Control system for steam generator

#This invention relates to a steam generator which comprises an evaporator for producing vapor from the feed-water and a super-heater for further heating the vapor produced and which generates steam through heat-exchange between feed-water and alkaline metal. More particularly, this invention relates to a control system for the steam generator for preventing liquid water content from entrance into the super-heater.

#A steam generator where heat-exchange is made between alkaline metal and feed-water is employed in a fast breeder reactor, for example. The alkaline metal is used as a coolant carrying the heat produced in the nuclear reactor to exterior. The steam generated by the steam generator drives a turbine generator.

#A rough construction of the steam generator is as follows: A closed tank receives at its one end a high temperature alkaline metal fed from the nuclear reactor while feeds from its other end the alkaline metal cooled as a result of heat exchange back to the nuclear reactor. The closed tank is passed through by a plurality of heat transfer tubes each of which permits the feed-water to pass therethrough. The thermal energy of the alkaline metal is transferred to the feed-water through the heat transfer tube thereby producing water vapor.

#FIG. 1 is a schematic diagram showing the construction of a fast breeder type nuclear power station as an example of a plant which employs a separate type steam generator.

#FIG. 2 is a block diagram showing an embodiment of a control system according to the present invention in which a permissible lower limit temperature is established in accordance with the vapor pressure at the output of the evaporator.

#FIG. 3 is a graph illustrating the relationship among the vapor pressure, the saturated temperature, and the permissible lower limit temperature.

#Referring now to FIG. 1, there is schematically shown a fast breeder type nuclear power station as one example of a nuclear power plant employing a so-called separate type steam generator. In the figure, reference numerals 14 and 15 designate a nuclear reactor and a control rod for controlling an output of the nuclear reactor, respectively. Alkaline metal such as liquid sodium is used as a coolant of the nuclear reactor 14, and it is introduced to the nuclear reactor by a pump 1. The heat produced in the nuclear reactor 14 is carried out of the nuclear reactor by the primary coolant sodium to exterior and is introduced into an intermediate heat exchanger 13 where the heat is transferred to the secondary coolant. Also in the case of the fast breeder reactor, liquid sodium is used as the secondary coolant. A pump 2 is used to cause the secondary coolant sodium to flow. The steam generator mentioned above is comprised of an evaporator 11 and a super-heater 12. The secondary coolant sodium flows through the primary side of the steam generator, while water flows through the secondary side of the same. Steam produced from the water as a result of heat exchange between the water and the secondary coolant sodium is supplied through an adjusting valve 5 to a turbine 6 to drive the same. A pressure control device 9 controls the opening of the adjusting valve 5 so as to provide a given pressure of the steam supplied to the turbine 6. Ps designates the pressure at the inlet of the adjusting valve 5. A detailed explanation of the pressure control device 9 is omitted because it it not essential to this invention. Reference numerals 3 and 4 designate a feed-water pump and a feed-water valve, respectively, and reference numeral 10 designates a control system essential to the present invention. The control system 10 serves to control the amount of feed-water in response to the pressure PE and the steam temperature TE at the outlet of the evaporator 11. More particularly, the control system 10 controls the opening of the feed-water valve 4. The schematic construction of the fast breeder type nuclear power station as one form of nuclear power plants using the separate type steam generator, is as just mentioned.

