They are simple reactors that rely on uranium natural decay like real life nuclear plants do. fission=cracking/breaking. uranium decays into more stable elements releasing heat in the process.
In Adro it is a 1x1x1 cube-ish block that makes plasma out of water and uranium (applies heat to water).
Indispensable if you want to go into plasma stage (shields, mfm drives, railguns, plasma cannons,...)