The Collaborative International Dictionary of English v.0.48:

BWR \BWR\ n.
   a boiling water reactor; a type of nuclear reactor that uses
   water as a coolant and moderator; -- the steam produced can
   drive a steam turbine and produce electrical power.

   Syn: boiling water reactor.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

BWR
    n 1: a nuclear reactor that uses water as a coolant and
         moderator; the water boils in the reactor core and the
         steam produced can drive a steam turbine [syn: boiling
         water reactor, BWR]