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

In summary, the energy required for fusion reactions is much greater than that for fission reactions due to the need to overcome the strong repulsion between hydrogen nuclei. However, fusion fuels, such as hydrogen, are much more stable than fission fuels, allowing for a higher energy output per kg of fuel. On average, a fusion reaction yields more energy than a fission reaction.
  • #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
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
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
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
Thanks for the details!
 
  • #6
canopus said:
Thanks for the details!

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

1. Why is more energy needed for fusion than fission?

The main reason is that fusion involves combining two atoms to form a heavier atom, while fission involves splitting a heavier atom into two lighter atoms. In order for fusion to occur, the nuclei of the atoms must overcome the strong repulsive forces between them. This requires a significant amount of energy, whereas in fission, the nuclei are already unstable and splitting them releases energy.

2. How much more energy is needed for fusion compared to fission?

The amount of energy needed for fusion is significantly greater than that needed for fission. In fact, fusion reactions require millions of times more energy than fission reactions. For example, the fusion reaction that powers the sun requires temperatures of millions of degrees and pressures millions of times greater than those needed for fission reactions in nuclear power plants.

3. Can fusion reactions be controlled like fission reactions?

Currently, fusion reactions cannot be controlled in the same way as fission reactions. Fission reactions can be controlled by moderating the speed of the neutrons released, while fusion reactions require extremely high temperatures and pressures to overcome the repulsive forces between nuclei. However, scientists are working on ways to control fusion reactions for potential use in energy production.

4. Why is fusion considered a more sustainable source of energy?

Fusion reactions use hydrogen isotopes, which are abundant in seawater and can be extracted in unlimited quantities. This makes fusion a potentially sustainable source of energy, unlike fission reactions which rely on finite resources such as uranium. Fusion reactions also produce less radioactive waste compared to fission reactions.

5. What are the challenges in achieving fusion energy?

The main challenges in achieving fusion energy are creating the extreme conditions of temperature and pressure needed for fusion reactions to occur and sustaining those conditions for a long enough period of time to produce a net energy gain. Additionally, controlling and harnessing the energy released from fusion reactions is also a major challenge. Scientists are actively researching and developing new technologies to overcome these challenges and make fusion energy a viable source of clean and sustainable energy.

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