The Chernobyl nuclear reactor had much stronger containment structures beneath it than previously believed, making it unlikely that molten uranium fuel melted through the floor and into the dirt, according to American nuclear experts.

The experts also said that even if the melting fuel breached the building's concrete foundation, radioactive contamination probably would be stopped by the dirt or spread only very slowly if it reached the water table.

Based on new design information about Chernobyl station No. 4 released Wednesday by the Nuclear Regulatory Commission and the Environmental Protection Agency, it appears that the reactor had thick concrete -- possibly six to eight feet -- under the reactor core, then a double sandwich of thin concrete slabs and pools of water. Beneath them lay another thick slab that was the foundation of the building.

Previously, experts analyzing the accident were using plant diagrams obtained from the International Atomic Energy Agency. They showed one six- to eight-foot layer of concrete for the Chernobyl reactors.

Harvard nuclear expert Richard Wilson, who has made some of the more conservative assessments of events at Chernobyl, said yesterday that some fuel probably reached melting temperature -- about 5,000 degrees Fahrenheit -- and some may have broken out of the reactor's graphite core and dripped down to the first layer of flooring.

But he said it would take substantial amounts of molten fuel to break down the concrete floor, partly because the fuel might have spread out in a thin layer across the 1,600-square-foot floor like water spilling across a floor. Thus no large concentration of hot fuel would have been likely to build up, he said.

Many U.S. reactors have flooring with a surface-area less than one-tenth that of the floor thought to be in the Chernobyl unit. In most U.S. reactors, the base foundation is twice as thick as that at Chernobyl.

Victor Stello, an NRC deputy director, said the sandwich of pools and concrete slabs beneath the wide first floor would make it even less likely that a "burn-through" could take place.

U.S. studies of reactions between molten nuclear fuel and concrete show that even as much as 12 feet of concrete can be breached if the conditions are right. If fuel at thousands of degrees somehow became concentrated beneath the core, and the pools of water had been drained by the accident, 12 feet of concrete could be breached in a day or two, according to one expert's calculation.

German nuclear officials began the international speculation about the "burn-through" Wednesday when they said they feared that molten fuel had burned through the bottom of the building. The officials said they were questioned by the Soviets about how long it would take to burn through concrete and how quickly contamination might move through groundwater.

But those officials said they did not know the structure or depth of the concrete under the core and could not estimate the likelihood of fuel breaking out of the building.

Groundwater contamination experts, including Wayne Bliss of the EPA's Las Vegas radiation section, said contamination reaching underground water tends to move very slowly. Bliss said the travel time from a contaminated area to a river hundreds of feet away could be months and years, not days.