Nuclear bombs

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i've always wondered how the split the uranium atom to creat a chain reaction in a nuclear bomb
 

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Nabeshin
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Again, a simple google search will produce many answers to your query on varying levels of complexity, for whatever you can understand. Please do not post general points like this that can easily be looked up.
 
Astronuc
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When the U-235 nucleus splits it gives off two neutrons that can split two more nuclei, but when U-238 (more than 99 percent of the naturally-occurring U atoms) splits it doesn't give off any neutrons. So most of the technical problems in developing the bomb were figuring out how to separate the material atom-by-atom and obtain a chunk of pure U-235. Once you have that, if it's a large enough sample that most of the spontaneously released neutrons will be internal and not near the surface, it's easy to get a chain reaction. Just put two separated pieces of the material together to make the total mass enough, at the moment when you want it to be enough, and it will explode.
 
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When the U-235 nucleus splits it gives off two neutrons that can split two more nuclei, but when U-238 (more than 99 percent of the naturally-occurring U atoms) splits it doesn't give off any neutrons. So most of the technical problems in developing the bomb were figuring out how to separate the material atom-by-atom and obtain a chunk of pure U-235. Once you have that, if it's a large enough sample that most of the spontaneously released neutrons will be internal and not near the surface, it's easy to get a chain reaction. Just put two separated pieces of the material together to make the total mass enough, at the moment when you want it to be enough, and it will explode.
This isn't completely accurate:
U-238 Can split, and it gives off neutrons as well. The problem is that U-238 is only likely to do so when bombarded with neutrons that have more energy than are produced with the fission of U-235 or U-238. With slower neutrons U-238 is likely to capture them and get converted to Pu-239.
Large Hydrogen bombs have a mantle of depleted uranium around them, that will readily fission when bombarded by the high-energy neutrons produced by the fusion reaction.
 
Astronuc
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Both U-235 and U-238 release 2 or 3 neutrons, with an average value in between. U-238 is more likely to absorb neutrons (to become U-239 which decays to Np-239, which decays to Pu-239) than fission, except for fast neutrons with E typically > 1 MeV.

U-235 has a high fission cross-section for neutrons with thermal energies, < 0.1 eV, and is morely likely to fission than simply absorb neutrons.

In nuclear explosions, the mass of fissile material is compressed into a supercritical mass and the fission process is really prompt supercritical - hence the rapid fissioning within microseconds - before the mass is dispersed.
 
LURCH
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i've always wondered how the split the uranium atom to creat a chain reaction in a nuclear bomb
The process of splitting the atom has been described as, "simply putting more energy in one place then can theoretically been there." Some reactions are caused (in some bombs) by placing two half-spheres of uranium at opposite ends of the tube with no air in it. And an explosive charge at one end of the two fires one of these hemispheres down the tube, like a bullet down the barrel of the gun. This is called the "uranium gun" type of bomb. Where the two hemispheres slam into each other, a great deal of energy is concentrated, as heat. If you think of the uranium atoms as bags for holding energy, this is like over stuffing those bags till they burst.

The other design uses lands shaped charges all the way around a ball of uranium to focus a large explosion inward (implosion), thereby achieving the same access of energy in a small place which causes the atoms to fracture. There is a fairly good description of both of these devices on http://science.howstuffworks.com/nuclear-bomb.htm".
 
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