[en] Purpose: To improve a neutron shield effect and a safety of a reactor. Constitution: A reactor container disposed within shield concrete through pedestals, characterized in that shield rings are mounted on the outer peripheral surface of the reactor container for shielding scattering of neutrons upwardly of the reactor container, the mounting position being set to a position closing a gap between the pedestals and the reactor container and to a position closing a gap between the reactor container and a safety container covering the reactor container. (Nakamura, S.)