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Englandd12 t1_iyv70ef wrote

Hey...finally something I can answer. I'm doing a PhD in Nuclear fusion materials. We ideally want to do the DT reaction due the fact it has the highest specific reactivity at lower temperatures even when compared to DD, DH etc reactions. It also produces a free neutron, which we can use for tritium breeding using a lithium breeder blanket, so the intent is we can produce self-sustaining tritium within the chamber.

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DoubleDot7 t1_iyvfwfj wrote

>it has the highest specific reactivity at lower temperatures

Does that mean that deuterium-tritium reactions need less energy for the reaction to start? Is that because tritium is less stable than the other isotopes?

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RobusEtCeleritas t1_iyvicsj wrote

It means that the Coulomb barrier is a little bit lower. It's unrelated to the stability tritium, it's just possible to make this reaction occur at a reasonable rate at lower temperatures.

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financial2k t1_izsx87h wrote

How much lower is this temperature?

Is the main motivation of using Tritium the lower temperature or actually the breeding reaction?

How far apart is the fusion temperature from the fusion ignition temperature and how is each one defined?

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RobusEtCeleritas t1_izt7by2 wrote

>How much lower is this temperature?

Here is the reactivity as a function of temperature for a few candidate reactions.

>Is the main motivation of using Tritium the lower temperature or actually the breeding reaction?

The main motivation is the temperature. Obtaining fuel for DD is not an issue, because there's plentiful deuterium in nature (seawater, for example). It's a nice benefit, and quite important for tritium, which is not found naturally in large amounts. We have to produce tritium somehow, and having the reactor breed its own fuel is a nice way to do that.

>How far apart is the fusion temperature from the fusion ignition temperature and how is each one defined?

Not sure what you mean here.

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financial2k t1_iztmlqb wrote

Thanks.

How far apart is the fusion temperature from the fusion ignition temperature and how is each one defined?

This was answered in a comment below somewhere. perhaps even by you

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[deleted] t1_iyvj5ib wrote

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treefor_js t1_iyvuhla wrote

This is not correct. Temperature is a measurement of the average kinetic energy of the particles - you express plasma temperature in units of eV (energy unit). If one element is heavier then it'll have a slower average velocity. DT reactions require lower temperatures to achieve their highest cross section for fusion reactions. Meaning you need to put less energy into the system.

  • HEDP plasma physicist
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ChipotleMayoFusion t1_iyw8hte wrote

Isn't that exactly what Robus said? The DT reaction is favorable because it reaches high reactivity at lower temperatures. https://upload.wikimedia.org/wikipedia/commons/thumb/d/d0/Fusion_rxnrate.svg/330px-Fusion_rxnrate.svg.png

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treefor_js t1_iyw9ick wrote

That was the conclusion, yes. However, the reasoning was not correct.

Edit: the other thing to note here is not just that it takes a lower temperature to reach higher reaction cross sections but the loss mechanisms that scale with temperature as well. It's a balancing act to keep the plasma warm to use the fusion products to keep burning the fuel without it cooling off rapidly. Bremsstrahlung radiation - x-rays generated by accelerated charged particles, is the main culprit here.

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ChipotleMayoFusion t1_iywmfex wrote

Ok, thanks for the clarification. Maybe I misunderstood what his post was getting at. I have heard that proton-Boron is basically impossible because the brems losses at the temperature where reactivity is sufficient will always be higher, or almost always higher. I think this is what you are saying, you can't just focus on the temperature. Sam Wurzel had a great talk on this at APS 2021, clarifying Qeng vs Qsci and how that changes depending on your recirculating power fraction and other factors.

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treefor_js t1_iywn305 wrote

Oh nice. I didn't get a chance to go to that one. Came down with a stomach bug for a day or two in Pittsburgh. Also wish I had time to go to the commercial fusion breakout this year, but alas. There's always next year.

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ChipotleMayoFusion t1_iywnxfg wrote

I'm glad you were able to attend at all, a lot of the US national labs people were not there due to COVID travel rules.

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treefor_js t1_iywoopg wrote

I sat in on one of the MagLIF sessions and I think there was one live talk with like 10 recorded ones. It was a weird conference. Basically just networked with university folks. So much better turn out this year with national lab folks returning.

