated for a hydraulic control unit accumulator having a pump head of more than 5 MPa.  
         [0037]     In a further aspect of the present invention, there is also provided a hydraulic system for driving a control rod drive mechanism comprising:  
         [0038]     a control rod drive water pump;  
         [0039]     a purge line connected to the discharge side of the control rod drive water pump for feeding a coolant to a control rod drive mechanism;  
         [0040]     a charging pump disposed independently of the purge line so as to charge a water at a predetermined pressure;  
         [0041]     a charging line connected to a discharge side of the charging pump and including a pressure gauge on a downstream side thereof; and  
         [0042]     a plurality of hydraulic control units connected to the charging line through a charging header and the purge line, each of the hydraulic control units including an accumulator for storing pressurized water using high-pressure gas and a scrum valve through which the accumulator is connected to the control rod drive mechanism.  
         [0043]     In this aspect, the pressure gauge may be disposed on a downstream side of the charging pump so as to monitor a pressure for accumulating the water in a manner such that in a case of detection of a reduction in the pressure, the charging pump is started and in a case of detection of a completion of the charging operation, the charging pump is stopped.  
         [0044]     It is desirable that the charging pump is a pump dedicated for a hydraulic control unit accumulator having a pump head of more than 15 MPa.  
         [0045]     According to the present invention of the characters mentioned above, an additional pump is disposed for charging the HCU accumulator with water, independent of the control rod drive water pump. Note that a low-pressure dedicated pump or a booster pump is employed as the additional pump. With such an arrangement, the additional pump performs all or a part of the operation or function of charging the HCU accumulator of the hydraulic control unit with water. This enables the performance of the control rod drive water to be maintained for a long time and relaxes the high pump head specifications required thereof. Accordingly, a control rod drive water pump of the kind employed in BWR plants can be employed in the present invention. This improves reliability of the control rod drive water pump, i.e., suppresses deterioration in this pump. Furthermore, this reduces costs and power consumption.  
         [0046]     Furthermore, with the system according to the present invention, the control rod drive water pump does not need to operate at the high discharge pressure at all times. This improves the useful life of the control rod drive water pump, as well as reducing costs thereof. Furthermore, the system according to the present invention may include the water lines with reduced maximum pressure specifications, except for the charging line, thereby reducing costs thereof.  
         [0047]     In addition, the present invention has the advantages of: relaxation of the pressure-difference conditions for the CRD system flow control valve (conditions for suppressing erosion); relaxation of the pressure-difference conditions for the flow regulator/orifice of the RIP/CUW pump purge line; relaxation of the pressure-difference conditions for the control rod drive water pump mini flow line orifice; relaxation of the pressure-difference conditions for the control rod drive water pump mini flow line manual needle valve (conditions for suppressing erosion); and operation of the control rod drive mechanism with reduced driving force, thereby allowing the costs of the system to be further reduced.  
         [0048]     The function of the charging pump for charging the HCU accumulator with water does not require high-speed operation. Accordingly, a small-capacity and high-head pump can be employed with no difficulty. Thus, the system employing such a pump has a sufficient function of charging the HCU accumulator with water for driving the control rods in a sure manner.  
         [0049]     The nature and further characteristic features of the present invention will be made clear from the following descriptions made with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0050]     In the accompanying drawings:  
         [0051]      FIG. 1  is a block diagram showing a configuration or structure of a hydraulic system for driving a control rod drive mechanism according to a first embodiment of the present invention;  
         [0052]      FIG. 2  is a block diagram showing a configuration or structure of a hydraulic system for driving a control rod drive mechanism according to a second embodiment of the present invention; and  
         [0053]      FIG. 3  is a block diagram showing a configuration or structure of a hydraulic system for driving a control rod drive mechanism according to a third embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0054]     A hydraulic system for driving a control rod drive mechanism according to embodiments of the present invention will be described hereunder with reference to the accompanying drawings. Further, it is to be noted that the hydraulic system for driving the control rod drive mechanism of the present invention may be described hereunder merely as control rod drive hydraulic system for the sake of convenience.  
