Abstract:
A team turbine control device. A main steam system runs between a nuclear reactor and a steam turbine and includes a main stream isolation valve (MSIV), a main steam control valve, and a turbine by-pass system with a turbine by-pass valve. A calculating means generates a main steam pressure control signal and a reactor dome pressure control signals. A pressure control single changeover means changes a pressure control signal from the reactor dome pressure signal to the main steam pressure signal when the MSIV closes to restrain the abrupt decrease of stream in the main steam system effectively.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to steam supply in a nuclear power plant. Steam moves from a nuclear reactor to a steam turbine through a main steam system with main steam isolation valves for isolating the nuclear reactor.  
           [0003]    2. Description of the Related Art  
           [0004]    Japanese Non-examined Patent Publication No. 9-80195discloses a steam turbine control device for a nuclear power plant. FIG. 8 is a basic block diagram of the main steam system and a turbine by-pass system of a nuclear power plant.  
           [0005]    Steam generated in a nuclear reactor  1  is supplied to a steam turbine  8  through a main steam system  61 . There are a plurality of, for example ten, main steam isolation valves (MSIV)  2  settled inside and outside of a primary containment vessel (PCV)  3  containing a nuclear reactor  1 , and a main steam header  4 , a main steam stop valve  5 , and a main steam control valve (CV)  6 , in the main steam system  61 .  
           [0006]    The main steam isolation valves  2  operate independently from other pressure control valves to isolate the nuclear reactor  1 . The main steam isolation valves  2  are open in the usual state of operation, and close to automatically seal the inside and outside of the primary containment vessel  3  when there is possibility that reactor coolant may flow out of the primary containment vessel  3 . The main steam isolation valves  2  thus close, for example, if an accident internal or external of the primary containment vessel  3  arises.  
           [0007]    The main steam stop valve  5  blocks the steam from reaching a steam turbine  8  when suspending the steam turbine  8 . The main steam control valve  6  adjusts the discharge of steam that is generated in the nuclear reactor  1  and that flows into the steam turbine  8 . The steam then rotates the steam turbine  8 , and a generator  9 , directly linked with the steam turbine  8 , generates an electric output.  
           [0008]    A turbine by-pass system  62 , independent of the main steam system  61 , branches from the main steam system  61  at the main steam header  4  and connects to a condenser  10  via the turbine by-pass valve  7 .  
           [0009]    A steam system supplies drive steam for a turbine grand steam evaporator  12 , a reactor feed water pump turbine  13 , and a steam jet air ejector  14 , from the main steam system  61 . The turbine grand steam evaporator  12  supplies a little steam to the space of a grand sealing part of the steam turbine  8 , i.e., the boundary portion with the open air, in order to seal the grand part of a turbine  8 . The steam jet air ejector  14  continuously extracts noncondensing gases, such as hydrogen and oxygen, from air in a condenser  10  or in exhaust gas of the steam turbine  8 . The steam jet air ejector performs an action like spraying the steam supplied from the turbine by-pass system  62  and sends the noncondensing gas to an off-gas system and thereby maintains the vacuum of the condenser  10 .  
           [0010]    During normal operation, the main steam control valve  6  adjusts the reactor dome pressure, when a reactor dome pressure detector  11 , installed in the nuclear reactor  1 , detects the reactor dome pressure. A turbine by-pass valve  7  is kept completely closed then.  
           [0011]    If an accident occurs at a startup or a shutdown of the nuclear power plant, or a electric transmission system, the position of the main steam control valve  6  is restricted, and the turbine by-pass valve  7  adjusts the pressure of the nuclear reactor  1 .  
           [0012]    [0012]FIG. 9 is a block diagram for explaining the conventional example of the steam turbine control device of the nuclear power plant of FIG. 8.  
           [0013]    The steam turbine control device described below controls the positions of the main steam control valve  6  and the turbine by-pass valve  7 .  
           [0014]    A reactor dome pressure signal from the reactor dome pressure detector  11  is inputted into the steam turbine control device  19  and is compared with the pressure setting of the reactor dome pressure setter  23 . A first pressure deviation calculating unit  24  then calculates the pressure deviation. A reactor dome pressure control calculating unit  25  receives the pressure deviation calculated by the first pressure deviation calculating unit  24  and sends a signal proportional to the deviation as a pressure control signal  29  to a first low value selector  18 .  
