3 ms·

> 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 Eart

> 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. [1] https://www.google.com/search?q=tritium+price https://www.google.com/search?q=tritium+price [2] https://www.uxc.com/p/tools/FuelCalculator.aspx https://www.uxc.com/p/tools/FuelCalculator.aspx

zdragnar 2y 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.
- DennisP 2y ago
  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/ 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.)
  - credit_guy 2y ago
    > 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/S0920379615303598 https://www.sciencedirect.com/science/article/abs/pii/S09203...
    - DennisP 2y ago
      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.
      - credit_guy 2y ago
        > CFS for example uses FLiBe molten salt Ok, let's talk about that. For those who are not familiar, CFS stands for Commonwealth Fusion Systems, as startup with links to MIT. CFS aims to build a fusion reactor similar to ITER, but many times smaller, the secret sauce being that they use superconductors to achieve high magnetic fields. Back in 2022 some of the MIT guys got an ARPA-E grant to investigate the use of FLiBe to achieve atritium breeding ratio higher than 1 [1]. The results are in [2], they were published in January 2025. Here are some quotes: > The long-term goal of LIBRA is to demonstrate a TBR ⩾ 1 in a large volume (1000 kg ∼ 500 l) of FLiBe molten salt using D–T neutron generators. Note that a full-scale LIB in an ARC-class FPP will require ∼250 000 l of FLiBe, hence the importance of understanding tritium behavior in large salt volumes. ARC is the fusion reactor designed by CFS. This paper states that it will need 250000 liters of FLiBe. This is an insane amount. To understand how large this amount is, consider this: this ARPA-E project that took 3 years, used a quantity of 100 ml, so 0.1 liters. Anyway, what breeding ratio was achieved? 3.57 x 10^(-4), or 0.0357%. It's a long way to go from here to 1. I'm not saying it's impossible, but too many things related to fusion are just "engineering details". [1] https://arpa-e.energy.gov/programs-and-initiatives/search-all-projects/liquid-immersion-blanket-robust-accountancy-libra https://arpa-e.energy.gov/programs-and-initiatives/search-al... [2] https://iopscience.iop.org/article/10.1088/1741-4326/ada2ab/pdf https://iopscience.iop.org/article/10.1088/1741-4326/ada2ab/...
- pfdietz 2y ago
  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.
aptitude_moo 2y ago
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_fusion#Reactant_sourcing https://en.m.wikipedia.org/wiki/Deuterium%E2%80%93tritium_fu...

zdragnar 2y 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.

DennisP 2y ago
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/ 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.)
- credit_guy 2y ago
  > 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/S0920379615303598 https://www.sciencedirect.com/science/article/abs/pii/S09203...
  - DennisP 2y ago
    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.
    - credit_guy 2y ago
      > CFS for example uses FLiBe molten salt Ok, let's talk about that. For those who are not familiar, CFS stands for Commonwealth Fusion Systems, as startup with links to MIT. CFS aims to build a fusion reactor similar to ITER, but many times smaller, the secret sauce being that they use superconductors to achieve high magnetic fields. Back in 2022 some of the MIT guys got an ARPA-E grant to investigate the use of FLiBe to achieve atritium breeding ratio higher than 1 [1]. The results are in [2], they were published in January 2025. Here are some quotes: > The long-term goal of LIBRA is to demonstrate a TBR ⩾ 1 in a large volume (1000 kg ∼ 500 l) of FLiBe molten salt using D–T neutron generators. Note that a full-scale LIB in an ARC-class FPP will require ∼250 000 l of FLiBe, hence the importance of understanding tritium behavior in large salt volumes. ARC is the fusion reactor designed by CFS. This paper states that it will need 250000 liters of FLiBe. This is an insane amount. To understand how large this amount is, consider this: this ARPA-E project that took 3 years, used a quantity of 100 ml, so 0.1 liters. Anyway, what breeding ratio was achieved? 3.57 x 10^(-4), or 0.0357%. It's a long way to go from here to 1. I'm not saying it's impossible, but too many things related to fusion are just "engineering details". [1] https://arpa-e.energy.gov/programs-and-initiatives/search-all-projects/liquid-immersion-blanket-robust-accountancy-libra https://arpa-e.energy.gov/programs-and-initiatives/search-al... [2] https://iopscience.iop.org/article/10.1088/1741-4326/ada2ab/pdf https://iopscience.iop.org/article/10.1088/1741-4326/ada2ab/...
pfdietz 2y ago
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.

aptitude_moo 2y ago

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_fusion#Reactant_sourcing https://en.m.wikipedia.org/wiki/Deuterium%E2%80%93tritium_fu...