Yes, RADIUSES is a Scrabble word!

Letter values in Scrabble US: R 1, A 1, D 2, I 1, U 1, S 1, E 1, S 1.

Is it a valid word?

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

the length of a line segment between the center and circumference of a circle or sphere
a straight line from the center to the perimeter of a circle (or from the center to the surface of a sphere)
a circular region whose area is indicated by the length of its radius
the outer and slightly shorter of the two bones of the human forearm
support consisting of a radial member of a wheel joining the hub to the rim

Also: r, wheel spoke.