A '''Blue Fusion Reactor''' is a nuclear generator that uses fusion to provide a large amount of energy. 4 of them appear in Deus Ex.
Blue Fusion Reactors are never truly explained in game, but they were likely reverse-engineered from alien technology (much like the Grays and B13 Antimatter Reactors) as official documents call it an artifiact, and it is codenamed ROS172-E (ROS may stand for Roswell.) They are likely very expensive to build and maintain, as they are only seen under the Nanotech Infusion Chamber at Area 51.
Blue Fusion Reactors are fairly small, only the size of a Human Being, and most of that size is devoted to the Isotope Reservoir. Even for their size, the reactors generate enormous amounts of energy; they are capable of supporting a plasma field around Bob Page, powering the Nanotech Infusion Chamber, and powering 3 different Universal Constructor|universal constructors at the same time. Fusion Reactors cannot be destroyed physically, with either the Dragon's Tooth Sword|Dragons Tooth Sword or Explosives. Every device has a numpad which is locked with the code '''7243'''. Once all 4 reactors are shut off, the player must hit the Infusion Control Switch, which will destroy the Infusion chamber (and by extension Helios) in a violent explosion and kill Bob Page. Afterwards, the Illuminati ending to Deus Ex will trigger.