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.
"This particular accident certainly was human error," an IAEA safety analyst said. "I doubt it could have occurred without some very foolish mistakes" on the part of the reactor's operators.
"At the same time, this is a difficult kind of reactor. It can be run safely--they've done it for 13 years--but under certain circumstances, it is difficult to manage," this analyst said.
Difficult to Turn Off
"Just its size--more than 40 feet across--makes it difficult to shut down. You can turn part of (the core) off while other parts are still critical."
The first of these large graphite reactors began generating electricity at a Leningrad plant in 1973. It is essentially a massive block of graphite with 1,659 vertical channels into which uranium-filled fuel rods are inserted. The graphite "moderates" or slows the neutrons emitted from the uranium fuel, enabling them to split other uranium atoms and produce the chain-reaction of fission fuel that generates the reactor's heat.
Water is pumped past the fuel rods and emerges as steam and water. The steam is then separated out and used to drive the plant's turbine generators.
In its basic concept, the Chernobyl reactor resembles the world's first reactor--an "atomic pile," it was called then--of graphite and uranium built under a University of Chicago stadium in 1942, at the start of the Manhattan Project, the program that developed the first atomic bomb. It differs radically from most nuclear power plants in the West, which use water as both the moderator and coolant.
This difference accounts for the Chernobyl design's tendency to race out of control when cooling water is lost or blocked by steam bubbles. Normally, cooling water absorbs some of the neutrons released by fission. Wherever bubbles form and water is absent, these neutrons come into play and accelerate the chain-reaction.
"This is obviously not a desirable state of affairs," according to an introductory American text on nuclear engineering by John R. Lamarsh, and published in 1975. "(This) gives an increase in power, which would give rise to additional boiling, more voids, a further increase in reactivity, and so on, until much of the liquid is boiled away and the core melts down."
Reaction Can Be Stopped
In most water-cooled reactors, the opposite effect occurs. A loss of cooling water means a loss of the moderator as well, and neutrons are no longer slowed enough to maintain fission. Instead of accelerating, the nuclear chain-reaction stops.
Even in conventional reactors, intense heat is still emitted from decaying radioactive wastes in the fuel. This is sufficient to damage the core severely, as happened in 1979 at the Three Mile Island nuclear plant near Harrisburg, Pa.
At Three Mile Island, however, there was no devastating explosion and no significant release of radioactivity. The damaged core remained inside its heavy, steel pressure vessel, and the steel-and-concrete building surrounding the reactor trapped the radioactivity that leaked from the cooling system.
At Chernobyl, the use of combustible graphite in the core and the absence of a containment structure account for the massive scale of radioactive contamination that was released into the environment, according to safety engineers familiar with the report.
The report notes that even though fuel did not actually melt--an assertion that has surprised Western experts--50 million curies of radioactive materials, or 3.5% of the total contained in the reactor core, escaped over the 11 days from April 26 through May 6. The report said this estimate has a margin of error of 50%, which means that the actual total could have been as little as 25 million curies or as much as 75 million curies.
About a fourth of the contamination, or 12 million curies, was released on the day of the accident, April 26. Analysts believe most of this came from the initial explosion. The remaining three-fourths, or as much as 38 million curies, poured out of the reactor over the next 10 days, as military helicopters dumped 5,000 tons of sand, lead, clay, boron and dolomite on the reactor to smother the graphite fire.
Second Wave of Radiation
Soviet data point to a second crisis that began nearly a week after the accident, on May 2, and lasted for four days: After bottoming out at 2 million curies on May 1, the daily output of contamination rose sharply the next day and climbed steadily to 8 million curies on May 5, before it dwindled to almost nothing the following day. Nearly half of all the radioactivity released from Chernobyl, or 24 million curies, spewed from the burning reactor on these four days.
The Soviet report attributes this surge partly to a delayed escape of radioactive iodine that was cooking out of the smoldering core. The report also shows that 18% of all the iodine and 12% of the cesium in the reactor core--the two contaminants that present the greatest threat to human health--escaped into the atmosphere before the fire was effectively extinguished May 6. In view of this, one analyst observed, "You would be hard put to imagine a worse reactor accident."
Contrary to some reports published in the United States, the Chernobyl reactor had no containment building.
According to experts familiar with the development of Soviet RBMK reactors, they were seen as a means of developing nuclear power rapidly, without the need for massive steel pressure vessels and containment buildings that conventional, water-cooled reactors required and Soviet industry had difficulty building in the 1970s and early 1980s.
Heavy, reinforced-concrete "isolation spaces" protected the Chernobyl reactor's cooling system in the event of a pipe breaking, but not the reactor itself, apparently because Soviet engineers never envisioned an accident serious enough to disrupt the core.
"The reactor was very well protected from pipe breaks, but what was not protected was the reactor vault," a senior IAEA safety engineer said. The reactor vault is the high-ceiling room immediately above the reactor. The top of the reactor forms the floor of the vault, which the explosion obliterated.
As to whether a standard containment structure such as those used in Western countries and on some of the Soviets' pressurized-water reactors would have made a difference at Chernobyl, the engineer said that "it probably would have made a huge difference."
He said that such a containment housing might have sustained some "penetrations" or cracks from flying debris, and allowed some radioactive waste to escape, but it would have trapped most of it.
"The standard U.S. containment is roughly an equal mixture of steel and concrete," the engineer said. "These do not fail catastrophically."