Abstract:
The present invention aims to reduce the total exposed dose of the workers inside a reactor well during the annual inspection of a nuclear reactor. In order to achieve the above-mentioned aim, the present invention provides a boiling water reactor, comprising a first piping for pouring water of a reactor vessel to a reactor well located above the reactor vessel via a pump and a cooler, a second piping for pouring water of a spent fuel storage pool to the reactor well via a pump and a filter/demineralizer, and a third piping for pouring the water of the spent fuel storage pool to the spent fuel storage pool via the pump and the filter/demineralizer.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a boiling water reactor and a method of purifying reactor well water.  
         DESCRIPTION OF THE RELATED ART  
         [0002]    The operation to decontaminate a reactor well performed during an annual plant inspection maintenance (hereinafter referred to as annual inspection) for an advanced boiling water reactor (hereinafter referred to as ABWR) is one of the exposure operation for workers. The total exposed dose of the worker should desirably be as low as possible. In this regard, a technique for lowering the exposure of workers inside a reactor well is disclosed in Japanese Patent Laid-Open No. H9-138294.  
         SUMMARY OF THE INVENTION  
         [0003]    As a reason for increasing radiation dose of a reactor well, it is considered that radioactive materials adhering to the surface of the fuel rods are exfoliated during refueling (taking-out of fuel, or fuel shuffling loading operation) at annual inspection, and are brought into the reactor well.  
           [0004]    In a boiling water reactor (hereinafter referred to as BWR) including ABWR, a part of the fuel rods burned during power operation of the reactor is transferred from the reactor pressure vessel to the reactor well during annual inspection. Then, it is stored in a spent fuel storage pool (hereinafter referred to as SFP). In the ABWR, at least the reactor pressure vessel (hereinafter referred to as RPV), the reactor well, and the fuel pool are filled with water during transfer of the fuel rod. The flow of water during this period is approximately divided into two mentioned below.  
           [0005]    First, fluid of a residual heat removal system (hereinafter referred to as RHR) that runs from the pressure vessel to the reactor well by the driving of the RHR system pump is cooled by a RHR system cooler, and is provided to the reactor well from a sparger. The other is called a fuel pool cooling cleanup system (hereinafter referred to as FPC). First, fluid is drawn into a surge tank from SFP and the reactor well. Then, the water returns into the SFP as FPC system fluid via a FPC pump, a FPC filter/demineralizer and a FPC cooler.  
           [0006]    In these two flows, the FPC draws in water from the SFP and the reactor well, and returns the same to the SFP. This means that, of the water returned from the FPC to the SFP, water with same flow rate as the flow rate being drawn in from the reactor well to the FPC flows from the SFP to the reactor well. The spent fuel with the possibility of exfoliating radioactive substance from the surface thereof is stored in the SFP. Therefore, the radioactive substance exfoliated from the surface of the spent fuel becomes radioactive floats, and may flow into the reactor well with the flow from the SFP to the reactor well. When the radioactive substance flows into the reactor well, the radioactive float inside the reactor well sediments, and adheres to the wall surface or the side surface of the reactor well, thereby increasing the radiation dose of the reactor well.  
           [0007]    The present invention aims to reduce the total exposed dose of the workers inside the reactor well during the annual inspection of a nuclear reactor.  
           [0008]    The embodiment for achieving the above-mentioned object comprises a first piping for pouring water of a reactor vessel to a reactor well located above the reactor vessel via a pump and a cooler, a second piping for pouring water of a spent fuel storage pool to the reactor well via a pump and a filter/demineralizer, and a third piping for pouring the water of the spent fuel storage pool to the spent fuel storage pool via the pump and the filter/demineralizer.  
           [0009]    According to the present embodiment, the water purified by the filter/demineralizer located in the second piping could be provided to the reactor well. Therefore, the water of the reactor well could be purified more than in the case of not pouring water purified by the filter/demineralizer located in the second piping to the reactor well. Therefore, the radiation dose of the reactor well could be reduced. By doing so, the total exposed dose of the workers inside the reactor well during the annual inspection of a nuclear reactor could be reduced.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a structural view of embodiment 1; and  
         [0011]    [0011]FIG. 2 is a structural view of embodiment 2. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0012]    (Embodiment 1)  
         [0013]    [0013]FIG. 1 indicates the system of an ABWR applied with the present embodiment. FIG. 1 shows the operating condition of RHR  13  and FPC  24  during an annual inspection of the ABWR.  
