Control Rods

Six control rods are positioned symmetrically and at the same radial position throughout the core. The rod pins are filled with boron-carbide balls similar to those used in the Ultimate Shutdown System. The control rods are powered by fine motion control rod drives to handle most power-need manipulations and allow more freedom to select the most desireable insertion depth.

During SCRAM, the main drives disconnect and the control rods are allowed to "free fall" into their final core positions. Drive-in motors are soon activated to drive into the core any control rods that may have become stuck in the assembly channels.

One control rod assembly can independently shut down the reactor. Thus, the ALMR core has a 6:1 redundancy for shutting down the reactor with control rods. Redundancy is the term used to describe the condition in which more than one safety system is working to ensure that one safety concern is adressed. It is also an inverse measure of the probability that the accident situation will not be alleviated by the safety systems. For example, it is more probable that a reactor with one control rod system will not be able to stop an accident than a reactor with six independent control rod systems. Thus a 6:1 redundancy is better than a 3:1 redundancy.