Coating could take burnables out of nuclear fuel
When nuclear plant operators power up their reactors, the uranium-dioxide fuel pellets emit a hail of neutrons that pass through the walls of the long zirconium tubes that hold the fuel.
To control the neutrons, engineers currently mix boron or gadolinium, which "eat" neutrons, with the uranium pellets. However, the mixing process requires a facility regulated for radioactive materials use, increasing the fuel's cost by up to 30 percent; adding these elements to the uranium-dioxide can lower its melting point, degrade its efficiency, and requires expensive separation operations when the fuel is spent. In addition, operators of some reactor types add boron to the water, which can hasten part corrosion.
A group of UW-Madison, Westinghouse Corporation and Sandia National Laboratories researchers is developing a new method that could be both cost-effective and kinder to the reactor's environment. "Our concept is to take the boron or gadolinium out of the fuel pellets and put it on the outer walls of the tube," says Senior Scientist Kumar Sridharan. The group, which also includes Professor Mike Corradini, Associate Scientist Mark Anderson, and materials science and engineering undergraduates Scott Harrington and Alissa Johnson, received a two-year, $616,000 Department of Energy Nuclear Energy Research Initiative grant to conduct the work. The UW-Madison role includes working closely with Sandia researchers to optimize the surface alloying process through experiments and computer simulations, and preparing and characterizing samples, coordinating the project and conducting educational efforts.
During the project's first year, Sandia researchers used an ion beam surface treatment process to surface-alloy test samples of zirconium with gadolinium and will do the same with boron in the grant's second year. So far, the researchers have demonstrated that by using a high-energy ion blast, they can alloy a layer of gadolinium only one-50th the width of a human hair onto zirconium, the fuel tube material. Currently, their Westinghouse colleagues are testing inch-square samples of various compositions in an autoclave, which simulates a reactor's thermal and corrosion conditions, for up to two months. "That's where we'll know how long this thing's going to last in a nuclear reactor environment," says Sridharan.
And though it must withstand reactor conditions, the coating isn't supposed to last a long time. "The initial period, the first six months, is when you want it to do its job," he says. After that, the reactor stabilizes, the fuel ages, the neutron storm turns into a light drizzle, and the gadolinium or boron coating isn't needed.
At present, the alloying process can treat only a 4-by-4-inch square of zirconium, but researchers also are studying manufacturability issues, including those related to alloying on a tube's curved surface. "It's reasonable that it could be economical," says Mark Anderson.