NO MATTER WHERE WE were from or how sophisticated our background, all of us were astonished by the construction blitz we witnessed in Oak Ridge. There, in what was once a sleepy little pocket of the rural South, where change always had come slowly, 35,000 construction workers virtually attacked the landscape overnight. In just 30 months, a town and three plant sites were constructed, and Oak Ridge emerged as the fifth largest city in Tennessee - 17 miles long, seven miles wide, with a population topping 75,000. And with the sixth-largest bus system in the nation. By 1944, Oak Ridge was using 20 percent more electricity than New York City.

The earliest workers lived in Army trailers and hutments, when they were available, and rooms, attics and garages rented from nearby farmers when they were not. Some workers slept in abandoned outbuildings, in barns, in cars, and some pitched tents in parks and school yards until housing was available. In an attempt to appease scientists and their wives, lest they grow disgusted and leave, the Army built 3,000 "permanent" houses called cemestos, which became status symbols.

They were well-built, two- and three-bedroom homes with coal furnaces, fireplaces, hardwood floors and porches and furnished with new electric appliances that were virtually unattainable during the war. The cemestos, made of fiber board coated with a cement-asbestos mixture, were vaguely assigned according to a person's importance to the mission - mainly key scientists and engineers, top management, doctors and Army brass. And wherever possible, they were tastefully built on roomy lots, most of which were wooded.

(When the layoffs began after the war, the houses once reserved for VIPs became open rentals, which led to the creation of some interesting neighborhoods. In 1946, for instance, we left our $29-a-month three-bedroom prefab and moved into a Ph.D.'s three-bedroom, $46-a-month cemesto three houses down from the mayor, who lived next door to a plant superintendent and across the street from a janitor at an elementary school. My mother and the ex-mayor still live on that street.)

Prefabs - the ugly, boxy, one-door flattops that were transported to lots on flatbed trucks and built on wooden stilts - were considered the second-best housing. They, too, were equipped with refrigerators and ranges and also with beds, built-in cabinets, bookcases and warm-morning coal stoves. Coal was delivered free to everyone. Electricity and water were free. Trash pickup was free. City buses and work buses were free. Most streets were mud and gravel, and 163 miles of boardwalks, which ambled through neighborhoods and wooded recreation sites, were made of scrap wood.

The Army trailers, about 7,000 of them crammed into two locations, were mostly for construction personnel. They had no running water; some had no electricity. Oil stoves were used for heat and cooking. Kitchen sinks drained into open barrels, which were emptied when the daily sewage truck arrived. Slop jars used during the night were emptied next morning at bathhouses. My future wife, then 8, lived with her sister and parents in such a trailer for two months waiting for their house to become available.

"The trailer looked like a palace to me," said her mother, Peggy Hildebrand, now 79, "because we had lived in the woods in a tent for three days before we got that."  Some married couples waiting for housing were forced to live separately in men's and women's dormitories, and no opposite-sex visitors were allowed in rooms. Some wives were arrested for visiting their husbands' rooms.

At the bottom of the wartime housing scale were the hutments, reserved mostly for black workers and unskilled laborers. Hutments were ugly, 16-foot- square, un-insulated plywood units with no electricity, no plumbing, no furniture except for footlockers, and no glass in the windows. There was only one door, and a pot-bellied stove was the only heat. Many residents walked a block or more to bathhouses for toilets and showers. Four people and sometimes five lived in them - 16,000 people in all, and not for a week or a month or a season, but for the duration of the war.

In keeping with practices in the South at the time, blacks were segregated, rode in backs of buses and got the worst jobs. And in the earliest days, they weren't allowed to live with their spouses or have their children with them or leave their compounds after 10 p.m. Five-foot fences topped with barbed wire separated the men's hutments from the women's. At night, patrolling MPs sometimes pulled down bleeding men from the fences.

