Medium Fusion Destructive Device
From Halopedia, the Halo wiki
Medium Fusion Destructive Devices (MFDD) are a rather inclusive category of portable tactical nuclear weapons, encompassing destructive yields from 15 kilotons to 30 megatons. These weapons are used for both military and mining purposes, and are often contained in a cylindrical casing which can be affixed to a missile or detonated manually. MFDDs have a relatively simple and unimaginative operating architecture - consisting of a bare cylinder with a keypad. They were used during the Human-Covenant War by the United Nations Space Command as part of their fallback strategy, serving as territory denial for encroaching enemy forces or being used to destroy large concentrations of troop formations on worlds deemed unsalvageable.
The MFDD class encompasses the following:
In a radio conversation heard on the Halo: Reach map Glacier, one of the two ONI agents involved suggests killing Dr. Catherine Halsey with an MFDD to prevent her from interfering with their secret operation, and to erase any recognizable remains.