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[deleted] t1_iyvl308 wrote

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TinnyOctopus t1_iyvpco3 wrote

As you suppose, it's the sourcing of the tritium that's the problem, but I think you're underselling the difficulty. For the DT fusion, the plasma composition can be mostly D, which is difficult but not impossible to purify out of naturally occurring water (prevalence is generally about 1 in 10,000 to 100,000 hydrogen atoms). Tritium is about 1 to 10^18th hydrogen atoms, which is a million million times less common than even the uncommon deuterium. Which means tritium needs to be manufactured, and at a certain point, the amount of energy being put in to make the tritium fuel will become prohibitive, making the economics of a T-T rector nonviable.

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UWwolfman t1_iz0py64 wrote

>Does that mean that deuterium-tritium reactions need less energy for the reaction to start?

Yes and no.

Focusing on energy "needed to start" misses the key physics and related engineering issue.

When two fusion reactants (such as deuterium and tritium) collide, that vast majority of the time they do not fuse. Instead they scatter off of each other. This is true even if they have enough energy to fuse. The reactants only occasionally fuse when they collide. The specific reactivity is a measure of how often they fuse.

While the scattering collisions conserve energy, they lead to thermal conduction. Without some sort of thermal insulation, a fusion-ing plasma will cool off quickly and fizzle due to this collision induced thermal conduction. In fusion we call this insulation (energy) confinement.

So the key issue for engineering an economical fusion power plant is not providing enough heat to the fuel we can do this, but instead it's about building a good enough insulator to keep the plasma warm.

The more reactive the plasma, the more leaky the our insulator can be. It turns out the deuterium-tritium has the highest reactivity of all fusion fuels.

>Is that because tritium is less stable than the other isotopes?

It's complicated, but I'd argue is has more to do with He-4 being a very stable isotope. There are other factors, such a a relatively low coulomb barrier compared to high-Z reactants.

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cloudjianrider t1_iywwbdc wrote

You have just blown my mind. Free neutrons? Tritium breeding with a lithium breeder blanket!?! There is so much I do not know.

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Atechiman t1_iyyxvig wrote

When you do the reaction as dt you are left with a neutron. This neutron can be used with lithium to convert certain hydrogen into tritium basically.

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[deleted] t1_iyw1nal wrote

Are thorium/U233 reactors actually viable for power production or is it like the old industrial myths in the 80’s about 100 mpg motors being quashed by the oil companies?

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echawkes t1_iywbcdh wrote

It is certainly possible, and experimental reactors have been built to show that it could work.

Sixty or seventy years ago, people thought uranium was scarce, and that we would need breeder reactors (either on thorium/uranium or uranium/plutonium fuel cycles) to make nuclear power viable. However, breeder technology was never fully developed because uranium turned out to be a lot more plentiful and cheap than people expected. There just hasn't been any compelling reason to develop a new technology to make uranium when it's so much easier and cheaper to just dig it out of the ground.

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WazWaz t1_iyx9md4 wrote

Uranium is scarce, but the demand for it is low - known reserves would last about 5 years if it was our only electricity generation method (of course, we'd start using breeders, recycling, etc. if that was the case, and probably find more reserves too).

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echawkes t1_iyxs0g7 wrote

I wouldn't say that uranium is scarce. Significant deposits of uranium are found on every continent (except Antarctica, so far). In a list of the elements that have more than trace quantities in the earth's crust, uranium appears around the middle.

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WazWaz t1_iyzg1py wrote

As I said, if we used it for all our electricity, it would last 5 years. That's pretty scarce, considering how little we use.

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Atechiman t1_iyyywgd wrote

A kilogram of Uranium can generate ~24 terrawatt hours. World reserves of Uranium is 8,000,000 tonnes (8 billion kilograms) . 192 billion terrawatt hours. Worldwide electrical consumption is 23 thousand terrawatt hours.

It would take roughly 6.26 million years to burn through uranium reserves.

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Malkiot t1_iyznf3t wrote

You can't look at world reserves of uranium. You have to look at world reserves of U235 which makes up about 0.76% of all Uranium. You also can't take the total amount, but have to take the commercially viable amount and the amount of energy Uranium contains cannot be converted 1-to-1 to electrical energy.

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>The world's present measured resources of uranium (6.1 Mt) in the cost category less than three times present spot prices and used only in conventional reactors, are enough to last for about 90 years. This represents a higher level of assured resources than is normal for most minerals.

Source: World Nuclear Association an organization promoting nuclear energy.

From our current perspective, when comparing to our previous industrial development, 90 is pretty good. But nowhere near enough in the long term and we'd have to fall back to renewable again unless we use breeder reactors which would improve the sustainabiliy of nuclear or figure out fusion.

So, while nuclear does have some advantages from present knowledge, we may as well skip the 90-year nuclear phase and go for renewables straight away.

