Comment by gene-h | Hacker News Reader

Comment by gene-h | Hacker News Reader

Parent Full thread

>There is some radiation with some designs, but the basic idea of fusing hydrogen into helium works without radioactivity. Per unit energy D-T fusion, the fusion reaction most likely to be viable near term, generates more neutrons than the fission of U235. The U235 fission reaction[0] releases 3 neutrons and 202.5 MeV of energy, 8.8 MeV escapes as antineutrinos, so let's call it 193.7 MeV, so 64.5 MeV per neutron.

D-T fusion generates 17.6 MeV and 1 neutron, so 17.6 MeV per neutron. So for the same amount of energy produced it makes about 3.5x more neutrons. Worse yet, in D-T fusion, 14.1 MeV of the energy released is in the kinetic energy of the neutron. Compare this to the 8.8 MeV energy of all three neutrons in a fission reaction. Neutron activation and damage are significant engineering concerns for fusion reactors.

[0]https://en.wikipedia.org/wiki/Uranium-235 [1]https://en.wikipedia.org/wiki/Deuterium%E2%80%93tritium_fusi... https://en.wikipedia.org/wiki/Uranium-235

Thank you for some much needed solid information.

And then you need to use that neutron to breed new tritium from lithium-6, which is only about 2% of lithium. And since our only feasible source of tritium is a single tritium atom from that single neutron from a D-T fusion reaction that consumes a tritium, we had better be damn efficient at making sure the neutrons go only to breeding new tritium. Which is good, because then they cause less neutron radioactivity of all the rest of the reactor parts. But bad, because it's pretty unrealistic to capture all the neutrons via lithium blanket.

Which means we need ultimately need an outside tritium source, external from the fusion reactions, which means another neutron source, which means, fission reactors!

The whole idea of controlled terrestrial fusion being an inexpensive source of electricity is just absolutely baffling to me. As is the idea that it will somehow be a good extraterrestrial energy source, or power some sort of spaceship. Fission is far more realistic, likely to be compact, etc. Direct conversion of aneutronic fusion is complete sci-fi at the moment, we may as well be talking warp drives.

"Per unit energy D-T fusion, the fusion reaction most likely to be viable near term"

Ok well, I see the use of tritium only as a step towards normal hydrogen as fuel in fusion reactors. Tritium as fuel is just the "easy" target and yes, it is in limited supply and causes problems. But "in theory" we could have higher temperatures and not be dependant on tritium.

Reply on news.ycombinator.com