Comment by zdragnar
5 days ago
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...
That's why fusion blankets for D-T reactors use lead or beryllium as neutron multipliers. CFS for example uses FLiBe molten salt. Doing it this way a tokamak can not only sustain its own tritium supply, but periodically provide startup fuel for additional reactors.
Initial tritium load for a small, high-field reactor like CFS is much smaller than for ITER. And I'll note that the paper you linked has this conclusion:
> The preliminary results suggest that initial operation in D–D with continual feedback into the plasma of the tritium produced enables a fusion reactor designed solely for D–T operation to start-up in an acceptably short time-scale without the need for any external tritium source.
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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.