[credit_guy]

Sorry, I got carried away and did not finish my argument. I mentioned that the initial Chinese nukes originated with a design stolen by the Soviets from the US. There is no slam-dunk evidence for that (at least of the declassified type), but it appears highly likely that the US bomb that was the "inspiration" was the W-33 one [1]. That was a U-235 bomb that was so miniaturized that it fit into an 8-inch artillery shell. 2000 of those were produced and stockpiled. They remained in service until the end of the Cold War.

They were not very high yield, only about 10 kT. But it's very likely the design was exceptionally safe, considering that so many were built, and the idea for these nukes were to be used in a field of battle and fired using regular (although quite large) howitzers.

Two earlier designs were the Mk-8 and Mk-11 [2]. Both were of the Hiroshima type, using only U-235, and had a yield of up to 30 kT. A few dozen were built of both types.

Bottom line: uranium-only bombs could be made safe. They could also be made to have very high yield, up to 300 kT, as shown by the Chinese test Chic-6.

I don't have direct proof that a Hiroshima-type bomb could be made to be both high-yield and safe, because all major nuclear powers preferred to use Plutonium. But I think it's very plausible that if Plutonium had not been successful during WW2, the US, and the rest of the other nuclear powers, would have found ways to build large and safe such uranium bombs, where the uranium would have been either U-235 or U-233.

[1] https://en.wikipedia.org/wiki/W33_(nuclear_warhead)

[2] https://en.wikipedia.org/wiki/Mark_11_nuclear_bomb

[philipkglass]

The first Chinese fission bomb was an implosion bomb that used U-235 instead of Pu-239 as nuclear fuel (search page for "235" and "implosion"):

https://nsarchive2.gwu.edu/nukevault/ebb488/

Uranium implosion is far more efficient than a uranium gun-type bomb, and is the only demonstrated way to make a high yield (hundreds of kilotons) fission bomb. The Ivy King device from the US was the largest known uranium-only bomb. It used implosion:

https://en.wikipedia.org/wiki/Ivy_King

[credit_guy]

You are right, I stand corrected.

I read about the Chinese bombs in "Atomic Adventures" by James Mahaffey, but reading again, I see that only the Chic-4 bomb originated from the US W-33 warhead, which was copied by the Soviets and became their 3BV3 bomb. All these had a yield of about 10 kT. As I mentioned in my previous reply, there were gun-type uranium bombs with up to at least 30 kT.

I think your original point was that in order to get yields 10 times as high, you would need much more uranium, and this would be unsafe.

The Ivy Mike bomb that you mentioned had a fairly crude, but effective, safety measure: a chain containing boron (very strong neutron poison) in the middle of the assembly, to be removed only immediately prior to deployment. I don't see why this would not work with a gun-type design.

Still, you could say that the implosion brings together fissile metal from all directions, and at huge speed too, while the gun-type brings the metal from only 2 directions and at much lower speed, and therefore the implosion can assemble a much higher hypecritical mass. So inherently the implosion design can result in a higher yields. And that is absolutely true, and it is very likely the reason that nuclear powers prefer the implosion design, even in some weapons that use uranium.

From the post-WW2 history, it looks like gun-type was used specifically for smaller yields. An artillery shell should not have a megaton yield, for the simple reason that you want to survive after you fire it.

Yet, from the same book I learned that during the Manhattan project 20000 explosions were used to understand and fine-tune the implosion design, and for each explosion that happened at least 20 were analyzed on paper before. More than 1000 scientists and engineers worked on nailing that design, and it was by far the most expensive part of the entire Manhattan project.

Immediately following WW2, the US switched to a different method of uranium enrichment, that made uranium cheaper to produce than plutonium. I don't know if it was cheaper by mass or by yield, probably the first.

Still, let's imagine an evolutionary path where the US finds itself after the end of WW2, with a tried, tested and practical design based on uranium and the gun-type, and which needed a fissile material that was getting much cheaper. And knowing that it's possible to get an alternate design based on plutonium, but with that required an unknown amount of additional fundamental research and then engineering effort. That would have very likely been the situation if von Neumann did not get involved.

A lot of organizations faced with such a dilemma choose the incremental gains from an existing design, rather than exploring a potentially revolutionary, but risky alternative.

