Nuclear fusion discussion
I'm a big fan of nuclear fusion as a concept and hope to shift toward doing active research in the field at some point.
I'd like to open this discussion to talk about topics regarding nuclear fusion as a future energy source. To start, I'll lost a couple of ongoing fusion efforts I'm familiar with.
ITER:Of course the biggest fusion project is ITER, the massive multinational collaboration which is building a massive tokamak reactor in France. Unfortunately ITER will never produce power for average people, as it's purely a test reactor with no plans to be connected to the grid. The following effort to build a functional grid connected reactor, DEMO, isn't set to be built until at least 2050. This has resulted in a considerable number of private ventures trying iut experimental alternative approaches.
HELION:At the time of writing this, there's quite a bit of buzz surrounding Helion energy, both because of the ambitious timeline theyve recently proposed as well as the investment of Sam Altman of OpenAI fame. Helion uses an FRC topology, which I personally think is a really cool idea. Basically it's a tokamak without the physical shell around it, and is kept sustained by the internal plasma physics. Helion also has another interesting quirk, they are not pursuing the typical DT fuel strategy, but are instead planning to use DD fusion to breed He3 and use DHe3 fusion as the primary energy source. I think this is a good idea because DHe3 fusion is "aneutronic", whereas DT fusion produces high energy neutrons that are somewhat of an unsolved problem to deal with. I wonder though, how they intend to deal with the inevitable tritium pollution that DD fusion creates, and how they will separate that out before Iit creates neutrons anyway.
TRIALPHA:In addition, another major company TriAlpha Energy, also pursued FRCs, hoping to use an alternative proton-boron11 mix to achieve aneutronic operation. I think they've sort of pivoted toward being more a neutron source than working toward breakeven.
HB11: A recent proposed approach is HB11, which is also going for proton-boron fusion. Now with Tri Alpha this seemed really dubious, because hydrogen boron has a much lower cross section for fusion than other options, even the DHe3 that Helion is doing. In addition, boron has way more electrons than hydrogen, so a proton boron plasma has more electrons with causes more bremsstrahlung loss. HB11, however, thinks they can overcome this through high energy laser acceleration. They want to use a high power laser to shoot a fuel pellet into a target. This supposedly will work much better than heating the stuff, because the laser will impart a specific impulse and thus the thermal spectrum of the impact will have a much higher Q factor centered around the cross sectional peak. I'm not really convinced on this, just because I feel like that thermal spectrum would only last for the first few atomic layers of impact before it doesn't really matter amymore.
CFS: The next option I would consider to be one if the most popular fusion startups is Commonwealth Fusion Systems. They have what I'd consider the most conservative approach, they are attempting to build a Tokamak design like ITER, but hope to reduce the size considerably by taking advantage of advances in superconductor technology with REBCO tapes.
W7X:The next reactor type I'll mention was in the news a lot a few years back, the Wendelstein-7X in Germany. This is a stellarator design, the crazy twisted car wreck of a thing you may have seen before. The stellarator is shaped that way so that it doesn't require an induced current like the tokamak to have magnetic helicity, because the shape does that automatically.
ZAP:Another well liked dark horse is Zap Energy. They're not as flashy as the other reactors but seem to be working off solid physics that have been proven out over many years. They're trying to do sheared flow z-pinches, which is basically creating a lightning bolt that's perfectly straightened out and super dense.
DPF:One more somewhat obscure option is Eric Lerner's Dense Plasma Focus approach. I'm a little puzzled by this option because it seems to be the exact opposite of Zap, where they make an incredibly twisty lightning bolt instead of a straight one.
FUSOR/POLYWELL:There are a couple reactor types that get mentioned often but are more or less obsolete are the Fusor and the Polywell. A Fusor is a neat device that can be built to fit on a desktop and still produce actual fusion reactions, but has a fundamental design flaw of a physical electrode inside the plasma that introduces too much conductive heat loss. The Polywell is a more advanced concept thay tries to create a "virtual" cathode with orthogonal magnetic mirrors, but I think after many years of experimentation researchers were unable to validate the formation of such a virtual cathode.
