Fuel for the future: Finding the best materials for Gen IV reactors
sing some current nuclear reactor materials in the high-temperature, high-pressure reactors of tomorrow is a little like trying to cook a steak with a flamethrower. The operating environments are so extreme that both the reactor materials — and your dinner — would fail.
A U.S. Department of Energy (DOE) initiative called Generation IV calls for new nuclear energy systems that are economical, safe and reliable and could be commercially deployed by 2030. One of the reactor concepts under the plan is a supercritical
water reactor (SCWR),
which combines high pressure and high temperatures to convert water into its supercritical state. This super-heated water drives a turbine and a generator converts the heat into electricity.
“The main reason for using supercritical water is that you operate at higher temperatures and pressures and you get better system efficiency,” says Assistant Professor Todd Allen. “For the same amount of heat out of your fuel, you translate that into more electricity out of your turbine. The challenge is, by going up in temperature and pressure, it’s a more aggressive environment, so the materials challenges are more severe than they would be for today’s boiling water reactor.”
With funding from the DOE Office of Nuclear Energy, Science and Technology, Allen and Senior Scientist Kumar Sridharan are studying some of the materials challenges associated with fuel containment and other components for future SCWRs.
In the temperature, radiation and pressure range that an SCWR operates, says Allen, there’s no obvious first-choice material. Initially, the researchers are evaluating three classes of materials — austenitic steels, ferritic martensitic steels and nickel-based alloys. Ferritic martensitic steels oxidize the most but are very stable against radiation; the oxide layer on austenitic steels tends to be thinner, but the nickel in these alloys can lead to transmutation products. Over long periods, the oxides can flake and fall off. “If it falls off, then it makes it easier for you to corrode new metal,” says Allen. “And ultimately, what we’re trying to do is protect as much of the original alloy as possible.”
They also learned that nickel-based alloys don’t corrode much; however, when exposed to radiation, they generally become brittle and fail.
Given that information, the two are focusing on ferritic martensitic steels and austenitic steels. “We’re trying to find ways to keep the oxide layer on the ferritic martensitic steels stable to make it thinner; the austenitic steels we want to keep the oxide layer thin like it is, but make it more stable,” says Allen.
The researchers have tested materials treated with nanometer-sized
yttria-titanium oxide
particles to improve their strength. Including
the oxide particles dropped the corrosion rate
by about half based on a comparable, non-treated material. In addition, they are attempting to implant a variety of elements into the surfaces of the steels and to dramatically change their
surface structure to understand what character-istics make them work well.
They also treated the austenitic steel alloy
800-H to change the orientation of its grains relative to each other so that the energy of the boundaries was much lower. “That treatment really improved the stability of the oxide in a 1,000-hour test,” says Allen.
So that they can change both a material’s chemical composition and its physical nature, Sridharan and Allen recently bought two pieces of equipment for their laboratory. With an arc melter, they can make their own test alloys; with a shot peen, which essentially shoots little balls at a surface, they can study how surface treatments might change how materials corrode and stress crack. “It puts the surface under compression, and when it does that, cracks don’t open up because stress is trying to close the cracks,” says Sridharan. “And this is just a guess, but it should also be able to change the grain orientation.”
Ultimately, they hope to find materials that have adequate corrosion resistance. “The stable materials form a thin, stable oxide that does not limit heat transfer too much, they don’t stress-corrosion crack, and their response to radiation is adequate (meaning that they don’t get brittle), and that they don’t have unacceptable volume changes (some materials tend to swell up under radiation),” Allen says.
The materials he and Sridharan are studying for SCWRs also are candidates for other proposed Generation IV reactors, including lead-cooled and molten-salt reactors. In their new laboratory, a SCW corrosion cell, built by Associate Scientist Mark Anderson, came online in June; in the future, the lab also will include a pot system to study corrosion in the lead-cooled reactor concept and pot and loop systems to study corrosion and heat transfer for the molten-salt reactor concept. “We’re the one place in the country that has the ability to look cross-system at how some of these materials perform,” says Allen.
