Is Fusion in Our Future?

Humans have relied on nuclear fusion from the very beginning, but only indirectly, as the ongoing fusion reactions that power the sun have provided the light and warmth that made the earth habitable for life in the first place. Harnessing this power directly, though, is a matter of more controversy and greater difficulty. Although the use nuclear fusion as an energy source has appeared possible, however remotely, since the United States detonated the first fusion bomb (also known as a hydrogen bomb or a thermonuclear weapon) in 1951, an article in this week’s Science (subscription required) outlines the logistical challenges that may preempt any use of this potential energy source.

The article—written by William E. Parkins, who passed away in October 2005 after submitting his paper—describes the barriers to nuclear fusion as engineering problems, not physics problems. Although the science behind nuclear fusion is sound, engineers of a nuclear fusion power reactor would have to find a way to generate and then contain the extremely high temperatures necessary for nuclear fusion to occur and then find a way to remove the additional heat released in the reaction to both generate power and prevent the nuclear reactor from overheating.

As the name implies, nuclear fusion involves joining two atoms into one, most commonly the fusion of two hydrogen atoms to form a helium atom (although the process really just involves the nuclei of the atoms, for the sake of simplicity I’m using the term “atom” and “nucleus” interchangeably). Nuclear power reactors currently in use generate power by taking advantage of the opposite process, nuclear fission, which is the breaking up of larger atoms into smaller ones. It may at first seem unintuitive that two opposing processes can both release energy. In fact, it seems to violate one of the most fundamental concepts learned in chemistry 101: if a reaction in one direction releases energy, the reaction in the opposite direction should consume energy. The reason this works, though, is because atoms of different sizes behave differently, as illustrated by this diagram from Lawrence Livermore National Laboratory Public Affairs:


The nuclear stability of an atom depends on the number of protons and neutrons (collectively called nucleons) in its nucleus. The most stable nuclei are those of iron (56 nucleons) and nickel (62 nucleons). As one moves away from this point, heading toward larger or smaller nuclei, nuclear stability decreases. Since transitioning from a less stable to a more stable nucleus releases energy, fission generates power by breaking up nuclei larger than iron or nickel, and fusion does the same by combining nuclei of smaller atoms. The largest single energy transition occurs between hydrogen and helium, enabling such powerful phenomena as stars and hydrogen bombs.

Nature has taken care of the physics for us, but has left us with quite an engineering problem: to make this reaction occur, temperatures of roughly 100,000,000°C must be generated. Not only is this a difficult feat in itself, but such high temperatures have to be contained and safely dispersed. The fusion reaction itself then generates heat, and although this heat is used to generate power, removing it efficiently enough to prevent the reactor from melting down is not trivial. All of these problems seem to require expensive solutions, as Parkins details:

A 1000 MWe plant requires a thermal power of about 3000 MW, 20% of which must be absorbed by the vessel wall. If we assume an average heat transfer rate of 0.3 MW/m², the vessel wall and blanket-shield each must have an area of 2000 m². To absorb the 14 MeV neutrons and to shield against the radiation produced requires a blanket-shield thickness of ~1.7 m of expensive materials. This is a volume of 3400 m³, which, at an average density of about 3 g/cm³, would weigh 10,000 metric tons. A conservative cost would be ~$180/kg, for a total blanket-shield cost of $1.8 billion. This amounts to $1800/kWe of rated capacity--more than nuclear fission reactor plants cost today. This does not include the vacuum vessel, magnetic field windings with their associated cryogenic system, and other systems for vacuum pumping, plasma heating, fueling, “ash” removal, and hydrogen isotope separation. Helium compressors, primary heat exchangers, and power conversion components would have to be housed outside of the steel containment building--required to prevent escape of radioactive tritium in the event of an accident. It will be at least twice the diameter of those common in nuclear plants because of the size of the fusion reactor.

Scaling of the construction costs from the Bechtel estimates suggests a total plant cost on the order of $15 billion, or $15,000/kWe of plant rating. At a plant factor of 0.8 and total annual charges of 17% against the capital investment, these capital charges alone would contribute 36 cents to the cost of generating each kilowatt hour. This is far outside the competitive price range.

Although there is a lot of jargon here, the take-home message is that generating power from nuclear fusion will be expensive and inefficient, or at least it will be using current technologies. Based on this analysis, Parkins draws an interesting conclusion. Because fusion power seems unfeasible, funding priorities should be shifted toward studying fusion in terms of basic science:

New physics knowledge will emerge from this work. But its appeal to the U.S. Congress and the public has been based largely on its potential as a carbon-sparing technology. Even if a practical means of generating a sustained, net power-producing fusion reaction were found, prospects of excessive plant cost per unit of electric output, requirement for reactor vessel replacement, and need for remote maintenance for ensuring vessel vacuum integrity lie ahead. What executive would invest in a fusion power plant if faced with any one of these obstacles? It's time to sell fusion for physics, not power.

I appreciate his focus on basic science, and I have written previously that scientists need to work harder to sell their work on its intrinsic value, not just on potential applications. Still, I am reluctant to give up on nuclear fusion so quickly. If anything, we as a society have proven ourselves incredibly inept at predicting what new technologies will make the seemingly impossible now possible.

In the meantime, though, it looks like we’re going to have to leave fusion up to the real expert: nature. As the sun continues to provide us with an almost unending potential source of energy, it would be a shame not to take full advantage of it and harness the most powerful force of nature in a different way. I imagine that some of this funding being spent on fusion research, while still very important, could go a long way in further developing solar power and other renewable resources. With the need to cut down greenhouse gas emissions at any cost becoming increasingly clear, we need to hedge our bets and develop a truly multifaceted strategy for developing new energy sources.