Yes, FUSION is a Scrabble word!

Letter values in Scrabble US: F 4, U 1, S 1, I 1, O 1, N 1.

Is it a valid word?

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

an occurrence that involves the production of a union
the state of being combined into one body
the merging of adjacent sounds or syllables or words
a nuclear reaction in which nuclei combine to form more massive nuclei with the simultaneous release of energy
the combining of images from the two eyes to form a single visual percept
correction of an unstable part of the spine by joining two or more vertebrae; usually done surgically but sometimes done by traction or immobilization
the act of fusing (or melting) together

Also: nuclear fusion, nuclear fusion reaction, optical fusion, spinal fusion.