Why is confinement such an issue for fusion and not fission?

  • #1
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Hello,

Why is confinement a big problem in fusion and not in fission? Is it simply due to the fact that in fission, we're injecting a neutron that is neutral, and in fusion we're pushing together charged particles? If so, why is that facet not taken care of by reaching the critical ignition temperature and density? Is there perhaps another reason why confinement issues are so heavy?

Thank you.

EDIT: another source states it's because the particles are blown apart once one fusion has gone through, due to the shockwave; but what would be different in a fission process?
 
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Answers and Replies

  • #2
Is it simply due to the fact that in fission, we're injecting a neutron that is neutral, and in fusion we're pushing together charged particles?
Partly - the fissioning atoms are also ready to split and just need a small 'nudge' of an extra slow neutron. There are also lots of spare neutrons flying around from the fission process and you only need just over one neutron per splitting atom to hit another one to give a chain reaction.
There is a much higher barrier to forcing a couple of hydrogen atoms together and it isn't a chain reaction in the same sense.

If so, why is that facet not taken care of by reaching the critical ignition temperature and density?
It is - you need the confinement to reach the density.


another source states it's because the particles are blown apart once one fusion has gone through, due to the shockwave; but what would be different in a fission process?
Unless you are having a really bad day there shouldn't be much shockwave in a fission reactor. The power density in a fission process isn't that high - you can work out how much energy you get from each fission U atom and the proportion of fissioning atoms in the fuel.
 
  • #4
Drakkith
Staff Emeritus
Science Advisor
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Its a fundemental difference in the two reactions. Heavy nuclei, such as Uranium, have already been fused together from hydrogen and other elements in stars, but they are so heavy that they are unstable due to the repulsive positive force of all those protons. (90+) This is why they are already radioactive. Similar to how providing a spark will cause a chain reaction in gasoline, providing a neutron or two will start a chain reaction in fissionable materials.

Fusing two nuclei together is much more difficult because we have to do what a star normally does and make two nuclei that repel each other get close enough to fuse. The problem isn't simply getting nuclei to fuse, but to get them to do it in a manner that will release more energy than it took to get them to fuse together in the first place. There are so many ways to lose energy while trying to do this that we have yet to overcome it.

Check out fusion power on wikipedia, it explains the issues and has links to other useful sites.
 

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