"Fusion is the energy of the future...and always will be."I recently attended a seminar at the MIT Plasma Science and Fusion Center from a director of the ITER project about the prospects of a commercial fusion reactor. I decided to write a post about why fusion always seems to be 50 years away and what needs to happen to make it a reality. What it comes down to, is that it requires a huge initial investment that nobody wants to pay for.
|The SimCity 2000 transit advisor understands the challenges facing the fusion plants in his city.|
The most-studied system for fusion is the tokamak (from the Russian for toroidal chamber with magnetic coils), which is a big donut-shaped chamber with a plasma inside, with a strong applied magnetic field that keeps the plasma confined in thin ring-shaped region in the middle of the torus. The plasma is heated up by driving it with electric fields and inducing Joule heating. If the plasma gets hot enough, it can begin to fuse hydrogen into helium, which produces a lot of excess energy in the form of neutrons, gamma rays, and overall heat. The heat is used to boil water to run a turbine. While the sun typically fuses four protons into helium (the p-p chain), the most accessible reaction is deuterium-tritium fusion, which has a lower energy barrier. Tritium is extremely rare naturally, but it can be produced by surrounding a nuclear reactor or the tokamak itself with lithium, which becomes activated by the neutron flux and then decays into tritium.
|The inside of the Joint European Torus tokamak, with a view of the plasma.|
The sad graph
In 1976, fusion researchers in the US wrote a report about future prospects for fusion power, suggesting that with appropriate funding fusion power would be realized within 15 to 30 years. They also predicted that with current levels of funding, they would not be able to achieve their goals in the foreseeable future. The actual funding reality since 1976 has been even bleaker than that. So it's not just scientists always saying that it's 20 years away and not making any progress, it's the lack of investment that prevented those 20 years from counting down. Their prediction was correct.
The next step in this direction is ITER (International Thermonuclear Experimental Reactor), a giant tokamak being built in France. Its main goal, from what I've read, is to generate ten times more power than it requires to operate (although this still won't make it viable as a power plant). It has gone massively overbudget and is now expected to run a tab of 20 gigaeuros. I think, moreso than any plasma science objectives, this will shed a lot of light on what is required to engineer a facility of this magnitude.
|ITER, dream and reality.|
At the fusion seminar I attended, a director of ITER was talking about his ideas for a commercially viable fusion reactor, which would have to be much bigger than ITER. One of the reasons tokamaks are so expensive is that they have to be really big. The fusion reactor would require about 500 megawatts just to maintain the fusion reaction: beyond that, power can be sold to the grid. However, if it's just selling an extra 50 megawatts after that 500, the electricity would have to be extremely expensive to cover the costs of the plant. It is estimated that a fusion plant would have to generate at least 2.5 gigawatts of electricity (slightly more than Hoover Dam) in order to sell the electricity at a cost comparable to current power sources. Thus, the minimum sensible infrastructure investment is that which is needed to make a plant that big. Not all of this goes into the generator itself, a lot of it goes into the building that houses it, as well as the plumbing necessary to extract tritium such that the reactor can keep making its own fuel.
The price tag that he quoted was 30 billion dollars, plus maintenance costs and morgtage payments over the next sixty years totalling over 100 billion, but it would produce enough power that that electricity could be sold at grid prices. A debate arose in the seminar about the economics of choosing the ideal initial size, about which costs scaled super-linearly with size and which scaled sub-linearly. It was pointed out that the first fission reactor was not sufficiently large to produce electricity that could be sold at a reasonable price, but the fact that it demonstrated that the technology was viable lead to investments in bigger nuclear plants. After the first viable fusion reactor is built, it won't be as difficult to build the next one.
The speaker claimed that what was required for this to actually exist was a rapid increase in fossil-fuel prices that would be driven by scarcity and increased energy usage in China and India. He cited $200/barrel as roughly the price at which a 30 billion dollar fusion plant would seem like a not-crazy investment. However, I've heard this story before; the rise in prices in the last decade made it viable to extract oil from fracking and from the Canadian tar sands and didn't give us a renewable energy revolution. Someone in the audience mentioned that General Electric was now developing its coal power technology, even dirtier than oil, to placate the rising energy demand in China.
The National Ignition Facility
I'm mainly discussed magnetic confinement fusion, but I'll also mention the National Ignition Facility (NIF) that was built for fusion research and then hastily re-purposed. The idea was to fire an extremely powerful laser at a small deuterium-tritium pellet such that it rapidly compressed and heated up until it was so hot and dense that fusion ignited. To this end, they built the world's most powerful laser array that was basically in inverted Death Star, with all the lasers focused on a little target at the center. They gradually ramped up the power, occasionally publishing papers about the behaviour of shock waves and the radiation emitted from these tests, and just as they were on the verge of getting powerful enough for fusion, that aspect of the project was halted and the facility was turned into a materials characterization facility and a way to test whether the fuel in nuclear bombs still works, without detonating the bombs themselves. I really wish they'd keep trying for fusion.
|NIF from the outside and inside.|