Why the Chernobyl reactor failed. Soviets tell Vienna meeting of design flaws, operator errors

Major design deficiencies, together with human error, combined to cause the Chernobyl accident, the Soviets told a meeting of the International Atomic Energy Agency in Vienna last week. The delegation from the Soviet Union said it expects the entombment of the crippled reactor to be completed by late September or early October. It hopes that Chernobyl units one and two will be returned to service before the end of this year, but is still not certain when unit three will be able to restart.

Like Three Mile Island, Chernobyl points to one of the weakest links in nuclear power plant operation -- the operators themselves. But in describing the causes of the accident, Valery Legasov, who headed the Soviet delegation at the Vienna meeting, also spoke of ``a tremendous psychological mistake on the part of the designers of this reactor.''

The designers had failed to foresee the combination of mistakes and deliberate violations of safety regulations that had caused the initial explosion, Mr. Legasov said. He pointed to several weaknesses in the design of the Soviet water-cooled graphite-moderated reactor. These include its inherent instability, the complexity of its cooling system, the need for a complicated control system and the very high temperatures reached in the graphite core during operation.

The Soviet Union had used this reactor design because it had difficulty manufacturing components for apressurized water reactor, to which, he said, priority was now being given.

But a string of operator errors went hand in hand with the design problems. The Soviet operators deliberately placed the reactor in jeopardy in order to conduct an experiment.

Ironically, the experiment was safety-related and the information that the Soviets wanted was relatively trivial. They wanted to see how long the electricity supply to the main coolant pumps would last with one of the two turbine generators disconnected from the nuclear steam supply system, and to test the new voltage regulator that was needed to supply enough current to the pumps.

In order to conduct the experiment, the Soviets switched off some key safety and monitoring systems. But they went ahead when they must have known that conditions inside the reactor were giving cause for serious concern, and when they knew that the control rods (the devices used to control the chain reaction) had been withdrawn to a position where operation should not have been allowed.

Preparations for the experiment began many hours before the accident. The operators had increased water flow through the core by turning on two standby cooling pumps in addition to the six already operating. This filled the core with water and lowered the pressure in the coolant circuit, making it easier for the water to boil.

The operators began the experiment by disconnecting the turbine generators. Half the main coolant pumps, those driven by the turbine generators, began to slow down. Massive boiling of the cooling water resulted as the flow of coolant through the core decreased.

Because of the way the core was designed, the boiling of the water coolant caused the heat generated by the nuclear reaction in the core to increase dramatically, generating more steam, which generated more heat, in a disastrous vicious cycle.

Since steam is a very poor conductor of heat, the temperatures in the core rose very rapidly. The operators tried to insert the control rods to shut down the chain reaction, but were unsuccessful.

A massive steam explosion damaged the core. Subsequently the steam also reacted with the metal in which the fuel was contained to produce hydrogen. A number of hydrogen explosions followed, resulting in some 30 separate fires around the plant.

Finally, when the core's graphite became exposed to the air, it too began to burn.

The author is deputy editor of Nuclear Engineering International.