Welcome back. This is part three of a series on a possible Silver Bullet that just might be The Answer to the problem of global climate change and the energy crisis. Part 1: Coming Together laid out my overview of the situation and suggested there may be an answer taking shape: nuclear fusion. Part 2: It's Elementary was a broad and greatly simplified discussion of the basics of how atoms work, and how we might put them to use to solve our problems. Today's installment is going to be about the current state of the art for achieving nuclear fusion - the promises and the problems.
The joke is, Fusion is the power for the future - and always will be. The field has a long history of success being Just Around The Corner. Still, there has been steady progress over the decades of work and there are indications that practical fusion may finally be within reach.
Get comfortable. This is going to take a little bit of reading but I hope you'll find it worth it. This will set the stage for Part 4 where I'll get into the really exciting news about fusion.
The concept of nuclear fusion is simple enough. Squeeze the nuclei from two lightweight atoms together in the right way, and you'll get nuclear fusion, with energy being released. The problem with doing it consistently is that A) atomic nuclei are very small so making them collide is a more miss than hit proposition, and B) nuclei have positive electric charges which repel each other. If they do collide, they have to do it hard enough to overcome that repulsion before they'll fuse.
SOLAR FUSION
The sun does it 24/7 without a hitch, aside from occasional flares and sunspots. Of course the sun does it with really high temperatures which convert the atoms to plasma. The high temps make the atoms move very fast so they hit hard, under tremendous gravitational pressure which keeps them crowded together so they can hardly avoid colliding at those high speeds, amid sheer bulk that keeps heat from escaping too quickly so the fusion can continue. Tom Ligon (of whom more will be said later) describes the sun as "a gravitational spherical confinement device."
LASER FUSION
A different approach is needed on the surface of the earth, obviously. One is Inertial Confinement Fusion, where fuel (usually a deuterium-tritium mix) is encapsulated in small pellets and blasted by high energy lasers or other beams from multiple converging directions simultaneously. This A) heats the fuel to millions of degrees in an instant, and B) creates a shock wave that squeezes the pellet hard enough to cause nuclear fusion. There have been a number of laser-based ICF projects in the last 30 years or so, but none have been able to develop beams that can fire rapidly enough, long enough, to produce fusion efficiently enough to reach break-even, where more energy is coming out than is going in. Efforts continue, but a machine that can demonstrate proof of concept is still years away.
MAGNETIC FUSION
Magnetic Confinement Fusion is where most of the attention is these days, so I'm going to give it the most space here. Like the sun, the fusion fuel is heated to millions of degrees in the form of a plasma. This means the electrons are stripped away from the atoms, leaving just the positively charged nuclei. Unlike the sun, the plasma is very thin - a near vacuum - and instead of being confined by gravity, extremely strong magnetic fields interact with the charged atoms to confine them inside a reactor vessel shaped like a doughnut - a Tokamak. (Really cool image here.) The magnetic fields do two things: they keep the plasma from touching the sides of the reactor which would cool it below fusion temperatures, and they squeeze it hard enough to produce collisions which result in fusion. At least, that's the theory. Tokamaks do consistently achieve fusion these days, but the problem is A) making the reaction sustainable and B) reaching a level where more power is coming out than has to be put in to start the reaction.
Magnetic Confinement Fusion goes way back - to the 1950s. The latest iteration in a long line of reactor designs is currently under construction in France, due to power up in 2016. It's called ITER. (International Thermonuclear Experimental Reactor was the original aconym, but it's now meant to be the Latin word Iter, meaning journey, direction, or way because "thermonuclear" didn't focus-group well.) There's an article in Physics World Vol. 19 No. 3 March 2006 issue that gives a good overview of the project which I'm going to use heavily here. Here's a quote that states the case for building a working fusion reactor along the lines the ITER project envisions:
Nuclear fusion offers a potentially safe, environmentally friendly and economically competitive energy source. To operate for a whole year generating about seven billion kilowatt hours of electricity, a fusion plant would use just 100 kg of deuterium and three tonnes of lithium - releasing no greenhouse gases in the process. A typical coal-fired power station, in contrast, devours three million tonnes of fuel and produces some 11 million tonnes of carbon dioxide to yield the same annual output.
The ITER project is the great hope for harnessing the power of fusion. A 6 minute youtube video link here gives an idea of the hopes riding on this project, and how it will work using footage of the JET tokamak. An international group of countries is putting €5bn into this, so there's a lot at stake. (They're only getting around to building ITER now because the countries behind it spent years fighting over where it would be located.) But, with all the hope, there's also a great deal of hype.
ITER is just the intermediate step before the construction of a proof of concept fusion power reactor - which might lead to working fusion reactors by 2050. It's intended to answer some of the critical questions on how to build a working tokamak style fusion reactor. The parameters of the design incorporate some less pleasant consequences of the tokamak approach. For one, the expectations are that this reactor will have to run on deuterium-tritium fuel, the easiest combination of atoms to fuse.
Deuterium-tritium fusion produces neutrons and alpha particles. Of the two, the alpha particles are easier to deal with; the neutrons are another story. The Physics World article gives the details.
Of all the possible combinations of light elements that could be used for terrestrial fusion, the deuterium-tritium reaction proceeds at the highest rate for the lowest temperature and is therefore the best candidate for a fusion power plant. Each reaction yields an energy of 17.6 MeV, which is shared by an alpha particle and a neutron. The neutrons, carrying most of this energy, escape the confining fields and are captured in the walls of the tokamak where they generate heat. As a result, coolant circulating through the walls can be passed through a heat exchanger to produce steam in order to drive turbines, as in a conventional power station. The walls also double as a "breeding blanket" in which neutrons react with lithium to produce further tritium.
