At the end, the Number 4 unit was down to only six control rods, with 205 rods withdrawn. The experimenters allowed this dangerous condition to develop even though they had deliberately bypassed and disconnected every important safety system, including the emergency core-cooling system. They had also disconnected every backup electrical system, down to and including diesel generators, that would have allowed them to operate the reactor controls in the event of an emergency. That reduced the electrical supply to the reactor's water pumps, which in turn reduced the flow of cooling water through the reactor. In the coolant channels within the graphite-uranium fuel core, the water began to boil.
Graphite facilitates the fission chain reaction in a graphite reactor by slowing neutrons.
Coolant water in such a reactor absorbs neutrons, thus acting as a poison. When the coolant water in the Number 4 Chernobyl unit began turning to steam, that change of phase reduced its density and made it a less effective neutron absorber. With more neutrons becoming available and few control rods inserted to absorb them, the chain reaction accelerated. The power level in the reactor began to rise. The operators noticed the power surge and realized they needed to reduce reactivity quickly by inserting more control rods.