Nuclear reactor

A control rod comprises an absorber and a reflector, and they are disposed in a back to back arrangement. Control rods are disposed in adjacent with each other at the circumference of fuel assemblies in a reactor core, each rotatably around the central axis as a center of the control rod. Each of the control rods is rotated around the center axis as a center thereby varying the ratio between the face of the absorber and the surface of the reflector occupying a plane of projection from the center of the reactor core for controlling the reactivity of the control rods. With such a constitution, a driving mechanism for driving the control rod vertically and a guide tube for guiding the vertical movement of the control rod are no more necessary, and the size of the reactor container can be reduced. Further, equipments other than the control rod drives can be installed in a space above the reactor core. Further, since a striping phenomenon is not caused, it is no more necessary to take a countermeasure for the striping phenomenon. (I.N.)