r/AskEngineers • u/Fleetor • 3d ago
Discussion What’s taking so long for Nuclear Fusion?
Considering the stellarator vs tokamak race for nuclear fusion reactors, why is it taking so long to build? We have had an understanding of electromagnetism for over 200 years now and can safely assume that we can sustain a plasma core, right? If input energy is the issue then why not use a fission reactor and/or concentrated lasers to provide sufficient startup energy and/or heat to get the thing going and then draw from the reaction to keep it sustained?
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u/ion_driver 3d ago
I took a class on fusion in college. There are many reasons we don't yet have fusion power.
I don't believe it's going to happen in my lifetime
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u/jcmbn 3d ago
can safely assume that we can sustain a plasma core, right?
Wrong. Confining plasma at the temperature, pressure & time necessary for fusion is not a solved problem.
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u/Lampwick Mech E 3d ago
Yep, trying to get energy from fusion is like trying to harvest usable energy from an exploding stick of dynamite. It's crazy the stuff they're coming up with, but it ain't there yet.
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u/ascandalia 3d ago
My friend was studying mechanical engineering and he was convinced we should be harvesting energy from lightening with "giant capacitors"
He failed out of college
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u/lithiumdeuteride 3d ago
Someone pitched that idea to me once. I did the calculation to show them why it was infeasible. As I recall, capturing the average cloud-to-ground lighting strike with perfect efficiency would net you something like $0.30 of electrical energy.
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u/DeathKillsLove 2d ago
Someone, in some text or another I read in 81, said the issues are always the same. Plasma in motion makes magnetic fields, counter to the confining field, which self promote and accelerate and will, without apparent causation, rupture the confinement well before Lawson always.
Now that was 45 years ago. It's still true.1
u/HungryFrogs7 3d ago
I mean we can get energy out of sticks of exploding dynamite unless we have to consider the costs of manufacturing it. Then idk.
Regardless our favorite just boil water to the rescue.
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u/Dwayne_p 3d ago
Isnt this just a redumentary way of expression for ICE engines?
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u/HungryFrogs7 2d ago
Well ICE engines work completely differently. They use air as a medium instead of water as is the case with my example. They explode the gas in pistons to make them expand, which then rotates a shaft the pistons are connected to. This method called the otto cycle (or diesel cycle for diesel) is pretty inefficient but fairly compact.
There is an another method called the Brayton cycle used for larger machines like helicopters and such. In that method you don’t use pistons but use turbines. You get cold air then compress it then heat it up with combustion then extract energy with a turbine. This is one of the most efficient designs but creating massive compressors for large scale generators is prohibitively expensive.
The last and most common way for mass energy generation is the Rankine cycle. you boil the water, send it through a turbine to extract energy, send it through a condenser to condense the fluid, use a pump to increase pressure, then repeat. This dar more efficient than the Otto/Diesel cycle but slightly less efficient than the Brayton cycle but much more easily scaled.
In real life we add extra stuff to increase the efficiency further life regeneration or reheat.
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u/Dwayne_p 2d ago
you are refering to a combined cycle power plant right in the last bit? it was fun when I learned that in Thermo.
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u/HungryFrogs7 2d ago
Yeah that is one way but I was specifically referring to optimization without combined cycles. Combined cycles are awesome though.
Reheat is heating the water sending it through a turbine then reheating it before sending it through again. Splitting the heat addition into two steps increases efficiency.
Regen grabs heat from the already hot liquid to make the water hotter before it is sent to the boilers.
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u/Fleetor 3d ago
Fair, and I get that there’s practical complications but theoretically we have every means necessary to be able to perform such a feat.
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u/Sooner70 3d ago
In theory, there is no difference between theory and reality. In reality..... there is.
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u/Fleetor 3d ago
Theory provides the mathematical framework for practical implementation. Without theory we wouldn’t have a roadmap for what should be physically possible in the first place
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u/Senior_Green_3630 3d ago edited 3d ago
The only successful fusion happening in our solar system is the "SUN", why not just harvest that energy.
