Twenty-nine years ago the now-defunct Atomic Energy Commission announced its decision.

At the behest of President Truman, it would spend $19 million for 200,000 sparsely populated areas in Aiken and Barnwell counties, S.C. After moving out 6,500 residents, E.I. duPont de Nemours & Co. would build a top-secret defense plant for unspecific purposes.

The plant's primary mission then and now -- to produce the deadly plutonium and tritium necessary to build nuclear warheads -- was cloaked in secrecy in the early 1950s.

But as its 30th anniversary approaches, variety of factors -- declassification of public documents, environmental impact statements, congressional investigations and mounting public concern over radiation -- have brought the plant's functions into the open.

"It's not a mystery any more," says Dan Ross, a Du Pont environmental monitor at the plant and chairman of the South Carolina Republican Party. "Everybody knows what goes on there. After 25 or 30 years, people down here sort of live and die with the plant."

Life and death, of course, is what the Savannah River Plant (SRP) is all about.

The United States now produces an estimated three nuclear devices a day, in addition to servicing old ones. That explains the frantic, 24-hour, seven-day-a-week schedule at SRP.

Observers of the nuclear weapons industry -- such as journalist Howard Morland, who wrote "The Secret of the H-Bomb" for the Progressive magazine last summer -- speculate that newer weapons require even higher amounts of plutonium and tritium than older ones.

The Savannah River Plant plays a key role in turning normally harmless chemicals into warheads. It processes uranium into plutonium pellets and tritium, which are trucked to bomb-assembly plants around the country.

An escorted trip over the plant's heavily guarded 300 square miles reveals a variety of expensive -- and dangerous -- nuclear facilities, and some surprises. There are:

Five nuclear reactors, three of them active, in which the primary work of producing weapons-grade plutonium and tritium is done. Inside the stainless-steel reactor tanks, a complicated atom-splitting process transforms uranium. The reactors, spokesmen say, are not substantially different from commercial reactors operated by power companies. Only the steam generators and turbines are missing.

The country's only two operating reprocessing plants, where reusable plutonium and uranium are extracted from used nuclear fuels.

Facilities that recover heavy water from raw water at one ounce per 52 gallons. Heavy water, which contains heavier isotopes, helps regulate the speed at which uranium atoms are bombarded by particles that cause them to split. No other nuclear plant in the country uses heavy water.

Thirty-three separate 1.3-million-gallon storage tanks containing highly radioactive liquid waste, a residue of the plutonium production process that will stay dangerous for 250,000 years.

Deer, wild turkey, the world's largest bass and about 300 alligators drawn by warm water released by the reactors. A crew from the TV show "Wild Kingdom" recently visited to film the gators -- and the government insisted on scrutinizing the script and film for possible security leaks.

The remains of two towns, Ellenton and Dunbarton, which were moved out when the plant was commissioned. Most of the area is grown over, but the outline of asphalt streets and curbstones can be seen.

While any trip through the plant with Department of Energy personnel is punctuated by assurances of its safety, it is coming under more severe questioning than ever -- including some from the department's own acting director of the Office of Nuclear Materials Production, Dr. Charles Gilbert.

Gilbert told a Senate Armed Services Committee in March that employes in the two reprocessing facilities are routinely overexposed to radiation.

"The existing facility is marginal in meeting the current criteria for containment of radioactive material," Gilbert said in a closed session. "Most of the equipment at Savannah River is over 25 years old. It must be updated."

His testimony came less than a year after the House Armed Services Committee warned of the plant's "galloping obsolescence."

Environmentalists have long criticized the on-site storage of the 22 million gallons of highly radioactive nuclear waste, which must be solidified before it can be moved. There have been several spills.

Recently the House authorized spending $3 million to design a waste-solidification facility, which could be operating by 1989. Final cost could run between $2 billion and $3 billion, and plant officials say it would take 12 to 13 years, working 24 hours a day, just to solidify the waste on hand. The government hasn't selected a permanent site for the solidified waste.

The plant also has been criticized for radiation leaks. Dr. Ernest Sternglass of the University of Pittsburgh, a radiological physicist, has reported that cancer rates over the past three decades have increased dramatically in the Barnwell-Aiken area.

Sternglass links the increases to radiation discharges from the plant, but government spokesmen say local residents are exposed to less radiation than residents of many areas get naturally from the sun and other sources.

Plant spokesmen defend their environmental monitoring. SRP takes 30,000 samples a year from milk, vegetables, birds and even deer that area hunters kill on designated hunting days. The Environmental Protection Agency verifies the research.

Nearly everyone involved with the plant must be concerned with security precautions aimed at keeping plutonium out of irresponsible hands. It takes about 15 pounds of plutonium to make a small atomic device.

The plant is guarded by a 300-member security force, the largest at any DOE facility. While it has never been subjected to an attack, that is no longer considered an impossibility.

Security is especially tight on days the truck convoys assemble to transfer the manufactured plutonium 239 and tritium to New Mexico, Tennessee and other facilities where warheads are assembled.

"The routes and dates are totally classified and they've got SWAT teams and everything else in here," said SRP spokesman David Peek. "We've never had a problem."