Today, veterans of the project proudly speak of how they helped to forge China’s nuclear shield. A museum, still forbidden to foreigners, tells visitors that the weapons were made to fend off American and Soviet aggressors encircling China. A statue of Mao gazes paternally over the square of the main town, where thousands of people still live.

“At the time, China’s social conditions and international position were a bit like North Korea now,” said Liao Tianli, a writer who visits Jinyintan once or twice a year and has interviewed scientists who worked on the project. “For many people, it was purely in the spirit of ‘I’ll do whatever Chairman Mao tells me to.’ ”

Yet even as the scientists have won recognition for their work, lifting some of the secrets of Plant 221, other dark parts of its past remain muffled in official silence and censorship. Building nuclear weapons here came at a grievous cost, and a few survivors and researchers have tried to exhume the layers of history unmentioned in the memorials and displays.

The herders and farmers who were moved for the project endured starvation, executions and brutal expulsions. Political paranoia engulfed the project itself, and thousands of scientists and technicians were persecuted. Some veterans have said that the nuclear workers were not adequately protected against radiation, or given effective care after they fell ill from cancer.