NIF:One option that is sort of tangential is the NIF, which you might have heard technically produced more energy than it produced. I dont think its necessarily going to go anywhere, mostly because it's more a weapons program than an energy program, but I think the chirped pulse amplification technology they use is really cool.
GENERAL-FUSION:And finally I'd be remiss if I didn't mention the very highly funded and publicized General Fusion. I definitely give them points for pure childlike wonder. The original pitch was they were going to take a giant swirling tornado of molten metal, fire a ball of plasma into the eye of the storm, then smash the whole thing from all sides with a hundred giant hammers. To be honest is such a wild concept that I don't really know if it really makes any sense or if it's a fever dream. It's undergone a few revisions after finding out that certain parts of its concept just weren't going to work. This doesn't inspire a ton of confidence, but also shows flexibility in their thinking.
There's definitely lots of other companies with other variations, but this gives a general idea of the huge range of ideas and approaches being pursued. I think it's a really cool field to explore and I'd love to hear all your thoughts about it.
It might be viable in the long-term, but fusion will come too late to address climate change. Even if we tackle the technical problems (of which there are many), that doesn't make fusion scalable or financially viable. It took PV/wind 5+ decades to sort out the financial side.
But more broadly, fusion seems to be frequently misrepresented by its advocates - for instance, "net gain" seems to opportunistically switch between meaning "energy in vs energy out" and "electricity in vs electricity out".
All in all, fusion is a great for physics research - and I'm happy to drain other peoples' VC funds dry to make that research happen - but I'm seriously concerned whenever I see anyone genuinely buying into the hype and thinking fusion is remotely useful for climate change, because then they're less willing to support the 'imperfect' solutions that are actually viable.
This is, unfortunately, completely correct. I'm a big proponent of fusion research but in the mean time, I personally believe we'll need fission reactors in the mean time. We shouldn't completely count on fusion research either, as we don't yet know up until what point we can switch our energy sources from there, and how much.
We may have been able to already have successful fusion reactors now if we maintained the funding, interest and energy (which all declined over a couple of decades in the past), but that ship has sailed already.
I agree with this statement, but unfortunately that doesn't seem to be happening in the U.S. at least. We are instead investing most heavily in wind and solar, with other renewables which all have peak and off peak generation issues. But we're also investing in natural gas. I believe one of the most common ways that gas is sold is that it can be "modernized" by switching to a clean fuel like hydrogen for off peak power production, but that hand-waves the current hurdles of generating, storing and transporting hydrogen in a carbon neutral way.
Basically every single 'renewable' power generation that I am aware of has unsolved problems to deal with that prevent them from fully replacing fossil fuels in the current state. The plan to deal with those issue, as far as I am aware, is to wait for some new technology to be invented to solve the problem. I find it very frustrating because investment in fission reactors today is a viable solution to all of those issues.
tl;dr we've had the solution to climate change for 70 years but we're still too scared of it to use it. The nuclear waste issue of mass scale fission is 10x less than coal, and we accepted the solution for coal of literally just releasing the waste into the air.
I totally agree that I don't think fusion will be the fix for climate change.
But I disagree about the financial side. Every form of energy we use is subsidized so every form of energy is already nonviable, because abstract wind energy is more expensive than real life subsidized wind energy. Which is to say, anything becomes affordable, if a society collectively decides it needs to be. At least when your talking about things like food, water, electricity, etc.
...except it isn't. If there's an order-of-magnitude issue or two in the fundamental engineering then fusion is just dead economically, because subsidies are only as powerful as the government paying for them. Governments that do sufficiently-stupid things can and do drive themselves bankrupt.
More importantly, this doesn't actually justify focusing on fusion - if we wanted reliable renewable electricity everywhere and the costs be damned, we have that right now: it's called geothermal, and the limiting cost is boring holes down to the hot stuff. There's no actual technical blocker for geothermal, unlike fusion.
I agree, with some caveats.
I think renewables are a slightly complex topic, in that there's a big difference between e.g. "80% renewables" and "100% renewables" - a completely renewable grid does need some sort of firming, so the more renewables you build the less sense it makes to remove the last coal/gas plants.
