Patent Number: 043022871
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a method for operating a nuclear reactor to increase nuclear reactor power and especially a method for increasing nuclear reactor power without danger of failure of the fuel rods disposed in a core of a nuclear reactor. A core of a nuclear reactor is provided with a plurality of fuel rods where each fuel rod is assembled such that a plurality of fuel pellets are packed in a clad tube. A rapid increase of nuclear reactor power causes pellet-clad-mechanical-interaction in a recognized linear heat generating rate range (indicated as PCI in the following description) between the fuel pellets and the clad tube due to volume expansion of the fuel pellets and causing excessive strain in the clad tube. Thus, there is a possibility of a failure of the fuel rod, which means that one or more than one openings, cracks or pin holes appear on the clad tube so that nuclear fission products are relieved into a surrounding coolant from the fuel rod through the openings, cracks or pin holes. Regarding the failure of the fuel rod, it has been proposed, as described in Japanese Published Patent Application No. 50-143,999, and U.S. Pat. No. 4,057,466 to Thompson et al, to restrain the rate of increase of the linear heat generating rate when the linear heat generating rate is increased above the linear heat generating rate at which the PCI begins. As disclosed in the Thompson et al patent, in order to properly condition the fuel rods so as to avoid PCI failure, the rate of increase of the fuel power or linear heat generating rate within the pellet-cladding interaction range is maintained below a critical rate of increase. The critical rate of increase necessarily requires a long term of at least several days to attain the desired operating power level for the nuclear reactor and the length of time to attain such level cannot be shortened due to the limitation on the rate of increase. SUMMARY OF THE INVENTION It is therefore an object of the invention to provide a method for operating a nuclear reactor which overcomes the disadvantages of the prior art by increasing the power to attain the rated power level within a short period of time while also preventing failure of the fuel rods. In accordance with the present invention, there is provided a method for controlling the operation of a nuclear reactor to increase the reactor power in a range in which pellet-clad-mechanical-interaction occurs comprising the steps of increasing the reactor power from a power level in which pellet-clad-mechanical-interaction begins to take place up to a predetermined power level for the nuclear reactor and controlling the rate of increase of reactor power to a rate no less than 0.15 KW/ft/hr., and no greater than a predetermined critical rate so as to shorten the time necessary to raise the reactor power to the predetermined power level without causing pellet-clad-mechanical-interaction damage of the fuel elements.