Atomic Bombs - why are they so hard to make?

Factual Questions
1

Don’t worry guys, I’m no Ted Kazinsky. I’m just wondering the general process behind the making of the first atomic bomb. I’m pretty ignorant but here are my basic questions.

  1. What role did Einstein’s theory of relativity play in the making of the bomb?

    I know E=MC2(squared) meant that a very small amount of mass (M) could be converted into a very large amount (that M times the square of the speed of light) of Energy (E). Is that all there was? I know that is over simplifed but is it that general relationship between matter and energy that gave people the idea that an enormouns bomb could be made from a little matter?

  2. How did they do it? How did they take a theory and turn it into a physical reality?

    I know it had something to do with splitting the atom (nuclear fission). But how did they do it? I know that they used a “particle accelerator”. But what is that, what did it look like. And I think it was uranium that was first used. But what did that look like and where did they get it? Is just a brick a of metal, or a tiny piece rock??

  3. Why is it so difficult for other other nations, and more specifically rogue individuals to make there own atomic bombs?

    Is it the finding the necessary atomic ingredients? Why is that so hard? Or is the techonology involved in making the bomb, the particle accelerator, the detenator or whatever it is, very difficult. Is it only a matter of time before someone could make one in their living room and blow up NYC, or do you need millions of dollars of labratories and equipment.

What is up with A-Bombs??? I know that is a ton of questions. Some of you will no doubtedly want me to channel my curiousity into a trip to the library and read a book but I just thought I’d ask some of the smartest people I know (or kinda know) their basic thoughts on the matter. Thanks.

2

First off, let me summarize how an atomic bomb works. When a fissionable nucleus (like uranium or plutonium) captures a neutron, it breaks down into smaller atoms, releasing more neutrons. When another nucleus captures a neutron, it will decay, releasing more neutrons, and so on.

The trick to building a bomb, or, if your intentions are more peaceful, a nuclear reactor, is to get the fissionable materal to reach critical mass, the point where the neutrons emitted by each fission event have a large probability of being captured and causing another fission event which releases more neutrons, and so on. This is called a chain reaction. Basically, you need to get enough uranium in a small enough space.

For a power plant, you want that chain reaction to occur at a slow simmer, so you just need to put a bunch of relatively pure uranium together in a pile. (Indeed, it’s called a “pile.”) But for a bomb, you want a runaway reaction that will cause a lot of fission in a very short time, releasing a lot of energy before the thing vaporizes itself. To do that, you need a precisely controlled implosion to cram the uranium together. The implosion device is a carefully-constructed hunk of explosive packed around the uranium.

To answer your specific questions:

  1. Einstein’s relativity just says that it’s possible to convert mass into energy. It that wasn’t true, the Bomb wouldn’t work. However, discoveries in atomic physics were required to know that it was possible to make an atomic bomb.

  2. I suggest the film “Fat Man and Little Boy” It does a great job telling the story of the making of the first bomb. Plus it has John Cusack and Dwight Schultz (Howlin’ Mad Murdock from the A-Team).

  3. There are two difficulties for anyone who wants to build a bomb. First, you need an appropriate fissionable material. The desirable characteristics are purity and isotopic ratios. You want isotopes (like U-235) that will produce and capture lots of neutrons. This stuff is not easy to refine, and it don’t come cheap. Also, people tend to keep a pretty close eye on where uranium is mined and what is done with it. Then you need that implosion device I talked about–which is not easy to make. Happily, these requirements mean that if you want an atomic bomb, you have two basic options: 1) steal it, or 2) get a wealthy government to back your efforts.

3

If you just want a basic idea of what goes into fabricating an atomic bomb, I’d suggest reading Tom Clancy’s The Sum of All Fears. It’s a fictional account of a terrorist organization recovering a lost Israeli bomb and improving it into a two-stage fusion bomb. Granted, it’s fiction, but it gels with most of what I’ve read in non-classified nonfictional accounts and it is a hell of a lot more entertaining and less technical than a bunch of obscure scientific and military journal articles. Clancy admits he intentionally fudged a few minor technical details but they’d probably only matter to an engineer actually building a bomb, not a layman trying to get a general understanding of nuclear weapons and their proliferation.

