# Help Me Build an Atomic Bomb

I have to remove a tree stump in my backyard.

But seriously, I’m a math dummy and a part-time history buff. I’ve read several books on American history and WW2 (currently, William Manchester’s “The Glory and the Dream” which is friggin’ awesome), and these books often take a few paragraphs to explain how the original A bombs worked.

I still don’t get it completely, I think I need a few more paragraphs. Maybe a good link or website.

Here’s what I know so far:

First, you get two lumps of “Uranium-235”, a very special kind of uranium, the uranium at Wal-Mart isn’t good enough.

These lumps of uranium should be hemispherical in shape and of unequal size say, five pounds and seventeen pounds. (Why?)

Then you smash these two lumps of “Uranium-235” into each other really hard. Like, maybe you could shoot one of them out of a cannon at the other one. This causes a massive explosion which can be seen from Nevada to Texas.

(That gives me the most pause, is it really just banging rocks together?)

Apparently this process also involves making a ‘pile’ under the Chicago Squash Courts. I suppose I could handle that…

And that’s what I know so far. If anyone can fill any gaps in my knowledge I’d be grateful. There’s a family of mean-tempered wasps who’d like to borrow that tree stump for the summer and negotiations aren’t going well.

Oh boy, where to begin…

First off, the method you describe is basically accurate and is called ‘the gun method’. It was used for the Hiroshima bomb because it was considered nearly 100% reliable though not as powerful as the other design, ‘implosion’. Basically just a different way of doing the same thing, squishing a fissionable material (Uranium or Plutonium) until it chain reacts (i.e. goes bang, like, really loudly).

Implosion was used on the Nagasaki bomb. This, by the way, is the reason that the Hiroshisa bomb was called ‘Little Boy’, because it was long and thin (because it had a big gun inside it) and the Nagasaki bomb was called ‘Fat Man’, because it was big and round (because it had a round core surrounded by high-explosives which when detonated imploded it).

Anyway, the bit you mention about the two halves being of unequal size I’ve never heard of. The gun method was immediately abandoned after the war because implosion made bigger explosions (and they figured out how to make the design reliable) so I don’t know much more about it.

The type of Uranium is crucial. Uranium, by the way, is a metal not a rock. U-238 is the naturally occuring kind and won’t fission. U-235 will but, luckily, it is appallingly difficult & expensive to make, as is Plutonium. Most of the \$4 billion that The Manhattan Project spent went to U-235 & Pu creation.

As for how to build an implosion device the only real hard part, besides obtaining fissionable materials, is that the high explosives have to be aimed at the core very precisely and have to all detonate together within a few millionths of a second.

As for how fission itself works, Uranium is the heaviest naturally occuring element (its twice as heavy as lead). Plutonium is even heavier. The atoms of U-235 are especially unstable and, basically, when compressed really quickly neutrons are knocked off each atom. This releases a bit of energy. The neutrons then go on to hit other atoms knocking off more neutrons releasing more energy creating a chain reaction.

Do a search on any search engine and you’ll find everything you want (and don’t) want to know.

You should read the Nuclear Weapons FAQ from the Federation of American Scientists:

http://www.fas.org/nuke/hew/Nwfaq/Nfaq0.html

I will also note that every country that has obtained a sufficient mass of weapons-grade material has successfully created a nuclear weapon.

Okay, in the “gun type” bomb, you have a large, but subcritical mass of Uranium. Subcritical means that the mass is not large enough to cause the majority of the metal’s own emitted neutrons, to strike other U-235 atoms, which would cause a chain reaction, and therefore detonation.

In Little Boy, they had a spherical subcritical mass with a cone-shaped indentation in one side. The “Gun” assembly, on detonation, literally ‘fired’ a cone-shaped “bullet” of Uranium into the sphere’s recess. The two masses, when combined, were then slightly greater that critical mass, which then caused the explosion.

That’s why you have two pieces of U-235.

However, it’s wasteful and inefficient. The majority of the bomb’s fissile material is wasted because the explosion essentially blows it apart faster than some of it can react.

For a Plutonium weapon, the “gun” system cannot work- the critical mass forms too easily, and the material blows apart before more than a small fraction has had a chance to react.

In these cases, the Plutonium is shaped into a sphere. Sometimes it’s hollow, sometimes it’s solid, sometimes it’s a terribly complex mathematically-derived spherical construct that’s hard to picture visually, and even harder to describe.

Here, an outer sphere of sophisticated explosives compacts the Plutonium mass, almost instantly increasing it’s density many times over, which not only creates the critical mass, but also the “sphere” of explosive force adds a measure of the needed “delay” to allow the fissile material to more fully react before it’s all blown to smithereens.

Nowadays, the Plutonium devices are no longer even weapons in their own right. They’re simply “fuzes” which, when detonated, ignite a far larger reaction using more Uranium, deuterium, tritium, and other exotic materials. These are called “multistage” or “thermonuclear” weapons. The yeild from the Plutonium “fuze” may only be in the tens of kilotons, but the second (and third) stages can be in the multi-megaton range. Plus have the added… I suppose ‘benefit’ is a poor word here… of requiring less hard-to-obtain nuclear material per weapon for a given yeild.

