If it is about reaction and action, reaction force is the counterpart to action force. In other words: it is a force opposite to action force. The question is: what is action force? if the action force is the centrifugal force, the reaction force can just be the centripedal force, as the centripedal force is always opposite to the centrifugal force in every moment of the circular motion!
But why you are interest in reaction force? according to the question above, the speed has to be determine and you can find the speed out by converting the centrifugal force to the unit v.