Marc Estrin

HOW TO MAKE AN ATOMIC BOMB. IN FACT, TWO.

August 8, 2005

As a Nagasaki Day entry, I want to tell you the story of the Manhattan Project, the grand scheme, as it were, the event terrain which has so affected our lives. Like all grand schemes, it began with an insight, and an insight about that insight.

The light—which grew to be “brighter than a thousand suns”—first dawned in Leo Szilard’s brain, in 1939, at a street crossing in London. If there were an element, he reasoned, which would fission when struck by a neutron, and which, in fissioning, would release more neutrons—then a “chain reaction” was possible.

Imagining such a chain, I always think back to the story (clearly apocryphal) my father told me when he was teaching me chess. The putative king of some foreign country was so pleased by his subject, the putative inventor of chess, that he offered him any reward he pleased. The wily strategist played humble, and said, “I have few desires. But if you would put a grain of wheat on the first square, and two on the second, and four on the third, and so on, I would much appreciate it.” “Nothing easier,” said the innumerate king, and ordered his steward to attend to it. Point implicitly made, the story deteriorates rapidly, as the computation of 264 is indicated, and the immoderacy of the consequences ensue: “more wheat than exists on earth,” “more wheat than stars in the sky”, and so forth—an accumulation even William Gates might turn down.

But in a chair reaction situation, rapid multiplication of neutrons is a fact, two producing four, four eight, until, in a rather small space, in microseconds of time, unthinkable neutron densities can occur. And when each of these neutrons is capable of splitting some special, neutron-sensitive atomic nuclei, a lot of action can ensue.

It turns out that when these extraordinary elements split, the masses of their products do not add up to the original mass, but come out slightly smaller. Enter Einstein, thirty-five years earlier. 1905, the famous E = mc2. Here is another profligacy of number, the speed of light being so great, and the squaring of that speed so extravagant. Numbers aside, Einstein’s equation says, simply, “even a very small amount of mass can be turned into a colossal amount of energy.” The trick is to do it.

In 1938 Otto Hahn and Fritz Strassman demonstrated that uranium had split into lighter atoms under neutron bombardment, and physicists ran back to their labs to bombard uranium with everything they could throw at it. In 1939 Lise Meitner and Otto Frisch figured out an explanation of the process, which they named “nuclear fission”. It seemed uranium might provide a real-world demo for Einstein’s equation. In 1940, Glenn Seaborg detected something strange while bombarding uranium in his Berkeley cyclotron. The unexpected guest was not a lighter fission product, but a heavier new element, never before seen on earth, plutonium, the result of neutron capture—without fission—by uranium. It turned out that plutonium, too, was able to fission under neutron bombardment.

Two heavy elements – uranium and plutonium—both capable of splitting under neutron bombardment, and releasing excess neutrons and Einsteinian energy. One might think uranium the preferable beast, since it was relatively plentiful. But it turned out that the actually performing uranium was U235 —a “contaminant”, 0.7%, of U238 metal from the mines. Because they were isotopes of the same element, the two could not be separated chemically. Because they were so similar in weight, it was only with great difficulty that they could be separated physically. And without separation, the mix was pretty bland, explosion-wise. But the path was clear: if U235 could be isolated and stockpiled, or if enough Pu239 could be produced, either of them might be used to tickle the Einstein equation to climax.

Szilard had one other key insight, (one wonders if there is something in Hungarian water)—the notion of critical mass. The reason all the uranium on the planet does not set itself off and explode is that it is too dilute: should a stray neutron split a uranium atom the secondary neutrons produced would fizzle out before they could enter and split another atom: no chain reaction would occur. Same with plutonium. But—what if those atoms were concentrated—brought together in a small space? Then, secondary neutrons might penetrate their close neighbors, and tertiary neutrons theirs, and the chain would begin to rattle. Szilard called this “critical mass”. How much uranium, how much plutonium would you have to pack into how much space for critical mass? That was unknown.

Enrico Fermi decided to find out. In 1942, there was only unseparated uranium to play with, and that is what he did. Where do you play? In a stadium. In a squash court under the west stands at Stagg Field at the University of Chicago. On December 2, 1942 at three in the afternoon, the consequences of piling up enough uranium in one place became clear: embedded in 771,000 pounds of graphite, piled brick by brick, 80,590 pounds of uranium oxide and 12,400 pounds of uranium metal “went critical”, and began to produce a potentially enormous flux of neutrons—till Fermi dropped the neutron-absorbing control rods into the pile. The wily Italian, using Szilard’s original notions, had invented a nuclear reactor. The afternoon did more than demonstrate the truth of theory: one of the byproducts of pile reaction was plutonium. Fermi had also invented a “breeder”. On December 2, 1942, the nuclear age had begun. Today, a Henry Moore sculpture sits commemoratively on the site. Could Fermi have blown up the entire city of Chicago? Possible, but unlikely—according to his 6” slide rule. Since word was that “Fermi never makes a mistake,” breath-holding physicists hung out on the balcony and watched. And Chicago didn’t know anything about it.

Neutron sources, nuclear fission, uranium, plutonium, the idea of critical mass. Everything material and intellectual was in place to make a bomb. Still needed was the catalyst of will.

