The immediate cause of the Chernobyl accident was a mismanaged electrical-engineering experiment.
Engineers with no knowledge of reactor physics were interested to see if they could draw electricity from the turbine generator of the Number 4 reactor unit to run water pumps during an emergency when the turbine was no longer being driven by the reactor but was still spinning inertially. The engineers needed the reactor to wind up the turbine; then they planned to idle it to 2.5 percent power. Unexpected electrical demand on the afternoon of April 29 delayed the experiment until eleven o'clock that night. When the experimenters finally started, they felt pressed to make up for lost time, so they reduced the reactor's power level too rapidly. That mistake caused a rapid buildup of neutron-absorbing fission by products in the reactor core, which poisoned the reaction. To compensate, the operators withdrew a majority of the reactor's control rods, but even with the rods withdrawn, they were unable to increase the power level to more than 30 megawatts, a low level of operation at which the reactor's instability potential is at its worst and that the Chernobyl plant's own safety rules forbade.
At that point, writes Russian nuclear engineer Grigori Medvedev, "there were two options: increasing the power immediately, or waiting twenty-four hours for the poisons to dissipate. [Deputy chief engineer Dyatlov] should have waited...But he [had an experiment to conduct and he] was unwilling to stop...He ordered an immediate increase in the power of the reactor."
Reluctantly the operators complied. By 1 a.m. on April 26, they stabilized the reactor at 200 megawatts. It was still poisoned and increasingly difficult to control. More control rods came out. A minimum reserve for an RBMK reactor is supposed to be 30 control rods. At the end, the Number 4 unit was down to only six control rods, with 205 rods withdrawn.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.