Reactor control was by use of boron-steel or steel control rods. Boron steel was typically used for shut-down or bulk control, whilst mild steel ‘grey rods’ were used for trimming or power regulation. Typical rod design was as a series of about five articulated sections to permit insertion even in the event of disruption or movement of the core graphite. As is the case with many Magnox features, each station used a unique design of control rod operation and drive mechanism.

Further protection was provided to enable emergency shut-down in the form of boron ball shut-down systems. These comprised hoppers of boron-steel balls located above a thimble in an interstitial channel. Opening of the hopper dropped the balls into the core, inserting a large amount of negative reactivity. Operation of the boron ball shut-down mechanism was not terminal to reactor operation, and the devices were tested on a routine basis. The boron ball devices had a high degree of resilience against core disruption.

Boron dust injection mechanisms were retro-fitted as a final means of holding down the reactivity post-trip to prevent re-criticality. The mechanism for injecting boron carbide and boron trioxide powders in an air flow would have been manually connected to the reactor (preventing inadvertent automatic deployment). Use of the boron dust system would have irreversibly shut down the reactor.