Patent Number: 041705165
Section: claims

1. A method of operating a nuclear reactor during the period when power of the nuclear reactor is raised, comprising the steps of raising the power from zero W/cm after reactor shutdown up to 240 W/cm of a linear heat generating rate of nuclear fuel elements containing UO.sub.2 fuel pellets surrounded by a zirconium alloy and located in a core of the nuclear reactor utilizing a first control means providing coarse power regulation, raising the power of the nuclear reactor in a range over the linear heat generating rate 240 W/cm utilizing a second control means providing fine power regulation, and stopping the power rise to the nuclear reactor and holding the power at a fixed level at least once in a range below a linear heat generating rate corresponding to a length of a shutdown period of time of said nuclear reactor before the linear heat generating rate reaches 240 W/cm, and wherein the power of said reactor is held constant at least once somewhere in the range zero to P which satisfies the relation ##EQU7## where P is a linear heat generating rate at which said nuclear fuel elements initiate the Pellet Clad Mechanical Interaction; T.sub.s is the shutdown period of the reactor; P.sub.0 is a linear heat generating rate which causes said nuclear fuel elements to induce Pellet Clad Mechanical Interaction for the first time, after said reactor is restarted when the reactor shutdown period T.sub.s is zero; and T.sub.so is the reactor shutdown period when P=0. 2. The method of operating a nuclear reactor according to claim 1, wherein the power of said nuclear reactor is raised by pulling out from said core, control rods which are the first control means and are inserted therein. 3. The method of operating the nuclear reactor according to claim 1, wherein letting T denote a period of time during which the power of the nuclear reactor is held constant, .theta. denote a temperature of fuel pellets in said nuclear fuel elements, K.sub.1 denote a constant of proportion, K.sub.2 denote a constant, the power of the nuclear reactor is held constant for the holding period of time which satisfies EQU T.gtoreq.-K.sub.1 .theta.+K.sub.2. 4. the method of operating a nuclear reactor according to claim 3, wherein the power of the nuclear reactor is held constant for the holding period of time which satisfies EQU T .gtoreq.-0.01875 .theta.+17.625. 5. the method of operating a nuclear reactor according to claim 1, wherein said linear heat generating rate is increased by varying a flow rate of a coolant to be supplied to said core. 6. The method of operating the nuclear reactor according to claim 1, wherein when the linear heat generating rate of said nuclear fuel elements has become at least 240 W/cm, the linear heat generating rate is increased in a proportion of at most about 1.8 W/cm/hour, to raise the power of the nuclear reactor. 7. The method of operating a nuclear reactor according to claim 1, wherein the fuel pellets and the surrounding zirconium alloy satisfy the relation EQU G.sub.a /D.sub.i =0.024 8. A method of operating a nuclear reactor during the period when power of the nuclear reactor is raised, comprising the steps of raising the power from zero W/cm after reactor shutdown up to 240 W/cm of a linear heat generating rate of nuclear fuel elements containing UO.sub.2 fuel pellets surrounded by a zirconium alloy and located in a core of the nuclear reactor by manipulating control rods, raising the power of the nuclear reactor in a range over the linear heat generating rate of 240 W/cm by a coolant control means for controlling a flow rate of a coolant supplied to the core, and stopping the power rise of the nuclear reactor and holding the power at a fixed level at least once in a range below a linear heat generating rate corresponding to a length of a shutdown period of time of the nuclear reactor before the linear heat generating rate reaches 240 W/cm, and wherein the power of said reactor is held constant at least once somewhere in the range zero to P.sub.1 which satisfies the relation ##EQU8## where P.sub.1 is a linear heat generating rate at which said nuclear fuel elements initiate the Pellet Clad Mechanical Interaction; T.sub.s is the shutdown period of the reactor; P.sub.o is a linear heat generating rate which causes said nuclear fuel elements to induce Pellet Clad Mechanical Interaction for the first time, after said reactor is restarted when the reactor shutdown period T.sub.s is zero; and T.sub.so is the reactor shutdown period when P=0. 9. The method of operating the nuclear reactor according to claim 8, wherein the power of the nuclear reactor is held constant for the holding period of time which satisfies EQU T.gtoreq.-0.01875.theta.+17.625. 10. The method of operating the nuclear reactor according to claim 8, wherein after the linear heat generating rate of said nuclear fuel elements has become at least 240 W/cm, the linear heat generating rate is increased in a proportion of at most about 1.8 W/cm/hour, to raise the power of the nuclear reactor. 