Abstract:
There is provided a nuclear power plant having a steam turbine controller. The nuclear power plant includes a main steam supply system and a turbine by-pass system. The main steam supply system guides steam from heat generated by the nuclear reactor to a steam turbine. The turbine by-pass system is branched from the main steam supply system at a main steam header. A main steam control valve is equipped with the main steam supply system and adjusts steam pressure in the main steam supply system supplied to the steam turbine. A turbine by-pass valve is used to by-pass steam to the turbine by-pass system. A regulating controller generates first and second opening/closing signals for the main steam control valve and the turbine by-pass valve, and an ON-OFF controller generates a third opening/closing signal for the turbine by-pass valve. The third opening/closing signal has priority over the second opening/closing signal.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a nuclear power plant, and more specifically to a nuclear power plant capable of adjusting the position of a turbine by-pass valve based on the steam pressure generated in a nuclear reactor. 
     2. Description of the Related Art 
     FIG. 1 is a diagram showing a main steam system and a turbine by-pass system of a nuclear power plant. 
     A main steam supply system  01  is constituted such that steam generated in a nuclear reactor  1  is supplied to a steam turbine  8  via a main steam header  4 , a main steam stop valve  5 , and a main steam control valve (CV)  6 , respectively. 
     Specifically, steam from the nuclear reactor  1  is supplied to the main steam header  4  disposed outside a primary containment vessel  3 . The steam thus supplied to main steam header  4  then flows to the steam turbine  8  via the main steam stop valve  5  and the main steam control valve  6 . The main steam stop valve  5  isolates steam in the steam turbine  8  in case of stopping operation thereof, and the main steam control valve  6  adjusts the flow rate of steam to the steam turbine  8 . The steam from the nuclear reactor  1  rotates the steam turbine  8 , and a generator  9  connected directly to the steam turbine  8  generates electric power. 
     Steam that passes through the steam turbine  8  is then guided to a condenser  10 . Cooling water such as seawater enters the condenser  10 , and a heat exchange is made between the cooling water and the steam. Steam thus cooled is condensed to water and is circulated back to the nuclear reactor  1 . 
     A turbine by-pass steam supply system  02 , independent from the main steam system  01 , branches from the main steam header  4 . The turbine by-pass system  02  guides steam from the main steam header  4  to the condenser  10  via the turbine by-pass valve. 
     In a regular operation mode of the nuclear power plant, steam pressure generated in the nuclear reactor  1 , which is specifically pressure on the main steam header  4  detected by a main steam pressure detector  2  or pressure detected by a reactor dome pressure detector  11 , is adjusted by the main steam control valve  6  in order to meet a predetermined pressure value. The turbine by-pass valve  7  is fully opened in this situation. Meanwhile, when the nuclear power plant is in a starting or a stopping mode, or when an accident happens to a power supply system, the position of the main steam control valve  6  restricted. In this situation, the turbine by-pass valve  7  adjusts the main steam pressure  2  in the main steam header  4 . 
     Further, when a load is deprived, such as load isolation of the generator  9  and turbine trip, turbine-trip, or the like, both the main steam stop valve  5  and the main steam control valve  6  are closed rapidly, stopping the steam flow to the steam turbine  8 . This causes an increase in the pressure in the nuclear reactor  1  and of the main steam. To relax this pressure, the turbine by-pass valve  7  rapidly opens and the main steam is bypassed to the condenser  10 . 
     A conventional turbine controller for the nuclear power plant is explained referring to FIG. 2. A regulating controller in the steam turbine controller  12  controls the position of the main steam control valve  5  and the turbine by-pass valve  7 . 
     Main steam pressure signals are output signals from the main steam pressure detector  2  connected to the main steam header  4  and enter the steam turbine controller  12 . The signals thus entered are compared to the predetermined pressure value in a main steam pressure setter  23 , and a pressure deviation signal  29  is carried out by a first pressure deviation calculating unit  24 . Here, the pressure deviation signal  29  is entered into a pressure control calculating unit  25 , and a pressure control signal  30 , which is proportional to the pressure deviation signal  29 , is input into a first low value selector  18  as a pressure control signal  30 . 
     In the first low value selector  18 , the pressure control signal  30  is compared to a velocity/load control signal from a speed/load control calculating unit  15 , a load limit signal from a load limiter  16 , and a maximum flow rate limit signal from a maximum discharge restriction unit  17 , respectively. After the comparison, the first low value selector  18  chooses a minimum signal from among those signals and outputs the minimum signal as a valve position demand signal  26  of the main steam control valve  6 . 
     Further, the pressure control signal  30  carried out by the pressure control calculating unit  25  and the valve position demand signal  26  of the main steam control valve  6  obtained by the first low value selector  18  are input into a first deviation calculating unit  20 , and a deviation signal is calculated. The maximum discharge restriction signal carried out by the maximum discharge restriction unit  17  and the valve position demand signal  26  of the main steam control valve  6  obtained by the first low value selector  18  are input into a second deviation calculating unit  21 , and a deviation signal is calculated. 
     The deviation signals from the first deviation calculating unit  20  and the second deviation calculating unit  21  are input into a second low value selector  22 . These deviations are then compared therein, and the lower signal is chosen as a valve position demand signal  31  of the turbine by-pass valve  7 . 
