Current designs for tokamak based fusion reactors usually consider deuterium-tritium reactions with a 50–50% mixture of D-T, to give a maximum power density, although design studies for pure D-D, He3-catalysed and T-catalysed tokamak fusion reactors have been performed. The letter considers a reactor burning a D-T mixture with an enhanced percentage of deuterium to increase the possibility of D-D reactions (a so-called T-catalysed D reactor). Most of the energy needed to maintain the burning plasma comes from D-T reactions, but the D-D reaction results in the generation of extra neutrons (or equally useful tritons or 3He nuclei), which can contribute to the overall neutron economy of the reactor. – A simple plasma energy balance for an ignited tokamak is used to study the usefulness of D-D reactions, with confinement times, safety factors, and beta limits as predicted by existing experimental devices. The authors consider both the improvements necessary in present plasma performance limits to produce useful D-D reaction rates as well as the effect of a lower plasma energy density on reactor size. – The object of the study has not been to consider the economics of such types of reactor, but rather to assess the potential of the D-D reaction to introduce some flexibility into a fusion reactor design. It is evident from design studies on next generation tokamaks that the overall neutron economy will be a major challenge in a fusion reactor and that the reduction of the necessary blanket breeding ratio could improve the feasibility of the overall design.