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.

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

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.

Coster-Mullen and I met in the darkened parking lot of a regional distribution center for a big-box retailer, some ten miles outside Waukesha. Dressed in Lee jeans and a tan shirt with the J. B. Hunt logo, he had titanium-frame glasses, blue-gray eyes, and a full head of silvery hair. The distribution center was the size of seven or eight football fields; fans roaring overhead and an enormous conveyor belt drowned out the beeps of cabs backing up to trailers. Coster-Mullen picked up his sheet for the night, which involved stops at Store 1950, in Streamwood, Illinois, and Store 1889, in downtown Chicago. He was to drop off a container filled with lawn furniture in Streamwood, and haul back “sweep” merchandise—cardboard boxes, defective items, coat hangers—from Chicago. We walked outside and hooked up Coster-Mullen’s truck to trailer No. 537427, with a solid click. It was seven o’clock on a Sunday night. He calmly recited a safety checklist (“My lights are on, my flashers are on”) and we set off.

Over the years, Coster-Mullen told me, he had held nearly a dozen jobs, including working at camera stores in and around Milwaukee; doing inventory control for the Beloit Corporation, which manufactured paper-making equipment; taking photographs of industrial equipment for Trane, the heating and air-conditioning company, and then for Mercury Marine, which makes high-quality engines for boats; working as a studio photographer for Pohlman Studios, in Milwaukee; and running his own photography business. These jobs had provided him with the skills, he says, that helped him solve the puzzle of the bomb. I asked him how he wound up driving a truck. “They are always hiring,” he said. “I figured if people with the brains of a squirrel could drive a truck, maybe I could drive a truck.”

The highway cut through scrubland, and by nightfall Coster-Mullen was driving past Old World Wisconsin, a tourist attraction that features restorations of prairie homesteads. Making long cross-country drives, Coster-Mullen said, had given him plenty of time to reëxamine the three-dimensional diagram of the bomb that he keeps in his head, like a Buddhist monk contemplating the Karmic wheel. His truck routes also made it easy for him to maintain connections with sources. Twelve years ago, Coster-Mullen pulled into a Wal-Mart parking lot in North Carolina and got into the car of a retired machinist in his late seventies, who showed him photographs of metal pieces that he had fashioned for the Trinity bomb, which was set off in the desert outside Alamogordo, New Mexico, in July, 1945. Coster-Mullen said that machinists often hid the fragments in their shoes and pants cuffs, in order to have something to show their grandchildren. Two years after meeting the machinist, in 1998, Coster-Mullen, while driving through Nebraska with three cars in front of him, figured out the exact shape and weight of the pieces of uranium inside Little Boy. “I’m sitting there with my pocket calculator, going, ‘If the core had this diameter, and the length is this, what’s the volume?’ ” he recalled. “I went, ‘That’s it!’ And then I got on the horn—urh-urh.”

Arriving at the drop-off point in Streamwood, we unhooked the truck’s electric and air lines, then turned the crank on the landing gear forty times. We picked up another container, got back in the truck, and headed south, toward Chicago. At four in the morning, we passed the Sears Tower. Coster-Mullen gingerly navigated the pillars inside an indoor parking garage and pulled up to the loading dock.

The trailer, which contained thirty-one thousand pounds of FAK—“freight of all kinds”—wasn’t ready yet, so we checked out the bales of sweep merchandise: crushed boxes of cookies, dented cans, ripped jeans. Finally, we hooked up the trailer and hit the road. As we headed north, Coster-Mullen explained to me the likely blast effects of a Hiroshima-size nuclear device exploding in a container truck in downtown Chicago. He said, “All you need to do is take two subcritical masses of uranium and smash them into each other to form a critical mass. Neutrons strike the heavy uranium nucleus, which splits, releasing a tremendous jolt of energy along with two or more neutrons, which split more nuclei, setting off a chain reaction that grows and grows and finally manifests itself as a huge fireball over a populated area, blinding, asphyxiating, incinerating, or crushing every living being within a five-mile radius.” As he elaborated on the scenario, the sun began to rise, and I fell asleep with my face against the window.

We arrived at Coster-Mullen’s home, in Waukesha, around eight o’clock that morning. He lives in a ranch house on a cul-de-sac in a pleasant subdivision. His wife, Mary, is a retired social worker who spends most of her time reading and knitting. They have two children together, and Coster-Mullen has a third from a previous marriage. Asters, black-eyed Susans, and coral bells blossomed beneath the trees in the back yard. Coster-Mullen, in anticipation of my visit, had arrayed his kitchen with some of his atom-bomb memorabilia, including a roof tile from the hypocenter of the Hiroshima blast, which he purchased for eighty-nine dollars from a former member of the U.S. radiation-survey team. He also owns a brick of graphite from Enrico Fermi’s lab at the University of Chicago—the site of the first reactor pile—which he was given by two physicists who were fans of his book; a small marine fossil that was underneath the Trinity bomb when it exploded in the New Mexico desert, which Coster-Mullen dug out of the ground himself during a visit to the site; silicone molds of the detonator for the Trinity bomb; a chunk of uranium; and a sphere of beryllium, a component of modern atomic bombs.

