4. Reactivity control devices

Three methods are used to control reactivity:

• control clusters, which help regulate reactivity during operation; if they fall by gravity, the reactor is shut down rapidly (or automatically shut down);

• chemical control by varying the boron concentration in the primary circuit water, which complements the action of the control clusters in regulating reactivity and, under steady-state conditions, makes it possible to achieve cluster configurations in the core that minimally affect the desired power distribution; at shutdown, the boron concentration is significantly increased to ensure a significant anti-reactivity margin during fuel change operations;

• consumable poison rods, which provide partial compensation for the excess reactivity introduced during each core reloading campaign;...