Yes, ACCRETION is a Scrabble word!

Letter values in Scrabble US: A 1, C 3, C 3, R 1, E 1, T 1, I 1, O 1, N 1.

Is it a valid word?

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

an increase by natural growth or addition
something contributing to growth or increase
(astronomy) the formation of a celestial object by the effect of gravity pulling together surrounding objects and gases
(biology) growth by addition as by the adhesion of parts or particles
(geology) an increase in land resulting from alluvial deposits or waterborne sediment
(law) an increase in a beneficiary's share in an estate (as when a co-beneficiary dies or fails to meet some condition or rejects the inheritance)