Yes, SEQUESTRATION is a Scrabble word!

Letter values in Scrabble US: S 1, E 1, Q 10, U 1, E 1, S 1, T 1, R 1, A 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

the act of segregating or sequestering
the action of forming a chelate or other stable compound with an ion or atom or molecule so that it is no longer available for reactions
a writ that authorizes the seizure of property
seizing property that belongs to someone else and holding it until profits pay the demand for which it was seized