Abstract:
A nuclear power plant of the invention is equipped, for example, with a reactor; a steam loop comprising high and low pressure turbines; a condenser for condensing steam discharged therefrom the low pressure turbine; a feedwater heater for heating feedwater supplied from the condenser; and a feedwater loop for leading feedwater discharged from the feedwater heater to the reactor, and an operation method thereof is characterized by augmenting a second reactor thermal power output in a second operation cycle of the reactor larger than a first reactor thermal power output in a first operation cycle; and making an increase ratio of extraction steam, which is led to the feedwater heater with being extracted from the steam loop, in the second operation cycle for the first operation cycle smaller than an increase ratio of the second reactor thermal power output for the first reactor thermal power output.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a nuclear power plant and an operation method thereof, and particularly, to the nuclear power plant and the operation method thereof preferable for augmenting a power generation capacity.  
         [0003]     2. Description of the Related Art  
         [0004]     In a conventional newly-constructed nuclear power plant, for example, a power output is augmented by improving either of a composition or a shape configuration of a fuel assembly, or the like, and increasing a main steam flow rate at an outlet of a reactor in order to augment the power output.  
         [0005]     A technology of such a conventional example is disclosed in Japanese Patent Laid-Open Publication Hei. 9-264983.  
         [0006]     When applying the conventional technology described above to an existing nuclear power plant, the main steam flow rate increases substantially proportional to an increase of the power output. In order to suppress an increase of the main steam flow rate, a feedwater temperature may be lowered; however, because if an extraction steam for heating the feedwater is simply totally decreased, a thermal efficiency is extensively deteriorated and the power output hardly increases, it is not realistic. Therefore, by the increase of the main steam flow rate is decreased a design margin of pressure vessel internals such as feedwater piping, a feedwater heater, a feedwater pump, and a steam dryer, and almost all instruments such as a main steam pipe, a high pressure turbine, a low pressure turbine, and a condenser. In a power plant using a normal boiling water reactor, the high pressure turbine is one of instruments that have a possibility of firstly losing a design margin due to the increase of the main steam flow rate. Also in a nuclear power plant system other than a boiling water reactor, there is a similar problem with respect to a plant having a comparatively small design margin of the high pressure turbine, and when applying a conventional technology to an existing nuclear power plant, there is needed a large scale improvement and change of the plant instruments.  
         [0007]     Consequently, it is strongly requested a nuclear power plant and operation method thereof that enable a power uprate of the plant without extensively changing a configuration of plant instruments with respect to the power uprate of an existing nuclear power plant.  
       SUMMARY OF THE INVENTION  
       [0008]     A first invention to achieve the above problem is, when making it one operation cycle a period from an activation of a nuclear power plant to an operation stop thereof for changing fuel; to augment a second reactor thermal power output in a second operation cycle larger than a first reactor thermal power output in a first operation cycle before the second operation cycle; and to make an increase ratio of extraction steam, which is led to a feedwater heater with being extracted from a steam loop, in the second operation cycle for the first operation cycle smaller than that of the second reactor thermal power output for the first reactor thermal power output.  
         [0009]     A second invention to achieve the above problem is, when making it one operation cycle a period from an activation of a nuclear power plant to an operation stop thereof for changing fuel; to augment a second reactor thermal power output in a second operation cycle larger than a first reactor thermal power output in a first operation cycle before at least not less than one operation cycle than the second operation cycle; and to make it smaller in the second operation cycle than in the first operation cycle, a ratio of extraction steam specifically from a middle area and outlet of a high pressure turbine (actually between from the outlet of the high pressure turbine to any one of inlets of a moisture separator, a moisture separator and heater, and a moisture separator and reheater) out of extraction steam, which is led to a feedwater heater with being extracted from a steam loop in the first operation cycle, for a main steam flow rate.  
         [0010]     In addition, a third invention to achieve the above problem is to augment a second reactor thermal power output in a second operation cycle of a reactor larger than a first reactor thermal power output in a first operation cycle before at least not less than one operation cycle than the second operation cycle and to make it in the second operation cycle smaller than the first operation cycle, a mass flow rate of extraction steam specifically from a middle area and outlet of a high pressure turbine out of extraction steam led to a feedwater heater with being extracted from a steam loop.  
