Coating could take burnables out of nuclear fuel
hen 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 additon, 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
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,”