Next-generation nuclear fuel withstands
high-temperature accident conditions
IDAHO FALLS - A safer and more efficient nuclear fuel
is on the horizon. A team of researchers at the U.S.
Department of Energy's Idaho National Laboratory (INL)
and Oak Ridge National Laboratory (ORNL) have reached a
new milestone with tristructural-isotropic (TRISO) fuel,
showing that this fourth-generation reactor fuel might
be even more robust than previously thought.
In the past three years, David Petti, director of the
Very High Temperature Reactor Technology Development
Office, and his team have studied the safety of TRISO
fuel. New insights come courtesy of post-irradiation
examination of the fuel, which has been a team effort
between INL and ORNL.
Their findings reveal that after subjecting the fuel
to extreme temperatures - far greater temperatures than
it would experience during normal operation or
postulated accident conditions - TRISO fuel is even more
robust than expected. Specifically, the team found that
even at 1,800 degrees Celsius (more than 200 degrees
Celsius greater than postulated accident conditions)
most fission products remained inside the fuel
particles, which each boast their own primary
"The release of fission products is very low," says
TRISO fuel particles are the size of poppy seeds.
Break one open, and it looks like the inside of a tiny
jaw-breaker. An outer shell of carbon coats a layer of
silicon carbide, which coats another layer of carbon and
the uranium center - where the energy-releasing fission
happens. Byproducts of the fission process have the
potential to escape the fuel, especially at very high
To study the fuel under accident conditions, Petti
and his team placed six capsules inside INL's Advanced
Test Reactor core, where they were subjected to neutron
irradiation. Then, controlled, high-temperature testing
of the irradiated fuel in furnaces at INL and ORNL
demonstrated that fission product release remains
relatively low at high temperatures postulated to occur
in accidents and beyond.
"This first series of TRISO test fuel has performed
above the team's expectations, both during its three
years in the ATR, and throughout the subsequent
high-temperature testing," says John Hunn, ORNL project
lead for TRISO fuel development and post-irradiation
"The ability of the fuel to retain fission products
at such high temperatures translates directly to
enhanced safety of the reactor," said Paul Demkowicz,
the technical lead for post-irradiation examination of
TRISO fuel for the Very High Temperature Reactor
program. "This sort of test data is important input for
reactor design and reactor licensing."
He and his team were able to identify the few
individual particles that did secrete cesium and isolate
them for further analysis. They did this by dissolving
the matrix that contained the particles - thousands in
each chalk-sized fuel pellet.
"We've developed a tool that uses computer-controlled
automation to sort through thousands of irradiated
particles and identify the rare defects," said Hunn.
"Careful study of these few defective particles, along
with the numerous particles that perform well, allows us
to complete the TRISO fuel development circle by
connecting the fabrication process and material
properties to performance in the reactor."
The insights gained will also "improve our ability to
fabricate even better particles in the future," said
Petti wants to further explore the fuel particles'
limits. "If the fuel performs well at 1,800 [degrees
Celsius], what about higher temperatures?" he said.
This revelation comes 11 years into INL and ORNL's
joint study of TRISO fuel, which began in 2002. TRISO
fuel was developed and used in Germany in the 1980s.
U.S. researchers have shown that their own version of
the fuel can achieve more than twice the burn-up levels
- that is, the amount of the fuel that is used to
release energy - clocking in at nearly 20 percent