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Why did the United States choose to use an untested uranium-fueled (Little Boy) bomb at Hiroshima instead of a plutonium-fueled (Fat Man) bomb, which had been successfully tested at Trinity? Surely the components for additional plutonium bombs were available, as evidenced by events at Nagasaki a few days later.

I understand that the Little Boy design is far simpler than the implosion-style Fat Man design, so that prior to testing, it would have been the safer bet to actually go off as intended. But once the Trinity test had been completed, why was the Little Boy still chosen for the first combat use?

Edited to add: The responses below have addressed the question of why the U.S. had faith that the uranium bomb would work. But I don't think they've addressed the question of whether there was some reason to prefer the uranium bomb over the plutonium bomb for that first use, and I'll be glad if anyone has some insight on this.

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    I suspect that it was simply that the Little Boy bomb was completed first and was, therefore, available to be deployed first.
    – Steve Bird
    Feb 21, 2023 at 23:32
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    Was there not also an issue about weight? Did not the early prototype weigh 10 tons? The story I've heard is that since in 1945 the only aircraft that could take a ten-ton bomb load was a Lancaster, and as the USAF didn't have any Lancasters, they would have had to sub the operation to the RAF. And given that a large part of the team at Los Alamos was made up of the so-called "Tube Alloys" team from Cambridge who were moved lock-stock-and-barrel to North America in 1942, heaven forbid it should have been seen more as a British operation than an American!
    – WS2
    Feb 22, 2023 at 14:27
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    @WS2 the original Thin Man gun design (U235) was too big to fit in anything but a Lancaster. Little Boy was much smaller and lighter because someone realized that instead of an off the shelf-ish gun design, one whose barrel only needed to fire once could be made significantly shorter and thinner walled than one that needed to last thousands of shots before being serviced. Little Boy was able to fit into a B29 without major difficulty. Feb 22, 2023 at 16:09
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    Given what you seem to be really asking, I think a clearer title would be "Why was an untested uranium bomb dropped on Hiroshima?" Feb 22, 2023 at 18:24

1 Answer 1

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Details below on why a U-235 bomb was used at all. As to why that was the first bomb; because it was ready first and departed the continental U.S. only hours after the Trinity test for Fat Man.

The target and bomb pre-assemblies (partly assembled bombs without the fissile components) left Hunters Point Naval Shipyard, California, on 16 July aboard the heavy cruiser USS Indianapolis, arriving on 26 July. The target inserts followed by air on 30 July.

The Fat Man bomb assemblies (yes, plural) began arriving on Tinian only 12 days later, July 28:

The first plutonium core was transported with its polonium-beryllium modulated neutron initiator in the custody of Project Alberta courier Raemer Schreiber in a magnesium field carrying case designed for the purpose by Philip Morrison. Magnesium was chosen because it does not act as a tamper. It left Kirtland Army Air Field on a C-54 transport aircraft of the 509th Composite Group's 320th Troop Carrier Squadron on 26 July and arrived at North Field on Tinian on 28 July. Three Fat Man high-explosive pre-assemblies (designated F31, F32, and F33) were picked up at Kirtland on 28 July by three B-29s: Luke the Spook and Laggin' Dragon from the 509th Composite Group's 393d Bombardment Squadron, and another from the 216th Army Air Forces Base Unit. The cores were transported to North Field, arriving on 2 August, when F31 was partly disassembled in order to check all its components. F33 was expended near Tinian during a final rehearsal on 8 August. F32 presumably would have been used for a third attack or its rehearsal.


In truth, the uranium bomb was both so straightforward in design and elegant in function that, once the necessary calculations had been made and checked, not even one person at Los Alamos thought it necessary to test it.

Additionally, as of early summer 1945, it was still taking about 6 months to enrich enough U-235 for a bomb. That calculus wouldn't change until an improved enrichment technique was proven in mid-September.

The design of the Fat Man plutonium bomb, on the other hand, was so complex and fraught that many were uncertain, even on the morning of the Trinity test, whether it would work. There was a pool amongst the scientists on the test result, with bets ranging from it being an embarrassing fizzle to sinking into the Earth's core. These bets were certainly, in part, tongue-in-cheek: but the uncertainty about the design was real.

Note from my link above that, at least in part, the point of dropping the Nagasaki bomb was to announce, loud and clear to every knowledgeable person in the World (of which there were a great many): "We solved the uranium enrichment problem. We can now drop these indefinitely about every 7 to 10 days." The Japanese heard that loud and clear.


To respond to some of the comments below: it is an error to think that, because both bombs use fissile fuel, that they were "basically similar". That is the exact opposite of the truth.

Due to the very different spontaneous fission rate of the uranium and plutonium fuel, the Los Alamos physicists realized early on that a plutonium bomb required a vastly different, and more complex, mechanism than the uranium bomb.

Little Boy, the uranium bomb at Hiroshima, was essentially a fancy gun barrel that slid down a track onto a fixed bullet. Other than fixing the bullet and firing the barrel, its a firearm design is fundamentally the same as every firearm of the past half millennium. Provided that an appropriate barrel speed is reached and maintained as the barrel approaches the bullet, the bomb will go off. Minor errors in calculation, or speed, simply modify yield as the fuel—uranium-235—is forgiving in its spontaneous fission rate.