       First Embodiment  
       [0055]      FIG. 1  is a block diagram showing a configuration of a control rod drive hydraulic system according to a first embodiment of the present invention.  
         [0056]     Referring to  FIG. 1 , a control rod drive hydraulic system  1  includes a control rod drive water pump  2  having a discharge port which is branched to a purge water line  3  for supplying cooling water to a control rod drive mechanism and a charging water line  4  for supplying water to an HCU accumulator so as to charge the HCU accumulator with water. The charging water line  4  is connected to a charging header  9  with an orifice  5 , a check valve  6 , an accumulator  7  and a pressure gauge  8 , provided in that order at suitable positions on the way of the line  4 .  
         [0057]     Multiple charging line branch pipes  4   a ,  4   b , . . . ,  4   n  are connected to the charging header  9 . Each of the charging line branch pipes  4   a ,  4   b , . . . ,  4   n  is connected to each of hydraulic control units (HCU)  10  ( 10   a ,  10   b , ---,  10   n ) to thereby form a charging line  11 .  
         [0058]     The hydraulic control unit  10  ( 10   a ) includes a drive water line  12  connected to the charging line branch pipes  4   a ,  4   b , . . . ,  4   n  of the charging line  11 . The drive water line  12  includes a charging line flow control valve  13  and a charging line check valve  14 .  
         [0059]     The drive water line  12  includes an HCU accumulator  15 . A nitrogen container  17  is connected to the HCU accumulator  15  through a connection line  16 .  
         [0060]     Furthermore, the drive water line  12  includes a scram valve  18  provided on the downstream side of the HCU accumulator  15 . The drive water line  12  is branched to a drive water branch pipes  12   a  and  12   b  downstream of the scram valve  18 . The drive water branch pipes  12   a  and  12   b  are connected to control rod drive mechanisms (CRDs)  20   a  and  20   b  through gate valves  19   a  and  19   b.    
         [0061]     The purge water line  3  for supplying cooling water to the control rod drive mechanisms  20   a  and  20   b  is connected to the scram valve  18  on the downstream side of the drive water lines  12   a  and  12   b , thus forming a CRD purge line  21 . Note that a flow meter  22  and a flow control valve  23  serving as a discharge valve are provided to the CRD purge line  21  on the discharge side of the control rod drive water pump  2 .  
         [0062]     With such an arrangement of the first embodiment of the present embodiment, the pump line  4   a  is connected to the charging line  11  on the upstream side of the pressure gauge  8 . Furthermore, a charging pump  24  is provided to the pump line  4   a . It is to be noted that the charging pump  24  has a function for charging the HCU accumulator  15  with water at a necessary predetermined pressure independently without any assistance.  
         [0063]     A high-head and small capacity pump such as a plunger pump or the like is employed as the charging pump  24 , for example. The charging pump  24  is employed as a dedicated pump for charging the HCU accumulator with water at a pump head of 15 MPa or more.  
         [0064]     With such an arrangement, the charging pressure in the HCU accumulator  15  is monitored using the pressure gauge  8  disposed downstream of the charging pump  24 . In a case of detecting a reduction of the charging pressure, the charging pump  24  starts to be operated. Upon completion of charging operation, the charging pump  24  is stopped.  
         [0065]     Specifically, a flow control valve  25  serving as a discharge valve is provided downstream of the charging pump  24 . The pressure gauge  8  is provided downstream of the flow control valve  25  so as to monitor the pressure at which the HCU accumulator  15  is charged with water. In a case of detecting a reduction in the pressure, the flow control valve  25  is automatically opened, and the charging pump  24  is started, thereby charging the HCU accumulator  15  with water at a necessary pressure. On the other hand, upon completion of the charging operation, the flow control valve  25  serving as a discharge valve is automatically closed, and the charging pump  24  is automatically stopped.  
         [0066]     Furthermore, the charging water line  4  includes a line switching valve  26  for switching the line between the line of the control rod drive water pump  2  and the line of the charging pump  24 . This enables the pump operation mode to be switched between two kinds of single (solo) pump operation modes, i.e., the pump operation using the control rod drive water pump  2  alone and the pump operation using the charging pump  24  alone.  
