The Collaborative International Dictionary of English v.0.48:

nuclear reactor \nuclear reactor\ n. (phys.)
   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.

   Syn: reactor, atomic pile, fission reactor, nuclear pile
        [WordNet 1.5 +PJC]

WordNet (r) 3.0 (2006):

nuclear reactor
    n 1: (physics) any of several kinds of apparatus that maintain
         and control a nuclear reaction for the production of energy
         or artificial elements [syn: nuclear reactor, reactor]