Not quite - the Hiroshima and Nagasaki bombs were different designs - the story goes that they tested the Nagasaki design but they didn’t bother testing the Hiroshima design because they were so sure it would work.
If I remember correctly, “Fat Man” was the Hiroshima bomb and was the simple “two pieces of enriched uranium” design and was so big they had to modify the Enola Gay to carry it. “Little Boy” was a more sophisticated compression design and was much smaller.
Whoops. I reversed the names. Thanks, Desmostylus.
Yeah, Fat Man used plutonium and used implosion instead of collosion to create a critical mass. The question was whether or not the implosion would work.
DESIGNING the bomb is easy… actually making work is a whole other matter… basically not only do you need the uranium, you have to surround it with high explosive.
Now here’s the tricky part… you have to make all the explosive around the uranium explode inward AT EXACTLY THE SAME INSTANT.
You get it right, you get fission…you get it wrong (even by a millisecond or two) and you get a mild “pop” by comparison and a lot of radioactive debris.
On paper it’s easy…in practice it takes a few dozen geniuses and a billion dollars in research money to pull it off.
Just my two cents - I know I’m over simplifying a great deal, I"m no expert but I think I have the main points basically right.
D.
Most of this has been covered in the above, but I though Id add my own 2 cents. This is from varous sources, icluding Rhodes’ two books on the A-bomb and the H-bomb:
1.) When I was a kid, schematic designs for both bombs were printed in encyclopedias.
2.) The hard part lies in the details, including the explosives and their configuratoions. But that Princeton student referred to above found out a lot of this for his senio thesis, and a magazine printed its own details back in th 1980s. Rhodes gives a lot of detals in his book, and so does Clancy in “The Sum of All Fears”. Clancy goes on to point out that, although he changed or omitted some details, he was apalled at how easy it was to find them.
3.) Rhodes points out that every nation who tried to build an atomic bomb has succeeded on the first try.
4.) My understanding iis that the basic “atomic Gun” configuration, or the Fat Man-type “cruched shell” configuration is fairly easy in principle, but isn’t efficient i its use of fissionable material. The real advantage of good engineering is literally more bang for the buck, hence the use of those minim-cyclotrons and alpha sources Clany and Rhodes describe.
5.) Apparently the Soviet scientists were so certaoin that theitr basic designs would work that they didn’t want to test them, but wanted to go directly to the more sophisticated less-material bombs. But Stalin insisted that they go with something certain to work. I suspect that he saw the immense propganda value in a first-timne-successful bomb. A fizzle from a more sophisticated bomb design would make the oviet Union look like nuclear dilletantes.
This is the most disturbing fact I’ve learned in the past month.
Think I’ll post this from memory, so by all means feel free to pounce on my errors.
I believe that the uranium type bomb (Little Boy) was NOT tested because although the design is simpler - at the time- the US could only manufacture enough U-235 to make that one bomb. Hence - no test.
Pu-239 is relatively “easier” to obtain because it is produced by what are known as “breeder reactors”. However, a Pu-239 type bomb is much more complex to detonate because it requires implosion and very precise timing. Plus (if I remember correctly) some exotic isotopes which are called “initiators” are needed to give a greater yield or perhaps even to allow fission to occur in the first place.
And as someone has stated, the separation of U-235 from the much more abundant U-238 is extremely difficult. Plus since their difference in mass is so minor (U-235 being 98.7 % the mass of U-238), this only adds to the difficulty.
Let’s consider a substance that is “easier” to separate - “heavy” water. It has an atomic mass of 20 whereas “ordinary” water has an atomic mass of 18. So, heavy water is about 10% heavier than the “ordinary stuff”. Just as in separating uranium isotopes it has to be done mechanically. I remember some time ago (decades), a professor demonstrating how an ice cube of “heavy” water sinks in regular water. He also said the ice-cube sized chunk of heavy water cost about 10 bucks. Again giving you the idea of how difficult mechanical separation is.
I tried keeping my discussion as general as possible because there’s no sense giving any terrorists a “head start” on any of this. If anything else, I think we have shown that building a A-Bomb is pretty damned difficult.
The magazine case you’re thinking of is probably the 1979 fuss over the Progressive and Howard Morland’s article on how an H-bomb works. Others (including Rhodes in Dark Sun) have reached different conclusions from Morland about the physics of the Teller-Ulam mechanism, though he seems to have got many of the other details right.
Clancy was also writing about an H-bomb.
For the technical background to a simple Hiroshima-style bomb none of those sources remotely match The Los Alamos Primer (California, 1992), the reprint of Serber’s 1943 lectures on the subject to physicists arriving on the mesa.