     The turbine by-pass valve position demand signal  31  output from the regulating controller  13  and the valve position demand signal  26  are entered into a valve position control unit  32  having an amplifier, and a deviation signal carried out by the valve position control unit  32  is entered into a servo valve  33 . The servo valve  33  controls the valve position of the turbine by-pass valve  7  to a value required by the steam turbine controller  12 , by adjusting the amount of oil in an oil cylinder  38  that operates turbine by-pass valve  7 . 
     The oil cylinder  38  connects a fast acting solenoid valve  37 ; the fast acting solenoid valve  37  accepts a fast open acting demand to turbine by-pass valve  36  and makes turbine by-pass valve  7  realize a rapid valve-opening operation in an emergency as well as in a performance test. In the regular operation mode, the fast open acting demand to turbine by-pass valve  36  is not generated, and therefore, the oil cylinder  38  is controlled only by turbine by-pass valve  7 . However, if the fast open acting demand to turbine by-pass valve  36  is generated due to detection of a power load unbalance such as a load isolation, the turbine by-pass valve  7  is fully opened regardless of the control signal from the servo valve  33 . Usually, a plurality of turbine by-pass valves  7  are equipped in a plant, however, only the valve which accepted the fast open acting demand to turbine by-pass valve  36  can be fully opened. 
     For reliability reasons, the main steam pressure detector  2 , the regulating controller  13  and the like are multiplexed. Therefore, FIG. 2 shows the case where the triplex main steam pressure detectors  2  and the triplex regulating controller  13  are arranged. The medium value among the output signal from the triplex main steam pressure detectors  2  are chosen by the first medium value selector  27 , and each of the triplex regulating controllers  13  operates the pressure control signal  30  and the valve position control unit  32  for the plant control. 
     Further, the number of turbine by-pass valves  7  varies from each nuclear power plant. The valve position control unit  32 , the servo valve  33 , the fast acting solenoid valve  37 , and the oil cylinder  38  are identical in each turbine by-pass valve  7 , and therefore, only one turbine by-pass valve  7  and the peripherals are illustrated in FIG.  2 . 
     In a nuclear power plant having multiplexed regulating controllers  13 , if one regulating controller  13  has a problem or an unusual condition in its regular operating mode, the other regulating controllers can compensate the unusual condition and maintain the operation. Moreover, if the unusual condition is found, the system can recover from any problems. However, if there is an unusual condition in hardware or software that affects all the regulating controllers  13  commonly, such unusual condition may not be found and the operation may continue. 
     If an unusual condition over plural regulating controllers happens, the ability to adjust the position of the turbine by-pass valve  7  is lost, and a turbine trip occurs before the unusual condition is detected, the turbine by-pass valve  7 , which is usually opened when the main steam stop valve  5  is fully closed, may not operate. Because the turbine by-pass valve  7  keeps closing in this situation, pressure inside the nuclear reactor  1  is rapidly increased and will be in critical thermal condition. 
     The present invention has been made in view of the above-mentioned circumstances and is intended to solve the above-mentioned problems. In particular, the purpose of the present invention is to provide a steam turbine controller for a nuclear power plant capable of avoiding a rapid increase in pressure in the nuclear reactor even if the function of the turbine by-pass valve is lost. 
     SUMMARY OF THE INVENTION 
     The present invention provides a nuclear power plant having a nuclear reactor, including: a first steam supply system connected between the nuclear reactor and a steam turbine, a second steam supply system branched from the first steam supply system and connected downstream of the steam turbine, a first valve in the first steam supply system for adjusting steam pressure to the steam turbine, a second valve in the second steam supply system for adjusting branched steam pressure, a first controller that generates a first opening/closing signal for the first valve and a second opening/closing signal for the second valve, and a second controller that generates a third opening/closing signal for the second valve, the third opening/closing signal having priority over the second opening/closing signal. 
     Here, the third signal may be generated if the second valve is closed and the pressure in the steam turbine decreases. The third signal may include an opening signal for the second valve. 
     Further, the third signal may be generated if the second valve is closed within a predetermined time period after receiving the second signal. The third signal may be released if the second valve is opened within a predetermined time period after receiving the third signal. 
     The second valve may be multiplexed, and each second valve may accept the second signal and the third signal. 
     Furthermore, the third signal may be released if the steam pressure from the nuclear reactor is in a predetermined value. The third signal may be generated only once. 
     The third signal may be generated at least when the plant is not in regular operating mode. The third signal may act to avoid closing both the first valve and the second valve. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several preferred embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
     FIG. 1 is a diagram showing a main steam system and a turbine by-pass system of a nuclear power plant. 
     FIG. 2 is a block diagram showing a steam turbine controller for a nuclear power plant. 
     FIG. 3 is a block diagram showing a steam turbine controller for a nuclear power plant according to a first embodiment of the present invention. 
     FIGS. 4A and 4B are graphs showing function of the ON-OFF controller  14 . 
     FIG. 5 is a block diagram showing an ON-OFF controller in the steam turbine controller for a nuclear power plant according to a second embodiment of the present invention. 
