Abstract: An assembly for controlling the release of neutrons in a nuclear reactor having the core split into two halves. A disk assembly formed from at least two circular disks is positioned substantially at the center of and coaxially with the core halves. Each disk is machined with an identical surface hole pattern such that rotation of one disk relative to the other causes the hole pattern to open or close. The disks may be formed from neutron absorbing material or moderator material. The holes may be provided with fissile material inserts to enhance reactivity when in the open position. A drive motor mounted adjacent the reactor and drive shaft operatively engaged between the drive motor and disks is used to rotate the disks.