Method of operating PWR reactor

In PWR type reactors, the reactor core has been directed to that of high burnup degree and long time operation cycle by increasing the concentration of U-235 and using gadolinia-incorporated fuels as burnable poisons for the improvement of fuel economy. As a result, upon assumed accidents in which one of control rods at the highest reactivity should be pop out during operation, the maximum value of the power distribution in the reactor core is greatly changed to reduce the safety margin of the reactor core. Accordingly, the reactor operation is conducted at less than about 25% of the rated power, while inserting a control rod bank having a control rod worth of about 1.0 to 3.0% Δρ, by a height about 10 to 30% relative to the axial length of the reactor core. Thus, the maximum value of the power distribution and the control rod worth upon popping out are reduced to less than about one-half as compared with the case of not inserting the bank. Further, since the axial off set in the power distribution is within a range of about 30%, the control rod pop out accident does not constitute the restricting condition to provide a margin in view of the plant operation. (N.H.)