Radiation studies key to nuclear reactor life, recycling
spent fuel
wo
projects to study advanced materials and fuel forms for both current
and future nuclear reactors received funding of approximately $1 million
under the Department of Energy Nuclear Energy Research Initiative (NERI).
The NERI program supports research and development under three Department
of Energy nuclear initiatives: Generation IV nuclear energy systems,
advanced fuel cycles, and nuclear hydrogen.
In one three-year project,
UW-Madison nuclear engineers will study the resistance of oxide, carbide
and nitride nuclear fuel “matrix” materials—the vessels
that contain nuclear fuel—to radiation damage. A second project
will exploit recent advances in computational power and technique to
develop computer models of how a reactor’s structural materials
behave as a result of long-term radiation exposure. The projects were
among 24 selected for total funding of $12 million; UW-Madison is among
five universities to receive funding for multiple projects.
Matrix materials are a key
element of future fast-spectrum reactors, which are capable of safely
and efficiently recycling spent nuclear fuel. The nuclear fission process
produces high-energy radioactive neutrons, called “fast”
because of their great energy. Current thermal reactors use a moderator
to reduce the neutrons’ velocity, making them capable of sustaining
the nuclear fission reaction using simpler fuel.
But to recycle and minimize
the waste impact of the spent fuel, you need to keep those neutrons
fast, says Assistant Professor Todd
Allen.
He and Professor James
Blanchard are studying proposed matrix materials such as zirconium
nitride or titanium carbide as a replacement for the current carbon
matrix used in gas-cooled reactor fuel. “Replacing a lot of the
carbon with zirconium atoms, for example, means you’ll slow down
neutrons less,” says Allen.
While they know that materials
in the nitride or carbide families are more effective at keeping neutrons
up to speed, what’s not clear is how the materials hold up under
constant radiation. Allen and Blanchard have constructed a radiation
damage test facility on one beam line of the department ion accelerator.
In an experiment that simulates long-term radiation exposure, they will
bombard their candidate materials with a high-energy ion beam and study
how each one holds up.
“It’s all in
the context of devising new fuel forms that will allow you to efficiently
recycle reactor fuel in a way that minimizes the net waste output from
the entire fuel cycle,” says Allen. “And the reason for
looking at recycle is to limit the number of underground repositories
you have to build.”
Another project involves
applying complex materials modeling to nuclear reactors. In it, Allen
and Dane Morgan, an assistant professor of materials science and engineering,
will incorporate the properties of iron, chromium and nickel into more
complete computer models of radiation damage in steel, a common reactor
structural material. Previously, a lack of computing power limited such
models to single pure materials like copper or iron. “People have
learned a lot about radiation damage,” says Allen. “But
you never build anything out of just copper or just iron.” The
effort may lead to structural materials that are better able to withstand
long-term exposure to radiation—in some cases, nearly 60 years.
In a reactor, high-energy
neutrons can knock the individual atoms in steel out of their normal
positions, bumping them elsewhere in the material or wedging them between
their normal positions. As a result, vacant spots form in the steel
and allow diffusion that can lead to unacceptable changes in shape or
creation of brittle materials. “It all happens because atoms diffuse
and form structures that either change the volume or make it brittle,”
says Allen.
And that means costly reactor
components may need to be replaced sooner than desired.
In addition, the researchers
have seen evidence that when vacancies cluster together and form larger
voids, the composition of the steel around those voids is different
from the composition of the material as a whole. “We’d really
like to know how to predict these local composition changes, but to
do that requires you to understand how these diffusion events happen,
and how they happen as a function of composition,” says Allen.
That’s where Morgan’s
computer models will come in handy. Current diffusion data is measured
at approximately 1,000 degrees Celsius, but reactors operate at much
lower temperatures. Researchers can only speculate that the diffusion
and composition changes occur in the same manner at lower temperatures.
Add radiation and the diffusion mechanisms become more complex. Morgan’s
models enable the researchers to calculate diffusion parameters in complex
materials under radiation.
Coupled with accelerated
ex situ reactor experiments, which are fast to perform but are not strictly
representative of the multi-year damage that occurs in a nuclear system,
the duo’s more accurate models can help researchers predict how
materials such as steel might behave over a reactor’s lifetime.
“And if you really start to understand the fundamental mechanisms
of radiation damage, you gain the ability to predict how changes to
the material could improve its properties,” says Allen. “If
you understand things on a fundamental basis, it’s easier to translate
your accelerated experiment into the performance of the real system.”
