Patent Number: 041705165
Section: summary

BACKGROUND OF THE INVENTION This invention relates to a method of operating a nuclear reactor, and more particularly to a method of operating a nuclear reactor which is in less danger of the failure of nuclear fuel elements. Hereunder, a method of operating a boiling water reactor will be described as an example. In the prior art method of operating the nuclear reactor, no special stipulation is made as to the power rise before the linear heat generating rate of nuclear fuel elements arranged in the core of the reactor (hereinbelow, simply termed the "linear heat generating rate") reaches about 240 W/cm. Where the nuclear reactor is operated at a linear heat generating rate of about 240 W/cm or above, provisions as stated below are made from the standpoint of preventing the failure of the nuclear fuel elements. When the linear heat generating rate is raised to or above about 240 W/cm for the first time, the power is gradually raised in a raising proportion of below about 1.8 W/cm/hour by the control of the flow rate of a recycling system. When the reactor power has been held constant at the linear heat rate of or above about 240 W/cm for about 12 hours, it is said that preconditioning has been made. Below the linear heat generating rate having been once preconditioned, the power rise can be carried out in a raising proportion of below (about 15% of the total power)/minute with preference taken of the flow rate control. Where the preconditioning is not made, control rods cannot be pulled out at the linear heat generating rate of or above about 240 W/cm. It has been thought that, when the power of the nuclear reactor is raised as stated above, no failure of the nuclear fuel elements will occur. As described later, however, we have confirmed that the nuclear fuel elements are in danger of failure even below 240 W/cm. SUMMARY OF THE INVENTION It is an object of this invention to suppress the failure of nuclear fuel elements arranged in a core in a nuclear reactor. Another object of this invention to suppress the failure of nuclear fuel elements arranged in a core in a nuclear reactor when the power of the nuclear reactor is raised. Still another object of this invention is to suppress the failure of nuclear fuel elements arranged in a core in a nuclear reactor when, after suspending the reactor operation, the nuclear reactor is restarted to raise the power thereof. Yet another object of this invention is to provide a method of operating a nuclear reactor which can simply achieve the suppression of the failure of nuclear fuel elements arranged in a core in the nuclear reactor. A feature of this invention resides in that, when the power of a nuclear reactor is raised, the power rise of the nuclear reactor is suspended at least once to hold the power of the nuclear reactor at the fixed level before the linear heat generating rate of nuclear fuel elements located in a core in the nuclear reactor reaches about 240 W/cm. Thus, the failure of the nuclear fuel elements arranged in the core can be suppressed. Another feature of this invention resides in that, when a nuclear reactor is restarted and has its power raised after shutdown thereof, the power of the nuclear reactor is held at a fixed level at least once within a range not exceeding a linear heat generating rate corresponding to the shutdown time of the nuclear reactor. Thus, a radiation damage which is caused in fuel pellets by suspending the operation of the nuclear reactor can be recovered, and the failure of nuclear fuel elements can be suppressed. Preferably, the power of the nuclear reactor is held at a fixed level at least once in a range of linear heat generating rates as satisfies: ##EQU1## where P denotes the linear heat generating rate at which the nuclear fuel elements packed in a core in the nuclear reactor initiate the Pellet-Clad-Mechanical-Interaction (hereinafter, shortly termed PCI), T.sub.s denotes the shutdown period of the nuclear reactor, P.sub.o denotes the linear heat generating rate which makes the nuclear fuel elements cause the PCI for the first time when the nuclear reactor is restarted, and T.sub.so denotes the shutdown period of the nuclear reactor at P=0.