Nuclear reactor core

Inside the core of a typical pressurized water reactor or boiling water reactor are fuel rods with a diameter of a large gel-type ink pen, each about 4 m long, which are grouped by the hundreds in bundles called "fuel assemblies".

Also inside the core are control rods, filled with pellets of substances like boron or hafnium or cadmium that readily capture neutrons.

One type uses solid nuclear graphite for the neutron moderator and ordinary water for the coolant.

In the Advanced Gas-cooled Reactor, a British design, the core is made of a graphite neutron moderator where the fuel assemblies are located.

Carbon dioxide gas acts as a coolant and it circulates through the core, removing heat.