4

As to 3.: The trick is to keep a supercritical mass of fissionable material (like U238) together long enough to make a big bang. As the reaction starts immediately the moment you have a supercritical mass, you tend to just get an atomic firecracker unless you can hold it together long enough. You could do this yourself by using inertia. (I.e. drop one subcritical mass on another from 3 stories high.) But by the time you got it set up you’d likely be dead from radiation sickness, and regardless, everything 'd have to go just right for it to work. In one of the bombs used in WWII (Big Boy?) I believe they fired pieces of plutonium into a central mass of it, from an encasing globe, simultaneously. That requires some serious engeneering. At present, fissionable material is hard to get, especially the stuff that works well in bombs. But if you apply enough resources, much like developing your own space program, you could build your own Bomb. There is strong speculation that if Irak hadn’t gone playing in their neighbor’s sandbox, they’d have one (an A-Bomb) by now…

5

Actually critical mass is the point that seperates bombs from reactors. Critical mass is the point at which uncontrolled chain reactions begin to take place. That means a bomb (or at least a fizzle or meltdown, aka China Syndrome). Reactors use control rods, usually cadmium or graphite, to temper the neutron flux and prevent the reactor from going critical. The idea is to keep the reactor just hot enough to produce sufficient steam to run the generator turbines effeciently.

6

Sorry for the simulpost.

I can take a stab at 3.

There are two considerations right off the bat that make it difficult to construct an A-bomb.

  1. The need for concentrated fissionable material. Early bombs used, I believe, U-238, which makes up only a tiny percentage of uranium ore. The concentration process is tedious and requires enormous amounts of raw material. Fission bombs can also use plutonium, but this again is tedious to produce. In either case, the only practical option for a homebuilder would be to somehow obtain ready-made fissionable material, but of course access to stocks of these materials is heavily controlled. OTOH, this may not be an impossible task; it is widely suspected that Israel obtained its first stocks by skimming off material reported as lost during production at a concentration plant in Apollo, PA.

  2. The need for extreme precision in the triggering mechanism. The fissionable material is held in a non-critical state until the moment of detonation; conventional high explosives are used to force the subcritical masses together to initiate the fission reaction. As the bulk of the fission events must take place more or less instantaneously to achieve a satisfyingly large ‘bang’, the high explosive charge must be shaped very precisely, and ignition controlled throught the use of extremely high-precision electronic switches. Once again, access to these switches is highly controlled, although numerous attempts have been made to obtain them by rogue states, and some attempts may have been successful.

There are other considerations as well, but other posters are surely more qualified to discuss them.

7

Again, that’s why you want to use something like U235…U238 isn’t really fissionable.

8

The engineering is pretty simple. The Little Boy Bomb had a “bullet” of U-235 fired into a “slug” of U-235. Both apart were relatively safe, but together they reached critical mass and the neutrons generated bumped into other U-235 molecules and released more neutrons and then bang.

The problem was gathering enough of the U-235 since it is only about 1% of the uranium in nature. Since they are both uranium with the same number of electrons, you cannot use chemical means to separate them. Therefore you must exploit the tiny diference is mass (3 neutron’s worth) and use a Calutron (basically a cyclotron) to separate it. (About a year ago the IEEE Spectrum magazine ran pictures of Iraq’s Calutrons…)

The plants used to extract the U-235 in WW2 were HUGE. Multiple football fields of interior space, and these huge plants only produced about 100 lbs of U-235 by the end of the war.

Plutonium, the material used in the Fat Man bomb is synthesized from uranium in a nuclear reactor. It is much easier to extract since you can use chemical means to separate it from the uranium.

