While I agree with your analogy, graphite is actually a moderator which (in a nuclear context) accelerates reactions. This was a factor for Chernobyl [1] where the rods had graphite tips, so a scram had a momentary rapid increase in reactivity. This burst of reactivity caused temperature to momentarily spike. The heat produced steam which increased pressure and the vessel exploded.
[1] http://www-pub.iaea.org/MTCD/publications/PDF/Pub913e_web.pd...