Trinity

Sixty years ago July 16th, the United States detonated the first atomic bomb in the test called "Trinity" in the New Mexico desert. It was the penultimate event of the Manhattan Project, which was to climax in the destruction of Hiroshima and Nagasaki and the end of the Second World War. Trinity began the atomic age.

The Trinity test was experimental confirmation of the implosion-type weapon design. In an implosion bomb, the fissionable core (plutonium, in this case) is compressed by simultaneous detonation of a surrounding sphere of high explosives. The explosives create a shock wave that moves inward at high velocity, squishing the subcritical softball-sized core into a supercritical golf ball sized core. In the supercritical core, the fissionable material chain-reacts explosively. The inward pressure of the implosion and a surrounding uranium tamper helps hold the core together a few fractions of a microsecond longer, prolonging the chain reaction and increasing the bomb yield.

The other atomic weapon design was the gun design. The gun design used a more-straightforward method of assembling a critical mass; it would merely shoot one subcritical mass (like a soda can-sized bullet) down a barrel into a second, ring-shaped subcritical mass. As the bullet met the rings, the united assembly became supercritical, would chain-react and explode. The Hiroshima bomb was the gun design, using uranium-235 as its fissionable material.

The scientists of the Manhattan Project had a high degree of confidence that the gun-type bomb would work. They had run numerous experiments to convince themselves of it. In one experiment, a smaller version of the U-235 bullet was literally dropped through U-235 rings to briefly create a barely-supercritical assembly from which confirming measurements could be made. This experiment had a peak power production of twenty megawatts! With the implosion design though, the scientists could not come up with an experiment that gave them the same degree of confidence that the bomb would really work. They did detonate one full-scale implosion assembly (sans plutonium) before Trinity, but their scientific instrumentation was insufficient to properly distinguish a perfect implosion from an asymmetrical one. In this case, only a full test would do.

Interestingly, the implosion design was for a long time an also-ran, second-tier concept. The notion of lensing explosives was very new and fraught with technical difficulties that the gun design didn't have to contend with. It was thought that the gun design would suffice for the U-235 bombs and the plutonium bombs alike, as measurements with U-235 and plutonium samples had confirmed. Only it didn't turn out that way, for the initial plutonium measurements were performed on a tiny speck of plutonium created in a cyclotron (particle accelerator). If you bombard a uranium target with neutrons for a month in your cyclotron, you can then process your sample chemically and extract a wee bit of plutonium. But there weren't enough cyclotrons in the world that could generate enough neutrons to make enough plutonium for a bomb. The way to mass-produce plutonium is not on the muzzle of a neutron gun; it's in a neutron oven, which is to say, a nuclear reactor. And that's why Hanford complex was built in Washington state. Of course it was a while before the reactors could be built, and operated, and uranium retrieved to be processed for plutonium extraction. And after all this effort and expense the atomic scientists measured the mass-produced plutonium and found that it would not work in the gun bomb.

How could this be? It turns out that when you bombard uranium with a low neutron flux (cyclotron), you get a nice "clean" plutonium (Pu) 239. But in the neutron-rich environment of a 250-megawatt reactor core, you produce a far higher proportion of Pu-240 isotope. Pu-240 has a higher rate of spontaneous fission, meaning that as the plutonium is sitting there it's throwing off a lot of neutrons. In the planned gun design, the plutonium bullet would be producing so many initiating neutrons that the triggered bomb would start an energetic chain reaction a moment too early, before the bullet had fully entered the target rings. The plutonium gun design would fizzle. While it was still technically possible to create a working plutonium gun bomb, the higher velocity necessary to get the bullet into the rings and avoid predetonation required a gun assembly too large to fit in any bomber aircraft. Thus, the only way to use all the plutonium that Hanford was producing for the project was to get it working in an implosion bomb. The parallel development track being spearheaded by George Kistiakowsky suddenly became very important.

These are but a smattering of the challenges that the Manhattan Project overcame on the way to Trinity. As daunting as the technical problems were, there was also the scale of the industrial base that had to be created to produce the nuclear ordnance. As mentioned, the Hanford complex was built to create plutonium. But the scientists of the Manhattan Project could not just assume in 1942 that any one type of fissionable material or any one bomb design would be successful. So they also conceived facilities dedicated to uranium processing. To create bomb-grade uranium requires meticulously separating uranium's lighter 235 isotope from the more-common U-238 (the ratio before enrichment is only 1:137). How would this be achieved? Again, at the outset the leaders of the Project could not say with any surety which industrial process would be successful in achieving isotope separation. So they built them all. A calutron separation facility was built. A gas diffusion facility was built. A thermal diffusion facility was built. As each facility began to operate (with varying degrees of efficiency), complex schedules were developed in which partially enriched uranium from one facility would be fed as an input into another facility for final enrichment. The industrial undertaking was so massive that in less than three years the Manhattan Project had built facilities that were comparable in expense and size to the entire U.S. automotive industry of that time. And the entire wartime production of this new, multi-billion dollar industrial base amounted to tens of kilograms of metal, in aggregate probably smaller than a single beach ball.

I do not expect to be writing a post in August on either anniversary of the bombings of Japan, but that should not be taken as dismissal of the hellish enormity of what the U.S. Air Force brought to pass on those two days. Yes, I have read entire books documenting this. But August 1945 did not come about out of the blue on a mad whim. It was bought and paid for in Manchuria, Nanking, Pearl Harbor, Bataan, the Coral Sea, Midway, Guadalcanal, Peleliu, Iwo, Okinawa, and a hundred other places.

I had lunch with two coworkers on Friday. Entirely by coincidence, both of these coworkers' fathers were WWII veterans, both having served in the Pacific theatre. Both also occupied postwar Japan, meaning that both men were slated to be part of the invasion of the mainland had the nuclear strikes not ended the war in August. Both coworkers expressed certainty that their fathers survived the war because of the atomic bomb.

Try to imagine it from the perspective of the soldier. One week you are stockpiling and training for Operation Downfall, the landings to rival Normandy, only this time onto the home soil that bred the kamikaze. The next week your commander tells you that a new superweapon, dreamed up by physicists, with ten thousand times the power of high explosives, has just been used on the enemy and they gave up. If Jesus Christ himself rolled up on skates to hit you with a pie, it would seem less fanciful. At that moment, how would the Bomb not seem like a miracle?