1,197,528. Controlling reactors. COMBUSTION ENG. Inc. 28 Aug., 1968 [15 Sept., 1967], No. 41184/68. Heading G1C. In a nuclear reactor safety control system, the control rods are released so as to shut down the reactor, when variables monitored, such as primary coolant flow, water level, load and neutron flux, vary from predetermined values. Each variable is monitored by several, independent, sensing devices which are arranged in groups, each group containing one sensing device for each of the variables. Each sensing device energizes an associated relay coil during normal reactor operation and the coil is deenergized to fail safe when the undesirable condition of the variable monitored occurs. The contacts controlled by the relay coils are arranged in a number of independent matrices, each of which incorporates the contacts of some of the relays. This results in " redundant instrumentation " which permits correct operation of the safety control system when only some of the sensing devices are functioning satisfactorily. In a further embodiment, the control circuit is formed of a number of isolated sections, each supplied from a separate energy source.