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I don't know everything about nuclear fusion so I have to ask: Is it actually renewable? In other words, are the byproducts able to form back into the "fuel" a

I don't know everything about nuclear fusion so I have to ask: Is it actually renewable? In other words, are the byproducts able to form back into the "fuel" at a reasonable rate with the energy input of the Sun? I know that a selling point of fusion is that there is such an abundance of fuel that this doesn't matter. But if we treat finite energy sources as infinite, exponential growth in our energy budget means that we will undoubtedly run out of energy, as is being done with forests and such. After all, I have a feeling people at the dawn of the industrial revolution thought the amount of coal available in the world would serve their needs "practically forever," until energy consumption scaled up by thousands of times.

hoseja 4y ago
By the point we've fused significant portion of Earths hydrogen, it will really not be a problem to hop over to Jupiter for some more. The scales are insane. Energy input of the Sun ALSO isn't renewable if you think like this.
marcyb5st 4y ago
So, the fusion we are talking about here is deuterium - tritium fusion as it should be the easiest to achieve. Deuterium is not a problem. A rough estimate says that there's enough of the stuff to cover 100% of the world needs for thousands of years. And it's easy to breed: surround the reactor with water so the hydrogen there can capture the stray neutrons. Tritium, on the other hand, is a problem. It is radioactive with a half life of ~12 years and so the little we have needs to be produced since we can't really accumulate it. Currently it is produced by conventional nuclear reactors. Additionally, breeding tritium is harder than deuterium and requires a blanket around the reactor that uses other materials to multiply the number of stray neutrons. For each atom of Tritium that is fused we could get somewhere between 1.1 to 1.7 with a theoretical maximum of 2 Tritium atoms so, finally answering your question, it is renewable. It's just hard, but a piece of cake compared to actually maintaining a stable fusion.
- ncmncm 4y ago
  Right: intractable, but much smaller than other problems.
  - marcyb5st 4y ago
    I don't see it as intractable. We already have two ways to do that at scale. One is proven (the fission reactors), the other one is proven but not in an actual fusion reactor yet. Iter will have such a blanket for tritium breeding. Intractable in my mind sounds more like something that you don't know how to even start.
    - ncmncm 4y ago
      How will you extract the tritium at PPB concentration from thousands of tons of flame-hot radioactive lithium hydroxide? Daily? Intractable means that anything you try is worse than not starting.
      - ncmncm 4y ago
        Oops, lithium hydride. (Damn autocorrect.)
- pfdietz 4y ago
  > Deuterium is not a problem. A rough estimate says that there's enough of the stuff to cover 100% of the world needs for thousands of years. Far more than that is available.
ncmncm 4y ago
People talking about fusion expect to "breed" tritium in their reactor. This takes the form of blasting GW of hot neutrons into a thousand (or ten-) tons of lithium hydroxide, and somehow extracting grams of tritium from it at parts-per-billion concentration. There is no choice about that: it is the only way to get enough tritium to keep operating.
- pfdietz 4y ago
  I don't know of any blanket design that uses lithium hydroxide.
  - ncmncm 4y ago
    Oops, lithium hydride. (Autocorrect strikes again!) Lithium hydroxide is what you get, as thousands of tons of caustic vapor, when it catches fire. What that does when you breathe it does not bear contemplation.
    - pfdietz 4y ago
      I don't know any that uses lithium hydride, either. The usual designs are things like Li, PbLi, and lithium containing ceramics.
      - ncmncm 4y ago
        Lithium hydride is like a ceramic, i.e. a high-melting-point solid. But whatever you use, you have to get extremely low concentration tritium out, somehow, to run the reactor on tomorrow.
        pfdietz 4y ago
        The second sentence is a good reason NOT to use hydrogen in your breeding material, since if you do you have to separate the tritium from it, and do it very rapidly.
BjoernKW 4y ago
Ultimately, no method of energy generation is truly renewable, including solar. The Sun will run out of fuel in five billion years, after all, give or take. However, for all practical intents and purposes, solar energy is renewable. The same holds true for nuclear fusion for at least a couple of hundred years, even considering growing energy consumption.
DennisP 4y ago
This article by a Berkeley physicist does the math on fusion fuel: https://dothemath.ucsd.edu/2012/01/nuclear-fusion/ https://dothemath.ucsd.edu/2012/01/nuclear-fusion/ Deuterium fuel is the most abundant. There's enough in your morning shower to supply all your energy needs for a year. There's enough in the oceans to last for billions of years. Fusion is as close to renewable as anything, because it'll last until the sun goes out. Right now most projects are also using tritium fuel, which has to be made from lithium. That's plenty abundant but not to the extreme of deuterium. But pure deuterium fusion is possible, just a little harder. And one prominent fusion startup, Helion, is actually using deuterium (along with helium-3, which is the waste product of deuterium fusion).

