Fourth Spy at Los Alamos Knew A-Bomb’s Inner Secrets

Historians recently uncovered another Soviet spy in the U.S. atomic bomb program. Fresh disclosures show he worked on the device’s explosive trigger.

Fourth Spy at Los Alamos Knew A-Bomb’s Inner Secrets
Blueprint of Fat Man, the atomic bomb that the United States dropped on Nagasaki on Aug. 9, 1945. It shows external measurements rather than specifics of the bomb’s inner workings, such as its firing circuits.Credit...Los Alamos National Laboratory

Last fall, a pair of historians revealed that yet another Soviet spy, code named Godsend, had infiltrated the Los Alamos laboratory where the world’s first atom bomb was built. But they were unable to discern the secrets he gave Moscow or the nature of his work.

However, the lab recently declassified and released documents detailing the spy’s highly specialized employment and likely atomic thefts, potentially recasting a mundane espionage case as one of history’s most damaging.

It turns out that the spy, whose real name was Oscar Seborer, had an intimate understanding of the bomb’s inner workings. His knowledge most likely surpassed that of the three previously known Soviet spies at Los Alamos, and played a crucial role in Moscow’s ability to quickly replicate the complex device. In 1949, four years after the Americans tested the bomb, the Soviets detonated a knockoff, abruptly ending Washington’s monopoly on nuclear weapons.

“It’s fascinating,” Harvey Klehr, an author of the original paper, said in an interview. “We had no idea he was that important.”

The documents from Los Alamos show that Mr. Seborer helped devise the bomb’s explosive trigger — in particular, the firing circuits for its detonators. The successful development of the daunting technology let Los Alamos significantly reduce the amount of costly fuel needed for atomic bombs and began a long trend of weapon miniaturization. The technology dominated the nuclear age, especially the design of small, lightweight missile warheads of enormous power.

Mr. Seborer’s inner knowledge stands in contrast to the known espionage. The first Los Alamos spy gave the Soviets a bomb overview. So did the second and third.

The new clues suggest that Mr. Seborer’s thievery “could have been unique,” Alex Wellerstein, a nuclear historian at the Stevens Institute of Technology in Hoboken, N.J., said in an interview. “That doesn’t mean it was — just that it could have been.”

Mr. Klehr, an emeritus professor of politics and history at Emory University, said the new information cast light on a furtive boast about the crime. Last fall, in the scholarly paper, the two historians noted that Mr. Seborer fled the United States in 1951 and defected to the Soviet bloc with his older brother Stuart, his brother’s wife and his mother-in-law.

The paper also noted that an F.B.I. informant learned that a communist acquaintance of the Seborers eventually visited them. The family lived in Moscow and had assumed the surname Smith. The visitor reported back that Oscar and Stuart had said they would be executed for “what they did” if the brothers ever returned to the United States.

In a recent interview, Mr. Klehr said he originally thought the spies were exaggerating their importance. But, he added, the new information on Oscar’s technical knowledge suggested otherwise, although Stuart’s exact role in the spy case remains unclear. “Maybe it wasn’t hyperbole,” Mr. Klehr said.

Last fall, the historians described the Seborers as a Jewish family from Poland that, in New York, became “part of a network of people connected to Soviet intelligence.” Both Oscar and Stuart attended City College, “a hotbed of communist activism,” the historians wrote.

Stuart took a math class there in 1934 with Julius Rosenberg, they reported. In a notorious Cold War spy case, Mr. Rosenberg and his wife, Ethel, were convicted of giving the Soviets atomic secrets. In 1953 they were executed at Sing Sing Prison in Ossining, N.Y., orphaning their two sons, ages 6 and 10.

The scholarly paper, written with John Earl Haynes, a former historian at the Library of Congress, appeared in the September issue of Studies in Intelligence. The journal, a C.I.A. quarterly, is published for the nation’s intelligence agencies as well as academic and independent scholars.

A Times article about the historians’ paper ran on Nov. 23, a Saturday. Four days later, a reporter sent the scholarly paper to Los Alamos and asked if the lab’s archive had any photos of Mr. Seborer or relevant documents.

Two weeks later, on Dec. 10, the lab emailed 10 pages of newly declassified documents from 1956. The material consists mainly of a correspondence between a top security official at Los Alamos and the lab’s branch of the Atomic Energy Commission, a federal agency that oversaw the weapons development site. The letters discussed an F.B.I. investigation of Mr. Seborer’s espionage but gave no specifics on what he may have delivered to Moscow. Instead, the exchange dwelled on the secrets available to him.

The documents include pages from a 1945 Los Alamos telephone directory as a way of confirming the suspect’s lab employment.

All three previously known Los Alamos spies told the Soviets of a secret bomb-detonation method known as implosion. The technique produced a bomb far more sophisticated than the crude one dropped on Hiroshima on Aug. 6, 1945. A prototype of the implosion device was tested successfully in the New Mexican desert in July 1945, and a bomb of similar design was dropped on Nagasaki weeks later, on Aug. 9. Four years later, the Soviets successfully tested an implosion device.

The early bombs relied on two kinds of metallic fuel, uranium and plutonium. The bomb that was dropped on Hiroshima worked by firing one cylinder of uranium fuel into a second one, to form a critical mass. Atoms then split apart in furious chain reactions, releasing huge bursts of energy.

In contrast, the implosion bomb started with a ball of plutonium surrounded by a large sphere of conventional explosives. By design, their detonation produced waves of pressure that were highly focused and concentrated. The waves crushed inward with such gargantuan force that the dense ball of plutonium metal was compressed into a much denser state, triggering the atomic blast.

