When the AZ-5 button was pressed, the insertion of control rods into the reactor core began. The control rod insertion mechanism moved the rods at , so that the rods took 18 to 20 seconds to travel the full height of the core, about . A bigger problem was the design of the RBMK control rods, each of which had a graphite neutron moderator section attached to its end to boost reactor output by displacing water when the control rod section had been fully withdrawn from the reactor, i.e. when a control rod was at maximum extraction, a neutron-moderating graphite extension was centered in the core with columns of water above and below it.