West German scientists said today that Soviet diplomats have approached them seeking advice on how to cope with a "hot molten mass" that may have burned through the concrete floor of the Chernobyl nuclear plant and endangered the region's underground water supplies with radioactive contamination.

Thomas Roser, director of the German Atom Forum, a nuclear industry lobby, said in an interview that Soviet diplomats appealed to him Tuesday to find specialists for urgent information on "how to handle something extremely hot that may have melted through the nuclear plant floor."

This scenario would fulfill the fears of many nuclear scientists about a "China syndrome" occurring at Chernobyl. The term, which became the title of a popular movie, refers to an uncontrolled nuclear reaction melting the concrete floor and dropping the core into the earth.

Roser said that while the Soviet officials did not provide precise details on what was happening at the site, the increasingly likely prospect of a meltdown in which the white-hot radioactive core seeped into the earth and perhaps the water table was "extremely bad news."

He said the vast quantity of uranium at Chernobyl, which is thought to be in excess of 200 tons, or double the amount that large western reactors can contain, was bound to be "incredibly hot" even two weeks after the accident. The reactor core is thought to have been heated by an enormous graphite fire that West German specialists believe may have consumed as much as 2,000 tons of the carbon protective material inside the plant.

"Imagine 2,000 tons of charcoal burning continuously in that time and you can figure how hot that uranium must have become," he added.

Nuclear Regulatory Commission officials in Washington were skeptical of reports that the reactor core might have melted through the floor, noting that the Soviet reactor's design was more likely to avert a melt-through than a typical U.S. design.

[According to NRC official Victor Stello, the floor of the Soviet reactor vault covers about 1,600 square feet, providing room for molten material to spread out and cool. A U.S. reactor floor, by contrast, typically provides only about 144 square feet of "spread out" room, which would keep the molten material from cooling.]

Roser, who said he was contacted earlier by Soviet representatives wanting to know how to fight a graphite fire and what kind of equipment was required to work in highly contaminated areas, said that Soviet diplomat Alexander Chagayev would only present his request "in a hypothetical manner."

The Soviets apparently followed West German advice in fighting the fire by dropping huge amounts of boron, lead and sand on the burning plant from helicopters to smother the flames and cut off oxygen.

Soviet authorities also have ordered two robots from the Nuclear Technical Services company in Karlsruhe, West Germany. The robots, operated with radios and cameras up to a half mile away, can withstand high radioactivity but not high temperatures.

The robots, approximately the size of compact cars, can approach the plant with television cameras to provide on-site investigation of a nuclear accident. They have artificial limbs that can pick up contaminated soil samples and shovel earth as well as radioactive material.

But Roser noted that the robots cannot operate in extreme heat. "We believe the Soviets may have mastered the fire, but the core area is extremely hot, which would make the meltdown plausible depending on the thickness of the protective concrete floor."

Roser said the Soviets would not reveal the thickness of the floor. But Hermann Rininsland of the German Nuclear Research Center in Karlsruhe said that this kind of Soviet reactor has a bottom layer only six or seven feet deep, in contrast to western reactors, which have floors containing as much as 20 feet of concrete.

If such a meltdown has occurred at the Chernobyl plant, the molten radioactive core could seep into the water table and eventually into nearby rivers to contaminate water supplies in the entire region. The reactor is located near a lake and a tributary of the Dnieper River and any release of radioactivity into those water sources could reach Kiev, about 80 miles away, the German specialist said.

The urgency of the Soviet request on how to deal with a heated core and reports from Moscow describing workmen burrowing tunnels near the plant have persuaded West German scientists that the Soviets are striving desperately to curb the spread of the highly radioactive core below ground.

Once tunnels are dug close to the plant, Roser said, "the Soviets could either harden the area with concrete or freeze it with liquid nitrogen.

"The Soviets have no way of knowing what's going on in the reactor core, because it's beneath all that sand now. But they have to do something quickly to lower the heat," Roser said.