Nuclear fusion as an electrical power source is rather like some people’s plans for after they win the lottery. They’re sure it’s coming, and they’re sure it’s going to be great, but somehow it never seems to happen.
Actually, that’s not a very fair comparison, because nuclear fusion really does seem to be on the way, while most people will never get rich in a lottery. On the other hand, a lot of lottery winners will have come and gone before fusion finally becomes a viable energy source.
However, on still another hand (hmm, that makes three), almost all of our present energy sources are ultimately derived from nuclear fusion, because nuclear fusion is, literally, the power of the sun.
Fission, as you may recall from last week’s column, is the splitting of a very large atomic nucleus into two smaller nuclei. The two smaller nuclei mass less, even put together, than the large nucleus did. That missing mass turns into energy–lots of
Fusion is the opposite process. In nuclear fusion, two small nuclei are combined to form one larger nucleus. Oddly enough, when you’re dealing with very light elements like hydrogen and helium, two small nuclei actually have more mass than a single nucleus of twice the size. Once again, after fusion, there’s a missing mass, and it’s that mass that turns into energy.
How much energy? Well, ever hear of the hydrogen bomb?
Fusion is a lot harder to achieve than fission, because, as a little experimentation with magnets will show you, like charges repel. If you try to shove two hydrogen nuclei together–each consisting of a single positively charged proton–they’re not going to like it. The sun overcomes this mutual distaste by the simple procedure of cramming everything together to a density 100 times that of water, then heating it up to 15 or 20 million degrees. At that temperature the particles get quite excited, rush around frantically and run into each other with great force. But even in the centre of the sun, it’s estimated that a proton will exist on the average for 10 billion years before it’s finally fused with another. (And I thought I’ve been single a long time…)
So in order to initiate fusion on Earth, physicists have to not only match conditions at the centre of the sun, they have to improve on them. One way is to use deuterium or tritium, both isotopes of hydrogen–that is, they both have a single proton in their nucleus, like hydrogen, but they have one and two extra neutrons, respectively. Fusion of these nuclei occurs millions of times faster than the Sun’s basic proton-proton reaction.
In order to create the high temperatures required for fusion, physicists must turn the deuterium and tritium into a plasma. This is the fourth state of matter, above the familiar three of solid, liquid and gas; it’s a “soup” in which electrons and nuclei are completely separated from each other. Trouble is, this plasma is at a temperature of 100 million degrees Celsius. Obviously you can’t let this touch the walls of the container you’re trying to keep it in.
One solution to keep the plasma from vaporizing the machinery that created it is to contain it in a magnetic field, or “bottle,” shaped like a doughnut. (Well, scientists call it a “torus,” but it looks like a doughnut to me.) This kind of fusion reactor is called a tokamak, and it’s been the most popular over the years.
A newer kind of fusion reactor uses what is called “inertial confinement.” Basically this relies on the inertia of the fuel to keep it together after it has been compressed to the high density necessary to achieve fusion, which is accomplished by focusing extremely powerful lasers on glass-walled pellets containing the fuel. The beams evaporate the outer layers of the pellets and cause the rest to implode, compressing the fuel.
Still, it’s the tokamak that has had the most support, and the tokamak which has so far had the most success. In fact, just a year or so ago physicists at Princeton University announced they have developed a fusion reactor capable of producing as much energy as it consumes. This point, called “break-even” is an important stepping stone toward the development of a commercial fusion reactor.
Fusion is an immensely attractive energy source for a number of reasons. For one thing, its basic fuel can be obtained from one of the most plentiful substances on earth, ordinary seawater. (Deuterium exists naturally in seawater; although tritium does not, it can be produced from relatively common lithium during normal operation of the reactor.)
For another, fusion would be a very clean form of energy, producing far fewer and far less dangerous wastes than fission. (Fission reactors produce plutonium; fusion reactors produce helium. Which would you rather have in your back yard?)
The trouble is, energy-producing fusion is proving to be an immensely tough technology to develop. The Princeton project has been underway for 15 years and has cost $1 billion, and scientists caution fusion could still be decades away from commercial use. That was why the flap a couple of years back over so-called “cold fusion” was so great.
It would certainly be convenient if fusion could be induced in a test-tube instead of requiring duplication of conditions at the centre of the sun, wouldn’t it? Unfortunately, while something may have been happening in that test tube, fusion apparently wasn’t it–at least not in any exploitable form.
Still, the race is on, and it really does appear that it will only be a matter of time before fusion is the energy choice of a “new generation.”
Considering the environmental and economic costs of our present methods of energy production, it can’t come a moment too soon.