         [0011]     In addition, a fourth invention to achieve the above problem is to augment a second reactor thermal power output in a second operation cycle of a reactor larger than a first reactor thermal power output in a first operation cycle before at least not less than one operation cycle than the second operation cycle and to make it small, a temperature rise amount at a high pressure feedwater heater placed at a more downstream side than specifically a main feedwater pump out of a plurality of feedwater heaters.  
         [0012]     In addition, a fifth invention to achieve the above problem is to augment a second reactor thermal power output in a second operation cycle of a reactor larger than a first reactor thermal power output in a first operation cycle before at least not less than one operation cycle than the second operation cycle and to stop at least not less than one loop of an extraction steam pipe specifically from a middle area and outlet of a high pressure turbine out of extraction steam led to a feedwater heater with being extracted from a steam loop. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  is a schematic drawing of a heat balance of a boiling water reactor plant of an embodiment of the present invention.  
         [0014]      FIG. 2  is a schematic drawing of a heat balance of the boiling water reactor plant before a power uprate.  
         [0015]      FIG. 3  is a schematic drawing of a heat balance of the boiling water reactor plant in applying a conventional power uprate method.  
         [0016]      FIG. 4  is a schematic drawing of a heat balance of the boiling water reactor plant in bypassing a feedwater heater of an embodiment of the present invention.  
         [0017]      FIG. 5  is a schematic drawing 1 of a relationship between an operation cycle and a reactor thermal power output, a main steam flow rate, and an extraction steam flow rate.  
         [0018]      FIG. 6  is a schematic drawing 2 of a relationship between an operation cycle and a reactor thermal power output, a main steam flow rate, and an extraction steam flow rate.  
         [0019]      FIG. 7  is a schematic drawing of a heat balance of a pressurized water reactor plant of an embodiment of the present invention.  
         [0020]      FIG. 8  is a schematic drawing of a heat balance of the pressurized water reactor plant before a power uprate.  
         [0021]      FIG. 9  is a schematic drawing of a heat balance of the pressurized water reactor plant in applying a conventional power uprate method.  
         [0022]      FIG. 10  is a schematic drawing of a heat balance of the pressurized water reactor plant 0  in bypassing a feedwater heater of an embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     Here will be described an embodiment where the present invention is applied to the boiling water reactor of one of direct-cycle nuclear power plants.  
         [0024]      FIG. 1  shows a heat balance example of a boiling water reactor (BWR) after a power uprate according to the present invention, and  FIG. 2  shows a heat balance example of the boiling water reactor before a power uprate.  FIG. 3  shows a heat balance example of the boiling water reactor after a conventional power uprate.  FIG. 4  shows an example for realizing a heat balance of the present invention shown in  FIG. 1 . Although in  FIG. 1  an extraction steam amount is reduced by placing a valve at a middle area of an extraction pipe, a method shown in  FIG. 4  is applied when there is no space at the middle area of the extraction pipe and placement cost of the valve is high. In addition, each of  FIGS. 5 and 6  show a conceptual drawing of an operation cycle of an embodiment of the present invention. Meanwhile, in  FIGS. 1, 2 , and  3  are represented a reactor thermal power output as Q, each mass flow rate of water and steam as G, and each enthalpy of water and steam as H; and the reactor thermal power output Q and a mass flow rate G are expressed in each ratio (%) for a reactor thermal power output of a reactor before a power uprate and a steam flow rate at an outlet of a reactor pressure vessel described in  FIG. 2 , and an enthalpy is expressed in a numeral with a unit of kJ/kg. In addition, each embodiment of the present invention shows a normal operation condition; and operation conditions of an activation, stop time, transient state, and furthermore accident are excluded.  
         [0025]     The embodiment of the present invention is shown in  FIG. 1 , and the conceptual drawing of the operation cycles for complementing the embodiment is shown in  FIG. 5 .  FIG. 1  is a drawing schematically showing the heat balance example in a case of performing the power uprate in the boiling water reactor that comprises a recirculation pump and a jet pump within a reactor pressure vessel  1  and has a main steam pipe  2 , a high pressure turbine  3  and low pressure turbine  5  connected to the main steam pipe, and a moisture separator  4  between the high pressure turbine and the low pressure turbine.  FIG. 5  contrasts relationships between an operation cycle and a reactor thermal power output, a main steam flow rate (steam flow amount flowing in the main steam pipe from the reactor pressure vessel), and an extraction steam amount together with a conventional power uprate method. Meanwhile, one operation cycle is defined as a period from an activation out of a stop condition of a reactor operation to a stop thereof for a fuel change.  
