The control plates, in the form of two thin, nuclear poison-bearing concentric cylinders, are located in an annular region between the outer fuel element and the beryllium reflector. These plates are driven in opposite directions to open and close a window at the core mid-plane. Reactivity is increased by downward motion of the inner cylinder and the upward motion of the four outer quadrant plates. The inner cylinder is used for shimming and power regulation and has no fast safety function. The outer control cylinder consists of four separate quadrant plates, each having an independent drive and safety release mechanism. All control plates have three axial regions of different neutron poison content designed to minimize the axial peak-to-average power-density ratio throughout the core lifetime. Any single quadrant plate or cylinder is capable of shutting the reactor down.