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The Chicago Pile experiments used natural uranium slugs that were ~1.5 inches in diameter. The slugs were surrounded by graphite to slow down the naturally occurring neutrons to be able to affect other uranium slugs.

My question is: if the natural uranium's neutrons had to go through a graphite moderator to be able to affect other natural uranium slugs, why make them ~1.5 inches in diameter?

In other words, why not make the natural uranium slugs a much more smaller size, so that more neutrons be produced? If neutrons from a certain slug cannot affect other atoms within the slugs in a meaningful way, why make slugs of ~1.5-inch diameter?

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    $\begingroup$ Have you calculated the mean free path of a ~2 MeV neutron in uranium oxide? $\endgroup$
    – Jon Custer
    Commented Aug 26, 2023 at 2:41
  • $\begingroup$ @JonCuster I have not, but I get your point. You are saying the slugs must be thick enough to "catch" the neutrons? $\endgroup$
    – Young Jun Lee
    Commented Aug 26, 2023 at 2:43
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    $\begingroup$ Fermi had a very good idea of the various cross sections of interest, having done a variety of experiments with Uranium over the previous years. If many neutrons can get out of 1.5 inch diameter slugs with zero or perhaps one scattering event there is no need to make the slugs smaller. So, what is the mean free path? If more than a centimeter or so, things are just fine - the slug is small enough for neutrons to get out to be moderated by the graphite. Also remember that CP-1 was unenriched, naturally occurring uranium oxide, so the ~2MeV neutrons could fission the U-238. $\endgroup$
    – Jon Custer
    Commented Aug 26, 2023 at 2:59
  • $\begingroup$ @JonCuster I see, so they can theoretically be smaller, but Fermi must have had some experimental reason for making the slugs the size they are. Thank you! $\endgroup$
    – Young Jun Lee
    Commented Aug 26, 2023 at 3:05
  • $\begingroup$ In retrospect, it has something to do with the neutron diffusion equation. $\endgroup$
    – Young Jun Lee
    Commented Feb 29 at 14:32

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There were extensive experiments done before they built CP-1 to find the (mostly) optimal material dimensions. They built many "subcritical piles" which consisted of a neutron source, graphite blocks, and uranium slugs. These subcritical piles were smaller and could not become critical by themselves, hence the need for the neutron source. From these subcritical piles, they could measure things such as the fermi age and the four-factor parameters. Once they determined a geometry that could become critical, they proceeded to build CP-1. CP-1 was not a random experiment, they had a pretty good idea of exactly how big it would be before it would go critical.

If you want to learn more about subcritical piles, do a web search on how to measure the Fermi Age in graphite piles.

Edit: here is a very similar question answered earlier - How was Enrico Fermi able to predict a power output from his uranium and graphite Chicago Pile 1 (CP-1) reactor?

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  • $\begingroup$ Hello, thank you for your answer. I have read the post you have linked, and while there was no direct reason for why the slugs of Uranium were a particular size in the particular link, I can get a sense that Fermi did so using rigorous theory and experimental data. However, with reactor designs such as CANDU, I also get the impression that advancements in knowledge have made it so natural uranium reactors can be created more efficiently. Perhaps by using deuterium, the slug sizes can be decreased so that individuals can attempt to replicate CP-1 feasibly. Thank you. $\endgroup$
    – Young Jun Lee
    Commented Sep 4, 2023 at 0:37

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