nuclear reactor(n = noun.artifact) reactor - (physics) any of several kinds of apparatus that maintain and control a nuclear reaction for the production of energy or artificial elements;

   Any of several devices that maintain and control a sustained nuclear fission chain reaction, for the production of energy, heat, or artificial elements, or for research purposes. The main fuel sustaining the reaction and consumed by the process is typically uranium or plutonium. [WordNet 1.5]