POWER March 2021 - 26

NUCLEAR
The Allure of TRISO Nuclear
Fuel Explained
Tristructural isotropic (TRISO) particle fuel has long been used in high-temperature
gas-cooled nuclear reactors, but it is seeing a resurgence as a result of other
applications. Modern TRISO fuel designs are under consideration for an assortment
of advanced reactors, including high-temperature reactors and microreactors, and
even as accident-tolerant fuel for light water reactors.
Sonal Patel
I
n October 2020, when the U.S. Department
of Energy (DOE) announced
it chose TerraPower and X-energy to
each receive $80 million in initial federal
funding under the agency's Advanced Reactor
Demonstration Program (ARDP) to
build their two distinct advanced nuclear
reactors and begin operating them within
seven years, the advanced reactor community
was abuzz about the agency's larger
scope for X-energy. Under the award,
Rockville, Maryland-based X-energy will
deliver a commercial four-unit power plant
(likely in Washington state) based on its
Xe-100 reactor design-an 80-MWe/200MWth
pebble-bed high-temperature gascooled
reactor (HTGR). But X-energy will
also leverage the award to deliver a commercial-scale
fuel fabrication facility for
its proprietary TRISO-X TRi-structural ISOtropic
(TRISO) particle fuel, technology it
developed under the 2015 DOE Advanced
Gas Reactor (AGR) Fuel Qualification Program.
As some industry observers noted,
the award re-establishes a longstanding
commitment by the DOE to render the
U.S. into a technology purveyor of the
specialized nuclear fuel form that is expected
to quickly gain traction around the
world as new advanced reactor designs
emerge on the global power scene.
TRISO Is Complex, but Not
Complicated
As the DOE explained to POWER in
February, TRISO is essentially a " robust,
microencapsulated
fuel
form " developed
originally for use in HTGRs in the
1950s. Modern " particles " -which can
be " about the size of a poppy seed " -
typically consist of a spherical fissile
" kernel " sheathed by several layers of
pyrocarbon (PyC) and a silicon carbide
(SiC) layer. These particles can then be
packed together into different forms,
most prominently cylindrical pellets or
billiard ball-sized spherical fuel forms,
26
called
" pebbles, " using a resinated
graphite matrix material (Figure 1).
Perhaps TRISO's biggest benefit is that
each particle acts " as its own containment
system thanks to its triple-coated
layers, " the DOE explained. " This allows
them to retain fission products under all
reactor conditions. " TRISO particles are
also especially robust. " TRISO fuels are
structurally more resistant to neutron irradiation,
corrosion, oxidation, and high
temperatures (the factors that most impact
fuel performance) than traditional
reactor fuels, " the agency said. And
most importantly, they are safe. " Simply
put, TRISO particles cannot melt in a reactor
and can withstand extreme temperatures
that are well beyond the threshold
of current nuclear fuels, " it said.
While several exotic kernel compositions
and different matrix materials have
been proposed to date, so far, the DOE's
TRISO fuel qualification efforts have focused
on what it calls " conventional "
TRISO particles. These have a kernel
composed of a mixture of uranium carbide
and uranium oxide, which the industry
colloquially refers to " as uranium
oxycarbide " (UCO).
" When the uranium in a uranium dioxide
(UO2
) molecule splits, the oxygen
ions released can react with carbon in
the pyrolytic layers to form carbon monoxide
(CO) gas, " the DOE said. " At very
high burnup, the accumulated CO gas
may exert excessive pressure on the
particle coatings and cause them to fail.
The UCO mixture limits the amount of
free oxygen released from the kernel,
enabling higher burnups to be achieved,
as has been confirmed during post-irradiation
examination of TRISO fuel, " it said.
Because TRISO enables a high temperature
output in advanced reactor designs,
it could potentially boost the use of nuclear
energy beyond electricity generation
into energy sectors, for example, that are
currently dominated by fossil fuels such
www.powermag.com
1. This U.S. Department of Energy (DOE)
graphic shows different TRi-structural ISOtropic
(TRISO) particle fuel forms. On the right
is a micrograph of a coated particle with exposed
kernel and coating layers. Also shown
are examples of spherical (pebble) and cylindrical
(compact) fuel forms, and prismatic
graphite blocks. Source: DOE
as process heat, oil shale and sand reprocessing,
petroleum refining, desalination,
and hydrogen production. Meanwhile,
owing to TRISO's fuel form flexibility,
and despite its origin and historic use in
HTGRs, the conventional particle design
could be well-suited to several types of
advanced, high-temperature reactors
(HTRs), including the fluoride salt-cooled
HTRs, and microreactors. More exotic
compositions (that still retain the fundamental
TRISO coating structure) are also
being considered as accident-tolerant fuel
for light water reactors.
A 50-Year Developmental Legacy
The modern TRISO particle has evolved
through a series of improvements in
particle design, coating layer properties,
and kernel composition. According
to the DOE, the concept of nuclear fuel
microspheres with refractory coatings to
contain fission products can be traced to
the early years of the nuclear age, when
they were developed for the 20-MWth
Dragon reactor, a prismatic-core HTGR
that achieved critically in the UK in 1964.
" The first fuel charge included fissile
particles consisting of several consecutive
layers of pyrocarbon with different
POWER | March 2021
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POWER March 2021

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