In such an alternate history, would the scientists be able to increase the yield of a gun-type uranium bomb to 100 kT, or 500 kT? The boosted fission design was developed between 1947 and 1949, and there is no reason it would not work with a gun-type bomb. Once a boosted version of a gun-type is developed, a version that uses a lot of boosting and an additional U-238 temper around the core can deliver a lot of extra-yield, without increasing the U-235 mass, and the chance of pre-explosion. I'm sure motivated scientists could have come with many more ideas.

Here's a similar scenario of incremental changes to an existing design: after WW2, Admiral Rickover chose the pressurized water design for his submarines. He was a very smart man, and I have no doubts that he made the right choice. However, there are hundreds of possible reactor designs, and for applications other than submarines it is very likely other designs could be much better, yet 75 years later, and PWR is by far the most widespread design.

[philipkglass]

Yet, from the same book I learned that during the Manhattan project 20000 explosions were used to understand and fine-tune the implosion design, and for each explosion that happened at least 20 were analyzed on paper before. More than 1000 scientists and engineers worked on nailing that design, and it was by far the most expensive part of the entire Manhattan project.

The lion's share of the project spending was for uranium enrichment at Oak Ridge. The Los Alamos experiments and all R&D together accounted for only 8% of the Manhattan Project cost:

https://blog.nuclearsecrecy.com/2013/05/17/the-price-of-the-...

The gun-type Little Boy bomb had a yield of 12-15 kilotons from 63 kilograms of highly enriched uranium:

https://en.wikipedia.org/wiki/Little_Boy

The Ivy King uranium implosion bomb had a yield of 500 kilotons from 60 kg of HEU:

https://en.wikipedia.org/wiki/Ivy_King

That's about 35 times more efficient without even having to manufacture tritium for boosting (and tritium is, gram for gram, by far the most expensive material in the nuclear weapons complex.) I think that implosive fission bombs would soon have been developed after the war even if they hadn't been part of the original Manhattan Project effort. I do think that with a less intense USA-USSR rivalry after the war it's plausible that thermonuclear weapons based on radiation implosion would not have been developed, or developed much later.

[credit_guy]

I think you are agreeing with me.

Regarding the expensive part. The actual word in the book ([1], p 174) was "difficult", not expensive, I misremembered.

  > The United States Corps of Engineers at the secret Los Alamos Laboratory, consisting of 1,500 scientists, engineering specialists, and precision machinists, solved the problem of spherical shock-wave propagation in 1945 in the Manhattan Project. It was the most difficult task in the entire project, in which highly abstract theoretical physics was turned into a weaponized device. It took twenty thousand test explosions to perfect the technique, and for every test explosion there were probably twenty experimental configurations that were found to be not worth testing.

I am not disputing that implosion is much more efficient than the gun type. Even the Nagasaki bomb had a much higher efficiency than the Hiroshima bomb. In the Hiroshima bomb only 1.4% of the uranium underwent fission. In the Nagasaki bomb, 17% of the plutonium did that, plus an astounding 4% of the unenriched uranium temper.

However, imagine von Neumann did not get involved in the project. There's a good chance it would have taken the team a few more months to solve the implosion puzzle. If such had been the case, Groves and Oppenheimer would have pulled the plug, and focused fully on the uranium design.

Now, fast forward a few more months until the end of WW2, when Oppenheimer and the vast majority of the scientists and engineers at Los Alamos had gone home. Norris Bradburry is the new boss. The US Navy comes and tells him they want to set up Operation Crossroads in the Pacific, in mid 1946, and he needs to provide 3 atomic bombs for that, and maybe a spare or two. What would he do? Tell them that he needs some extra time to work on some potential great alternative design, or reply "Yes, Sir" and go ahead and make, with whatever handful of scientists and engineers he had left, a few replicas of the Little Boy? My guess is that he would have chosen the second option.

In such a scenario, when the Soviets build their own atomic bomb, it is also a gun-type, because that's the working design they steal.

It is perfectly reasonable that the US sees implosion as a game changer (as it was) and perfect the design after the war, but the urgency is down by a factor of 10, as is the scientific manpower. A large number of scientists refused to work on atomic weapons after the end of WW2, now aware of their monstrous effects.

And here we both agree that it's quite plausible that the arms race would between the US and the Soviet Union would have been less intense. It would have taken a few extra years to get to the Ivy King level of technology, and by that point the ICBM precision would have been high enough that radiation implosion thermonuclear weapons would not have been really needed.

[1] https://www.google.com/books/edition/Atomic_Adventures/wxokD...

[pfdietz]

> You are right, I stand corrected.

Is anyone else getting a LLM vibe from this?

[credit_guy]

I haven’t used any LLM to write my comments.