Reactivity control
AGR (Advanced Gas cooled Reactor). General Presentation of the British Nuclear Fleet

Reactivity is controlled by three different physical systems:

• a system of control rods inserted according to piloting requirements, supplemented by a system of control rods to maintain reserves in hot shutdown situations;

• an automatic system for rapid insertion of anti-reactivity agents by injecting nitrogen from beneath the core. In a first step, carbon dioxide is flushed out of the 163 channels located in the interstices between the fuel channels, then in a second step, from a new supply, all the channels in the core are filled. This system can keep the core subcritical for several hours;

• a manually triggered system for injecting 3 mm-diameter absorbent beads. The beads are introduced from above, pushed by carbon dioxide, and fill 32 channels in the gaps between the graphite...