[deleted] t1_j18tmw2 wrote
The reason people consider fusion to be a "holy grail" energy source is because it is capable of creating its own fuel.
The other poster is correct - if you irradiate 7Li with neutrons, it will create tritium through a nuclear reaction. Previously, we've done this by putting the 7Li near nuclear reactors, which releases a lot of neutrons. However, the fusion process naturally creates a very large number of neutrons as well.
The idea is that you put the 7Li near the fusion source, and allow it to build up tritium as you produce energy. Then, you would extract the tritium from the 7Li and use it to make more fuel.
The bigger problem is that we've learned to heavily rely on Li ion batteries since we first started planning fusion experiments back in the late 80s. So there will be a Li resource competition between energy production and energy storage.
This is a wild coincidence - somehow, the chemical properties of Li are appropriate for energy storage, while its (completely separate) nuclear properties are appropriate for energy production.
zimirken t1_j18v10a wrote
The amount of lithium you'll be consuming to breed tritium is orders of magnitude less than you think. One ton of duterium fused with one ton of tritium produces the same energy as 29 billion tons of coal.
Lasarte34 t1_j18vxk4 wrote
How many tons of lithium are needed to generate a ton of tritium? (I figure nothing close the billion tons of coal, but I would love to know the math still)
EvanDaniel t1_j18xaek wrote
That ratio is just the atomic mass ratio.
One atom of lithium-6 (or 7) produces one atom of tritium. Atomic masses are 6 (or 7) for the Li, 3 for the tritium.
So for one ton of tritium you need 2 tons (or a little more) of lithium. And some amount more than that of beryllium, though I don't know what ratio that's proposed to be used at.
Game_Minds t1_j198wil wrote
A ton of tritium would also go a pretty long way for the cost of the exchange
Wyrggle t1_j19u0pg wrote
It's a molar ratio. In a perfect world, you need the same number of atoms as opposed to mass.
Neutron flux from fission reactors is on the order of 10^19 to 10^25 neutrons/cm^2/s depending on location in the core and would light be similar for fusion reactors. So you're correct you'd need the same amount of source material to generate 1 ton of tritium from 2 tons of Li-6 with 100% efficiency. However, you'd lose tritium via decay and hydrogen diffusion along with uncaptured neutrons paint through the Li-6 target.
mfb- t1_j190vsb wrote
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.
pretendperson1776 t1_j1965vh wrote
I'm absolutely going to give this problem to my math class after the break!
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.
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.
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
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.
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!
Nescio224 t1_j19jovd wrote
>The reason people consider fusion to be a "holy grail" energy source is because it is capable of creating its own fuel.
Fission reactors can create more fuel than they consume as well. They are called breeder reactors. However the fuel for fission reactors is already so cheap that most commercial operators are not interested making it even more efficient. So what if fusion reactors can create their own fuel (which remains to be proven)? It doesn't matter if the reactors themselves are too expensive. Fusion is considered a "holy grail" because commercial large scale fusion is not yet real. When it becomes real, it will stop being a "holy grail", because there is almost no advantage to nuclear.
mystlurker t1_j19sve9 wrote
Isn’t part of the holy grail aspect of fusion that it lets you have a nuclear power source that has much lower radioactivity concerns than fission, making it easier to deal with both from a safety perspective but also a public acceptance perspective?
All the futurology stuff around fusion I saw always talked about how it was an unlimited CLEAN energy source. Nuclear power stopped growing partly due to insane costs and partly due to waning public acceptance after multiple disasters. It a fusion reactor exploded there may be major loss of life but it wouldn’t make the surrounding environment toxic for long periods, or at least that’s the idea.
Obviously if you need lithium it’s clearly not truly unlimited, but the idea of something you could scale out much faster than solar/wind is rather appealing.
Nescio224 t1_j1a3d5k wrote
>much lower radioactivity concerns than fission, making it easier to deal with both from a safety perspective but also a public acceptance perspective?
