atomic pile

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An early type of nuclear reactor whose core consisted of alternating layers of uranium pellets and graphite bricks, designed to create a sustained fission reaction.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun physics an early nuclear reactor fuelled by uranium and moderated with graphite

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a nuclear reactor that uses controlled nuclear fission to generate energy