The Atomic Bomb

I'm doing a report on the Manhattan Project and there are a few things I don't understand about the bombs.(1) In the Uranium model (Little Boy), there are two separate containers of Uranium. They get shot together and attain supercritical mass. Looking at the diagram on this page (Page not found), I don't see where the neutron initiator is in the Uranium model. If I'm not mistaken, once supercritical mass is achieved, there's still needs to be a neutron initiator to get the chain reaction going.
(2) There's also talk about how the two Uranium pieces needed to brought together quick enough so that spontaneous fission doesn't occur and fizzle the bomb out. I don't understand why the period of time where the one piece is shot into the other is particularly susceptible to spontaneous fission. Wouldn't spontaneous fission be possible in the plane ride up there and where it was being stored in the first place?
(3) At what level are each of the two Uranium pieces at while they're separated (not yet critical, critical...)?
(4) How come neutrons that are reflected back from the neutron deflector in either the Uranium or the Plutonium models don't bring about a premature fission?
(5) When the bomb eventually explodes, is the fission controlled or is controlled nuclear fission only used for testing in labs and stuff?
EDIT: (6) Why are neutron initiators needed in the first place? Why not just successfully get the fissionable material to supercritical mass and then spontaneous fission set off the chain reaction?I'm a little confused. Thanks!
(...and sorry if this is the wrong forum.)[This message has been edited by Tsegamla, 10-19-2003]

As I recall the neutron source is actually in the center of the larger uranium mass, like the core of an apple. It's like conventional explosives. In order to get a really big bang you need to have the chain reaction occur as quickly as possible. If it's spread out over time, the bang is smaller. It's like C4 - you can burn it on a campfire safely, or use it to bring down a building. It's the same chemical reaction in both cases, all that's different is the rate of reaction.The uranium is fissing constantly anyway, it's just doing so very slowly. When you put them into a critical mass with a neutron source, you get a speedy chain reaction that delivers all the energy of slow decay very quickly. Sub-critical, you could say. They're just decaying like normal U-238. I would assume because the masses aren't critical - there's not enough U-238 atoms in one place to support a runaway reaction. By definition it's an uncontrolled nuclear reaction. Controlled reactions occur in reactors where they use cesium rods to absorb neutrons to slow the reaction. Because you'd have to wait for a reaction to spontaneously start. In a bomb that's going to hit the ground if it doesn't go off that's waiting too long. Using a neutron source means that you're sure the bomb will go off.[This message has been edited by crashfrog, 10-19-2003]

Thanks,
If the two Uranium containers are both subcritical, why are neutron deflectors needed? Since they're subcritical, a stray neutron from container A hitting container B wouldn't start a chain reaction or vice-versa."In a Uranium bomb, the neutron deflector serves as a safeguard to keep an accidental supercritical mass from occurring by bouncing the stray neutrons from the `bullet' counterpart of the Uranium mass away from the greater mass below it (and vice- versa)." - that site I linked earlierHow could a stray neutron cause supercritical mass to occur if it's only slamming into the material at subcritical mass?

Oooh, a subject to match my avatar!

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How come neutrons that are reflected back from the neutron deflector in either the Uranium or the Plutonium models don't bring about a premature fission?

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The reflector serves two functions. One is reflect neutrons which effectively reduces how much fissile material is needed to reach criticality as less neutrons are "lost". The second is that it forms a buffer against the explosion, thus slowing the expansion of the fissile core and extending the duration of the detonation and producing a larger yield. A sub-critical mass will simply not produce enough neutrons through spontaneous fission, even with some of those reflected.

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When the bomb eventually explodes, is the fission controlled or is controlled nuclear fission only used for testing in labs and stuff?

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A nuclear blast is very much uncontrolled nuclear fission - once it's set going there's nothing going to stop it. Controlled nuclear fission is what you get in a reactor where you want to be able to regulate power and shut it down if needed.

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Why are neutron initiators needed in the first place? Why not just successfully get the fissionable material to supercritical mass and then spontaneous fission set off the chain reaction?