         [0014]    Each section will be described first. A reactor well  2  is located above a reactor pressure vessel  1 . The PRV  1  links with the reactor well  2  by removing a PRV top head (not shown). When transferring the fuel from the RPV  1  to a SFP  4 , the reactor well  2  is totally filled with water in order to shield the radiation generated by the fuel with water, and the fuel is transferred inside water from the RPV  1  to the SFP  4  via the reactor well  2 . The reactor well  2  and the SFP  4  are connected by an openable gate  3 . The gate  3  is opened when transferring the fuel. In FIG. 1, the gate  3  indicated by the dotted line means that it is opened. The SFP  4  storing a spent fuel  50  is totally filled with water during operation of the plant and also during the annual inspection.  
         [0015]    A RHR  13  includes a pump for residual heat removal system (hereinafter referred to as RHR pump)  11 , a heat exchanger  12 , a valve  31  and a valve  34 . The system water traveling from RPV  1  through RHR  13  is sprayed to the reactor well  2  with a sparger pipe  14 . The RHR  13  is connected to a feed water sparger  55  located inside the RPV  1  with a short-cut pipe  53 . The short-cut pipe  53  includes a valve  54 .  
         [0016]    The FPC  24  includes a pump for fuel pool cooling and filtering system (hereinafter referred to as FPC system pump)  21 , a FPC heat exchanger  23 , a filter/demineralizer for fuel pool cooling cleanup system (hereinafter referred to as FPCF/D)  22 , and a valve  32 . From the SPF  4  to a skimmer surge tank (hereinafter simply referred to as surge tank)  5 , the water from the SFP  4  is introduced via an inlet  51 , and from the reactor well  2  to the surge tank  5 , the water exiting from an outlet  52  provided to the reactor well  2  is introduced. The water inside the surge tank  5  is sprayed to the SFP  4  from the sparger pipe  25  via the FPC  24 . The inlet  51  and the inlet  52  include a gate  7  and a gate  6  before the surge tank  5 , respectively, so that the flow rate of the water from each of the inlets to the surge tank  5  could be altered independently.  
         [0017]    The RHR  13  and the FPC  24  are connected by a pipe  15  and a pipe  26 . The pipe  15  includes a valve  35 . The pipe  26  includes a valve  37 . The system water of the RHR  13  could be supplied to the FPC  24  with the pipe  15 . The system water of the FPC  24  could be supplied to the RHR  13  with the pipe  26 .  
         [0018]    The gate  3  is provided between the reactor well  2  and the SPF  4 . The gate could be opened and closed. The gate  3  indicated by the dotted line means that it is opened in FIG. 1.  
         [0019]    In the figures, the valves colored in black indicates the condition where the valves are closed, and the ones colored in white indicates the condition where the valves are opened. Also, each arrows and statements of the flow rate in the figures indicates the flow direction of the system water and its flow rate.  
         [0020]    The process of the present embodiment will be described. The FPC  24  is already in operation during operation of the nuclear reactor. The RHR pump  11  is started under the condition where nuclear reactor is shutdown, the valves  31 ,  34  and  35  are closed, and the valve  54  is opened. The water inside the RPV  1  is circulated to the RPV  1  from the sparger  55  via the RPV  1 , the RHR pump  11 , the heat exchanger  12 , the short-cut pipe  53 , and the valve  54 , and is cooled by the heat exchanger  12 . Next, the reactor well  2  is filled with water, and the pool gate  3  is opened, and the EPC valve  37  is opened, the EPC valve  26  is closed.  
