6. Primary and secondary circuits

The core cooling gas circuit is complex. Its design required many iterations, and was only finalized after the first reactors had been commissioned. The graphite itself is no longer cooled solely by the fuel channels; hot spots on internal structures and components must be specifically cooled. The CO ₂ circulation circuit must therefore both ensure cooling of the fuel elements and conduct heat to the exchangers, but also ensure cooling of the graphite, the vessel and the structures and components placed inside. This has been achieved with deflector structures to guide the flow of gas into the caisson.

The cold gas is pushed by blowers at the exchanger outlet, half of which is guided to a cavity beneath the core to feed the fuel assemblies, and the other half along the radiological and thermal shields protecting the primary...