           [0015]    The first low value selector  18  compares the pressure control signal  29  to a speed/load control signal from a speed/load control calculating unit  15 , a load restriction signal of a load limiter  16 , and a maximum discharge restriction signal from a maximum discharge restriction unit  17 . The speed/load control signal from the speed/load control calculating unit  15  controls the speed, i.e., the rotational number of the steam turbine  8 , and the load of the generator  9 , i.e., the electric output. The first low value selector  18  then chooses the lowest value signal among these signals and outputs it as a position demand of the main steam control valve  6 .  
           [0016]    Moreover, a first deviation calculating unit  20  generates the deviation signal between the pressure control signal  29  calculated by the reactor dome pressure control calculating unit  25  and the position demand signal to the main steam control valve  6 . A second deviation calculating unit  21  generates the deviation signal between the maximum discharge restriction signal calculated by the maximum discharge restriction unit  17  and the position demand of the main steam control valve  6 . The two deviation signals from the first and second deviation calculating units  20 ,  21  are inputted into the second low value selector  22 , which outputs the lower value of the two deviation signals as a position demand signal of the turbine by-pass valve  7 .  
           [0017]    In addition, the reactor dome pressure detectors  11  are generally multiplexed to improve reliability, and in FIG. 9, a first medium value selector  27  selects a medium value of the triplex reactor dome pressure detectors  11  as a signal to be used for control.  
           [0018]    In the conventional steam turbine control device of the nuclear power plant described above, in a usual operating state, the main steam control valve  6  is adjusted, based on the pressure signal from the reactor dome pressure detectors  11  installed in the nuclear reactor  1 , to control and fix the pressure of the nuclear reactor  1 . But in that case, if an accident detected by, for example, are actor isolation signal detector (not shown) inside or outside of the primary containment vessel  3  occurs, and if the main steam isolation valves  2  are automatically in a fully closed position at the time of the accident, the pressure of the nuclear reactor  1 , i.e., the reactor dome pressure, will rise abruptly.  
           [0019]    In this case, the main steam control valve  6  and the turbine by-pass valve  7  open, and the drive steam of the turbine grand steam evaporator  12 , the reactor feed water pump turbine  13 , and the steam jet air ejector  14  decrease abruptly. FIGS. 10 a - 10   c  are signal time charts for explaining this situation.  
           [0020]    If the main steam isolation valves  2  in FIG. 8 are fully closed, the pressure signal from the reactor dome pressure detectors  11  installed in the nuclear reactor  1  goes up as shown in FIG. 10 a.  In FIG. 10 a,  the ordinate axis shows pressure and the abscissa axis shows time.  
           [0021]    Since at that time the pressure deviation which is the output of the first pressure deviation calculating unit  24  of the steam turbine control device  19  rises, the pressure control signal  29  calculated by the reactor dome pressure control calculating unit  25  goes up as shown in FIG. 10 c.  In FIG. 10 c,  the ordinate axis shows an output of the signal and the abscissa axis shows time. The output of the first low value selector  18  goes up until it is restricted by either the speed/load control signal, the load restriction signal, or the maximum discharge restriction signal. Then, the main steam control valve  6  will open according to an increase of the pressure control signal  29 .  
           [0022]    On the other hand, if the first low value selector  18  restricts the pressure control signal  29 , the position demand signal to the main steam control valve  6  also becomes restricted, and the deviation signal between the pressure control signal  29  and the position demand signal to the main steam control valve  6  calculated by the first deviation calculating unit  20  goes up.  
           [0023]    Therefore, since the output of the second low value selector  22  goes up until it is restricted by the deviation signal between the pressure control signal  29  and the maximum discharge restriction signal calculated by the second deviation calculating unit  21 , the position demand signal to the turbine by-pass valve  7  goes up as shown in FIG. 10 c,  and the turbine by-pass valve  7  will open.  
           [0024]    [0024]FIG. 10 b  is a signal time chart of the reactor dome pressure control signal  36 . In FIG. 10 b,  the ordinate axis shows the output of the signal and the abscissa axis shows time.  
           [0025]    If the main steam control valve  6  and the turbine by-pass valve  7  open as mentioned above, since the steam remaining in the main steam system  61  downstream of the main steam isolation valves  2  flow into the steam turbine  8  or are directly collected by the condenser  10 , the drive steam of the turbine grand steam evaporator  12 , the reactor feed water pump turbine  13 , and the steam jet air ejector  14 , i.e., main steam pressure, goes down abruptly, as shown in FIG. 10 a.    
