atomic pile vs chain reactor

atomic pile and chain reactor both are nouns.

Word	Noun	Adjective	Verb	Adverb
atomic pile	Yes	No	No	No
chain reactor	Yes	No	No	No

As nouns, atomic pile and chain reactor are synonyms defined as:

atomic pile and chain reactor: a nuclear reactor that uses controlled nuclear fission to generate energy

Other synonyms of atomic pile include atomic reactor, pile.

atomic pile (noun)	chain reactor (noun)
a nuclear reactor that uses controlled nuclear fission to generate energy	a nuclear reactor that uses controlled nuclear fission to generate energy