[credit_guy]

Slow down a bit.

First: it took many years for the Ulam-Teller design to be discovered. Teller came up with the idea of a fusion bomb in 1942. The actual Ulam-Teller design was invented in March 1951, basically 9 years later. In this 9 years, for at least 2 years, people were searching frantically for a workable design. It's very easy to say, after the fact, that "X-Rays thermal transport would have been discovered as well" because we know this is what worked in the end. But before the fact, nobody was looking for X-Ray implosion.

Second: During WW2, plutonium was considered superior to uranium because it was cheaper to manufacture. U-235 was being manufactured via a very expensive separation process. But U-233 can be bred in a thorium reactor just like Plutonium is bred in a uranium reactor. They both come with their challenges (U-232 for U-233, Pu-240 for Pu-239), but in a scenario where the Manhattan project did not figure the implosion design in a hurry, the US would have shifted the resources to U-233. Here's a quote from wikipedia [1]

  > A declassified 1966 memo from the US nuclear program stated that uranium-233 has been shown to be highly satisfactory as a weapons material, though it was only superior to plutonium in rare circumstances. It was claimed that if the existing weapons were based on uranium-233 instead of plutonium-239, Livermore would not be interested in switching to plutonium. 

Now, I have no doubts that fission boosting would have been discovered. But with fission boosting, bombs would have gotten to the 1 MT yield, and that is plenty destructive for all war scenarios.

Crucially, such a weapon did not need to use any type of implosion.

In a scenario where the US does not develop the implosion knowledge because it can build a 1 MT weapon without, then it is not at all a given that someone else would have looked for an implosion-based design of a thermonuclear weapon. We would still have had the layer-cake design, but that is not a game changer, and it's not clear that the extra yield was worth the extra complexity.

Now, in today's world: Most of US's nukes have yields around 100 kT [2]. They are thermonuclear but the same effect can be achieved with (boosted) fission bombs. The largest current US nuke, the B83, has a yield of 1.2 MT, but the US is looking to retire it, and replace it with B61, with a maximum yield of 400 kT.

My point is that a superpower can service all its deterrence needs with nuclear weapons with yields that are achievable with boosted fission.

Why did we then go and build thermonuclear monster bombs in the 50's and 60's? Because at the time the ICBM precision was limited, and you needed something with a mile-sized fireball to make up for the lack of precision. But the ICBM technology advanced at an incredible pace. If we had been 10 years late coming with the design for a thermonuclear bomb, it would not have been needed at all.

Yet another thing is this: the destructive power of a nuke does not grow linearly with its yield. Ten bombs with a 100 kT yield are more destructive than on single bomb with a 1 MT yield. Oppenheimer knew that, and advised the US to focus on more rather than bigger. He was not listened to, and the US build both more and bigger. But we do know that you can devastate the world with 100 kT bombs, just as much as you can devastate it with 1 MT bombs.

[1] https://en.wikipedia.org/wiki/Uranium-233#Weapon_material

[2] https://en.wikipedia.org/wiki/W76

[pfdietz]

Those very large fission bombs are going to have much more than 1 critical mass of material, and so would be very dangerous in accidents. Implosion weapons can be designed so they cannot undergo a nuclear explosion unless the implosion occurs as designed, with all the charges going off at the proper very short intervals in time.

[credit_guy]

Not really.

For this it's instructive to look at China's nuclear program [1]. Wikipedia does not mention, but the origin of China's program is a uranium bomb design that was developed in the US, stolen by the Soviets and passed unchanged to the Chinese. The first 7 Chinese tests were all based on uranium bombs. The 6th one was a real, two-stage, thermonuclear bomb, with a yield of 3 MT. But the one before it, the Chic-5 test was a boosted fission design with a yield of 300 kT.

300kT exceeds the upper limit of any of the tests done by India, Pakistan or North Korea, and they are all considered bona-fide nuclear powers.

[1] https://en.wikipedia.org/wiki/List_of_nuclear_weapons_tests_...

[pfdietz]

Where does any of that contradict what I wrote?

[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.