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

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    Energy Fission Fusion
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Discussion Overview

The discussion centers on the energy requirements for nuclear fusion compared to nuclear fission, exploring the underlying mechanisms and energy outputs of each process. It includes theoretical aspects and conceptual clarifications regarding the stability of fusion fuels versus fissionable materials.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • Some participants propose that fission does not require energy to occur because fissionable substances are already unstable and can be triggered by neutrons.
  • Others argue that fusion fuels, such as hydrogen, are stable and require significant energy input to overcome strong repulsive forces between nuclei, necessitating high temperatures for fusion to occur.
  • A participant questions why fusion reactions yield more energy than fission reactions, noting the energy released per reaction for both processes.
  • It is mentioned that while individual fission reactions release more energy (approximately 215 MeV for U235), fusion reactions can provide more energy per kilogram of fuel due to the lower mass of fusion reactants.
  • A later reply references cold fusion experiments, suggesting that they may not require much energy, although this is presented without further elaboration.

Areas of Agreement / Disagreement

Participants express differing views on the energy dynamics of fusion and fission, with no consensus reached on the implications of cold fusion or the comparative energy outputs in practical applications.

Contextual Notes

The discussion does not resolve the complexities of energy requirements and outputs, and it relies on specific definitions and assumptions about stability and energy release in nuclear reactions.

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

PS: I apologize for my bad English,
 
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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
 
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.
 
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
 
Thanks for the details!
 
canopus said:
Thanks for the details!

Heh, in the cold fusion experiment you don't need much either.
 

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