Comment by credit_guy
4 days ago
> return on the initial investment.
It's not only initial investment. Half of the fusion fuel is tritium, which is one of the most expensive substances on Earth (a google search finds that the price of tritium is about $30k per gram [1]). For comparison, fission reactors need enriched uranium, and that costs only about $4000 per kilogram [2]. People have the idea that fusion produces many times more energy than fission, probably because fusion bombs have a higher yield than fission bombs. This is not true. The most typical fusion reaction involves one deuterium and one tritium and yields 17.5 MeV from a total or 5 nucleons. A fission reaction involves one neutron and one atom of U-235 and yields 190 MeV from 236 nucleons. So fusion yields about 4.3 times more energy per nucleon. That's respectable, but in the popular imagination fusion yields 100 or 1000 times more energy than fission, so the fuel cost can be neglected. Nothing could be further from the truth.
The myth of unbounded / free energy from fusion comes from being able to use any old hydrogen atoms, rather than the much rarer deuterium and tritium.
Perhaps one day we'll get there, but I worry that the current advancements using the rarer isotopes will end up proving to be a dead end on that road, much like so many attempts at GAI. In the short term I suspect we'd have better odds with getting thorium reactors to be economical.
Deuterium is not rare at all. There's enough in your morning shower to provide all your energy needs for a year.
https://dothemath.ucsd.edu/2012/01/nuclear-fusion/
Tritium is rare but lithium isn't, and we can make tritium from lithium using the neutrons from fusion. (We also get tritium from fission plants, which is how we'd build the first fusion reactors.)
> we can make tritium from lithium using the neutrons from fusion
Each fusion reaction consumes one tritium atom and produces one neutron. If that neutron hits a lithium atom, it can split that and produce a tritium atom. If everything goes perfectly and there are no losses, then you get a 100% replacement of all the tritium that you consume. If you have a 90% replacement ratio (highly optimistic), you essentially lower the cost of your tritium fuel by a factor of 10, so from $30000 per gram to $3000 per gram, or $3 MM per kilogram.
> We also get tritium from fission plants
Yes we do. Mainly from Candu reactors. There are 49 Candu and Candu-like reactors in the world, and each produces less than 1kg of tritium per year. According to [1] a 1 GW fusion power plant would consume about 55 kg of tritium per year. So you'd need to run more than 50 fission power plants to operate one fusion power plant. Most people who dream of fusion think that fission will become irrelevant, not that you'll need 50 fission power plants for each fusion power plant.
[1] https://www.sciencedirect.com/science/article/abs/pii/S09203...
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No, it comes from foolishly thinking that the cost of fuel will dominate cost of energy. That doesn't require fusion of protons; deuterium and lithium are cheap.
I don't know much about this but I assume that the tritium will be created somehow while fussion is done [1]
[1] https://en.m.wikipedia.org/wiki/Deuterium%E2%80%93tritium_fu...