Showing posts with label Little Boy. Show all posts
Showing posts with label Little Boy. Show all posts

Friday, September 9, 2016

Norks finally get the bomb

Took 'em five test shots before it worked right.  The first four Nork tests had yields in the one kiloton area.  That's a fizzle in most people's books.  Where as the United States was able to produce TWO functional nuclear weapons in 1945 with only one test shot.   The Little Boy gun type uranium bomb was so well understood that it was sent into action without a test, and  produced a 20 kiloton yield that devastated Hiroshima.   Fat Man, the far more tricky implosion type plutonium bomb, was tested once in Nevada before being dropped on Nagasaki.  In action, it worked properly, with a 20 kiloton yield, and vaporized Nagasaki.
   Getting a fission bomb to explode is tricky.  You have to assemble a critical mass of fissionables, either by gun style assembly or implosion, and hold it together long enough (nanoseconds) for the neutrons to fission the fissionables.  If the energy released in the first few nanoseconds blows the bomb to bits,  you don't get a 20 kiloton yield, you get a fizzle.
    We let the Norks run off five nuclear tests, and finally they got it together, achieved city smashing yield, and we did nothing to stop them.  

Wednesday, March 4, 2015

With weapons grade fissionables anyone can build a bomb.

There are two fissionable isotopes practical for nuclear weapons, Uranium 235 and Plutonium 239.  U235 is found in nature, but it's scarce and very difficult to concentrate.  Only seven tenths of one percent of natural uranium is the readily fissionable isotope U235, the rest is relatively inert U238.  Plutonium, with a half life of  only 24 thousand years, is not found in nature.  Any plutonium present when earth was created, 4.5 billion years ago has decayed to lead by now.  Plutonium can be made from Uranium in a nuclear reactor. 
  To make a uranium fission bomb, you have to concentrate the fissionable U235 up from the 0.7% found in nature to 90% or so.  That is hard to do.  Chemistry won't help, because U238 and U235 are both Uranium and any chemical process will effect both isotopes alike.  Chemistry won't separate them. 
   Concentration of U235 relies upon physical processes that work on the slight difference in mass.  The Iranians are using centrifuges.  They react the Uranium with fluorine creating uranium hexafluoride gas.  The gaseous Uranium is fed into the centrifuge and the very high G forces of the centrifuge cause the heavier U238 to sink to outside and the lighter U235 to rise to the center, where it is skimmed off.  A single pass thru a centrifuge will raise the concentration of U235 a small amount.  To achieve weapons grade, 90%, you make many passes thru the same apparatus.   It takes a lot of centrifuges to get enough 90% U235 for a bomb.  The Iranians have 6800 on line and 19000 a building. 
   A bomb is merely a "critical mass" of U235.  The exact value of critical mass used to be a top secret, but nowadays it is probably 25 kilograms or so, call it 55 pounds or more.  Create a chunk of U235 of critical mass and you have a nuclear explosion, right then and there. 
   To produce a nuclear explosion at the target, rather than in the factory,  make two sub critical masses, keep them separate until the bomb is on target, then slam them together, hard, creating a critical mass and a nuclear explosion.  Standard design puts one sub critical mass in a gun, and fires it at the other sub critical mass.  This was the design of "Little Boy", the bomb used on Hiroshima.  They worked out the design of "Little Boy" in the 1940's with nothing but slide rules and Munroematic adding machines.  They were sufficiently confident of the design that they didn't waste any U235 on a test shot in Nevada.  "Little Boy" was shipped to the Marianas and dropped on Hiroshima. 
   In short, once you have enough weapons grade fissionables, say 55 pounds of 90% U235,  you have a bomb.  Just takes a little ordinary work with machine tools and it's ready to go. 
   Which means, as long as the Iranians have 6800 or maybe 19000 centrifuges, they can produce a working fission bomb in a matter of weeks. 
   The Obama team, negotiating with the Iranians, is willing to let the Iranian keep all their centrifuges.