Yes, NUCLEUSES is a Scrabble word!

Letter values in Scrabble US: N 1, U 1, C 3, L 1, E 1, U 1, S 1, E 1, S 1.

Is it a valid word?

NUCLEUSES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a part of the cell containing DNA and RNA and responsible for growth and reproduction
the positively charged dense center of an atom
a small group of indispensable persons or things
(astronomy) the center of the head of a comet; consists of small solid particles of ice and frozen gas that vaporizes on approaching the sun to form the coma and tail
any histologically identifiable mass of neural cell bodies in the brain or spinal cord
the central structure of the lens that is surrounded by the cortex

Also: cell nucleus, core group, lens nucleus.

Not playable in Scrabble US: KARYON.