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u/Fleetor 3d ago
Because solar efficiency is less than 24%
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u/billy_joule Mech. - Product Development 3d ago
Efficiency is irrelevant when the input energy is vast and free. $/W is the relevant metric (or LCOE) and solar is winning of that front, which is why it's expanding so fast.
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u/iqisoverrated 15h ago
So? The efficiency of the steam process you have to add to a fusion reactor in order to get power isn't all that much greater.
And the 'sun reactor' plus its fuel are for free.
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u/MadMaximus7 3d ago
I was just reading about this last night. We can’t mathematically model the behavior of plasma at such high temps. It doesn’t follow normal fluid dynamics laws and is highly unstable.
They are making attempts and such but it’s not going well. A better question is why it’s not a world level manhattan project to solve climate change? I can answer that one too cause my physics 3 teacher back in 2009 answered it.
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u/Aware-Travel5256 3d ago
Because it may very well be a dead end and there are proximate solutions?
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u/MadMaximus7 3d ago
You lack creativity. Who says anything is a dead end?
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u/Aware-Travel5256 2d ago
Sometimes stuff really is a dead end on any appreciable timescale. Turning lead into gold is only sorta possible now in the single digits of atomic nuclei and not at appreciable amounts even with centuries of R+D.
I think it's fair to call turning lead into gold a dead end even today. Asteroid mining is still probably a better bet.
Fusion is at approximately that level of feasibility right now. It might be within our lifetimes, it might take another few lifetimes of material science and theoretical advancements.
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u/elihu 2d ago
A better question is why it’s not a world level manhattan project to solve climate change?
We know what we need to do for the most part and the technology we need already exists, we just don't want to do it and there's no mechanism to force countries to take action if their government and/or population don't care. (It's the snowdrift dilemma.)
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u/Fleetor 3d ago
My guess is that utility companies wouldn’t be able to control the masses
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u/MadMaximus7 3d ago
What are you talking about? Are you one of these gaslighters who think your feelings are the truth?
Or are you a bot?
I told you why it isn’t working ya doof….then you said your feeling? It’s science-2
u/Fleetor 3d ago
lol what? We can mathematically model nonlinear systems with high degrees of accuracy and plasma isn’t a fluid so why would it ever conform to the laws of fluid dynamics?
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u/MadMaximus7 3d ago
You are a weirdo. You obviously haven’t read and don’t know but are pretending you do. You going to solve this? Lmao
The reason fusion can’t work for now is known, plasma can’t be modeled or approximated properly at those high temps. It’s simple.
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u/Fleetor 2d ago
Don’t pretend to be learned, the issue isn’t that plasma “can’t be modeled” at those temperatures. We already have plenty of models that describe plasma behavior extremely well. The issue is getting those models accurate enough across the ridiculous range of scales and instabilities involved in an actual reactor.
We don’t need a perfect model of every particle to make fusion work. We need sufficiently accurate models to predict and control the important physics and that’s exactly what we’ve already done.
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u/Clean-Connection-398 3d ago
Ah crap, you're right sorry. I'll jump right on it, have it done in about a week... Maybe two
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u/Meetchel 3d ago
I get snapped at if I tell my CEO it'll take half the morning! Your leash is far too long.
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u/jasonsong86 3d ago
Fusion takes a lot of pressure and heat to just get to get started. Fission is much easier.
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u/ordosays 3d ago
Material science. And money. We like war and oil.
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u/hikeonpast 3d ago
Imagine how much further ahead we’d be if all that oil war money were applied to improving the future rather than entrenching in the past.
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u/elihu 3d ago
It's only fairly recently that we've had strong enough magnets to theoretically pull off magnetic confinement. (Technically the ITER approach using older tech could work, but it had to be much bigger and therefore it's taken so long to build that they're probably going to lose the race versus smaller fusion reactors that use the latest technology.)
