I e-mailed John and he confirmed that it was the cobalt content that was being referred to by Agnew.
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Role of the tamper is the same in both devices: hold the reacting material together longer, reflect neutrons back into the core. Improves efficiency. They used unenriched U metal for the FM tamper.
I don't recall seeing any explanation of why they used WC in LB instead of U, though they also studied U. My guess (half-memory?) is that they were worried about excess neutrons in the core causing predetonation possibilities. U has a real background neutron rate, WC does not.
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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.
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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.
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So I have a declassed doc on this in which that sentence is redacted but it’s obviously what is said re: neutrons from natrl U. Let me get out of class and I will send.
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https://twitter.com/nuclearanthro/status/935011250811035649?s=21 … It’s from one of the glasstone classed docs on nukes. Will get full citation later.
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So much math in nuclear physics If only it was easier to calculate Z in my back yard breeder reactor shielding. Half of that statement is true I hate math and skipped half of Glasstone because it was using Greek symbols
End of conversation
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