Patent Number: 059636114
Section: summary

BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to a steam separator, a nuclear power generation plant, and a boiler apparatus equipped with a separator/injector having a two-phase flow accelerator nozzle for guiding a two-phase flow of mixed liquid and vapor components in the interior thereof and accelerating the same, a liquid-phase capture means for capturing the liquid phase of the thus accelerated two-phase flow, and means for increasing the pressure of this liquid phase and imparting a recirculation drive force thereto. The description herein relates to nuclear power plants in general, taking a boiling-water reactor as an example with reference to the accompanying drawings. Referring to FIG. 27, a reactor container 106 of a boiling-water reactor (BWR) is configured of a reactor pressure vessel 102 accommodating a core 101, a drywell 103 that contains this reactor pressure vessel 102, and a wetwell 105 having a pressure suppression pool 104. In addition, this nuclear power plant comprises a turbine 107, a main steam line 108 that supplies steam to that turbine 107, a main condenser 109, a condensate pump 110, a feedwater pump 111 that supplies feedwater to the reactor pressure vessel 102, a feedwater heater 112, a feedwater pipeline 113, a reactor recirculation system 114 that causes changes in the quantity of core coolant that recirculates therethrough, a control rod drive system 115 that controls the output, a standby coolant system 116 that operates when the reactor has been isolated by valves, a residual heat removal system that removes residual heat when the reactor is halted, and an emergency core coolant system (ECCS) that operates during emergencies. Existing BWRs use a forced recirculation method by which coolant is sent through the core by the reactor recirculation system 114. This reactor recirculation system 114 is configured of a recirculation pump 117 and a jet pump 118. In an emergency, the recirculation pump 117 has a certain amount of inertia and takes about five seconds to stop, so the cooling efficiency of the coolant has to rely on a relatively weak natural circulatory system. The ECCS is configured of a high-pressure core spray system 119 and a low-pressure core spray system 120 that also acts as the residual heat removal system. These operate together with a containment spray 121. The emergency core coolant system uses a condensate storage tank 122 or the pressure control pool 104 as a water source and supplies water into the core 101 by the rotation of a centrifugal pump driven by power supplied from emergency diesel generators 123, or sprays the water into the reactor container 106. During an emergency, a boric acid solution is dumped in by an SLC (Standby Liquid Control System) pump 125 from a SLCS tank 124 into a lower plenum of the reactor pressure vessel. In an advanced boiling-water reactor (ABWR), which is an improvement on the BWR, the external recirculation piping for the reactor recirculation system of the above described BWR is removed and recirculation through the core is enabled by the provision of a jet pump that is smaller than an internal pump, instead. The employment of an internal pump has various effects, such as a reduction in the pressure losses with respect to the flow of reactor coolant, in comparison with a BWR. A cross-sectional view illustrating the concept of the systems of this ABWR is shown in FIG. 28. A core 52 that is provided with a large number of fuel rod assemblies is disposed slightly below the center of a reactor presure vessel 51. A large number of control rod guidance tubes 53 are provided below this core 52, and an upper aperture of a shroud 54 that shapes the core 52 is closed by a shroud head 55. Stand pipes 57 of steam separators 56 are erected above the shroud head 55, and flat, rectangular steam dryers 58 are disposed above the steam separators 56. A control rod drive mechanism 59 is provided in a lower portion of the presure vessel 51, to drive the cross-shaped control rods within the core 52, using the inner surfaces of the control rod guide tubes 53. A plurality of internal pumps 60 are disposed in a base portion between the inner side of the reactor presure vessel 51 and the outer side of the shroud 54. The core 52 is supported by a core support plate 61 that supports a lower portion of the large number of fuel rod assemblies, an upper portion thereof is supported by an upper lattice plate 62, and the entire core is surrounded by the shroud 54. A main steam line 108 that sends steam that has been dried by the steam dryers 58 to a turbine is connected to the reactor presure vessel 51. Coolant flowing into the reactor pressure 51 from a feedwater line 113 is recirculated by the internal pumps 60. The reactor presure vessel 51 is mounted on and fixed to a pedestal, with a support skirt 63 therebetween. An upper aperture of the reactor presure vessel 51 is hermetically sealed by an upper lid 64. A cross-sectional view of one of the steam separators 56 disposed within the reactor presure vessel 51 is shown in FIG. 29. This steam separator 56 comprises swirl vanes 41 provided above each of the stand pipes 57 to impart a swirling motion to a two-phase flow of steam-water mixtures, and steam separator stages 42a, 42b, and 42c provided above the