meltdown vs cataclysm

meltdown and cataclysm both are nouns.

Word	Noun	Adjective	Verb	Adverb
meltdown	Yes	No	No	No
cataclysm	Yes	No	No	No

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

Other hypernyms of meltdown include calamity, catastrophe, disaster, tragedy.

meltdown (noun)	cataclysm (noun)
a disaster comparable to a nuclear meltdown	an event resulting in great loss and misfortune
severe overheating of the core of a nuclear reactor resulting in the core melting and radiation escaping	a sudden violent change in the earth's surface