         [0026]     In the operation cycle shown in  FIG. 5  an Nth operation cycle is before a power uprate method of the present invention is applied, and at this time the reactor thermal power output is Q=100%. A heat balance example before the power uprate is shown in  FIG. 2 . An (N+1)th operation cycle increases the reactor thermal power output by 5% and thereby makes Q=105%. An increase of the reactor thermal power output can be realized by any method of enlarging a pull-out amount of control rods in the (N+1)th cycle larger than in the Nth cycle; a reactor core flow rate in the (N+1)th cycle larger than in the Nth cycle with rising a rotation number of the recirculation pump; and changing a kind of a fuel assembly. In addition, because if applying the present invention, a temperature of feedwater supplied to the reactor pressure vessel lowers, it can also be expected that the reactor thermal power output naturally rises by coolant density feedback for core reactivity due to a lowering of a reactor-core-inlet coolant temperature. Meanwhile, in some plant an extraction steam flow rate and main steam flow rate in one cycle are changed as shown in  FIG. 6 . In a case of a plant adopting the operation cycle as shown in  FIG. 6 , it is assumed that the heat balance, extraction steam flow rate, main steam flow rate, feedwater heating amount, and the like are compared at an operation point where the main steam flow rate becomes maximum in the operation cycle excluding an activation, stop, accident/transient phenomenon occurrence time, and test operation.  
         [0027]     When increasing the reactor thermal power output, it is necessary to increase a feedwater flow rate or to widen an enthalpy difference of a coolant at an inlet/outlet of the reactor pressure vessel in order to remove the increment of thermal energy. The conventional power uprate method adopts the former method and increases the feedwater flow rate in proportion to the reactor thermal power output. A heat balance example by the conventional power uprate method is shown in  FIG. 3 . As a result, in the conventional power uprate method the main steam flow rate of the (N+1)th operation cycle shown in  FIG. 5  becomes 105%. The present invention adopts the latter method and is characterized by widening the enthalpy difference of the coolant at the inlet/outlet of the reactor pressure vessel by intentionally lowering a feedwater enthalpy at the inlet of the reactor pressure vessel. In order to lower the feedwater enthalpy at the inlet of the reactor pressure vessel, although it is available to decrease an extraction steam from a steam loop and thereby to decrease a steam amount sent to feedwater heaters, if simply totally decreasing an extraction steam amount, a thermal efficiency decreases and a power generation amount cannot be increased so much. Accordingly, by selectively decreasing an extraction steam amount from any of a middle area and outlet of the high pressure turbine (actually between from the outlet of the high pressure turbine to an inlet of the moisture separator), a steam amount flowing in the low pressure turbine is increased and thus the power generation amount is increased. Because much extraction steam from any of the middle area and outlet of the high pressure turbine is used at a feedwater heater placed at a more downstream side than a main feedwater pump, if changing a viewpoint, the power uprate method of the present invention becomes a method of decreasing a feedwater heating amount at the more downstream side than the main feedwater pump. Meanwhile, in a case of a plant where an original extraction steam amount from any of the middle area and outlet of the high pressure turbine is little, it is necessary to also decrease an extraction steam amount from the low pressure turbine in order to sufficiently decrease a feedwater temperature. If when even applying the present invention to such a plant, the extraction steam amount any of the middle area and outlet of the high pressure turbine is decreased more, some extent of effect can be obtained. In the embodiment, in spite of increasing the reactor thermal power output by 5% compared to that of the Nth cycle, the main steam flow rate can be made same as that of the Nth cycle. Although because the embodiment shows an ideal power uprate method, the main steam flow rates of the Nth and (N+1)th operation cycles are assumed same, they need not always be entirely same and may be increased within a range of an instrument design margin including the high pressure turbine.  