However, this is basically irrelevant for the present - anywhere with less than 50% renewables has basically none of this problem, and when it comes to tackling climate change reducing emissions by 10% now is far more important than getting rid of the last 10% in 20 years. Our deadline isn't a measure of time, it's a measure of emissions - if we dropped global emissions by half then we'd have roughly twice as long.
Nuclear has some places where it's a good choice (like Korea - ideally they'd use solar but they have no space domestically, and relying on Chinese solar farms causes geopolitical problems), and nuclear has a lot of benefits as a last 10%, but it's not a panacea and there are some places where it's not remotely viable (like Australia; it'd take 20+ years to get one operational which puts it past the climate deadline).
My main concern with nuclear is just that it's too easy to sink politically - fossil fuel corps can say "make nuclear, not renewables!", then 15 years into the nuclear project they can say "OH NOEZ nuclear is too dangerous, cancel it now!" to protect their coal/gas plants. And then we have neither renewables nor nuclear, and we're fucked climate-wise. There are other issues, but I'm not convinced that nuclear is worse than coal -
renewables > nuclear > coal.Practically speaking, there are some financial issues with nuclear (high capex, long build times, inherent government subsidies, "inefficiencies" from maintaining safety overhead), sure. But that's only an issue if nuclear distracts from renewables - I don't particularly oppose wave energy even though I think it'll be useless/infeasible.
There's no actual technical blocker on geothermal, and yet we're still not really doing it.
Even though we ARE doing it. But not doing it enough, apparently.
We could be building more nuclear fission plants, we could be investigating Enhanced Geothermal Systems, we could be looking into geoengineering, we could be doing all sorts of things that we aren't focusing on enough now.
People more influential than me have already decided what the "correct" things to focus on are. Which means there's no need for me to spend any time thinking about it.
I just like fusion research, and I don't really care that much about defending its merit. Anyone who isn't interested is totally in their right to ignore me.
That's because it's 1) not the cheapest option ever, and 2) solves a problem (firming renewables) that we don't actually need yet.
If we really wanted to solve climate change we'd price the externality - put in a carbon tax of $100/ton (and if you're squeamish, phase that in over a decade by upping it $10/ton each year), and watch the market sort itself out. But there's no political will for the easy solution, yet alone the hard and risky.
That's fair.
This is a good point. Climate change is far too pressing a matter to wait for fusion to come along and replace fossil fuels. I think fusion will become viable in our lifetimes, but the time to address climate change was decades ago and ongoing today.
Which wouldn't have been nearly as much of a problem had we not alienated fusion technology's older brother.
But now I suppose we have to deal with the cards we've been dealt. And fusion certainly won't be in the deck for a good while.
It's an exciting time for fusion science! As a nuclear scientist, I do feel it necessary to say that it's a very exciting time, but all of these potential approaches are just that - potential. We just breached the Lawson Criterion, and in a design that is useful for many things about fusion, but that is not a practical energy source. Certainly if we are talking about solving the greenhouse gas crisis, fusion is not the principal approach needed. I sincerely hope it works - both for it's elegance and efficiency - and it is looking better than ever...Just don't forget that there are other green energy sources that are vitally important too!
I'm of the opinion that fusion probably won't have a big impact on climate change. Either we will commit to making the necessary changes with the options we have now, or we'll just learn to accept the consequences of a warming planet as just the new normal.
What I think fusion is going to be useful for is what we decide to do next as a global civilization.
To be clear, I totally agree.
I'm not certain if, with our current understanding of physics, we will be able to build a practical net-positive fusion energy source. It releases massive amounts of energy, but requires massive amounts of energy too. That said, if we can make it work, it changes absolutely everything about our world, so it is 100% worth the attempt.
I'm not so sure of that - even if we built a fusion reactor which generates infinite heat for free, you'll still need to plonk a steam turbine on the end, which is still somewhat costly and requires maintenance - and steam turbines are not getting any cheaper.
But apart from that, Fusion is limited by distance just like any other sort of power - moving electricity isn't free, and even if the fusion plant doesn't care about transmission efficiency, the electrical wiring companies will, because it'll require lots more wire. So the best power plant doesn't just have low per-watt costs, it also has low costs to build the plant.