swirl vanes in three consecutive stages in the axial direction as steam separator means for separating the steam from the two-phase liquid-vapor flow. Each of the steam separator stages 42a, 42b, or 42c has a double structure of a revolving tube 43a, 43b, or 43c with an outer tube 44a, 44b, or 44c positioned on the outer sides thereof. There is a hook-shaped pickoff ring 45a, 45b, or 45c formed on an upper portion of each of the outer tubes 44a, 44b, and 44c, respectively. The description now turns to the operation of the steam separator 56. Coolant that has been boiled off by the heat of the fission reaction forms a two-phase liquid-vapor flow in which ordinary water and steam are mixed. It is distributed between the steam separators 56, which normally number between 200 and 300, and rises to the stand pipes 57. As shown in FIG. 29, the coolant within the stand pipes 57 forms a fluidized state called an annular flow. In other words, a liquid layer 48 covers the inner wall surface of each stand pipe 57 and a mixture of water droplets 49 and steam 50 flows within this liquid layer 48. A centrifugal force is forcibly imparted to the two-phase flow rising through the stand pipe 57 by the swirl vanes 41 disposed directly above the stand pipe 57, to turn it into a rotating flow. At this point, the liquid-vapor density ratio of the coolant under normal operating conditions of the boiling-water reactor is 1:21, and thus a useful difference is generated in the centrifugal forces that are imparted by the rotational action to each of the vapor phase and the liquid phase. This ensures that the low-density steam is positioned towards the center of the lowermost steam separator stage 42a, the high-density liquid forms the liquid layer 48 along the inner wall surface of the revolving tube 43a of this steam separator stage 42a, and both rise while rotating. This liquid layer 48 is carried upward along the inner wall of the revolving tube 43a against its own weight by the shear forces of the high-speed rotating flow near the center and is captured by the pickoff ring 45a which is a slit having a width that is designed to be substantially equal to the thickness of this liquid layer 48. Then, a thin annular portion between the concentric tubes 43a and 44a falls under its own weight. A breakdown ring 47 is provided partway along this flow path to prevent the intermixing of a large quantity of vapor bubbles, and the flow is sent on at a slower speed to an upper downcomer where it mixes with the surrounding liquid. The larger part of the liquid phase that has not been captured by the lowermost steam separator stage 42a is captured by the pickoff rings 45b and 45c of the subsequent steam separator stages 42b and 42c. Note that the apparatus is designed in such a manner that approximately 90% of the moisture extracted by the steam separator 56 from the steam passing through the steam separator 56 is removed by the lowermost steam separator stage 42a, and the mass ratio of water amidst the two-phase flow at the outlet of the steam separator 56 is suppressed to no more than 10%. More of the moisture in the steam that has passed through the steam separator 56 is removed by the steam dryer 58 disposed above each steam separator 56. A steam injector has recently attracted attention as a static jet pump to be used instead of the prior-art rotary pump. This steam injector has a compact structure, requires no power source for operation, and can also be made to have a discharge pressure that is higher than the steam pressure at the inlet thereof. An objective of the present invention is to make full use of the above characteristics in the application of a steam injector to a steam separator, to provide a steam separator which achieves substantially the same liquid-vapor separation effect as that of the above prior-art steam separator, and, at the same time, provide a higher discharge pressure. The recirculation method currently used in BWRs and ABWRs necessitates components such as a large-scale pump, which is a rotating mechanism, and a large-capacity inverter power source for controlling that pump. From various viewpoints such as structural cost, material resources, and regularly scheduled maintenance, this method increases the cost of the plant and causes breakdowns in the rotating mechanisms. In contrast thereto, a movement has recently been seen to implement a simplified BWR with a modified natural recirculation method for the core that does not require jet pumps and internal pumps. Because the electrical output thereof is small in comparison to the size of the plant, the construction costs and unit-power costs tend to increase. In a similar manner, it has become possible to design smaller, simpler equipment in pressurized water reactors (PWRs) and fast breeder reactors (FBRs) as well, by reinforcing the natural recirculation forces within steam generators. This is not limited to reactors; there is also a large demand for smaller, simpler installations having processes that separate out a liquid phase comprised within steam, such as boilers. SUMMARY OF THE INVENTION The present invention was devised in the light of the above described problems of the prior art, and has as an objective the implementation of a steam separator wherein a discharge pressure is higher than the steam pressure at