Yes, REACTION is a Scrabble word!

Letter values in Scrabble US: R 1, E 1, A 1, C 3, T 1, I 1, O 1, N 1.

Is it a valid word?

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

(chemistry) a process in which one or more substances are changed into others
an idea evoked by some experience
a bodily process occurring due to the effect of some antecedent stimulus or agent
(mechanics) the equal and opposite force that is produced when any force is applied to a body
a response that reveals a person's feelings or attitude
extreme conservatism in political or social matters
doing something in opposition to another way of doing it that you don't like

Also: chemical reaction.