         [0067]     Such a configuration enables selection of the pump operation mode in which the HCU accumulator  15  is charged with water using the charging pump  24  alone without using the control rod drive water pump  2 . In this pump operation mode, the HCU accumulator  15  of the hydraulic control unit  10  is charged with water using the charging pump  24  alone, which is a dedicated pump for charging the HCU accumulator with water.  
         [0068]     With the present embodiment, after the scram reset, the pump operation mode is switched from the pump operation mode using the control rod drive water pump  2  to the pump operation mode using the charging pump  24  alone, thereby enabling the HCU accumulator  15  to be charged with water at a necessary pressure. This enables the performance of the control rod drive water pump  2  to be maintained for a long time and relaxes the high pump-head specifications required thereof. Accordingly, a control rod drive water pump of the kind employed in conventional, i.e., existing, BWR plant can be employed in the present embodiment. This improves reliability of the control rod drive water pump and suppresses deterioration in this pump. Furthermore, the costs, power consumption, and the like can be reduced effectively.  
         [0069]     Furthermore, with the present embodiment, the control rod drive water pump  2  does not need to operate at the maximum pressure specification at all times, so that the useful life of the control rod drive water pump  2  can be improved with reduced costs thereof.  
         [0070]     In addition, the present embodiment allows the reduction in the maximum pressure specification of the lines other than the charging line  11 , so that the costs for these lines can be reduced. Furthermore, the present embodiment provides the following advantages of: relaxation of the pressure-difference conditions for the CRD system flow control valve (conditions for suppressing erosion); relaxation of the pressure-difference conditions for the flow regulator/orifice of the RIP/CUW pump purge line; relaxation of the pressure-difference conditions for the control rod drive water pump mini flow line orifice; relaxation of the pressure-difference conditions for the control rod drive water pump mini flow line manual needle valve (conditions for suppressing erosion); and operation of the control rod drive mechanism with reduced driving force to thereby allow the costs of the system to be further reduced.  
         [0071]     According to the described first embodiment, since the charging pump  24  for charging the HCU accumulator  15  with water does not require the functions of high-speed operation, a small-capacity and high-head pump can be employed as the charging pump  24  with no difficulty. Thus, the system employing such a pump can provide a sufficient function of charging the HCU accumulator  15  with water for driving the control rods in a sure manner.  
       Second Embodiment  
       [0072]      FIG. 2  is a block diagram which shows a configuration of a control rod drive hydraulic system according to a second embodiment of the present invention.  
         [0073]     As shown in  FIG. 2 , the control rod drive hydraulic system  1  according to this embodiment has a configuration, different from that of  FIG. 1 , in which the charging line  11  includes the charging pump  24 . The charging pump  24  operates along with the control rod drive water pump  2 , thereby charging the HCU accumulator  15  at a necessary pressure.  
         [0074]     That is, the charging pump  24 , serving as a booster pump for charging the HCU accumulator, is provided to the charging line  11  in series with the control rod drive water pump  2 . The charging pump  24  has pump head of 5 to 7 MPa or more, for example.  
         [0075]     Furthermore, the charging line  11  includes a bypass line  27  in parallel with the charging pump  24 . With such an arrangement, a bypass valve  27   a  is opened until the pressure drops to a predetermined value at which the control rod drive water pump  2  alone cannot charge the HCU accumulator  15 . That is, the HCU accumulator  15  is charged with water using the control rod drive water pump  2  alone through the bypass line  27  until this point in time.  
         [0076]     Accordingly, with the present embodiment, the charging operation is performed using the control rod drive water pump  2  alone until the water-charging pressure drops to a predetermined value, for example, and subsequently, the charging pump  24  operates as an additional booster pump, thereby enabling the charging operation at a necessary pressure in the final stage.  
         [0077]     Further, a flow control valve  28  is provided downstream of the charging pump  24  as a discharge valve or drain valve. This prevents the water, with which the HCU accumulator  15  has been charged at a necessary pressure, from flowing backward to the control rod drive water pump  2 .  
