Yes, MELTDOWN is a Scrabble word!

Letter values in Scrabble US: M 3, E 1, L 1, T 1, D 2, O 1, W 4, N 1.

Is it a valid word?

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

severe overheating of the core of a nuclear reactor resulting in the core melting and radiation escaping
a disaster comparable to a nuclear meltdown

Also: nuclear meltdown.