the inlet thereof, by using an injector type of steam separator instead of the steam separator of the prior art. Another objective of the present invention is to implement a great simplification of the equipment installed around the core of a reactor, by applying an injector type of steam separator, i.e., a separator/injector, to a nuclear power generation plant and by achieving both an increase in the performance of the steam separator for a two-phase flow at the core outlet and the establishment of a forced circulation in the core. A further objective of the present invention is to reduce the flow-rate of a recirculation pump of a boiler apparatus, and thus implement a simpler overall structure, by applying an injector type of steam separator to the boiler apparatus to convert the heat exchanger thereof from a natural circulatory system to a forced circulatory system. In order to achieve the above objectives, a first aspect of the present invention relates to a steam separator equipped with a separator/injector, wherein the separator/injector comprises: a two-phase flow accelerator nozzle having an inlet portion opening towards a source of a two-phase liquid-vapor flow and an outlet portion positioned higher than the inlet portion, for causing an acceleration of the two-phase liquid-vapor flow that flows into an interior portion thereof from the inlet portion and discharging the same from the outlet portion; a liquid-phase capture means connected to the outlet portion of the two-phase flow accelerator nozzle and having a guide wall formed as an inverted U-shape curve, in such a manner that the two-phase flow of steam-motor mixture that is discharged from the outlet portion of the two-phase flow accelerator nozzle is guided along a wall surface of the guide wall but is also capable of separating therefrom, wherein a difference in centrifugal forces that are imparted to a vapor-phase component and a liquid-phase component of the two-phase flow while the two-phase flow is being guided along the guide wall causes the liquid-phase component to be guided along the wall surface of the guide wall and be captured, whereas the vapor-phase component is allowed to separate from the guide wall; and a diffuser into which the liquid-phase component captured by the liquid-phase capture means is allowed to flow, which increases the pressure of the liquid-phase component as the liquid-phase component flows therethrough, and which discharges the liquid-phase component from an outlet side thereof. With this configuration, the two-phase flow of steam and water are accelerated by the two-phase flow accelerator nozzle, a strong centrifugal force is applied thereto, and thus the vapor and liquid are separated. In other words, the high-density water flows at substantially constant speed along the flowpath formed by the liquid-phase capture means, but the low-density steam separates from this flowpath, is released into a vapor space, and moves above the separator/injector. The water (liquid phase) that has flowed along the flowpath then flows into the diffuser, is decelerated (in accordance with the Bernoulli principle) as the cross-sectional area of the flowpath increases within the diffuser, and is then discharged out of the separator/injector through a diffuser outlet. During this time, the pressure of the discharge water is increased by the diffuser, so that the outlet pressure of the separator/injector can be made higher than the inlet pressure thereof. The nuclear power generation plant of a second aspect of the present invention is further provided with a foundation portion such that the separator/injector is erected upon the foundation portion, wherein the foundation portion comprises an upper plate and a lower plate positioned below the upper plate to form a space therebetween; the configuration being such that the inlet portion of the two-phase flow accelerator nozzle communicates with a space positioned below the lower plate, and the outlet side of the diffuser communicates with the space formed between the upper and lower plates. This provides complete separation between the two-phase flow that enters the separator/injector and the discharge water from the separator/injector. In a third aspect of this invention, the wall surface of the guide wall of the liquid-phase capture means is preferably formed in an arch-shaped curve orientated upward and at least one portion thereof is in the shape of a circular or elliptical arc. In addition, a side edge portion of the guide wall of the liquid-phase capture means is preferably bent in a direction to enclose the two-phase flow, so that the liquid phase does not flow out of the separator/injector from the underside of the flowpath. In a fourth aspect of this invention, riblet grooves are formed in the flow direction of the two-phase flow or the liquid-phase flow along at least one portion of an inner wall surface of the two-phase flow accelerator nozzle, the wall surface of the guide wall of the liquid-phase capture means, and an inner wall surface of the diffuser. This makes it possible to reduce frictional losses in the fluid in the vicinity of the inner wall surfaces. In a fifth aspect of this invention, the steam separator preferably further comprises an outer tube having an axis in the vertical direction comprising the