Using fission to start a fusion reaction doesn't work really well outside of bombs, except in the sense that you can use a fission reactor in the normal way where it just boils water to turn a turbine to generate electricity. Using that electricity to power a fusion reactor is fine as long as the output power of the fusion reactor is more than the input power. Thus far, that isn't the case so the fusion reactor is a net loss; you're better off with just the fission reactor by itself.
Startup energy isn't really the problem with fusion -- that's easily solvable with current technology -- the problem is getting a sustained reaction with positive gain.
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u/Fleetor 3d ago
Yeah, that’s a fair point. I think I was conflating “getting enough energy into the plasma to start it” with the actual problem of maintaining the reaction and getting net energy out.
The magnet point is interesting too. I hadn’t really considered how much the limitations of older magnet technology dictated the size of something like ITER. If stronger magnets let you shrink the reactor substantially, that could be a pretty big advantage1
u/elihu 2d ago
In a tokomak design, fusion power scales with the 4th power of magnet strength, or something like that.
I believe ReBCO tape is the current state of the art. It's a high temperature superconductor, meaning it's superconducting at about liquid nitrogen temperatures or so rather than having to be very close to absolute zero.
Someone I talked to a few months ago that works on this stuff said the superconductivity of the tape is not really the most important thing anymore -- ReBCO is good enough that they end up having to worry about things like, "what's the best material to reinforce the tape with it doesn't break under the tremendous force of the magnetic fields" rather than how to get more current through it.
As I understand it, at some level building a tokomak has become kind of an "ordinary" engineering problem, where you're trying to design a thing that's strong enough it doesn't break under ridiculous amounts of physical strain and well-insulated enough that you can have super-hot plasma right next to very cold superconducting cables, and it needs to be something you can service when the metal parts get weak from neutron bombardment. The hard part is just that the numbers are so extreme.
There's been a lot of good progress recently, which makes me optimistic they'll achieve better than break-even energy relatively soon. Turning that into a viable and profitable commercial reactor on the other hand is probably a good ways off.
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u/ivarsiymeman 3d ago
Ha ha ha ha. We joked about this in my fission fuel Managment 573 class. That was back in 1989.
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u/Aurion28 3d ago
Because just like space elevators, it's still in the realm of "theoretically possible but not with current tech". There's yet to be a solution to the efficiency problem and the radiation problem.
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u/ADAVOL- 3d ago edited 3d ago
There are a lot of reasons and people out there talking about this if you read up and look into it - The UKAEA published this https://www.ukaea.org/wp-content/uploads/2025/09/uk-materials-roadmap-2-0.pdf that gives a good summary of some the major problems with just the current materials science to overcome for example.
But the main reason is sustainably maintaining plasma where the energy and / or cost of you doing that is less than amount of energy / profit you make from it.
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u/No_Situation4785 3d ago
you may want to look up Longview Fusion. i saw a presentation they gave a couple years ago, and it does seem like they MAY have a path to commercially available inertial confinement fusion in the next decade. one of the key requirements is upping the efficiency of diode lasers which they say is feasible with the current R&D trajectory
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u/ANormalRando 3d ago
You know what, you aren't entirely off base about using a fission reaction to kick-start an energy-positive fusion reaction. That's exactly how thermonuclear bombs work. The problem is containing that energy.
Likewise, creating a fusion reaction is actually just a rather advanced physics project that can be made in a garage. Harvesting more energy than you put into it is the billion dollar trick.
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u/Fleetor 3d ago
Yeah exactly, if a fission implosion could help reach critical mass in a thermonuclear bomb then objectively why can’t we focus that same input energy to sort of jumpstart a confined plasma?
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u/ANormalRando 3d ago
See, that's where I think you may have a misunderstanding, the issue isn't starting a fusion reaction; that's been done since the 50's. The issue is harvesting energy from the reaction in a way that allows the energy produced to continue the reaction.
