The tritium intermix was an adjustable feature in the warp core reactors of Klingon starships in the late 24th century. Federation and Romulan starships in the same era were not designed with this feature.
In 2375, shortly before the Second Battle of Chin'toka, the chief engineer of the Klingon Bird-of-Prey IKS Ki'tang adjusted the tritium intermix on the ship's reactor to compensate for a containment problem in the warp core. The adjustment had the unintended side effect of protecting the Ki'tang from the effects of the Breen energy dissipater. Soon after, General Martok ordered every starship in the Klingon fleet to make the same adjustment to their reactor. Federation and Romulan starships could not be similarly adjusted and required a different defense for the Breen energy dampening weapon. (DS9: "When It Rains...")