John Coster-Mullen conducted a decade of research to build the first accurate replica of the Hiroshima bomb. Illustration by John Ritter

The single, blinding release of pure energy over Hiroshima, Japan, on August 6, 1945, marked a startling and permanent break with our prior understandings of the visible world. Yet for more than sixty years the technology behind the explosion has remained a state secret. The United States government has never divulged the engineering specifications of the first atomic bombs, not even after other countries have produced generations of ever more powerful nuclear weapons. In the decades since the Second World War, dozens of historians have attempted to divine the precise mechanics of the Hiroshima bomb, nicknamed Little Boy, and of the bomb that fell three days later on Nagasaki, known as Fat Man. The most prominent is Richard Rhodes, who won a Pulitzer Prize, in 1988, for his dazzling and meticulous book “The Making of the Atomic Bomb.” But the most accurate account of the bomb’s inner workings—an unnervingly detailed reconstruction, based on old photographs and documents—has been written by a sixty-one-year-old truck driver from Waukesha, Wisconsin, named John Coster-Mullen, who was once a commercial photographer, and has never received a college degree.

I first came across Coster-Mullen’s name in January of 2004, after I attended an exhibit by the artist Jim Sanborn, at the Corcoran Gallery of Art, in Washington, D.C. The show, called “Critical Assembly,” included what appeared to be spookily exact replicas of the interior mechanism of the first atomic bomb, which Sanborn had manufactured according to Coster-Mullen’s specifications. A year later, I read an article in the Bulletin of the Atomic Scientists that mentioned a six-hundred-mile trip Coster-Mullen had taken across the Midwest with a full-scale model of the Hiroshima bomb in the back of a Penske rental truck. He had built the replica with the help of his son, Jason, in his garage, basing it, in part, on his analysis of sixty-year-old screws, bolts, and fragments of machined steel that had been stored in rural basements and attics.

The mention of Coster-Mullen’s journey led me back to the November/December, 2004, issue of the Bulletin, which included a review of a book by Coster-Mullen titled “Atom Bombs: The Top Secret Inside Story of Little Boy and Fat Man.” The review, written by the eminent atomic historian Robert S. Norris, began, “For many years, Coster-Mullen has been printing his manuscript at Kinko’s (adding to and revising it along the way) and selling spiral-bound copies at conferences or over the Internet.” Norris clearly considered Coster-Mullen’s understanding of the bomb superior to his own. It was known that Little Boy and Fat Man brought together two masses of fissile material inside a bomb casing, forming a critical mass that set off a nuclear explosion. Little Boy shot one mass of highly enriched uranium into the other with a gunlike mechanism; Fat Man used explosives to squeeze together two hemispheres of plutonium. But the exact details of how these devices worked were unknown. Norris said of Coster-Mullen’s work, “Nothing else in the Manhattan Project literature comes close to his exacting breakdown of the bomb’s parts. Coster-Mullen describes the size, weight, and composition of many of Little Boy’s components, including the nose section and its target case; the uranium-235 target rings and tamper; the arming and fuzing system; the forged steel 6.5-inch-in-diameter gun barrel through which the uranium-235 projectile was fired at the target rings; and the tail section—to cite just a few.”

My own copy of “Atom Bombs” soon arrived in the mail, along with a sheet of testimonials from Harold Agnew, the former director of the Los Alamos Scientific Laboratory, who was aboard the Enola Gay when it annihilated Hiroshima (a “most amazing document”); Philip Morrison, one of the physicists who helped invent the bomb (“You have done a remarkable job”); and Paul Tibbets, the commander and pilot of the Enola Gay (“I was very much impressed”). “Atom Bombs” consists of densely interlocking sentences, nearly all of which contain dimensional information that contradicts the assertions of previous authorities. “A circular steel plate was positioned inside the 17.0"-diameter tail cylinder at the front of the tail tube and another towards the rear of the tube,” Coster-Mullen writes. “These allowed the tail to be slid over the 10.5"-diameter gun tube during assembly. The forward plate was positioned 26.5" in front of the aft plate and was welded to the front of the tail tube.”

Though the book’s specificity about dimensions, shapes, and materials was mind-numbing, the accumulation of detail was strangely seductive. As Coster-Mullen described how the different parts of the Hiroshima and Nagasaki bombs fit together, I felt that I could practically assemble an atomic weapon myself.

The text was followed by more than a hundred pages of declassified photographs extracted from half a dozen government archives, which showed the weapons at various stages of completion—surrounded by scientists in New Mexico or by tanned, shirtless crew members on Tinian Island, in the Western Pacific, just before the bombs were dropped. Coster-Mullen’s book concluded with thirty-five pages of end notes, including a hilariously involved discussion of the textural differences in the gold foil used to separate the plutonium hemispheres for the first atomic bomb, Trinity (dimpled), and the Nagasaki bomb (flat).

Coster-Mullen sees his project as a diverting mental challenge—not unlike a crossword puzzle—whose goal is simply to present readers with accurate information about the past. “This is nuclear archeology,” he told me, in a late-night phone call. “It’s like any other kind of archeology.” Though the government does not make a practice of providing Coster-Mullen with timely responses to his technical inquiries, no official has actively discouraged him from pursuing his research. After a period of mild equivocation, he decided to publish all the details he had uncovered about the mechanics and production of the bomb, even though the subject remains classified. “I was acting like a classification officer,” he recalls. “ ‘I can have the truth and you can’t.’ Who am I to say that?”

Among other things, Coster-Mullen’s book makes clear that our belief in the secrecy of the bomb is a theological construct, adopted in no small part to shield ourselves from the idea that someone might use an atomic bomb against us. Surely, hostile powers could easily obtain the kind of information that Coster-Mullen has acquired, however painstakingly, in his spare time. Any nation that can master the challenges of the atomic-fuel cycle and produce a critical mass of uranium or plutonium, as Iran is reported to be on the verge of doing, would have little difficulty in producing a workable bomb. Given a sufficient quantity of highly enriched uranium, a small number of engineers working for a terrorist group like Al Qaeda or Hezbollah could easily assemble a homemade nuclear device.

I recently wrote to Coster-Mullen and suggested that we take a trip across the country to visit his Little Boy replica, which is currently housed at Wendover, a decommissioned Air Force base in Utah. After some negotiation, we agreed to ride together on his late-night delivery route between Waukesha and Chicago. We would then drive to Wendover. Along the way, he would explain the inner workings of the first atomic bombs, and I would learn how he got it right and the experts got it wrong.