A cursory skimming suggests that in terms of revealing fundamental information not known publicly before it was a huge step. And there are a couple of key points that really were quite unobvious.
There is a parallel discussion along with the technical content that basically says that the information was gathered from unclassified sources (although some may well have been on the edge of inadvertent leaks rather than truly unclassified) and many of the arguments were more about freedom of the press than about the bomb.
The only people the technical content might have been useful to would heave been scientists in the USSR or China. Nobody else would (or does) have the resources to build an H-bomb. Knowing even the basics of a successful design is a major step. Just knowing it works is half the battle.
How about if one of the masses was dropped into place down an evacuated tube - presumably it could accelerate to a sufficient velocity to assemble the device fast enough that way?
John Aristotle Phillips produced an implosion-type fission bomb design as a term paper project at Princeton, using publicly available information, with various sources claiming it would or would not have worked. I can only imagine that this inspired “The Manhattan Project” (1986), in which a gifted teenager constructs a functioning (!) fission bomb out of stolen Plutonium, to enter in a science fair.
There was also the “Nth Country Experiment” from the 1960s, conducted by Lawrence Livermore, in which three recently-minted PhDs in physics with no experience in weapons design were given the task of designing an atomic weapon, again with only publicly available information, and succeeded in producing a “credible” implosion design within three man-years. I seem to recall there was at last a proposal to actually build and test the designed device, but this was never done, for various reasons, not the least being cost.
That’s no different in principle from a gun design. The question is just whether you can get faster than a gun that way. Considering that you can make a gun to shoot a projectile miles into the air, you’d need a miles-high drop tube, which is a bit difficult to construct. Oh, and if it did work, then you’d end up with your explosion happening miles under the surface, which probably isn’t what you had in mind for your device.
Nah, Earth’s gravity is plenty strong enough. Escape velocity at Earth’s surface is 11.2 km/s, while a basic gun-type weapon apparently only needs to fire the uranium “bullet” at about 300m/s. You might not even need to use an evacuated tube: according to this handy calculator, terminal velocity for a 26-kg cylinder of uranium ~10cm across could be as high as 380 m/s. That’s assuming a bullet-like drag coefficient of 0.3, which may or may not be accurate.
But hey, only one way to find out: I just need someone who is very good at catching things to hold a hollow uranium cylinder, and I’ll fly over and drop the matching piece from an airplane. Any volunteers?
What we call “a nuclear explosion” is simply a very fast fission chain reaction. The exact same one that heats water in a nuclear reactor. If you slammed two pieces together by hand, you can indeed create a nuclear chain reaction. You just won’t get much of an explosion.
The video showing an actor actually doing that, putting two pieces of Pu together and causing a fission chain reaction. The reason you can’t cause an explosion like that is due to PHYSICS. A mass of Pu large enough to go critical will start the chain reaction long before the missing piece slams into the other mass. Even if you fire it with the fastest gun possible, before the Pu reaches the target the entire thing will chain react, turning the entire mass into plasma or molten Pu or gas, which stops the explosion.
It doesn’t matter how much shielding or steel you put it in, Pu is so reactive, so fast to fission, with out a need for a moderator, long before the missing piece reaches the other piece, everything turns to plasma, and it happens so fast, the piece will never even reach the end of the gun barrel before it all is a radioactive molten/plasma/gas cloud emitting neutrons and gamma rays and x-rays and heat.
But no explosion.
Of course if you dropped a suitcase with enough Pu onto another suitcase, from a certain height, the resulting bang of plasma heat neutrons gamma radiation and light that would result (when the second suitcase was about two feet from the first, roughly), that event would certainly seem like an explosion to anybody close enough to experience the horrific scenario.
But it’s really hard to make even 10 tons of Pu explode, because it’s so reactive, long before it reaches a reaction like we think of with an A bomb, it would turn itself and whatever surrounds it to plasma, causing the Plutonium to blow apart, stopping the reaction.
Certainly a pile of Pu will react if it’s enough mass, but by it’s nature it keeps blowing itself apart, which is why you might see might look like an explosion, but it’s really just a lot of instant heat and energy being sprayed in all directions.
And while a human could certainly try to clang two large pieces together, if this human was holding enough mass of Pu to cause an explosion, the fission reaction would start long before they got the two pieces very close, and their hands would vaporize, along with most of their body, ending that little experiment.
It’s why those who own some Pu are very careful with it. If any sizable amount were to accidentally come together it would be very bad news for the container, and anybody near by.
That’s only true for gun-type weapons; Pu implosion weapons take advantage of a phase change that Pu undergoes under pressure, raising the density enough that a subcritical mass becomes supercritical as a result.
We are not talking about the well known and absolute factual technology that of course can turn 25 kilos of Pu or U235 into a bomb. The question was could anyone manually create an explosion by slamming two masses together by hand.
The problem with U235 is almost the same as with Pu, the fission just happens right before the critical mass is reached, once again our hapless human is now holding in his hands a very hot metal spraying him with gamma rays and x-rays and heat, and the entire mass explodes before it can create what we think of as a nuclear explosion.
The reason you have to shoot it together very fast, and have it firmly bound by a lot of very strong steel and neutron reflection is to keep it together long enough for the fission to reach that point where it goes boom.
Anyone (theoretically) can pile up enough Pu or U235 and get a chain reaction, or a molten pile of deadly nuclear fuel going. The real problems are how to keep it from melting through whatever it is sitting on, or to get it to go boom. The problem of making it go boom was solved long ago. The problem of how to keep a pile of chain reacting metal from eating through whatever it is sitting on is the real problem.
What goes with out saying is that anyone could manually bring together, in their hands, enough Pu (or U235) to cause a chain reaction. But nobody can do it fast enough, or in the right way, with just their hands, to create a nuclear bomb explosion.
Hell, most people couldn’t get that to happen even if they could use all the tools and technology in the world.
You say “no explosion,” but a solid mass of several kilos of Pu turning into plasma before it reaches the end of the gun barrel sounds like it would be a pretty impressive explosion.
And in fact the only difference between a city-killing multi-megaton explosion and a fizzle is one of degree. What you described above is indeed a fizzle, i.e. a supercritical mass of nuclear material that has disassembled itself after only a very small percentage of its nuclear energy has been released. Between “fizzle” and “glass parking lot” there is a smooth continuum in which greater and greater amounts of nuclear energy get released before the supercritical mass vigorously disassembles itself.
So going back to the OP:
The answer is yes, you could assemble two subcritical masses by hand and create a supercritical mass, but it would result in a fizzle. This is still a nuclear explosion, i.e. the energy that is released would all come from supercritical nuclear reactions, but the magitude of the blast would be tiny compared to a military-grade nuclear device.
Surprised no-one picked up on this already, but U238’s useless for bomb-making - which is a nuisance as it’s much more abundant than U235 and they’re no picnic to separate.
If you are building a fission device yes, but H-Bombs use a great deal of U-238, indeed are mostly constructed of it. Neutron radiation from the fusion reaction is enough to cause the U-238 to fission, and roughly double the yield of the weapon. For the purposes of the OP, U-238 is however useless. Indeed it is quite possible to buy on the web.
A better dramatization of what would happen isthis scene from* Fatman and Little Boy*. Although the movie itself is not all that great this scene was well done and is actually based on two separate but very real incidents, both occurring with the same Plutonium core (the so-called ‘demon core’). Unlike in the film they both happened after the war, but like in the film both men who caused them died horribly painful deaths…
