8.1.5 Availability of Additional Bombs
The date that a third weapon could have been used against Japan was no later than August 20. The core was prepared by August 13, and Fat Man assemblies were already on Tinian Island. It would have required less than a week to ship the core and prepare a bomb for combat.
By mid 1945 the production of atomic weapons was a problem for industrial engineering rather than scientific research, although scientific work continued - primarily toward improving the bomb designs.
The three reactors (B and D which went started up for production in December 1944, and F which started up February 1945) at Hanford had a combined design thermal output of 750 megawatts and were theoretically capable of producing 19.4 kg of plutonium a month (6.5 kg/reactor), enough for over 3 Fat Man bombs. Monthly or annual production figures are unavailable for 1945 and 1946, but by the end of FY 1947 (30 June 1947) 493 kg of plutonium had been produced. Neglecting the startup month of each reactor, this indicates an average plutonium production 5.6 kg/reactor even though they were operated at reduced power or even shut down intermittently beginning in 1946.
Enriched uranium production is more difficult to summarize since there were three different enrichment processes in use that had interconnected production. The Y-12 plant calutrons also had reached maximum output early in 1945, but the amount of weapon-grade uranium this translates into varies depending on the enrichment of the feedstock. Initially this was natural uranium giving a production of weapon-grade uranium of some 6 kg/month. But soon the S-50 thermal diffusion plant began feeding 0.89% enriched uranium, followed by 1.1% enriched feed from the K-25 gaseous diffusion plant. The established production process was then: thermal diffusion (to 0.89%) -> gaseous diffusion (to 1.1%) -> alpha calutron (to 20%) -> beta calutron (up to 89%). Of these three plants, the K-25 plant had by far the greatest separation capacity and as it progressively came on line throughout 1945 the importance of the other plants decreased. When enough stages had been added to K-25 to allow 20% enrichment, the alpha calutrons were slated to be shut down even if the war continued.
After Japan's surrender in August 1945, S-50 was shut down; the alpha calutrons followed in September. But K-25 was complete on August 15, and these shutdowns would have occurred in any case. At this point gaseous diffusion was incapable of producing weapon grade uranium, a planned "top plant" had been cancelled in favor of more beta calutrons. An expansion of K-25, called K-27, to produce a larger flow of 20% enriched feed was under construction and due to go in full operation by 1 February 1946. In October production had increased to 32 kg of U-235 per month.
In November and December additional beta tracks went on line, and the percentage of downtime for all beta tracks fell, boosting production further. Between October 1945 and June 1946, these improvements led to a 117% increase in output at Oak Ridge, to about 69 kg of U-235 per month.
It is very unlikely any more Little Boy-type bombs would have been used even if the war continued. Little Boy was very inefficient, and it required a large critical mass. If the U-235 were used in a Fat Man type bomb, the efficiency would have been increased by more than an order of magnitude. The smaller critical mass (15 kg) meant more bombs could be built. Oppenheimer suggested to Gen. Groves on July 19, 1945 (immediately after the Trinity test) that the U-235 from Little Boy be reworked into uranium/plutonium composite cores for making more implosion bombs (4 implosion bombs could be made from Little Boy's pit). Groves rejected the idea since it would delay combat use.
The improved composite core weapon was in full development at Los Alamos when the war ended. It combined two innovations: a composite pit containing both U-235 and Pu-239, and core levitation which allowed the imploding tamper to accelerate across an air gap before striking the pit, creating shock waves that propagated inward and outward simultaneously for more rapid and even compression.
The composite pit had several advantages over using the materials separately:
* A single design could be used employing both of the available weapon materials.
* Using U-235 with plutonium reduced the amount of plutonium and thus the neutron background, while requiring a smaller critical mass than U-235 alone.
The levitated pit design achieved greater compression densities. This permitted using 25% less than fissile material for the same yield, or a doubled yield with the same amount of material.
Production estimates given to Sec. Stimson in July 1945 projected a second plutonium bomb would be ready by Aug. 24, that 3 bombs should be available in September, and more each month - reaching 7 or more in December. Improvements in bomb design being prepared at the end of the war would have permitted one bomb to be produced for every 5 kg of plutonium or 12 kg of uranium in output. These improvements were apparently taken into account in this estimate. Assuming these bomb improvements were used, the October capacity would have permitted up to 6 bombs a month. Note that with the peak monthly plutonium and HEU production figures (19.4 kg and 69 kg respectively), production of close to 10 bombs a month was possible.