Most white townspeople were never aware of the blacks' living conditions. We saw them mostly in their workplace. Later, when the war was over and the Army was gone, Oak Ridge would become the first town in the South to integrate its school system. But during the '40s, the Army was not out to promote social change. Its mission was solely to complete a project as rapidly and with as little resistance as possible. The town was always of secondary interest.

FOR MANY REASONS, Oak Ridge was not an easy place to live. Unless you were a child. For the children, Oak Ridge was paradise.

There were 20 supervised playgrounds with age-group athletic leagues, arts and crafts and clubs and movies and Little Theater. Every summer was like free camp. Every school was new and modern (two national spelling bee champions came out of those schools).

The town was so secret that non-working townspeople - which included children - could neither leave nor enter without badges or special passes, and we were subject to search at the security gates coming and going. The adults hated the inconvenience, but we children thought the idea of a covert existence was adventurous.

We imagined German spies to be behind every tree. And sometimes, when we played hooky from school, we imagined we were German spies and escaped the fenced city "to freedom" over and under barbed-wire along a wooded border road, hiding in ditches and weeds from U.S. patrol vehicles. "Freedom" was a nearby country store where young "German spies" were sometimes known to spend their lunch money on bologna, crackers and Nehi sodas and get sick on a 20-cent pack of Chesterfields.

If you were 12 years old or older, you had to wear a resident's badge with your picture on it - to school, to the store, everywhere. For a school kid, having a badge was the mark of certain maturity, a source of pride, and it was proof that you had been counted and were a part of this wonderfully impervious place. I never got my badge - one of the major disappointments of my childhood - for the town's secret was exposed before I turned 12, and nothing was ever quite the same.

I was 10 when we moved there, one of three children. Mother, a former one- room-school teacher, and my father, a farmer's son, were $4-a-day cotton- mill hands in Stevenson, Ala., when they heard about the Manhattan Project . Like many in the South, they moved there and tripled their income.

I never knew what my father did except that he was a "chemical operator" at Y-12, one of three secluded, code-named plant sites. I know he was accidentally gassed once and crawled on his stomach to an adjoining room for help. I overheard the story in his hospital room - but not from him. He had signed a pledge of secrecy, and 19 years after the war ended, he went to his grave without ever talking to us about his job.

Mother also worked at Y-12, but in a different building. Even now, at age 85, she will say only that her job was to process uranium. "I know they don't do it now the same way they did it then," she told me one day recently, "but I gave my word." She took from her purse a discolored, dog-eared pledge card that read in part: "The U.S. government wishes to remind Dovie Ryan Searcy of the Secrecy Acknowledgment you signed when employed . . . and call your attention to the penalties (death or life imprisonment, maximum) for disclosing any information. . . ."

(In 1961, after working for most of 15 years in the "hottest" building at Y-12, mother was forced to transfer to a "clean" building because of her ''high body count." For months she was required to leave urine and feces samples on our front porch for lab pickups. Doctors were still getting a radioactive reading on her at her retirement in 1971 and as recently as five years ago when she was last checked. She is considered something of a phenomenon by researchers from the Department of Energy, and they have asked that she donate her body for research. In the spirit of her wartime youth, she signed the pledge.)

At Oak Ridge during the war, patriotism and secrecy had became religions. ''If you only knew," the Army kept reminding us, "how important this work is to our fighting boys. . . ."

BECAUSE OF THE SECRECY, MOST OF US didn't know enough to be scared. Everybody worked round the clock, six and seven days a week, and never asked questions. Even labor union leaders were persuaded to postpone plans to organize plant workers until after the war.

But some of those who did know weren't just scared, they were terrified - terrified, we later learned, that the project might fail or that something might slip or spill and blow up the building. Or the town. Or the state.

Col. Groves, the West Point engineer in charge of the Manhattan Project , who during the project was promoted to general, once told one of his engineers: "If the reactor blows up, jump in the middle of it and save yourself a lot of trouble."