|Style over substance|
Fusion woo refers to dreams of extracting energy from nuclear fusion in a simple, cheap, and safe way. While fusion energy might one day be available, all the signs are that it will require large, complex, and expensive machines, will require the use of radioactive tritium as fuel, and will produce at least some radioactive waste.
Such wishful thinking is popular because, given that fusion fuel resources range from large to effectively limitless, such a device would solve all of our energy headaches. If successful, such schemes would also provide very satisfying opportunities to say I-told-you-so to the scientific establishment. Fusion woo has been fueled by two developments. First, there are in fact several ways to produce nuclear fusion in relatively simple devices, even in a hobbyist's garage. These include the Farnsworth fusor and pyroelectric fusion. Unfortunately, the laws of physics ensure that these devices will always consume more energy than they can produce, although they can still be useful as a neutron source. Second, the widely publicized claims of cold fusion raised hopes of producing nuclear energy in small household devices, free from the disadvantages of conventional, large scale nuclear power plants.
It's not easy to ascertain the goals of fusion woo proponents. Some of them might just be fraudsters out to acquire some venture capital funding and then disappear. Some of them may genuinely believe that their approaches will work.
The mother of all fusion woo is cold fusion, which is so far out in left field that we have to refer you to our main article on cold fusion. This article will only cover approaches utilizing energetic particles.
The "safe" part: aneutronic fusion
Of all the possible fusion reactions, the one with the largest cross section, which also releases the most energy per reaction, is
- D + T → α + n
The alpha particle (α) can be thought of as your friendly neighborhood helium, a.k.a. 4He. The "n" stands for neutron. D stands for deuterium, which is doubly-heavy hydrogen. There's a lot of that around and it behaves itself. T stands for tritium, triply-heavy hydrogen, and trouble. It is radioactive, with a half-life of 12.3 years, so it does not occur naturally in any significant quantity. It has to be "bred" from lithium using neutrons produced by the D-T fusion reaction. This will probably work, but it complicates the reactor, and the mathematics of breeding is borderline. The radioactivity also makes tritium a health hazard, especially since hydrogen is volatile and biologically active. The neutrons (n) produced are also a problem. First, they require about 1 meter of shielding to protect the magnets. Second, they get absorbed by the structure of the reactor and make it radioactive. But, with the proper choice of structural material, the radioactivity can be minimized such that it is safe to recycle in about 100 years.
Those are all very good reasons to give serious consideration to alternative fusion reactions, of which there are several. As a rule, if you want to reduce your radiation headaches, the cost is even bigger headaches making your reactor work at all. There are three reasons for this.
- The required temperature is higher.
- The fusion power relative to power loss channels, especially Bremsstrahlung radiation, is lower, so you can tolerate less inefficiency.
- The energy density is lower, making the capital cost per MW higher.
For advocates of fusion woo, it makes little difference whether they ignore science and engineering constraints marginally or in a big way, so they usually go straight for the reaction which would produce the fewest neutrons:
- p + 11B → 3 4He
p is a proton (garden variety hydrogen). 11B is the most common isotope of boron, which is itself a reasonably common element. Note that none of the reactants or products are radioactive or rare. In particular, no neutrons are produced, so you could put a reactor into your basement and take it to the dump when you're done with it, right? Well, first you have to make sure that not too much deuterium or 10B gets into your fuel since they will also fuse and produce neutrons. But even with isotopically pure fuel there are several nasty side reactions, including these:
- 11B + α → 14N + n
- 11B + p → 11C + n
- 11B + p → 12C + γ
The first two produce neutrons (albeit low-energy) and the third produces hard gamma rays. The upshot of it is that even the "aneutronic" p-11B reaction would produce enough neutrons to require heavy shielding and disposal of radioactive waste, it would be about a thousand times less than the D-T reaction, which already has orders of magnitude lower problems with radioactivity than nuclear fission.
The figure of merit usually considered most indicative of the overall quality of a plasma confinement device is the product of the pressure and the energy confinement time (nTτ, the "triple product"). For p-11B fusion, the triple product would have to be 500 times larger than that required for D-T fusion (which is already extremely difficult to achieve). Worse yet, while one might imagine finding a way to increase the confinement time by plugging one hole after another, there is one sort of loss that is essentially impossible to stop: Bremsstrahlung radiation. The standard calculation under the most optimistic conditions indicates that in a p-11B plasma, the Bremsstrahlung loss will always be higher than the fusion power produced. The plasma cannot "burn", but will fizzle out. Some people have spent a lot of effort trying to find a way around this, for example by keeping the electrons colder than the ions, or by keeping the ions or electrons from relaxing to a Maxwellian (thermal) velocity distribution, but the energy required to maintain these special distributions is always greater than the fusion energy produced.
Less attention has been paid to the energy density, although it would also be a dramatic problem, even if you could solve the confinement problem somehow. For a given pressure with otherwise comparable conditions and reasonable approximations, the power density for p-11B would be 2,000 times lower than that for D-T. If the capital cost of a fusion reactor scales linearly with the plasma volume (a reasonable first cut), then electricity from p-11B would cost around 2,000 times more than electricity from D-T (though if we take into account that p-11B plasma may allow more efficient energy conversion, that could fall to "only" 400 times more). Even if you could make p-11B fusion work, why would you not choose to put in D-T fuel instead and produce hundreds of times more power for the same investment?
All of these considerations — the residual radioactivity, the low power density, but especially the Bremsstrahlung losses — make aneutronic fusion the biggest fusion woo. Except for cold fusion, of course.
