How do fusion scientists expect to produce enough Tritium to sustain D-T fusion (see text)?

Submitted by DanTheTerrible t3_zslth0 in askscience

In discussions of fusion research online, I keep finding statements that the preferred fusion fuel is a combination of Deuterium and Tritium. Deuterium is readily available in nature, but the Tritium has to be manufactured. The commonly stated scheme is to bombard Lithium-6 with neutrons from the fusion reaction, which will (usually?) cause the Lithium-6 nucleus to split into Tritium plus an alpha particle. What I can't understand is how this can produce enough Tritium to maintain fusion. One D-T fusion reaction produces one neutron, and one neutron causes one Lithium-6 nucleus to produce a Tritium ion -- for this to work, it seems we are requiring perfect 100% efficiency of fusion neutrons producing Lithium fuel. My engineering background refuses to believe it. 100% efficiency is a fantasy, some of those fusion neutrons are going to escape the Lithium blanket without reacting, and some will get involved in reactions that don't produce Tritium. Thus the fuel cycle I keep seeing described can't possibly produce enough Tritium to keep fusion going indefinitely. Is there some mechanism I haven't run across that produces extra Tritium or extra neutrons somehow?

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ivonshnitzel t1_j18rrka wrote

Short answer is that there are ways to make neutrons produce more than one triton. Neutrons can react with lithium-7 to produce tritium + a neutron (which can then go on to react with another lithium-6 or lithium-7 nucleus to produce more tritium). Neutron multipliers such as beryllium that react with neutrons to produce two neutrons can also be included in the tritium bleeding blanket.

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[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.

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zimirken t1_j18v10a wrote

Replying to [deleted] (#1,008,176)

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.

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EvanDaniel t1_j18xaek wrote

Replying to Lasarte34 (#1,008,383)

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.

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mfb- t1_j190vsb wrote

Replying to [deleted] (#1,008,176)

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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bawki t1_j196ku8 wrote

Replying to mfb- (#1,008,806)

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_j199jwc wrote

Replying to pretendperson1776 (#1,009,323)

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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mfb- t1_j19c9vw wrote

Replying to Game_Minds (#1,009,600)

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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kmacdough t1_j19ce3h wrote

As others have posted, there are ways to generate extra neutrons, e.g. with Lithium-7 or Berillium, but many are skeptical of the practicality. It adds more massive technical challenges to the still unsolved problem of net energy. Best case, it's hard to imagine these technologies coming of age in the next few decades.

You may be interested in Helion's approach of fusing Deuterium and Helium-3. Helium-3 is also rare, but can be produced in a similar manner from D-D fusion. The D-H3 fusion avoids many of the other challenges associated with managing and extracting energy from high-energy neutrons.

But fewer challenges is not none, and I wouldn't count on any of this bailing out our energy crisis. Modular fission makes a lot more sense for this purpose.

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Game_Minds t1_j19g1u2 wrote

Replying to mfb- (#1,009,848)

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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Nescio224 t1_j19jovd wrote

Replying to [deleted] (#1,008,176)

>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.

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NappingYG t1_j19nxcx wrote

Tritium is also a byproduct of Nuclear Power industry where heavy water used for moderation, like the Canadian designed CANDU reactors. In Darlington for example, there is an entire Tritium Removal Facility used for purification of tritated heavy water and exports tritium. If demand for tritium increases, so can tritium production

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SoylentRox t1_j19ocmt wrote

What stops you from replacing the breeding blankets/dissolved liquid tubes for lithium on a fusion reactor with uranium targets?

Would this allow you to breed plutonium or are the neutrons the wrong velocity for this?

What bothers me is that this is yet another long term showstopper for fusion. If fusion technology is dual use - meaning it can be used to make plutonium for nuclear weapons - it cannot be freely shared with most nations in existence on earth. Only a few rich ones who already have nuclear weapons or are considered 'trustworthy'.

Meanwhile by the time this happens, solar panels and batteries are so cheap they are almost a waste product and can be dumped by the pallet load to anyone anywhere.

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mystlurker t1_j19sve9 wrote

Replying to Nescio224 (#1,010,531)

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.

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Wyrggle t1_j19u0pg wrote

Replying to EvanDaniel (#1,008,492)

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.

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Nescio224 t1_j19ufka wrote

Replying to atomfullerene (#1,011,205)

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.