         [0028]     When there are a plurality of extraction points at any of the middle area and outlet of the high pressure turbine, an extraction point for decreasing an extraction steam amount is most effective if it is selected at the most upstream side. In this case although it is available to place an extraction pipe flow rate adjustment valve  10  for controlling the extraction steam amount and to decrease it, it is available to completely close at least not less than one loop of an extraction pipe. As a closing method, it is available to place a shut-off valve on the way of an extraction pipe or to plug it. When having completely closed the extraction pipes, control loop instruments of the extraction steam amount becomes unnecessary and operation control is also simplified. It depends on a heat balance and a power uprate range whether controlling the extraction steam amount or completely closing the extraction pipes (because when an extraction steam amount per extraction pipe is too much, a feedwater temperature lowers too much in a case of completely closing the extraction pipes, then adjust the extraction steam amount.). In addition, instead of placing a shut-off valve on the way of an extraction pipe, a feedwater flow rate flowing in a feedwater heater may be decreased. The embodiment is shown in  FIG. 4 . In the embodiment a feedwater heater bypass loop  11  is placed on the way of feedwater piping, and a part of feedwater is made to flow in the bypass loop  11 . A low temperature coolant flowing in the bypass loop  11  bypasses at least not less than one feedwater heater and then mixes with high temperature main feedwater. Thus a lowering of a feedwater temperature can be realized at an inlet of the reactor pressure vessel.  
         [0029]     Because when even augmenting the reactor thermal power output and increasing the power generation amount of a nuclear power plant, the embodiment can suppress an increase of a feedwater flow rate and a main steam flow rate, it can suppress an increase of a load burdened on a feedwater pipe, main steam pipe, and pressure vessel internal. Compared to a case of simply totally decreasing the extraction steam amount, the present invention can suppress the lowering of the thermal efficiency and obtain a larger power output. In addition, although in an extensive power uprate by a conventional power uprate method it generally becomes necessary to change the high pressure turbine, a power uprate range performable without a change of the high pressure turbine widens compared to the conventional method. Because if the feedwater temperature lowers, a thermal margin (corresponding to an MCPR (Minimum Critical Power Ratio) in a case of the BWR) of a reactor core increases, there is also a merit in an increase of a design margin compared to the conventional method. Because although in a power uprate a pressure loss and stability of the reactor core deteriorates, a void fraction of the reactor core becomes lower and an absolute value of void coefficient of the reactor core becomes larger in the power uprate method of the present invention, the pressure loss of the reactor core is reduced, and the deterioration of the stability of the reactor core is also suppressed. The decrease of the pressure loss of the reactor core means that an increase of a load by a power uprate on the jet pump and recirculation pump for recirculating a coolant can also be suppressed. Because an increase amount of generation steam in the reactor core also becomes small compared to that of the thermal power output, an influence on a carry under that occurs due to a steam entrainment into recirculation water is also small, and even in an extensive power uprate, it becomes easy to ensure a flow window. In a direct-cycle nuclear power plant other than the boiling water reactor is enabled a power uprate by a similar method.  
         [0030]     Table 1 shows a relationship among a reactor thermal power output, main steam flow rate, extraction steam flow rate, and feedwater enthalpy when applying the power uprate method of the embodiment to various output increase amounts. The reactor thermal power output and the main steam flow rate show ratios in the case of a reactor thermal power output 100%, and the extraction steam flow rate shows a ratio for the main steam flow rate in the case of the reactor thermal power output 100%. As seen from Table 1, even when making the reactor thermal power output 110%, the power uprate method of the present invention is widely applicable. A reason why the output is not shown only till 110% in Table 1 is that in the power uprate not less than this a change of the moisture separator and the like becomes necessary; if permitting the change of the moisture separator or being combined with a reactor pressure increase, an introduction of the moisture separator and heater, and the like, the power uprate method of the present invention is more extensively applicable.  
         [0031]     Meanwhile, generally in the boiling water reactor a reactor thermal power output is performable till an extent of a reactor thermal power output 102% only by improving measurement accuracy of a feedwater flowmeter and the like, and the present invention has a large effect for a power uprate exceeding the reactor thermal power output 102%. Furthermore, in the power uprate till an extent of a reactor thermal power output 105%, it is generally unnecessary to extensively change system instruments such as a change of the high pressure turbine. Using the present invention, particularly a large effect can be obtained because the change of the high pressure turbine becomes unnecessary even in the power uprate exceeding the reactor thermal power output 105%.  
                                             TABLE 1                       Reactor           Feedwater       thermal power   Main Steam   Extraction Steam   Enthalpy       output (%)   Flow Rate (%)   Flow Rate (%)   (kJ/kg)                                100   100   45   924       103   100   43   869       105   100   42   831       107   100   40   795       110   100   38   739       110   105   42   831                  
 
         [0032]     Next will be shown an embodiment applied to a pressurized water reactor (PWR) of one of indirect cycle nuclear power plants of the present invention.  