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WazWaz t1_iz13knp wrote

And to be clear, that 90 years is at present consumption rate. Nuclear is about 10% of world electricity use, so if it was 100% it would last 9 years. Electrify the road transport sector alone and that comes down to 5 years.

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LucubrateIsh t1_iyxdyvr wrote

Sure. It isn't necessarily all that different from other fast reactor designs like ebr-2. The idea usually gets combined with some other thorium salt ideas that I have no idea about the viability in terms of cost of

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juxt417 t1_iz00lel wrote

They are viable but they are having issues with the molten salt in the reactor.

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[deleted] t1_iyvlrlm wrote

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[deleted] t1_iyw3af8 wrote

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ProneMasturbationMan t1_iywfeqx wrote

>it has the highest specific reactivity at lower temperatures even when compared to DD, DH etc reactions

Why?

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RobusEtCeleritas t1_iyws6yx wrote

Lower Coulomb barrier, which means that the cross section begins to increase at lower energies, which means that the convolution of the cross section with a Maxwellian distribution function is higher at lower temperatures.

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redruben234 t1_iyy98oh wrote

Question about the lithium blanket: doesn't it seem like a potential future problem? Will the lithium get used up this way? It's already a valuable material

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RobusEtCeleritas t1_iyv79be wrote

The cross section for that reaction is very low over all energies. DT and DD have the most favorable Maxwellian-averaged cross sections as a function of temperature, meaning that they “turn on” at the lowest temperature, and therefore, it’s easiest to create the required conditions in a reactor.

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[deleted] t1_iyv6ywj wrote

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mfb- t1_iyvbd9b wrote

It's not about multiples of 4, it's about producing Helium-4 which is a very tightly bound nucleus.

If a reaction only produces a single nucleus then the released energy has to be released as photon, which means we need the electromagnetic interaction. That's weaker than the strong interaction, so these processes are rare. D-D fusion can produce He-4 + photon, but T + p and He-3 + n are much more common.

H + B-11 produces 3 He-4

D + T produces He-4 + n

D + He-3 produces He-4 + p.

H + D can only produce He-3 + photon, so we get a pretty bad reaction plus we need the electromagnetic interaction.

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Ishana92 t1_iyvalhh wrote

Can you explain why multiples of four matter here?

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Qrkchrm t1_iyvcg6u wrote

Just like electrons have orbital shells that affect chemical stability, the nucleus has quantum states with different binding energies. In general even numbers of protons and neutrons are higher binding energy that odd numbers. Helium 4 is particularly high binding energy, as it is double magic.

https://en.m.wikipedia.org/wiki/Magic_number_(physics)

https://en.m.wikipedia.org/wiki/Nuclear_binding_energy#/media/File%3ABinding_energy_curve_-_common_isotopes.svg

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DataHead127 t1_iyv93m5 wrote

All isotopes of hydrogen have one proton, deuterium has one neutron and tritium has two neutrons, so their ion masses are heavier than protium, the isotope of hydrogen with no neutrons. When deuterium and tritium fuse, they create a helium nucleus, which has two protons and two neutrons. The reaction releases an energetic neutron. Deuterium and tritium are isotopes of hydrogen. They reaches fusion conditions at lower temperatures compared to other elements and releases more energy than other fusion reactions. The current best bet for fusion reactors is deuterium-tritium fuel that is relatively easy to produce.

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NetworkLlama t1_iyvd13n wrote

Deuterium is easy, tritium is not. The entire world's supply is about 20 kg and it's only produced in a few reactors around the world. It decays rapidly with a half-life of only 12 years, making holding on to what you have a temporary prospect at best. Much of what is produced goes into keeping nuclear weapons active, and when ITER comes online, it will get much of the rest unless its lithium blanket for breeding tritium works spectacularly well.

The lack of easy tritium production is a major reason various projects around the world are looking at alternate means.

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the_geth t1_iyvg3in wrote

Really? I read about everywhere that it’s easy to produce with the lithium blanket (including in a classical fission reactor if needed). The reason we don’t have much production is simply… that we don’t need it (or rather, that the amount we have is enough for our CURRENT use)

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NetworkLlama t1_iyvlbjf wrote

Lithium blankets are still very much in the research stage. The US gets its tritium for it's nuclear arsenal by irradiating special rods called tritium producing burnable absorber rods (TPBARs) containing lithium-6 in a nuclear reactor, specifically Watts Bar Reactors 1 and 2 at the TVA. Each TPBAR is about ten feet long and less than half an inch in diameter. Over about 500 days of burning, each produces about 1.2 grams of tritium.