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binaryblade t1_j19v6p8 wrote

Wonderful explanations here about Li7 breeding and what not. I would just like to add that D-T can be often seen as a stepping stone to a D-D reactor. D-T is easier, that's why its the immediate goal, but the next step on the path is D-D which doesn't have these problems.

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Nescio224 t1_j1a3d5k wrote

Replying to mystlurker (#1,011,387)

>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.

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mystlurker t1_j1a4avk wrote

Replying to Nescio224 (#1,012,286)

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.

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camron67 t1_j1a6i5h wrote

Replying to kmacdough (#1,009,865)

Re Helion. Helium-3 is produced by tritium decay. The Darlington reactor is now recovering the helium-3 from the tritium they have in storage. Good to know there is a process that can use up the available fuel from all the available tritium that will decay before D-T fusion plants are in place.

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ClapAlongChorus t1_j1a7i0j wrote

Replying to Danne660 (#1,011,831)

oh really?? that's cool, I always assumed the Fusion revolution required a decent amount of fuel and that was never a big deal until massive batteries came along and started using lithium as fast as we could pull it out of the ground in Western Australia. Sometimes it's neat not knowing much because there are so many chances to learn

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DJ_Spark_Shot t1_j1a8vec wrote

Tritium is produced in the reactor as He-3 decays and the neutrons collide with deuterium. That said, I'd be curious why tritium would be a target atom. I suspect that is just an acceptable contaminant from the deuterium concentration process.

Tritium would set off fission reactions that cause radioactive byproducts. A significant reason to go fusion is the lack of radioactive waste products.

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Wings-N-Beer t1_j1a9mky wrote

CANDU reactors produce plenty of tritium. Deuterium is not quite “readily available in nature”, but can be produced through heavy water upgrading. Moderation in CANDU using D2O produces tritium. Periodically water swaps are completed, and the tritiated D2O is detritiated. The H3 is then packaged and sold.

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binaryblade t1_j1abhsy wrote

Replying to calebs_dad (#1,012,825)

Ignoring side reactions DD spits out about 1/4 of the energy and has a smaller cross section. There's also a 50/50 chance that reaction spits out tritium instead of helium 3.

It's much harder to do than DT hence the focus on DT, but not impossible.

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adventuringraw t1_j1abuxl wrote

Replying to ClapAlongChorus (#1,012,674)

There's been recent advances with MOF filters that energetically favors Lithium going through, kind of like ion channels in cells. It drastically decreases time and water cost in filtering out Lithium, and I guess it's being tested at scale now. It certainly won't solve Lithium supply constraints, but stuff like that's cool to look at... advances in one area of tech research potentially facilitating progress in others. It'll be interesting to see how things change over the next decade.

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Ulfgardleo t1_j1afdic wrote

Replying to [deleted] (#1,013,090)

other projects do not "completely dismiss" this issue, but they are researching other aspects of fusion. For example: that Wendelstein-7x does not investigate breeding is not because they don't see it as interesting, but because they want to show that stellarator confiment works and that we have finally understood plasma physics.

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DontWorryImADr t1_j1agg8b wrote

Replying to ClapAlongChorus (#1,012,674)

So for a comparison, a single EV battery requires something on the order of 63 kg of lithium. Assuming this is even remotely true, we then have references from Youtubers like Real Engineering who estimate a commercial fusion plant would need about 600g of tritium per day.

Switching from mass to equivalent atom counts and then back to relate, 600g of tritium is just shy of 200 moles (3.016 g/mol). Multiplying this by the molar mass of lithium (6.941 g/mol) gets us ~1,382g. I couldn’t easily find an efficiency rate, so let’s assume we’re terrible at conversion and only manage 1% conversion rate: a plant would require about 138kg per day.

So while the lithium usage by a commercial power plant would be noticeable if they became super common.. it would amount to about two EV batteries per day, per plant. Considering we may need over 30 million EV batteries per year by 2030, this would be a very small impact.

Edit: month —> day Edit2: I was lame, got the molecular mass of tritium (T2) rather than atomic mass, and ran with the math. All fixed now.

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RobusEtCeleritas t1_j1agwgi wrote

Replying to atomfullerene (#1,011,205)

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.

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matt7810 t1_j1ahrh8 wrote

Replying to Techsterr (#1,012,325)

Part of the reason D-T fusion is the primary candidate is that it occurs at temperatures and pressures orders of magnitude lower than He-3 based fusions. This makes Helions approach more difficult from a materials, heating, and magnetic field perspective.