           [0026]    Under the circumstance, the heating steam of the turbine grand steam evaporator  12  may lose, the amount of supply of the grand seal steam from the turbine grand steam evaporator  12  to the steam turbine  8  may fall in a short time, and this situation may damage on the steam turbine  8 .  
           [0027]    The vacuum drop of a condenser  10  becomes comparatively greater by rapid reduction of the drive steam of the steam jet air ejector  14 , because the ability of steam jet air ejector  14  to discharge the noncondensing gas goes down then.  
         SUMMARY OF THE INVENTION  
         [0028]    In view of the foregoing, it is an object of this invention to provide a steam turbine control device and the method for a nuclear power plant.  
           [0029]    This object can be achieved according to the present invention by providing, in one aspect, a steam turbine control device of nuclear power plant including:  
           [0030]    a main steam system connected to lead steam generated in a nuclear reactor into a steam turbine, comprising a main steamline, a main steam isolation valve, and a main steam control valve;  
           [0031]    a turbine by-pass system connected to by-pass the steam turbine, branched from the main steam system, and connected to a condenser, the turbine by-pass system comprising a turbine by-pass valve;  
           [0032]    a main steam pressure detector in the main steam system;  
           [0033]    a main steam pressure control calculating means for outputting a main steam pressure control signal dependant on the signal from the main steam pressure detector;  
           [0034]    a reactor dome pressure detector in the nuclear reactor;  
           [0035]    a reactor dome pressure control calculating means for outputting a reactor dome pressure control signal dependant on the signal from the reactor dome pressure detector;  
           [0036]    a main steam isolation valve fully closed position detector for outputting a pressure control change trigger signal when the main steam isolation valve is detected to be fully closed; and  
           [0037]    a pressure control signal calculating means for outputting a pressure control signal to control the position of the main steam control valve and/or the turbine by-pass valve comprising a pressure control signal changeover means to change over a pressure control signal from the reactor dome pressure control signal to the main steam pressure control signal when the main steam isolation valve is detected to be closed by the main steam isolation valve fully closed position detector.  
           [0038]    In another aspect, in a nuclear power plant including a main steam isolation valve in a main steam system between a nuclear reactor and a steam turbine, there is provided a control device for controlling at least one of a first valve and a second valve, the first valve being in the main steam system and the second valve being in a steam turbine by-pass system branched from the main steam system, the control device including:  
           [0039]    a first pressure monitor connected to the nuclear reactor;  
           [0040]    a main steam isolation valve position monitor connected to the main steam isolation valve;  
           [0041]    a second pressure monitor connected to the main steam system between the first valve and the main steam isolation valve;  
           [0042]    a control means for controlling the position of the first valve or the second valve based on a first pressure signal from the first pressure monitor during normal operation of the nuclear reactor, and based on a second pressure signal from the second pressure monitor when the second pressure monitor detects that the main steam isolation valve is closed.  
           [0043]    In another aspect, in a nuclear power plant including a nuclear reactor in a primary containment vessel, a steam turbine, and a main steam system between the nuclear reactor and the steam turbine, there is provided a method for controlling the steam turbine based on the position of a main steam isolation valve in the main steam line, the method including the steps of:  
           [0044]    monitoring the pressure in the nuclear reactor when the main steam isolation valve is open;  
           [0045]    closing the main steam isolation valve to isolate the primary containment vessel;  
           [0046]    monitoring the pressure in the main steam system downstream of the main steam isolation valve when the main steam isolation valve is closed; and  
           [0047]    controlling the amount of the supply of the steam to the steam turbine only in response to the pressure in the main steam system downstream of the main steam isolation valve when the main steam isolation valve is closed. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0048]    Other objects and aspects of the present invention will become apparent from the following description of an embodiment with reference to the drawings in which:  
         [0049]    [0049]FIG. 1 is a block flow diagram of a steam turbine control device of a nuclear power plant according to a first embodiment of the invention.  
