nuclear meltdown vs overheating

nuclear meltdown and overheating both are nouns.

Word	Noun	Adjective	Verb	Adverb
nuclear meltdown	Yes	No	No	No
overheating	Yes	No	No	No

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

nuclear meltdown: severe overheating of the core of a nuclear reactor resulting in the core melting and radiation escaping
overheating: excessive heating

nuclear meltdown (noun)	overheating (noun)
severe overheating of the core of a nuclear reactor resulting in the core melting and radiation escaping	excessive heating