Patent Number: 046474218
Section: summary

BACKGROUND OF THE INVENTION This invention relates to operation control methods for nuclear reactors, and more particularly it is concerned with an operation control method for a nuclear reactor that can have application in a nuclear reactor performing a daily load follow-up operation in which power control is effected by using control rods and liquid poisons. A pressure tube type nuclear reactor comprises a multiplicity of pressure tubes having fuel assemblies therein which are mounted in a calandria tank to extend through a moderator contained therein. A coolant flows through the pressure tubes. Power control of this type pressure tube type nuclear reactor is effected by inserting and withdrawing control rods in the moderator between the pressure tubes in the calandria tank, and adjusting the concentration of a liquid poison incorporated in the moderator in the calandria tank. In recent years, there has been a tendency to adopt a method of operation of a nuclear reactor which aims not only at producing a fixed reactor power for a base load but also at developing a reactor power which may vary depending on a fluctuation in load, by performing a load follow-up operation. The pressure tube type nuclear reactor of the type described hereinabove is not an exception, and research has been conducted into the possibilities of incorporating the load follow-up operation in this type of nuclear reactor. For example, a proposal has been made in Japanese Patent Laid-Open No. 141594/82 to incorporate the load follow-up operation in a pressure tube type nuclear reactor. The document referred to hereinabove shows in FIG. 5 thereof a load follow-up operation control system for a pressure tube type nuclear reactor which is designed to effect control of operation of a nuclear reactor in a manner to cope with demands for electrical power which vary from daytime to nighttime during a day by increasing power in the daytime and reducing it in the nighttime everyday. In a nuclear reactor using control rods and liquid poison concentration adjustments as control means, such as a pressure tube type nuclear reactor and pressurised-water reactor, control is effected to keep the reactor power in the range of allowable powers between an upper limit line and a lower limit line set above and below, respectively, a power fall line or a predetermined power fall rate (or a power rise line or a predetermined power rise rate) in accordance with a fall (or a rise) of the reactor power. Load follow-up operation control of a pressure tube type nuclear reactor will be described. In this control process, a high reactor power achieved in the daytime is reduced to a low power level in the nighttime by increasing the concentration of a liquid poison in the calandria tank. Insertion and withdrawal of the control rod are performed only when the reactor power tends to exceed the upper limit line or lower limit line of the range of allowable powers because they cause great damage to the fuel assemblies by bringing about sudden changes in reactor power. Operation of the control rods has a much higher rate of change in reactor power than adjustments of the concentration of the liquid poison, and has the risk of damaging the fuel assemblies in a high nuclear power range. Thus, one should refrain from operating the control rods as much as possible in the high power range. The load follow-up operation control of the pressure tube type nuclear reactor shown in FIG. 5 of the document referred to hereinabove aims at the reduction of the number of times of operation of the control rods during a load follow-up operation of the reactor. The control is effected by obtaining predicted values of changes with time of the reactivity from changes in the reactor power by using values of the reactor power set beforehand and data for analyzing the dynamic characteristics of the nuclear reactor, splitting the time for effecting power control into time units in accordance with the changing rate of reactivity obtained from the predicted values, and determining optimum values of the quantity of liquid poison to be injected or removed for each time unit, to thereby control the concentration of the liquid poison in the calandria tank to an optimum level at all times. Although this control process has achieved a success in reducing the number of times of operation of the control rods, the control rods are still operated for about 300 times to keep the reactor power to the vicinity of 50% when a load follow-up operation of the reactor is performed while maintaining the reactor power at a 50% level. SUMMARY OF THE INVENTION An object of this invention is to provide a method of operation control for a nuclear reactor enabling load follow-up operation control performed repeatedly to be simplified in process. Another object is to provide a method of operation control for a nuclear reactor capable of reducing the number of times of operation of control means for effecting coarse adjustments of the power of the nuclear reactor. One outstanding characteristic of the invention is that a reactivity introduced by operating control means in a first cycle of a load variation program is obtained, a manipulated variable of second control means for effecting fine adjustments of reactor power in a second cycle of the load variation program which follows the first cycle is obtained based on the reactivity introduced in the first cycle, and control of reactor power is effected by operating the second control means in the second cycle based on the manipulated variable obtained in the first cycle. The outstanding characteristic described hereinabove simplifies the process of effecting load follow-up operation control because power control is effected by operating the second control means in the next following cycle based on the reactivity introduced in the preceding cycle, thereby facilitating load follow-up operation control. Another outstanding characteristic is that a reactivity introduced by operating first control means in a first cycle of a load variation program is obtained, a manipulated variable of second control means for effecting fine adjustments of reactor power in a second cycle of the load variation program which follows the first cycle is obtained based on the reactivity introduced in the first cycle, and control of reactor power is effected by operating the second control means in the second cycle based on the manipulated variable obtained in the first cycle, when a change in the reactivity occurring in the second cycle becomes equal to a change in the reactivity occurring in the first cycle. The outstanding characteristic described hereinabove enables the number of times of operation of the first control means for effecting coarse adjustments of reactor power to be reduced much more than the first mentioned outstanding characteristic. It has been ascertained that, when load follow-up operation of a pressure tube type nuclear reactor is performed, if the load variation program has the same pattern for each and every day of the operation or if the operation is performed in accordance with the same load variation cycle every day, then changes in the reactor core reactivity have substantially the same pattern after the second day of operation. This phenomenon will be described. FIG. 1 shows changes in the reactivity in the reactor core of a pressure tube type nuclear reactor which occur when load follow-up operation is performed by varying the load every day. In this case, the load follow-up operation is performed in accordance with a load variation program 37A having an operation pattern (load variation cycle) in which an electrical power is reduced from 100% to 50% in one hour as indicated by a characteristic 1 (solid line) and kept at a 50% level for eight hours, followed by a rise from 50% to a 100% level in one hour after lapse of the eight hours and holding the electric power at the 100% level for fourteen hours. This operation pattern is repeated every day. Assume that the pressure tube type nuclear reactor has been operated to obtain 100% of electrical power until the load follow-up operation in conformity with the load follow-up operation program 37A is initiated. Then, if the electrical power changes as indicated by the characteristic 1, a thermal power of the nuclear reactor changes from 55% to 100% as indicated by a characteristic 2 (broken line) and the concentration of xenon produced in the reactor core by nuclear fission changes as represented by a characteristic 3 (one-dot-and-dash line). If the electrical power shows the changes represented by the characteristic 1, then the reactor core reactivity changes as indicated by a characteristic 4 (two-dot-and-dash line) under the influences of the changes in the xenon concentration and power coefficient. The reactor core reactivity indicated by the characteristic 4 is such that, except for the first day on which the mode of operation of the nuclear reactor is switched, changes occurring in the reactor core reactivity follow substantially the same pattern every day. This phenomenon occurs also when the pattern of load follow-up operation is switched from one with a range between a high electrical power of 100% and a low electrical power of 50% to one with a range between a high electrical power of 100% and a low electrical power of 70%. Stated differently, when the load follow-up operation is performed in accordance with the latter pattern, changes occurring in the reactor core reactivity follow substantially the same pattern after the second day following the first day of introduction of a change in pattern. The invention is based on the discovery that when load follow-up operation is performed repeatedly in accordance with the same pattern, changes occurring in the reactor core reactivity are substantially equal to each other after the same pattern of operation is repeatedly performed several times.