He handed me a leaflet that had been dropped over Japan by B-29 bombers in late July, 1945. “Attention Japanese People,” the leaflet says. “In the next few days, four (or more) of the cities named on the reverse side will be destroyed by American bombs. These cities contain military installations and workshops or factories that produce military goods. We are determined to destroy all of the tools of the military clique.”

On the kitchen counter sat something seemingly unconnected to atomic weapons: a hobbyist’s model of the Joan of Arc chapel, on the campus of Marquette University, in Milwaukee. In fact, Coster-Mullen told me, the model, which he completed in 1993, had helped spark his obsession with building his own bomb. He had built the model in the hope of launching a business. Marquette alumni and other visitors, he had figured, would eagerly buy replicas of the chapel and display them in their homes. Constructing the model was difficult, he recalled: “I was using dental picks and surgical 3-D glasses and I learned how to carve little eyes in the wood benches.” Like most of his business ideas, before and since, the project showed both a fanatical devotion to detail and a hazy grasp of what ordinary consumers might pay for. He placed the chapel models in local gift shops on consignment, but few sold. After this failure, Coster-Mullen decided to make replicas of something with wider commercial appeal.

In December, 1993, he persuaded his son, Jason, who was then seventeen, to accompany him on a road trip to the National Atomic Museum, in Albuquerque, where Coster-Mullen could examine the empty ballistic casing of an atomic bomb at first hand and make sketches that he could use to build an accurate scale model. After driving two thousand miles to the museum, he was distressed to find that the atomic-weapons area was closed for renovation. He protested until his contact at the museum finally appeared and let them in. He and Jason spent hours measuring the bomb casings on display. (In the early nineties, after the fall of the Soviet Union, no one was particularly disturbed by the sight of a father and son poking measuring tape inside the casings of fifty-year-old bombs.) The Coster-Mullens were soon measuring weapons casings around the country, including at the Wright-Patterson base, in Ohio; the West Point Museum, in the Hudson Valley; and the Smithsonian, in Washington, D.C. They also saw the Fat Man display at the Bradbury Science Museum, in Los Alamos.

During these and other excursions, Coster-Mullen discovered that much of the dimensional information about the bombs in history books was wrong. “Rhodes and others said that Little Boy is twenty-nine inches in diameter—but it’s not, it’s twenty-eight,” he said, in the friendly, matter-of-fact tone he uses to soften the force of his obsession. Wondering what other errors the historians had made, he began to attend reunions of the 509th Composite Group, the military unit that dropped the bombs. He went to his first reunion in 1994, in Chicago. Before the gathering, he wrote a draft of a pamphlet about the bomb and sent it to Frederick Ashworth, a naval commander who was in charge of the Fat Man weapon. “The Monday of the reunion week, I get this letter back from Admiral Ashworth, who, justifiably, took me to task,” Coster-Mullen recalls. “He said, ‘Either treat this subject with the seriousness it deserves or drop it.’ So I chose the former.”

Coster-Mullen spent the next ten years of his life mastering a body of recondite technical data. He extracted photographs from government archives and scrutinized them with a magnifying glass; he interviewed one retired machinist after another, as well as scientists and engineers. Researching the bomb provided Coster-Mullen with an outlet for a sensibility that might have been equally at home collecting tropical butterflies or double-print stamps. To suggest that Coster-Mullen is a garden-variety classification freak, however, is like comparing a high-school trumpet player to Miles Davis. Driven by his desire to solve a great puzzle, he is personally affronted by recycled information and secondary sourcing, which often leads him to express contempt for people who are lazier than he is—a category that includes virtually everyone.

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The first edition of “Atom Bombs” was completed in 2003. With the publication of the book, which has since undergone several hundred revisions, Coster-Mullen became a leading member of the loosely organized scholarly fraternity dedicated to challenging the ethic of secrecy behind the atomic security state. Its dozen or so members included Richard Rhodes; Chuck Hansen, a computer programmer whose Freedom of Information Act requests helped him assemble “The Swords of Armageddon,” a twenty-nine-hundred-page, seven-volume archive of documentary information about the U.S. nuclear arsenal; Howard Morland, who published the first detailed sketch of a thermonuclear weapon, in The Progressive, in 1979; and Carey Sublette, a programmer in California, who has posted a wealth of data about atomic weapons on the Internet.