     FIG. 6 is a timing chart for explaining advantages of the second embodiment. 
     FIG. 7 is a block diagram showing an ON-OFF controller in the steam turbine controller for a nuclear power plant according to a third embodiment of the present invention. 
     FIG. 8 is a timing chart for explaining advantages of the third embodiment. 
     FIG. 9 is a block diagram showing an ON-OFF controller in the steam turbine controller for a nuclear power plant according to a fourth embodiment of the present invention. 
     FIG. 10 is a timing chart for explaining advantages of the fourth embodiment. 
     FIG. 11 is a block diagram showing an ON-OFF controller in the steam turbine controller for a nuclear power plant according to a fifth embodiment of the present invention. 
     FIG. 12 is a timing chart for explaining advantages of the fifth embodiment. 
     FIG. 13 is a block diagram showing an ON-OFF controller in the steam turbine controller for a nuclear power plant according to a sixth embodiment of the present invention. 
     FIG. 14 is a block diagram showing signal decision units such as pressure detectors in FIG.  13 . 
     FIG. 15 is a block diagram showing an ON-OFF controller in the steam turbine controller for a nuclear power plant according to a seventh embodiment of the present invention. 
     FIG. 16 is a block diagram showing signal decision units such as pressure detectors in FIG.  15 . 
     FIG. 17 is a block diagram showing an ON-OFF controller in the steam turbine controller for a nuclear power plant according to an eighth embodiment of the present invention. 
     FIG. 18 is a chart for explaining advantages of the eighth embodiment. 
     FIG. 19 is a block diagram showing an ON-OFF controller modifying the ON-OFF controller shown in FIG.  17 . 
     FIG. 20 is a block diagram showing an ON-OFF controller in the steam turbine controller for a nuclear power plant according to a ninth embodiment of the present invention. 
     FIG. 21 is a block diagram showing an ON-OFF controller in the steam turbine controller for a nuclear power plant according to a tenth embodiment of the present invention. 
     FIG. 22 is a block diagram showing an ON-OFF controller in the steam turbine controller for a nuclear power plant according to an eleventh embodiment of the present invention. 
     FIG. 23 is a block diagram showing an ON-OFF controller in the steam turbine controller for a nuclear power plant according to a twelfth embodiment of the present invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Preferred embodiments of a steam turbine controller for a nuclear power plant of the present invention will now be specifically described in more detail with reference to the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
     FIG. 3 is a block diagram showing a steam turbine controller for a nuclear power plant according to a first embodiment of the present invention. 
     In first embodiment, an element capable of controlling opening/closing such as an ON-OFF controller  14  can operate the turbine by-pass valve  7  even if the regulating controller  13  malfunctions. The ON-OFF controller  14  rapidly opens the fast acting solenoid valve  37  independent from the regulating controller  13  that controls the position of the main steam control valve  6  and the turbine by-pass valve  7 . 
     The ON-OFF controller  14  includes an AND circuit  41  and an ON delay timer  43 . The AND circuit  41  receives a detection signal indicating that the turbine by-pass valve  7  is fully closed from a turbine by-pass valve fully closed position detector  39  and a detection signal indicating that the steam turbine  8  is tripped from a turbine trip detector  40 . The AND circuit  41  outputs a turbine by-pass valve non-operation detection signal  42  when both detection signals are recognized. 
     FIGS. 4A and 4B are graphs showing functions of the ON-OFF controller  14 ; that is, these figures are to explain advantages of the first embodiment of the present invention. As shown in FIG. 4A, the ON delay timer  43  outputs turbine by-pass valve fully opened demand signal  44  when the turbine by-pass valve non-operation detection signal  42  from the AND circuit  41  is entered during a certain period (ON delay time T) such as 0.1 second for example. Thus, the ON delay timer  43  can output the turbine by-pass valve fully opened demand signal  44  to the fast acting solenoid valve  37 . 
     If a turbine trip occurs while the regulating controller  13  operates normally, the turbine by-pass valve  7  can be controlled to open. This usually employs a time delay; it requires a time period from the generation of a detection signal from the turbine trip detector  40  to the opening operation of the turbine by-pass valve  7 , as shown in FIG.  4 A. Considering this time delay, the ON delay time T generated by the ON delay timer  43  can operate the turbine by-pass valve  7  effectively by providing a proper turbine by-pass valve fully opened demand signal  44  carried out from the turbine by-pass valve non-operation detection signal  42 . 
     According to the first embodiment explained above, the second control based on the on/off signal using the ON-OFF controller  14  as well as the first control based on the pressure signal using the regulating controller  13  control the turbine by-pass valve  7 . Thereby, the ON-OFF controller  14  can drive the fast acting solenoid valve  37  even if the regulating controller  13  has a problem such as a malfunction, and rapid increase of pressure in the nuclear reactor  1  can be avoided by ensuring opening operation of the turbine by-pass valve  7 . 
     A single ON-OFF controller  14  is employed in the first embodiment; however, the ON-OFF controller  14  can be multiplexed and a single turbine by-pass fully opened demand signal  44  can be chosen from among the multiple output signals. Specifically, the output signals from the turbine by-pass valve fully closed position detector  39  and the turbine trip detector  40  are input to the plural ON-OFF controllers  14 , and that the turbine by-pass valve fully opened demand signal  44  is output if at least one of the plural ON-OFF controllers  14  accepts a detection signals from both the turbine by-pass valve fully closed position detector  39  and the turbine trip detector  40 . 
