nuclear reactor vs apparatus

nuclear reactor and apparatus both are nouns.

Word	Noun	Adjective	Verb	Adverb
nuclear reactor	Yes	No	No	No
apparatus	Yes	No	No	No

As nouns, apparatus is a hypernym of nuclear reactor; that is, apparatus is a word with a broader meaning than nuclear reactor:

nuclear reactor: (physics) any of several kinds of apparatus that maintain and control a nuclear reaction for the production of energy or artificial elements
apparatus: equipment designed to serve a specific function

Other hypernyms of nuclear reactor include setup.

nuclear reactor (noun)	apparatus (noun)
(physics) any of several kinds of apparatus that maintain and control a nuclear reaction for the production of energy or artificial elements	equipment designed to serve a specific function
	(anatomy) a group of body parts that work together to perform a given function