[caseyavila]

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.

[marcyb5st]

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.

[pfdietz]

> 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]

Right: intractable, but much smaller than other problems.

[marcyb5st]

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]

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]

Oops, lithium hydride.

(Damn autocorrect.)

[BjoernKW]

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]

This article by a Berkeley physicist does the math on fusion fuel: 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]

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.

[ncmncm]

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]

I don't know of any blanket design that uses lithium hydroxide.

[ncmncm]

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]

I don't know any that uses lithium hydride, either.

The usual designs are things like Li, PbLi, and lithium containing ceramics.

[ncmncm]

(As you pointed out before, elemental liquid Li or Pb would interfere with magnetic containment. LiH is an example of a diamagnetic Li-rich material resistant to radioactivation (other than the desired 3H). We need a great deal of Li in the neutron-absorbing blanket to breed tritium fuel.)

1000 tons of lithium deuteride (half 6Li, half 7Li, all 2H) would cost ~$2B for the deuterium, plus a smallish fraction of that for the 6Li-enriched lithium. Any deuterium that picks up a neutron would become tritium, adding to what is got by fooling with the lithium. Maybe you economize with half-H, half-2H, for only ~$1B.

You have many reasons not to let your LiH catch fire, beyond that it cost you $1-2B and would totally destroy your $50B reactor and be deucedly hard to put out. It burns in air to LiOH, Li3N and H2, and reacts with any water, CO2, or nitrogen you might have hoped would douse it. Li3N further reacts with the hydrogen making lithium amide LiNH2, thence various unpleasant peroxides.

Regular LiH is solid at a more-familiar operating temperature under 400C, and liquid at what might thought an extreme 700C. The deuterides would raise the melting point some. You really want something in there to scavenge any metallic lithium, if molten, because that corrodes steel and silica.

[ncmncm]

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]

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.