     Furthermore, the turbine by-pass valve fully opened demand signal  44  is used to control the fast acting solenoid valve  37  in FIG.  3 . However, the turbine by-pass valve fully opened demand signal  44  can be applied to control the servo valve  33  instead. 
     FIG. 5 is a block diagram showing an ON-OFF controller  214  in the steam turbine controller for a nuclear power plant according to a second embodiment of the present invention, and FIG. 6 is a timing chart for explaining advantages of the second embodiment. An ON-OFF controller  214  employed in the second embodiment is constituted such that an OFF delay timer  45  is disposed downstream of the ON delay timer  43  of the ON-OFF controller  14  shown in FIG.  3 . This makes it possible to release the fully opened command based on the turbine by-pass valve fully opened demand signal  44  after a predetermined period from the opening operation of the turbine by-pass valve  7 . The other structures are identical to those explained in the first embodiment. 
     As shown in FIGS. 5 and 6, when fully opened condition of the turbine by-pass valve  7  is detected by the turbine by-pass valve fully closed position detector  39 , the turbine by-pass valve non-operation detection signal  42  is output from the AND circuit  41 . The turbine by-pass valve non-operation detection signal  42  is entered to the OFF delay timer  45  after a certain succession period, that is, after the on-delay period passes set in the ON delay timer  43 . If the off-delay period set in the OFF delay timer  45  passes, the turbine by-pass valve fully opened demand signal  44  is output from the OFF delay timer  45 . Thereby, the fully opened command based on the turbine by-pass valve fully opened demand signal  44  is released, restraining repeated opening/closing operations of the turbine by-pass valve  7  caused by the turbine by-pass valve non-operation detection signal  42  detecting continuously. 
     According to the present embodiment, if a turbine trip is detected by the turbine trip detector  40  and the fully closed condition of the turbine by-pass valve  7  is detected by the turbine by-pass valve fully closed position detector  39 , then the turbine by-pass valve non-operation detection signal  42  is output. The turbine by-pass valve non-operation detection signal  42  is reset when the turbine by-pass valve  7  is opened. That is, opening/closing operations caused by the continuous detection of output signals from the turbine by-pass valve fully closed position detector  39  is restrained while the turbine by-pass valve  7  is closing. Consequently, pressure inside the nuclear reactor  1  can be restrained based on the first opening operation of the turbine by-pass valve  7 . 
     FIG. 7 is a block diagram showing an ON-OFF controller  314  in the steam turbine controller for a nuclear power plant according to a third embodiment of the present invention, and FIG. 8 is a timing chart for explaining advantages of the third embodiment. 
     In the third embodiment, an ON-OFF controller  314  is employed corresponding to the plural turbine by-pass valves  7  such as three for example. On each turbine by-pass valve  7 , an OFF delay timer  45   a  having off-delay time T 1 , an OFF delay timer  45   b  having off-delay time T 2 , and an OFF delay timer  45   c  having off-delay time T 3  are connected, respectively. On the upstream side of these OFF delay timers  45   a ,  45   b  and  45   c , there is connected the AND circuit  41  via the ON delay timer similarly to FIG. 5, and the AND circuit  41  is capable of inputting detection signals obtained by the turbine by-pass valve fully closed position detector  39  and the turbine trip detector  40 . The other structures are identical to those explained in the first embodiment. 
     According to the present embodiment, if a turbine trip is detected by the turbine trip detector  40 , and the fully closed condition of the turbine by-pass valve  7  is detected by the turbine by-pass valve fully closed position detector  39 , then the turbine by-pass valve non-operation detection signal  42  is output from the AND circuit  41 . This turbine by-pass valve non-operation detection signal  42  is input to the ON delay timer  43 . If an output is made continuously during a certain time period, turbine by-pass valve fully opened demand signals  44   a ,  44   b  and  44   c  are generated corresponding to each turbine by-pass valve  7  using OFF delay timers  45   a ,  45   b  and  45   c.    
     Accordingly, after all the turbine by-pass valve fully opened demand signals  44   a ,  44   b  and  44   c  are generated, commands based on the turbine by-pass valve fully opened demand signals  44   a ,  44   b  and  44   c  are released shortly and all the turbine by-pass valves  7  are closed once. Thereby, it can be restrained that the turbine by-pass valves  7  repeat opening/closing operations caused by the continuous detection of the turbine by-pass valve non-operation detection signal  42 . 
     According to the present embodiment, after the opening operations of the turbine by-pass valves  7 , pressure build-up inside the nuclear reactor  1  can be restrained by closing the turbine by-pass valves  7 . Consequently, pressure inside the nuclear reactor  1  can be restrained based on the first opening operation of the turbine by-pass valves  7 . 
     Note that pressure inside the nuclear reactor  1  can be recovered by adjusting parameters of the OFF delay timers  45   a ,  45   b  and  45   c  connected to corresponding turbine by-pass valve  7  in this embodiment. Therefore, unique parameters can be set for individual turbine by-pass valves  7  so as to restrain the pressure increase. 
