Reactor operation method

The whole control rods are divided into a first group of control rods situated at the outermost periphery of the reactor core and a second group of control rods situated at the position other than the above. In the earlier half cycle of the rated power operation or at the operation point with strict thermal destruction value, the operation pattern is constituted with the second control rod group in the central portion of the reactor core, thereby suppressing the thermal restriction value and controlling the excess reactivity. In the latter half cycle of the operation or at the easy operation point with the thermal restriction value, the operation pattern is constituted with the first control rod group, thereby controlling the excess reactivity. As the excess reactivity is gradually lowered, the control rod at the outermost circumference is extracted for control. In this way, the accumulated neutron irradiation amount of the control rod inserted in the earlier half of the cycle is reduced to one-half to make the exchange internal of the control rod longer. At the control rod position in the outermost circumference, neutron leakage is large and nuclear lifetime is not reached even after long time insertion. The amount for the control rod replacing operation is reduced upon periodical inspection and the amount of the radioactive wastes formed is also reduced. (N.H.)