11. The method of operating a nuclear reactor according to claim 8, wherein the fuel pellets and the surrounding zirconium alloy satisfy the relation EQU G.sub.a /D.sub.i =0.024 12. The method of operating the nuclear reactor according to claim 8, wherein the power of said nuclear reactor is held constant for a period of time which satisfies the relation EQU T.gtoreq.-K.sub.1 .theta.+K.sub.2, 13. The method of operating the nuclear reactor according to claim 12, wherein the power of the nuclear reactor is held constant for the holding period of time which satisfies EQU T.gtoreq.-0.01875.theta.+17.625. 14. The method of operating the nuclear reactor according to claim 12, wherein the power of the reactor is held constant at a linear heat generating rate P.sub.1 which satisfies the relation ##EQU9## for a period of time T which satisfies the relation EQU T=-K.sub.1 .theta.+K.sub.2. 15. the method of operating a nuclear reactor according to claim 14, wherein the power of the nuclear reactor is held constant for the holding period of time which satisfies EQU T=-0.01875.theta.+17.625. 16. The method of operating a nuclear reactor according to claim 12, wherein a recovery percentage R.sub.n of self-irradiation damage of said UO.sub.2 pellets, which depends on the temperature .theta..sub.n of said UO.sub.2 pellets, is evaluated, and wherein when ##EQU10## is established, the linear heat generating rate of the nuclear fuel elements packed in the core is raised to 240 W/cm, where n represents the number of times which the power is held at a fixed level. 17. The method of operating a nuclear reactor according to claim 16, wherein after the linear heat generating rate of said nuclear fuel elements has reached at least 240 W/cm, said linear heat generating rate is increased at most about 1.8 W/cm/hour, to raise the power of said nuclear reactor. 18. The method of operating the nuclear reactor according to claim 16, wherein when ##EQU11## a linear heat generating rate P.sub.(n+1), which depends on the temperature .theta..sub.n of said UO.sub.2 pellets and which renders zero the width of the gap between said UO.sub.2 pellets and a fuel cladding of said nuclear fuel elements, is evaluated, and wherein when P.sub.(n+1) .gtoreq.240 W/cm, the linear heat generating rate of said nuclear fuel elements packed in said core is raised by the coolant control means. 19. The method of operating a nuclear reactor according to claim 18, wherein when P.sub.(n+1) .gtoreq.240 W/cm, the linear heat generating rate of the nuclear fuel elements packed in the core is raised to 240 W/cm. 20. The method of operating a nuclear reactor according to claim 18, wherein after the linear heat generating rate of said nuclear fuel elements has reached at least 240 W/cm, said linear heat generating rate is increased at most about 1.8 W/cm/hour, to raise the power of said nuclear reactor. 21. The method of operating a nuclear reactor according to claim 18, wherein when P.sub.(n+1) .gtoreq.240 W/cm, the power of the nuclear reactor is raised up to the linear heat generating rate P.sub.(n+1), whereupon the power of the nuclear reactor is held there for a fixed period of time. 22. A method of operating a nuclear reactor during the period when power of a nuclear reactor is raised so as to prevent Pellet Clad Mechanical Interaction of the fuel elements, comprising the steps of raising the power of the reactor by a first control means providing coarse power regulation from a linear heat generating rate of zero W/cm after reactor shutdown up to a linear heat generating rate of nuclear fuel elements containing oxide fuel pellets surrounded by a zirconium alloy and located in a core of the reactor which is an upper limit of operation of the first control means, raising the power of the reactor above the linear heat generating rate of the upper limit of operation of the first control means by a second control means for providing fine power regulation; and preventing Pellet Clad Mechanical Interaction of the fuel elements during the raising of the power of the reactor by stopping the raising of the power of the reactor and maintaining the power constant at least once in a range below a linear heat generating rate corresponding to a length of a shutdown period of the reactor before the linear heat generating rate reaches the upper limit of operation of the first control means, and wherein the power of said reactor is held constant at least once somewhere in the range zero to P which satisfies the relation ##EQU12## where P is a linear heat generating rate at which said nuclear fuel elements initiate the Pellet Clad Mechanical Interaction; T.sub.s is the shutdown period of the reactor; P.sub.o is a linear heat generating rate which causes said nuclear fuel elements to induce Pellet Clad Mechanical Interaction for the first time, after said reactor is restarted when the reactor shutdown period T.sub.s is zero; and T.sub.so is the reactor shutdown period when P=0. 