More than 27 million gallons of highly radioactive waste are stored at the federal Savannah River Plant in South Carolina in "obsolete and dangerous" tanks that are vulnerable to explosions and earthquakes, an environmental research group said yesterday.

The Environmental Policy Institute, releasing the results of a five-year study of the Energy Department bomb-production facility, warned that a major accident or natural calamity there would release enough radioactivity to kill thousands of people and permanently contaminate thousands of acres.

The report said the consequences could be as severe as those experienced 30 years ago in the Soviet Union when hundreds of square miles in the Ural Mountains were turned into a radioactive wasteland by what some scientists believe was an explosion of chemical and nuclear wastes.

"A potential for similar explosive accidents or loss of containment from earthquakes also exists at the Savannah River Plant site," the report said.

A DOE spokesman said the report revealed "no evidence of unsafe practices or procedures in the way waste is managed at Savannah River."

"The waste is now being stored in a safe and environmentally responsible way," spokesman Dave Devan said.

The Savannah River Plant, near Aiken, S.C., has produced weapons-grade plutonium since the early 1950s, storing its high-level nuclear waste in 51 underground tanks. According to institute researchers, the accumulated waste contains more than 800 million curies of radioactivity, or more than three-quarters of the radioactivity of all high-level U.S. military waste.

The report contends that hydrogen gas can build to explosive levels within the tanks, posing the threat of a blast "severe enough to destroy the tank and send millions of curies of radioactive waste spewing into the air and onto the land."

A 1978 safety analysis prepared by Du Pont which operates the plant, said that such an accident was possible but was not likely to happen more than once every 25,000 years.

Documents obtained by the institute indicate that a hydrogen blast took place 20 years ago in one of the plant's tanks after the gas built up to only 15 percent of the amount thought to be required for an explosion. The incident was not mentioned in Du Pont's 1978 safety analysis, and Devan said plant officials deny that such an accident occurred.

Researchers "misinterpreted the data," he said.

The report also questioned the ability of the tanks to withstand a severe earthquake. The U.S. Geological Survey has assessed the area as a "level 3" earthquake zone, meaning that major damage could be expected in the event of a quake.

"I think it is a matter of speculation whether the tanks could withstand even the moderate earthquake for which they were designed," said Arjun Makhijani, one of the report's authors. He cited documents indicating that some of the tanks have corroded or developed cracks and brittle areas.

Robert Alvarez, another author, said that leaks, spills and mishandling of high- and low-level nuclear wastes at the plant had already heavily contaminated the 300-square-mile reservation and underground water supplies.

In 1983 the Environmental Protection Agency warned that contamination at Savannah River posed an "imminent threat" to the Tuscaloosa aquifer, which underlies the plant and supplies drinking water to the south-central Atlantic Coast.

The plant "has been used more or less as a giant sponge," Alvarez said.