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The night the American B-29 warplanes came, Ryohei Nakane had been enriching uranium for Japan’s “super bomb.”

By the next morning — April 13, 1945 — all that remained of his samples and his laboratory at Riken Institute was charred, splintered wood and broken glass.

For nearly six decades, historians have been unable to solve one of the mysteries of Japan’s World War II A-bomb project: How close were Japanese scientists to building the bomb before the U.S. air raid stopped them?

All official records were believed to have been burned in the closing days of the war, forcing historians to piece together an answer from less reliable clues.

Now, long-lost wartime documents are setting the record straight.

The 23 pages of Imperial army papers returned to Japan last April offer convincing evidence that Japanese scientists were years from completing their 20-kiloton A- bomb — which would have had more force than the 15- kiloton bomb dropped on Hiroshima but less than the 22-kiloton device that hit Nagasaki.

Historians say that not only had Japan’s scientists underestimated how much of the rare isotope uranium-235 they would need for the bomb, they misunderstood the mechanics of an atomic explosion.

“The documents are one of a kind. We can finally prove that even if Japan had built a bomb, it would not have been powerful at all,” said Masakatsu Yamazaki, a professor of science history at the Tokyo Institute of Technology who analyzed the papers. “And it might have taken them another decade to complete one.”

Nakane has been telling a similar story for years.

“We were carrying out our research so leisurely. None of us thought we would finish before the war ended,” Nakane, 83, said in a recent interview at his Tokyo home.

More than a half-century after the war, Japan’s A-bomb project is only a historical footnote. Few Japanese have even heard of their government’s wartime nuclear program.

Scientists and military officers who were there have written memoirs and talked publicly about their work. But over the years, speculation and conspiracy theories have clouded the facts and raised doubts about the participants’ accounts.

Japan’s own efforts to build a bomb are difficult for many here to accept because of the bombings of Hiroshima and Nagasaki, and the widespread feeling that Japan would never have even considered such a brutal attack.

Every year, the two cities honor more than 200,000 people killed or wounded by the two U.S. atomic bombs, and the ceremonies are nationally televised.

As the only nation to have suffered a nuclear attack, Japan has been one of the most outspoken advocates for a global ban on nuclear weapons. It has vowed never to possess, build or trade nuclear weapons and has used this stance to upbraid countries with nuclear ambitions, including North Korea.

But Tokyo hasn’t always escaped its own criticism.

Last May, Prime Minister Junichiro Koizumi’s top aide, Chief Cabinet Secretary Yasuo Fukuda, appeared to signal a shift away from the antinuclear doctrine by saying Japan was not legally prohibited from having atomic arms.

China, South Korea and Russia immediately protested, prompting Koizumi to reiterate Tokyo’s long-standing policy against nuclear weapons. Fukuda later complained that he had been misquoted.

Surprisingly, the reaction in Japan was muted.

“I think younger Japanese might not be that strongly opposed since they were born after the war and probably have never even heard about their government’s wartime atomic program,” said Yuzo Fukai, an atomic energy expert and former Nihon University professor.

The treasure-trove of wartime papers could change that.

Sneaked out of the country just after the war by former University of Tokyo professor Kazuo Kuroda, who left for the United States, the papers were sent to Riken Institute, north of Tokyo, by Kuroda’s widow months after his death in Las Vegas in 2001.

The documents — the only surviving record of Japan’s A-bomb research — read like a blueprint for the bomb.

Among the papers are several pages of handwritten notes taken by an army officer during a June 1943 interview with Yoshio Nishina, the country’s leading physicist. Nishina, who had once worked with atomic pioneer and Nobel laureate Niels Bohr in Copenhagen, headed the project’s 100 scientists beginning in the early 1940s.

The papers show that Nishina believed he could fashion a bomb from 1 kg of weapons-grade U-235 with 1 to 2 tons of natural uranium ore.

But making U-235 proved to be no easy task.

By early 1945, Nishina’s team was still struggling to make U-235 from uranium hexafluoride gas in a leaky chamber using a technique known as gaseous thermal diffusion, said Yamazaki of the Tokyo Institute of Technology. In the United States, scientists years earlier had ruled out the technique as too unwieldy.

“Given the inefficiencies of that method, Nishina would have needed thousands of tons of ore,” Yamazaki said.

Years later, one of the project’s physicists, Tatsusaburo Suzuki, said they had managed to make only about 5 kg of impure U-235 — far short of what they needed for an atomic weapon.

Uranium wasn’t their only concern. Wartime rationing made equipment scarce and money even harder to come by, and scientists had to make do with only one industrial-size cyclotron and four smaller ones to purify fissionable uranium.

Scholars estimate that the army spent the equivalent of $500,000 — a pittance compared with the roughly $2 billion the United States shelled out for the Manhattan Project. A parallel Japanese navy project, which had no chance of success, cost $150,000.

So stretched were the country’s resources that, at one point, military leaders considered scrapping a battleship to supply steel to the army’s A-bomb team, said Nakane, the former scientist, who is now an honorary director at Riken.

But at the root of Japan’s failure was Nishina’s flawed theory about an atomic blast.

To generate an atomic explosion, Nishina knew he had to trigger a chain reaction of U-235. Experts agree that has to occur within 1/200th to 1/300th of a second. In the documents, Nishina says he thought he could do it in 1/20th to 1/30th of a second.

“That’s equivalent to the slow-fission reaction in an out-of-control nuclear reactor. An explosion of that magnitude wouldn’t be very strong at all,” Yamazaki said. “Only years after the war did he realize that his calculations were wrong.”

Not everyone believed the Japanese were so far off.

Beginning in the late 1940s, speculative reports citing U.S. and Japanese military intelligence sources concluded that Japan’s wartime government had successfully completed an atomic test explosion in August 1945 near a uranium ore processing plant in Hungnam, Korea.

However, postwar U.S. scientific missions to Korea, which was under Japanese colonial rule from 1910 to 1945, found no evidence to back those claims, said Walter Grunden, a professor at Bowling Green State University in Ohio who is finishing a book on the topic.

Grunden said neither U.S. spy photographs of the site nor corporate records from Nihon Chisso, the Japanese company that owned most of the plants in Korea during the war, reveal any A-bomb research activity.

Nakane agrees. When U.S. warplanes destroyed the Riken labs, Japan still hadn’t done any work on bombshell design or conducted a single experimental explosion, he said.

“Completing the bomb wasn’t Nishina’s priority. Advancing Japan’s scientific research and saving scientists from the war were more pressing concerns,” Nakane said.

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