Were they just “testing” two types of designs (gun barrel vs. implosion) or were they designed for different applications?
The Manhattan Project was working on a number of parallel development tracks. Nobody knew which particular method of making a bomb was the best so they tried different methods simultaneously. As it turned out, they came up with two different methods of producing bomb material and building a bomb that both worked and both happened to result in a working bomb at the same time. So they had two working bombs (actually three because one was used as a test) and they used them both.
Wasn’t Trinity also a gun barrel type device? If so, was the Nagasaki device untested prior to the drop?
**Little Nemo ** is correct.
To provide some background: The original idea for an atomic bomb was conceived as using U-235 for it’s fissile material. The process for separating the fissile isotope from the raw Uranium was the limiting factor in how fast a bomb could be created. During the process of building up enough U-235, the idea of breeding fuel for a bomb (turning U-238 into PU-239) was floated. It turns out that this was a relatively straightforward process, although there were a number of engineering obstacles that had to be overcome. Once enough PU-239 had been bred, it was a relatively simple process to separate it from the bulk U-238. However, building the implosion bomb was much more complicated than the simple gun-type U-235 bomb. So, the two projects succeeded in creating their two different bombs almost at the same time. The Fat Man bomb was the only one tested - the U-235 bomb was considered too simple to fail, and was “tested in combat.”
For a good account, read Rhode’s “The Making of the Atomic Bomb.”
No, the Trinity test was a Plutonium implosion design.
Because they were the only two bombs they had available, one from each of the two design paths they were working on.
The Hiroshima gun-type device however, was untested prior to the drop. It’s basically a pretty simple design, and my understanding is they were so confident it would work that they saw no need for wasting scarce fissile material on a test for that design.
To really simplify it
One type of bomb was much easier to build and have work right, but making the fuel was a pain in the ass.
The other type of bomb was much harder to make and much less likely to work right, but the fuel was much easier to make.
So, they covered both their bases.
The Original plan was for a plutonium gun type weapon, but they found that the spontaneous fission rate of plutonium was too high for gun-assembly, and the extra neutrons flying around would cause predetonation with such a slow insertion time, and a consequent fizzle would occur.
U235 doesn’t spontaneously fission nearly so much, which makes it suitable for gun-type designs. On the other hand, U235 has a much larger critical mass than Pu-239, and is quite a bit more difficult to separate out of natural uranium than plutonium is out of reactor grade uranium, which combined made uranium very unattractive when compared to plutonium in 1944-1945.
Plutonium is however, suitable for implosion assembly- the insertion time is much faster than gun-assembly, and the spontaneous fissions aren’t nearly the problem. However, implosion bomb design is drastically more complicated than gun-type bombs.
The Los Alamos scientists weren’t nearly so confident of the implosion design bombs (Fat Man & the Gadget), so in order to have a bomb that they knew would work, they built Little Boy, a U235 gun-type bomb which took 64 kg of 80% U235 to produce about 15 kilotons.
They also never tested the Little Boy gun design, while the Trinity test was an implosion test of the bomb later used on Nagasaki.
(for reference, Fat Man used 6.2 kg of about 98% Pu-239 plutonium and produced a 21 kiloton blast, which is quite a bit more efficient than Little Boy was)
And to add to this: the implosion type is only effective with Plutonium, because it depends on a characteristic of the physical (metallic) structure of the element plutonium, not shared by uranium: at a specific pressure, plutonium takes on a denser, more compact metallic structure. I don’t know if it’s even possible to force U-235 to implode as plutonium does, and it certainly was not with 1945 technology.
Allow me to recommend the best book ever written on this subject.
It’s by Richard Rhodes who also did an equally thorough and fascinating book on the hydrogen bomb. These books cover the history, technology, politics, everything. Wonderful stuff, just talking about it makes me want to go back and read it all over again. (Just check those reviews on Amazon!)
Really?
I could see why the helps make the implosion process easier for plutonium.
But given you could probably get the uranium denser before it spontaneously started its chain reaction (because you would have the time to do so), I would think it would be somewhat of a wash.
Now if the implosion process is really borderline I could believe it. And in the early days it probably was.
