For almost three decades, the Savannah River Plant, a unique defense production complex sequestered in the midst of a huge government-owned pine plantation south of here, has produced the plutonium and tritium used in America's nuclear bombs and warheads.

It also has produced as a byproduct more than 28 million gallons of high-level radioactive waste. Since the early 1950s, the daily output of liquid waste has been pumped into huge underground carbon-steel tanks for temporary storage.

Over the years, some of the older storage tanks have begun to wear out and have started to leak. While they are being replaced by new double-walled tanks, a small amount of radioactive liquid has seeped out of old tanks into the ground below. Officials here insist that so far, none of the leaks has posed a hazard to the surrounding area.

But the 80 million gallons of radioactive liquids, powder, salt and sludge in temporary storage here, at the Hanford Reservation in Rich-land, Wash., and at the Idaho National Engineering Laboratory near Idaho Falls, Idaho, constitute by far the nation's largest accumulation of nuclear waste.

And while Congress yesterday resumed wrestling with how to dispose of the highly radioactive spent fuel from U.S. atomic power plants, the federal government is moving ahead on a separate track to prepare its huge inventories of defense wastes for permanent burial.

The Energy Department -- which owns the three defense nuclear materials facilities -- is planning to start construction here in 1984 of a new $1 billion plant that will begin converting the high-level waste now held in Savannah River's 51 storage tanks into a solid form suitable for disposal.

"We expect to be able to start processing the waste at Savannah River Plant by the end of the decade," said Dr. Goetz Oertel, director of the Energy Department's defense waste program.

The new defense waste processing facility will solidify the radioactive waste stored here by combining it in a melting furnace with powdered borosilicate glass. The high percentage of borax in this type of glass makes it more heat resistant and impervious to water than ordinary glass, according to government experts.

The molten mixture, which will consist of 28 percent radioactive waste and 72 percent glass, will be poured into corrosion-resistant stainless steel canisters.

Once the glassy mixture hardens, officials said, it will be relatively impervious to damage and not subject to the leaks that have begun to plague some of the older storage tanks here.

The 10-foot-high canisters -- which Oertel said will have a surface temperature of about 90 degress centigrade and still emit a very high level of radiation -- will be stored temporarily here on the site until Congress decides on a permanent geological repository.

The decision to immobilize the radioactive wastes stored here in borosilicate glass followed lengthy studies by national laboratories of a variety of possible alternatives. The options ranged from mixing the wastes with ceramics that would incorporate them in their molecular structure to simply burying the wastes in concrete.

"We were interested first of all in protection against leaching," Oertel said.

This is particularly important in preparing radioactive materials for permanent underground burial, because it provides added assurance that dangerous radioisotopes will not escape hundreds of years from now if a fault develops in the geological repository and the nuclear waste is exposed to water.

"For these defense wastes, the borosilicate glass proved to be comparable in performance with other alternatives, and from a quality assurance standpoint, it is simpler and more reliable to make," Oertel said.

No decision has yet been made as to whether this same glassified form will be used for permanent disposal of the 49 million gallons of high-level waste stored at the government's original plutonium production facility at Hanford, Wash., or for the 3 million gallons stored near Idaho Falls.

The inspector general of the Energy Department, James R. Richards, has suggested in a new report that the combination of borosilicate glass and stainless steel containers may exceed any need, and has recommended the Energy Department reassess the possibility of using "a less durable but less expensive" process for disposing of the waste at the other two sites.

Another unresolved question -- one that remains to be settled by Congress -- is where these defense wastes finally will be buried.

The high-level waste that continues to accumulate here comes from Savannah River's two reprocessing plants, which handle the material produced in special atomic reactors.

The chemical processes used to separate out plutonium for use in nuclear weapons generates as a byproduct a stream of acidic radioactive waste, which is pumped directly from the reprocessing plants into underground storage tanks, where an internal maze of coils keeps it continuously cooled.

As the waste ages, the solution separates into a liquid portion and a brownish gray sludge that resembles axle grease. The sludge -- containing strontium 90, which remains intensely radioactive for centuries, and some highly toxic suspended plutonium solids -- accumulates at the bottom of these tanks.

The liquid portion -- containing dissolved salts that include cesium 137, another radioactive isotope that remains deadly for hundreds of years--is transferred after a year or two into another tank for evaporation into crystalized salt.

It is the sludge, with its strontium 90 and plutonium, that would be combined with the borosilicate glass in the new waste processing facility at the rate of two canisters a day. Plans also call for the radioactive cesium to be removed from the salt via another process. The cesium then probably will be disposed of as part of the glass mixture, and the salt will be mixed with concrete and buried here on the site.