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Is a "fusion bomb" realistic?

Bannanawamajama
Posts: 125
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4/8/2014 9:30:41 PM
Posted: 2 years ago
I was just watching Batman: The Dark Knight Rises. In it Bane threatens to blow up Gotham city using the reactor core taken from a nuclear reactor developed by Lucius Fox. At one point, when explaining this to the police, Fox mentions that this was to be the first nuclear fusion reactor in the world.

Based on my loose understanding of fusion and nuclear physics, I don't think this seems very realistic. A fission bomb works because every time you split a U-235 atom, it releases a few fast neutrons, each of which has the potential to destabilize another U-235. The result is that since 2-5 neutrons can be released per decay, the number of reactions, and thus energy release, grows exponentially. So a small amount of enriched uranium or plutonium can very quickly release a huge amount of power, and because its energy over time. That makes the big explodyness.

But in nuclear fusion, even though neutrons can be released, they don't drive the reaction. It is the collision of the atoms themselves, which is based on the heat and kinetic energy of each atom. So the rate of reactions should be a function of energy, and therefore there should be a linear, not exponential, growth of output. When you combine this with the huge amount of initial energy to start a meaningful reaction to begin with, its hard to imagine output power high enough to cause a spontaneous explosion. What seems more likely is the device slowly but steadily increasing in heat as the reaction proceeds, eventually damaging the core itself, and probably causing a small explosion when the container ruptures. This would not be an atomic bomb scale explosion, as was portayed in the movies, but more like a pipe bomb. Devastating, but not really city destroying.

Am I right in my understanding of this? What do you think of the idea of a "fusion bomb"? Sci-fi fluff to make action movies seem cool, or is there truth behind this?
Ore_Ele
Posts: 25,980
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4/8/2014 9:56:02 PM
Posted: 2 years ago
We've already had Fusion bombs since the 60's (if not 50's, I'm not good with dates, ask my wife).
"Wanting Red Rhino Pill to have gender"
Ore_Ele
Posts: 25,980
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4/8/2014 10:00:53 PM
Posted: 2 years ago
I haven't seen the dark knight rises, so I don't know what it was that he described to know if it was accurate.

However, Fusion works by compressing the fusion material (taking X amount of energy) to trigger the Fusion reaction (it is not done by Neutrinos, like Fission is, but by the actual atoms mergeing into larger atoms). As such, there is no chain reaction. You have to generate enough pressure (and heat) to cause the reaction to occur.

In real world fusion bombs, this is done by a small fission bomb (mind = nuclear blown!). The heat and pressure created by that is enough to ignite the fusion process.
"Wanting Red Rhino Pill to have gender"
Ore_Ele
Posts: 25,980
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4/8/2014 10:05:06 PM
Posted: 2 years ago
http://en.wikipedia.org...

For more reading.

Of course, if Baine was talking about using Fusion only (with no fission for a trigger), that is purely hypothetical.
"Wanting Red Rhino Pill to have gender"
Enji
Posts: 1,022
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4/8/2014 11:11:01 PM
Posted: 2 years ago
All bombs ever rely on the conversion of energy into explosion. TNT, for example, converts chemical energy into explosion whereas projectiles fired from a railgun convert kinetic energy into explosion.

Nuclear bombs convert atomic energy into explosion. In fission reactions, heavy isotopes like uranium and plutonium are split into smaller elements which combined weigh less than their initial counterparts, with the remaining mass appearing as energy. In a fusion reaction, light isotopes of hydrogen are fused together into helium atoms which again weighs less than the initial components with the remaining mass appearing as energy. A fusion bomb is as realistic as a fusion reactor (and actually quite a bit simpler). Fusion reactions are actually better at releasing energy than fission reactions (hence part of the interest in fusion reactors) and hydrogen bombs (which currently exist) are stronger than uranium or plutonium bombs. If cold fusion were discovered to be possible, then pure fusion bombs would be possible as well, but currently we can't do either.
Sswdwm
Posts: 1,398
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4/9/2014 5:05:39 AM
Posted: 2 years ago
At 4/8/2014 9:30:41 PM, Bannanawamajama wrote:
I was just watching Batman: The Dark Knight Rises. In it Bane threatens to blow up Gotham city using the reactor core taken from a nuclear reactor developed by Lucius Fox. At one point, when explaining this to the police, Fox mentions that this was to be the first nuclear fusion reactor in the world.

Based on my loose understanding of fusion and nuclear physics, I don't think this seems very realistic. A fission bomb works because every time you split a U-235 atom, it releases a few fast neutrons, each of which has the potential to destabilize another U-235. The result is that since 2-5 neutrons can be released per decay, the number of reactions, and thus energy release, grows exponentially. So a small amount of enriched uranium or plutonium can very quickly release a huge amount of power, and because its energy over time. That makes the big explodyness.

But in nuclear fusion, even though neutrons can be released, they don't drive the reaction. It is the collision of the atoms themselves, which is based on the heat and kinetic energy of each atom. So the rate of reactions should be a function of energy, and therefore there should be a linear, not exponential, growth of output. When you combine this with the huge amount of initial energy to start a meaningful reaction to begin with, its hard to imagine output power high enough to cause a spontaneous explosion. What seems more likely is the device slowly but steadily increasing in heat as the reaction proceeds, eventually damaging the core itself, and probably causing a small explosion when the container ruptures. This would not be an atomic bomb scale explosion, as was portayed in the movies, but more like a pipe bomb. Devastating, but not really city destroying.

Am I right in my understanding of this? What do you think of the idea of a "fusion bomb"? Sci-fi fluff to make action movies seem cool, or is there truth behind this?

It always unnerves me that bombs exist that use a nuke... as a friggin detonator!
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