separator/injector within the outer tube and an inner tube having an axis in the vertical direction disposed within the outer tube; wherein: the wall surface of the guide wall of the liquid-phase capture means is formed by part of an inner wall surface of the inner tube; the diffuser is formed to be in contact with an inner wall of the inner tube; and a space is formed between an inner wall of the outer tube and an outer wall of the inner tube in such a manner that the liquid-phase component discharged from the outlet side of the diffuser is capable of flowing therethrough. In this case, the lower portion of the two-phase flow accelerator nozzle is formed along the axial line of the inner tube, the outlet portion of the two-phase flow accelerator nozzle is formed in the vicinity of an inner wall of the inner tube, and the diffuser is formed in a helical shape with respect to the axial line of the inner tube. This configuration ensures that a centrifugal force is imparted to the two-phase flow discharged from the two-phase flow accelerator nozzle as it flows along the helical flowpath in the vicinity of the inner wall surface of the inner tube, the high-density water flows into the diffuser while being pressed against the inner wall surface, and the low-density steam is separated out towards the center in the axial direction and rises. Thus the two-phase flow is separated into liquid and vapor. The present invention also provides a nuclear power generating system in which this separator/injector is mounted. In other words, the fifth aspect of the present invention relates to a nuclear power generation plant using a boiling-water reactor, wherein the nuclear power generation plant comprises: a reactor pressure vessel; a plurality of fuel rod assemblies disposed within the reactor pressure vessel and through which a coolant flows; a shroud surrounding the plurality of fuel rod assemblies, within which is comprised a two-phase liquid-vapor flow that is created as the coolant flows within the plurality of fuel rod assemblies, and which is sealed by a shroud head at an upper end thereof; and a separator/injector erected above the shroud head; wherein the separator/injector comprises: a two-phase flow accelerator nozzle having an inlet portion opening towards the interior of the shroud and an outlet portion positioned higher than the inlet portion, for causing an acceleration of the two-phase liquid-vapor flow generated in the shroud that flows into an interior portion thereof from the inlet portion and discharging the same from the outlet portion; a liquid-phase capture means connected to the outlet portion of the two-phase flow accelerator nozzle and having a guide wall formed as an inverted U-shape curve, in such a manner that the two-phase liquid-vapor flow that is discharged from the outlet portion of the two-phase flow accelerator nozzle is guided along a wall surface of the guide wall but is also capable of separating therefrom, wherein a difference in centrifugal forces that are imparted to a vapor-phase component and a liquid-phase component of the two-phase flow while the two-phase flow is being guided along the guide wall causes the liquid-phase component to be guided along the wall surface of the guide wall and be captured, whereas the vapor-phase component is allowed to separate from the guide wall; and a diffuser into which the liquid-phase component captured by the liquid-phase capture means is allowed to flow, which increases the pressure of the liquid-phase component as the liquid-phase component flows therethrough, and which discharges the liquid-phase component from an outlet side thereof. During this time, recirculation is performed within the reactor by returning increased-pressure coolant that flows out of the diffuser back into the shroud. The wall surface of the guide wall of the liquid-phase capture means is preferably formed to be a smooth curve. The above configuration ensures that the two-phase flow is accelerated by the two-phase flow accelerator nozzle, the liquid phase thereof is subjected to a strong centrifugal force and is separated thereby, the water (liquid phase) then flows into the diffuser, is decelerated (in accordance with the Bernoulli principle) as the cross-sectional area of the flowpath increases within the diffuser, and the pressure thereof is increased, creating a recirculation drive force. In a sixth aspect of the present invention, the shroud head is formed as a double structure having an upper shroud head and a lower shroud head which is positioned below the upper shroud head to form a space between the upper and lower shroud heads; the inlet portion of the two-phase flow accelerator nozzle communicates with a space within the shroud that is positioned below the lower shroud head; and the outlet portion of the diffuser communicates with the space formed between the upper and lower shroud heads. The nuclear power generation plant of a seventh aspect of the present invention is further provided with a jet pump drive nozzle disposed in an upper portion of a downcomer portion surrounding the shroud; and a jet pump provided below the jet pump drive nozzle; wherein the configuration could be set in such a manner that, after the coolant discharged from the diffuser has passed through the space formed between the upper and lower shroud