Abstract:
The present invention is characterized by a reactor cooling system, which comprises a lower drywell which is a space for containing a bottom side portion of the reactor pressure vessel, the lower drywell being disposed in a lower portion of the reactor pressure vessel; reactor recirculation pumps for circulating cooling water in the reactor pressure vessel, the reactor recirculation pump being disposed in the bottom side portion of the reactor pressure vessel in such a manner that a side of a motor portion of the reactor recirculation pump is projected into the lower drywell; and heat exchangers disposed in the lower drywell, the cooling water circulated by the reactor recirculation pump passing through the heat exchanger, wherein number of the reactor recirculation pumps is 4 or 6, and the reactor recirculation pumps are arranged with nearly equal angular spacing.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a reactor cooling system comprising recirculation internal pumps (internal pumps: hereinafter, referred to as RIPS) for circulating cooling water in a reactor pressure vessel.  
         [0003]     2. Prior Art  
         [0004]     An RIP system in a conventional advanced boiling water reactor (hereinafter, referred to as ABWR) will be described below, referring to  FIG. 9 ,  FIG. 10  and  FIG. 11 .  
         [0005]      FIG. 9  is a plan view showing the arrangement of conventional RIP systems, and  FIG. 10  is a detailed plan view showing the arrangement of the conventional RIP systems, and shows the ¼-portion.  FIG. 9  and  FIG. 10  show the arrangement of the RIP systems in a lower drywell of a reactor containment.  FIG. 11  shows the system construction of a conventional power system.  
         [0006]     In the conventional ABWR, ten RIP systems are nearly equally spaced in the circumferential direction of the lower portion of the reactor pressure vessel, as shown in  FIG. 9 . Arc-shaped through-cutouts  19 ,  20  are formed at two positions in peripheral portions of an upper grid plate  17  and an upper shroud  16  so that the upper grid plate  17  and the upper shroud  16  do not interfere with the RIP  1  when the RIP  1  is withdrawn for maintenance and inspection. The reason why number of the through-cutouts formed is not ten, which is equal to number of the RIPs, is that there are two header pipes  18  having an arc shape in approximately {fraction (1/4)}-circumference for a core injection system which interfere with the withdrawal of the RIP. Therefore, the through-cutouts are formed at the minimal two positions.  
         [0007]     During RIP maintenance work, the recirculation internal pump  1  is turned in the circumferential direction in a downcomer region between the reactor pressure vessel  5  and the core shroud from a given installed position of the RIP  1  to one of the two cutout portions  19 ,  20  through which the RIP  1  is withdrawn. As shown in  FIG. 10 , each of the RIP systems is constructed by connecting one RIP  1  and one heat exchanger  4  disposed most closely to each other by connecting pipes  9  and  10 . The reason why one heat exchanger is combined with one RIP is that in a case where, for example, two RIPS (RIP (A) and RIP (B)) are connected to one heat exchanger, if the RIP (A) stops operation during normal operation, the RIP (B) continues to be operated and the operating RIP (B) can be not sufficiently cooled because part of cooling water passes through the stopping RIP (A). In order to avoid this problem, it is necessary to provide a check valve in the loop. However, provision of the check valve increases the pressure loss in the loop to decrease the flow amount of the cooling water. Therefore, in order to ensure soundness of the component and eliminate an influence of a single-failure, the RIP system has been constructed by combining one heat exchanger with one RIP.  
         [0008]     A construction of a conventional RIP power system will be described below, referring to  FIG. 11 . In a conventional ABWR, the reactor output power is changed by controlling rotating speed of the RIPs  1  contained in the reactor pressure vessel  5  to change the core flow rate. Control of pump rotating speed of the ten RIPs ( 1   a  to  1   j ) is performed using stationary variable frequency power supplies RIP-ASDs ( 2   a  to  2   j ) provided for the individual pumps. Regarding the RIP-ASDs ( 2   a  to  2   j ), the stationary variable frequency power supplies RIP-ASDs ( 2   a ,  2   b ) are connected to a bus line  3   a  installed in the power station; and the stationary variable frequency power supplies RIP-ASDs ( 2   c  to  2   e ) are connected to a bus line  3   b ; the stationary variable frequency power supplies RIP-ASDs ( 2   f ,  2   g ) are connected to a bus line  3   c ; and the stationary variable frequency power supplies RIP-ASDs ( 2   h  to  2   j ) are connected to a bus line  3   d.    
