Hey Doc – what can you tell me about the Trinity test?
Wow – the shortest questions can unlock the longest answers! Depending on how far back you want to go we could wrap this up in a paragraph, or in a book or two. Let’s try for something intermediate, shall we?
Let’s start with the fact that there were two different designs used in the two bombs dropped over Japan; the first weapon, dropped over Hiroshima, was a gun-type design that was considered to be so fool-proof that there was no need to test it. It’s the second weapon – the Nagasaki bomb – that was tested in advance because, even so late in the Manhattan Project and just 3 ½ weeks before it was to be used, there were still doubts that the implosion design it used would work. On top of that, the bomb was going to fission plutonium, the first synthetic element to be produced in large quantities, and which was produced in the core of a nuclear reactor, another new invention. Virtually everything about the bomb took the designers into new territory – it’s understandable that the designers thought a test was necessary.
Physicists knew that the lighter isotope of uranium, U-235, would fission and they’d calculated that plutonium (specifically, Pu-239) would fission just as readily, releasing as much energy as U-235. What made plutonium look attractive was that Pu-239 is produced when U-238 captures a neutron and decays to form plutonium; since U-238 constitutes over 99% of the uranium atoms on Earth (compared to only 0.72% for U-235) the vast majority of uranium atoms could be made into fuel for a nuclear weapon. And since plutonium and uranium are different chemical elements, separating plutonium from spent reactor fuel would be a lot easier than trying to pluck out enough of the occasional atoms of U-235 to make a weapon.
In a remarkably short time – just a few years – the scientists and engineers of the Manhattan Project designed and built the world’s first plutonium production reactor, learned enough about plutonium chemistry to figure out how to extract plutonium from dangerously radioactive spent reactor fuel, learned enough about radiation safety to do so without putting themselves at undue risk, as well as learning enough about the metallurgy of the brand-new element to start to work it into the core of a nuclear weapon. They did all of that in less time than it would take simply to do the paperwork to apply for a radioactive materials license to build a new reactor today. But I digress….
Another bit of necessary work involved finding out exactly how fissionable this brand-new element might be, and what they found was a bit sobering. Plutonium-239 certainly fissioned well, but they found a heavier isotope, Pu-240 that not only fissioned under a neutron flux, but it would fission spontaneously as well. Spontaneous fission is not uncommon – one example is U-238, where it’s used to help date rocks and minerals and has been used to help date the rate at which the Alps are rising. In this case, though, the same neutron capture that produced U-239 (which decayed through Np-239 to form Pu-239) could lead to the absorption of a second neutron, leading to Pu-240. Reducing the amount of time the fuel “cooked” in the reactor would reduce the amount of Pu-240 that formed, but it also reduced the amount of Pu-239 that was created.
The mechanics of the uranium bomb were simple – a subcritical mass of U-235 was shot down a tube into another subcritical mass of U-235; when they came together they formed a supercritical mass that fissioned, releasing tremendous amounts of energy quickly enough to cause a huge explosion. The simplicity of this design is why it wasn’t thought necessary to test.
What scientists quickly realized when they started looking at what’s called the “fission cross-section” (the likelihood that a given atom will fission) for Pu-239 was that in a gun-type device the neutrons produced from the spontaneous fission of Pu-240 would induce enough fissions in the Pu-239 to cause the bomb to detonate prematurely, before the halves were close enough to get a full nuclear yield. This would blow the weapon apart, scattering plutonium around the countryside but the yield would be nowhere near what they were hoping for. In other words, the simple and foolproof gun-type design wouldn’t work for plutonium.
The weapon designers were stumped until hitting on the idea of implosion – using explosive shock waves to squeeze the plutonium into criticality. But that would require focusing the shock waves from multiple charges of high explosives into a spherically symmetric shock wave, hitting from all sides simultaneously. This required developing explosive “lenses” to focus the shock waves as well as ultra-precise timing to make sure every charge went off at exactly the same instant. They wanted to test this design because the implosion weapon required so many novel and untested features and mechanisms to make it work properly; even a single failure or error would keep the weapon from functioning as intended.
There’s a lot about the Trinity test that’s fairly well-known: Fermi’s dropping scraps of paper at the right moment to get an idea of the weapon’s yield; Feynman’s looking at the detonation through the windshield of a parked truck; Teller’s last-minute concern that the heat and pressure might ignite the atmosphere; Oppenheimer’s quotation of Hindu scripture following the detonation; the radioactivity being detected as far away as Rochester NY, and more. These are all parts of the story, but there’s more that isn’t as widely known.
Preparing for the test was an arduous process – they needed to construct and pave roads to the site as well as string miles and miles of wires and cables for power, lighting, and instrumentation. The” Gadget” (as the device was nicknamed) was to be set off above-ground, requiring a tower to be constructed and lifting equipment installed. To help to accurately measure the device’s yield, the scientists decided to set off a test explosion using 100 tons of TNT about two months before the Trinity test. And as a tracer, a solution into which 1000 curies of beta-emitters and 400 curies of gamma-emitters had been dissolved was used to fill plastic tubes that were wound through the piles of TNT-filled boxes (although, I have to admit, I’m not sure what they were used to “trace”).
Concerns that the plutonium bomb would fail were sufficiently high that General Lesley Groves, the military head of the Manhattan Project, was worried that a “fizzle” could spread valuable plutonium around the desert where it would be lost. To guard against that Groves ordered the Los Alamos laboratory to build a mammoth 200-ton steel cylinder, inside of which the device would be detonated. “Jumbo” (as the cylinder was named) was 10 feet in diameter and 25 feet long; its 14-inch walls were so strong that it survived the nearby nuclear explosion a few days later and, when Groves later tried to destroy it with multiple 500-pound bombs, all that happened was that the ends were blown off. Jumbo is still in the desert.
Working up to the test itself was a bit nerve-wracking; just a few days before the test itself, in a dress rehearsal lacking only the nuclear materials, the explosives…did nothing. And then the same lack of “boom” followed a test of the firing unit. Luckily, both of these were simply due to worn-out test equipment due to heavy use in the weeks leading up to the test detonation; the new gear used for the test itself worked flawlessly a few days later.
The day of the test was also a bit nerve-wracking; it began with heavy wind and rain that threatened to postpone the test for days, at a time when the pressure to have a successful test was mounting. Luckily the storm passed through and, at 5:29 AM local time on July 16 the device detonated, releasing nearly as much energy as 25,000 tons of TNT in the tiniest fraction of a second.
The 100’ tower from which the device was suspended was almost completely vaporized and, over a radius of 300 m the heat fused the desert sand into a pale-green glass (I have a few pieces of this “trinitite” that are very slightly radioactive). The explosion also created a crater about 1.4 m deep and 80 m in diameter and the mushroom cloud eventually reached a height of 7 ½ miles.
On August 7 the untested uranium bomb was dropped over Hiroshima, working as perfectly as expected. Two days later a twin to the Gadget detonated over Nagasaki. The two weapons together caused more than 150,000 fatalities; on August 15 the Japanese surrendered, ending the war.