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mfb- t1_j190vsb wrote

Reply to comment by [deleted] in How do fusion scientists expect to produce enough Tritium to sustain D-T fusion (see text)? by DanTheTerrible

Producing the current global electricity demand (~2 TW) with fusion would need around 350 tonnes of tritium per year (assuming ~1/3 conversion of thermal power to electricity), which can be bred from 700 tonnes of lithium. The current global production is 80,000 tonnes per year, so even if we replace all power plants we only need 1%. You might want to go through more than 1% for isotope enrichment but you can still sell the "waste" lithium with different isotopic composition.

You can buy lithium for almost any price. If you massively overpay $100/kg for the lithium then buying 700 tonnes costs $70 million - for electricity that you sell for a trillion dollars or so.

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pretendperson1776 t1_j1965vh wrote

I'm absolutely going to give this problem to my math class after the break!

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mfb- t1_j199jwc wrote

It's more of a physics problem I think. Work with power*time, divide by the efficiency to get thermal energy, divide by the energy released per fusion reaction, multiply with the mass of tritium, get the lithium to tritium mass ratio from the breeding reaction. You also need some approximations on the way - the energy per reaction will depend on the reaction rates of tritium breeding, and lithium-6 and lithium-7 have different masses so we need that ratio there, too.

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bawki t1_j196ku8 wrote

It's just amazing to see these numbers, I was a bit curious about the breeding process and was hoping it wouldn't turn out to be a limiting factor.

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mfb- t1_j199r38 wrote

Getting one new tritium nucleus extracted per D-T reaction in the reactor won't be trivial, but getting the lithium is not an issue.

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Game_Minds t1_j1998kv wrote

Can't you also dope/enrich lithium back up to 7 relatively cheaply? Thought I read that somewhere too

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mfb- t1_j19c9vw wrote

Natural lithium is ~95% Li-7 already. If you do isotope separation then you want more Li-6 to produce tritium from the lower-energy neutrons that the Li-7 reaction left behind. Both breeding reactions destroy the lithium and leave behind helium and tritium.

Luckily 6 vs. 7 is a pretty large mass ratio (for uranium it's 235 vs. 238) and lithium is neither radioactive nor too toxic (although mercury is), so enrichment is much easier.

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Game_Minds t1_j19g1u2 wrote

Aha! I knew there was some trick. It's that it should be relatively easy to just take raw lithium or even possibly recycle lithium batteries and cheaply achieve the isotope ratios you want for tritium breeding (but the byproduct of the breeding process isnt leftover lithium). Thanks!

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