Reactor core insert assembly

Materials of variable neutron absorption rate are incorporated in the axial center of a reactivity control member in a reactor core insert assembly, and materials scarcely absorbing neutrons and thermalizing the neutron spectrum are incorporated at the upper and the lower portions. When the insert assembly is inserted to a fuel assembly, primary coolants are flown to the reactivity control members during operations. Since the boric acid concentration in the primary coolants is high at the early stage of the cycle and the absorption of thermal neutrons by boric acid is high, nuclear reactions are controlled in the adjacent region to the central portion and the peripheral region at the initial stage. Accordingly, the relative power in the axial center of the fuel assembly is decreased. On the other hand, since the thermal neutrons are scarcely absorbed and materials for forming thermal neutrons, that is, zirconium hydride are present in the upper and the lower portions of the reactivity control members, the nuclear reactions are vigorous in the adjacent region and the peripheral region thereof to compensate the reduction of the relative power in the axial center of the fuel assembly, thereby flattening the axial power distribution. (N.H.)