Fat Man, the Nagasaki bomb, uses plutonium fuel. The physics is (early) graduate level, so I won't go into details here: but the spontaneous fission rate of plutonium (specifically, the Pu-240 contaminant) is significantly greater than for enriched uranium. This means that the time period for assembling the fissile fuel must be much much less than for Little Boy. This required an implosion device, where several (16?) spherical sectors are fired inwards simultaneously, to assemble perfectly into a sphere in something like a microsecond. (Possibly even less; I haven't read up on the fine details in some years.) Minor errors in timing or assembling results in the fuel melting before it can go critical, and not reaching critical mass at all. One is left not with the debris of an exploded bomb, but simply a blob of melted plutonium and other bomb materials.

Finally, it's important that recall that a uranium bomb could have been ready in 1944, except that the uranium enrichment process of the time was so slow and inefficient. We should be glad of that - it being the hurdle that both Japan and Germany were unable to overcome in their bomb programs. The importance of even relatively tiny amounts of enriched uranium is why, at the end of the war, Germany tried (albeit unsuccessfully) to get its small supply to Japan by submarine, in hope that it might speed development of a Japanese bomb.

Page 66,67-68 Chapter Two - The Fission Bomb had to Come First (PDF):

Caltech physicist Richard Tolman suggested implosion as early as 1942, but the implosion method ... constituted an extremely complicated shockwave phenomena.
...
Generally an implosion device works in the following way: A subcritical core ... is surrounded by a shell of high explosives -- part of a lens structure that focuses the blast into a converging inward moving front. Electrical charges detonate the explosives nearly simultaneously, so the resulting blast is relatively symmetric, causing an even implosion of the core and compression of the fuel. Due to this compression, the core becomes super critical, and begins to expand outward, causing an explosion.
Modelling these process provided not merely a challenge, but in the summer of 1944 no one knew if an implosion would work at all.

Page 67 Chapter Two - The Fission Bomb had to Come First (PDF):

In the spring and summer of 1944, Emilio Segre's experimental physics group realized that spontaneous fission in Pu240 made the plutonium gun idea unworkable; it would not be fast enough to tolerate the added neutrons. ... A uranium bomb could be made by the summer of 1945, but probably only one. Thus the Laboratory turned to implosion as the only practical means of utilizing the plutonium available in the summer of 1944.

Example of how the scientists and engineers, not the bureaucrats, ran Los Alamos Page 70:

... the army recruited several high school graduates [SED] from all over the U.S. and sent them to Los Alamos. ... [k]nowing nothing about the purpose of the project .... One cycle took about three months to complete until Feynman obtained permission from Oppenheimer to inform SED's about the purpose of the project. Excited about fighting a war, the SED's quickly invented their own programs to speed the effort, and completed nine problems in three months.

Some additional bomb details, with thanks to Vorbis for digging them up:

  1. The purpose of the conventional explosives was to collapse it from the less dense delta phase to the more compact - but still solid - alpha phase (allotropes of plutonium are quite weird! nuclearweaponarchive.org/Nwfaq/Nfaq4-1.html)
  2. The conventional explosive shell was divided in 32 sections, as a truncated icosahedron (see here: en.wikipedia.org/wiki/Truncated_icosahedron#Applications )

From the first: my imagery above of the Fat Man bomb construction details, and of the details of a fission fizzle, may be off slightly.

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    The US judged -- correctly -- that dropping a single bomb would not cause surrender and would, in fact, convince the Japanese high command (the only people who counted) that the US only had the one bomb. We are all very, very lucky that this bluff with a second bomb -- that the US had many bombs -- was not called.
    – Mark Olson
    Feb 22, 2023 at 1:56
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    @MarkOlson: Nonsense. As per my link above: "Fat Man style bombs using Pu-239 for fuel could be readied approximately every 7 to 10 days; Thin Boy style bombs using thermal diffusion enriched Uranium (until the plant was shut down in mid-September, 1945.) could be readied approximately every 6 to 12 months; and Thin Boy style bombs using gaseous diffusion enriched Uranium (after the plant at Oak Ridge came online in mid-September 1945) could be readied approximately every 3 to 4 weeks." That's 2 to 3 more in August '45, and then 4 to 5 a month going forward. Feb 22, 2023 at 2:08
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    @WillO I'm an olde engineer. Electrical/electronic but the principles are similar enough in this case. The complexity of Fatman meant that 'working once' validated the design, but by no means guaranteed the reliability. Failure to operate at Nagasaki would have been of no great surprise to those most involved. || Most very major catastrophes occur when about 4 or 5 impossible things go wrong in concert. || The inverse - many complex things working - is not quite the same but approaches it as complexity rises. || ... Feb 22, 2023 at 10:10
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    @RussellMcMahon: A key difference between Los Alamos and both the Soviet space program and NASA post-1972 (or so) is that Los Alamos was run by the physicists and engineers. The latter two were both run by bureaucracies of scientific incompetents. As Feynman noted in his appendix to the Challenger disaster report, NASA's estimates of Space Shuttle error likelihood were off by at least 4 orders of magnitude; and that they were on the order of 1% and not 1 in a million or more. Feb 22, 2023 at 12:29
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    The difference is not the neutron cross sections, but the spontaneous fission rate. Plutonium's is much higher because of Pu-240 (remember, the Pu is chemically separated, not mass separated). The Pu-240 was on the order of a few percent or so, but the spontaneous fission rate is about 100,000 times larger. This leads to a much higher probability of predetonation in the "slowly" assembling gun-type device. See, for example, American Journal of Physics 78, 804 (2010); doi: 10.1119/1.3367757
    – Jon Custer
    Feb 22, 2023 at 13:34

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