Fusion Reactor use is a power generation device that use nuclear fusion. They considered extremely efficient, but they cannot generate enough energy at once like Antimatter reactors or Zero Point Energy Device.
Before the Bio-Digital Plague Fusion reactor were common on earth. The were rediscovered after the Data Ark was uncovered and by 3472, it was believed that more than half of [[earth] energy was being produced by fusion plants.
Takarans knowledge of controlled fusion is very advanced. They've been using fusion reactors to power everything from homes to cars to ships, and every building has its own little reactor to provide electrical power. They are using self-contained, little units that require no service whatsoever, that can run for up to a decade, before the they need to be swapped and refurbish for reuse.