What is the physics behind A-Bomb?

Krzysztof Zieleniewski (SKFiz)
Homepage: www.kwzieleniewski.wikidot.com


Summary

Kaa-boom! I talked about general aspects of nuclear weapons. First off, after discovery of neutron physicists immediately realized that a chain reaction should be possible, releasing enormous energy confined in atomic nucleus. But for manufacturing a working device they needed to overcome several scientific and engineering problems. For instance, coming up with the difference between 235U and 238U isotopes was perhaps the single most important step towards nuclear weapons. The Manhattan Project had practically unlimited money supply, which enabled the project to follow all unknown possibilities and finish the bomb just before the end of World War II.

The simpler layout can be roughly described as a cannon firing a piece of uranium into another one. However, this approach is inefficient (may disassemble before reacting, see predetonation). More advanced one is to squeeze material using immense pressures (several megabars) from conventional explosion. This needs advanced explosive lenses, wave-shaping, some tricks and precise timing of initiating nuclear reaction. The advantage is miniaturization of warheads by order of a hundred — from diameter of 60 inch down to 16" or even 6" in extreme cases. Once developed, a spherical nuclear bomb can be used to start thermonuclear reactions — responsible for shining of stars — in Teller-Ulam design.

No, there are no nuclear bombs in suitcases, but there were "manual" weapons, see Davy Crokett.

I also talked a little about effects of nuclear weapons. The most feared one — radiation — is also the fastest declining, due to atmospheric absorption (by a factor of distance to third power, $P \sim 1/d^3$ ). So it is important only in relatively small explosions, up to a few kilotons of TNT. Bigger ones produce shock and heat waves strong enough ($P \sim 1/d^{2,5}$ and $P \sim 1/d^2$) to kill everybody within deadly radiation range anyway.

There was also some discussion about moral, military or political aim in using nuclear weapons during WW2 as well as risks connected to uranium mining and whole nuclear power industry.


Further reading

I based my talk on several sources (loosly in order of relevance):

  • Nuclear Weapons FAQ (comprehensive description)
  • Richard Rhodes, Jak powstała bomba atomowa? (history of developing first nuclear weapons)
  • Article The H-bomb secret. How we got it — why we're telling it, The Progressive, Nov 1979 (avaible online here)
  • Nuclear Weapons Archive (description of various warheads and tests)
  • Cresson H. Kearny, Nuclear War Survival Skills (a book avaible online here)
  • Nuke xls (a spreadsheet calculating effects of a nuclear explosion), avaible for example here
  • We_nuke (a nuclear explosion effects simulator), avaible for example here
  • personal experience from trip to Chernobyl, see its page
  • numerous Wikipedia articles
  • Chinese Nuclear Forces and U.S. Nuclear War Planning (a book avaible online here)
  • Richard P. Feynmann, Ralph Leighton, Surely you're joking, Mr. Feynmann!
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