Nuclear material is a catch-all term for radioactive metals, like uranium, plutonium and thorium. This material is used primarily for radiation traps in settlements, but is also used for modifying power armor as well as adding optics when modifying weapons.
Nuclear material is typically found on the corpses super mutant suiciders after they have detonated, as well as enemies that have been "gooified" by a plasma weapon.
Nuclear material also found on the corpses of any creature that had the "glowing" subtype.