Yes, PLUTONIUMS is a Scrabble word!

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

Is it a valid word?

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

a solid silvery grey radioactive transuranic element whose atoms can be split when bombarded with neutrons; found in minute quantities in uranium ores but is usually synthesized in nuclear reactors; 13 isotopes are known with the most important being plutonium 239

Also: atomic number 94.

Not playable in Scrabble US: PU.