This person is kinda of a show-off. He wants to build a nuclear weapon simply to impress everyone because he's a smartass.

He means no harm, fissile and explosive components are replaced with inert materials so the contraption is completely safe. He just wants to show that he can do it.

Can he do it? In the sense "Is it legal for someone to build a safe but possibly functioning atomic device in their backyard? What could be the consequences?"

I expect him to receive some phone calls at the very least.

(Asking for a friend of a friend)

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    $\begingroup$ There is no such a thing as safe nuclear bomb. Without fissiles, it's not a bomb and proves nothing. Acquiring fissiles in quantity and purity high enough is THE challenge behind nuclear bombs. Nuclear weapons usually means bomb and delivery system to which I can tell you that building fully operational strategic bomber is outside of any single smartass' competence. You need a large team of smartasses, with large technological park. Also, I'm pretty sure building operational strat bombers might be illegal. $\endgroup$ – M i ech Aug 13 '17 at 17:13
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    $\begingroup$ I think Kim alteady know the answer. $\endgroup$ – Vincent Aug 13 '17 at 17:27
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    $\begingroup$ @Swier This article is definitely a very good answer, please write one. Secespitus: This is not a duplicate; the principle of nuclear reactors have nothing to do with nuclear weapons. A reactor is balancing out a nuclear chain reaction (A nuclear reactor will never explode like a nuclear bomb, its heat will simply melt through the material), a bomb tries to trigger an exponential growing chain reaction by holding supercritical masses together (implosion) before the generated heat simply separate them again. $\endgroup$ – Thorsten S. Aug 13 '17 at 18:18
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    $\begingroup$ @Thucydides The DPRK already knows how to build the bomb...? $\endgroup$ – Thorsten S. Aug 13 '17 at 22:08
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    $\begingroup$ @Miech: Without fissiles, a competent nuclear engineer can tell if a device has a feasible design. And in fact, such devices have been built for testing purposes. But the important verb is can tell. As in, that engineer won't tell you how close you got, and which bits were wrong. He'll be talking to the department of defense instead. And they'll be trying to figure out which bits you discovered on your own, which through open literature, and which through nefarious means. $\endgroup$ – MSalters Aug 13 '17 at 22:52

Several different issues:

  • As you guessed, getting fissile material and explosives would be illegal.
  • Getting the right electronics to trigger the non-existent explosives might also be illegal. There are export controls on many of these parts, and that could mean paperwork. Is the character going to lie in official paperwork?
  • Assembling the dual-use electronics into a weapon configuration is even less legal. It could be seen as a violation of the non-proliferation treaty, and surely countries are not allowed to weasel out ouf it by saying "it was a private citizen who did the design."

Note that quite a lot of industrialized countries have both fissile materials and high-end explosives. They promised that they wouldn't design bombs. And by and large other nations believe those promises -- while keeping in mind that Japan, South Korea, Germany, etc. could go nuclear in a hurry if they ever saw the political need to take such a step.

  • $\begingroup$ If the trigger is just electronics can't he just make his own? Or are there necessary components that just can't be found? $\endgroup$ – SilverCookies Aug 14 '17 at 8:54
  • $\begingroup$ @SilverCookies, making electronics is from scratch is rather difficult, and orders of the necessary parts will probably be monitored (if only because the government would suspect him of being a North Korean or Iranian agent). $\endgroup$ – o.m. Aug 14 '17 at 17:47
  • $\begingroup$ Thanks, just to clarify what kind of components would he need to obtain? I am no engineer but was under the impression one could make basic electronic devices like triggers and switches out of stuffs like Arduino and Rasberry. $\endgroup$ – SilverCookies Aug 15 '17 at 19:10
  • $\begingroup$ @SilverCookies, I don't know the details, either, but from news reports those high-speed switches are one key element. Computers have gotten faster by a factor of 1000 since I was young and when all is said and done, it was possible with WWII technology. Highly accurate blasting caps? $\endgroup$ – o.m. Aug 16 '17 at 4:55

This answer is mostly based on the following article: https://www.theguardian.com/world/2003/jun/24/usa.science

In the 60's the U.S. wanted to know if it was possible for a random country to build a nuclear bomb. In order to test this, they recruited two people (one of which Dave Dobson) with physics PhDs, but no further knowledge of how to build a nuclear bomb. They gave them an office and helped them conduct 'experiments' (they told them the outcome), but gave them no access to classified information. After two and a half years they had designed a nuclear bomb of the more advanced implosion variety:

The whole works, in great detail, so that this thing could have been made by Joe's Machine Shop downtown.

They were told the destructive power would be similar to the bomb dropped on Nagasaki.

