Yes, ASSIMILATION is a Scrabble word!

Letter values in Scrabble US: A 1, S 1, S 1, I 1, M 3, I 1, L 1, A 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

the state of being assimilated; people of different backgrounds come to see themselves as part of a larger national family
the social process of absorbing one cultural group into harmony with another
the process of absorbing nutrients into the body after digestion
a linguistic process by which a sound becomes similar to an adjacent sound
the process of assimilating new ideas into an existing cognitive structure
in the theories of Jean Piaget: the application of a general schema to a particular instance