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The chain of events began April 25, when plant operators began reducing the reactor's power level so that they could run the turbine experiment. In order to have the machinery operate at low power levels, the workers switched off the automatic control system, thus removing emergency restraints designed to prevent the reactor from going out of control. When power dropped too low for the test, technicians took steps to bring it back up. They also removed too many control rods. Thirty rods must be inserted in the reactor's nuclear-fuel assembly at all times in order to regulate the chain reaction, but Soviet workers took out all but six to eight of them. The operators closed off another safety system that would have automatically shut the reactor down when the turbines stopped. This, said one Soviet official, was a violation of the ) "most sacred rule."
At 1:23 a.m. on April 26, the workers began the actual experiment by stopping power to the turbine. Just prior to that, the flow of the water that normally cooled the reactor was reduced and certain safety devices were disengaged. The reactor immediately began to overheat dangerously, but since the emergency cooling system had been shut off some twelve hours earlier, there was no backup. Within seconds, there was a tremendous power surge that caused two explosions, blew the roof off the reactor building and ignited more than 30 fires around the plant. The damaged reactor core and the graphite surrounding it began burning at temperatures as high as 2,800 degreesF. The fire sent a plume of radioactive debris into the upper atmosphere while Soviet fire fighters in helicopters frantically tried to extinguish the blaze by dumping 5,000 tons of boron, lead and other material on the reactor core. They did not succeed in putting out the fire until twelve days after the accident.
While telling their chilling tale of worker incompetence, Soviet officials were careful to defend both the concept of nuclear power and the overall safety of Soviet reactors. Valeri Legasov, first deputy director of the Kurchatov Atomic Energy Institute, insisted that the cause of the accident was human negligence of such extraordinary proportions that a recurrence was improbable. "The sequence of human actions was so unlikely," he said, "that the engineer (who designed the plant and its safety systems) did not include such a scenario in his project." Though Soviet officials are now re- evaluating their nuclear-safety program and considering more remote locations for new plants, the report makes a strong pitch for continued use of nuclear power, calling it "essential" for future energy needs.
Not all Western experts were satisfied with the quality of the information in the Soviet report. Victor Gilinsky, a nuclear consultant based in Bethesda, Md., and a former member of the U.S. Nuclear Regulatory Commission, said he did not find the "technical discussion up to the standard you would expect." Frank Congel, a current staff member at the NRC who is in Vienna this week for the IAEA symposium, generally praised the report but said he intends to pose a lot of questions about Soviet contamination statistics and procedures.