Soviet Reactor Badly Flawed, Experts Say

The Chernobyl reactor that exploded last April in the Soviet Union was fundamentally flawed in its design and would never have been considered acceptable in Western countries, senior American and European nuclear safety authorities said here Monday.

Soviet officials acknowledged that serious flaws in the reactor’s design played an important role in the disaster, along with elementary mistakes made by its operating crew. But they maintained that technical remedies now being applied to similar power plants in the Soviet Union will ensure that the accident will not be repeated.

Five-Day Symposium

The officials made their remarks as a five-day symposium on the Chernobyl accident, the worst in the 44-year history of nuclear energy, opened here under the auspices of the International Atomic Energy Agency. About 550 safety and health specialists are attending from 50 of the agency’s 110 member states.

Valery A. Legasov, a prominent designer of power reactors and head of the Soviet delegation, opened the meeting with a five-hour presentation on the causes and effects of the April 26 disaster, which has so far killed 31 people. The presentation was praised by Western specialists for its candor and wealth of detail.

‘Negative Aspects’

“The accident assumed catastrophic proportions because . . . all the negative aspects of the reactor’s design” were brought into play by operator errors, Legasov said.

But later, at a news conference, he cautioned against overemphasizing the reactor’s design weaknesses.

“The only guilt of the designer is that he could not foresee the awkward and silly acts of the operators,” Legasov, the first deputy head of the I.V. Kurchatov Institute of Atomic Energy in Moscow, insisted.

The Chernobyl unit was one of 14 virtually identical water-cooled, graphite reactors that produce just over half the Soviet Union’s nuclear electric power and account for about 5% of its electric generating capacity.

The defect in these reactors on which Western safety authorities have begun to focus is a tendency for the nuclear fission reaction to race out of control if large steam bubbles or “voids” are allowed to form as cooling water flows through the core, as happened at Chernobyl. This is known as the positive void effect.

‘Extremely Improbable’

The Soviet government report submitted to the International Atomic Energy Agency said the prime cause of the accident was “an extremely improbable combination of violations of instructions and operating rules committed by the staff of the unit.”

The report added that the accident “assumed catastrophic proportions” because the positive void effect allowed the reactor’s power to race out of control, rising 330 million watts in three seconds.

Western specialists said the Soviets were familiar with this feature of their graphite reactors but were confident that the system was designed so that it would not come into play.

“They knew this was undesirable,” a senior member of the U.S. delegation said in an interview. He added that “they wrote a lot of papers about how they kept it under control.”

Unacceptable in West

This official, who like other members of the U.S. group asked not to be identified by name or agency, said this feature would “absolutely not” be considered acceptable by Western regulatory agencies.

He noted that conventional, water-cooled power reactors used in the West, as well as in the Soviet Union, display the opposite tendency--a “negative void effect,” in which the fission reaction dies down if operators inadvertently allow steam bubbles to develop in the core.

“To be very precise about it,” he said, “at certain points in the fuel life of a U.S. plant, you can have a slight positive void coefficient, but nothing like this (Soviet) plant.”

Giovanni Naschi, the head of reactor safety in Italy’s nuclear agency, told reporters that “from the Soviet description, we realize there were not only human errors but that it was the system itself, this instability that had a primary role in the disaster.”

Britain’s Experience

Walter Marshall, the chairman of Britain’s main utility, the Central Electricity Generating Board, said British engineers became aware of several major weaknesses in these reactors in the mid-1970s, when the government considered building a series of similar units, to be called steam-generating heavy water reactors.

After two visits to the Soviet Union, specialists with the British government’s development agency, the National Nuclear Corporation, produced a report in 1977 that listed at least five weaknesses in the safety of Soviet graphite reactors.

Marshall, head of the British delegation, said the report concluded that such reactors could not be made both sufficiently safe and economical in Britain, and that it urged Soviet engineers to take remedial measures to improve the safety of these reactors.

“The response was noncommittal,” Marshall said in a conversation with reporters. He said some improvements were eventually made “but obviously not enough.”

Denies Seeing Report

At a news conference later, Legasov, the head of the Soviet delegation, said he was not familiar with the British report.

Besides the positive void effect--the tendency to produce uncontrolled bursts of power when steam bubbles develop--British engineers found the reactor to be cumbersome and difficult to control, of questionable structural integrity and enmeshed in a plumbing system of bewildering complexity, Lord Marshall said.

In addition, British experts believed that the Soviets ran the graphite core of such reactors at inordinately high temperatures.

“These were fundamental defects that we found unacceptable,” Marshall said. “For all practical purposes, this was also an assessment of whether these reactors would be acceptable by American standards. In 1977, that report was saying it’s not on.”

The British later decided against building similar reactors, in part because a version judged sufficiently safe proved uneconomical to build and run.

‘Designer Failed’

Marshall said that although the dividing line between human error and technological flaws is hard to draw with any precision in the Chernobyl accident, “as a matter of principle you want to design a system that presents the most modest challenge possible to the operator. In this task, the designer failed.”

Soviet officials have indicated that Chernobyl-model reactors--a scaled-up version of plutonium production reactors developed 30 years ago for nuclear weapons--were built to circumvent stubborn deficiencies in Soviet heavy industry.

The Soviet Union had great difficulty, still not fully overcome, in building the massive pressure vessels, steam generators and other major components of ordinary pressurized water reactors.

At the same time, central planners in Moscow were demanding a rapid expansion of nuclear electric power in the 1970s and on into the 1980s to satisfy a growing need for electricity in the Western, or European, third of the Soviet Union, where most of the country’s industry and most of its people are found.

Half Shut for Repairs

Legasov said about half of the remaining 13 operable reactors of the Chernobyl type are currently shut down for safety modifications. The modifications include increasing the number and length of control rods and installing an automatic emergency shutdown system that cannot be turned off by operators, as happened at Chernobyl.

To counter the positive void effect, the enrichment of the fuel--its fraction of fissionable uranium 235--will be increased to 2.4%, from 2.0%.

These and other steps will make the units less economical to run, reducing their effective power output by 10%, Legasov said.

“We in the Soviet Union are convinced that what happened at Chernobyl was an exceptionally improbable event, and that these measures we are taking reduce the likelihood of this improbable event (recurring),” Legasov added.

An American safety specialist said in an interview: “We hope it doesn’t happen again. We are here to find out what exactly they’re doing to make sure it doesn’t.”