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In the Manhattan project they originally planned to use plutonium for their gun-type bomb and gave the design the name "Thin Man". However it was later determined that reactor-bred Pu-239 would inevitably be contaminated with Pu-240 whose high spontaneous fission rate would have caused a gun-type bomb to predetonate and fizzle, releasing only a small portion of its energy, unless the two pieces of plutonium were to be brought together at impractically high speeds and would have required a barrel too long to fit in any bomber. As such they switched gun-type bomb development to using U-235 which would eventually result in Little Boy while the use of plutonium was confined to implosion-type weapons and was the fissile material used in Trinity and Fat Man.

The logic behind the unusability of of Thin Man was that the Pu-240 impurities had a high spontaneous fission rate and produced fast neutrons with every spontaneous fission. These neutrons could then trigger a nuclear chain reaction when the two pieces of plutonium where near enough to constitute a critical mass but before they had reached their optimal position. The energy from this reaction would blast the two pieces of plutonium apart and quickly end the reaction before more than a tiny fraction of the plutonium had undergone fission, producing a fizzle that would have released an amount of energy impressive by the standards of chemical explosives but far below the kilotons of a proper detonation.

However U-235, the material used in Little Boy, also undergoes spontaneous fission and releases neutrons, albeit at a much lower rate as it has a much lower probability of decaying by spontaneous fission and a much longer half-life making decays of any kind much less frequent than in plutonium.

My question is, was it possible for Little Boy, or for that matter any U-235 gun-type weapon, to undergo the same type of predetonation and fizzle that made Thin Man nonviable if they were unfortunate enough to have a U-235 atom undergo spontaneous fission during the window where the two masses of uranium are close enough to sustain a chain reaction but have not reached the configuration where such a reaction would split the most atoms and release the most energy?

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    $\begingroup$ It's not that spontaneous fission in these amounts of material are something you turn on or off. It happens all the time anyway, at a known rate. The statistical fluctuation is known, small, and calculatable. So it's not that a single neutron more or less makes any difference. It would require a "large" statistical deviation, and that is "very" unlikely. $\endgroup$
    – rfl
    Jul 15, 2021 at 5:13

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In the Little Boy, spontaneous fission within the U-235 was occurring continuously, but the two pieces of U-235 were both carefully designed to be subcritical- that is, they were deliberately shaped and set so far apart that the neutrons released by spontaneous fissions escaped the bulk of the U-235 without setting off more fissions, thereby avoiding a premature runaway chain reaction and explosion. They were also deliberately shaped so that when the gun assembled them, the resulting shape and size of the U-235 mass would assuredly be critical.

In this effort the bomb designers were aided by the fact that the U-235 which had been purified from the U-235/U-238 mixture did not have contaminants in it which had spontaneous fission rates high enough to rule out the feasibility of the gun design. They had enough confidence in their design that they felt it unnecessary to proof-test the Little Boy, and so the first time a Little Boy design exploded was when it was dropped on Hiroshima.

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  • $\begingroup$ I am aware that the individual pieces of uranium were subcritical but the same was true of the Thin Man design, the problem was that when the two pieces were brought together there was a brief window were they were close enough to constitute a critical mass but not yet in their final positions, during this time stray neutrons from Pu-240 could trigger a premature reaction and blast the bomb apart before it could produce enough energy. My question is if uranium gun-type designs have that same window and if they could predetonate if a stray neutron were to be generated during that frame. $\endgroup$ Jul 15, 2021 at 4:44
  • $\begingroup$ The criticality experiments included the effects of stray neutrons from cosmic rays, etc.; this was all rolled into the determination of the critical mass for U-235. I recommend Serber's book, The Los Alamos Primer, which goes into this in considerable detail. $\endgroup$ Jul 15, 2021 at 5:23
  • $\begingroup$ So why was plutonium unfeasible if stray neutrons could be dealt with? Did the amount of spontaneous neutrons somehow make a difference? I thought a single neutron was all that was needed to ignite a chain reaction. $\endgroup$ Jul 15, 2021 at 5:37
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    $\begingroup$ I have not read Serber’s book but I do understand critical mass. The problem with plutonium wasn’t when the masses were subcritical, it was when the 2 pieces of plutonium were close enough to be a critical mass even if there was still a slight separation between them, the stray neutrons triggered a chain reaction at this stage and blew the bomb apart early. Even in fizzles there must be a critical mass and a nuclear chain reaction, albeit a suboptimal one. I am wondering if the same could happen with U-235 if the 2 pieces are almost but not quite in position and a stray neutron is generated. $\endgroup$ Jul 15, 2021 at 6:29
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    $\begingroup$ Serber's book will probably answer your questions. The key issue here is that it takes a finite amount of time for a chain reaction to go to completion and a finite amount of time for the gun to assemble the critical mass. Artful design allows the assembly process to go to completion fast enough that there is no time for the chain reaction to get underway early, even with a stray neutron background- eliminating the possibility of a fizzle. $\endgroup$ Jul 15, 2021 at 15:42
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Within a given time, the probability of spontaneous fission for the isotopic composition of uranium in a weapon is very low compared to the probability of spontaneous fission for the isotopic composition of plutonium in a weapon. The slower assembly time for a U gun-type weapon was accepted based on the low probability of pre-ignition due to spontaneous fission. For a Pu weapon, the probability of pre-ignition is too high for the slower gun-type assembly, so the faster explosively driven implosion design was developed. There is a small probability of a fizzle from pre-ignition from spontaneous fission for both U and Pu weapons.

The isotopic composition of plutonium in a weapon includes Pu-240 that has a relatively high rate of spontaneous fission. The presence of Pu-240 is unavoidable for Pu produced in a nuclear reactor. Pu production reactors at Hanford and Savannah River, used relatively short exposure times for the fuel before removing it for reprocessing to recover the Pu, to avoid excessive buildup of Pu-240.

In contrast to the U in a weapon that is highly enriched- and therefore "purified"- the Pu recovered from reprocessing is not enriched, so the Pu-240 is not removed prior to use in a weapon.

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  • $\begingroup$ Also, the half-lives of Pu-239 & Pu-240 are much smaller than that of U-235 (& U-238), and the rate of spontaneous fission in Pu-240 is ~87 million times higher than that of U-235. $\endgroup$
    – PM 2Ring
    Apr 25 at 15:48
  • $\begingroup$ Yes, thanks. Wikipedia has some useful information under "spontaneous fission". $\endgroup$
    – John Darby
    Apr 25 at 15:50

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