I would add that if you want to know about the chain of discoveries that led up to the Atomic Bomb, read “The Making of the Atomic Bomb” by Richard Rhodes. It covers the whole process from the discovery of the Neutron (More important than Einstein’s theory in creating a bomb) to Hiroshima and Nagasaki.

9

The physics behind it is well understood now, and the role the physisists played was more along the lines of explaining what is happening and why so that the designers can put the pieces together in the correct manner. The physisists had to understand the nature of something they couldn’t see, and had experience with only indirectly and beyond the scale of things in everyday experience. It is similar to the problem modern physisists face in explaining quantum theory, in that the problems are so abstract it takes a very special type of mind to even understand what is believed so far. Read some of Richard Feynman’s stuff to get an idea of the level of intellect needed.

I don’t know if particle accelerators were used in the manhatten project, but they are basically tubes surrounded by electromagnetic coils that are fired in sequence to accelerate particles down the tube to a target material. Watching the results of the collisions would give them greater understanding.

Once you have the critical mass of uranium or plutonium, the hard part is making it not explode, or more specifically, making it explode only when you want it to. The critical mass for uranium is higher than that for plutonium, so plutonium is the prefered material. Plutonium is not only radioactive, it is very poisonous in very small quantities, so working with it is very difficult. The basic idea behind an atomic bomb is having a bunch of pieces of plutonium kept seperate, and then putting them all together at once to make it go boom. Needless to say precision is critical, so the casting and machining equipment has to be quite advanced, and with all the plutonium dust in the air the environment is pretty deadly. Assembling the bomb and having the means to detonate the ring of conventional explosives surrounding the pieces of plutonium in a manner that will make certain all the bits will slam together just right is really tough. The early bombs just brute-forced it and used a whole lot of plutonium, which made the bombs bigger and heavier, and therefore tougher to direct and control. Control is what makes a bomb useful.

10

I seemed to understand that the explosion used to cause the impact was also required to cause the initial burst of neutrons, though I’m not aware of how that would be possible from a chemistry standpoint. I just didn’t think that the half-life of Uranium or Plutonium was enough to cause a lump of material to self-detonate.

Of course, if the explosion around it was intense enough, the density would increase drastically to the point where just a few hundred decays would be all that is necessary to start the reaction, since each decay releases three neutrons itself.

Indeed, physics “caused” the bomb by noticing that fission of a U atom created (lead? Strontium, I can’t remember) and that the leftover parts weren’t adding up masswise. Thus, some of the mass either escaped or was converted into energy.

Of course, I could be making this up, but that was my understanding of the whole thing.

11

I used to work at a rather infamous DOE plant called the “Mound,” located in Miamisburg, OH. We made and tested triggers for nuclear weapons. And I’m here to tell you, triggers ain’t easy to make. The amount of facilities, lab equipment, infrastructure, and professional & support personnel needed to make triggers is simply mind-boggling. (It makes rocket science look like child’s play.) The facility has over 400 acres, 120 buildings, and its own fire department. In other words, you couldn’t do it in your basement.

12

Actually, Podkayne had it right the first time. This is a common misconception. A nuclear reaction is said to be critical if each fission produces exactly enough neutrons which are absorbed causing subsequent fissons to continue the reaction at the same rate. In other words, each fission releases a neutron that produced one more fission in the next generation.

When a reactor is critical, it is at a steady-state, and the power produced is constant. If the reactor is sub-critical, then the power is decreasing. A super-critical reactor is one in which the power is (you guessed it) increasing. This is a perfectly normal state of affairs. It happen when the demand for power from the reactor increases, or when the reactor is being brought online from being shutdown. After the period of supercriticality while the power is increasing, the reactor returns to critical at the higher power and is in a stead-state once again.

An A-bomb, of course is also an example of a supercritical reaction, just of a much higher degree of criticallity. The point is, that when you see a seen in a movie where the engineer runs screaming into the room saying that “The reactor is CRITICAL!!! She’s gonna blow!” then you should know that the script writer is clueless about nuclear power.