         [0033]      FIG. 7  shows a heat balance example of the pressurized water reactor of the present invention after a power uprate, and  FIG. 8  shows a heat balance example of the pressurized water reactor before a power uprate.  FIG. 9  shows a heat balance of the pressurized water reactor after applying a conventional power uprate method. Each of  FIGS. 5 and 6  shows the conceptual drawing of the operation cycle of one embodiment of the present invention. Meanwhile, in  FIGS. 7, 8 , and  9  are represented a reactor thermal power output as Q, each mass flow rate of water and steam as G, and each enthalpy of water and steam as H; and the reactor thermal power output Q and a mass flow rate G are expressed in each ratio (%) for the reactor thermal power output and steam flow rate (steam amount flowing in a secondary main steam pipe from a steam generator) of a reactor before the power uprate described in  FIG. 8 , and an enthalpy is expressed in a numeral with a unit of kJ/kg. Meanwhile, a heat exchange amount at a steam generator is an amount where a heat leak in a primary loop is subtracted from a reactor thermal power output, and because a normal heat leak amount is sufficiently small compared to the reactor thermal power output, the heat exchange amount at the steam generator and the reactor thermal power output are assumed equal.  
         [0034]     The embodiment of the present invention is shown in  FIG. 7 , and the conceptual drawing of the operation cycle for complementing the embodiment is shown in  FIG. 5 .  FIG. 7  is a drawing schematically showing a heat balance example in the pressurized water reactor that comprises a reactor pressure vessel  1 , a steam generator  13  transferring heat generated at a reactor core within the reactor pressure vessel to a secondary loop, a main steam pipe  2  leading secondary loop steam going out of the steam generator, a high pressure turbine  3  and low pressure turbine  5  connected to the main steam pipe, and a moisture separator and heater  12  between the high pressure turbine and the low pressure turbine.  FIG. 5  contrasts relationships between an operation cycle and a reactor thermal power output, a main steam flow rate, and an extraction steam amount in a case of using the embodiment together with a conventional power uprate method. Meanwhile, one operation cycle is defined as a period from a reactor activation to a reactor operation stop for a fuel change.  
         [0035]     In the operation cycle shown in  FIG. 5  an Nth operation cycle is before an power uprate method of the present invention is applied, and at this time the reactor thermal power output is Q=100%. A heat balance example before the power uprate is shown in  FIG. 8 . An (N+1)th operation cycle increases the reactor thermal power output by 5% and thus makes Q=105%. An increase method of the reactor thermal power output can be realized by any method of enlarging a pull-out amount of control rods in the (N+1)th cycle larger than in the Nth cycle and changing a kind of a fuel assembly. Meanwhile, in some plant an extraction steam flow rate and main steam flow rate in one operation cycle are also changed as shown in  FIG. 6 . In a case of a plant adopting the operation cycle as shown in  FIG. 6 , it is assumed that a heat balance, extraction steam flow rate, main steam flow rate, feedwater heating amount, and the like are compared at an operation point where the main steam flow rate becomes maximum in the operation cycle excluding an activation, stop, accident/transient phenomenon occurrence time, and test operation.  
