PURPOSE: To control output peeking and a decellerating material temp. coefficient without using a large number of IFBA rods in a nuclear core.

CONSTITUTION: The nuclear core 14 of a nuclear reactor is constituted of a first group of fuel rods containing a fissionable substance but not a combustible absorbing substance and a second group of fuel rods containing a fissionable substance and two combustible absorbing substances. The number of the fuel rods of the first group is larger than that of the fuel rods of the second group. Two combustible absorbing substances are separately coated or present as a mixture. One of the combustible absorbing substance is an erbium-containing substance such as erbium oxide and the other one of them is a boron-containing substance such as zirconium diborate and the erbium-containing substance may be mixed with the fissionable substance.