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[keptin]

[disclaimer, I'm not an expert]  I've always been interested in how things work and thought I might share about the mechanical simplicity of the big bad bomb...namely, Little Boy, a gun-type fission bomb.  Not to scare you, but to educate you, because doom prophecies aside, it's fun science.  Also, this is a scifi forum, so it's sort of thematic  

So, I've heard it said before that if it weren't for the difficulties of obtaining weapons grade fissile materials, low yield nuclear weapons could more or less be made in someone's backyard.  I sent out on a wiki-venture to see just what is entailed.  After some digging, it seems that gun-type fission bombs are in fact fairly simple in construction.  Perhaps not quite DIY assembly, but it's not nearly as complex as most imagine.

They sort of work like this,


A set of uranium rings >1 critical mass is shot toward a <1 critical mass and slightly too thick uranium "plug" surrounded by a neutron reflector.  
                                               ========
||||||||||
                         ------>            DDDDDDDD
||||||||||
                                               ========

What that means is that due to their shape and location within the device, there aren't enough neutrons in a small enough space for the mass to go supercritical, but when brought together and encased in a neutron reflector...
========
||||||||||
DDDDDDDD
||||||||||
========
...boom.

The simplicity of this design means that the only limiting ingredient is weapons grade uranium-235.  It's not very efficient and it's a terribly unsafe design in terms of accidental detonation, but it does work.  It took a lot of research to understand the science behind it, but mechanically, it's very simple.  Thankfully, manufacturing usable quantities of the gooey rich center (90% uranium-235, not very gooey) is very difficult to this day. Even more fascinating are all the different ways nuclear fuel is made and used for good, like generating electricity.  I love learning/teaching this stuff if anyone has questions.

[WKOB]

 Similarly an early firearm was frighteningly easy to build but power was still limited to the government because the government controlled the material needed, black powder.
You don't get the weapons grade material you need to build a nuclear weapon of substantial payload with out attracting some attention, so they're not so really easy to manufacture.

But this thread is about the science, not the politics, so carry on. Neat explanation of it, though, you could teach this stuff to elementary school kids.

[keptin]

Ah yes, good point!  I didn't want to get into the politics behind it, but suffice it to say that advancing technology certainly makes the manufacture of controlled materials easier.

Neat explanation of it, though, you could teach this stuff to elementary school kids.

haha, some parents might frown upon that

[Dr.Noid]

You can also just watch youtube to have a prof explain it all to you:

It's a really nice series.

[keptin]

While the principles hold true, his noted assembly of Little Boy is incorrect per wiki.  It was not two half-spheres coming together, but as I described above.  For some time, it was mistakenly thought that there was a small wedge or cone driven into a larger sphere mass.  The way I understand it, given the total mass of the uranium used, a large sphere would already be greater than one critical mass and of the right geometry to achieve a supercritical reaction on its own.

You definitely couldn't have a supercritical mass just sitting there encased in a neutron reflector.

[Thaago]

Aaaaaaaaand everyone's on a watch list

Yeah criticality accidents are terrifying: https://en.wikipedia.org/wiki/Criticality_accident

This was a criticality accident that caused the first ever meltdown. Be warned that the wikipedia article is a bit graphic.

Spoiler

https://en.wikipedia.org/wiki/SL-1

[close]

[naufrago]

The thing about Uranium-235 is, it requires a fairly large-scale operation to get an appreciable amount of it. There's a reason only the first couple of nuclear bombs used it: it's easier to enrich the Uranium (specifically the Uranium-238, which is the most abundant Uranium isotope by far) into Uranium-239 in a neutron-rich environment (like in a nuclear reactor, first fueled by Uranium-235), which then beta decays into Neptunium-239, and ultimately into Plutonium-239 (which can be used to enrich even more Uranium-238 to make more Plutonium). Plutonium-239 not only has the advantage of being much easier to produce than Uranium-235, it's also more fissible than Uranium-235, meaning less is required to create the same energy.

To have any chance of making usable fissible material in your backyard, you'd need a nuclear reactor. So, a catch-22? Mmmm.... not quite. Here's a story of a Boy Scout who created a makeshift nuclear reactor in his mom's shed, and dangerously irradiated a large number of people in the process: http://www.dangerouslaboratories.org/radscout.html (and by all means, heed the warnings at the top of the page and DON'T TRY ANY OF WHAT HE DID. I'm actually questioning whether I should be posting this link in case one of you nutters gets any funny ideas. =P But it's a really interesting story and worth a read, regardless.)

It's questionable whether that would be sufficient for enriching enough Uranium into a useable fissible material, if it's possible to obtain enough Uranium to enrich, or even purify the Uranium into something usable since, thankfully, his experiment was discovered before any worst-case scenarios were realized. But his story shows that someone creating a nuke in their backyard may actually be possible. Slightly worrying, but thankfully most people aren't crazy enough to try. [EDIT: I should also mention that it'll more likely just generate a bunch of radiation and not produce the desired results. You'd have to ask someone more informed than me.]


As for why I'm interested in nuclear stuff, it's because my grandfather was one of the folks who worked to deplete the Uranium to produce the Uranium-235 used in the first atomic bombs (he was a Chemist). That's part of where my insight into the process of creating Uranium-235 comes from. Also, there was a very informative documentary of the Manhattan Project on The History Channel.

[GUNINANRUNIN]

My grandfather was an engineer who brought nuclear barges through the Panama Canal.

Some of this stuff sounds familiar to me since we always talk about his Army days.

We always work our way around to this topic whenever we talk about fission.

[LordHerpDerpington]

...aaaand everybody who read or posted on this thread is now on a CIA watchlist. Congratulations, gentlemen.

[GUNINANRUNIN]

Please don't necro.