Why do we need much more energy for fusion than fission?

  • Thread starter canopus
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  • #1
canopus
Why do we need much more energy for fusion than fission?

PS: I apologize for my bad English,
 

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  • #2
chroot
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Welcome to PF, canopus!

You don't need any energy at all to get fission to occur -- the fissionable substance is already at the top of its activation energy curve and is thus unstable. You can imagine that a fissile nucleus already has internal pieces which are repelling each other naturally, and it just takes a trigger, like a neutron, to allow it to break apart.

Fusion fuels, like hydrogen, are already quite stable. You have to push them up and over a very large activation energy before they will fuse, eventually releasing more energy than you put in. The hydrogen nuclei repel each other very strongly, so you have to use very large temperatures to make them move very fast. When they move fast enough, their kinetic energy overcomes their repulsion and allows them to fuse.

- Warren
 
  • #3
canopus
Hi! Thanks for your explanation! Well, i also wonder, why do we have more energy after a fusion reaction than a fission reaction? I might be wrong, i know, there is no fusion reactor because of the strong energy it spreads.
 
  • #4
chroot
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An individual fission of a U235 nucleus liberates approximately 215 MeV of energy.

http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/u235chn.html#c3

An individual deuterium-tritium fusion reaction liberates about 17.6 MeV of energy.

http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/fusion.html#c2

However, the fusion reactants are much less massive than the fission reactants, so you can actually obtain much more energy per kg of fuel from fusion than from fission:

http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html#c5

All of these details can be seen on the graph of nuclear binding energies here:

http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html#c2

- Warren
 
  • #5
canopus
Thanks for the details!
 
  • #6
Mk
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canopus said:
Thanks for the details!
Heh, in the cold fusion experiment you don't need much either.
 

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