     FIG. 9 is a block diagram showing an ON-OFF controller  414  in the steam turbine controller for a nuclear power plant according to a fourth embodiment of the present invention, and FIG. 10 is a timing chart for explaining advantages of the fourth embodiment. 
     In the fourth embodiment, an ON-OFF controller  414 , which includes a self-holding circuit  51 , and a signal decision unit such as a pressure detector  52  for example are included in the ON-OFF controller  14  shown in FIG.  3 . The self-holding circuit  51  includes an OR circuit  49  and a wipe out circuit  50  capable of calculating “NOT” and “AND” and is connected to the downstream of the ON delay timer  43 . The pressure detector  52  inputs a second main steam pressure signal  65 , which is chosen by a second medium value selector  64  as the medium value among signals from the main steam pressure detector  2 , and a release command based on the turbine by-pass valve fully opened demand signal  44  by considering the detection signal from the pressure detector  52 . The other structures are identical to those explained in the first embodiment. 
     In the system as constituted above, if the detection of the fully opened condition of the turbine by-pass valve  7  is made using turbine by-pass valve fully closed position detector  39  as well as the detection of the turbine trip condition, the turbine by-pass valve non-operation detection signal  42  is output from the AND circuit  41 . The turbine by-pass valve non-.operation detection signal  42  is input to a self-holding circuit  51  via the ON delay timer  43 . The turbine by-pass valve fully opened demand signal  44  is held by the self-holding circuit  51  and is released by a release signal from the pressure detector  52  when the pressure detector  52  detects the main steam pressure signal  65  to be equal or less than a predetermined value (α). The pressure detector  52  is employed for the reason such that pressure inside the nuclear reactor  1  is adjusted to an acceptable value when the turbine by-pass valve  7  is opened and the pressure inside the nuclear reactor  1  is decreased. 
     According to the fourth embodiment, if the main steam pressure signal  65  is depressed to a certain value after suppressing a pressure peak of the main steam caused by a turbine trip, the fully opened turbine by-pass valve  7  can be reset automatically. 
     The main steam pressure detector  2  is preferably multiplexed for increased reliability. Therefore, detection signals from the triplex main steam pressure detector  2  are input to the ON-OFF controller  414 , and the medium value is chosen by the second medium value selector  64 . The second main steam pressure signal  65  obtained by the second medium value selector  65  is used as a release signal for the self-holding circuit  51 . 
     FIG. 11 is a block diagram showing an ON-OFF controller  514  in the steam turbine controller for a nuclear power plant according to a fifth embodiment of the present invention, and FIG. 12 is a timing chart for explaining advantages of the fifth embodiment. In the present embodiment, a one-shot circuit  55  receives a detection signal from the turbine by-pass valve fully closed position detector  39  and outputs a processed signal to AND circuit  41 . The one-shot circuit  55  includes a wipe out circuit  54  having a NOT circuit and an AND circuit and an ON delay timer  53  disposed parallel to the wipe out circuit  54 . The other structures are identical to those explained in the first embodiment. 
     By employing the one-shot circuit  55  thus constituted, the continuous opening/closing operation of the turbine by-pass valve  7  caused by the valid turbine by-pass valve fully opened demand signal  44  is restrained even if the main steam pressure signal  65  is decreased. Further, the turbine by-pass valve fully opened demand signal  44  is reset whereby the turbine by-pass valve  7  is fully opened. Consequently, the system can be operated only by the turbine by-pass valve fully closed detection signal  42  at first. 
     According to the fifth embodiment, the continuous opening/closing operation of the turbine by-pass valve  7 , such that the turbine by-pass valve  7  is fully opened and the command from the turbine by-pass valve fully opened demand signal  44  is released and such that the turbine by-pass valve  7  is fully closed and the command from the turbine by-pass valve fully opened demand signal  44  is effective, can be restrained. 
     FIG. 13 is a block diagram showing an ON-OFF controller  614  in the steam turbine controller for a nuclear power plant according to a sixth embodiment of the present invention, and FIG. 14 is a block diagram showing signal decision units such as pressure detectors  58 ,  59  and  60  in FIG.  13 . In the sixth embodiment, a plurality of turbine by-pass valves  7  such as three for example is employed, and an ON-OFF controller  614  for controlling the turbine by-pass valves  7  is constituted as explained below. 
     The ON-OFF controller  614  includes a second medium value selector  64 , three pressure detectors  58 ,  59 , and  60 , an AND circuit  41 , an ON delay timer  43 , an OR circuit  49 , a self-holding circuit  151 , and three AND circuits  61   a ,  61   b  and  61   c.    
