Alex WellersteinVerified account

Guess first questions is why are those components different than those on the Fat Man device? Is it some requirement inherent in gun type weapon? The need for steel and hence the alloys vs an implosion weapon? They both have a tampers though..?

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.

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.

I love the next line about “well, water immersion will give an upper limit also”