Yes, RATIONAL is a Scrabble word!

Letter values in Scrabble US: R 1, A 1, T 1, I 1, O 1, N 1, A 1, L 1.

Is it a valid word?

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

consistent with or based on or using reason
of or associated with or requiring the use of the mind
capable of being expressed as a quotient of integers
having its source in or being guided by the intellect (as distinguished from experience or emotion)

Also: rational number.