The amount of "radioactive waste"(=radioactive resources) would be comparable to a breeder reactor. Newer reactor designs are much safer than old ones. If you eliminate the most common risks in the design, you can easily reduce the chance of a meltdown to practially zero. The development cycle for nuclear power is much slower than for other technologies, because of their high lifetimes and low numbers. Imagine if we stopped developing cars after the first 4 designs, with the biggest charge being a few hundred cars, because they were too unsafe. That's where we are with nuclear. If you can drop the mean time between meltdown incidents from every 10 years to every 1000 years worldwide, then that does matter. People want to make you believe this technology is inherently unsafe and the designs can't be improved. That is completely false.
>All the futurology stuff around fusion
Most of the futurology stuff is made by people who have no clue what they are talking about.
> It a fusion reactor exploded there may be major loss of life but it wouldn’t make the surrounding environment toxic for long periods, or at least that’s the idea.
This study concludes that the relocation of people after the Fukushima nulcear accident was not justified on the grounds of radiological health benefit. Also that "long period" is "only" about 100 years. Nuclear has still the lowest death rate of all alternatives.
>Obviously if you need lithium it’s clearly not truly unlimited
Lithium is abount 10 times more abundant than Uranium, so supply is not an issue.
> but the idea of something you could scale out much faster than solar/wind is rather appealing.
Why are you assuming that you can build fusion reactors faster than fission reactors or solar/wind? There are no commercial fusion reactor yet and all existing designs are very early prototypes. The data to make that conclusion doesn't exist yet. Not to mention that fusion is at least 20 years away (as always) and we need a solution now.
mystlurker t1_j1a4avk wrote
You are misinterpreting my post, I’m not saying I necessarily agree or support various aspects of this, just explaining why there is this mythical aspect to fusion.
Facts about fission haven’t done well to change public perception. And public perception has an outsized impact on government policy.
Fusion theoretically offers the upsides of fission without the downsides and it theoretically offers better scaling than existing renewables. But as you said this is all theory. But that is what makes it the holy grail, in theory it has major upsides but it’s far off from production. Dismissing the human element here is to dismiss a large part of what defines the allure of fusion.
[deleted] t1_j19y0sh wrote
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RobusEtCeleritas t1_j1ah5rr wrote
There will be activation of reactor components, of course. But that will not result in nearly the amount of radioactivity per unit mass as the fission products in spent fission fuel.
[deleted] t1_j1adh64 wrote
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[deleted] t1_j1ae48v wrote
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atomfullerene t1_j19qxu4 wrote
Don't breeders produce fissile materials that could be used in weapons? I thought proliferation concerns were the main thing keeping them from being more widely adopted.
Nescio224 t1_j19ufka wrote
Yes, but that depends on the specific breeder design. Besides, if someone really wants nuclear weapons, there are a thousand different paths. Just look at north korea. The wikipedia article on breeder reactors states the reason why there is not more interest in breeder in the first paragraph: >Breeders were at first found attractive because they made more complete use of uranium fuel than light water reactors, but interest declined after the 1960s as more uranium reserves were found,[2] and new methods of uranium enrichment reduced fuel costs.
Breeders could extract 100 times more energy from the same fuel rod than an LWR can, but even at 1% efficiency LWR's are efficient enough that fuel cost is not an issue. That's just how OP the energy density of the fuel is.
[deleted] t1_j1bp9jc wrote
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RobusEtCeleritas t1_j1agwgi wrote
If you can breed fuel for a reactor, you can inherently breed fuel for a weapon too. Any spent fission fuel can in theory be reprocessed, and have material diverted for weapons purposes.
But that's why organizations like the IAEA closely monitor fuel cycles for proliferation concerns.
[deleted] t1_j1b9rhg wrote
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[deleted] t1_j1923qw wrote
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ngiotis t1_j19ykhk wrote
Well surely we won't need as many batteries if we're using fusion instead of solar so it balances out more
LordVile95 t1_j1a4izn wrote
Batteries are trying to move away from lithium when a viable alternative is found though due to them being spicy
[deleted] t1_j1bbdv2 wrote
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