Yes, RAD is a Scrabble word!

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

Is it a valid word?

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

a unit of absorbed ionizing radiation equal to 100 ergs per gram of irradiated material
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)