hoseja 4y ago

By the point we've fused significant portion of Earths hydrogen, it will really not be a problem to hop over to Jupiter for some more. The scales are insane. Energy input of the Sun ALSO isn't renewable if you think like this.

marcyb5st 4y ago

So, the fusion we are talking about here is deuterium - tritium fusion as it should be the easiest to achieve. Deuterium is not a problem. A rough estimate says that there's enough of the stuff to cover 100% of the world needs for thousands of years. And it's easy to breed: surround the reactor with water so the hydrogen there can capture the stray neutrons. Tritium, on the other hand, is a problem. It is radioactive with a half life of ~12 years and so the little we have needs to be produced since we can't really accumulate it. Currently it is produced by conventional nuclear reactors. Additionally, breeding tritium is harder than deuterium and requires a blanket around the reactor that uses other materials to multiply the number of stray neutrons. For each atom of Tritium that is fused we could get somewhere between 1.1 to 1.7 with a theoretical maximum of 2 Tritium atoms so, finally answering your question, it is renewable. It's just hard, but a piece of cake compared to actually maintaining a stable fusion.

ncmncm 4y ago
Right: intractable, but much smaller than other problems.
- marcyb5st 4y ago
  I don't see it as intractable. We already have two ways to do that at scale. One is proven (the fission reactors), the other one is proven but not in an actual fusion reactor yet. Iter will have such a blanket for tritium breeding. Intractable in my mind sounds more like something that you don't know how to even start.
  - ncmncm 4y ago
    How will you extract the tritium at PPB concentration from thousands of tons of flame-hot radioactive lithium hydroxide? Daily? Intractable means that anything you try is worse than not starting.
    - ncmncm 4y ago
      Oops, lithium hydride. (Damn autocorrect.)
pfdietz 4y ago
> Deuterium is not a problem. A rough estimate says that there's enough of the stuff to cover 100% of the world needs for thousands of years. Far more than that is available.

ncmncm 4y ago

People talking about fusion expect to "breed" tritium in their reactor. This takes the form of blasting GW of hot neutrons into a thousand (or ten-) tons of lithium hydroxide, and somehow extracting grams of tritium from it at parts-per-billion concentration. There is no choice about that: it is the only way to get enough tritium to keep operating.

pfdietz 4y ago
I don't know of any blanket design that uses lithium hydroxide.
- ncmncm 4y ago
  Oops, lithium hydride. (Autocorrect strikes again!) Lithium hydroxide is what you get, as thousands of tons of caustic vapor, when it catches fire. What that does when you breathe it does not bear contemplation.
  - pfdietz 4y ago
    I don't know any that uses lithium hydride, either. The usual designs are things like Li, PbLi, and lithium containing ceramics.
    - ncmncm 4y ago
      Lithium hydride is like a ceramic, i.e. a high-melting-point solid. But whatever you use, you have to get extremely low concentration tritium out, somehow, to run the reactor on tomorrow.
      - pfdietz 4y ago
        The second sentence is a good reason NOT to use hydrogen in your breeding material, since if you do you have to separate the tritium from it, and do it very rapidly.

BjoernKW 4y ago

Ultimately, no method of energy generation is truly renewable, including solar. The Sun will run out of fuel in five billion years, after all, give or take. However, for all practical intents and purposes, solar energy is renewable. The same holds true for nuclear fusion for at least a couple of hundred years, even considering growing energy consumption.

DennisP 4y ago

This article by a Berkeley physicist does the math on fusion fuel: https://dothemath.ucsd.edu/2012/01/nuclear-fusion/ https://dothemath.ucsd.edu/2012/01/nuclear-fusion/ Deuterium fuel is the most abundant. There's enough in your morning shower to supply all your energy needs for a year. There's enough in the oceans to last for billions of years. Fusion is as close to renewable as anything, because it'll last until the sun goes out. Right now most projects are also using tritium fuel, which has to be made from lithium. That's plenty abundant but not to the extreme of deuterium. But pure deuterium fusion is possible, just a little harder. And one prominent fusion startup, Helion, is actually using deuterium (along with helium-3, which is the waste product of deuterium fusion).