[1wd]

Feynman has an interesting story about critical mass:

> Los Alamos was going to make the bomb, but at Oak Ridge they were trying to separate the isotopes of uranium ... he saw them wheeling a tank carboy of water, green water - which is uranium nitrate solution. He says, “Uh, you're going to handle it like that when it's purified too? Is that what you're going to do?" They said, “Sure -- why not?" "Won't it explode?" he says. Huh! Explode?" ... he noticed certain boxes in big lots in a room, but he didn't notice a lot of boxes in another room on the other side of the same wall ... what you would have to do to fix this. It's rather easy. You put cadmium in solutions to absorb the neutrons in the water, and you separate the boxes so they are not too dense ...

https://calteches.library.caltech.edu/34/3/FeynmanLosAlamos....