         [0078]     Furthermore, with the system according to this embodiment, the pressure gauge  8  is also provided downstream of the charging pump  24 . The pressure at which the HCU accumulator  15  is charged with water is monitored with the pressure gauge  8 . The hydraulic system according to the present embodiment has a controlling structure in which in a case of detection of a reduction in the pressure, the charging pump  24  is started. Furthermore, with such a controlling structure, upon completion of the charging operation, the charging pump  24  is stopped.  
         [0079]     Specifically, the flow control valve  28  serving as a discharge valve is provided on the downstream side of the charging pump  24 . The pressure gauge  8  is also provided on the downstream side of the flow control valve  28  so as to monitor the pressure at which the HCU accumulator  15  is charged with water. In a case of detecting a reduction in the pressure, the flow control valve  28  is automatically opened, and the charging pump  24  is started, so that additional pressurizing is thereby applied to the water with which the HCU accumulator  15  is charged. On the other hand, upon completion of the charging operation, the flow control valve  28  is automatically closed, and the charging pump  24  is automatically stopped.  
         [0080]     The other components are generally the same as those of the first embodiment. Accordingly, the corresponding components shown in  FIG. 2  are denoted by the same reference numerals as those shown in  FIG. 1 , and description thereof will be omitted.  
         [0081]     With the system according to the second embodiment described above, the control rod drive water pump  2  and the charging pump  22  are started in order after the scram reset, and these two pumps operate in cooperation with each other. Alternatively, these two pumps may be started at the same time. At any rate, either arrangement enables HCU accumulator  15  to be charged with water at a necessary pressure.  
         [0082]     With such an arrangement, the charging pump  24  is employed as a dedicated pump for charging the HCU accumulator  15  with water. The charging pump  24  serves as an additional pump for charging the HCU accumulator  15  with water at necessary pressure. Thus, such an arrangement enables the performance of the control rod drive water pump  2  to be maintained for a long time, as well as relaxing the specifications required thereof with respect to the high pump head. Furthermore, a control rod drive water pump of the kind employed in conventional BWR plants can be employed as the control rod drive water pump  2  according to the present embodiment. This improves reliability, i.e., suppresses deterioration in the control rod drive water pump, and reduces costs and power consumption.  
         [0083]     In addition, with the present embodiment, the control rod drive water pump  2  does not need to operate at the maximum pressure specification at all times. This improves the useful life of the control rod drive water pump  2 , as well as reducing costs thereof.  
         [0084]     Furthermore, this second embodiment allows the reduction in the maximum pressure specification of the lines other than the charging line  11 . This enables costs for these lines to be reduced. Furthermore, this embodiment has the advantages of: relaxation of the pressure-difference conditions for the CRD system flow control valve (conditions for suppressing erosion); relaxation of the pressure-difference conditions for the flow regulator/orifice of the RIP/CUW pump purge line; relaxation of the pressure-difference conditions for the control rod drive water pump mini flow line orifice; relaxation of the pressure-difference conditions for the control rod drive water pump mini flow line manual needle valve (conditions for suppressing erosion); and operation of the control rod drive mechanism with reduced driving force, thereby allowing the costs of the system to be further reduced.  
         [0085]     Further, the charging pump  24  for charging the HCU accumulator  15  with water does not require the function of the high-speed operation. Accordingly, a small-capacity and high-head pump can be employed as the charging pump  24  according to the present invention with no difficulty. Thus, the hydraulic system employing such a pump has a sufficient function of charging the HCU accumulator  15  with water for driving the control rods in a sure manner.  
         [0086]     With the present embodiment, the pressure of the charging line is monitored. In a case of detecting a reduction in the pressure, the charging pump serving as an additional pump is started, thereby enabling the water to be pressurized at a predetermined pressure.  
       Third Embodiment  
       [0087]      FIG. 3  is a block diagram which shows a configuration of a control rod drive hydraulic system according to a third embodiment of the present invention.  
         [0088]     As shown in  FIG. 3 , the control rod drive hydraulic system  1  includes the cooling water line  21  having the control rod drive water pump  2  and the charging line  11  having the charging pump  24  for scram, independent of each other.  