         [0009]     Electric power generated by a turbine generator is transmitted to the outside of the power station though a circuit breaker  8   a ,  8   b  and a power transmission line  6   a ,  6   b . Part of the generated electric power is distributed to the inside of the power station through the bus line  3  and the branched bus lines  3   a  to  3   d . Power supplies for driving the recirculation pumps  1  are three systems of the normal power supply from the bus line  3 , a diesel-driven generator  7   a  and a diesel-driven generator  7   b.    
       SUMMARY OF THE INVENTION  
       [0010]     Although most of conventional nuclear reactors have been large-sized, improvement of middle- and small-sized nuclear reactors is recently studied in addition to the large-sized nuclear reactors. In the middle- and small-sized nuclear reactor, it becomes more difficult to perform maintenance and inspection of RIPs and heat exchangers disposed in a lower drywell because the size of the lower drywell becomes smaller.  
         [0011]     Further, it is desirable that a pump runner of the RIP can be withdrawn as easily as possible because maintenance and inspection of the runner is performed by being withdrawn out of the nuclear pressure vessel.  
         [0012]     An object of the present invention is to provide a reactor cooling system of which maintenance and inspection can be easily performed by solving the above problems.  
         [0013]     The present invention is characterized by a reactor cooling system, which comprises a lower drywell which is a space for containing a bottom side portion of the reactor pressure vessel, the lower drywell being disposed in a lower portion of the reactor pressure vessel; reactor recirculation pumps for circulating cooling water in the reactor pressure vessel, the reactor recirculation pump being disposed in the bottom side portion of the reactor pressure vessel in such a manner that a side of a motor portion of the reactor recirculation pump is projected into the lower drywell; and heat exchangers disposed in the lower drywell, the cooling water circulated by the reactor recirculation pump passing through the heat exchanger, wherein number of the reactor recirculation pumps is 4 or 6, and the reactor recirculation pumps are arranged with nearly equal angular spacing.  
         [0014]     Further, the present invention is characterized by a reactor cooling system, which comprises a lower drywell which is a space for containing a bottom side portion of the reactor pressure vessel, the lower drywell being disposed in a lower portion of the reactor pressure vessel; reactor recirculation pumps for circulating cooling water in the reactor pressure vessel, the reactor recirculation pump being disposed in the bottom side portion of the reactor pressure vessel in such a manner that a side of a motor portion of the reactor recirculation pump is projected into the lower drywell; a lower shroud for containing fuel rods therein, the lower shroud being disposed inside the reactor pressure vessel; and an upper shroud mounted on the lower shroud, the upper shroud having an outer diameter lager than an outer diameter of the lower shroud, wherein a runner of each of the reactor recirculation pumps driven by the motor portion is disposed in an inner bottom portion of the reactor pressure vessel and between an inner periphery of the reactor pressure vessel and an outer periphery of the lower shroud, and a through-cutout capable of passing the runner therethrough is formed corresponding to each of the runners at a position just above the runner in an outer peripheral side of the upper shroud. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0015]      FIG. 1  is a vertical cross-sectional view of a reactor containment, and shows an embodiment in accordance with the present invention.  
         [0016]      FIG. 2  is a cross-sectional view of a transverse section of the central portion of a reactor pressure vessel, and shows an embodiment in accordance with the present invention.  
         [0017]      FIG. 3  is a view showing the arrangement of reactor recirculation pumps and a heat exchanger in a reactor containment, and shows an embodiment in accordance with the present invention.  
         [0018]      FIG. 4  is a diagram of a power supply system for the reactor recirculation pumps, and shows an embodiment in accordance with the present invention.  
         [0019]      FIG. 5  is a vertical cross-sectional view of a reactor pressure vessel, and shows an embodiment in accordance with the present invention.  
         [0020]      FIG. 6  is an enlarged view of the portion (A) of  FIG. 5 , and shows an embodiment in accordance with the present invention.  
         [0021]      FIG. 7  is a cross-sectional view of an upper shroud and a lower shroud which shows portions where through-cutouts are formed, and shows an embodiment in accordance with the present invention.  
         [0022]      FIG. 8 ( a ) is a view showing an example of a conventional arrangement of reactor recirculation pumps, heat exchangers, secondary cooling water inlet pipes and secondary cooling water outlet pipes, and the conventional arrangement is shown for comparison with an embodiment of the present invention shown in  FIG. 8 ( b ).  
