Yes, FISSION is a Scrabble word!

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

Is it a valid word?

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

reproduction of some unicellular organisms by division of the cell into two more or less equal parts
a nuclear reaction in which a massive nucleus splits into smaller nuclei with the simultaneous release of energy

Also: nuclear fission.