         [0021]    Next, the RPV top head (not shown) is removed. The RHR valves  34  and  35  are opened, and the valve  54  is closed. By doing so, the water inside the RPV  1  is circulated to the reactor well  2  from the sparger pipe  14  via the RHR  13 . Also, the system water of the FPC  24  is supplied to the RHR  13  via the valve  37  and the pipe  26 , and is sprayed to the reactor well  2  from the sparger pipe  14 . The fuel assembly (not shown) inside the RPV  1  is transferred to the SFP  4 . Also, by opening the valve  35 , the system water of the RHR  13  is sprayed to the SFP  4  from the sparger pipe  25  via the pipe  15 .  
         [0022]    Next, each of the RHR  13  and FPC  24  system will be described.  
         [0023]    In RHR  13 , the reactor water sucked out from the RPV  1  by the RHR pump  11  is sent to the heat exchanger  12  and is cooled. The water exiting the heat exchanger  12  is branched beyond the valve  31 , one of which is sprayed to the reactor well  2  from the sparger pipe  14  via the valve  34 , and the other is sprayed to the SFP  4  from the sparger pipe  25  via the valve  35 . In the present embodiment, the flow rate of 1900 m 3 /h of the RHR  13  is divided into 1400 m 3 /h for the side of the sparger pipe  14 , and 500 m 3 /h for the side of the sparger pipe  25 . By doing so, the reactor water flow rate brought in from the RPV  1  to the reactor well  2  is decreased from 1900 m 3 /h to 1400 m 3 /h. The change of the flow rate distribution is carried out by controlling the opening of the valve  34  and the valve  35 .  
         [0024]    In FPC  24 , the skimmed water of the SFP  4  flowing into the surge tank  5  from the inlet  51  via the gate  7 , and the skimmed water of the reactor well  2  flowing into the surge tank  5  from the inlet  52  via the gate  6 , is purified by running through the FPCF/D  22  with the FPC pump  21 , and is cooled by running through the FPC heat exchanger  23 . The water exiting the FPC heat exchanger  23  is transferred to the sparger pipe  14  through a valve  37  and a tie line  26 . In the present embodiment, 500 m 3 /h of purified water purified by the FPC  24  is supplied to the sparger pipe  14 .  
         [0025]    After transferring the fuel assembly inside the RPV  1  to the SFP  4 , various checks concerning the reactor are carried out. After carrying out the checks, the fuel assembly is loaded to the RPV  1 , the top head of the RPV  1  is closed, the gate  3  is shut, and the water inside the reactor well  2  is drained, by reversing the process described heretofore. With this, the annual inspection is completed.  
         [0026]    According to the present embodiment, the water passing through the FPCF/D  22  could be supplied to the reactor well  2 . Therefore, the water of the reactor well  2  could be purified more than in the case of not supplying the water passing through the FPCF/D  22  to the reactor well  2 . Therefore, the radiation dose of the reactor well could be reduced. As a result, the total exposed dose of the workers inside the reactor well during the annual inspection of a nuclear reactor could be reduced.  
         [0027]    Also, by including the pipe  15  for supplying the system water of the RHR  13  to the FCP  24 , and the pipe  26  for supplying the system water of the FCP  24  to the RHR  13 , one of the system water could be sprayed from the sparger pipe of the other system. By doing so, the distribution of the flow rate sprayed from the two sparger pipes could be altered. Of the components sprayed with water, that is, the reactor well  2  and the SFP  4 , the effect of cooling and water purification could be enhanced for the component with larger flow rate of sprayed water. That is, when the amount of sprayed water for the reactor well is increased, the water quality of the reactor well could be enhanced with the purified and cooled sprayed water. Also, when the amount of sprayed water for the SFP is increased, cooling of the SFP could be achieved with purified and cooled sprayed water. The control of the flow rate distribution is carried out by altering the opening of the valve  36  and the valve  37 .  
         [0028]    The spent fuel storage pool  4  may be purified at the same time, by opening a base valve  36  of the FPC pool sparger pipe and supplying purified water to a SFP sparger pipe  25 .  
         [0029]    When annual inspection is completed and the reactor enters the power output operation, the pool gate  3  is closed, and in the FPC  24 , a tie line base valve  37  is closed, the base valve  36  of the FPC pool sparger pipe is opened to cool and purify only the spent fuel storage pool  4 .  
