Fusion reactor

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.

Speculation
According from the vista of conventional physics, the deuterium reactor uses supermagnetic fields and extremely high temperatures to fuse together deuterium nuclei to form energy.