I do research tangential research and one thing I've heard (take with a grain of salt) is that they don't publish nearly as many results as other fusion companies. This may not mean anything, or it could mean they don't have favorable results thus far.

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__Pers t1_j1aly4r wrote

>Is there some mechanism I haven't run across that produces extra Tritium or extra neutrons somehow?

Yes. In addition to breeding tritium from (n,t) reactions on 6Li, you can multiply neutrons through, e.g., n + 9Be -> 2*4He + 2 n, and can also breed tritium without losing a neutron through (n,t) reactions on Li7. Both can, in principle, lead to tritium production systems capable of replacing the fuel at the rate at which it's consumed.

Such systems haven't been demonstrated at scale, however, and would have to be co-located with the power plant, leading to all sorts of complications (not to mention NIMBY concerns) but there is nothing fundamental that is stopping such a recovery system from being designed.

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DontWorryImADr t1_j1b07jo wrote

Replying to starmartyr (#1,014,801)

It better be, considering the volume of waste if all those batteries need replacement every 10 years. That would be the order of 1.89 billion kg of lithium every battery replacement cycle based upon 2030 numbers. Considering some of the issues with lithium, that would be all sorts of bad.

I don’t know that commercial scale recycling of said batteries is truly ready, but hence why it’s a big area of examination and study when it comes to converting transportation away from fossil fuels.

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zekromNLR t1_j1b9g5z wrote

Replying to Splatterman27 (#1,008,898)

Those processes are how a D-T fusion plant would capture energy. About 80% of the energy output of D-T fusion is in the neutron, and the other 20% are probably required to keep the plasma hot anyways. As the neutrons slow down and go through nuclear reactions in the breeding blanket, they will give up their kinetic energy as heat, which can then be used to boil water and drive a steam turbine.

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zekromNLR t1_j1ba8rm wrote

Replying to Danne660 (#1,011,831)

Well, you would need a quite large inventory of lithium in the reactor to capture a large fraction of the neutrons, but it would only be consumed at a slow rate. Even assuming only 20% of the fusion power comes out as net electricity output (the rest being either lost as waste heat or needed to keep the fusion going), a 1 GW D-T fusion power plant would consume only about 275 kg of tritium per year, which would correspond to a lithium consumption of about 600 kg per year, depending on the specific mix of lithium isotopes.

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lo53n t1_j1bb1ke wrote

Replying to zekromNLR (#1,015,823)

Its so strange, after all those years we still convert kinetic energy to electricity via steam turbine. Is there even any feasible option to phase out steam energy or use more direct conversion?

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RobusEtCeleritas t1_j1bejog wrote

Replying to [deleted] (#1,016,191)

RBMK fission reactors are completely different things than what we're talking about here. There's plenty of information available on what caused the Chernobyl accident, none of which is relevant to this conversation.

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Bluerendar t1_j1bfw1h wrote

Replying to Techsterr (#1,012,325)

There's two main bottlenecks I see:

  1. Energy extraction. The way their system works, it is basically impossible to be efficient extracting energy from the heat produced like how ITER (or, planned future commercial reactors using the principles in ITER) is planning. This is because the extremely strong, temporary magnetic fields they are producing use up enormous amounts of energy, which recycling from heat would be horribly inefficient - ITER uses much weaker semi-permanent fields (in comparison to Helion, ITER's fields are no joke either), which are much less energy-intensive to maintain. Therefore, Helion proposes to generate energy directly from the magnetic fields involved - the fusion process itself produces much of the energy from the motion of charged particles, which the magnetic confinement will capture as magnetic flux - thereby also recycling the energy from the magnetic fields they produce. So far, they haven't demonstrated capturing the energy back out, which will be very difficult (but, at my cursory look, physically reasonable, just difficult to do) and very difficult to do efficiently enough for their proposal to work.

A slightly different but equivalent explanation for the efficiency issue is that Helion uses much higher temperatures for their fusion - this means more energy in, which needs to be recaptured to be efficient, and following thermodynamics, the higher temperature at which the reactor can capture the energy, the higher efficiency of recycling. Magnetic fields capture the energy immediately after fusion at peak temperatures. Heat capture would be much lower temperature in comparison, which makes everything horribly inefficient.