13

There is a small neutron source used to kick things off after the critical mass is brought together. You could wait for spontaenous decay to kick off the reaction, (It only takes 1 neutron!), but they kick in a neutron to make sure the thing happens at the correct time.

It is especially critical for an implosion weapon since the force of the explosion may separate the material before the reaction has a chance to get going. A terrorist’s “Little Boy” probably does not need a neutron trigger since you can wait a while for the reaction to get going.

Crafter_Man may not even be able to tell us, but I can think of about 6 components that could plausibly be called nuclear “triggers”

  1. If we are talking about fusion weapons, there is a small implosion-type fission device used to set off the fusion reaction.

  2. The shaped charges of High Explosive in an implosion device could be the “trigger”.

  3. The detonators that make the shaped charges go off could be the “trigger”.

  4. The X-Unit that makes sure that all the charges go off at the proper time could be called the trigger. (The krytons – small vaccum tube switching devices – used in these also got Iraq in trouble 10-15 years ago.)

  5. The neutron source mentioned above – which is still highly top-secret – could also conceivably be called a trigger.

  6. Actually, I guess the radar altimeter could also be the trigger, though I know that these are made in Kansas City.

14

>>"I don’t know if particle accelerators were used in the manhatten project, but they are basically… "

As I recall, Isotope separation in WW2 was done using gas diffusion. The raw uranium was chemically converted to a gas (Uranium hexafloride?? i can’t remember for sure). The gas molecules with a U235 have a slightly lower mass, so at a given temp they have a slightly higher average velocity and will diffuse through materials slightly faster.
-Luckie

15

What ever would give you the ridiculous idea that atomic bombs are hard to make? Every single country that has seriously attempted to build an atomic bomb has succeeded. Hell, even I could make a nuclear weapon if I had access to a critical mass of plutonium.

I suggest you go read the Nuclear Weapons FAQ:

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

In particular, read section 2 about weapon design.

16

To clear up a few things:

Fissile isotope separation is by far the most expensive and time-consuming part of making a bomb. Natural Uranium is 99.3% [sup]238[/sup]U and 0.3% [sup]235[/sup]U. Both of these isotopes are fissionable, i.e. they can be made to split apart by hitting them with a neutron. Only [sup]235[/sup]U is fissile though, i.e. it can sustain a chain reaction. [sup]238[/sup]U will only fission when hit with a neutron whose energy is >1MeV; however most of the neutrons produced by the fission only have an energy of about 700keV.

To make a bomb you basically need to have pure fissile material, which means doing isotope separation, and this is notoriously hard. The K-25 plant in Oak Ridge, TN that made the enriched Uranium for the Manhattan project used roughly as much electricity as New York city.

Making a bomb out of Plutonium (produced in nuclear reactors by bombarding [sup]238[/sup]U with neutrons) is somewhat easier than making one out of natural Uranium, but you still have to do isotope separation. Plutonium, much like Uranium, has both fissile and non-fissile isotopes ([sup]239[/sup]Pu is the most commonly used fissile isotope). You cannot make pure [sup]239[/sup]Pu in a reactor though–[sup]238[/sup]Pu and [sup]240[/sup]Pu are also produced in large enough quantities to make the Pu that comes out of a reactor unusable directly in a bomb. [sup]240[/sup]Pu in particular is problematic because (IIRC) it decays relatively often by spontaneous fission, which will set off your bomb too quickly causing it to fizzle.

On to the next thing: critical mass is a rather ambiguous property, and there isn’t really such thing as “The critical mass of [sup]235[/sup]U.” If you hear someone say that the critical mass of [sup]235[/sup]U is 50Kg (or whatever), what they really mean is that the critical mass of a pure metallic sphere of [sup]235[/sup]U with no neutron moderators or reflectors at room temperature is (whatever). Any change in geometry, material composition, temperature, etc. can have quite a large effect on the amount of fissile material required for criticality.