Sounds to me like you’re looking for a primer on the guts of the matter, the actual nuclear reaction. It is certainly NOT the same as banging two ordinary things together, not even two bottles of nitroglycerin. That all happens at the chemical level, outside the nuclei of the associated atoms.

In any sample of radioactive material, U-235 included, there are always a certain number of reactions taking place spontaneously. Nuclei (the lumps of protons and neutrons at the center) this large have a tendency to fall apart because of all the positively-charged protons pushing against each other. “Fall apart” is a bit of a euphemism, it’s rather more violent than that.

Lots of smaller stuff gets ejected at high speeds, like a couple of smaller atoms (Krypton and Xenon, f’rinstance), an alpha particle (basically a Helium nucleus), some high-energy gammas and stuff (I’m sure our resident physicists here can chime in with the whole list of up-muons and strawberry-flavored quarks and whatnot ;)) - and, most importantly to the bang we’re looking for, a few fast-moving spare neutrons.

In most ordinary samples, these guys will get slowed down (or “thermalized”: at the atomic level, fast random motion is what heat looks like, so once they’ve slowed down to the speed of most other things around them they’re thermal - i.e., at the same temperature), or harmlessly absorbed by some impurity, or leak out the outside and into the scientist, or something. But if you get enough U-235 reeaaally close together, those oafish extra neutrons smack into neighboring nuclei hard enough to split them apart - and then they give up their own couple of fast neutrons, etc., etc.

Pretty soon (and we’re talking, within a few trillions of a second), there’s lots of nasty subatomic particles plowing around in there really fast. In the inverse of what I said before, that - on the macroscopic scale - shows up as heat. Lots of heat. Heat, and X-rays, and expansion of core and bomb casing and a few cubic miles of air/dust/water into that big pretty cloud you see in all the pictures. Neat, huh?

As a moderately interesting side note, this is almost precisely the same thing that happens inside the Navy’s nuclear power/propulsion plants. There’s just a whole ton of safety equipment added to ensure that the reaction never starts taking place too quickly. And in over 40 years of trying, they’ve never lost one man to a reactor accident.
[sub]Oh, and thanks for jumping in and beating me while I was typing all this up, Doc. Grrr.[/sub]

In the USA all the instructions are patended. Any old patent library (ucsd HAS ONE) has all the patents you need. e.g The Erickson patent.

Thats as far as i’LL go with info.

I found this on the Internet a while ago.

[friggin’ huge post removed --Chronos]

[Edited by Chronos on 05-28-2001 at 05:24 PM]

Nor had a serious accident in over three thousand reactor-years. Unfortunately, that last bit is an over-simplification so great as to be essentially false.

Naval Nuclear Reactors are designed in a manner that even without safety equipment, the worst possible condition you can get would be prompt-critical, resulting in a steam explosion, followed by a decay-energy meltdown. You simply can’t get a weapon-type critical geometry out of a Naval PWR. Further, Navy reactors use a thermal fuel, rather than a fast fuel like you’d find in a weapon. Specifically, the U[sub]235[/sub] is held in a matrix mixed with discrete and distributed poisons, surrounded by water that moderates neutrons down to thermal energy, where they can then cause a chain reaction.

The density of the coolant water regulates how many neutrons are moderated, and how many escape the core before achieving thermal energy (migration distance). Hot water is less dense:. fewer neutrons are moderated:. power decreases, and vice versa. This is the characteristic that makes a PWR inherently stable. Once you’ve removed the water, you’ve got very few thermal nuetrons:. no more chain reaction. You’ll still have a significant amount of energy from the decay of short-lived fision daughter elements, which will likely destroy the core if the heat isn’t removed quickly. If you’ve got sufficient decay heat (depends on power history), you might get enough fissile material gathered in the bottom of the core vessel to generate a long-term, low energy reaction, but that’s damned unlikely, requiring an astronomical power history followed by a disasterous main coolant rupture, such as a non-isolable double-ended shear.

[Moderator watch ON]

Studi, please don’t quote an entire published work here… That’s copyright violation, and could get you (and us!) into a lot of trouble. Considering that you found this information on the Internet anyway, you could have saved everyone a lot of trouble by just posting a link to the original page. That failing, you could have rephrased the same information in your own words, or quoted a small, relevant excerpt from some source. Just don’t post an entire copyrighted work (webpage, poem, short story, etc.), unless you, yourself, own the copyright.

how is Pu235 heaver then U235?

P has an additional electron - yes but it also has a proton instead of a (slighltly heaver by about the weight of an electron) neutron.

I say they weigh the same.

My post, my post! My kingdom for my post!

Yeah, Chronos, you’re right. I actually thought that the file no longer existed on the Internet.

Meh.

Studi