         [0036]     When increasing the reactor thermal power output, it is necessary to increase a primary coolant flow rate into the reactor pressure vessel and a secondary feedwater flow rate into the steam generator or enlarging an enthalpy difference of a primary coolant at an inlet/outlet of the reactor pressure vessel and that of a secondary coolant at an inlet/outlet of the steam generator in order to remove the increment of thermal energy. The conventional power uprate method adopts the former method and increases the primary coolant flow rate and the secondary feedwater flow rate in proportion to the reactor thermal power output. A heat balance example by the conventional power uprate method is shown in  FIG. 9 . As a result, in the conventional power uprate method the main steam flow rate of the (N+1)th operation cycle shown in  FIG. 5  becomes 105%. The present invention adopts the latter-method and is characterized by enlarging the enthalpy difference of the secondary coolant at the inlet/outlet of the reactor pressure vessel with intentionally lowering a secondary feedwater enthalpy at the inlet of the steam generator. In order to lower the feedwater enthalpy at the inlet of the reactor pressure vessel, although it is available to decrease an extraction steam from a steam loop and thereby to decrease a steam amount sent to the feedwater heaters, if simply totally decreasing an extraction steam amount, a thermal efficiency decreases and a power generation amount cannot be increased so much. Accordingly, by selectively decreasing the extraction steam amount from any of a middle area and outlet of the high pressure turbine (actually between from the outlet of the high pressure turbine to an inlet of the moisture separator and heater), a steam amount flowing in the low pressure turbine is increased and thus the power generation amount is increased. Because much extraction steam from any the middle area and outlet of the high pressure turbine is used at a feedwater heater placed at a more downstream side than a main feedwater pump, if changing a viewpoint, the power uprate method of the present invention becomes a method of decreasing a feedwater heating amount at the more downstream side than the main feedwater pump. Meanwhile, in a case of a plant where an original extraction steam amount from any of the middle area and outlet of the high pressure turbine is little, it is necessary to also decrease the extraction steam amount from the low pressure turbine in order to sufficiently decrease a feedwater temperature. Even if when applying the present invention to such a plant, the extraction steam amount from any of the middle area and outlet of the high pressure turbine is decreased more, some extent of effect can be obtained. In the embodiment, in spite of increasing the reactor thermal power output by 5% compared to that of the Nth cycle, the main steam flow rate can be made same as that of the Nth cycle. Although because the embodiment shows an ideal power uprate method, the main steam flow rates of the Nth and (N+1)th operation cycles are assumed same, they need not always be entirely same and may be increased within a range of an instrument design margin including the high pressure turbine.  
         [0037]     When there are a plurality of extraction points at any of the middle area and outlet of the high pressure turbine, an extraction point for decreasing the extraction steam amount is most effective if it is selected at the most upstream side. In this case although it is available to place an extraction pipe flow rate adjustment valve  10  for controlling the extraction steam amount and to decrease it, it is also available to completely close at least not less than one loop of an extraction pipe. As a closing method, it is available to place a shut-off valve on the way of the extraction pipe or to plug it. When having completely closed the extraction pipes, control loop instruments of the extraction steam amount becomes unnecessary and operation control is also simplified. It depends on a heat balance and a power uprate range whether controlling the extraction steam amount or completely closing the extraction pipes (because when an extraction steam amount per extraction pipe is too much, a feedwater temperature lowers too much in a case of completely closing the extraction pipes, then the extraction steam amount is adjusted.). In addition, instead of placing a shut-off valve on the way of an extraction pipe, a feedwater flow rate flowing in a feedwater heater may be decreased. The embodiment is shown in  FIG. 10 , and it shows an example for realizing a heat balance of the present invention shown in  FIG. 7 . Although in  FIG. 7  an extraction steam amount is reduced by placing a valve at a middle area of an extraction pipe, a method shown in  FIG. 10  is applied when there is no space at the middle area of the extraction pipe and placement cost of the valve is high. In the embodiment a feedwater heater bypass loop  11  is placed on the way of feedwater piping, and a part of feedwater flow is made to flow in the bypass loop  11 . A low temperature coolant flowing in the bypass loop  11  bypasses at least not less than one feedwater heater and then mixes with high temperature main feedwater. Thus a lowering of a feedwater temperature can be realized at the inlet of the reactor pressure vessel.  
         [0038]     Because when even augmenting the reactor thermal power output and increasing the power generation amount of a nuclear power plant, the embodiment can suppress the increase of the feedwater flow rate and the main steam flow rate, it can suppress the increase of a load burdened on the feedwater pipe, main steam pipe, and steam generator. It is also enabled to lower the reactor pressure vessel inlet temperature of a primary loop without increasing the primary coolant flow rate, and in this case it is more effective to suppress the increase of a load burdened on the steam generator and a load on the primary coolant pump is also reduced. Furthermore, if the reactor pressure vessel inlet temperature of the primary loop lowers, a thermal margin (corresponding to a DNBR (Departure from Nuclear Boiling Ratio) in the case of the PWR) of a reactor core increases, there is also a merit in an increase of a design margin compared to the conventional method. In an indirect-cycle nuclear power plant other than the pressurized water reactor is enabled a power uprate by a similar method.  
         [0039]     Thus, although the embodiments of the present invention are described, the invention is not limited thereto, and various variations are available without departing from the spirit and scope of the invention.