         [0050]    [0050]FIGS. 2 a - 2   c  are signal time charts of signals for explaining the function and effect of the first embodiment of the invention described in FIG. 1. FIG. 2 a  shows a chart of reactor dome pressure and main steam pressure. FIG. 2 b  shows a signal time chart of a reactor dome pressure control signal  36  and a main steam pressure control signal  37 . FIG. 2 c  shows a signal time chart of a pressure control signal  29  that determines the position of the main steam control valve  6 .  
         [0051]    [0051]FIG. 3 is a block flow diagram of a steam turbine control device of a nuclear power plant according to a second embodiment of the invention.  
         [0052]    [0052]FIG. 4 a  is a chart of reactor dome pressure and main steam pressure for explaining the function and effect of the second embodiment of the invention described in FIG. 3.  
         [0053]    [0053]FIG. 4 b  is a chart of reactor dome pressure and main steam pressure in the first embodiment of the invention described in FIG. 1.  
         [0054]    [0054]FIG. 5 is a block flow diagram of a steam turbine control device of a nuclear power plant according to a third embodiment of the invention.  
         [0055]    [0055]FIG. 6 is a block flow diagram of a steam turbine control device of a nuclear power plant according to a fourth embodiment of the invention.  
         [0056]    [0056]FIG. 7 is a block flow diagram of a steam turbine control device of a nuclear power plant according to a fifth embodiment of the invention.  
         [0057]    [0057]FIG. 8 is a system diagram of a main steam system and the turbine by-pass system of the nuclear power plant concerning this invention or the related art.  
         [0058]    [0058]FIG. 9 is a block flow diagram showing an example of a conventional steam turbine control device of a nuclear power plant.  
         [0059]    [0059]FIG. 10 is a signal time chart of signals for explaining the problem of the conventional steam turbine control system shown in FIG. 9. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0060]    Embodiments of the steam turbine control device of the nuclear power plant concerning this invention are explained below. Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views.  
         [0061]    [0061]FIG. 1 is a block flow diagram showing a first embodiment of the present invention. in this embodiment, a steam turbine control device  19  has a control means to restrain a fall of the main steam pressure, i.e., the pressure of the main steam header  4  or the main steam system  61 , when the main steam isolation valves  2  are closed. A changeover means changes over, as a pressure control signal, from the reactor dome pressure control signal side to the main steam pressure control signal side when the main steam isolation valves  2  are closed.  
         [0062]    A second pressure deviation calculating unit  31  calculates with the main steam pressure signal  35  from the main steam pressure detector  26  installed in the main steam header  4  or the main steam system (main steam line)  61  as shown in FIG. 8 and the signal from a main steam pressure setter  30 .  
         [0063]    A main steam pressure control calculating unit  32  calculates the pressure deviation signal from the second pressure deviation calculating unit  31  and outputs the main steam pressure control signal  37 .  
         [0064]    A main steam isolation valve fully closed position detector  34  detects that the main steam isolation valve  2  that isolates the main steam system from the nuclear reactor  1  is fully closed.  
         [0065]    A pressure control signal changeover unit  33  changes over the pressure control signal  29  from the reactor dome pressure control signal  36  to the main steam pressure control signal  37  when the main steam isolation valves  2  are fully closed. That is, when the pressure control signal changeover unit  33  receives the pressure control change trigger signal  38  from the main steam isolation valve fully closed position detector  34 , the connection state of the pressure control signal changeover unit  33  is changed over from between a-c to between b-c, and then the signal outputted as the pressure control signal  29  from the pressure control signal changeover unit  33  is changed over from the reactor dome pressure control signal  36  to the main steam pressure control signal  37 .  
         [0066]    Since some of the physical structure in this embodiment may be similar to the conventional structure shown in FIG. 9, the same reference characters are given to the same composition.  
         [0067]    [0067]FIGS. 2 a - 2   c  are signal time charts for explaining the function and effect of the first embodiment acquired by constituting as shown in FIG. 1 mentioned above. FIG. 2 a  shows a signal time chart of the rector dome pressure and the main steam pressure.  
         [0068]    As shown in FIG. 2 a,  if the main steam isolation valves  2  are fully closed and the pressure control signal  29  is changed over to the main steam pressure control signal  37  by the pressure control signal changeover unit  33 , the main steam pressure detected by the main steam pressure detector  26  below the main steam isolation valves  2  falls according to the lost of steam supply, so the main steam pressure control signal  37  calculated by the main steam pressure control calculating unit  32  using the deviation signal between the main steam pressure signal and the signal from the main steam pressure setter  30  becomes zero or less. Therefore the pressure control signal  29  denotes fully closed position command, that is, the main steam control valve  6  and the turbine by-pass valve  7  are thus closed.  
