Fusion reactor
From Halopedia, the Halo wiki
Template:FOF-1 The Deuterium Fusion Core is the thermonuclear deuterium/deuterium hydrogen-isotope power source for United Nations Space Command frigates. It may be used aboard further UNSC starships, but is not explicitly stated.
The Deuterium Fusion Core may be related to the also UNSC Fusion Core, although this hasn't been confirmed.