Reactor core for use in BWR type reactor

Purpose: To increase the nuclear life and remarkably decrease the number of exchanges for control rods. Constitution: Long life control rods using only hafnium metal rod as the long life neutron absorbent are arranged at the center of a control rod cell in the reactor core, which are inserted into the reactor core during reactor operation and used as the control rods for conducting power conditioning and the reactivity control. This enables to increase the nuclear life of the control rods and remarkably reduce the number of exchange for the control rods. On the other hand, control rods having hafnium only in the upper portion of them and boron carbide as the neutron absorbers at other portions are disposed in a not-control cell in the reactor core, which are utilized as the control rods that are withdrawn from the reactor core during reactor operation, while inserted into the reactor core upon reactor shutdown. This can increase the life of the control rods even when the upper end thereof undergoes neutron irradiation during reactor operation. (Kawakami, Y.)