Civilian sources are primarily from CANDU reactors, but building more of these can be problematic as they're heavy-water reactors (they produce tritium by deuterium neutron capture) and are considered to be proliferation risks, raising both political and legal problems. They also don't produce that much. According tothis paper on sourcing tritium for fusion use, the CANDU 6 reactor, a 700 MW design, can generate only 130 grams of tritium per year, though not all of this can be captured.

According to ITER's own numbers, 800 MW of fusion-generated electricity will require 300 grams of tritium per day. Lithium blankets are the most promising way to get this done, but they present their own technical challenges. This is why research is happening on other approaches like laser confinement and Z-pinch to find ways of using just deuterium.

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the_geth t1_iyw8h5l wrote

Super interesting, thank you for the thoughtful answer!

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beaded_lion59 t1_iywmi7r wrote

Who’s working on Z-pinch fusion now? I did D-D fusion in the 80’s in a Z-pinch, but the process would require a lot of pulse-power advances to make it practical for energy production.

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beaded_lion59 t1_iyxuv6e wrote

I did this at a 7 TW pulsed power system at a company in the Bay Area using deuterium gas. The system could do 3 shots/day. Sandia’s shot rate is probably less. They’re more like NIF at Livermore.

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Relevant_Monstrosity t1_iyviaco wrote

There's probably both a lack of demand, and necessary capitalization to meet potential demand. So you can look for projects to increase the supply to kick off as new users come online.

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ukezi t1_iyvjnee wrote

It's more of a in theory it should be "easy". There are still a lot of details to figure out.

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the_geth t1_iyw8nz0 wrote

I see. It’s interesting this was not (until I hear it here) mentioned as a potential problem.

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MortalPhantom t1_iyvhxac wrote

I know it's rare but then why was it used on watches? Even current watches some of them use tritium and you can find them prelaatively cheap 500-1000 usd, and I doubt tritium is a considerable part of that price.

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karlnite t1_iyvm70b wrote

Canadian nuclear plants pay millions a year to remove tritium from our cooling and moderator water. It’s a pesky problem and we are always looking for ways to get rid of tritium. We currently keep barrels upon barrels of our most tritiated water sitting around to decay off so we can use the deuterium content again safely one day. We would pay fusion companies to take our tritium away. They could set up a device to harvest it from our reactors or waste just like we do for the global supply of medical isotopes currently. We have determined a myriad of medical isotopes that could be harvested, we just need investors willing to set up the systems to harvest them who have connections to medical companies that can utilize the isotopes (our industry doesn’t have the experience to do this all ourselves as we make power and that is hard enough). I don’t see why we couldn’t do that for tritium too.

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NetworkLlama t1_iywq9xk wrote

There's probably not as much tritium in that water as you think. According to this paper on sourcing tritium, a 700 MW CANDU-6 reactor can make only about 130 grams of tritium per year. There probably aren't dozens of kilograms of tritium sitting in those barrels.

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karlnite t1_iywuaul wrote

That’s very likely, I have not done the math to calculate to mass of tritium in the systems.

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chemhobby t1_iyvlxfh wrote

the amount used in gaseous tritium light sources is absolutely miniscule

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NetworkLlama t1_iyvmmco wrote

A tiny fraction of a gram is used, with a maximum of 25 millicuries allowed per timepiece. A little bit goes a long way for that purpose.

Edit: I looked up some numbers, and the amount of tritium in any given timepiece is apparently measured in micrograms, not even milligrams. I'm having trouble finding exact amounts, but a very rough calculation based on a specific activity of 9650 Cu/g and 25 millicuries results in .025 mCu-g/9650 Cu = 2.6 micrograms per watch. That's a very, very tiny amount.

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vokzhen t1_iyx4njh wrote

For those who don't grasp what this means very well, those numbers mean a gram of tritium is enough for about 400,000 watches.

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ukezi t1_iyvjtw7 wrote

Before tritium paint they used radium to get the phosphor to glow. The problem is that radium is highly toxic and carcinogenic.

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boycey10802002 t1_iyvl6qk wrote

That I had heard the horror stories of. It just didnt seem to make sense to me to use a super-limited resource like tritium for watches and thought titanium would be a more likely candidate for a mid-level, rugged watch.

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Lazzer555 t1_iywefn7 wrote

Tritium is only used for making the hands and face of the watch glow in the dark, titanium would be used for something like the body of the watch.

