Gentle correction: Chobham is not reactive. It mimics some of the effects of reactive armor, by redirecting some of the energy of a HEAT round against the shaped charge jet itself (when the ceramic shatters into dust), but it is not truly reactive. As one poster put it:
Thats why you'll see soldiers riding on top of an M1 but never a T72 -the top of the T72 is covered with explosive reactive bricks.