         [0069]    At this time, the steam flow into the steam turbine  8  or the condenser  10  is lost, and a rapid reduction of the steam that remains in the main steam system  61  downstream of the main steam isolation valves  2  can be prevented, as shown in FIG. 2 a.  Therefore, the steam remaining in the main steam system  61  is supplied to the turbine grand steam evaporator  12  as its heating steam, and a fall in the amount of supply of the grand sealing steam from the turbine grand steam evaporator  12  to the steam turbine  8  can be restrained.  
         [0070]    Moreover, in this embodiment, a rapid reduction of the drive steam of the steam jet air ejector  14  is prevented, and the vacuum drop in the condenser  10  can be restrained in an action like an atomizer by the steam flow into the steam jet air ejector  14 .  
         [0071]    Consequently, the concept of this embodiment can be applied easily to any plant, even if it is an established conventional plant, without troubles concerned to the fully closed position of the main steam isolation valves  2 .  
         [0072]    In addition, FIG. 2 b  shows a signal time chart of the reactor dome pressure control signal  36  and the main steam pressure control signal  37 , and FIG. 2 c  shows a signal time chart of the pressure control signal  29  that determines the position of the main steam control valve  6 .  
         [0073]    [0073]FIG. 3 is a block flow diagram showing a second embodiment of the present invention. In this embodiment, the steam turbine control device as mentioned in the first embodiment shown in FIG. 1 has an additional control means to restrain the abrupt change at the time of the changeover of a pressure control signal from the main steam pressure control signal  37  to the reactor dome pressure control signal  36  when the signal of the main steam isolation valve fully closed position detector  34  is cancelled.  
         [0074]    In the steam turbine control device, a pressure control deviation calculating unit  43  calculates the deviation between the reactor dome pressure control signal  36  and the pressure control signal  29 . A NOT circuit (logic reversal circuit)  39  outputs a signal when the pressure control change trigger signal  38  from the main steam isolation valve fully closed position detector  34  is in a OFF state. A one-shot circuit  40  outputs a signal in an instant when an output arises from the NOT circuit  39 . A relay contact  42  closes in an instant when the signal from the one-shot circuit  40  is outputted, and a bias signal generator  41 , in the case that a signal is inputted, outputs a signal whose initial value is the value of the inputted signal and that decreases by a certain rate of change.  
         [0075]    The signal from the bias signal generator  41  is inputted to the first pressure deviation calculating unit  24  as an additive signal, and then the output of the first pressure deviation calculating unit  24  becomes the deviation signal added a bias.  
         [0076]    In this embodiment, the abrupt change of the pressure control signal  29  may be restrained when the pressure control signal  29  returns to the reactor dome pressure control signal  36  by the pressure control signal changeover unit  33 .  
         [0077]    [0077]FIGS. 4 a - 4   b  are signal time charts for explaining the function and effect of the second embodiment. The ordinate axis shows pressure and the abscissa axis shows time. FIG. 4 a  is a signal time chart of the reactor dome pressure and the main steam pressure for explaining the function and effect of the second embodiment of this invention described in FIG. 3. FIG. 4 b  is a chart of the reactor dome pressure and the main steam pressure in the first embodiment of this invention described in FIG. 1.  
         [0078]    In the second embodiment, once the pressure control change trigger signal  38  does not exist after the main steam isolation valves  2  are fully closed, the pressure control signal  29  changes over from the main steam pressure control signal  37  to the reactor dome pressure control signal  36  by the pressure control signal changeover unit  33 .  
         [0079]    If there is non-zero deviation between the main steam pressure control signal  37  and the reactor dome pressure control signal  36  at this time, the pressure control signal  29  changes abruptly according to the deviation, and the reactor dome pressure can be changed as shown in FIG. 4 b  according to the first embodiment of this invention.  
         [0080]    To the contrary, according to the second embodiment as shown in FIG. 3, the pressure control signal  29  can change over stably and restrain the change of the reactor dome pressure as shown in FIG. 4 a,  by adding the deviation between the pressure control signal  29  and the reactor dome pressure control signal  36  as bias to the reactor dome pressure control signal  36  side and by decreasing the bias by a certain rate of changeover gradually.  
