1. Teller's triumph?
2. Gone fission
3. Credit - or blame?
Japan surrendered shortly after this A-bomb obliterated
Nagasaki on Aug. 9, 1945. On Aug. 6, Hiroshima was the first city
destroyed by an atomic weapon. By then, Edward Teller had been thinking
about the vastly more powerful H-bomb for four years.
To understand the difficulty of inventing the
H-bomb, take a mental journey to 1945. Atomic ("fission") bombs have
just closed World War II with a double-bang. For more than two years,
Los Alamos, New Mexico has been world headquarters for physicists,
home to the intense intellectual debates and frantic engineering needed
to build the atomic bomb.
the exhilaration at Los Alamos is soured by the realization that
the "project" has killed more than 100,000 people and forever tipped
the military balance toward offense. A week after Nagasaki, Manhattan
Project leader J. Robert Oppenheimer is "guilty, weary and depressed,
wondering if the dead at Hiroshima and Nagasaki were not luckier
than the survivors, whose exposure to the bombs would have lifetime
effects," wrote Richard Rhodes in his history of the hydrogen bomb.
The leaders of the bomb project write that "the safety of this nation...
cannot lie wholly or even primarily in its scientific or technical
prowess. It can only be based on making future wars impossible"
(see p. 203, "Dark Sun... " in the bibliography.
While Los Alamos continues working on the
atomic bomb, and takes faltering steps toward a hydrogen bomb, many
physicists, satisfied that their goal is met and uncertain about
building a hydrogen bomb based on the vastly greater energy of fusion,
return to their universities.
Nuclear energy comes from two
Fission ("splitting") occurs
when the nucleus of large, unstable atoms, like uranium and plutonium,
break into smaller atoms, releasing energetic radiation and neutrons.
Fission powers the "atomic" bomb that destroyed Hiroshima, and all nuclear power reactors.
Fusion ("joining") occurs when
light atoms, primarily isotopes of hydrogen, fuse into larger atoms,
releasing fantastic quantities of energy. Fusion powers the sun
and "hydrogen" bombs, which are called "thermonuclear" for the intense
heat needed to overcome electrical repulsion between positively-charged
hydrogen nuclei. Fusion, however, is extremely difficult to control;
although billions have been spent to tame fusion for electricity,
practical reactors are decades away.
In bombs, the two forms of nuclear energy are
often blended. Most fission bombs are "boosted" with fusion fuel.
All fusion bombs are triggered by fission bombs, and most contain
a second fission bomb, called a "spark plug."
In 1946, Edward Teller departs for the University
of Chicago. One of several brilliant Hungarian refugees who contributed
mightily to the atomic bomb, Teller had earned his Ph.D. under the
eminent German physicist Werner Heisenberg and moved to the United
States in 1933, after the Nazis took power in Germany.
Teller's interest in the hydrogen bomb dated to 1941, when Italian physicist Enrico Fermi floated the idea. Although the fusion bomb (called the 'super') got some attention, leaders of the Manhattan Project decided to defer the difficult challenge of fusion until they had learned to make a fission weapon from uranium and plutonium. (For one thing, fission could be tested in the laboratory, while fusion requires conditions more like the center of the sun than the top of a New Mexican mesa. And the physicists understood that a fusion bomb would need a fission trigger.)
While the Manhattan Project tried to tackle first things first, Teller dwelled on fusing two isotopes of hydrogen --deuterium and tritium. (Isotopes are atoms that are chemically identical but have a different neutron count and different masses.) "Even during the war he was troublesome," says physicist Herbert York. "He wanted to work on fusion, but the job was fission, and he quit in a huff several times."
Under earthly conditions, electrical repulsion
prevents hydrogen nuclei from fusing. In the sun, however, enormous
gravitation squeezes hydrogen nuclei until they fuse into helium.
Gobs of energy are released when a bit of their mass is converted
to energy according to Einstein's famous equation, E=MC2
(Energy equals mass times the speed of light, squared).
The Apache H-bomb test, July, 8, 1956 on Eniwetok
atoll. In 1963, health concerns about radioactive fallout led to
a ban on atmospheric testing. Photo:
Scientists had known since the 1930s about fusion in the sun, but fusion refused to be "tamed" into a bomb on Earth, and even after the war, fusion was not the focus at Los Alamos. Then the Cold War intensified: The 1948 Berlin blockade, the 1949 Soviet atomic bomb test, and the start of the Korean war in 1950 created new political realities. In 1950, President Harry Truman made the H-bomb a national priority.
In June, 1950, however a long series of calculations proved that Teller's super design would fail. The calculations, made in those pre-computer days with slide rules and mechanical calculators, were incredibly complex, says Carey Sublette, author of Nuclear Weapons Frequently Asked Questions and operator of the Nuclear Weapon Archive website. "There are a lot of processes involved that could pull the outcome in different directions, and all are significant, so you can't simplify the problem by assuming things away, as you frequently can do in science .... Very complicated computations were needed to chart what was going to happen."
The answer, delivered by mathematician Stanislaw Ulam, was that the fusion fuel would either not start fusing, or the fission trigger would blow the fuel apart too soon. Then, in January, 1951, Ulam, who had immersed himself in matters thermonuclear, suggested using energy from the fission bomb to compress, not heat, the fusion fuel.
Although Teller had long argued "compression does not matter," Ulam realized, according to Rhodes, that "Compression works in thermonuclear fuels in much the same way it works in fission fuels, squeezing nuclei closer together and therefore improving their chances of interacting" (Dark Sun, p. 464). Compression also makes the fuel easier to heat with thermal radiation and slower to cool, Sublette adds.
Although Teller soon scented success, shock from the atomic bomb might not compress the fuel evenly, and the secondary might still be destroyed too soon. So Teller built on Ulam's idea by suggesting that the compression could come from thermal X-rays from the primary bomb, not the shock wave.
In the Teller-Ulam hydrogen bomb design,
high explosive compresses fission fuel in the "primary" stage. Intense
X-rays from the atomic explosion move through the radiation channel,
vaporizing the uranium pusher-tamper, which acts like an inside-out
rocket, compressing the fusion fuel and spark plug to extreme density.
The spark plug becomes a second fission bomb, heating the fusion
fuel and igniting the fusion reaction. The uranium shield protects
the secondary from destruction while fusion occurs; the explosion
is over in microseconds. Diagram:
Adapted from Carey Sublette, Nuclear Weapon Archive
The different was subtle, but critical. Radiation moves at the speed of light, much faster than a shock wave, and radiation can be directed to compress the fuel evenly from all directions so quickly that fusion can occur before the shock wave destroys the secondary.
Teller then contributed another idea: placing a "spark plug" of uranium or plutonium in the center of the fusion stage. The spark plug, compressed by the radiation implosion, would fission, heating and igniting fusion in the already compressed fusion fuel. The result, the "Teller-Ulam" design for a thermonuclear weapon, remains the standard design 50 years after it was invented.
The stage was set for the monster, Mike. On Nov. 1, 1952, the world's first hydrogen bomb created a mushroom cloud 100 miles across and proved what physicists suspected - that while there was an upper limit to the size of fission bombs, hydrogen bombs could be made as big as you wished.
Teller? Ulam? Who invented the "Teller-Ulam" design?