nuclear submarine vs sub

nuclear submarine and sub both are nouns.

Word	Noun	Adjective	Verb	Adverb
nuclear submarine	Yes	No	No	No
sub	Yes	No	Yes	No

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

Other hypernyms of nuclear submarine include pigboat, submarine.

nuclear submarine (noun)	sub (noun)
a submarine that is propelled by nuclear power	a submersible warship usually armed with torpedoes
	a large sandwich made of a long crusty roll split lengthwise and filled with meats and cheese (and tomato and onion and lettuce and condiments); different names are used in different sections of the United States

nuclear submarine (verb)	sub (verb)
	be a substitute