Explanation

Centripetal acceleration, $a_{c} = v^{2}/R$. Where v is the speed of an object and R is the radius of the circle. It is always directed towards the centre of the circle. Since v and R are constants for a given uniform circular motions, therefore the magnitude of centripetal acceleration is also constant. However, the direction of centripetal acceleration changes continuously. Therefore, a centripetal acceleration is not a constant vector.