Tangential acceleration is the reason of a particle travelling faster or slower in a circular motion. A uniform motion is one in which the velocity remains constant. As a result, the tangential component of the acceleration is zero.
An item travelling in a circular path at a constant speed has a constant centripetal acceleration. Due to the continual change of direction, the radial acceleration is not constant.
If ar = 0, there is no radial acceleration and circular motion is not possible So ar ≠ 0
If at≠ 0 the motion is not uniform as angular velocity will change
So ar ≠ 0 and at = 0 for uniform circular motion