At the end of the article, Dave notes the following about a nuclear bomb:

It turns out it's not overwhelmingly difficult. There are some subtleties that are not trivial ... but an awful lot has been published. If you were a grad student today, and you reviewed the literature, a lot of pieces would fall into place.

So yes, if he's driven enough and has a solid physics background, he can most likely pull it off.

Another example, a student who designed a bomb as a physics term paper: http://people.com/archive/a-princeton-tiger-designs-an-atomic-bomb-in-a-physics-class-vol-6-no-17/ or this wiki article
He faced no criminal charges. However, part of his paper was confiscated, as well as his mockup. As for how difficult it was, he states:

It’s very simple. Any undergraduate physics major could have done what I did.


If there's no fissile material, it's not a nuclear bomb, thus, as you said, perfectly safe. It's perfectly legal to make a scale model of a nuclear bomb. Just make sure that the phrase in front is used at all times ("scale model of...")

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    $\begingroup$ A nuclear bomb is characterised by more than just the fissiles. The triggering mechanism is highly classified and protected in its own right. That alone will get you locked up. Not speculation - there have been multiple sentences handed out (Nazir Ahmed Vaid, Richard Kelly Smyth). Then you've got the conventional explosives. Explosives are not illegal per se, but you need a license. And you're not going to get one for the particular type you need for atomic weapons. So: no fissiles, no trigger, no firing mechanism - basically an empty shell. $\endgroup$ – MSalters Aug 13 '17 at 23:05
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    $\begingroup$ @MSalters - while it's true that there is regulation on detonators (particularly exploding bridge wire detonators and related technologies) that are capable of initiating the large numbers of nearly-simultaneous explosions that are required for (some designs of) atomic bomb, that doesn't mean its impossible to acquire them legally, and they aren't classified, so you can actually find the design of the original EBW detonators used during the Manhattan project. Both Vaid and Smyth weren't prosecuted for having such detonators, but for attempting to export them to non-permitted countries. $\endgroup$ – Jules Aug 14 '17 at 2:43
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    $\begingroup$ ... as long as you don't attempt to export them, in the US at least, you'll be just fine legally speaking. $\endgroup$ – Jules Aug 14 '17 at 2:43

Building an old "Hiroshima style" A-bomb is not difficult once you get the fissile material, that's the reason why they're trying to regulate (read: "make pretty much impossible") to make nuclear reactors for countries that do not already have them. Once you lay your hands on enough fissile material building a bomb is within capabilities of (almost) any "smartass".

Getting the fissile is the problem.

After you've done that all you have to do is to put a certain quantity (the "critical mass") all together in a place.

Small problem is you have to be fast otherwise it will fuse in your hands ("predetonates" is the technical term). Reason for this is chain reaction starts in the surfaces facing each other, but it's still unable to self sustain, so the surfaces will melt (fast in a matter of milliseconds) and reaction will stop. It will begin again if you keep bringing back together until the whole mass is fused and starts digging a (very radioactive!) hole in the floor.

In the original bomb they had two half-spheres, each under critical mass, that were brought together by an explosive charge (i.e.: one was the target and the other was shot at it with something resembling a short cannon).

Once you have a ball of enriched Uranium bigger then critical mass chain reaction will not stop so easily due to inertia of Uranium that will remain in place long enough to generate a nice Atomic Mushroom (highly venomous variety).

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    $\begingroup$ A small correction: not only you need to bring critical mass in place, but you also need to held all that U together. The longer you able to held it, the more uranium will undergo fission and less will uselessly pollute the area, being scattered by explosion force before it had chance to react. $\endgroup$ – Revolver_Ocelot Aug 13 '17 at 19:16
  • $\begingroup$ In the original bomb ... There were two quite different designs for the first atomic bombs. The one you describe and one using the gun type design, which is arguably easier, but less useful for advanced weapons and consider unsafe (an unsafe bomb - who'd have thought ? :-) !) $\endgroup$ – StephenG Aug 13 '17 at 22:17
  • $\begingroup$ The gun-type bomb didn't use two hemispheres of uranium. They used a ring and a cylinder instead, which let them get a higher supercritical mass, and do so more reliably. $\endgroup$ – Mark Jun 3 '18 at 18:44

How hard? Depends on the reliability you want.

Take two near-critical masses of U-235, machined so they don't need precise assembly (put a cone on one, an inverted cone on the other--if they aren't aligned perfectly they'll align themselves), put one one in your left hand, one in your right. Slam them together.

Your death is certain. There's a reasonable chance you take a city with you.

  • $\begingroup$ You can't slam them together fast enough. You're almost certain to get a predetonation that lethally irradiates you, possibly accompanied by a small explosion (of the "blows your hands off" variety, not the "level a city block" variety). $\endgroup$ – Mark Jun 3 '18 at 18:46

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