The control rod insertion mechanism moved the rods at 0.4 m/s, so that the rods took 18 to 20 seconds to travel the full height of the core , about 7 meters. A bigger problem was a flawed graphite-tip control rod design, which initially displaced neutron-absorbing coolant with moderating graphite before introducing replacement neutron-absorbing boron material to slow the reaction. As a result, the SCRAM actually increased the reaction rate in the upper half of the core as the tips displaced water. This behavior was known after a shutdown of another RBMK reactor induced an initial power spike, but as the SCRAM of that reactor was successful, the information was not widely disseminated. A few seconds after the start of the SCRAM, the graphite rod tips entered the fuel pile. A massive power spike occurred, and the core overheated, causing some of the fuel rods to fracture, blocking the control rod columns and jamming the control rods at one-third insertion, with the graphite tips in the middle of the core. Within three seconds the reactor output rose above 530 MW. [ 24 ] :31 The subsequent course of events was not registered by instruments; it is known only as a result of mathematical simulation. Apparently, the power spike caused an increase in fuel temperature and massive steam buildup, leading to a rapid increase in steam pressure. This caused the fuel cladding to fail, releasing the fuel elements into the coolant, and rupturing the channels in which these elements were located.