         [0081]    [0081]FIG. 5 is a block flow diagram showing a third embodiment of the present invention. In this embodiment, the steam turbine control device in the above-mentioned embodiments, as shown in FIG. 1 or FIG. 3, further comprises a holding means for holding the pressure control change trigger signal  38  by the signal of the main steam isolation valve fully closed position detector  34  and a canceling means for canceling the pressure control change trigger signal  38  by manual operation or the signal of the main steam isolation valve fully open position detector  48 .  
         [0082]    In the third embodiment, the main steam control device further comprises a self-holding means  46  to hold the pressure control trigger signal  38  which is once in an ON state, having a first NOT circuit (logic reverse unit)  39 , an OR circuit  44 , and an AND circuit  45 . A manual reset operation unit  47  outputs a signal to the AND circuit  45  of the self-holding means  46  so that the self-holding state of the self-holding means  46  can be canceled. This situation as mentioned above shows an example in this embodiment where the main steam isolation valve fully position state detector  48 , that detects the fully open position of the main steam isolation valves  2 , is not included.  
         [0083]    In the third embodiment described above, it is possible to hold the state that the pressure control change trigger signal  38  is in an ON state and the pressure control signal  29  is changed over to the main steam pressure control signal  37 , and it is also possible to cancel the pressure control change trigger signal  38  by the manual reset operation unit  47 . So it is enabled to change over the pressure control signal  29  to the reactor dome pressure control signal  36  side manually according to the judgment of the operating staff.  
         [0084]    We may, in the third embodiment shown in FIG. 5, transpose the manual reset operation unit  47  for the series circuit of the main steam isolation value fully open position detector  48  and a second NOT circuit  55 , to cancel the self-holding means  46 .  
         [0085]    In the third embodiment described above, it is possible to hold the state that the pressure control change trigger signal  38  is in an ON state and the pressure control signal  29  is changed over to the main steam pressure control signal  37 , and it is also possible to cancel the pressure control change trigger signal  38  when the main steam isolation valves  2  are fully opened. The pressure control signal  29  can be changed over to the reactor dome pressure control signal  36  side automatically when the main steam isolation valves  2  are detected to be fully open.  
         [0086]    [0086]FIG. 6 is a block flow diagram showing a fourth embodiment of the present invention. In this embodiment, in the steam turbine control device as mentioned in the third embodiment shown in FIG. 5, the manual reset operation unit  47  is substituted for a series circuit of a pressure switch  49  and a NOT circuit  39  to cancel the signal of self-holding means  46 . The pressure switch  49  is activated when a pressure signal input is less than and equal to a pressure setting α, wherein α is a fixed value that is enough to be decompressed, for example, 1 MPa. The pressure switch  49  detects the reactor dome pressure signal  28  is not more than a fixed value α. A canceling means cancels the pressure control change trigger signal  38  when the reactor dome pressure falls to be equal to and less than α enough to be decompressed.  
         [0087]    In this embodiment, it is possible to hold the state that the pressure control change trigger signal  38  is in an ON state and the pressure control signal  29  is changed over to the main steam pressure control signal  37 . After that, the nuclear reactor  1  is to be decompressed for a shutdown operation of the nuclear reactor  1 . It is thus possible to cancel the pressure control change trigger signal  38  when the decompression operation of the nuclear reactor  1  is detected to be finished, and then the pressure control signal  29  can be changed over to the reactor dome pressure control signal  36  automatically with a shutdown operation of the nuclear reactor  1 .  
         [0088]    [0088]FIG. 7 is a block flow diagram showing a fifth embodiment of the present invention. In this embodiment, the steam turbine pressure detectors  26  in the steam turbine control device in the first or second embodiment shown in FIG. 1 or FIG. 3 are multiple, two or three. Specifically, in this embodiment, two main steam pressure detectors  26  and the second medium value selector  50  are added to the construction of the above-mentioned embodiments, and the main steam pressure signal  35  is replaced by the signal of medium value selected by the second medium value selector  50  from the triplex main steam pressure detectors  26 .  
         [0089]    In this embodiment, even when the main steam isolation valves  2  are fully closed and one system breaks down among three systems of the main steam pressure detectors  26 , the main steam pressure signal  35  is normally outputted to the second pressure deviation calculating unit  31 .  
