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Chernobyl Design Flaws Made Accident Worse, Soviet Report Concedes

August 23, 1986|ROBERT GILLETTE | Times Staff Writer

VIENNA — Human error was the overriding cause of the Chernobyl nuclear accident, but the reactor's design made it a difficult one to manage, according to nuclear safety experts who have read the Soviet Union's government report on the disaster.

These analysts say that Soviet authorities appear to recognize that operator errors at the Chernobyl plant on the night of April 25-26 were not the sole cause of the accident, and that technical flaws in the reactor's design contributed to the worst accident in the 44-year history of nuclear energy.

In particular, they said, a distinctive feature of the Chernobyl design, which sets it apart from conventional nuclear power plants in most of the world, is its tendency to generate a sudden and uncontrollable burst of power if large steam bubbles, or "voids," are allowed to form in the reactor core, as they did before the accident.

This peculiarity of the Chernobyl type of graphite reactor, called a positive void effect, is now seen as a decisive factor in the accident, one that transformed successive blunders on the part of Soviet operators over a period of hours into a catastrophe.

Death Toll Revised

Thirty-one people have died as a result of the accident; 203 others are still suffering from acute radiation sickness, and 135,000 people had to be evacuated from the area around Chernobyl and Kiev, the Soviets have reported.

"This was what caused the accident," a senior analyst associated with the International Atomic Energy Agency said. "It's like removing the control rods. You increase the fissioning that's going on."

He and other officials interviewed at the Vienna-based agency and the missions of its member states asked not to be identified by name.

As reactor operators prepared to carry out a planned safety test involving one of the plant's eight turbine-generators, they inadvertently let steam voids form in the reactor's cooling water as it passed through the core, according to the Soviet report. The effect was akin to pressing a car's gas pedal to the floor.

As the fission accelerated, the reactor's heat output rose 330 million watts within three seconds. This triggered explosions of steam and hydrogen gas in the core that destroyed the reactor, blew the roof off the building and started a graphite fire in the core that spewed radioactive wastes into the atmosphere for the next 11 days.

The 382-page report places heavy emphasis on the negligence of the reactor operators, who were said to have proceeded with the planned test despite clear and mounting evidence of erratic behavior in the reactor that should have led them to shut it down.

Design Flaws Conceded

In apparent recognition that design flaws increased the reactor's vulnerability to human error, however, the report gave several technical fixes that are to be made in the 21 graphite-moderated reactors of similar size and design that are still in operating or being built in the Soviet Union. These RBMK reactors, as the Soviets designate them, currently produce about half of the Soviet Union's electricity from nuclear power and 5% of its total generating capacity.

The changes are to include lengthening and increasing the number of control rods, improving the reactors' monitoring and automatic-shutdown systems, boosting the capacity of cooling pumps and increasing the enrichment--the proportion of fissionable uranium-235 in the fuel.

These modifications are intended to make similar reactors less sensitive to the kinds of errors committed by the Chernobyl operators. For instance, by increasing the percentage of uranium-235 in the fuel to 2.4% from its design level of 2%, Soviet engineers expect to reduce the tendency for power bursts to occur in the presence of steam bubbles.

The higher enrichment, however, will make the fuel more expensive to produce and such plants less economical to run. Some analysts, noting that the void effect is well known to nuclear engineers and that Soviet designers must have taken it into account when they set the original level of fuel enrichment, speculated that Soviet authorities may have knowingly sacrificed a measure of safety in an attempt to hold down operating costs, and perhaps to make this novel reactor design more attractive to central planners in Moscow.

There is no indication in the Soviet report of plans to enclose the remaining Chernobyl-type reactors in steel-and-concrete structures. IAEA analysts believe that such a housing, which is hugely expensive, would have greatly reduced the amount of radioactive wastes blown out by the initial explosion on April 26 and released by the subsequent fire in the reactor's graphite core.

The Soviet report is to be discussed at a post-accident review conference in Vienna, beginning Monday, under the auspices of the IAEA. About 600 technical experts from most of the agency's 110 member-states are expected to take part in the five-day meeting.

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