Nuclear fusion is perpetually 30 years away. Why? What is the problem that engineers have yet to crack? Is it that current reactor designs are too inefficient to recoup the energy used to create the reaction?
Thanks,
Rob
There are a number of problems, but the ‘trick’ I think is getting a sustained (and contained) reaction that produces more energy than it takes to initially start.
I think it’s perpetually 30 years out because folks in the past have been wildly optimistic as to the engineering challenges and our ability to leap over them.
-XT
Containment. Getting something hot and dense enough to fuse is fairly easy and we’ve been doing it since the 1950’s in H-Bombs and the like. Getting something hot enough and then keeping it in some sort of sustainable container so we can use it for something other then blowing people up is much harder.
How do you cause fusion? By squeezing hydrogen really really really hard. The hydrogen fuses into helium, and releases lots and lots of energy. That energy causes the stuff you’re fusing to expand lots and lots. In the Sun, gravity does this trick. In a fusion reactor, you use magnets, lots and lots of magnets. But as you release energy you’ve got to squeeze harder and harder to keep that ball of fusing nuclei together, or else the fusion stops.
If you just want a big boom, you can easily cause fusion by setting off a fission bomb, the tremendous pressure from the fission explosion will cause fusion, then the fusion reaction will blow everything to hell, but you don’t have to contain it because you want that big explosion. Trying to contain that fusion explosion with a bunch of magnets would be pretty difficult.
And of course, you’ve got to use magnets, because once you get to the heat and pressure neccesary to cause fusion, any physical containment vessel would be long since melted. You’re talking about temperatures as hot as the Sun here. But of course, really strong magnets require a lot of energy to run. Of course, you can run the magnets with the energy you get from the fusion reactor, problem solved. Except if the energy you get out is less than the energy you need to power the magnets, your problem is back. And this is where we are currently, with the added problem that even our current nearly-break-even reactors cost incredible amounts of money and so the cost of generating electricity this way would be really really high, even if we had a reactor that broke the break-even barrier. So a working break-even fusion reactor isn’t a panacea unless you can build one cheap enough to get within the ballpark of the cost of conventional energy.
This is the basic point. Any element below roughly silicon will undergo nuclear fusion given the right temperature and pressure – but except for the isotopes of hydrogen and helium-3, those temperatures and pressures are unattainable on earth except in the middle of an atomic explosion.
There have been reactors fusing deuterium and tritium into helium-4 since I was young. But it takes a great deal of energy to set up the necessary conditions – energy that has to be pumped into the reaction before any fusion occurs. It’s only a few years ago that we got the first mathematical break-even point reaction – one in which as much power was obtained from the reaction as was pumped into it. And this for only a relatively short time frame…
“Technological” break-even – the point at which a reliable supply of energy equalling or exceeding that pumped in is obtained on a sustainable scale – remains a future hope. Though it would be nice if fusion research was not the unwanted redheaded stepchild of a Department of Energy which seems fixed on promoting the oil and gas industry at all costs.
(Indeed. Congress’s last omnibus appropriations bill killed a lot of basic physical sciences, including the US’s involvement in the ITER project.)
Another problem you face is neutron flux. While fusion doesn’t directly make radioisotopes, it makes boatloads of free neutons that themselves can irradiate components and people (damaging both).
So containment (and managing neutron flux) is the problem. How do things change when you get away from tokamak designs and use laser containment a la the National Ignition Facility? Or is that just an expensive experiment to see if laser containment works? Or did the powers that be kill that too?
Thanks,
Rob
The above posts are talking about “magnetic confinement” fusion reactors. There’s another type of machine which doesn’t have all of those problems -Inertial Confinment fusion
This machine uses a battery of lasers to subject a tiny fuel pellet to a blast of light, which causes it to get crushed to the point of ignition. There have been machines that have gotten very close (or maybe exceeded) break-even.
[slightly OT hijack]
Are new nuclear reactors being built in the US?
[/hijack]
The University of Rochester has been running a Laser Fusion Lab since the 1970’s. They’ve also been on the edge of containing a sustained reaction for 30 years without getting there.