Photovoltaics might have more long-term cost potential than fusion, since they're dirt-cheap to build a single generating site with, and because they don't have moving parts that wear themselves out so easily.
To be clear: If we can net positive energy output by any meaningful amount (not a few percent net positive, but much more), it will absolutely change everything. PV and other solar technologies are intermittent, while nuclear is a firm power supply. Where PV is limited by space (and inherent inefficiencies due to physics and chemistry), nuclear offers a massive power output on a very small footprint, both literally and from a waste perspective. Even fission produces relatively little waste per unit of power (albeit quite important waste to watch and store). When you look at all the problems and costs associated with various power sources, nuclear is very appealing for several reasons, and absolutely must be a part of the green future energy economy. It is no panacea, and I absolutely support massive PV and wind development, but it is a vital "always on baseline power" tool for the future.
Thank you for posting this, reading it makes me realize how absolutely limited and archaic my knowledge of fusion energy is, is there a primer you'd recommend to get the basics so I can begin to approach what you wrote?
I dont have a good one really, most of my knowledge comes from disparate sources.
If you want to learn some of the fundamental plasma physics involved, I think the book Introduction to Plasma Physics and Controlled Fusion by Chen. As someone who kinda struggles with mathematical formalism I think it's pretty good at giving intuitive explanations for things along with the math.
If you want more practical info, the book Principles of Fusion Energy, An Introduction to Fusion Energy for Students of Engineering and Science is really good for learning about fusion technologies like the tokamak and stellarator.
A lot of the companies I mentioned are doing somewhat cutting edge work so the best real info would just be from whatever they release in their own. I got most of my information on them from hanging around the r/fusion subreddit.
Heavy ion fusion never gets talked about
https://en.wikipedia.org/wiki/Heavy_ion_fusion
but if you were serious about inertial confinement fusion as an energy source instead of a way to test hydrogen bombs without a fission stage it is the obvious way forward.
Lasers are terribly inefficient and leave a huge amount of heat in the laser glass which takes a lot of time to remove. Sure you can make one shot a day but a power plant needs at least one shot a second if not many times more than that.
The problem is that heavy ion ignition requires a big machine, probably 100 or so beamlines about a kilometer long. Of course a commercial scale laser fusion power plant is a big machine too but you can do ignition experiments with a smaller laser.
If some billionaire wanted to spend $40B on something crazy they could build a prototype heavy ion power plant and figure they’d ned to rework it a few times to get it running; it reminds me of the attitude of the Douneray fast breeder project in the UK. (Let’s hope they can avoid the mistake of throwing random waste into an otherwise forgotten shaft.) It is no so clear you could make it work with lasers.
I've been teying to read up on laser wakefield accelerators recently with the idea that it might be useful for this kind of thing in the future.
I always thought wakefield acceleration was about energy not luminosity, but I could be wrong.
The kilometer long requirement for conventional accelerators comes from needing a few GeV so that energy is transferred efficiently into the outer layers of the pellet. The "100 beamlines" requirement comes from the luminosity requirement, a beam with too many particles in it is going to spread apart from the space charge.
Maybe if you can accelerate the beam more quickly there is less time for space charge to blow up the beam but if you are using lasers to accelerate particles you brought laser inefficiency back into the system.
D + T has the problem of creating many high energy neutrons that are hard to deal with, practically these might be multiplied with Be and then used to breed more T from Li.
Another path is to irradiate U238 or Th232 with hard neutrons from fusion, the U238 will fission directly at a high rate, otherwise you get Pu239 or U233 which burn in a thermal spectrum. You avoid some fast reactor problems but add fission problems to your fusion problems — and lose the benefits of fast reactors burning up all the actinides and having no long term waste problems. (e.g. your fission products are mainly gone in 1000 years. If you can perfect Cl molten salt reactors you might even burn up the worst fission products and have waste that decays in just 30 years…. See https://www.moltexenergy.com/)
DT neutrons are indeed a big issue, which is why alternative fuels are somewhat attractive.