     The second medium value selector  64  receives detection signals from the main steam pressure detectors  2 , and chooses the medium value for output. There may be, for example, three pressure detectors  2 . The pressure detectors  58 ,  59  and  60  are used as signal decision units and detect that the main steam pressure signal  65  is equal to or more than a predetermined value (β) and thereby output detection signals  67 ,  68  and  69 . The AND circuit  41  receives the turbine by-pass valve fully closed detection signal detected by the turbine by-pass valve fully closed position detector  39  and the turbine trip detection signal detected by the turbine trip detector  40 , and outputs a turbine by-pass valve non-operation detection signal  42  when both the turbine by-pass valve fully closed detection signal and the turbine by-pass valve non-operation detection signals are detected. The ON delay timer  43  receives the turbine by-pass valve non-operation detection signal and outputs a signal to the OR circuit  49  after a certain time period (ON delay time). The self-holding circuit  151  includes a wipe out circuit  150  that carries out a “NOT” and an “AND” operation. Each of the AND circuits  61   a ,  61   b , and  61   c  receives both the output signal from the self-holding circuit  151  and the pressure detection signal  68 ,  69  or  70 , and when both signals are detected, outputs a turbine by-pass valve fully opened demand signal  44   a ,  44   b  or  44   c . The other structures are identical to those explained in the first embodiment. 
     As shown in FIG. 14, the pressure detector  58  includes a turbine by-pass valve pressure deviation calculating unit  73  that receives an output signal from a turbine by-pass valve pressure setter  70  and the main steam pressure signal  65  to calculate a pressure deviation signal  76 , and a turbine by-pass valve pressure comparator  79  that receives the pressure deviation signal  76  and compares it with a predetermined value (β1) thereby outputting the result as a detection signal  67 . 
     The pressure detector  59  and the pressure detector  60  have the same basic structure as the pressure detector  58  in the present embodiment. 
     According to the sixth embodiment, the system can detect a fully opened condition when the turbine trip happens, and can detect if the main steam pressure is equal to or more than a predetermined value (β) as to each turbine by-pass valve  7 . The system thus outputs a fully opened demand signal to each turbine by-pass valve  7 . Therefore, unnecessary pressure control can be avoided by opening and closing required valves considering pressure increase and decrease thereof, and the valves can be controlled similarly to a regulating control depend on a pressure deviation signal. 
     Further, as shown in FIG. 13, a self-holding circuit  151  accepts a manual reset operation signal  57  from a manual reset operation means  56 , and the commands to the turbine by-pass valves  7  can be released by inputting the manual reset operation signal  57  to the wipe out circuit  50 . 
     FIG. 15 is a block diagram showing an ON-OFF controller  814  in the steam turbine controller for a nuclear power plant according to a seventh embodiment of the present invention, and FIG. 16 is a block diagram showing signal decision units such as pressure detectors  86 ,  87  and  88  in FIG.  15 . 
     In the present embodiment, instead of the pressure detectors  58 ,  59  and  60  disposed upstream of the AND circuits  61   a ,  61   b  and  61   c  as shown in FIG. 13, the pressure detectors  86   a ,  87  and  88  are employed. Further, instead of the second medium value selector  64  between the main steam pressure detectors  2  and the pressure detectors  58 ,  59  and  60  in FIG. 13, a pressure detector  52  and a pressure detector  83  are employed. The pressure detector  83  accepts an output signal from the pressure detector  52  and a signal predetermined by the main steam pressure setter  82  and carries out a main steam pressure deviation signal  84 . The main steam pressure deviation signal  84  thus calculated is input to the pressure detectors  86 ,  87  and  88 . 
     As shown in FIG. 16, the pressure detector  86  includes a turbine by-pass valve pressure deviation calculation unit  123  and a turbine by-pass valve pressure comparator  129 . The turbine by-pass valve pressure deviation calculation unit  123  accepts the main steam pressure deviation signal  84  and a signal (γ 1 ) predetermined by the turbine by-pass valve pressure setter  120  and calculates a turbine by-pass valve pressure deviation signal  126 . The turbine by-pass valve pressure comparator  129  accepts the pressure deviation signal  126  and outputs a pressure detector signal  89 . 
     The pressure detector  87  and the pressure detector  88  have the same basic structure as the pressure detector  86  in the present embodiment. 
     The turbine by-pass valve pressure setters  120 ,  121  and  122  are preset such that pressure values γ1, γ2 and γ3 corresponding thereto have different values and let the turbine by-pass valve  7  open and close in a predetermined sequential manner. Thereby, a continuous pressure control can be made. 
     When the functions of the pressure detectors  86 ,  87  and  88  are to be recovered, disconnect margins X 1 , X 2  and X 3  can be set with respect to the predetermined values in order to avoid repeating operation around the predetermined values. 
     A self-holding circuit  251  in the ON-OFF controller  814  inputs a manual reset operation signal  57  from the manual reset operation means  56  (not shown in FIG.  15 ). Because the manual reset operation signal  57  is applied to the wipe out circuit  50  in the self-holding circuit  251 , release of commands can be realized. 
     According to the seventh embodiment, unnecessary pressure control can be avoided by opening and closing required valves considering pressure increase and decrease thereof, and the valves can be controlled similarly to a regulating control depending on a pressure deviation signal. Furthermore, an operator can recover the system by using the manual reset operation means  56 , after confirming a stable condition of the nuclear reactor. 
     FIG. 17 is a block diagram showing an ON-OFF controller  914  in the steam turbine controller for a nuclear power plant according to an eighth embodiment of the present invention, and FIG. 18 is a chart for explaining advantages of the eighth embodiment. 
