refill vs fill up

refill is a noun but fill up is not a noun.

Word	Noun	Adjective	Verb	Adverb
refill	Yes	No	Yes	No
fill up	No	No	Yes	No

As verbs, fill up is a hypernym of refill; that is, fill up is a word with a broader meaning than refill:

Other hypernyms of refill include fill, make full.

refill (noun)	fill up (noun)
a commercial product that refills a container with its appropriate contents
a prescription drug that is provided again

refill (verb)	fill up (verb)
fill something that had previously been emptied	make full, also in a metaphorical sense
	become full
	eat until one is sated
	fill or stop up