This is compounded by the fact that the boron control rods at Chernobyl were tipped with graphite (to increase volume/displacement at lower cost), so that when the reaction started to run away as the Xenon built up, the operators attempted to SCRAM, but the graphite tips on the leading edge of the control rods acted as neutron moderators before the the boron was in place, further accelerating the reaction and causing the reactor explosion.