I could see a potential setup where fission based power plants are used as breeders to create tritium, and that tritium is allowed to decay to He3 which is used for DHe3 fusion. If you set the temperature target higher up there's a peak where DHe3's cross section is higher than DD, so with an even fuel mix you'll get a lot of DHe3 burn and not much DD burn.
I was reading the other day that it was expected that liquid metal fast breeders, if they are ever commercialized, are expected to have tritium get into the coolant which is not a problem because the coolant doesn't have any hydrogen in it and it is not hard to extract the tritium.
Before 1990 or so the expectation was that people wanted to roll breeders out rapidly and people were concerned that we couldn't make plutonium fast enough; interest in fast reactors mostly collapsed at that point but what interest has remained has focused around the ability of fast reactors to completely consume plutonium and other actinides. Even today people still do some optimization analyses of systems that combine fast and thermal reactors because if you do think throughput matters, the large critical mass for a fast assembly is an economic problem.
Since the late 2000's there has been interest in molten salt reactors based on the U233 cycle where the issue of launching the thing is difficult, that is, there is not much U233 in the world. You can trade Pu239 or U235 for U233 but neither of those is terribly attractive for numerous reasons (Pu doesn't dissolve in F salts, proliferationphobia means mixing a lot of U238 with your U235)
The D+T reactor that breeds U233 could be attractive but that, of course, reveals that D+T reactors are a proliferation problem insofar as they are a powerful neutron source even if they don't inherently require U enrichment or Pu fuel processing.
ITER will probably be the last project on this list to actually fire. It's just too damned big and already obsolete.
I though you'd failed to mention MIT's ARC and SPARC, but I guess CFS is where that's at now. Been a few years since I checked in. The interface (blanket) is the most critical structure, even over the geometry of the field or performance of the superconductor, and theirs is the only design that really solves it. Everything else will burn out in mere months of operation.
Yeah, there are a couple companies doing tokamaks, but I feel like CFS is just the one that seemed to be the most advanced over the years.
I believe Zap is also doing a similar liquid metal breeding blanket, the most recent render of their design concept now includes that.
But it is something I saw Helion get criticized for. That the reactor, as it's been shown in videos, doesn't look like it's built to last.
I agree with most of your post's takes especially about the ITER and NIF essentially being only tangentially (if at all) applicable to civilian energy.
In regard to pB fusion processes and other processes which use exotic fuels (e.g. not deuterium-tritium based fusion) all must be done in non-equilibrium non-thermal processes. The Bremsstrahlung losses are too high to ever actively obtain excess heating (and thus power) out of the fusion process. As an aside, if you look at the literature on the topic, the yields from pB laser fusion experiments have been steadily growing as people figure out the nuances of getting the accelerated proton cloud just right.
A couple fusion topics not mentioned:
There's an effort in Hungary (NAPLife) to use gold nanorods embedded in material to induce fusion via amplification from the nanorod EM response within the target. It's tantalizing, but still ongoing work. I'm biased since some of the folks there are acquittances.
Muonic catalytic fusion. I don't think there's any commercial efforts in this direction right now, but if a reliable source of muons can be engineered, then muonic-molecules vastly reduce the threshold to allow "cold" fusion (not the crackpot stuff) to occur a low temperature. There's been a little bit of discussion of building a muon collider in the particle physics community, so muon-catalyzed fusion technology might benefit from the engineering if that project ever gets off the ground.
Cool, I hadn't heard of NAPLife.
Muon fusion is pretty cool as an application of non-electron leptons. I would think in order to be useful you'd need naturally occurring muons. If you just made them kn real time, then you're basically just shifting the energy usage over to a different step. I think muons do get created in space through cosmic radiation, so it might be an interesting option in space based power.
I just feel it's too soon for venture capitalist projects in fusion. The timeline for return feels to great at this time and I feel many companies are just feeding off the buzz taking in money while ignoring the practicality of fusion. D-T fusion is still the most attainable reaction and I don't see how, for example, D-D for He3 and then D-He3 becomes viable at this time with regard to sustained operating temps, the spatial energy density of the plasma, and cost. The same goes for p-B11 for many of the same reasons.
With that said, as a research field it's incredible. I look forward to the day we solve some of these issues, I just don't know when we will solve them (something something in 30 years).
