Seeking an energy miracle: the traveling wave reactor runs on nuclear waste - at least in theory.
We need "energy miracles" to combat political inaction on climate change said Microsoft Chairman Bill Gates at the 2010 TED conference--a conference of inspirational speakers spreading ideas broadly related to technology, entertainment and design. Nuclear energy is touted as a large-scale solution that doesn't produce greenhouse gases in its electricity production (although its life cycle does). But while the U.S.'s 104 nuclear reactors produce 20% of the country's electricity, the expansion of nuclear power came to a halt here after nuclear accidents at Three Mile Island in Pennsylvania in 1979 and Chernobyl in the Ukraine in 1986. Safety concerns and the accumulation of highly radioactive spent nuclear fuel with no definite long-term storage plans remained seemingly insurmountable obstacles to an increased nuclear future.
But Gates is gambling undisclosed millions of his own money (not his foundation's) on TerraPower, a Bellevue, Washington, offshoot of the private global invention company Intellectual Ventures that's developing a new type of nuclear reactor. TerraPower asserts that its traveling wave reactor (TWR) will produce zero greenhouse gas emissions, minimize risks of nuclear proliferation and create less waste than conventional nuclear reactors.
"The advantages of the TWR are that it reduces the amount of uranium enrichment needed per unit of electricity generated and it can utilize the fuel more efficiently without reprocessing," says Ed Lyman, a senior staff scientist with the Global Security Program at the Union of Concerned Scientists. "Out of the whole spectrum of concepts, this is the best way to go."
The TWR concept was first introduced in the 1950s, and studied by Edward Teller and Lowell Wood in 1995, but today's supercomputers can actually simulate the TWR down to the neutrons.
Most nuclear reactors today are light water reactors (LWR), cooled by water and fueled by uranium-238, or U-238 (the most common naturally occurring uranium isotope) and 4% to 5% U-235. Since mined uranium is less than 1% U235, it must undergo expensive and energy-intensive enrichment to reach the concentration of U-235 needed for LWR fuel. The byproduct of this process is depleted uranium. In the reactor, neutrons collide with U-235, which fissions, releasing energy and additional neutrons that set off a chain reaction. Some neutrons collide with U-238, producing plutonium-239 and more energy to create heat, which generates steam that turns a turbine and produces electricity. Every 18-24 months, LWRs need to be refueled, leaving behind spent fuel that is mostly uranium. The plutonium and uranium in spent fuel can be separated out and used for more fuel through reprocessing, but because plutonium can potentially be used for nuclear weaponry, reprocessing was banned in the U.S. in 1977.
Unlike the LWR, the TWR uses mostly depleted uranium as fuel, though some enriched uranium is needed to start the chain reaction and produce neutrons. These neutrons convert the depleted uranium into plutonium-239 (breeding), which is then fissioned (burned) to produce energy. The TWR can theoretically burn 60 to 100 years without refueling or removing waste. TerraPower maintains that the 700,000 metric tons of depleted uranium sitting at U.S. nuclear plants today could provide energy for 3,000 years.
Because of higher operating temperatures, thermal efficiency and fuel density, the TWR consumes less uranium than a LWR for each unit of electricity generated and is 40 times more efficient in fuel utilization. It produces fewer greenhouse gas emissions, uses less water because it is cooled by sodium and produces five times less waste than LWRs. The goal is to burn down the waste further, but what waste remains can be blended with depleted uranium to make new TWR fuel. And since enrichment and reprocessing could eventually be phased out, TWRs will be more economical than LWRs, too.
Most media still report that the TWR burns like a candle through a fixed core, but Roger Reynolds, technology advisor at TerraPower, explains that the candle model is too mechanically difficult and does not use all the neutrons efficiently. TerraPower is now focused on a cylindrical "standing wave" reactor based on the same principles. Fuel is radially shuffled in and out of the breed-burn region but the reactor does not need to be opened or shut down during the process.
"The biggest issues now are materials," says Reynolds, "because no materials last 60 years." The company is currently developing and testing materials that can withstand high radiation exposures, so that the reactor components can last as long as the fuel.
TerraPower plans to have its first demonstration reactor, the 500 megawatt (MW) electric TP-1, running by 2020. "We won't really know if it works until then," Reynolds says. Since TP-1 will probably be built outside the U.S. TerraPower is in discussion with China, Russia and India, as well as the companies Areva, Hitachi, Toshiba, GE and Westinghouse. If TP1 succeeds, TerraPower would then produce 1,000-1,500 MW commercial models.
Around the world, nuclear reactors are being developed with improved sustainability, economics, safety, reliability and nonproliferation. "Ninety percent of what is being proposed involves reprocessing" says Lyman.
CONTACTS: TerraPower, www.ter rapower.com; Union of Concerned Scientists,www.ucsusa.org/nuclear we apons_and_global_security.
RENEE CHO is director of research at INFORM, Inc. and a freelance environmental writer.
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|Date:||Mar 1, 2011|
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