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[deleted] t1_iyvjddl wrote

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zutnoq t1_iyvlecc wrote

20 kg is probably not as small an amount as you might think. The energy equivalent of that mass is around 500 TWh of which something on the order of half a percent is released in the reaction IIRC, so say something on the order of 2 TWh worth of energy of which you could extract say 1 TWh worth of electricity (if we assume 50% efficiency which might be optimistic but probably not orders of magnitude off). Global yearly electricity production/usage is currently around 22 TWh so scaling up the production of tritium to meet this demand seems more than feasible to me.

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NetworkLlama t1_iyvn7k3 wrote

I explained in another comment the details and challenges of production. In short, scaling up is difficult and requires either special rods that produce 1.2 grams per 10-foot rod per 500 days of irradiation in a light-water reactor or else heavy water reactors that can create up to 130 grams per 700 MW reactor per year. It's extremely inefficient either way.

Lithium blankets are being researched, but they're not yet proven.

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Real-Patriotism t1_izffbc6 wrote

I have Tritium in my watch that makes it glow in the dark. While probably a minuscule amount, why would this be done if there is such a low supply of Tritium?

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NetworkLlama t1_izfjlpq wrote

Because it's micrograms and because the current supply somewhat outweighs the demand. Most fusion concepts now under investigation rely on tritium but it's still years away from needing significant amounts. Once fusion power is a reality, tritium is going to be in enormous demand (ITER is expected to use most of the world's supply just in experiments) unless we have a way to generate it more freely, such as with lithium-6 blankets.

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ChipotleMayoFusion t1_iywaugd wrote

The DT reaction that most groups believe is the economical choice for fusion given our current technology level. It is not the easiest fusion reaction to do, I believe that would be muon catalyzed fusion. It is not the cleanest fusion reaction, that would probably be proton-boron which just makes three alpha particles and rarely makes a neutron. It's not the fusion reaction with the most available fuel, that would be pure hydrogen fusion like the sun does. DT seems to be the sweet spot when you combine all the relevant factors together.

The reactivity curve of DT is favorable compared to other similar options like DD and DHe3. It occurs at lower temperatures, which means it is easier to build a fusion reactor to reach those conditions. For most power plant schemes you want to reach ignition of the fuel, meaning the energy coming out of the fuel also heats the fuel more than the external heat source, like lighting a match. It is a lot easier to build a power plant with a fuel that is more like gasoline, where a little spark sets it ablaze, rather than a block of rubber, which requires significant heating to get it to burn.

The other big advantage of DT is that half the fuel is very abundant. Deuterium is relatively easy to find, you can buy it at Praxair in T-cylinders. You need to breed tritium in the fusion reactor itself, but the consumable is lithium, which is also pretty easy to find. Because it's a nuclear reactor each power plant is only burning maybe hundreds or thousands of kilos of lithium a year, which is nothing on the world scale. If we can design a plant that has a viable tritium breeding cycle, and there are many proposals that seem promising, this should be solvable. This is likely a lot easier than making a fusion reactor that achieves 10x higher temperatures, which is what you need to do a straight DD reaction.

Source: I work at one of those private fusion companies. Engineer, not a physicist.

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financial2k t1_izsz9rh wrote

Thanks.

wait. So the better the isolation i.e. energy confinement the lower the ignition temperature?

And you will never get any fusion reaction below a certain temperature, because Temperature is really the Bolzman distribution of particle mv2 and particles faster than X at temperature Y just don't exist due to quantum stuff Z?

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ChipotleMayoFusion t1_izt0f54 wrote

Yes, that's the Lawson Criterion. The product of density, temperature, and confinement time has to be above a threshold to reach ignition.

The second thing I don't know, thats outside my expertise.

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ProneMasturbationMan t1_iyweuvw wrote

Please define "cleanest" fusion reaction?

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joalheagney t1_iyyy54d wrote

Free neutrons penetrate shielding materials like crazy (because they are uncharged) and cause secondary nuclear reactions (because they pack a lot of mass and energy) when they are captured by something. Those reactions leave radioactive decay products. So your entire reactor becomes radioactive.

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financial2k t1_izszje8 wrote

I always wondered how the breeding works then. Don't the free neutrons go anywhere in a Tokamak and are most likely to be captured by the surrounding atoms with the highest coloumb surface?

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financial2k t1_iztmxre wrote

I've just seen this, which answered most of my questions on fusion: https://www.youtube.com/watch?v=BzK0ydOF0oU&t=19s

It shows the biggest problem for the D-T fusion is actually Beryllium. So despite all that progress and engineering wizardry there are still fundamental hurles that don't allow Tokamak D-T fusion to scale.

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