Ever hear of Elugelab? Until Oct. 31, 1952, it was an island on Eniwetok Atoll in the Pacific Ocean. Then it vanished, consumed in the fireball of the world’s first hydrogen bomb.

The 82-ton device, nicknamed “the Sausage,” created a radioactive mushroom cloud 60 miles across and a crater over a mile wide. At more than 10 megatons, the blast exceeded the power of all the high explosives detonated in both world wars.

The test, code-named “Ivy Mike,” introduced the world to thermonuclear bombs, two-stage weapons that use a fission bomb to compress and heat a fusion fuel, like deuterium. The resulting amplification of explosive power, in theory, is only constrained by the size of the device, and has nearly infinite destructive force.

The 1952 test also intensified the Cold War, starting Soviet scientists on a race to find a similar super bomb. On Nov. 22, 1955, the Soviet Union exploded its own thermonuclear device.

For historians, there’s the rub. How did Moscow acquire the secret of the hydrogen bomb? A new book — “The Nuclear Express” (Zenith, 2009) — purports to have the answer: an unnamed Soviet agent, code-named PERSEUS, working at Los Alamos.

The authors of the book — former nuclear weapons designer Thomas C. Reed and Los Alamos physicist Danny B. Stillman — argue that the secret behind a thermonuclear bomb could have been conveyed in a single phrase: “radiation implosion.”

Those two words are the heart of the breakthrough that Edward Teller and Stanislaw M. Ulam secretly published at Los Alamos in March 1951.

The idea of a spy as the source of the H-bomb secret is intriguing enough that a panel of experts met at Harvard last week (May 14) to discuss it.

The gathering, in a basement room of the Knafel Building on Cambridge Street, included two historians and a physicist from Harvard. It was the last Sakharov event of the academic year, in a series — the Sakharov Seminar on Human Rights — sponsored by the Davis Center for Russian and Eurasian Studies. The event was co-sponsored by the Cold War Studies Seminar.

Joining the panel via videoconferencing was Stanford University military historian David Holloway, author of “Stalin and the Bomb.”

Davis Center senior fellow Mark Kramer, who directs Harvard’s Cold War Studies Project, was immediately skeptical of the claim that the H-bomb secret came from a Soviet mole at Los Alamos.

“The Nuclear Express” is not footnoted, he said, and its authors give “little indication of how they came to their conclusion.”

Holloway was equally skeptical. But he said the book again raises a question much-discussed and so far unanswered: How did the Soviets arrive at their own version of the breakthrough Teller-Ulam idea?

Holloway offered four hypotheses:

Soviet physicists were tipped off by an isotopic analysis of radioactive debris from the “Ivy Mike” test. (Not true.)

The Soviets, including physicist and future peace activist Andrei Sakharov, did it themselves. (Persuasive.)

Intelligence slipped to the Soviets in the late 1940s by atomic spy Klaus Fuchs — though not a workable hydrogen bomb design — contributed in some way to inspiring a Teller-Ulam configuration. (Possible.)

Or the Soviet breakthrough — as argued in “The Nuclear Express” — was provided by a spy. (No real evidence.)

Richard Wilson, Harvard’s Mallin-ckrodt Professor of Physics Emeritus, recalled that his friend Sakharov “clearly said he was not sure” about the origin of the Soviet version of the H-bomb, “but he thought it was independent.”

And Sakharov was of the opinion, Wilson said, that “no information came from the United States.”

Atom historian Priscilla McMillan, A.M. ’53, a Davis Center associate and author of “The Ruin of J. Robert Oppenheimer,” said Sakharov “came close to claiming credit” for the Soviet H-bomb, but he honored to his death the secrecy he swore to in the 1940s.

“The big secret was himself,” she said, but it is likely that Sakharov was the one who thought of Soviet-style radiation implosion.

Fuchs as a factor was unlikely, said McMillan, since his theoretical outline lacked the idea of compression — the “bomb in a box” — required for a thermonuclear weapon.

Another factor comes into play, said Wilson, a 60-year radiation scientist who joined the Harvard faculty in 1955: the way scientists work. “They often forget where their ideas came from,” he said — and can rarely keep a secret anyway.

Wilson speculated that even limited information, from Soviet intelligence sources or other means, could have been enough to trigger the Soviet’s own H-bomb breakthrough. Or perhaps it was stimulus enough, he said, for the Soviet Union to know that the other side already had the answer. (To that, Holloway agreed.)

Kramer, the Cold War historian, thinks the answer to the puzzle might lie in Holloway’s second hypothesis — that the Soviets did it themselves — combined with an analysis of fallout from the U.S. “Castle Bravo” H-Bomb test.

“Bravo” was exploded on March 1, 1954, on Bikini Atoll in the Pacific. A miscalculation of the fusion reaction made it a much bigger event than expected — 15 megatons, the largest U.S. nuclear device ever touched off. In seconds, the fireball bloomed to 3 miles in diameter. In hours, debris showered on Marshall Islanders 100 miles away.

As for evidence that a spy was the key to a Soviet H-Bomb, said Kramer: There is none.