The reactor used cadmium-clad steel control rods. Made from neutron-absorbing cadmium, these could restrict or halt the reaction. Three rods penetrated the reactor vertically, held in place by a clutch to form the scram system. They were suspended from steel cables that were wound around a drum, and held in place by an electromagnetic clutch. If power was lost, they would drop into the reactor, halting it. The other four rods were made of boron steel and horizontally penetrated the reactor from the north side. Two of them, known as "shim" rods, were hydraulically controlled. Sand-filled hydraulic accumulators could be used in the event of a power failure. The other two rods were driven by electric motors.