         [0023]      FIG. 8 ( b ) is a view showing an arrangement of reactor recirculation pumps, heat exchangers, secondary cooling water inlet pipes and secondary cooling water outlet pipes, of an embodiment in accordance with the present invention.  
         [0024]      FIG. 9  is a view showing a conventional example, and corresponds to  FIG. 2 .  
         [0025]      FIG. 10  is a view showing a conventional example, and corresponds to  FIG. 3 .  
         [0026]      FIG. 11  is a view showing a conventional example, and corresponds to  FIG. 4 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]     Embodiments in accordance with the present invention will be described below, referring to  FIG. 1  to  FIG. 8 . Repetition of description on things common to the conventional ones will be avoided by attaching common reference characters as far as possible.  
         [0028]     As shown in  FIG. 1 ,  FIG. 4  and  FIG. 5 , four RIPs (reactor recirculation pumps)  1  are arranged in a lower head portion (a bottom portion side) of a reactor pressure vessel (RPV)  5  with 90° spacing in the peripheral direction. Number of the RIPs may be six. Even number, four or six, of the reactor recirculation pumps are arranged in the outer peripheral side of the reactor pressure vessel.  
         [0029]     The reactor recirculation pump  1  comprises a motor portion  70  and a pump portion  71 , and a runner  72  disposed in the pump portion  71  is detachably supported by a driving shaft  73  extending from the motor portion  70 . The motor portion  70  of the reactor recirculation pump  1  is attached to the reactor pressure vessel (RPV)  5  so as to project from the outer side bottom portion of the reactor pressure vessel (RPV)  5 , and the pump portion  71  is placed so as positioned inside the reactor pressure vessel (RPV)  5 .  
         [0030]     The reactor recirculation pumps  1  (RIPs  1 ) forcedly circulate coolant (liquid such as cooling water or the like) of the reactor inside the reactor pressure vessel (RPV) to promote heat removal and steam generation in the core, and serve the function of controlling reactor output power by increasing and decreasing the core flow rate.  
         [0031]     In a case where the RIPs of the conventional ABWR, in which the RIP heat exchangers are connected to the RIPs in a one-to-one relationship, are applied to a middle- and small-reactor, it can be considered from only the viewpoint of geometrical arrangement that the minimum necessary arrangement and number of RIPs is a structure of arranging three RIPs with 120° spacing in the peripheral direction. In the case of three-RIP structure, it is practical from the viewpoint of unitization and operability that three RIP heat exchangers are also installed.  
         [0032]     Since the four-RIP structure is employed in the present invention, the system can be simplified by sharing one RIP heat exchanger  4  between the two RIPs though the minimum necessary number of the RIPs is increased by one.  
         [0033]     Therein, the sharing of the RIP heat exchanger is on the premise that partial operation of the RIPs will be not performed. This is to be described in detail later.  
         [0034]     The lower drywell  81  is arranged in the center of the reactor containment  80 . The reactor pressure vessel (RPV) is installed so that the bottom portion side of the reactor pressure vessel (RPV) is placed into a space of the lower drywell. Since the lower drywell  81  is formed large enough for the outer diameter of the reactor pressure vessel  80  and deep enough for the bottom of the reactor pressure vessel (RPV), the lower drywell  81  can contain the reactor recirculation pumps  1 , the heat exchangers  4 , the secondary cooling water inlet pipes  13  and the secondary cooling water outlet pipes  14 .  
         [0035]     A space  82  having a small diameter is formed under a central bottom of the lower drywell  81 . This space  82  is a room for installing burn-up control rods  83 . The space  82  is partitioned from the lower drywell  81  by a mesh-grid bottom plate  84 . That is, the floor of the lower dryer  81  is formed by the mesh-grid bottom plate  84 .  
         [0036]     In the lower drywell  81 , the reactor recirculation pumps  1 , the heat exchangers  4 , connecting pipes  9  and  10 , the secondary cooling water inlet pipes  13  and the secondary cooling water outlet pipes  14  are arranged as shown in FIGS.  8 ( a ) and  8 ( b ). In the conventional large-sized reactor, these components are intricately arranged, as shown in  FIG. 8 ( a ). However, in the middle- and small-sized reactor in accordance with the present invention, these components are arranged apart from one another. The inside of the lower drywell  81  is narrowed by placing these components. However, since the lower drywell  81  in accordance with the present invention is less intricate and has the extra space compared to the conventional lower drywell, maintenance and inspection work of the reactor recirculation pumps  1 , the heat exchangers  4 , the secondary cooling water inlet pipes  13  and the secondary cooling water outlet pipes  14  can be easily performed inside the lower drywell  81 . Since there are radiant rays leaking from the reactor pressure vessel (RPV) inside the lower drywell  81 , it is important to shorten the working time by improving workability of the maintenance and inspection work.  
