Yes, THORIUM is a Scrabble word!

Letter values in Scrabble US: T 1, H 4, O 1, R 1, I 1, U 1, M 3.

Is it a valid word?

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

a soft silvery-white tetravalent radioactive metallic element; isotope 232 is used as a power source in nuclear reactors; occurs in thorite and in monazite sands

Also: atomic number 90.

Not playable in Scrabble US: TH.