heads, the coolant is guided into the jet pump through the jet pump drive nozzle. The recirculatory force of the discharge water from the separator/injector makes it possible to reduce the number of jet pumps used in the prior art. In an eighth aspect of the present invention, a pipeline that branches off from a feedwater pipeline that links the reactor pressure vessel to a feedwater pump is connected to the jet pump drive nozzle. This makes it possible to supply water from the feedwater pump through the jet pump drive nozzle to the jet pump. If necessary, the nuclear power generation plant also comprises a feedwater pump for supplying water to the reactor pressure vessel; a feedwater pipeline linking the reactor pressure vessel to the feedwater pump; and a branch pipeline branching off from the feedwater pipeline and communicating with the jet pump drive nozzle, wherein the feedwater pump supplies water to the jet pump through the branch pipeline and the jet pump drive nozzle. In a nuclear power generation plant of a ninth aspect of this invention, pressure is increased to control the circulation flow-rate in sequence from the interior of the downcomer portion, the interior of the shroud, the inlet portion of the separator/injector, to the outlet portion of the separator/injector by controlling the flow-rate and discharge pressure of water supplied from the feedwater pump to the jet pump, thereby controlling the thermal output generated within the reactor pressure vessel. This makes it possible to increase the pressure in sequence from the downcomer portion, a lower plenum, the core, an upper plenum, the inlet portion of the separator/injector, to the outlet portion of the separator/injector by, for example, increasing the flow-rate and discharge pressure of water supplied from the feedwater pump to the jet pump, to increase the core circulation flow-rate, thereby controlling the thermal output of the core. In a tenth aspect of the present invention, the jet pump could be driven at the start-up of the plant by mixing a flow of feedwater supplied from at least one of a pump in a residual heat removal system and a pump in a reactor water clean-up system with a flow of feedwater supplied from the feedwater pump to the jet pump. The nuclear power generation plant of an eleventh aspect of this invention further comprises a recirculation flow-rate control valve disposed in the outlet portion or the inlet portion of the jet pump; and a flow-rate control means that uses at least one of an electrical generator output signal, a main steam flow-rate signal, a neutron flux output signal, and a jet pump pressure difference signal as an input signal, calculates a suitable recirculation flow-rate and corresponding degree of opening of the recirculation flow-rate control valve therefrom, and outputs a valve-opening signal. This adjusts the recirculation flow-rate of the core in accordance with the setting and adjustment of the degree of opening of the valve as appropriate in accordance with the electrical output required of the nuclear power generation plant. The nuclear power generation plant of a twelfth aspect of this invention further comprises a feedwater pump for supplying water to the reactor pressure vessel; a feedwater line connected to the reactor pressure vessel to the feedwater pump; a flow-rate adjustment valve provided in the feedwater pipeline; and a branch pipeline branching off from the feedwater pipeline and communicating with the jet pump drive nozzle, wherein the flow-rate control means controls a water level of the reactor by calculating a suitable degree of opening of the flow-rate adjustment valve and outputting a valve-opening signal to the flow-rate adjustment valve. This monitoring and control of the water level of the nuclear reactor makes it possible to maintain the core water level to be substantially constant. Alternatively, the flow-rate control means could control the flow-rate of water supplied to the reactor by calculating a suitable feedwater flow-rate and outputting a rotational frequency signal to the feedwater pump. The nuclear power generation plant of a thirteenth aspect of the present invention further comprises a jet pump drive nozzle disposed in an upper portion of a downcomer portion surrounding the shroud; a bell mouth opening towards the downcomer portion in close proximity to the jet pump drive nozzle; a straight pipe having one end connected to a leading edge of the bell mouth and another end connected to an inlet side of the jet pump; and a jet pump provided below the jet pump drive nozzle; wherein after the coolant, which has accumulated in the upper shroud head without passing through the diffuser, has passed through the bell mouth and the straight pipe via the jet pump drive nozzle, the coolant is guided into the jet pump. This configuration imparts a recirculatory drive force of the coolant flowing through the downcomer portion. A fourteenth aspect of the present invention relates to an ABWR characterized in that the shroud head is formed as a double structure having an upper shroud head and a lower shroud head which is positioned below the upper shroud head to form a space between the upper and lower shroud heads; and the ABWR is further provided with: a downcomer pipe having an upper end portion opening upwards in an upper