         [0030]    (Embodiment 2)  
         [0031]    In the present embodiment, the height of the gate  6  at the side of the reactor well is raised to inhibit overflow from the side of the reactor well  2  to the surge tank  5 , and the height of the gate  7  at the side of the spent fuel storage pool  4  is lowered to increase overflow from the side of the spent fuel storage pool  4  to the surge tank  5 .  
         [0032]    The structure of each sections are the same as those in Embodiment 1, therefore the explanation will be omitted. Also, the process of the annual inspection is the same as that in Embodiment 1, therefore the explanation will be omitted.  
         [0033]    The present embodiment differs from Embodiment 1 in that the openings of the gate  6  and gate  7  are altered, and the flow rates flowing to the surge tank  5  from the reactor well  2  and the SFP 4 , respectively, are altered. In the present embodiment, flow rate of the gate  6  at the side of the reactor well is zero, the overflow flow rate from the gate  7  at the side of the spent fuel storage pool is 500 m 3 /h, and the amount of water sprayed from the sparger pipe  25  of the SFP  4  is 500 m 3 /h supplied from the RHR  13  via the piping  15 . The valve  36  is fully closed.  
         [0034]    According to the present embodiment, the same effect as that in the Embodiment 1 could be obtained. Moreover, the flow rate flowing into the spent fuel storage pool  4  from the SFP sparger pipe  25 , and the flow rate flowing out from the spent fuel storage pool  4  through the gate  7  equals at 500 m 3 /h, so that the flow rate of the pool gate  3  becomes zero. Therefore, there exists no flow from the spent fuel storage pool towards the reactor well. This means that the amount of floating radioactive crad inside the SFP  4  generated by transferring the fuel from the RPV  1  to the SFP  4  at the start of the annual inspection or the like flowing into the reactor well could be reduced more than in the case where a flow is generated from the SFP  4  to the reactor well  2 . Therefore, the radiation dose of the reactor well could be reduced. By doing so, the total exposed dose of the workers inside the reactor well during the annual inspection of a nuclear reactor could be reduced.  
         [0035]    (Embodiment 3)  
         [0036]    The present embodiment is an embodiment altering the overflow flow rate from the gate  7  at the side of the spent fuel storage pool in Embodiment 2. The structure of the present embodiment is the same as that in Embodiment 2, therefore the explanation will be omitted. Also, the method being applied is the same, therefore the explanation will be omitted.  
         [0037]    The difference between the present embodiment and Embodiment 2 will be explained. The overflow flow rate from the gate  7  at the side of the spent fuel storage pool is set at a flow rate exceeding 500 m 3 /h. In the present embodiment, the flow rate is set at 600 m 3 /h.  
         [0038]    According to the present embodiment, the flow rate of the pool gate  3  which was zero in Embodiment 2 becomes 100 m 3 /h. By doing so, a flow from the reactor well  2  to the SFP  4  is generated. Therefore, the amount of floating radioactive crad inside the SFP  4  generated by transferring the fuel from the RPV  1  to the SFP  4  at the start of the annual inspection or the like flowing into the reactor well could be reduced more than in the case where a flow is generated from the SFP  4  to the reactor well  2 . Also, because a flow from the SFP  4  to the reactor well  2  is generated, the amount of floating radioactive crad flowing in from the SFP  4  to the reactor well  2  could be restrained, even when convection is generated from the difference in water temperature. Therefore, the radiation dose of the reactor well could be reduced. By doing so, the total exposed dose of the workers inside the reactor well during the annual inspection of a nuclear reactor could be reduced.  
         [0039]    Each of the valves in Embodiment 1 through Embodiment 3 have the capability of not only opening and closing fully, but also altering their opening from 0% to 100%. By doing so, the amount of water sprayed to the reactor well  2  and the SFP  4 , and the flow rate of the pipe  15  and the pipe  26  could be adjusted. Also, the openings of each of the valves are adjusted by hand in each of the embodiments, but the valves may be ones including power such as electric motors.  
         [0040]    According to the present invention, the total exposed dose of the workers inside the reactor well during the annual inspection of a nuclear reactor could be reduced.