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As mentioned by crashfrog initiators add a level of control over starting the chain reaction - mostly to ensure that it starts in the right place in the core so as not to scatter the core before the chain reaction reaches maximum yield. The initiator is more important in plutonium devices because, ironically, the spontaneous fission rate (and mean number of neutrons per fission) means that it is more likely to go in an uncontrolled manner when critical mass is reached than uranium.Oh, and Crashfrog got one thing wrong. 235U is the fissile material in bombs, not 238U. Though 238U is often used as the reflector.Alan

The point is that a critical mass doesn't need to be all in one lump ... criticality is defined by the amount of material "in contact" by means of neutron exchange. You can keep things subcritical by either adding distance (difficult to do in a bomb) or shielding between bits of fissile material. The reflector is a very good shield, and as it serves another purpose too it does the shielding job without adding weight or added complexity.

Oops. My mistake. Thanks for pointing it out.

Sorry I got to this so late, I didn't see it until today. While I do not think a gun barrel bomb requires an initiator, one would help jumpstart the chain reaction.

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(2) There's also talk about how the two Uranium pieces needed to brought together quick enough so that spontaneous fission doesn't occur and fizzle the bomb out. I don't understand why the period of time where the one piece is shot into the other is particularly susceptible to spontaneous fission. Wouldn't spontaneous fission be possible in the plane ride up there and where it was being stored in the first place?

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What you want to avoid is a (relatively) slow collision between the subcritical components. Otherwise the incomplete fission would reduce the explosive yield. What they mean by spontaneous fission is an explosion triggered by neutrons from spontaneous fission (rather than an initiator). This would reduce the fission mass confinement before an acceptable level of chain reaction has been completed.If the masses aren't close enough, the early chain reaction will generate enough heat and explosive force to separate the unreacted uranium, thus choking off the exponential progression of the chain reaction. With the mass in a smaller volume, the exponential progression proceeds at a faster rate. This allows for less required fissionable material for a given explosive yield.This problem is so pronouced in plutonium type bombs that implosion of a sphere of plutonium is the only way to get the required mass into the critical volume fast enough to prevent a fizzled explosion.

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(3) At what level are each of the two Uranium pieces at while they're separated (not yet critical, critical...)?

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I presume by level you mean subcritical, critical, supercritical.Subcritical is the answer.

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(4) How come neutrons that are reflected back from the neutron deflector in either the Uranium or the Plutonium models don't bring about a premature fission?

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Reflectors aren't perfect reflectors, even if they were, the lack of critical mass would mean the number of neutrons present wouldn't be enough tip the chain reaction into the required exponential curve.Reasons for reflectors:Think about a fission based explosion in terms of exponential chain reaction - each neutron triggering two more neutrons, in an exponential progression.Now, realize that many of the neutrons will not hit another nucleus, thus be wasted. The more material they have to travel thru the higher the chance of colliding with another nucleus. The more fissionable material you can get into a fixed volume, the higher the chance that a neutron will hit another nucleus, rather than escape. Using a reflector (U238/Lead), will enable you to get a second chance with the neutrons that have left the fissionable mass, by reflecting them back into the fissionable mass. This will reduce the critical mass needed for a successful nuclear explosion.A second benefit of reflectors is to confinement. The mass of the reflector helps keep the fissionable material in a more confined space, until the exponential progression has exhaused most fissionable fuel.

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(5) When the bomb eventually explodes, is the fission controlled or is controlled nuclear fission only used for testing in labs and stuff?

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Define controlled.The bombmaker would consider the exponential chain reaction, such that the majority of the fissionable material is converted into chain reaction breakdown products, giving a maximum energy yield, to be controlled.A nuclear reactor officer on a sub would consider an exponential chain reaction a very bad thing, therefore uncontrolled.

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(6) Why are neutron initiators needed in the first place? Why not just successfully get the fissionable material to supercritical mass and then spontaneous fission set off the chain reaction?

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More efficient use of fissionable material - exponential reaction rates (first derivative) are slow, at first (relatively), progressing to more and more rapid. Since slow reactions can cause the fissionable material lose confinement (due to explosively dissipation) before the exponential progression has consumed an acceptable amount of the fissionable material, this is undesireable.
What the initiator does is start the reaction a little further along the exponential curve, into the more rapid portion of the reaction. This allows for less required fissionable material for a given explosive yield.