There was a significant flaw in the design of the control rods that were inserted into the reactor to slow down the reaction rate by neutron absorption. In the RBMK design, the bottom tip of each control rod was made of graphite and was shorter than necessary. Only the upper part of the rod was made of boron carbide, which absorbs neutrons and thereby slows the reaction. With this design, when a rod was inserted from the fully retracted position, the graphite tip displaced neutron-absorbing water, initially causing fewer neutrons to be absorbed and increasing reactivity. For the first few seconds of rod deployment, reactor core power was therefore increased, rather than reduced. This feature of control rod operation was counter-intuitive and not known to the reactor operators.