Tritium

Tritium is a basic atomic element, it is an isotope of hydrogen. Its nucleus consists of a proton and two neutrons.

Tritium, similar to deuterium, was used as fuel in matter-antimatter reactions aboard starships like the USS Enterprise-D. (TNG: "Galaxy's Child")

In late 2375, tritium proved to be extremely important in helping the Federation Alliance to win the Dominion War. A Klingon chief engineer aboard the IKS Ki'tang adjusted the tritium intermix to compensate for a Warp Core containment problem. During the Second Battle of Chin'toka, the tritium made the ship invulnerable to the Breen energy dampening weapon. (DS9: "When It Rains...")

The use of tritium in this way by Klingon vessels suggests that they are powered by something similar to a D-T cycle fusion reactor, such as that tested in the experimental ITER power plant.