The nuclear accident in the Soviet Union began with a probable cooling-system failure on Friday, followed by a nuclear core meltdown through Saturday and a "violent" chemical explosion and fire by Sunday, according to a rough picture drawn yesterday by U.S. government and outside experts.

The chain of disastrous events led to a breach in the Chernobyl nuclear power plant's relatively meager containment structure and a vast radioactive cloud being dispersed into the atmosphere. The experts said the cloud probably now covers much of eastern and northern Europe including the northwestern part of the Soviet Union, and may extend well into the Arctic basin.

The fire that began Sunday, they said, continued burning yesterday with no immediate prospect of being extinguished. A Soviet official in Moscow, however, said yesterday that the fire was out, diplomatic officials reported.

The reactor involved, which became operational in 1983, is the newest of four reactors at Chernobyl. It is the second largest reactor in the Soviet Union and of a design that had been modified for safety after the 1979 accident at the Three Mile Island plant in Pennsylvania.

In the absence of detailed reports from the Soviet Union about what happened at Chernobyl, experts used radiation fallout data, spy satellite pictures and knowledge of the kind of reactor involved to piece together an outline of the world's worst nuclear power plant disaster.

Casualties from the accident are uncertain, but the Soviets have reported two dead and 197 hospitalized. U.S. government officials refused in briefings yesterday to give any estimate of casualties, but there were private reports of hundreds of deaths.

Because the accident is believed to have released very large amounts of radiation into the air -- perhaps 1,000 to 1 million times that vented from Three Mile Island in 1979 -- the accident is being described by several experts as near the "worst case" pictured in a National Academy of Sciences report a decade ago.

Ed Zebroski, chief nuclear scientist for the Electric Power Research Institute in Palo Alto, Calif., said this accident could be worse than the one portrayed in that report because the graphite core of the reactor continues to burn and may do so for days or weeks.

Experts said they could not project the wider medical implications within hundreds of miles of the site without further data from the Soviet Union. U.S., Swedish and Finnish officials continued to say there was little or no danger in their countries.

"We can only speculate about what the initiating event was," said Harold R. Denton, director of reactor regulation for the Nuclear Regulatory Commission, in a briefing at the State Department yesterday. "But it's quite clear that whatever happened caused a loss of coolant" at the plant.

That would make the key event in the accident fundamentally the same as that which has caused most other major nuclear accidents, including the Three Mile Island incident.

Nuclear plants use controlled nuclear reactions to create heat, which can run a turbine to create electricity. But uncontrolled nuclear fission can heat the radioactive fuel to the melting point, which could turn cooling water to highly pressurized steam and blow open the vessel containing the fuel.

The loss of cooling water around the hot fuel would result in such heat that the fuel -- uranium packed into long rods -- could begin to melt, along with the zirconium metal rod encasing the fuel, over a period of hours.

Richard Wilson, a nuclear physicist at Harvard University and specialist in Soviet reactor systems, said the loss of coolant at Chernobyl also could have created a condition in which the fuel rods, reacting with air and water, would have liberated hydrogen gas.

Hydrogen is extremely explosive, and could have ignited suddenly to blast away the cement shield over the fuel rods, as well as the relatively weak walls of the building.

Such an explosion would have been similar to the one at Three Mile Island, which produced sudden pressures against the walls of about 28 pounds per square inch (psi). The containment vessel at Three Mile Island, designed to hold above 60 psi, stopped the blast. But at Chernobyl, the floor, walls and roof would have burst with only a few psi.

A violent hydrogen gas explosion would have spewed smoke and nuclear debris, including light and heavy radioactive elements, high into the air. These elements were detected in Sweden beginning Sunday.

More easily vaporized radioactive elements, such as krypton, cesium and iodine, would have been swept out in this explosion, and by now have been largely released from the plant, a senior administration official said.

The plant "looks like a chimney and will burn until it burns itself out" or until its air supply is cut off and the fire smothered, he said.

What continued to burn was the heavy graphite core surrounding the fuel rods. One expert compared the fire to one in a coal pile, in which there is a hot burning core and a great deal of fuel around it.