DE2953818

A method and arrangement for operating a boiling water nuclear reactor and fuel assembly designs for such reactor wherein the reactor is operated with uniquely located control rods that serve the primary functions of power shaping and reactivity control. A second and separate distinct group of control rods is withdrawn when the reactor is at power and the control rods of this group serve the primary function of reactor shutdown. The design of the fuel assemblies and the selected patterns of fuel assemblies and control rods make the separation of control functions feasible. The power shaping and reactivity control control rods are located in low power regions of the core designated control cells. The design of the fuel is such that the control rods of the control cells may remain in fixed positions in these cells during the operating cycle until withdrawal for burnup reactivity compensation.