surface of the upper shroud head, for guiding downwards a liquid phase of coolant that has accumulated on the upper surface of the upper shroud head, without being captured by the liquid-phase capture means; an impeller provided in a lower portion of the downcomer pipe for rotating in such a manner that coolant flowing downward within the downcomer pipe is discharged towards a lower portion of the shroud; and an internal pump for driving the impeller; wherein the outlet side of the diffuser communicates with a space formed between the upper and lower shroud heads, and coolant that is discharged from the outlet side of the diffuser flows down through the space formed between the upper and lower shroud heads and into a downcomer portion on an outer side of the downcomer pipe, then is discharged to a lower portion of the shroud. The nuclear power generation plant of a fifteenth aspect of this present invention further comprises a recirculation flow-rate control means that uses at least one of an electrical generator output signal, a main steam flow-rate signal, a neutron flux output signal, and a core support plate pressure difference signal as an input signal, calculates a suitable recirculation flow-rate and a suitable rotational frequency of the impeller therefrom, and outputs a suitable rotational-frequency signal with respect to the internal pump. This configuration adjusts the recirculation flow-rate in the core by setting and regulating the rotational frequency of the pump as appropriate in accordance with the electrical output required of the nuclear power generation plant. The recirculation flow-rate control means controls the rotational frequency of the internal pump by performing calculations in accordance with overall proportional integral differential (PID) control relating to a difference from a predetermined water level based on an input reactor core water-level signal. This makes it possible to control the rotational frequency of the internal pump to maintain the core water level to be substantially constant. The nuclear power generation plant of a sixteenth aspect of the present invention is further characterized in that each of a shroud casing forming a side portion of the shroud and a core support plate forming a lower portion of the shroud is formed as a double structure; and the plant further comprises a first coolant circulation pathway formed so as to mutually communicate the space between the shroud heads, a space formed between the double shroud casings, and a space formed between the double core support plates, through which flows coolant discharged from the outlet portion of the diffuser; a water rod provided in the interior of the fuel rod assemblies, within which coolant flows; a first coolant guide pipe formed so as to communicate the first coolant circulation pathway with an outlet portion at a lower end of the water rod, for guiding increased-pressure coolant, which has been discharged from the outlet side of the diffuser and which is flowing through the first coolant circulation pathway, to the outlet portion at the lower end of the water rod; and a hole formed in a side surface of the water rod, for ejecting coolant that is flowing within the water rod to the exterior of the water rod. This configuration makes it possible to cause an increase in the liquid-phase flow-rate of the two-phase flow in the core, by allowing the high-pressure discharge water from the separator/injector to flow into the fuel rod assemblies. In the nuclear power generation plant of a seventeenth aspect of the present invention, the coefficient of thermal expansion of the material configuring the first coolant guide pipe and the coefficient of thermal expansion of the material configuring the water rod are set to be different in the vicinity of a connective portion between the first coolant guide pipe and the water rod. In the nuclear power generation plant of an eighteenth aspect of the present invention, labyrinth grooves are provided in the first coolant guide pipe and the water rod in the vicinity of a connective portion between the first coolant guide pipe and the water rod. This prevents leakage of the coolant by increasing the resistance of the flowpath of leaking coolant. The nuclear power generation plant of a nineteenth aspect of the present invention further comprises a second coolant guide pipe provided within a control rod tube positioned below the fuel rod assemblies, for guiding coolant that is outside the shroud into a lower tie plate of the fuel rod assemblies; and an orifice provided in a second coolant circulation path that is formed by the second coolant guide pipe, for locally constricting the flowpath thereof. This suppresses any increase in the pressure losses in the coolant flowpath. In a nuclear power generation plant of a twentieth aspect of the present invention, holes are provided in a side surface of an inner shroud casing of the double shroud casings, and a side surface of a channel box of the fuel rod assemblies. This makes it possible to even out the coolant density within the fuel rod assemblies. A twenty-first aspect of the present invention relates to a nuclear power generation plant using a boiling-water reactor, wherein the nuclear power generation plant comprises: a steam generator comprising a lower casing surrounding