What I don’t get about Helion is that D + D can proceed by two branches: one produces T, the other He3. In principle you could extract those but D + T and D + He both go faster than D + D so you might not get much of the desired products out of the breeder. No matter how you slice it, you have to deal with T and the neutrons that come with it. Assuming your breeder is really producing enough product you can either use the T in D + T reactors or you can store the T and wait for it to decay into He3.
D + D is particularly interesting for an civilization existing outside the “frost line” in the sense that outer solar system and interstellar objects have a lot of water by mass not to mention carbon (Pluto has huge amounts of frozen carbon monoxide.). You could in principle take Pluto apart and make lots of large O’Neil colonies or little Ringworlds, though digging deep enough to find rock might be hard. Charon has stones and metals so it maybe you could bring the stuff together but you want to avoid reaction propulsion as much as possible and instead conserve as much material as possible.
A,D + D fusion economy could let people like that live independent of stars particularly if “rouge planets” like Pluto or like Jupiter with moons are common. That kind of civilization could make it to the next star but might be entirely indifferent to dry inner system worlds like Mars and the Earth and would be unlikely to have a working space shuttle to explore a world like Earth where getting off the planet needs a much more performant rocket than landing on the many tens of outsert solar system objects accessible with something like the SpaceX starship upper stage.
I'm totally with you, I like Helions approach a lot but I don't think I've seen much explanation on how they expect to extract tritium from the reactor.
Tritium is actually kind of a problem for Helion. They're banking on DHe3 to avoid needing a ton of shielding for the neutrons, but I don't see any mechanism to get the tritium out before it fuses.
Before these new companies I had assumed it would be the opposite. That we'd use DD specifically to breed tritium and immediately use it, amd the helium would just be a nice byproduct for stuff like cryogenic. Which makes a bit more sense because DT has the highest cross section, so if you put your target there, then whatever tritium you get will burn up pretty quickly, but the helium will have a smaller cross section so it'll stick around longer.
Targeting DHe3 though will result in both DT and DHe3 burn, which requires you to be have power extraction schemes for both ions and neutrons. Or, maybe it doesn't require it but then your just getting all the bad parts of DT without any of the energy.
For the speculative interstellar scenario note that He3 is a permanent fuel whereas T isn't. That is, if interstellar travelers come across some plutoid object they could build a really huge D-D machine that possibly fuels D-T machines for local use. You can't store T for a 100 or 1000 year voyage but you can store He3. Similarly the weight of a fusion machine for a spacecraft would be more constrained than for a breeder machine.
One factor in the development of commercial fusion reactors that often gets overlooked are the materials behind it. Having worked with insertion devices and superconducting magnets in tangential physics fields, the pain inflicted by quenching is still very much alive.
Some things that I'd like to see discussed are magnet fatigue in pulsed tokamaks and high temperature superconductors, which I believe are an important part of reliable, economically efficient reactors. Maybe some learning could be taken from CERN with their approaches following the most recent dark period. But one "unique" effect in pulsed tokamaks is going to be the effect of radiation on superconductors, which is addressed in some papers.
Furthermore, plasma-facing materials is a topic yet to be refined to a high degree. Currently, if I recall correctly, ITER employs tungsten, which although appropriate due to a high melting point, still suffers from fuzz and having it's lattice structure compromised by helium atoms.
In short, I do believe that fusion can be a fair option in a few decades, but for the moment being, I myself hope fourth generation fusion reactors can pick up the slack. I haven't read much on fast neutron reactors, which I believe would offer a myriad of benefits over common thermal neutron reactors due to more efficiently making use of "wasted" energetic potential, but the challenges that sort of reaction are still not entirely clear to me.
One thing I'm sort of torn on is neutronic vs aneutronic fusion.
As you say, the neutrons that come out of DT fusion are MeV energy, and it's a huge problem dealing with embrittlement that I dont think we've found a solution for yet.
But on the other hand there's a lower cross section and the problem of how to separate out your energized helium if it's ionic and trapped by the magnetic fields. There's some ways to do it, but they're more experimental and I don't know to what extent they've been proven out.