Fusion reactor

The Deuterium Fusion Core is the thermonuclear deuterium hydrogen-isotope power source for United Nations Space Command frigates. It may be used aboard further UNSC starships, but is not explicitly stated.

Since it is called the Deuterium Fusion Core, it should be believed that the Deuterium Fusion Core uses Deuterium-Deuterium Fusion in which 2 Deuterium Isotopes combine to form a Helium-3 Isotope and a neutron. Another kind of Fusion is Deuterium-Tritium Fusion in which a Deuterium Hydrogen Isotope combines with a Tritium Hydrogen Isotope and forms helium-4 and a neutron, however Deuterium-Tritium Fusion creates less energy compared to Deuterium-Deuterium fusion. It is possible that older UNSC Vessels uses Deuterium-Tritium Fusion cores.

Modern day nuclear power plants uses Nuclear Fission in which a Uranium-235 or Plutonium atom gets smashed and split to create energy, Fission is way less efficient compared with fusion. Modern day fusion reactors are on experimental stage.

The Deuterium Fusion Core may also be related to the UNSC Fusion Core, although this hasn't been confirmed.