Patent Number: 052934110
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a nuclear reactor power control method and a nuclear reactor power control device, in particular, for applying to a boiling water type nuclear reactor. The boiling water type nuclear reactor comprises a feed water system including a turbine driven reactor feed water pump (TDRFP) and a motor driven reactor feed water pump (MD-RFP). In normal operation, with two turbine driven reactor feed water pumps, the feed water is supplied to the reactor. The motor driven reactor feed water pump is also used as a back-up for the turbine driven reactor feed water pump. When one turbine driven reactor feed water pump is abnormally tripped and the motor driven feed water pump is not started, the flow of feed water becomes short and a nuclear reactor scram is caused due to the decrease of water level of the nuclear reactor. Scram can be considered as a general plant trip. It is known to control the power of a nuclear reactor by changing the flow of the recirculated water. This controls the amount of voids in the water between fuel elements and a void will absorb neutrons to a greater extent than water. Therefore a reduction in the recirculated water by reducing the speed of the core cooling fluid pump will increase the amount of voids and thus decrease the power. A method for avoiding such nuclear reactor scram is explained in Japanese Patent Application Laid-Open No. 55114998(1980). When both the water level of the nuclear reactor decreases below a fixed level and a mismatching between the flow of the feed water supplied to the nuclear reactor and the flow of the steam discharged from the nuclear reactor appears, the method directly controls a recirculation flow control device with the mismatched amount and thereby decreases the nuclear reactor power. Thereafter, the flow of the steam decreases and the water level of the nuclear reactor rises. Thereby the nuclear reactor scram is avoided. This method detects the shortage of feed water flow due to non-start of the motor driven reactor feed water pump by measuring the water level drop of the nuclear reactor. Further, although different from the abnormality example of the above feed water pump trip, Japanese Patent Application Laid-Open No. 49-37094 (1974) discloses a method for avoiding the nuclear reactor scram caused by the water level decrease of the nuclear reactor due to the decrease of feed water flow. The objective deals with the problem when there is a decrease of the feed water flow due to breakage of the feed water piping. The method is to lower the nuclear reactor power when both the feed water flow decreases below the steam flow by more than a fixed value and the water level of the nuclear reactor decreases from a normal water level by more than a fixed value. As the operations for lowering the nuclear reactor power, there are decreasing the speed of the recirculation pump and the insertion of selected control rods. SUMMARY An enlargement of the opening domain of the boiling water type nuclear reactor is now studying. The enlargement of the operating domain includes the hatched domain portion shown in FIG. 14 into the operation range of the nuclear reactor. Thereby the operation of the nuclear reactor at a low core flow and a high nuclear reactor power (for example, at a core flow of 85% and a nuclear reactor power of 100%) is enabled. However, even if the operating domain is enlarged, there can occur the abnormal state as explained above, that is, the trip of one turbine driven reactor feed water pump and the non-start of the motor driven reactor feed water pump standing by. Therefore the inventors have studied the phenomenon appearing in the enlarged operating domain when the above abnormality occurred. As the result, under the condition that the nuclear reactor was operating in the enlarged operating domain, when the above abnormality happened, it was found out that the nuclear reactor possibly went to a scram even if the nuclear reactor power was lowered based upon the nuclear reactor water level and the mismatch between the feed water flow and the steam flow. The above study is explained below. FIG. 15 shows a characteristic curve of the above nuclear reactor scram. This characteristic curve was taken when the above abnormality happened under the operating condition of a core flow of 85% and a nuclear reactor power of 100%, and the feed water flow suddenly decreases by the trip of one turbine driven reactor feed water pump. However since the non-start of the motor driven reactor feed water pump is determined by the nuclear reactor water level, the decrease of the nuclear reactor power is delayed. In particular, in the case where the determination is made based upon the mismatch between the feed water flow and the steam flow and based upon the nuclear reactor water level, such as disclosed in Japanese Patent Application Laid-Open No. 49-37094 (1974), the decrease of the nuclear reactor power is further delayed. For this reason, the initiation of the steam flow decrease is delayed, and the nuclear reactor water level lowers below a fixed value of nuclear reactor scram so that the nuclear reactor goes to scram as shown in FIG. 15(B). One object of the present invention is to provide a nuclear reactor power control in which the decrease of the steam flow at the feed water pump trip is performed quickly and the possibility of the nuclear reactor scram is reduced. Another object of the present invention is to provide a nuclear reactor power control in which an over decrease of the steam flow at the feed water pump trip is suppressed. A further object of the present invention is to provide a nuclear reactor power control in which an overspeed, that is run out, of the feed water pump during its operation, which would produce a trip, is prevented. A still further object of the present invention is to provide a nuclear reactor power control in which the control to lower the nuclear reactor power is facilitated after the decrease of a second pump that supplies cooling water to the core in the nuclear reactor. One feature is to decrease the speed of a second pump supplying cooling water to the core in the nuclear reactor based on a trip signal that is generated when a first pump supplying feed water is tripped, and to thereby decrease the nuclear reactor power. Another feature is to perform a first operation to decrease the speed of a second pump supplying cooling water to the core in the nuclear reactor based on a trip signal that is generated when a first pump supplying feed water is tripped to decrease the nuclear reactor power to a first fixed value and to perform a second operation to lower the nuclear reactor power to a second fixed value that is lower than the first fixed value. A further feature is to reduce the nuclear reactor power to a level that prevents run-out of the other first pumps in operation. Another feature is to switch a set value of the nuclear reactor power used for controlling the nuclear reactor power to a second fixed value that is lower than a first fixed value used at the moment when a first pump is tripped. Since the second pump speed is reduced based upon a trip signal generated when the first pump is tripped, the steam flow is decreased substantially simultaneously with the decrease of the feed water flow. Thereby, the decrease of the steam flow is carried out in an early stage when the feed water pump is tripped and the possibility of nuclear reactor scram is reduced. Since the first operation reduces the nuclear reactor power down to a first fixed value through reduction of the speed of the second pump and the second operation rapidly reduces the nuclear reactor power, that perform run-back, to a second fixed value lower than the first fixed value, the decreasing range of the nuclear reactor power by the speed reduction of the second pump is limited. Thereby, unnecessary enlargement of the decreasing range of the steam flow is suppressed. Since the nuclear reactor power is decreased to a level that prevents the run-out of the other first pumps in operation, the run-out of the first pumps in the operation is prevented. Since the set value of the nuclear reactor is switched to the second fixed value level lower than the first fixed value used at the trip of the first pump, the control to reduce the nuclear reactor power after the runback is facilitated.