Fuel retention refers to the deposition of fusion fuel in the plasma-facing wall components. Carbon, which is traditionally used in fusion experiments, retains fusion fuel because of its high affinity for hydrogen. The tokamaks ASDEX Upgrade and JET are now testing the metals tungsten and beryllium as wall materials which reduce fuel retention.
While fuelling refers to supplying a fusion plasma with the necessary reactants, fusion experiments normally utilise deuterium plasmas, while fusion power plants will burn deuterium-tritium plasmas.