The new documents show that Mr. Seborer worked at the heart of the implosion effort. The unit that employed him, known as X-5, devised the firing circuits for the bomb’s 32 detonators, which ringed the device. To lessen the odds of electrical failures, each detonator was fitted with not just one but two firing cables, bringing the total to 64. Each conveyed a stiff jolt of electricity.

Glen McDuff, a Los Alamos scientist, once characterized the bomb’s firing circuits as “unbelievably complicated.”

A major challenge for the wartime designers was that the 32 firings had to be nearly simultaneous. If not, the crushing wave of spherical compression would be uneven and the bomb a dud. According to an official Los Alamos history, the designers learned belatedly of the need for a high “degree of simultaneity.”

Possible clues of Mr. Seborer’s espionage lurk in declassified Russian archives, Mr. Wellerstein of the Stevens Institute of Technology said in an interview. The documents show that Soviet scientists “spent a lot of time looking into the detonator-circuitry issue,” he said, and include a firing-circuit diagram that appears to have derived from spying.

The diagram shows an implosion bomb. Several labels of the schematic diagram appear first in English, then Russian. In a 2012 analysis, Mr. Wellerstein described the order as “betraying their obvious roots in espionage.” The English labels include “Power Supply” and “Fusing Unit.” In a follow-up analysis, Mr. Wellerstein concluded that Igor Kurchatov, the head of the Soviet bomb project, drew the schematic for Lavrenty Beria, the head of Stalin’s secret police.

The Soviet diagram was dated June 1946, four months after Mr. Seborer left Los Alamos. It shows pairs of wires running from an electrical controller to detonators on the bomb’s exterior — a clear echo of the American reliance on redundant firing circuits.

“I don’t know if any of the other known spies would have had access” to the kind of secrets that Mr. Seborer worked on routinely at Los Alamos, Mr. Wellerstein said. The Americans spying for Moscow, he added, “might have filled a gap.”

The main appeal of implosion was that it drastically reduced the amount of bomb fuel needed. The dense metals were hard to obtain and far more valuable than gold. The Hiroshima and Nagasaki bombs were roughly equal in destructiveness — but fuel for the Nagasaki bomb weighed just 14 pounds — one-tenth the weight of the fuel for the Hiroshima bomb. The secret of implosion thus represented the future of atomic weaponry.

Slowly, nuclear experts say, bomb designers cut the plutonium fuel requirement from 14 pounds to about two pounds — a metal ball roughly the size of an orange. These tiny atomic bombs became enormously important in the Cold War, because their fiery blasts served as atomic matches to ignite the thermonuclear fuel of hydrogen bombs.

The atomic bomb dropped on Hiroshima killed some 80,000 people. But hydrogen bombs can pack 1,000 times the destructive energy — a terrifying fact of atomic life that generated widespread fear of mutual annihilation. A single SS-18 missile, the Cold War’s deadliest Soviet weapon — Western intelligence agencies called it Satan — could easily fire 10 or more hydrogen warheads halfway around the globe.

If the 1956 documents shed light on Mr. Seborer’s crime, they do little to explain why the United States kept the nature of his job and likely espionage secret for 64 years.

One possibility was domestic politics. Several atomic spy scandals shook the nation in the early 1950s, starting with the arrest of the first Los Alamos spy. His testimony led to the capture of the second, and to the execution of the Rosenbergs. The anti-Communist hysteria of the McCarthy era reached a fever pitch between 1950 and 1954. President Dwight Eisenhower, who had put himself above the fray, began to fight back with information leaks and administrative fiats.

Mr. Klehr of Emory said it was late 1955 when the F.B.I. first uncovered firm evidence that Mr. Seborer had been a Soviet spy, prompting the inquiry that led to the Los Alamos correspondence of Sept. 1956. A presidential campaign was then underway, and the last thing President Eisenhower needed was another spy scandal. The same held true in 1960, when Mr. Eisenhower’s vice president, Richard Nixon, fought John F. Kennedy for the White House.

“It’s entirely possible — let sleeping dogs lie,” said Mr. Klehr. “But you can’t prove it.”

A more likely reason for the silence, he added, was the flimsy nature of the evidence against Mr. Seborer. Nothing in publicly released F.B.I. files suggests that warrants had been obtained for wiretaps, a legal requirement. Nor did the bureau know the identity of the courier between Mr. Seborer and his Soviet controllers. Finally, the two prime suspects were in Russia, out of reach. It was doubtful that the lawmen “could even get an indictment,” Mr. Klehr said.

He noted, too, that much F.B.I. information about the Seborers had come from a hugely successful undercover operation known as Solo, which had infiltrated the American Communist Party in the 1950s and continued the monitoring as late as 1977. Most likely, the bureau wanted to do nothing that might risk revealing the identities of its informants.

Mr. Wellerstein of the Stevens Institute of Technology noted that the F.B.I. also remained silent in the case of the third Los Alamos spy, Theodore Hall. By the time his espionage was identified publicly, in 1995, he had moved to England, and Mr. Hall was never charged with betraying his country.

“Reflexively, they keep this stuff secret,” Mr. Wellerstein said of the bureau.

While the F.B.I. remains tight-lipped, the C.I.A. has shown considerable pride in helping publicly unmask the fourth spy at wartime Los Alamos, perhaps in part because more than seven decades have now passed since Mr. Seborer first entered the secretive site in the New Mexico wilderness.

On Jan. 24, the intelligence agency gave Mr. Klehr and Mr. Haynes an award at C.I.A. headquarters in Virginia for “an outstanding contribution to the literature of intelligence.” Mr. Klehr, 74, said he and his colleague were delighted with the official recognition of their work. Even so, he said the two men foresaw a need for more research, despite having stumbled on the Seborers seven years ago and having already done much to unveil the hidden drama.

“There’s still a lot to learn,” he said.

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