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FUTURE REACTOR MATERIALS
Can The Next Generation Take The Heat?

ORNL is the U.S. leader in developing materials for 21st century, high-temperature nuclear plants that will produce power and hydrogen.


Advanced materials that will be used in components of Generation IV reactors must be able to survive intense neutron exposures, corrosive environments, and high temperatures.
 

Oak Ridge National Laboratory has long been a world leader in materials research, and now with the resurgence of interest in nuclear energy, ORNL has a leading role in developing and selecting materials for the next generation of nuclear power plants. According to the vision of the U.S. government for 2015 and beyond, these plants will produce hydrogen as well as electricity, efficiently providing clean energy for the next generation of cars, trucks, homes, and factories. To achieve the vision, the Department of Energy has called upon ORNL to help cross one huge hurdle: finding materials that can withstand high temperatures, high radiation levels, high pressures, and harsh chemical conditions.

As the National Technology Director for Materials for DOE's Generation IV Reactor Program, ORNL's Bill Corwin understands the challenge. DOE's Office of Nuclear Energy, Science and Technology, has the U.S. lead for the International Generation IV Nuclear Energy Systems Initiative, which includes 10 partners from 9 other nations, as well as the European Union.

Corwin is building a team to resolve the materials issues for the four reactor concepts that DOE favors out of the 122 concepts considered in a recent international road-mapping process. The four selected concepts are the Very High Temperature Reactor (VHTR), the leading candidate for hydrogen production; the Gas-Cooled Fast Reactor (GFR); the Lead-Cooled Fast Reactor (LFR); and the Supercritical Water-Cooled Reactor (SCWR), a much-higher-efficiency variant of the boiling-water reactor.

"We understand that a limiting factor for these reactors will be the materials used in their operation," Corwin says. "Reactor materials will be exposed to very high temperatures, intense neutron radiation, and corrosive environments—in many cases, all three at once."

DOE's goal for Gen IV nuclear energy systems is a 60-year life rather than the 40-year life of today's light-water reactors. Because some of these reactors will produce process heat for hydrogen generation, the equation for materials compatibility changes.

"In a future hydrogen production plant, a reactor may be connected by a long pipe to a chemical plant to produce hydrogen, using the reactor's heat to drive a thermochemical separation cycle. The best heat transfer medium must be selected to get the process heat from the reactor to the chemical plant. Some of the reactants used to make the hydrogen at high temperatures are really nasty."

Producing Hydrogen Economically

To produce hydrogen economically, a reactor must operate at extremely high temperatures. Thus the VHTR has been selected for future hydrogen production plants. "The VHTR has the highest priority among the U.S. reactor concepts," Corwin says, "because it fits into the President's plan for the hydrogen economy."

In the envisioned hydrogen economy, hydrogen will be used in fuel cells to propel automotive vehicles and power buildings. By making hydrogen the fuel of choice for transportation and building sectors, the nation will be less reliant on imported fossil fuels and will, subsequently, reduce its emissions of climate-altering carbon dioxide. Because most hydrogen today is obtained from natural gas, producing significant greenhouse gases as a by-product, DOE plans to use nuclear reactors to produce hydrogen in an environmentally friendly fashion.

DOE's Office of Nuclear Energy plans to build a VHTR, also called the Next Generation Nuclear Plant (NGNP), by 2015 or soon thereafter. "Between now and 2009 it will cost about $200 million to answer the research questions about different materials and to select or develop and then qualify the needed materials for the NGNP," Corwin says.

ORNL principal investigators (PIs) working on materials research and development (R&D) for the selected advanced reactor types are Randy Nanstad, radiation effects; Bob Swindeman, high-temperature materials and alloys; Roger Stoller, microstructural analysis and modeling; Jim Corum (retired) and Tim McGreevy, high-temperature design methods; Tim Burchell, graphites; Lance Snead, ceramics; James Klett, carbon-carbon composites; and Dane Wilson, materials compatibility. As National Technology Director for Gen IV Materials, Corwin, working with ORNL PIs and representatives from other national laboratories responsible for leading the design activities for the various reactor concepts, has put together an integrated plan for materials R&D for the four reactor concepts.

For the NGNP, ORNL researchers are examining existing materials because of the tight deadline. They are considering candidate materials they developed for DOE's fusion and fossil energy programs because they can survive higher temperatures. Those materials include 9 chrome-1 moly vanadium steel and tungsten-vanadium steel (developed by ORNL's Ron Klueh as a reduced activation alloy for fusion devices) and a variant on the Hastelloy material developed primarily at ORNL years ago for nuclear reactor components.

"We are trying to identify a material that might be used in a compact heat exchanger, which could consist of thin, closely spaced, parallel layers containing alternating, perpendicularly oriented micro-channels," Corwin says. "The helium cooling the reactor would pass through the module in one direction and transfer its heat to, say, molten salt passing through in the other direction on its way to the hydrogen production plant."

Graphite Structure

Unlike the other three reactor concepts, the NGNP will have a graphite structure to contain the fuel and moderate its neutrons to sustain heat-producing fission reactions."We are working with graphite manufacturers to help them make graphites that are sufficiently similar to nuclear-qualified graphites produced in the 1970s," says Tim Burchell of ORNL's Metals and Ceramics Division. "We want to show that the VHTR graphites behave in a predictable manner during irradiation, based on knowledge from the past 40 years. Then we'll develop fundamental materials physics models so we can predict with confidence the material properties in the new graphites."

Many of the metal components used inside current reactor vessels would not survive the 1200oC temperatures that might occur during an accident. Carbon-carbon composites could be candidates for these components, but ORNL researchers must determine how well they will perform under long-term radiation exposure and long-term and short-term oxidation. One potential problem is that if air enters the hot reactor internals, the composites would be converted to carbon dioxide. Silicon carbide is also being studied for potential use in control rods, which absorb the reactor's neutrons.

As they race the clock to meet America's energy needs, Corwin and his team members are absorbed by the challenging project of finding materials that can take the heat of a 21st-century nuclear reactor.

 

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