AIKEN, S.C. (AP) _ Disposing of accumulated radioactive waste in the U.S. nuclear weapons complex, and making the plants safe, is a job that will cost up to $110 billion, and is one that Joe Mellen doesn't take lightly.

Mellen is technical supervisor at the Defense Waste Processing Facility, a $1.3 billion project designed to make glass out of what he calls the ''hottest stuff,'' high-level radioactive waste stored on the 300-square-mil e Savannah River Plant near Aiken, S.C.

Once the plant is fully operational in 1992, it ''will take 12 years to catch up'' and dispose of the 33 million gallons of radioactive waste that has built up at Savannah River since its reactors began operation in 1954. It will take 600 people and $60 million a year to operate.

Similar glass techniques have been tried out in France, Britain, Japan and West Germany, and if they work at Savannah River, they will be used to dispose of an even greater backlog of nuclear waste at the Hanford Site, near Richland, Wash., said Mellen.

The Energy Department, which runs the complex, decided to try it first in South Carolina, said Mellen, because ''if there were a catastrophic accident at Savannah River, such as an earthquake, radioactive contaminants might reach the water table and possibly the river itself,'' alongside the plant.

Hanford is in an arid environment, so the danger is not thought as acute.

Turning the waste into glass, rather than allowing it to sit in metal tanks, will provide a wide margin of safety, said Mellen, because ''if someone blew up a glass canister house, we would have a containment and cleanup problem'' but face little danger of contamination entering the air and water supply. The glass is virtually insoluble.

Mellen, wearing a business suit and a white construction hat, twisted and ducked as he led visitors through chambers that will be closed to humans after operations begin.

''These doors are labyrinthine to keep the telephone poles out in a tornado,'' he said. The building contains 10,000 tons of reinforced steel, protection against earthquakes.

''When most of these buildings were built, no one was even thinking about earthquakes,'' said Mellen. The waste tanks, which require a crew of 200, ''were not designed to the same earthquake standard that we use now.''

Flying buttresses and other seismic insurance have been added to Savannah River's three operational reactors, which started running in 1954. They have been closed down for safety concerns and the first is not expected to return to production until early next year. The department expects to complete a new generation production reactor at the plant by the turn of the century.

The massive concrete waste processing facility, 360-feet long, 100 feet high and 117 feet wide, contains an inner ''canyon,'' where all the radioactive waste will be processed behind four-foot thick walls.

The heart of the operation is a melter, costing $19 million. The processed waste, rich in highly dangerous cesium, will be mixed with a fine sand called borosilicate ''frit,'' and heated to 2,100 degrees Fahrenheit inside stainless steel canisters 10-feet tall and two-feet in diameter.

The canisters, untouched by humans, will be moved through tunnels to a device which uses 75,000 pounds of pressure and an electric charge of 240,000 amps to weld in a steel cap that is slightly wider than the container neck.

''They weld cable-car rails together like this in San Francisco,'' said Mellen.

The plant can process 1 1/4 canisters a day, and each will be moved separately by a vehicle weighing 255,000 pounds to an adjacent warehouse able to store 2,286 canisters temporarily.

Because a permanent underground repository will not be ready before the year 2003, at a geologically stable site in the West, authorities must build another warehouse. Each canister is expected to generate the heat of a 470- watt light bulb, and the building is designed in such a way that air will flow over them and cool them.

In 1990, said Mellen, ''we will begin simulating a cold run, using a non- radioactive stand-in for the chemicals. It is going to take us 19 months to prove to ourselves that we are ready to go radioactive.''

''Experience has taught us that caution is the way to go,'' he said.

The facility will process only about 1 percent of the radioactive waste, ''the hottest stuff,'' giving off gamma rays. Lower level radioactive waste, which do not emit such lethal rays, are buried in carefully monitored yards.

Radioactive contamination is a problem at most of the 17 major plants in the nuclear weapons complex. The worst is at Hanford, where atomic reactors secretly manufactured material for the bombs during World War II.

Hanford stores 37 million gallons of radioactive wastes. According to an Energy Department report last July, environmental, safety and health costs at Hanford will run to $633.2 million through 1995 and up to $46 billion over 50 years, almost half of the total for the entire nuclear weapons complex.

Cleaning up contamination at Savannah River Plant could cost $3.7 billion through 1995 and up to $8.8 billion more through 2010, said the report.

Another highly publicized problem exists at the Rocky Flats Plant near Denver. Low-level wastes, such as gloves and rags, had been shipped from Rocky Flats to another federal facility at Idaho Falls, Idaho, until Idaho governor Cecil Andrus closed state borders to the shipments in October.

Since then, seven steel-lined rail cars carrying 140 drums with 55 gallons of waste each have been parked at Rocky Flats.

Colorado Gov. Roy Romer has threatened to close the plant until federal officials find a solution.

The material was earmarked for the Waste Isolation Pilot Plant near Carlsbad, N. M., but that has not opened because Congress adjourned this fall before formally ceding it from the Bureau of Land Management to the Energy Department. Department officials say they do not want to store radioactive wastes at the site until they actually have the deed.