     In the present embodiment, a pressure control calculating unit  85  and pressure control calculating output detectors  92 ,  93  and  94  replace the pressure deviation detectors  86 ,  87  and  88  disposed between the pressure deviation calculator  83  and the AND circuits  61   a ,  61   b  and  61   c.    
     The pressure control calculating output detectors  92 ,  93  and  94  for detecting δ 1 , δ 2  and δ 3  detect that the output signals  84   a ,  84   b  and  84   c  from the pressure control calculating unit  85  are equal to or more than a predetermined value δ, and output signals  95 ,  96  and  97 . These signals  95 ,  96  and  97  are compared to the output signal from the self-holding circuit  251  in AND circuits  61   a ,  61   b  and  61   c , thereby outputting the turbine by-pass valve fully opened demand signals  44   a ,  44   b  and  44   c  to the corresponding turbine by-pass valves  7 . 
     The pressure control calculating output detectors  92 ,  93  and  94  are preset such that detected values δ1, δ2 and δ3 corresponding thereto have different values and let the turbine by-pass valve  7  open and close in a predetermined sequential manner. Thereby, a continuous pressure control can be made. Further, the predetermined values δ1, δ2 and δ3 are set to 5%, 35% and 65%, for example. When the functions of the pressure control calculating output detectors  92 ,  93  and  94  are to be recovered, disconnect margins X 1 , X 2  and X 3  can be set with respect to the predetermined values in order to avoid unwanted operations around the predetermined values. 
     FIG. 18 shows the case where the turbine by-pass valve  7  is fully opened when the pressure deviation output signal  84   a  takes δ 1 , δ 2  and δ 3 , and turbine by-pass valve  7  is fully closed when the pressure deviation output signal  84   a  takes δ 1 -X 1 , δ 2 -X 2  and δ 3 -X 3 . 
     The self-holding circuit  251  in the ON-OFF controller  914  receives a manual reset operation signal  57  from the manual reset operation means  56  (not shown in FIG.  17 ). Because the manual reset operation signal  57  is applied to the wipe out circuit  50  in the self-holding circuit  251 , release of commands can be realized. 
     According to the eighth embodiment, the turbine by-pass valve  7  can be opened and closed constantly by the same timing with respect to predetermined pressure values even if the predetermined pressure values are changed. Furthermore, an operator can recover the system by using the manual reset operation means  56 , after confirming a stable condition of the nuclear reactor. 
     FIG. 19 is a block diagram showing an ON-OFF controller  1014  modifying the ON-OFF controller  914  shown in FIG.  17 . In FIG. 19, the pressure deviation calculator  83  and the pressure control calculator  85  are omitted. On the contrary, output signal  98  from the main steam pressure setter  82  as well as the main steam pressure signal  65  are sent to pressure detectors  102 ,  103  and  104 , and output signals to the AND circuits  61   a ,  61   b  and  61   c  are generated. This modification results in similar effects to the embodiment shown in FIG.  17 . 
     FIG. 20 is a block diagram showing an ON-OFF controller  1114  in the steam turbine controller for a nuclear power plant according to a ninth embodiment of the present invention. In the present embodiment, pressure detectors  99 ,  100  and  101  replace the pressure detectors  70 ,  71  and  72  shown in FIG.  14 . 
     Explaining about the pressure detector  99  for example, the system is constituted such that the main steam pressure setter signal  98  and a turbine by-pass valve pressure setter bias (ε 1 )  108  are input to a turbine by-pass valve pressure set adder  105 , and the output from the turbine by-pass valve pressure set adder  105  is then entered to the pressure deviation calculator  73 . 
     The pressure detector  100  and the pressure detector  101  have the same basic structure as the pressure detector  99  in the present embodiment. 
     According to the ninth embodiment, the turbine by-pass valve  7  can be opened and closed constantly by the same timing with respect to predetermined pressure values even if the predetermined pressure values are changed. 
     FIG. 21 is a block diagram showing an ON-OFF controller  1214  in the steam turbine controller for a nuclear power plant according to a tenth embodiment of the present invention. In the present embodiment, a valve position demand signal changeover unit  122  is disposed between the servo valve  33  and the valve position control unit  32 . A self-holding circuit  126  includes an OR circuit  124  and a wipe out circuit  125  and receives signals from the manual reset operation means  56  and the ON delay timer  43 . The self-holding circuit  126  generates a valve position demand signal changeover signal  127  and sends that signal  127  to the valve position demand signal changeover unit  122 . Either a turbine by-pass valve fully opened demand signal  212  from the valve position control unit  32  or a fully opened demand signal  120  is output to the servo valve  33  as a servo valve input signal  123 . 
     The turbine by-pass valve fully opened demand signal  44  controls the fast acting solenoid valve  37  in FIG.  21 . However, similarly to the first embodiment, the turbine by-pass valve fully opened demand signal  44  can be applied to control the servo valve  33  instead. 
     According to the present embodiment, the turbine by-pass valve fully opened demand signal  44  is input to the self-holding circuit  126  and the valve position demand changeover signal  127  is output. Connecting condition of the valve position demand signal changeover unit  122  is then changed from “a to c” condition to “b to c” condition. Therefore, the servo valve input signal  123  is replaced by the fully closed demand  120  from the turbine by-pass valve fully opened demand signal  121  used in the regular operating mode. A command signal from the self-holding circuit  126  can be released by the manual reset operation signal  57  from the manual reset operation means  56 . 
