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Yet the week's gaiety could not conceal that the Soviets' handling of the tragedy had created a severe diplomatic setback for Gorbachev, who has been trying to give the West the impression of openness and public debate. Gorbachev missed an opportunity to turn a potential public relations disaster into a triumph of Soviet good-neighborliness and statesmanship. Had he recognized the international dimensions of the radiation leak soon enough, had he thought through the consequences of trying to keep the catastrophe a secret and had he openly invited foreign scientists and technicians to help put out the fire, Gorbachev might have scored a brilliant diplomatic success. But by acquiescing to the Soviet instinct for glum silence, he showed anew that he remains very much a creature of the stolid system that brought him to power.
Although the Soviets kept virtually silent about the origins of last week's accident, Western experts in Moscow and elsewhere were gradually piecing together the probable sequence of events that led to disaster (see diagram). The trouble seems to have begun Saturday, April 26, when a mishap caused a loss of the water that continuously cools the uranium fuel rods in the reactor's core. With the coolant gone, superheated steam could have triggered ) a series of irreversible reactions leading to a meltdown of the fuel and a blast that ripped through the roof of the building that housed Unit No. 4.
As outside air rushed in, oxygen in the atmosphere would have fueled a raging fire in the graphite, which burns like coal when ignited, throwing a plume of volatile radioactive elements into the air. U.S. officials calculated that the particulates and gases surged nearly a mile high, where they were caught by prevailing winds and then blown over a wide swath to the northwest.
Though the accident was a type of core meltdown, the ultimate nuclear power nightmare, U.S. experts also called it a burnup. Meltdowns technically occur in reactors containing pools of water. When the water boils away, the molten core sinks into the earth in the so-called China syndrome, a term used by scientists, and popularized by the 1979 movie of the same name, that mordantly suggests that the radioactive mass might plunge all the way through the earth. The Chernobyl plant had no such pool, by contrast, and engineers expect the reactor to be consumed by intense heat.
The four huge RBMK-1000 reactors at Chernobyl were mighty but in many ways outdated machines. "It's a crude technology," said a senior Administration official. "They haven't changed it in 30 years." Although capable of producing 1,000 MW of power (vs. 850 MW for a typical U.S. nuclear generator), the Chernobyl unit had some design features dating back to the atomic pile that Enrico Fermi used in 1942 to create the world's first chain reaction at the University of Chicago's Stagg Field. Both systems employed graphite to moderate the nuclear reaction. Most U.S. units regulate with water instead. About half of all Soviet reactors employ graphite rather than water.