         [0090]    In the fifth embodiment, three sets of the main steam pressure detectors  26  may be replaced by two sets of the main steam pressure detectors  26 , and the second medium value selector  50  may be replaced by a high value selector  51  (not shown) which chooses the high value of the outputs of the two main steam pressure detectors  26 . In this case, the main steam pressure signal  35  is replaced by the signal of the higher value chosen by the high value selector  51  among the signals from the doubled main steam pressure detectors  26 . In this structure, even when the main steam isolation valves  2  are fully closed and one system breaks down between two systems of the main steam pressure detectors  26 , a fall in the main steam pressure signal  35  can be restrained.  
         [0091]    This invention is not limited to these embodiments described above.  
         [0092]    For example, a state indicator may displays the changeover state of the contact of the pressure control signal changeover unit  33  in the first or second embodiments shown in FIG. 1 or FIG. 3. This structure enables an operating staff to recognize the changeover state of the pressure control signal  29  easily.  
         [0093]    With the form of the embodiments described above, the means to restrain the fall in the main steam pressure when the main steam isolation valves  2  are closed is constructed by the means to change over the pressure control signal  29  from the reactor dome pressure control signal  36  to the main steam pressure control signal  37 . For example, the pressure control signal changeover unit  33  can be replaced by a control means to control the main steam control valve  6  and/or the turbine by-pass valve  7  shown in FIG. 8 when the main steam isolation valves  2  are fully closed.  
         [0094]    Furthermore, in the third embodiment shown in FIG. 5, the inputs of the self-holding means  46  have two signals of the main steam isolation valve fully closed position detector  34  and one of the manual reset operation unit  47  and the main steam isolation vale fully open position detector  48 , which can be replaced by the three signals of the main steam isolation valve fully closed position detector  34 , the manual reset operation unit  47 , and the main steam isolation valve fully open position detector  48 .  
         [0095]    And the input of the AND circuit  45  in the third embodiment shown in FIG. 5 may further comprise the input of a series circuit of the NOT circuit  39  and the pressure switch  49  described in the fourth embodiment shown in FIG. 6. In this case, the reactor pressure signal  28  maybe inputted to the other end of the series circuit of the NOT circuit  39  and the pressure switch  49 .  
         [0096]    Moreover, although the third, fourth, and fifth embodiments shown in FIG. 5, 6, and  7  explain the case where the first embodiment shown in FIG. 1 is used as a base, which can similarly explain the case where the second embodiment shown in FIG. 3 is used as a base.  
         [0097]    Furthermore, at least one of the speed/load control calculating unit  15 , the load limiter  16 , the maximum discharge restriction unit  17 , the first and second low value selectors  18  and  22 , the first and second deviation calculating units  20  and  21 , the reactor dome pressure setter  23 , the first and second pressure deviation calculating units  24  and  31 , the reactor dome pressure control calculating unit  25 , the first and second medium value selectors  27  and  50 , the main steam pressure setter  30 , the main steam pressure control calculating unit  32 , the pressure control signal change unit  33 , the first and second NOT circuits  39  and  55 , the one-shot circuit  40 , the bias signal generator  41 , the relay contact  42 , the pressure control deviation calculating unit  43 , the OR circuit  44 , the AND circuit  45 , the manual reset operation unit  47 , the pressure switch  49  activated when the pressure of its input signal is less than and equal to a, the high value selector  51 , may be hardware or a stored program memory and a CPU (central processing unit) which can read the content of the memory and calculate, or means similar to these.  
         [0098]    The first and the second indicator are hardware; the means for displaying the state, the memory storing the state signal, the software program, and the CPU reading and processing the content of the memory; or means similar to these.  
         [0099]    In the fifth embodiment shown in FIG. 7, the main steam pressure detectors  26  are doubled or tripled, but they may be multiple more than three.  
         [0100]    While there has been particularly shown and described with reference to the preferred embodiment thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details can be made therein without departing from the spirit and scope of the present invention.  
         [0101]    According to the present invention, since the main steam control valve and the turbine by-pass valve are fully closed when the main steam isolation valves are fully closed, it can be prevented to decrease abruptly the mass of the drive steam of the turbine grand steam evaporator, reactor feed water pump turbine, and the steam jet air ejector. The steam turbine control device of the nuclear power plant concerning this invention realizes to utilize the steam that remains in the main steam system effectively.