Yes, BALLAST is a Scrabble word!

Letter values in Scrabble US: B 3, A 1, L 1, L 1, A 1, S 1, T 1.

Is it a valid word?

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

any heavy material used to stabilize a ship or airship
coarse gravel laid to form a bed for streets and railroads
an attribute that tends to give stability in character and morals; something that steadies the mind or feelings
a resistor inserted into a circuit to compensate for changes (as those arising from temperature fluctuations)
an electrical device for starting and regulating fluorescent and discharge lamps

Also: ballast resistor, light ballast.

Not playable in Scrabble US: BARRETTER.