Although not the first such reunion here, time is increasingly precious, as a vast majority of the workers are into their 80’s, Ms. Kelly said.

“What’s happening is significant, because 20 years from now, very few, if any, of the veterans of the Manhattan Project will be alive,” she said. “So it’s important to get the oral history, the first-hand accounts.”

The average age of those at Los Alamos, considered the brain trust of the Manhattan Project, was 25, Ms. Kelly said. She estimated that 20 percent remained alive. “I’m overwhelmed and engulfed with memories,” Paul Numerof, who was in a special engineers detachment, said. “It was a tense, exciting time for all of us. I felt like I was in the presence of scientific royalty.”

Dr. Numerof recalled attending Monday night meetings led by J. Robert Oppenheimer.

Aside from nostalgia, the reunion also commemorates the restoration of Manhattan Project sites at Los Alamos. They were given a federal grant in 1999, but a forest fire in 2000 heavily damaged them.

In celebration of the restoration, the Atomic Heritage Foundation and Los Alamos Historical Society have sponsored events like bus tours of the region and talks by experts like Richard Rhodes, who wrote the prize-winning “Making of the Atomic Bomb,” and Thomas O. Hunter, director of the Sandia National Laboratories in Albuquerque.

Not everyone is completely enamored of the reunion.

Jay Coghlan, executive director of Nuclear Watch New Mexico, a watchdog group in Santa Fe, said nuclear weapons continued to be produced, in a far murkier geopolitical landscape.

“I’m not going to begrudge a bunch of old fellows as reliving their war years,” Mr. Coghlan said. “Every generation has to operate under the exigencies of their time.