In the Energy Shadows, the US Continues Push Toward Nuclear Fusion

Fusion is an old energy dream. Free power, or something like it. It's smashing atoms without the waste or risk, with efficiencies untouchable by any other power generation scheme known to humans. It figures then that it should be really hard to do.

The United States currently supports three major fusion research centers, but that might not be the case for long. According to a report drafted by the Fusion Energy Sciences Advisory Committee, two of those centers could be mothballed in the near future: the Alcator C-Mod reactor at MIT by 2015, and by 2025, either the DIII-D in San Diego or the National Spherical Torus Experiment in Princeton, New Jersey.

Instead, the report recommends shifting US fusion support to the long in-development ITER project, an international collaboration being built in the south of France. With costs touching $50 billion, ITER is easily the most expensive scientific undertaking in history, beating out the Large Hadron Collider by about $45 billion. It's also been beset by a long history of delays: First hatched as a Reagan-Gorbachev initiative in 1985, construction finally began in 2007, with completion anticipated in 2019.

Nearly a century after it was first suggested by physicists trying to get a handle on Einstein's E = mc2, fusion remains one of the biggest experimental challenges in physics. Doing fusion in its most basic form is easy, but the looming challenge is in initiating a fusion reaction with less energy than the reaction produces.

The problem is that fusion reactions are just fundamentally difficult to begin, at least outside of the guts of our Sun (and other main-sequence stars), where it happens nonstop. The goal is in fusing two atomic nuclei together, deuterium and tritium in the case of the ITER project, but the components of atomic nuclei are fundamentally opposed to the idea of fusion.

Protons, the constituent particles of nuclei along with neutrons, are repelled from other protons. That's just electromagnetism. Protons nonetheless hold together in nuclei thanks to another force, the strong nuclear force. This force is immensely, staggeringly powerful, but only works at staggeringly small distances. So to fuse a pair of nuclei, physicists have to overcome the repulsive energy of electromagnetism at its absolute strongest, at the distance in which electromagnetism becomes subsumed by the strong force.

When a pair of relatively light nuclei fuse, the resulting nuclei will have less mass than the particles had going in, as the attractive strong force gives way to the electrostatic repulsive force at the larger distances across larger nuclei. This extra mass is converted to energy with an efficiency many millions of times better than burning coal.

Just a single fusion event between one deuterium nucleus and one tritium nucleus should release about 18 MeV (18,000,000 eV) of power. For a combustion/chemical reaction between a couple of atoms, we might expect a mere 3 or 4 eV.

That said, it's still a whole lot easier to just burn some coal (or initiate a fission-type nuclear reaction). The ITER reactor will consist of a 23,000 ton, 37 feet tall doughnut-shaped containment vessel. Within this vessel a plasma of deuterium and tritium will be heated to around 180,000,000 degrees Fahrenheit, at which point the particles should be energized enough to begin fusing with each other and releasing energy. The neat thing is that nothing is wasted; anything "extra" just goes toward more reactions.

The reactor core of a fusion nuclear power plant is surrounded by a blanket of sorts and a region of supercooled space. The blanket absorbs the released energy (in the form of bolting neutrons) and in turn heats water. Superheated water yields steam, steam yields pressure, pressure yields energy. The ITER reactor should generate about 500 megawatts of power during for every 400 second pulse of activity.

So that $50 billion starts to make a bit more sense, anyway. The promise of fusion power hasn't waned very much over the past century of free-energy dreaming, but its necessity as a safe (read: self-limiting), waste-free energy generation scheme has only grown.