The "simple and cheap" part: alternate concepts
Once cold fusion and aneutronic fusion are off the table, most of the rest has a scientific kernel deep down inside, but the PR department oversells the potential in a manner somewhere between "aggressive" and "criminal".
The best hope of producing net energy from nuclear fusion is the tokamak. It probably really will work, but it may also prove to be an order of magnitude more expensive than alternatives, whether they be nuclear fission, solar plants in desert regions, or burning coal and living with climate change. The best argument for continuing the research anyway is that it could turn out to be better than expected, and it will only cost a tiny drop to find out, compared to our expenditures on energy supply.
Among the factors that make tokamaks (and their less popular brothers the stellarators) expensive is that they are toroidal (big and complex), and that the plasma pressure is only about a tenth of the magnetic field pressure. Most of the alternatives try to attack both of these issues. A toroidal field is really a good idea because it's the only way to avoid loose ends, but if you could produce it in a cylindrical machine, you might be able to make it a lot smaller and cheaper. This avenue is called a "compact toroid", of which there are two major lanes: Field-Reversed Configurations (FRC) and Spheromaks. FRCs also have the advantage of inherently having a plasma pressure nearly as high as the magnetic field pressure. (Don't let anybody tell you the plasma pressure can be higher than the magnetic field pressure. The mathematical proof of that fact is called the "virial theorem".)
How to recognize fusion crankery
It is quite easy to recognize fusion woo. The following things are big red flags:
- No neutron production, especially when claiming to use pure deuterium as fuel. D-D fusion produces tritium in half of its reaction, the tritium then undergoes D-T fusion to produce neutrons. The other half of D-D fusion reactions produces 3He and a neutron. Genuine aneutronic fusion reactions require exotic fuels (such as helium-3) and conditions far more extreme than those achieved at the largest 'Big Science' fusion facilities.
- Use of particle accelerators as the primary means of sustaining the plasma. There are fundamental reasons which prevent such systems from ever generating net power.
- Use of fuel other than a deuterium-tritium mixture, in particular, proton-boron fuel. The conditions required for p-11B fusion are so extreme that Bremsstrahlung losses would dwarf any power produced from fusion.
- CrossFire Fusion Reactor
- Tri Alpha Energy - A California company developing aneutronic fusion machines
- Focus Fusion - while the approach is a variant of the "mainstream" magnetic confinement fusion and might not be completely useless, its proponents make claims of being able to eventually achieve net energy gain from proton-boron fusion, which is strongly suspected to be physically impossible.
These are items that, while not overtly being woo themselves, are often presented by their promoters in ways that make them woo.
- ICF — Inertial Confinement Fusion (ICF), where extremely powerful laser pulses are focused on tiny pellets of fuel to implode them. This closely models the processes happening in thermonuclear weapons. The primary goal of these facilities is to make it possible to design new nuclear weapons and verify the reliability of existing ones without the need for test explosions. It's highly unlikely such a device would ever be used for power production, but describing these projects as energy research rather than stockpile maintenance is far more politically acceptable. ICF facilities include the National Ignition Facility in the U.S. and the Laser Mégajoule in France. It is the claims of being energy research that result in ICF being classified as soft woo.
- Focus Fusion — Also known as dense plasma focus (DPF) fusion. This approach definitely achieves fusion, and has been claimed to produce 4 times the fusion events per unit energy of an ICF device at 1/12,000th the power level using D-D and not the more expensive but better D-T fuel. Thus, if the results are real, it has been empirically shown to be closer to net energy generation than ICF, although ICF itself is far from reaching break-even.
DPFs may be useful as an X-ray or neutron source independent of fusion energy production. There is much speculation that the device is woo because the group pursing it eventually wants to burn p-11B fuel and features this fuel prominently, although their device currently burns D-D fuel. Additional criticism is levied at the group's leader, Eric Lerner, for his rejection of the mainstream consensus regarding the Big Bang, which leads to more doubt as to the group's legitimacy.
Will it ever work?
The prospects for any such low-cost fusion technology look very bleak. Back in 1995, a graduate physics student at MIT, Todd Rider, wrote an extensive Ph.D. thesis which investigated many possible ways of producing energy from fusion in plasma far from thermodynamic equilibrium, i.e. in a way that does not require sustained extremely high temperatures and pressures. None of the dozens of approaches he studied could be expected to generate net power, even under highly optimistic assumptions. However, this paper does have a loophole in that it does not investigate "transient nonequilibrium burning systems which try to produce enough fusion power before the particle distributions equilibrate". There is of course some small chance that he made serious errors that were not spotted by him nor his reviewers, or that there is some approach he did not consider, and energy gain from a steady state fusion system far from thermodynamic equilibrium is possible after all.
Some of the devices produced by fusion woo companies could find applications in places where intense neutron sources are required — for example, in the transmutation of nuclear waste.
The approach to fusion power which receives the majority of funding is magnetic confinement fusion (MCF), which uses powerful superconducting magnets to create a spiral magnetic field to stabilize the plasma and may not be woo. This approach is used in the ITER facility under construction in France, which could produce net power within the next decade. Commercial application, however, is still far in the future (after 2050). The Polywell, the brainchild of Robert "interstellar ramjets" Bussard, is also a kind of magnetic confinement fusion; the jury is still out on the Polywell's efficacy, but the U.S. Navy has continued to show an interest in it.
The second approach is inertial confinement fusion (ICF), where extremely powerful laser pulses are focused on tiny pellets of fuel to implode them. See above.
- Nuclear fusion gets boost from private-sector startups — Science News review private research