There are no magic solutions. They may be no solutions with our technology.
I would’ve said iron rather than silicon. Is there some reason the elements between silicon and iron won’t fuse, even given the right temperature and pressure?
They might fuse, but the resulting nucleus is unstable. Also, the energy curve becomes negative for heavier elements (it requires more energy to fuse them than is released when they do).
The physics problems with tokamaks are pretty well understood, the problems now are primarily to do with the engineering and developing suitable materials. The general rule is that the bigger the tokamak plasma the less energy – relatively – it takes to confine it. That’s why the new world machine, ITER, will be about twice the size in all dimensions than the biggest existing machine, JET. ITER is designed to reach the break even point – more fusion energy given out than energy put in to heat and confine the plasma but it’s not designed to produce electricity. It is a test bed for developing the technologies needed for a power plant.
One problem is that tokamaks are pulsed devices – they are essentially big transformers – and to get really long pulses – tens of minutes – they need superconducting magnets. Not easy technology.
The new materials are needed to survive the neutron flux without themselves becoming highly radioactive. Lots of ideas but doing things on an industrial scale is again not easy.
When you get down to it the basic problem is that there has not been enough money put into practical research – while oil prices were low and no-one was worrying about green house gas emissions – there was no real incentive. What money there was went into the basic science – essential of course – but not a lot into solving the engineering problems and working out how to build an economic power station.
To an extent the world has now woken up and ITER is supported by China and India who are definitely looking to the future 30, 40, 50 years ahead.
Inertial Confinement is another line of research but it has it’s own engineering problems and it is still some way behind magnetic confinement. The lasers are massive and extracting the energy will be even harder than with a tokamak.
None in the past 20 years or so. But lots of new applications are in the works.
Silly children!
We already have Fusion,
Just not useful Fusion!
Google for “Farnsworth Fusor” sometime, nu? 
They will, but they won’t produce more energy then it takes to bind them togeather, so while you can have fusion of heavier materials, you can’t have a fusion chain reaction (so no reactors or bombs), where the energy released by one fusion event fuels a similar reaction in neighboring nuclei.
I’m not particularly sure it’s all worthwhile.
Any reactor is nothing more than just another version of a steam engine. It isn’t some magical “we fuse hydrogen and it gives off electricity”. No, current nuclear reactors do nothing but create a large amount of heat (just a fancy “fire”), which then turns water into steam, which turns a turbine to generate electricity.
So what are we asking this incredibly difficult sustained hydrogen reaction to do? Exactly the same thing. The only thing this fancy controlled explosion is doing is boiling water.
Billions and billions of dollars to find a fancier way to boil water. :rolleyes:
Well fancier in that it doesn’t produce greenhouse gas or long-lived radioactive waste (and I think it can breed its own fuel from the high neutron flux as well). So if it could ever be go to work, it would at least arguably be better then our current method of boiling water.
I hope I am not being wooshed…
All the current ways of boiling water are either infeasible on a large scale (hydro, wind, solar), too expensive (wind, solar, nuclear fission [sort of]), soon to be too expensive (oil, gas, coal), or bad for the environment (hydro [sort of], oil, gas, coal, nuclear fission [sort of]).
No one knows whether nuclear fusion will be an improvement on all these, but it is certainly worth looking for.
And by the way - there are some fusion reactor designs that produce electricity directly, without the intermediate step of boiling water.
Up to and including silicon, fusion of two atoms of the same element is exothermic. Silicon fuses to produce iron (and nickel). The elements between silicon and iron will fuse with lighter elements in exothermic reactions, but fusion with themselves is endothermic, as of course is any fusion reaction involving iron or anything heavier. So there’s pretty much a theoretical limit at silicon – even though you can produce on paper the results of nice pretty silicon+helium exothermic fusion, there’s no way to keep the actual substances in any feasible physical quantities to exothermic reactions. (Plus, as noted, pretty much everything beyond helium requires temperatures and pressures to fuse that are beyond our present capacities to produce at controlled, sustained levels.) But the basic reactions involving hydrogen and helium are also the greatest energy producers.