23. The method of operating a nuclear reactor according to claim 22, wherein the upper limit of operation of the first control means is 240 W/cm, and after the linear heat generating rate of said nuclear fuel elements has reached at least 240 W/cm, the linear heat generating rate is increased by the second control means at most about 1.8 W/cm/hour, to raise the power of said nuclear reactor. 24. The method of operating a nuclear reactor according to claim 23, wherein the first control means for providing a coarse power regulation includes control rods and manipulating the control rods to raise the power of the reactor up to the linear heat generating rate which is an upper limit of operation of the control rods, the second control means providing fine power regulation including coolant control means and controlling the flow rate of a coolant supplied to the core for raising the power above the linear heat generating rate of the upper limit of operation of the first control means. 25. The method of operating a nuclear reactor according to claim 22, wherein the fuel pellets and the surrounding zirconium alloy satisfy the relation EQU G.sub.a /D.sub.i =0.024 26. The method of operating the nuclear reactor according to claim 22, wherein the power of said nuclear reactor is held constant for a period of time which satisfies the relation EQU T.gtoreq.-K.sub.1 .theta.+K.sub.2, 27. The method of operating the nuclear reactor according to claim 26, wherein the power of the nuclear reactor is held constant at a linear heat generating rate P.sub.1 which satisfies the relation ##EQU13## for a period of time T which satisfies the relation EQU T=-K.sub.1 .theta.+K.sub.2. 28. the method of operating a nuclear reactor according to claim 26, wherein the power of the nuclear reactor is held constant for the holding period of time which satisfies EQU T.gtoreq.-0.01875.theta.+17.625. 29. The method of operating the nuclear reactor according to claim 26, wherein a recovery percentage R.sub.n of self-irradiation damage of said fuel pellets, which depend on the temperature .theta. of said fuel pellets, is evaluated, and wherein when ##EQU14## is established, the linear heat generating rate of said nuclear fuel elements packed in said core is raised to the linear heat generating rate which is the upper limit of operation of said first control means, where n represents the number of times which the power is held at a fixed level. 30. The method of operating a nuclear reactor according to claim 29, wherein when ##EQU15## a linear heat generating rate P.sub.(n+1), which depends on the temperature .theta..sub.n of said fuel pellets and which renders zero the width of a gap between said fuel pellets and a fuel cladding of said nuclear fuel elements, is evaluated, and wherein when P.sub.(n+1) .gtoreq. (the linear heat generating rate which is the upper limit of operation of the first control means), the linear heat generating rate of said nuclear fuel elements packed in said core is raised by the second control means. 31. The method of operating a nuclear reactor according to claim 30, wherein when P.sub.(n+1) .gtoreq. (the linear heat generating rate which is the upper limit of operation of the first control means), the linear heat generating rate of said nuclear fuel elements packed in the core is raised to the upper limit of operation of the first control means. 32. The method of operating a nuclear reactor according to claim 30, wherein when P.sub.(n+1) &lt; (the linear heat generating rate which is the upper limit of operation of the first control means), the power of said nuclear reactor is raised up to said linear heat generating rate P.sub.(n+1), whereupon the power of said nuclear reactor is held there for a fixed period of time. 33. The method of operating the nuclear reactor according to claim 32, wherein the first control means for providing a coarse power regulation includes control rods and manipulating the control rods to raise the power of the reactor up to the linear heat generating rate which is an upper limit of operation of the control rods, the second control means providing fine power regulation including coolant control means and controlling the flow rate of a coolant supplied to the core for raising the power above the linear heat generating rate of the upper limit of operation of the first control means. 34. The method of operating a nuclear reactor according to claim 33, wherein the upper limit of operation of the first control means is 240 W/cm, and after the linear heat generating rate of said nuclear fuel elements has reached at least 240 W/cm, the linear heat generating rate is increased by the second control means at most about 1.8 W/cm/hour, to raise the power of said nuclear reactor.