Nuclear fission/fusion

[JustStuit]

Which nuclear process releases more energy? Fusion usually takes higher pressure and temperature so does it release more energy? I take it fission is more practical for a means of energy since it can be done cheaper.

[Janus]

Fusion gives you more "bang for the buck" in terms of percentage of mass converted to energy.

[JustStuit]

The mass for fusion comes from the protons (and maybe neutrons?) becoming smaller - where does it come from in fission?

[JustStuit]

Also, what radioactive byproducts come from fusion of hydrogen and hydrogen to helium?

[swansont]

The mass for fusion comes from the protons (and maybe neutrons?) becoming smaller - where does it come from in fission?

 

They don't really become smaller. What's in play here are the forces involved, the strong nuclear force (attractive for all nucleons) and the electrostatic force (repilsive between protons). If you form a bound system, because of an attractive force, energy is released from that system.

 

The nuclear force has a finite range, so it saturates at about 60 nucleons (in the vicinity of iron). Up to that point, if you were to build a nucleus one particle at a time, you keep releasing more and more energy, because the nuclear force is stronger than the electrostatic force, and they all attract the new particle added. But for heavier nuclei, the net nuclear force stops getting bigger (new nuclei are not attracted to all other nuclei) but the electrostatic force keeps growing. So you start releasing less energy with each new nucleon. Eventually you reach a point where nuclei just can't stay together (for long anyway) because the repulsive forces are too large.

 

So fusing heavy nuclei won't release energy (Janus's comment was a little too general; the reference no doubt applies to very light nuclei like Hydrogen isotopes)

 

Heavy nuclei release energy when they fission because you have reduced the electrostatic repulsion present. Fusion releases energy because of the large attractive net force.

[[Tycho?]]

Just look up fission and fusion on wikipedia. After you've done that, then you can ask questions.