         [0037]     One heat exchanger  4  may be provided for one reactor recirculation pump  1 . However, by providing one heat exchanger  4  for two reactor recirculation pumps  1 , spare area inside the lower drywell  81  is increased to make the inspection work easier.  
         [0038]     Further, the nuclear power plant can be substantially simplified because of small number of the reactor recirculation pumps, and because of small numbers of the connecting pipes  9  and  10 , the secondary cooling water inlet pipes  13  and the secondary cooling water outlet pipes  14  are also small, and particularly because of only two heat exchangers  4 , which can be understood from FIGS.  8 ( a ) and  8 ( b ).  
         [0039]      FIG. 4  shows an embodiment of a power supply system for RIP control units in accordance with the present invention.  
         [0040]     Pump rotation speeds of the four reactor recirculation pumps  1  (RIPS  1   a  to  1   d ) are controlled by the stationary variable frequency power supplies (RIP-ASDS). Partial operation of the reactor recirculation pumps  1  (RIPs) is not performed, and all the RIPs are stopped at once when at least one of the four RIPs stops during normal operation. Since the driving power supply for the RIP needs not to be divided, driving power is supplied to all the four RIPs from one power source of a bus line  3  inside the power station. Therefore, the system can be simplified.  
         [0041]     However, employing of the single-train power supply system can be considered possible only in the RIP system that RIP rotation speed can be maintained for a necessary time period or longer by mechanical inertia at an event of loss of power (tripping of the four RIPS) or the like to sufficiently moderate thermal influences on the fuels by relaxing a rapid change in the core flow rate. In a case where thermal influences on the fuels can not be moderated at tripping of the four RIPS, employing of a two-train power supply system and provision of an MG set (diesel-driven generator) are required.  
         [0042]     Further, although the present embodiment has the RIP-ASDs  2   a  to  2   d  for the individual pumps, the plant can be further simplified by reducing the number of the RIP-ASDs as small as possible because the partial operation of the reactor recirculation pumps  1  is not performed.  
         [0043]     Maintenance and inspection of the runner  72  in the reactor recirculation pump  1  will be described below, referring to  FIG. 6  and  FIG. 7 .  
         [0044]     Description will be made including the internal constructions of the reactor pressure vessel (RPV)  5  because the runner  72  is placed inside the reactor pressure vessel (RPV)  5 .  
         [0045]     The reactor pressure vessel (RPV)  5  is composed of a vertically long cylinder portion, a spherical bottom head and an upper head.  
         [0046]     The lower shroud  23  placed in the reactor pressure vessel (RPV)  5  has a cylindrical body portion and is so placed that the body portion becomes concentric with the reactor pressure vessel. The lower end side of the lower shroud  23  is supported by an inner bottom portion of the reactor pressure vessel (RPV)  5 . The pump portion  71  and the runner  72  are placed between the outer periphery of the lower shroud  23  and the inner periphery of the reactor pressure vessel (RPV)  5  and near the inner bottom portion of the reactor pressure vessel (RPV)  5 . The portion placing the pump portions  71  and the runners  72  is a narrow annular groove vertically extending.  
         [0047]     The core support plate  25  is arranged in a sublevel of the lower shroud  23 , and the upper grid plate  17  is supported in the upper portion of the lower shroud  23 . A region between the core support plate  25  and the upper grid plate  17  mainly corresponds to the core. Fuel rods  15  placed in the region are supported by the core support plate  25  and the upper grid plate  17 . The upper grid plate  17  is fixed to the lower shroud  23  by grid attaching bolts  29 .  
         [0048]     The upper shroud  16  has a cylindrical body portion. The upper grid plate  17  is arranged in the lower end side of the body portion of the upper shroud  16 , and an upper shroud fringe portion  32  is arranged in the upper end side of the upper shroud  16 . The upper grid plate  17  and the upper shroud fringe portion  32  are fixed to the upper shroud by welding.  
