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At school I have learned that there are two very similar fusion reactions, which physicists (and science fiction authors) hope can be used as an energy source:

$$^2_1\mathrm{H}+^3_1\mathrm{H} \rightarrow ^4_2\mathrm{He}+^1_0\mathrm{n} + (17.589\,\mathrm{MeV})$$

and

$$^2_1\mathrm{H}+^3_2\mathrm{He} \rightarrow ^4_2\mathrm{He}+^1_0\mathrm{p} + (18.353\,\mathrm{MeV})$$

Does the fact that the reaction which requires helium-3 produce protons instead of neutron, imply that entirely different fusion reactors would need to be build, to make the different fusions reactions happen, or would a facility built to fuse tritium with deuterium work perfectly fine if you instead gave it helium-3 and deuterium?

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  • $\begingroup$ That was fastly accepted. Nevermind. I wanted to look at reaction rates to learn a bit of something and I did. There are two very important factors - Coulomb barrier - it is 2x higher for 3He reaction. Then there is an astrophysical S factor (kind of reaction probabity) and it is huge for d+t (14MeVb) and huge for 3He+3He (5MeVb) and d+3He (10MeVb). If there in no surpise in S factor, then one must go from 760 milion Kelvin to 8 bilion Kelniv temperature to maintain same reaction rates. Then- magnetic field will have different problem to confine higher velocities. Basically. $\endgroup$ – jaromrax Apr 12 '17 at 14:35
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Neutrons, being uncharged, will escape the plasma. These can cause significant damage to vessel walls (one portion of the ITER fusion experiment is to test materials). One idea people hope for is to use the neutrons to breed more tritium for fuel from lithium (ITER also hopes to test this https://www.iter.org/mach/TritiumBreeding ).

Then, to get energy out of all this, somehow we need to extract heat from the plasma or whatever the neutrons are bombarding.

The other reaction produces protons, so one doesn't need to worry about neutrons (except maybe from side reactions), and the heat will remain in the plasma. So the engineering decisions will be different.

With small test reactors it is probably possible to test either reaction if the temperature and pressure conditions can be reached. However for building a fusion generator, the engineering solutions to the above issue will almost assuredly depend on the reaction.

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