It is no accident that European scientists were the first to imagine that Germany had already begun developing a bomb. German thought, German art, German philosophy, German science monopolized peak after peak in their cultural heritage. Consider: the atomic bomb is the ironic legacy of Beethoven. And Goethe, Kant, Schopenhauer, Einstein and Mann. If German Geist could consistently produce such giants of the mind, then surely Germany—even Nazi Germany—must be well on her way to exploiting original discoveries made by German scientists on German soil. That America must play catch-up to an already advanced German bomb project was the most universal, most powerful assumption, practically a certainty, the catalyst for the gargantuan striving of the Manhattan Project. No matter that many top Jewish scientists had been exiled. Planck was there. Von Laue was there, Hahn was there, Weiszäcker was there—Heisenberg was there. All you needed was one Heisenberg.

The Europeans were motivated, but Roosevelt had been sitting on Uranium Committee reports for almost a year. The British, however, their senses sharpened by a rain of German bombs, were not so nonchalent. In July of ‘41, a group of British scientists visited the States to try to provoke more urgent activity. Important research had been done for a year, both on the design of a weapon, and on the separation of uranium isotopes. But with bombs falling, and the country stretched to its limits, Britain could simply not move on to the massive manufacturing necessary to bring about a weapon before the end of the war. Persuaded by some high-level scientific politicking, on October 9, 1941, FDR agreed to put all possible resources into the expeditious development of a nuclear bomb. The Manhattan Project was born.

This was a double race, both with Germany, and between the two known fissionable metals. Which would be ready faster—a uranium bomb or a plutonium one? There was no telling, so both would be simultaneously pursued. Two huge secret cities were created, employing tens of thousands of workers, none of whom had any idea what they were working on. Oak Ridge, Tennessee was charged with separating out U235, and Hanford, Washington with the creation of plutonium. These astonishing industrial operations were to feed their products into the brain of the operation—Site Y at Los Alamos. There, on the New Mexican mesa, physicists, chemists, metallurgists and ordnance experts worked together to design and produce a weapon that could be carried in an aircraft, and, when dropped, might actually explode.

Once the theoreticians had determined the likely critical masses involved, the initial design plans were fairly straightforward. A large gun would be built inside a bomb case. Using well-understood engineering, a sub-critical bullet of uranium or plutonium would be fired into a sub-critical target of same, and the two, coming quickly together, would form a critical mass, a neutron flood, a consequent explosion. The only problem was that, for almost a year, there was not enough U235 to work with, and almost no plutonium at all. Oak Ridge’s progress was slow—uranium would probably work well, but not enough separated U235 could be produced out to make more than one weapon during the likely duration of the war. Hanford began at essentially zero—by the fall of ‘43, there were only milligram quantities of Pu available: all the plutonium on earth could be placed on the head of a pin. Still, as the reactor technology pioneered by Fermi matured, it looked as if quantity production of plutonium would be less problematical in the long run than uranium separation. The health risks were larger, as we will see, but plutonium seemed the more likely basis for a substantial nuclear arsenal.

All great enterprises have their crises. In July of ‘44, there were finally gram quantities of plutonium to work with, and it was discovered by Emilio Segrè that the metal available from the reactors was actually a mixture of isotopes. One of them, Pu240, was an alpha emitter, and source of “background” neutrons, creating a million and a half spontaneous fissions each hour. With such pollution, the gun design would simply fail: that thin hail of unwanted neutrons would condemn any plutonium gun to fizzling predetonation. Separating plutonium isotopes would be even more difficult—and much more dangerous—than separating uranium. That tack was out of the question. The race seemed lost, the project hopeless.

Oppenheimer was devastated, and considered resigning as Lab Director. But Seth Neddermeyer came to the rescue, and was able to salvage the plutonium bomb with a new design. The “implosion bomb” replaced the gun model: only implosion could develop critical mass quickly enough to avoid predetonation. It was Neddermeyer’s implosion idea that was tested at Trinity in July of ‘45. It was his implosion idea that destroyed Nagasaki in August.

Implosion design was far trickier than the familiar ballistics of cannons. A ball of sub-critical plutonium had to be compressed in microseconds, absolutely symmetrically, in order not to fizzle. (Fizzling, by the way, meant exploding with the force of five or six tons of TNT, rather than fifty or a hundred.) Achieving this almost instantaneous spherical compression provided the drama of the last phase of the work. Explosive lenses had to be developed, shaped charges of various materials which would focus the otherwise unpredictable shock waves. A tampering container was needed to reflect escaping neutrons back into the fission. A timing circuit and detonating system had to be invented which could trigger the jacket of lenses with fantastic accuracy so that pressure waves from all directions would converge in step towards their target. An internal initiator was needed at the center of the plutonium core to provide an exquisitely timed burst of neutrons to initiate the process. And finally, diagnostics had to be developed, ways of measuring all these processes—wave profiles, neutron flux, materials stress: super-high speed cameras, new x-ray devices, magnetic sensors, radiation detectors—all this without being able to test the crucial materials in full-scale assembly.

There was much doubt that such an intricate scheme could succeed; there was far more confidence in the uranium gun design. Since not enough U235 existed for both test and bomb, the uranium bomb was not tested until Hiroshima. It worked—as expected. Not so with with implosion bomb. After much debate, it was decided to gamble more than half of the world’s existing plutonium to run a test of the real implosion device. In May of ‘44, site selection began, and in August the Trinity site was chosen.

We know the result. Since it was by then known that Hitler had no bomb and thus the raison d’etre for the project no longer existed, there was still a chance to avoid using the bomb against human targets. Szilard and others fought hard to stop the work. But the reasoning in my last entry prevailed, and here we are, where we are, today. And so very much the worse for us.