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An all-semiconductor reactor safety system is described which consists of a dc amplifier, an adjustable trip level bistable circuit, a diode logic circuit, and a power switching circuit. The dc amplifier is a simple emitter follower transistor stage. Three transistors are used in the bistable circuit, two in a Schmitt-trigger arrangement and one as an output drive. The diode logic circuit uses a combination of OR and AND gates to give 2-out-of-3 coincidence of the bistable circuit outputs. A unijunction transistor and two silicon controlled rectifiers are the active elements in the power switching circuit. These components are particularly well suited for the on-off control needed in reactors and the circuitry is reasonably fail-safe.