But given how small they can make the fellas these days, I’d WAG that you could amp up the implosion process to still get something workable, though the bomb would be significantly bigger, and if you’ve got the plutonium, why bother?
Which brings up people always fearing terrorist getting plutonium.
A rouge nation with a good bit of funding? Yes a problem. A terrorist cell getting some plutonium, not nearly so much IMO.
Give me a decent amount of money, a handful of good hands on people, a back of the envelope physicist or two, a place to work without being stopped and nice pile of enriched uranium, and I am somewhat confident we could make something that likely work. It would be pretty darn big…buttt…
Give me a shitload of money, some more folks, and plutonium. What, you want an implosion device? Awww crap, this is going to be a royal PITA.
A a definite second on the Rhodes book.
The Manhattan project was absolutely amazing, and the book REALLY brings it out. It has some good geeky stuff in it, but even the everyday common man would appreciate reading it I think.
I look back at that and the Apollo program, then I look around today and shake my head sometimes.
Plutonium would make a nice dirty bomb (although there are probably better, more easily obtainable isotopes for a terrorist radiological weapon.)
FWIW,
Rob
I’m only aware of one rouge nation, but I admit I sure wouldn’t want them getting nukes.
In general, the difficulty with the implosion plutonium devices is that the imploding explosive sphere needs to be perfectly machined and the explosive panels perfectly simultaneously detonated. Computer-controlled machine tools were developed to aid in this process; it’s fairly high-tech.
From everything I can find it is possible to make a uranium-235 implosion-type device (although plutonium certainly appears to be the preferred material).
For example, China’s first nuclear detonation, in 1964, is described as a “pure-fission U-235 implosion fission device”.
Red China has a slightly different tint.
hmmm
makes you wonder why they took the middle road there. must be some reason, but its not obvious to me at moment.
Will have to do some pondering.
Short answer is:
neither.
The Hiroshima bomb was a gun design. The Nagasaki bomb was an implosion design.
The gun design is straightforward and fairly easy to test with sub-critical assemblies. The Manhattan Project scientists did that for a few months, to refine the design as much as they could. The tests showed that the bomb would work, and that it didn’t need a “live” test.
However, the gun bomb can’t use plutonium. You may be able to make a U235 implosion bomb, but tests showed that you can’t make a pure plutonium gun bomb.
By August 1945, though, we only had enough U235 for one bomb. The stuff is a PITA to make. We used at least 2 different processes to get to semi-pure U235; I think that scientists continued to refine that throughout the 1950s.
Conversely, we could make lots (relatively speaking) of Pu, but we needed implosion to use it. You can make Pu in a nuclear reactor; that’s one of the reasons Fermi and others did reactor design. Reactors make neutrons, and neutrons “convert” uranium to plutonium. So you have an easy way to make the material.
But implosion is a PITA. They worked on it for 2 years without much progress, until British explosives research and American chemical know-how combined to develop the right techniques, literally weeks before the end of the war. Scientists did some testing with plutonium, but they couldn’t reproduce implosion conditions in the lab and didn’t have the computing power to model it.
So, they needed a live test. Less, I think, to prove that it worked than to calibrate and correlate their theoretical work with observable results. The US (and USSR) continued to test bombs long after they had the computing power, again to do calibration and correlation. For this, underground tests work as well as atmospheric ones, and of course they’re much more “observable”.
A weird symmetry. U235 bomb: easy to test, no material. Pu bomb: easy to make material, bitch to test. The scientists at the beginning made a choice to pursue both avenues. I hesitate to say that this was a “fortunate” decision, but it does point out that single-mindedness is not always the way to go.
My impression is that the modern fission devices that act as “starters” for our current thermonuclear(fusion) bombs use a careful combination of U235 and Pu. Similarly, atomic subs use a fuel that is “enriched uranium”; that is, a combination of U235 and U238. So there’s still a use for U235.
The Making of the Atomic Bomb by Richard Rhodes is my source; read it for all the gooey details.
Nitpick: General Leslie Groves, the military commander of the Manhattan Project, who was not a scientist, made that decision. Groves decided in this, and many other cases, to pursue many different parallel strategies, knowing that the stakes were too high to go down any dead ends.
What would have happened if it had failed? go like a dirty bomb? Just a dud and nothing happened?