The alpha particles, in contrast, are confined by the magnetic field and transfer their energy to the deuterium and tritium fuel ions via Coulomb collisions.
There's a lot going on here to get those alpha particles and neutrons emitted. It starts with the way those magnetic fields get funky with the deuterium-tritium plasma. A toroidal field sends the plasma circling around the doughnut-shaped vacuum chamber; a poloidal field tries to make it spin in circles perpendicular to the circular toroidal path. Combining the effects sends the particles in the plasma tracing a spiral path around the tokamak. Here's a schematic of the hardware. There's some extra special fun and games with the magnetic fields to set up regions inside the tokamak where undesirable particles can get moved out of the plasma stream and trapped.
Just figuring out all of the ways plasma can interact with magnetic fields, contaminants, etc. and keep them from diverting energy away from fusion is a major headache. It has to be kept within a specific range of temperature and pressure conditions for fusion to take place. Instabilities in the plasma's distribution, maintaining the electric currents induced in it by the magnetic fields, contamination from materials outgassing under the conditions in the reactor; these and many other problems have been real challenges. The alpha particles formed by the fusion reaction are supposed to make the process self sustaining. They 'ignite' the plasma when their production rises to levels where - trapped inside the plasma by the magnetic fields - the collisions they produce are enough all by themselves to keep the plasma hot enough and dense enough to sustain the fusion reaction. But, they are just helium atoms with the electrons removed. If they're allowed to build up in the plasma, eventually they'll 'smother' the reaction. One of the things the ITER reactor is supposed to study is what happens as a fusion reaction runs and all the products from that reaction build up. Up till now, the records for sustained fusion reactions are measured in seconds. A practical power reactor will have to do a lot better than that! ITER is expected to sustain fusion reactions for periods long enough to begin finding out what happens.
And those neutrons? Because they don't have an electric charge, the magnetic fields of the tokamak have no effect on them. They go shooting off with the energy from their creation until they strike the walls of the reactor chamber. Blankets of lithium inside the chamber walls help soak them up - and also get transmuted to tritium which makes more fuel for the reactor. Heat also results from these collisions - and a circulating cooling fluid transfers that heat away from the reactor to where it can be used to create steam to power steam turbine generators. But, between the bombardment from the neutrons, the effects of the high temperature plasma, and other factors, the walls of the chamber take a beating.
What this means in practice, is that the chamber walls of the tokamak are going to be bombarded by particle radiation which over time will erode them and make them radioactive. The radiation will be nothing like that from a fission reactor, but it's not negligible. Operational scenarios for tokamak reactors expect that the reaction chambers will have to shut down for periodic replacement or refurbishing. Reactor life can only be estimated at this point. A separate facility is being built in Japan just to study the effects of neutron bombardment under simulated fusion conditions so that materials and designs for a working power reactor can be developed.
The article in Physics World gives a much more complete idea of all the challenges involved, so I suggest reading it if you want a deeper understanding of the matter. If you'd like a simpler image of what is being attempted though, try this:
Fusion by Magnetic Confinement is like trying to take an extra heavy white-hot lightning bolt made of atoms, wrestle it into a circle, and keep it spinning while you twist it to squeeze out power - even as it's chewing away at your tools and trying to wriggle out of your grip.
If that sounds like a daunting prospect, it is. But, the people involved think they have a good idea of where they are, where they have to get to, and a road map to get there. If they succeed, mankind will have gained access to an energy source that will only be limited by our ability to build and maintain them. Fuel supplies are not going to be a problem; no more green house gasses, no worrying about loose nukes or runaway reactors, and the radioactive waste problem will be far easier to manage than the kind produced by fission reactors. It won't make wind or solar power redundant, but it will do what they can't: provide a base power capacity of high level in a concentrated package on demand that can be plugged into the existing power grid. The picture painted in the Physics World article glosses over a few more troubling details, but that last advantage is still pretty hard to argue with.
Magnetic Confinement Fusion still has a long ways to go, but it could finally begin delivering on a promise that's over half a century old at this point. Or, it could turn out to be technological blind alley IF the approach I'm going to describe tomorrow gets a fair shake and proves out. We might not only get practical fusion power sooner and in a much nicer form, we might get the Solar System as an added bonus. Stay tuned for Part 4: the Dynamic Alternative.
ADDENDUM: For an example of why we need something as soon as possible, this story from the NY Times shows what the world is up against.
CIVITAVECCHIA, Italy — At a time when the world’s top climate experts agree that carbon emissions must be rapidly reduced to hold down global warming, Italy’s major electricity producer, Enel, is converting its massive power plant here from oil to coal, generally the dirtiest fuel on earth.
Marco Di Lauro for The New York Times
Italy’s Civitavecchia power plant is converting from oil to coal.
Over the next five years, Italy will increase its reliance on coal to 33 percent from 14 percent. Power generated by Enel from coal will rise to 50 percent.
And Italy is not alone in its return to coal. Driven by rising demand, record high oil and natural gas prices, concerns over energy security and an aversion to nuclear energy, European countries are expected to put into operation about 50 coal-fired plants over the next five years, plants that will be in use for the next five decades.
emphasis added