Yes, ATRIUMS is a Scrabble word!

Letter values in Scrabble US: A 1, T 1, R 1, I 1, U 1, M 3, S 1.

Is it a valid word?

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

any chamber that is connected to other chambers or passageways (especially one of the two upper chambers of the heart)
the central area in a building; open to the sky