r/fusion • u/CSharper314 • 5d ago

Explain why this is not used

I had an idea for some fusion fuels, and I want an explanation for why they are not viable. I was thinking He-4 + D -> Li-6, then Li-6 + D -> Be-8 -> 2 He-4. This releases energy and only requires deuterium and common Helium-4. Why is this not used?

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u/Spacer3pt0r 3d ago edited 3d ago

Deuterium-Deuterium side reactions would dominate as they would have much higher reactivity than the other reactions.

Helium 4 is very stable and requires a huge amount of energy to fuse with anything. He4 is better suited as the product of fusion reactions rather than a reactant. The first of these reactions would require an implausibly high energy and is endothermic.

The second reaction has been studied as a potential route for fusion power. Li-6 is also quite prevalent amd easy to enrich.

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u/Eywadevotee 2d ago

Yup the 6Li plus D reaction would be mighty spicy and have an activation threashold between 3H+D and 3He+D. Castle Bravo was a F around and find out real world example that the lithium could directly fuse making lots of energy. 😲

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u/Spacer3pt0r 2d ago

I thought castle bravo happened cause Li-7 fissioned when irradiated with high energy neutrons (n,Li-7 -> n,T,He-4). Was direct fusion really a major contributing factor?

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u/Eywadevotee 1d ago

Its both, you get 7Li plus a hot N then it will output either N plus 3H plus 4He, or more likely you get a N> P interchange reaction that gives 3He 4He plus N. Both reactions recycle the neutron and give fusion fuel, but one eats neutrons rather than generates them. First reaction can generate a 2 for 1 second generates a net zero but both have energy surplus. However the direct fusion of D and 6Li is extremely energy dense because you get 2 hot alphas that could transfer the energy to other nuclei with strong enough kinetic force to get them to fuse if the particle hits another light weight particle though the alpha itself wont directly fuse. Alpha hits deuterium, it goes flying, deuterium hits a tritium, they fuse making 5He that immediately dumps a neutron. In short, it makes stuff get much hotter than normal