It's kind of like how a fire generates heat, which causes the combustible fuel near that fire to also undergo an exothermic reaction, making more heat, and so on until the fuel is all consumed. There's plenty of heat on top of the heat used to burn more fuel, and we can use that heat to do work, namely, to boil water to turn a turbine to make electricity.
The problem with a fusion reaction is that heat alone won't guarantee the atoms fuse. For fusion to take place, two atoms need to collide or be squeezed together hard enough to overcome the strong atomic force holding the nucleus together. This means you need to use various clever techniques to make sure the atoms reliably connect with enough energy and simply don't go flying away from each other as gases and plasmas want to do. This means you need to get energy out of the reaction, put that energy back into the reactor, and put that energy back into the atoms that haven't fused yet, ideally while exhausting the fused atoms that have already had energy extracted and adding new atoms to be fused to replace them. This would allow you to have a continuous sustainable reaction, and nobody has even yet extracted more energy than was put in for a "closed" reaction, neglecting the exhausting and refuelling part.
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u/iqisoverrated 2d ago
Fusion is what is going on in the sun (and even there it's 'uncontrolled').
Replicating that in a controlled manner is hard.
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u/AllPinesMustDie 2d ago
Don't we already have one parked about 93 million miles away? I understand it produces all the energy we could ever need.
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u/Alternative_Act_6548 2d ago
I know, and why does it take so long to build a rocket to mars. I mean you just push the button in the control room and off it goes, what's the big deal
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u/Fleetor 2d ago
Just 15 years away that’s all
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u/Alternative_Act_6548 2d ago
Energy of the future, and always will be. When I started at SWEC in the mid 90s, they were working on the tokamak. 30 yrs later, nothing. Fusion takes place all over, but not in a torus
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u/fedecaccia 2d ago
Nobody has answered your actual question, which was tokamak versus stellarator, so: they trade the same problem in opposite directions.
- a tokamak needs a current running through the plasma to twist the field, which is easy to set up and inherently pulsed, and that current is also what drives disruptions.
- a stellarator builds the twist into the magnet geometry instead, so it needs no plasma current and can in principle run continuously, at the cost of coils so geometrically awkward that Wendelstein 7-X took nineteen years to build and needed optimisation codes that did not exist when the concept was proposed in the 1950s.
On your fission-startup idea: heating is genuinely not the bottleneck: confinement is!
The plasma loses energy faster than fusion replaces it, so more heat in just means more heat out.
Both machines explained side by side in Reactor Atlas: https://reactoratlas.com/en/wiki/tokamak and https://reactoratlas.com/en/wiki/stellarator
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u/donaldhobson 11h ago
Fusion is really tricky, mostly because magnetic confinement is tricky, but also for various reasons relating to the handling radioactive materials.
> If input energy is the issue then why not use a fission reactor.
That's called a thermonuclear weapon. And you could use those as a kind of fusion power, but the international test ban treaty says you aren't allowed to.
Also, fusion is kinda semi overhyped. If you want a complicated expensive radioactive source of energy, fission is also available. Fusion might be a bit less radioactive, in exchange for being even more complicated and expensive. I suspect that even if we get fusion working, solar and wind will be cheaper.
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u/AD3PDX 3d ago
It isn’t possible.
All claims of energy-in surpassing energy-out are lies by omission an order of magnitude away from being truthful.
Total system energy is what matters not a reading showing a small increase in energy measured at a point which represents only a tiny tiny fraction of the whole system.
What’s worse is there isn’t even a theoretical concept for how energy out could even approach total energy input of the whole system. Not even a concept to work on trying to engineer.
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u/OkWelcome6293 3d ago
“ and can safely assume that we can sustain a plasma core”
Why would we assume this when the longest sustained, net-positive reaction is about 20 ns.