What if there was an inexhaustible form of carbon-free, clean energy that was available 24/7, rain or shine, and was no larger than existing fossil fuel plants? Is that something you might be interested in? What I'm talking about is fusion energy. The easy joke is that fusion energy is the power source of the future and always will be, but what are its real prospects?. This is not a subject of unbiased speculation for me, I've devoted the last 35 years of my career to harnessing this tricky power source. So what have we been up to and where are we going?
Let's start at the beginning. Fusion is a form of nuclear energy that powers the sun and the stars and produces the elements of the periodic table by combing light elements into heavier ones. The discovery of fusion solved a vexing problem -- where did the sun's heat come from. After geologists figured out how old the earth was, there was no known mechanism that could keep the sun so hot for so long. On the sun, hydrogen fuses to produce helium, releasing a tremendous amount of energy -- more than a million times as much energy as when hydrogen is burned chemically. The hydrogen isotope we would use for a power source on earth is called deuterium and has one extra neutron in its nucleus, compared to ordinary hydrogen whose nucleus is a single bare proton. The deuterium extracted from 10 gallons of water would weigh about a 1/10 ounce and could supply enough electricity to last an average U.S. consumer about 15 years. And there's a lot of water.
Fusion reactions are slow until the fuel is heated to unimaginably high temperatures. At that point, the electrons in the fuel atoms are all stripped from their nuclei and the gas becomes a plasma, the fourth state of matter. The most promising approach for commercial fusion energy uses powerful magnetic fields to insulate this hot plasma from nearby material walls. Using these techniques, we've attained the necessary plasma densities and temperatures, over 300 million degrees, far hotter than the core of the sun. Experiments to date, have produced about as much fusion power as they consume and a simple scale-up in size would yield net energy production. ITER, an experiment to do just that, is under construction by an international consortium that includes the U.S..
The advantages of fusion energy go well beyond an abundance of fuel. The need for a carbon-free source should be obvious to everyone of course and by eliminating fossil fuels it also dispenses with other pollutants, as well as the hazards of mining, refining and transportation. How does it stack up to other alternate energy sources like wind or solar? Fusion would provide electricity in large central stations, simply replacing the heat from combustion with another energy source, so it would run all day in any weather, eliminating the need for expensive energy storage systems or expensive modification of the electricity grid. And unlike proposed forms of biomass energy, there would be no significant land or water use -- energy would not be competing with food for these precious resources.
How about fission, the nuclear power source we use now? Fission and fusion can claim many of these same advantages, but fission comes with some serious and all too apparent risks. Fission plants are fueled with enough enriched uranium to run for a year or two. The highly radioactive byproducts accumulate, which by themselves produce an enormous amount of heat that can't be turned off. The recent disaster at Fukushima was a consequence. When the earthquake and tsunami damaged the plant and all sources of electricity to run the cooling pumps, the reactor cores melted and released radioactive products into the environment. Some of these radioactive products are chemically volatile and biologically active -- they can move through the environment and accumulate in living organisms. In contrast, a fusion reactor would never have more than a few seconds of fuel in it at any time -- there is no way it could melt down. The metal structures that make up a fusion reactors would become mildly radioactive over time and would need to be isolated, but after about a hundred years, the materials could be recycled or buried -- no permanent waste disposal would be required.
Perhaps the most frightening aspect of a global fission industry is the risk of nuclear proliferation. The technology for enriching uranium for reactor fuel is largely identical to what is required to make a bomb (hence our concern with Iran's program). Alternately, only the tiniest fraction of the plutonium that is produced in fission reactors is needed to make a nuclear weapon. By comparison, the proliferation risks of fusion are minimal and easily detected. Fusion energy would pose little danger to the public health, to property or water supplies, nor would it threaten social trauma.
Fusion is the big winner in what economists call "external costs," costs that are borne by society as a whole and that don't factor into the costs of production. So why aren't we running our homes with fusion energy? Well, it turns out to be a very hard problem. We've made some dramatic breakthroughs and a lot of slow steady progress -- between 1970 and 2000, the energy produced by each pulse of a fusion experiment increased by a trillion times. Computational power increased over the same period by "only" a factor around a million. Of course, the semiconductor industry could sell individual transistors and make a profit, but fusion won't be profitable until it reaches full scale. Scientists working in the field agree that you could build a fusion reactor that produces a lot of energy, but whether in the end, fusion can be cost competitive and reliable is still an open question.
What's next? The steps required to harness fusion power are well known but will take some time and some money. The ITER experiment needs to be completed and operated. You'll read about cost overruns and schedule slips -- its multi-national management has been slow and inefficient. It would have been faster and cheaper if one nation had stepped up to lead by itself, but none did so we'll have to live with what we've got. We need to learn more about the behavior of hot plasmas, more about its interactions with material surfaces and more about the behavior of structural materials in a fusion environment. Fusion won't come cheap, but the costs need to be put into context -- in the U.S. alone, direct expenditures for energy are on the order of 1.5 trillion dollars per year, about 10% of the U.S. economy. We're spending less than 0.03% of that on fusion research. With a reasonable level of investment, the power source of the future could really be the power source of the future.
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