An on-line digital computer is used to improve operating safety and efficiency of a large production reactor at the Savannah River Plant. About 3250 instrument sensors are scanned in a 4½-min cycle; signals and computed results closest to limits are displayed to the reactor operator. Automatic alarm programs identify an exceeded limit, a change in normal fuel-coolant temperature distribution, or a failed sensor. Demand programs print temperature distributions and flux curves used to adjust reactor power distribution. Computer results are used with existing written procedures for control of reactor operation.