  1. Scaling up production of energy. Helion makes relatively low-energy "bursts" of fusion - to make the energy generation appreciable, they have said they need the bursts to cycle at something like 1000+ times per second (I forget the exact number they gave). Right now, they've demonstrated fusion at 1 time per minutes, and the magnetic confinement and lauch (without fusion) at closer to, but still far from, that frequency. This means everything involved - the production of magnetic fields using their superconducting electromagnetics powered by massive capacitor banks, injection of fuel, launch process, collection of energy probably back into capacitor banks, needs to long-term reliably happen at these frequencies. As an example, in practice, one thing that probably needs to happen is the capacitors need to be done away with altogether (outside of startup and net energy capture) and the electrical energy generated needs to mostly directly power the electromagnets - cycling that much energy at those frequencies would overheat capacitors, needing multiple banks to run instead.
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mrwolfisolveproblems t1_j1bph0g wrote

Replying to DontWorryImADr (#1,015,069)

Battery end of life with EVs is the 1000 pound gorilla in the room that no one wants to acknowledge. All these states passing laws to ban sales of ICE vehicles have put zero thought into it that’s for sure. Not to mention the huge cost to consumers of said replacements. So insane to me that these problems are not close to being solved with EVs being jammed down everyone’s throat. I guess necessity is the mother of all invention, so hopefully mass EV adoption will drive solutions to these problems. End of sidebar.

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FinndBors t1_j1bq2hs wrote

Replying to lo53n (#1,015,966)

You are looking for aneutronic fusion: https://en.wikipedia.org/wiki/Aneutronic_fusion

He3 is required but rare, or Boron fusion but needs a ton more input energy than D-T fusion. The sibling comments regarding Helion Energy is an attempt to do this using He3 and a way to synthesize it since it is very rare. The way they synthesize it is D-D fusion which does produce neutrons.

Regarding He3, people talk about mining this on the moon since it is less rare there, but IMO, its a stupid idea since it isn't like mining an ore where there is concentrations of it, it's weakly spread out all over the surface.

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cdstephens t1_j1bq2qw wrote

Replying to graebot (#1,008,151)

No, fusion devices cannot lead to an uncontrolled chain reaction. The reason is because the plasma needs to be confined in order to maintain the appropriate density and temperature; the Sun uses gravity to confine the plasma. In contrast, if the magnetic fields were turned off in a magnetic fusion device, the fusion plasma would just expand outwards into the wall and then cool down.

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AeternusDoleo t1_j1d635j wrote

Replying to graebot (#1,008,151)

There would be a danger of a localized kaboom, once the plasma loses containment it disperses - explosively. But I'm going to assume the amount of fuel in the reactor is going to be minimal, after all you're after a controlled fusion reaction. Once the fuel is spent or the pressure/temperature is too low to sustain fusion, the reaction ends.

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mrwolfisolveproblems t1_j1ehkcm wrote

Replying to FRCP_12b6 (#1,018,976)

If an EV battery is so degraded it can provide a few hours of runtime in a car what meaningful use will it have to the grid? Has anyone actually tested this at reasonable scale beyond a simple demonstration? Who is going to pay for the infrastructure to connect all these old batteries to the grid? That grid storage argument is just thrown out there for PR. It would take decades to get off the ground and we’re going to have millions of dead battery packs in 10 years.

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SoylentRox t1_j1fxj19 wrote

Replying to roguetrick (#1,031,643)

You have to absorb neutron flux regardless.

This is one argument in favor of aneutronic designs, that fusion may not ever be practical if the reaction emits a significant amount of neutrons. Hydrogen-B11 for instance. I understand it's immensely harder to do though, requiring much higher temperatures.

An aneutronic design could be designed to fail if someone tries to breed plutonium.

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mrwolfisolveproblems t1_j1gdayr wrote

Replying to FRCP_12b6 (#1,026,947)

So a 100kwh pack becomes 60kwh. A thousands of them together gives you 60MW for 1 hour. Peak load demand can swing 20-40,000 MW for 10+ hours at a time. That’s just peak demand, forget about base load, and that’s just in a regional area (say Texas for example) An extra 20,000 MW for 10 hours is 200,000,000 kWh. You would need 3.33 million old battery packs all tired together and synced to the grid. Not to mention every day they will lose capacity and eventually be useless even for grid storage.

TLDR: need to find a way to recycle them into new batteries like we do for lead acid batteries.

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