         [0049]     The diameter of the outer periphery of the upper shroud is formed larger than that of the outer periphery of the lower shroud. That is, the diameter of the body portion of the upper shroud is formed larger than that of the body portion of the lower shroud. In the outer peripheral side of the upper shroud, through-cutouts  19 ,  20 ,  21  and  22  capable of passing the runners  72  therethrough are formed at positions just above the runners  72  so as to correspond to the individual reactor recirculation pumps  1 . Since the reactor recirculation pumps  1  are arranged with 90° spacing, the through-cutouts  19 ,  20 ,  21  and  22  are also arranged with 90° spacing. Each of the through-cutouts  19 ,  20 ,  21  and  22  is vertically formed over the wall of the upper shroud from the upper grid plate  17  to the upper shroud fringe portion  32 . Each of the through-cutouts  19 ,  20 ,  21  and  22  is arc-shaped so as to match with the shape of the runner  72 , but the through-cutout having another shape may be acceptable if the runner  72  can pass through. Although each of the through-cutouts  19 ,  20 ,  21  and  22  looks as if it were formed by being cut out from the outer peripheral side, the through-cutout in the body portion of the upper shroud  16  is formed by pressing the appropriate positions of the body portion toward the inner side.  
         [0050]     A shroud head  24  is placed on the upper side of the upper shroud. Steam separators  27  are arranged on the shroud head  24  through stand pipes  26 . A rim body  33  is provided in the outer peripheral side of the shroud head  24 , and a rim body fringe portion  31  is provided in the lower end of the rim head  33 . The shroud head  24  is mounted on the upper shroud by putting the rim body fringe portion  31  on the upper shroud fringe portion  32 . The shroud head  24  is fixed to the upper shroud by fastening the rim body fringe portion  31  and the upper shroud fringe portion  32  using long shroud head bolts  30 . A steam dryer assembly  28  is arranged above the steam separators  27 .  
         [0051]     The maintenance and inspection work of the runners  72  of the reactor recirculation pumps  1  is performed as follows. Initially, the upper head of the reactor pressure vessel (RPV)  5  is removed. Next, the steam separators  27  are removed after removing the steam dryer assembly. The upper shroud supporting the fuel rods and the core portion of the lower shroud are not removed from and left in the reactor pressure vessel (RPV)  5 .  
         [0052]     Then a tool for removing the runner is lowered down from the upper side of the reactor pressure vessel (RPV)  5 , and the runner is taken off from the pump portion  71  existing in the bottom portion of the narrow annular groove formed between the outer periphery of the lower shroud  23  and the inner periphery of the reactor pressure vessel (RPV)  5  using the tool, and the runner is taken out to the outside through the through-cutout while the runner is being held with the tool, and then the maintenance and inspection work is performed.  
         [0053]     The series of jobs relating to taking-out of the runner for maintenance and inspection are difficult to perform, but the runner can be taken out to the outside of the reactor pressure vessel (RPV)  5  by lowering the tool directly downward through the through cutout, and then by pulling the tool directly upward after taking off the runner from the pump portion.  
         [0054]     In the past, after lowering down the tool through the through-cutout, the tool is transversely moved along the annular groove up to a desired runner. Then, after taking off the runner using the tool, the tool is returned to the position of the through-cutout again by transversely moving the tool while holding the runner. After that, the runner is drawn out through the through-cutout.  
         [0055]     In the present invention, since the runner can be taken out by lowering directly downward through the through-cutout and then by pulling directly upward the tool after taking off the runner, the workability is extremely better compared to the conventional work because there is no need to transversely moving the tool along the annular groove, which is different from the conventional work.  
         [0056]     The core injection system header pipe  18  is limited within a range somewhat narrower than 90 degrees, as shown in  FIG. 2 . The reason why the core injection system header pipe is formed so as to fall within the range smaller than 90 degrees is to avoid that the through-cutout provided in the body portion of the upper shroud  16  interfere with an end portion of the core injection system header pipe  18 . The core injection system header pipe  18  is arranged near and along the inner peripheral surface of the body portion of the upper shroud. Since the through-cutouts provided in the body portion of the upper shroud are formed by being pressed toward the inner side, the inner side portion of the through-cutout may hit the core injection system header pipe  18 .  
         [0057]     The core injection system header pipes  18  are arranged at two positions, but may be arranged at four positions. In a case where the core injection system header pipe  18  is in a range larger than 90 degrees, the four reactor recirculation pumps  1  can not be arranged with equal spacing. In order to uniformly cool the core  15  in the reactor pressure vessel (RPV)  5 , it is preferable that the reactor recirculation pumps  1  are arranged with equal spacing.  
         [0058]     As having been described above, according to the present invention, it is possible to provide a reactor cooling system of which maintenance and inspection can be easily performed.