         [0089]     That is, the charging line  11  includes the charging pump  24  having a function of charging the HCU accumulator  15  with water at a necessary pressure independently. The charging line  11 , including the charging pump  24 , is provided, independent of the cooling water line  21 , which has the control rod drive water pump  2 .  
         [0090]     Furthermore, with regard to the charging line  11  according to this third embodiment, the pressure gauge  8  is also provided downstream of the charging pump  24 . The pressure at which the HCU accumulator  15  is charged with water is monitored with the pressure gauge  8 . The hydraulic system according to this embodiment has a controlling structure in which in a case of detection of a reduction in the pressure, the charging pump  24  is started. Furthermore, with such a control configuration, upon completion of the charging operation, the charging pump  24  is stopped.  
         [0091]     Specifically, a flow control valve  29  serving as a discharge valve is provided downstream of the charging pump  24 . The pressure gauge  8  is provided downstream of the flow control valve  29  and monitors the pressure at which the HCU accumulator  15  is charged with water. In a case of detecting a reduction in the pressure, the flow control valve  29  is automatically opened, and the charging pump  24  is started, thereby charging the HCU accumulator  15  with water at a necessary pressure. On the other hand, upon completion of the charging operation, the flow control valve  29  is automatically closed, and the charging pump  24  is automatically stopped.  
         [0092]     On the other hand, with the cooling water line  21 , purge water is supplied to the control rod drive mechanisms  20   a  and  20   b.    
         [0093]     The other components are generally the same as those of the first embodiment. Accordingly, the corresponding components shown in  FIG. 3  are denoted by the same reference numerals as those shown in  FIG. 1 , and description thereof will be omitted.  
         [0094]     The hydraulic system according to the third embodiment described above has a function of charging the HCU accumulator  15  with water using the charging pump  24  alone, independent of the control rod drive water pump  2 . With such an arrangement, the HCU accumulator  15  of the hydraulic control unit  10  is charged with water using the charging pump  24  alone, which is a dedicated pump for charging the HCU accumulator with water.  
         [0095]     Thus, the pump in operation is switched from the control rod drive water pump  2  to the charging pump  24  after the scram reset. Then, the HCU accumulator  15  is charged with water at a necessary pressure using the charging pump  24  alone. Thus, such an arrangement enables the performance of the control rod drive water pump  2  to be maintained for a long time, as well as relaxing the specifications required thereof with respect to high pump head. Furthermore, a control rod drive water pump of the kind employed in conventional BWR plants can be employed as the control rod drive water pump  2  according to the present embodiment. This improves reliability, i.e., suppresses deterioration in the control rod drive water pump, and reduces costs and power consumption.  
         [0096]     Furthermore, with the present embodiment, the control rod drive water pump  2  does not need to operate at the maximum pressure specification at all times. This improves the useful life of the control rod drive water pump  2 , as well as reducing costs thereof.  
         [0097]     In addition, this third embodiment allows the reduction in the maximum pressure specification of the lines other than the charging line  11 . This enables the costs for these lines to be reduced. Furthermore, the present embodiment has the advantages of: relaxation of the pressure-difference conditions for the CRD system flow control valve (conditions for suppressing erosion); relaxation of the pressure-difference conditions for the flow regulator/orifice of the RIP/CUW pump purge line; relaxation of the pressure-difference conditions for the control rod drive water pump mini flow line orifice; relaxation of the pressure-difference conditions for the control rod drive water pump mini flow line manual needle valve (conditions for suppressing erosion); and operation of the control rod drive mechanism with reduced driving force, thereby allowing the costs of the system to be further reduced.  
         [0098]     Further, the charging pump  24  for charging the HCU accumulator  15  with water does not require the function of high-speed operation. Accordingly, a small-capacity and high-head pump can be employed as the charging pump  24  according to the present invention with no difficulty. Thus, the system employing such a pump has a sufficient function of surely charging the HCU accumulator  15  with water for driving the control rods.  
         [0099]     It is further to be noted that the present invention is not limited to the described embodiments and many other changes and modifications may be made without departing from the scopes of the appended claims.