radioactive dust vs fallout

radioactive dust and fallout both are nouns.

Word	Noun	Adjective	Verb	Adverb
radioactive dust	Yes	No	No	No
fallout	Yes	No	No	No

As nouns, radioactive dust and fallout are synonyms defined as:

radioactive dust and fallout: the radioactive particles that settle to the ground after a nuclear explosion

radioactive dust (noun)	fallout (noun)
the radioactive particles that settle to the ground after a nuclear explosion	any adverse and unwanted secondary effect
	the radioactive particles that settle to the ground after a nuclear explosion