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After ITER, Many Other Obstacles for Fusion Power
17 January 2013 1:10 pm
The body responsible for fusion research in Europe has published a road map to get it from ITER—a giant international reactor under construction in France which will be the first to produce useful amounts of energy—to an industry-ready prototype fusion power plant by 2050. Although the successful operation of ITER, still more than 6 years away, will be considered a major breakthrough for fusion energy, the new road map from the European Fusion Development Agreement (EFDA) includes a daunting list of the technical hurdles that fusion scientists and engineers still face over the next few decades.
Fusion reactors use the power source of the sun and stars—fusing together isotopes of hydrogen—to produce energy. To do this they must compress and heat a plasma of fusion fuel to prodigious temperatures, at least 150 million°C, using powerful magnets, radio waves, and particle beams. It takes so much energy to get a plasma up to a temperature at which fusion occurs that no reactor has yet produced net energy gain.
ITER is expected to break through that barrier and generate 500 megawatts from a 50 MW input for periods lasting a few minutes. But it will be only a scientific demonstration; ITER won't generate any electricity. That job will be left for its successor, the prototype power plant DEMO. Fusion researchers are just starting to think about designs for DEMO but it is looking increasingly likely that it won't be a global collaboration like ITER, whose members are China, the European Union, India, Japan, Russia, South Korea, and the United States.
Korea announced recently that it was beginning preliminary design work on a next-step reactor called K-DEMO. China is already working on a design for an intermediate step between ITER and DEMO called the China Fusion Engineering Test Reactor. And now EFDA has laid out its own path to DEMO. The agency doesn't preclude international collaboration but has designed the road map so that any research will fit within the confines of the E.U. fusion budget for 2014 to 2020, although that may have to be revised once the overall E.U. budget is agreed upon later this year.
The EFDA road map acknowledges that ITER is the key to progress toward fusion power and so all efforts should be made to ensure its success, including researching various operating scenarios on smaller existing reactors. The biggest unanswered question, the road map says, is how to remove heat exhaust from a future machine. ITER and other similar modern reactors, known as tokamaks, have a structure at the bottom known as a divertor which, among other things, removes spent fuel from the plasma vessel. As the only place in the vessel where the plasma deliberately touches a solid surface, it must also absorb a lot of heat. ITER's divertor is made of stainless steel and coated with tungsten. This should work in a research reactor which operates at lower power and for at most a few minutes at a time, but DEMO will generate several gigawatts of power continuously and that heat load may be too much for a standard diverter.
The road map says researchers must work on other designs as a backup. These could involve alternative shapes that spread out the area of contact to reduce the heat load or allow the plasma to radiate more heat before coming into contact with the diverter. Alternatives should be tested on an adapted existing tokamak or a purpose-built test facility, EFDA says.
Another big unknown is what material to use for the structure and lining of DEMO's plasma vessel and other plasma-facing components. Fusion produces high-energy neutrons and the bombardment from DEMO will be intense. The neutrons knock atoms in solids out of position, weakening them and making them radioactive. Research is needed to find materials that can stand up to decades of sustained neutron bombardment, but there is no existing neutron source intense enough to test them. A design for an accelerator-based neutron source is being developed as an adjunct to the ITER project, but EFDA thinks something is needed sooner.
EFDA also wants more work done on the so-called "tritium blanket," sections of the plasma vessel wall in which neutrons from the reactor convert lithium into tritium, one of the fusion fuels. Alternative blanket designs should be developed in case the one to be tested on ITER is not successful. The road map also calls for greater involvement from industry in preparations for DEMO, since it will have to take over fusion development once DEMO is complete, and for plasma theory and modeling to be strengthened.
As an ultimate backup plan, the road map advocates continuing research on stellarators, an alternative fusion reactor scheme that fell out of favor when tokamaks came on the scene in the late 1960s. Germany's Wendelstein 7-X stellarator, which is due to be finished next year, could provide the model for a later energy-producing version dubbed HELIAS.