It was an understandable fear. Despite assurances by Nobel Prize-winning physicists, nobody knew for sure what might happen, how powerful the new ''stuff" would be. They were working in an area where there was no experience. On the eve of the first test in the southern New Mexico desert, Robert Oppenheimer, in charge of the secret assembly site at Los Alamos, quietly set up two evacuation plans with the New Mexico governor, just in case - one for the general area, another for the entire state. And some technicians working on the test at Los Alamos that day were stunned to hear the project's top physicist, Enrico Fermi, take side bets on whether they would ignite the atmosphere.

Only a handful knew it, but on Nov. 4, 1943, just 14 months after the first parcel of land had been purchased in Tennessee, the world's first full-scale graphite nuclear reactor went critical at X-10. Criticality, a controlled nuclear chain reaction, was achieved on a grand scale. Nine miles to the east, the electromagnetic separation plant (code name Y-12) and later the gaseous diffusion separation plant (K-25) began turning out U-235, the weapons-grade uranium, the "stuff" for which our secret atomic city had been built.

As it was processed, the uranium was stored in a specially built bunker called Katie's Kitchen near X-10, disguised as an abandoned barn and silo. The storage room, with thick concrete walls and a heavy vault door, was built into the earth. The silo, hooded at the top, was actually a guard tower.

By the winter of 1945, the plants were producing daily about seven ounces of U-235, and in March, the first of several small clandestine shipments to Los Alamos left Site X in the form of green salt or powder. By late July, Los Alamos had about 30 pounds of the enriched uranium. More than enough.

ON AUG. 6, A MONDAY, WE WERE JUST SITTING down for lunch when my father heard President Truman come on the radio. We huddled around the set. A B-29, he announced, had dropped a new kind of bomb on Hiroshima, a bomb more powerful than 20,000 tons of conventional explosives - and the main component had come from Oak Ridge, Tenn.

"It's a bomb!" my father shouted. "We've been making an atom bomb!" My sister, Mary Glenn, began to cry, partly out of fear and partly because she had been told by my father that they were making paper dolls at the plants.

As quickly as you could say "atomic bomb," the secret of my home town was out - and no town was ever quite so proud. Oak Ridge, not even in existence when the war started in 1941 (and at war's end, still not shown on maps of Tennessee), suddenly was known all over the world. We who had uprooted ourselves from our homes all over America to live here in secrecy, we who were from virtually every walk of life, we who had joined hands for three years in the '40s and lived under military dictatorship to help win a war, we were suddenly reading about ourselves in the newspapers and hearing about our town in radio broadcasts.

As soon as Truman finished making his announcement, neighbors spilled into their yards and formed snake lines in the streets. Car horns and fire hall sirens sounded and firecrackers exploded (how did they ever get them through the security gates, I wondered). Wives telephoned their scientist husbands at the plants: "Hey, now I know what you've been working on!"

Waldo Cohn, then a young biochemist at X-10, and some of his colleagues knocked off work, drove to the center of town and became engulfed in a spontaneous parade. They stuck their heads out of car windows and waved and shouted words they had been forbidden to utter:

"Uranium!"

"Atoms!"

"Nuclear fission!"

"Radioactivity!"

In a matter of hours the Knoxville newspapers had extra editions on the streets ("Power of Oak Ridge Atomic Bomb Hits Japan"), selling for $1 apiece. Bootleggers sold out before dark (it was a dry county, as were most in Tennessee). Street dancing lasted well into the morning.

Three days later, another bomb, this one with a plutonium core from a second secret city, Hanford, Wash., was detonated above Nagasaki. Two days after that, World War II was over.

When the announcement came, I joined some friends for the celebration. We ran through the streets shouting and beating dishpans with spoons and waving an American flag until we came upon an unsmiling man, a scientist, mowing his lawn.

"Hey," we shouted, "don't you know the war's over?"

"I know," he said, "but a lot more than the war is over. We've got to live tomorrow, too, you know." We thought he was a grump. We didn't know what he knew.

All we knew was what we read and heard on the radio: We had been working on one of the most important missions in history - and the world would never be the same because of it. It was the largest, most extraordinary scientific experiment in history, the first time that mankind had ever handled radioactivity on such an enormous scale. We all felt a little famous, the nuclear scientists and the laborer, the homemaker and the schoolboy. It didn't matter that half the town didn't know what an atom was, that, in fact, only about 1 percent of the people working there knew what was going on. We were all in it together, and we all had kept the secret - even if we didn't know what the secret was.

Our euphoria lasted for days. It was like having a romance with history. Oak Ridge had helped save a million lives, we were told, and had ushered in the Atomic Age. And what an age it would be:

In 20 years, we would run our homes and factories with nuclear energy, drop a pill in our gas tanks and drive for a year. Ships would sail the Pacific on a nuclear charge no bigger than a lump of coal. We would power rockets with it, and spaceships, and cure cancer. An unlimited supply of energy was now available.

How could we know then that almost none of that would happen and that, instead, billions of dollars would be spent for nuclear arsenals? How could we know that 50 years later, nuclear waste problems would be unsolved, that the nation would not trust atomic energy?

Our celebrity began to fade when the horrid details from Japan began to emerge - two cities scorched by heat, buildings leveled for miles, more than 100,000 dead or missing, thousands maimed, thousands more exposed to lethal doses of radioactive fallout. Once the faces of the victims were shown across America in Movietone News clips, it was hard to continue cheering. Now, the genie we let out of the bottle had to be dealt with. Not just for a few years or for a lifetime, but forever.

IT WAS IN A MOVIETONE CLIP THAT I FIRST saw those big, secret, windowless buildings that emitted that ominous yellow glare at night. Somewhere in those buildings was where the atom was split, where they produced the Uranium 235 that fueled a bomb four times hotter than the sun.

Those plants are still there, still off-limits mostly. But now, rather than being created, nuclear weapons are being dismantled. The Army barracks are gone, the soldiers are gone, and the boardwalks and hutments, and trailer camps and mud roads. Our first house and the thousands of other hurry-up wartime prefabs are gone, broken down like cardboard boxes soon after the war and removed like trash.