The most practical fuel for a thermonuclear reactor, both from economical and nuclear considerations is
The deuterium atom is widely used in prototype fusion reactors.
The current best fuel for fusion reactors is deuterium-tritium. This fuel reaches fusion conditions at lower temperatures compared to other elements and releases more energy than other fusion reactions.
They are used as a tracer in nuclear fusion reactors in order to slow down the neutrons in heavy water-moderated fission reactors.