a heat exchanger formed of heat-exchange pipes having inlets and outlets for a primary coolant, and an upper casing provided connected to the lower casing and having a steam outlet for supplying steam to a turbine; a reactor container communicating with the steam generator and surrounding the primary coolant and a fuel rod assembly; and a separator/injector provided above the lower casing; wherein the separator/injector comprises a two-phase flow accelerator nozzle having an inlet portion opening towards the interior of the lower casing and an outlet portion positioned higher than the inlet portion, for causing an acceleration of a two-phase flow of the primary coolant that flows into an interior portion thereof from the inlet portion and discharging the same from the outlet portion; a liquid-phase capture means connected to the outlet portion of the two-phase flow accelerator nozzle and having a guide wall formed as a inverted U-shape curve, in such a manner that the two-phase flow that is discharged from the outlet portion of the two-phase flow accelerator nozzle is guided along a wall surface of the guide wall but is also capable of separating therefrom, wherein a difference in centrifugal forces that are imparted to a vapor-phase component and a liquid-phase component of the two-phase flow while the two-phase flow is being guided along the guide wall causes the liquid-phase component to be guided along the wall surface of the guide wall and be captured, whereas the vapor-phase component is allowed to separate from the guide wall; and a diffuser into which the liquid-phase component captured by the liquid-phase capture means is allowed to flow, which increases the pressure of the liquid-phase component as the liquid-phase component flows therethrough, and which discharges the liquid-phase component from an outlet side thereof. In this case, it is preferable that the nuclear power generation plant further comprises an inner casing surrounding the heat exchanger within the lower casing; wherein discharge water exhausted from the diffuser is guided into the heat exchanger through a space formed between the lower casing and the inner casing. This makes it possible to improve the heat-transfer characteristics by creating a forced circulation of fluid on a secondary side in the steam generator of this PWR. A twenty-second aspect of the present invention relates to a boiler apparatus comprising: a pressure vessel surrounding heat-transfer tubes that configure a heat exchanger and a combustor for heating the heat-transfer tubes; a recirculation pump for recirculating a fluid that flows through the pressure vessel; and a separator/injector provided above the heat-transfer tubes; wherein the separator/injector comprises a two-phase flow accelerator nozzle having an inlet portion opening towards the interior of the pressure vessel and an outlet portion positioned higher than the inlet portion, for causing an acceleration of a two-phase liquid-vapor flow that flows into an interior portion thereof from the inlet portion and flows through the heat exchanger, and discharging the same from the outlet portion; a liquid-phase capture means connected to the outlet portion of the two-phase flow accelerator nozzle and having a guide wall formed as a inverted U-shape curve, in such a manner that the two-phase flow that is discharged from the outlet portion of the two-phase flow accelerator nozzle is guided along a wall surface of the guide wall but is also capable of separating therefrom, whereby a difference in centrifugal forces that are imparted to a vapor-phase component and a liquid-phase component of the two-phase flow while the two-phase flow is being guided along the guide wall causes the liquid-phase component to be guided along the wall surface of the guide wall and be captured, whereas the vapor-phase component is allowed to separate from the guide wall; and a diffuser into which the liquid-phase component captured by the liquid-phase capture means is allowed to flow, which increases the pressure of the liquid-phase component as the liquid-phase component flows therethrough, and which discharges the liquid-phase component from an outlet side thereof. This makes it possible to cause a reduction in the flow-rate of the recirculation pump by creating a forced circulation of fluid within the boiler. The configuration of the present invention enables the following effects. In other words, it makes it possible to implement a steam separator that is equipped with a separator/injector that enables an outlet pressure that is higher than the inlet pressure thereof, in addition to the steam separation capability of the prior art. Installing this separator/injector in a nuclear power generation plant or boiler apparatus makes it possible to separate steam and water from a two-phase flow and achieve a forced circulation in the core, without requiring the complicated configuration of the prior art. This means that the number of items of dynamic recirculation equipment required in the art, such as recirculation pumps or internal pumps, can be reduced, which leads to a huge reduction in the equipment and structural resources of the entire apparatus, rationalization and simplification of the apparatus, and also a reduction in the time and costs involved in construction and maintenance.