I am trying to understand this, so the nose of little boy acted as a neutron reflector as well?
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When you are assembling multiple crits in an inefficient way, you want to make sure the thing doesn't really start until it is supposed to start. The U-235's surface was coated with cadmium to absorb stray neutrons as well.
Here's a nice bit from a 1944 report. Same report notes that they wanted to play with the cobalt levels to see how it affected the ductility of the tamper, but that they wanted to be conservative with what was understood. https://www.osti.gov/opennet/servlets/purl/16135471.pdf …pic.twitter.com/ZTcDbIsRvT
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A slightly later 1944 doc indicates the different levels of Co they experimented with for the gun program (3, 6, 9%). From what I can tell, John thinks it was 6% based on an experimental report, not I think, bc something said the final version was 6%. https://www.osti.gov/opennet/servlets/purl/16135456.pdf …pic.twitter.com/wLR0cdBXVy
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Unrelated, but interesting: in a report on "tests we should do to make sure LB is safe-ish," one of the fears is that it could go critical from having lots of (water-filled) people near it. How to test the limit? Gather lots of people, see what happens! https://www.osti.gov/opennet/servlets/purl/938436.pdf …pic.twitter.com/AADzL5JM6P
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I love the next line about “well, water immersion will give an upper limit also” -
I mean it's not like water immersion of fissile material can cause problems...pic.twitter.com/UfoQeooTOY
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Isn't that the basis of the early "water boiler" experiments
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The water boiler was an actual reactor design (if a simple one). This was just them putting HEU in water to make measurements — much less thought seems to have gone into the criticality dangers.
End of conversation
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With 6% being the best to manufacturer and safe to not have a criticality problem, I would lean to that number as well
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Thank you for tracking down and sharing! This is awesome!
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