#Turning now to FIG. 4, an example of temperature distributions of the secondary coolant sodium end of the water along the evaporator 11 and the super-heater 12 are graphically shown in the drawings. In the graph, the abscissa shows the axial distance of the steam generator the left and right portions of which show the respective axial distances of the evaporator and the super heater. The water flows in the steam generator from right to left and the secondary coolant sodium from left to right in the graph. The ordinate shows the water temperature TF and the sodium temperature TN. The curves depicted by solid lines designate the temperature distributions in the steam generator when the flow rate of the secondary coolant sodium is 100 %, while the curves depicted by dotted lines indicate those when the flow rate of the secondary coolant sodium is reduced. The flat region in those temperature distribution curves existing between the super heat portion and the evaporator portion correspond to the coolant pipe between the evaporator 11 and the super-heater 12. As seen from the graph, when the secondary coolant sodium passes through the steam generator at the 100 % flow rate, the sodium temperature TN drops from about 510.degree. to 320.degree. C, while the water temperature TF rises from about 250.degree. to 490.degree. C. In the graph, the water is in a liquid state during the right end rising portion A-B of the curve TF and is in a combination of liquid water and vapor during the flat portion B-C following the right end rising portion A-B. The flat portion B-C of the curve TF shows the water temperature distribution substantially at the middle portion of the evaporator 11. The temperature of the water containing liquid water and vapor is about 335.degree. C at the flat portion B-C, and the pressure PE at the outlet of the evaporator 11 is 140 Kg/cm.sup.2. The temperature 335.degree. C, here, is the saturation temperature Ta(PE) of the water which is determined decidedly when the pressure PE is 140 Kg/cm.sup.2. The curve TF rises again near the outlet of the evaporator 11. The inflection point C where the flat portion B-C of the curve TF is bent upward is the super heat starting point at which the mixture of liquid water and vapor is turned into vapor steam. The water must be in a vapor steam state at the inlet of the super-heater 12 in order to prevent liquid water from entering the super-heater 12. Since there is no heat exchange between the water and the sodium in the region between the outlet of the evaporator 11 and the inlet of the super-heater 12, the respective temperatures TF and TN remain constant in this region. Thus, the water must be vapor steam at the outlet of the evaporator 11 in order to prevent the liquid water from entering the super-heater 12. Accordingly, the axial lengths of the respective parts of the steam generator and the temperatures of the sodium and the water must be so selected that the super heat starting point of the water is positioned in the evaporator 11. The vapor steam is additionally heated in the super heater to be superheated vapor steam. On the other hand, the temperature of the secondary coolant sodium gradually drops due to the heat exchange between the coolant sodium and the water. It is important to place the super heat starting point in the intermediate region of the evaporator 11. If the flow rate of the secondary sodium is reduced, however, the super heat starting point shifts from the point C to the point C' placed in the vicinity of the outlet of the evaporator 11, and hence the temperature TE of the water falls. Note that if the temperature TE of the water at the outlet of the evaporator 11 falls to the level of the saturation temperature Ta(PE), there is a high possibility of entrance of the water in liquid state into the super-heater 12. In FIG. 4, the respective curves indicated by dotted lines show the sodium temperature TN and the water temperature TF when the flow rate of the secondary sodium is reduced. It will be seen from the dotted-line curve TF that the temperature of the water is decreased. It is further seen from this graph that the vapor temperature at the outlet of the super-heater 12 changes little while the water vapor temperature TE at the outlet of the evaporator 11 greatly drops. Calculation was made of how the sodium flow rate should be reduced in order to cause the water vapor temperature TE at the outlet of the evaporator 11 to fall to the level of the saturation temperature Ta(PE), on the assumption that the sodium flow rate is 100 % and the water vapor temperature TE at the outlet of the evaporator 11 is 370.degree. C. This calculation shows that the limit of the sodium flow rate to be reduced is about 5 % if no control is applied to the system. The water vapor temperature TE at the outlet of the evaporator is sensitive in change due to various disturbances in addition to the reduction of the sodium flow rate. This is one of features of the steam generator. From the foregoing description, it will be seen that it is most effective to monitor the reduction of the water vapor temperature TE at the outlet of the evaporator which is remarkably responsive to disturbances, for the purpose of preventing liquid water from entering the superheater 12. The conceivable reasons why the vapor temperature TE at the outlet of the evaporator 11 is more sensitive to disturbances than the vapor temperature at the outlet of the super-heater 12 are as follows:

#a. The temperature difference between the sodium and the water at the outlet of the evaporator is larger than that at the outlet of the super-heater;