Finally, neutron sources: trypically neutron sources are composed of Beryllium + a strong alpha emitter, such as [sup]241[/sup]Am or [sup]239[/sup]Pu. When struck by an alpha particle, Beryllium will turn into Carbon and release a neutron. I think that in bombs the Beryllium and alpha emitter are somehow joined together by the detonation of the explosives.

17

Ok, this is getting a little too complicated. All you need is an appropriate amount of pure fuel, and a trigger. Everything else is just a matter of when and how big you want your boom to be. With a pure fuel (say 235), you can theoretically set it off with a hammer, provided you don’t mind being the only victim.

Two problems with this, besides the obvious. One, it might not work when you expect it to. You might have to hit it a couple times. You might hit it, the lift the hammer again, and it goes off before you hit it again. It might go off by itself before you hit it the first time. It’s all a matter of chance.

As a result, most bomb-grade fuel isn’t pure, but is instead doped a little to prevent things like this from happening. Last thing you want to do is visit the afterlife because you happened to be the unlucky guy that loaded the bomb onto the plane. Instead, you want your missles to meet a certain requirement for purity so you know that not only will they not only be stable until you trigger them, but they also won’t bounce harmlessly off the enemy’s lawn.

The other problem is the size of the boom itself. You don’t want to set off a nuclear war only to have your ICBM’s make a cute little “poof” noise. You want them to take out cities and small countries. You want them to level as much as they possibly can if you have to use them. This doesn’t necessarily mean having a pure fuel either, because if it’s too pure, it will blow before the optimum number of nuclei have reacted. Doping helps prevent this by “spacing” the nuclei far enough apart that as many of them as possible have a chance to react in the instant it takes the neutrons to move from one to another.

The trigger part is another story. Compared to making the fuel, setting up a trigger is akin to wiring your house. Not terribly hard, but you don’t want joe shmoe doing it either. The basic idea is to wrap the fuel with standard explosives, then light them off so that the resulting explosion smashes the fuel into a small, dense area. Then the fuel reacts, and the reaction takes care of itself.

The hard part is getting it right. The older bombs are easier to explain, if you’ve seen them. They are spherical, with wires hanging out of panels all around them. Each wire has to trigger the exposion at the exact right time to ignite the panel it’s attached to. A millisecond either way, and part of the fuel wont fire, or you may not get a reaction at all. Sad as it is, if the timing is completely off, all you may end up with is a conventional explosion from the trigger itself.

Just for the record, it’s not that difficult for rogue nations to build bombs. The US gov usually likes to say that it’s hard for them to get the fuel. It’s not. The hardest part is getting the people to work on them. While I’m sure there are a few crazies out there that are more than willing and capable of building a bomb, they are surprisingly few and far between.

Remember, it’s not a one man, basement-type operation. You need some impressive equiptment to prepare the fuel. You need operators to run the equiptment. You need someone to test, or at least calculate the proper arrangement of the fuel to the trigger, as well as the delivery mechanism. You need a lot of highly skilled technical people that believe that the bomb is the best answer to whatever problem you have. These tend to be few, as most highly skilled, technical people have better things to do than risk getting bombed out of existance if your enemy discovers you’re working on the bomb and decides to slow you down a bit.

For an example of how hard it is to build a bomb if you have the right people, look at the recent example of india and pakistan. Both countries have long had the skilled people to build a weapon. It was only when tensions reached a boiling point that they did it, and ignited one. Not too long after, pakistan did the same. How? They were able to drum up enough fear of each other that both sides were able to convince enough people to work on it to protect their countries.

18

I’m having a hard time finding it in the archives, or in any other search, but I know we talked about a high school kid that was building an atomic weapon in a shed in his backyard. Help me find this!

19

Since when has this been top secret? I learned at school that they used tritium as the neutron source!

20

The documentary Trinity And Beyond: The Atomic Bomb Movie has the only film I’ve seen of the moment of fission, and I have no idea how they got it; it’s pretty cool, though. You can see the explosion’s pressure forcing the nuclear material into a smaller and smaller package, until there’s a sudden white flash from its center…