What is the acceleration of an object that has a velocity of 18 m/s and is moving in a circle of radius 30m?

Well, first of all, 18 m/s is the object's speed, but not its velocity.
Velocity consists of the object's speed AND direction, and this
particular object's direction is constantly changing, since it's
moving around a circle.  So its velocity is constantly changing too.

Centripetal acceleration  =  (speed)² / (radius)

                                           =  (18 m/s)² / (30 m)

                                           = (324 m²/s²) / (30 m)

                                           =       10.8 m/s²

                                       (about  1.1 G)