Yes, RADIAN is a Scrabble word!

Letter values in Scrabble US: R 1, A 1, D 2, I 1, A 1, N 1.

Is it a valid word?

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

the unit of plane angle adopted under the Systeme International d'Unites; equal to the angle at the center of a circle subtended by an arc equal in length to the radius (approximately 57.295 degrees)