The five-day international meeting in Vienna to review the Chernobyl nuclear power station disaster closed Friday in a mood of self-congratulation. The Soviet delegation was pleased with the atmosphere of ``solidarity, rather than criticism'' that had characterized the meeting, said Valery Legasov, head of the Soviet team.
Western experts, for their part, were impressed at the revealing and frank account of the causes and implications of the accident given by the Soviets at the meeting of the UN's International Atomic Energy Agency.
In his closing statement Hans Blix, director general of the IAEA, said a great deal had been learned about the management and containment of a severe accident and about medical and decontamination measures. The meeting had ``not addressed the political question -- which you were not asked -- whether the risks connected with nuclear power are unacceptably high,'' he said. Dr. Blix expressed the hope that this question would be addressed at a special session of the agency scheduled for later this month.
The experts at the hearings believed the precise sequence of events that led to the April 26 accident, as detailed by the Soviets. But a number of questions and nagging concerns remained: How could such a gross abuse of operating procedures come about and how can it be prevented in the future? And do the modifications now being made in the Chernobyl-type RBMK design really bring these units up to safety standards comparable with those prevailing outside the Soviet Union?
The Soviet presentations answered the key question of whether the accident was attributable to human error or to faults in the reactor design. Human error led to the accident, the Soviets said. But shortcomings in the reactor design exacerbated the speed and scale of the catastrophe. The mass of detailed technical information which emerged will be analyzed at the special session beginning Sept. 24.
Mr. Legasov made some unexpectedly candid off-the-cuff remarks. Just after the accident, for example, Moscow was informed the operators had the reactor under control. But, he said, at that stage there was no reactor left to control. Only when a government-appointed team arrived on scene did the extent of the disaster became clear.
Nevertheless, once the accident took place, the Soviets' response was superb, as everyone in Vienna agreed. They did all the right things to try to contain the radioactivity. They distributed iodine tablets speedily and the evacuation of people was carried out calmly and quickly. The 49,000 inhabitants of the nearby town of Pripyat were, for example, completely cleared in 2 hours and 45 minutes and firemen worked selflessly to get the fires under control.
Legasov also did not hold back from outlining design shortcomings inherent in the RBMK design, and admitted that the control system designers were at fault in failing to anticipte and build in systems to guard against such a chain of unlikely actions by operators.
Although the graphite-moderated RBMK design had some advantages, it also had a number of shortcomings: the possibility that there might be a ``positive void coefficient of reactivity'' -- that is, if the cooling water boils excessively, the reactor power rises very rapidly; uneven power distribution in the core, requiring a complicated control system; the high temperatures reached in the graphite and other structures in the core during normal operation; and the presence of slightly radioactive steam in the turbine.
Several factors conspired to put an abrupt and explosive end to the short operating life of Chernobyl Unit 4.
Most important, psychologically, was a mood of complacency about safety inspired by the unit's good operating record. At the time of the accident, responsibility for operation of the unit seems to have been given over to a team of electrical engineers from a ``commercial electrotechnical company'' charged with carrying out relatively trivial tests on the plant's electrical systems. They seemed ignorant of reactor physics and oblivious of the safety implications of what they were doing. It seems that one reason for the haste with which the operators worked was that the failure to get the test done that night would have meant a further year's delay.
In order to rectify some design problems, the Soviets are installing a system to limit the extent to which the control rods can be withdrawn; increasing the number of control rods (a temporary measure to be replaced later by the use of higher enriched fuel); and installing a system to monitor core reactivity to ensure automatic shutdown in case of danger.
Regarding human error, the Soviets admitted to deficiencies in training, and have shuffled the leadership of commercial nuclear power to ensure there is no chance of standard operating practices being ignored again.
Clearly, the Soviet Union remains committed to the continued development of nuclear energy. The country needs the electricity desperately. But it is likely that building of the RBMK reactor line will cease with plants now operating or at an advanced stage of construction.
The Soviets are expected to turn more and more to pressurized water reactors -- the type that dominates in the United States.
The author is deputy editor of Nuclear Engineering International.