What kind of atomic bombs were anticipated after Nagasaki?

By the beginning of World War 2 all major industrial countries had the basic Physics knowledge to design and construct a little-boy type U235 bomb - given sufficient enriched uranium. The basic mechanism - use high explosive to fire a machined cylinder down a tube into a hollowed-out sphere hollowed-out sphere down a tube at a prepared cylindrical insert to construct a critical mass - was so simple that the engineers of the Manhattan project saw no need to even test it.

However the thermal diffusion technique for enriching Uranium, the only process known and fully engineered until very late in 1945, even with the vast industrial resources of the United States, could only produce sufficient U-235 for such a bomb every 6 to 12 months.

Note in particular that this production problem was well known also to the scientists and engineers of both Japan and Nazi Germany; and that all these teams also knew that everyone else knew. One of the reasons motivating Japan's refusal to surrender after Hiroshima was their knowledge that a second such bomb would not be available to the U.S. for another 6 to 12 months; which was actually true. What Japan did not realize is that the Los Alamos team had managed to build a bomb using Pu-239, as well as the ability to isolate sufficient Pu-239 for additional such bombs on a 7 to 10-day cycle, AND perfect a more efficient means of enriching Uranium (though not until a month after the war ended).

Recognizing the production problems associated with thermal diffusion early in the war, the Manhattan Project executive was able to:

1. Assign a top priority for scarce nickel and copper resources to the R&D team attempting to perfect a much more efficient gaseous diffusion enrichment process. The facility was designed and built in parallel to the rest of the Manhattan Project and went into production in mid-September 1945. The imminent completion of this design was not estimated until sometime in Spring 1945.

2. Assemble at Los Alamos an astounding collection of Scientific, Mathematical, and Engineering talent that invented, designed, and perfected a means of utilizing the much more easily obtained Plutonium-239 in a Fat-Man style bomb. Numerous scientific and engineering challenges were overcome to achieve this. The resulting mechanism was so complex that the need for a full test was decided on - the Trinity Test.

So, after Nagasaki on August 9, 1945, both types of bombs were in production:

• 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;

• 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.

Note that the announcement of Japanese surrender on Sep. 15 immediately resulted in a reallocation of resources away from additional bomb construction. The first priority immediately became "How fast can we get the boys back home and working civilian jobs again?". You must remember this when doing research, as this global change in strategic direction occurred literally overnight.

Anyone interested in a worms-eye view of the vast undertaking that was Los Alamos, including some of the mathematical challenges associated with the Fat Man bomb design, will enjoy Richard Feynman's memoirs Surely You're Joking Mr. Feynman - Adventures of a Curious Character. (or, as told to Ralph Leighton).

It seems that the intention had been to build Fat Man devices for the planned subsequent missions even before the first Trinity test on 16 July 1945.

I haven't been able to find online copies of most of the relevant memos, but they are quoted in Barton J. Bernstein's Eclipsed by Hiroshima and Nagasaki: Early Thinking About Tactical Nuclear Weapons.

The plan to use atomic weapons of the Fat Man plutonium-cored type was mentioned by J Robert Oppenheimer in a memo to General Groves on 7 May 1945:

It seems to us that our first obligation, as long as hostilities continue, and possibly for some little time beyond that, will be uninterrupted production of the gadget models so far developed. ... Our plans for this are for the most part based on the Assumption that the Trinity test and the first combat trial of the Fat Man will come reasonably close to expectation. both in date and in performance, and that the overwhelming majority of the gadgets ... will be Fat Men.

• J Robert Oppenheimer to General L.R Groves, May 7, 1945, Los Alamos Records [Bernstein, 1991, p153]

In that memo, Oppenheimer suggested that the Little Boy device might have special tactical uses, since it was was apparently expected to deliver less explosive power that the Plutonium-cored Fat Man. As far as I could find, he didn't expand on what these tactical uses might be, simply stating the possibility of

"using the Little Boy for certain specific missions ... at present unknown to us."

• J Robert Oppenheimer to General L.R Groves, May 7, 1945, Los Alamos Records [Ibid]

According to a paper titled The First Nuclear Weapons, the reason was simply that Little Boy was very inefficient, and required a large critical mass. If the Uranium-235 were used in a Fat Man type bomb, the efficiency would have been increased by more than an order of magnitude.

It seems that, immediately after the Trinity test, in a memo dated July 19, 1945, Oppenheimer suggested removing the Uranium from the original Little Boy device to be reworked into uranium/plutonium composite cores for more implosion bombs (the paper linked above states that 4 implosion bombs could have been made from Little Boy's core).

General Groves rejected the idea since it would delay combat use, and Little Boy was deployed on the Japanese city of Hiroshima on 6 August 1945. However, it seems that there was no intention to build further uranium-cored devices at that point.

Source

• Bernstein, Barton J: Eclipsed by Hiroshima and Nagasaki: Early Thinking About Tactical Nuclear Weapons, in International Security, Vol. 15, No. 4 (Spring, 1991), pp. 149-173

It was a matter of production volumes. As of mid-1945, the Manhattan Project was still dependent on the very slow electromagnetic process at the Y-12 plant at Oak Ridge for producing highly enriched uranium (HEU). When Little Boy was detonated, it expended at least 90% of the HEU that was available.

However, plutonium was being produced much faster, at the rate of several bombs-worth of material per month, at the Hanford Site. The Fat Man implosion-based design also required less material for the same explosive yield, so it simply made sense to concentrate on it.

Nowadays, there is far better technology for uranium enrichment, which also produces far less nuclear waste than plutonium production, but that wasn't the case in 1945.