     In a conventional system, the turbine by-pass valve  7  cannot be opened and closed by the fast acting solenoid valve  37  when the regulating controller  13  is under an unusual condition such as a malfunction or the like, even if the fast acting solenoid valve  37  is controlled based on the turbine by-pass valve fully opened demand signal  44 . However, the present embodiment can realize full-opening and full-closing of the turbine by-pass valve  7  by means of inputting fully opened demand signal  120  as the servo valve input signal  123 . 
     FIG. 22 is a block diagram showing an ON-OFF controller  1314  in the steam turbine controller for a nuclear power plant according to an eleventh embodiment of the present invention. In the ON-OFF controller  1314 , a reactor power signal  62  is input to the ON-OFF controller  1314 , and a signal decision unit  63  judges whether the output pressure of the reactor  1  is equal to or more than a predetermined value (ζ). If the detected-output pressure of the reactor  1  indicates that the turbine by-pass valve  7  has to be opened, the output signal  66  from the signal decision unit  63  is sent to the AND circuit  41  together with the detection signals from the turbine by-pass valve fully closed position detector  39  and the turbine trip detector  40  and thereby the turbine by-pass valve non-operation detection signal  42  is generated. 
     As the reactor power signal  62 , an output signal from an adjusting unit of the nuclear reactor, a discharge signal of the feeding water, a discharge signal of the main steam, an output signal from the generator, a pressure signal from a first stage pressure of the steam turbine and the like of a nuclear power plant can be applied. 
     According to the present embodiment, opening operation of the turbine by-pass valve  7  can be restrained under the condition where output energy is relatively low and opening operation of the turbine by-pass valve  7  is not required. 
     FIG. 23 is a block diagram showing an ON-OFF controller  1414  in the steam turbine controller for a nuclear power plant according to a twelfth embodiment of the present invention. In the twelfth embodiment, pressure signals from the reactor dome pressure detector  11  which indicate pressure inside the nuclear reactor  1  are employed instead of the main steam pressure signal s  2  in FIG.  3 . Thus, the pressure signal  28  and the ON-OFF control pressure signal  65  are generated. 
     According to the present embodiment, rapid pressure increase of the nuclear reactor  1  can be restrained in a nuclear power plant having the reactor dome pressure detector  11 , as well as a nuclear power plant having the main steam pressure detector  2 . 
     The foregoing discussion discloses and describes merely a number of exemplary embodiments of the present invention. As will be understood by those skilled in the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims. Thus, the present invention may be embodied in various ways within the scope of the spirit of the invention. 
     Especially, the above-explained elements such as speed/load control calculating unit  15 , load limiter  16 , maximum discharge restriction unit  17 , first low value selector  18 , first deviation calculating unit  20 , second deviation calculating unit  21 , second low value selector  22 , main steam pressure setter  23 , first pressure deviation calculating unit  24 , pressure control calculating unit  25 , first medium value selector  27 , AND calculator  41 , OR calculator, wipe out circuit  50 , self-holding circuit  51 , pressure detector  52 , wipe out circuit  54 , one-shot circuit  55 , manual reset operation means  56 , pressure detector  58 , pressure detector  59 , pressure detector  60 , AND circuit, pressure deviation switch  63 , second medium value selector  64 , turbine by-pass valve pressure setters  70 ,  71  and  72 , turbine by-pass valve No. 1  pressure deviation calculators  73 ,  74  and  75 , turbine by-pass valve No. 1  pressure comparators  79 ,  80  and  81 , main steam pressure setter  82 , pressure deviation calculator  83 , pressure control calculating unit  85 , pressure deviation switches  86 ,  87  and  88 , pressure control calculating output detectors  92 ,  93  and  94 , pressure detectors  99 ,  100  and  101 , turbine by-pass valve pressure set adders  105 ,  106  and  107 , turbine by-pass valve pressure deviation calculators  111 ,  112  and  113 , turbine by-pass valve pressure comparators  117 ,  118  and  119 , valve position demand signal changeover unit  122 , OR circuit  124 , wipe out circuit  125 , self-holding circuit  126  and the like are not limited to be constituted as hardware; these elements can be stored in a memory or a part of a CPU (Central Processing Unit), which can read data from the memory and calculate for the following processes, or the like. 
     The same function can be realized by installing programs into a computer. 
     Optical disks such as a DVD, a MO or a CD-ROM, magnetic disks such as a floppy disk and a hard drive disk, and other storage devices including a semiconductor memory can, be applied for installing the functions. 
     Further, multiplex regulating controllers  13  are employed in the embodiments; however, same advantages can be expected if a single regulating controller  13  is applied. 
     As described above in detail, the present invention makes it possible to provide a nuclear power plant capable of avoiding a rapid increase of pressure in the nuclear reactor even if the function of the turbine by-pass valve is lost. 
     The entire contents of Japanese Patent Application P2000-108622, filed Apr. 10, 2000, are incorporated herein by reference.