Hi Anoop,
Use the event
AT SELECTION-SCREEN.
if P_R1 = 'X'
then do the validations for the fields related to radio button P_R1
and if they are not filled then trigger an error message,
elseif P_R2 = 'X'
then do the validations for the fields related to radio button P_R2
and if they are not filled then trigger an error message,
endif,
Hope this solution will help you.
Regards,
Phani